| Title of Invention | A CHANNEL COMMUNICATION DEVICE AND A METHOD FOR A CDMA COMMUNICATION SYSTEM |
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| Abstract | A traffic channel transmission device for a COMA cmmunication system using a plurality of coding rates and orthogonal codes, determines a present channel condition and adactively selects a coding rate and an orthogonal code according to the determination. In the device, a channel receiver receives a channel signal and a controller analyzes the received signal to decide an environment of a channel in service and generates a coding rate select signal and orthogonal code information according to the decision result- A channel transmitter include* a channel encoder for encoding transmission data at a coding rate selected according to the coding rate select signal and an orthogonal modulator for generating an orthogonal code according to the orthogonal code information to spread the encoded data with the generated orthogonal code, whereoy the channel transmitter adaptively encodes and spreads the transmission data according to the channel environment- The orthogonal code information includes a number and a length of the orthogonal code. |
| Full Text | RATE CONTROL DEVICE AND METHOD FOR CDMA COMMUNICATION SYSTEM 1. Field of the Invention The present invention relates to a channel data transmission/reception device and method for a CDMA communication system, and in particular, to a device and method for adaptively controlling a rate of channel data according to a channel environment. 2. Description of the Related Art At present, CDMA (Code Division Multiple Access) communication systems are implemented according to the IS-95 Standard. With the progress of the mobile communication technology, the subscribers to the mobile communication service are increasing in number and there are many demands for the various services. To meet the subscribers' demands, there have been proposed many methods. FIG. 1 illustrates a structure of a forward traffic channel transmission device for the CDMA communication system, wherein the traffic channel includes a fundamental channel and a supplemental channel. Referring to FIG. 1, a channel encoder and puncturing part 10 encodes and punctures input data and outputs symbol data. A convolutional encoder or a turbo -1 A- encoder can be used for the channel encoder and puncturing part 10. A symbol repetition part 20 repeats the respective encoded symbol data for the input data having different bit rates so that they have the same symbol rate. An interleaver 30 interleaves an output of the symbol repetition part 20. A block interleaver can be used for the interleaver 30. A long code generator 91 generates long codes for the user identification, which are differently assigned to the respective subscribers. A decimator 92 decimates the long codes so as to match a rate of the long codes to a rate of the symbols output from the interleaver 30. A mixer 40 mixes the encoded symbols output from the interleaver 30 with the long codes output from the decimator 92. A signal mapping part 50 maps binary data output from the mixer 40 into 4-level data by converting data "0" to "+1" and data "1" to "-1". An orthogonal modulator 60 modulates data output from the signal mapping part 50 with an orthogonal code. A Walsh code can be used for the orthogonal code. In this case, the Walsh codes of lengths 64,128 and 256 bits can be used, A spreader 70 spreads the orthogonal modulation signal output from the orthogonal modulator 60 by combining it with spreading sequences. PN (Pseudo-random Noise) sequences can be used for the spreading sequences. Accordingly, a QPSK (Quadrature Phase Shift Keying) spreader can be used for the spreader 70. A gain controller 80 controls a gain of the spread signal input from the spreader 70 according to a gain control signal Gc, In operation, when the convolutional encoder is used for the channel encoder and puncturing part 10, a coding rate is 1/3 and a constraint length k is 9, -2- for the case of an IS-95 system. Therefore, one input data bit is encoded into three encoded bits (i.e., three symbols) in the channel encoder and puncturing part 10 (which performs 1/3 rate convolutional encoding or 1/3 rate forward error correction (FEC)). A purpose of using the FEC is to apply a coding gain to a channel so as to compensate for an increase in a BER (Bit Error Rate) of a mobile station (for the case of a forward link) and a base station (for the case of a reverse link), with respect to a channel having a reduced SNR (Signal-to-Noise Ratio) due to an increase in a signal path loss, a noise and an interference. However, the CDMA communication system cannot provide a good communication service, when the mobile station is located at an outer service area of the base station or is in a bad channel environment. In this case, it is preferable to change the coding rate in order to provide the good communication service even in bad channel environment. That is, when the channel SNR is reduced due to the bad channel environment or the increased distance between mobile station and the base station, it is preferable to use the coding rate (or FEC rate) lower than the present coding rate of 1/3. More specifically, when the distance between the base station and the mobile station increases, a reception device is very susceptible to the path loss or the noise on the link channel and the interference, so that the channel SNR is reduced unless a transmission device increases the transmission power or performs a pertinent compensation. Therefore, when the traffic channel transmission device with the fixed channel structure of FIG, 1 has the increased BER (Bit Error Rate) due to the reduction in the SNR, the base station increases a forward link traffic power in order to compensate for the increase in the BER. Therefore, it is preferably that the -3- channel gain of the 1/3 coding rate should be lower than that of the lower coding rate. According to circumstances, the channel gain is lower by about 0.2-ldB than that of a scheme with a lower coding rate of 1/6. For example, the forward reception power of a mobile station using the 1 /3 coding rate is lower by about 1 dB than that of a mobile station using the 1/6 coding rate, when the mobile station is in the long distance from the base station or in the bad channel environment. Therefore, the base station should increase the forward link transmission power, resulting in a waste of the transmission power and a low communication performance. Unlike the channel transmission device with the fixed channel structure of FIG. 1, a channel transmission/reception device for a 3rd generation multicarrier CDMA system proposed in the TIA/EIA TR45.5 conference has a scheme for transmitting and receiving the respective channel data by distributing them to the multicarrier. For example, when three carriers are used and a rate 1/3 encoder is used, the multicarrier scheme encodes the respective input data bit into three encoded bits (i.e., symbols) using the rate 1/3 encoder and are divisionally transmits the encoded bits using the three carriers after repetition and interleaving. This is well disclosed in Korean patent application No. 61616/1997 filed by the applicant of this invention. Here, the respective carriers have a bandwidth of 1.2288Mhz (hereinafter, called 1.25Mhz for short) which is identical to the IS-95 channel bandwidth. Therefore, the three carriers have the bandwidth of 3.6864Mhz in total, which is identical to three channel bandwidths. The forward link of the 3G multicarrier system can employ an overlay method where it shares the same frequency band with the IS-95 forward channel. - 4 - In this case, it may be interfered with the IS-95 system. In addition, it is preferable to use the coding rate lower than the present coding rate of 1/3, even when the channel SNR is reduced due to the bad channel environment or the increased distance between the mobile station and the base station. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide device and method for adaptively changing a rate of channel data according to the channel environment in a CDMA communication system. It is another object of the present invention to provide a traffic channel transmission device for a CDMA communication system having a plurality of coding rates and orthogonal codes, which determines a present channel condition and adaptively selects a coding rate and an orthogonal code according to the determination, and a method for operating the same. It is further another object of the present invention to provide a traffic channel transmission device for a CDMA communication system having a plurality of coding rates and orthogonal codes, which selects the coding rate and the orthogonal code according to control information transmitted from a transmission device, and a method for operating the same. It is still another object of the present invention to provide a traffic channel transmission device for a multicarrier CDMA communication system having a plurality of coding rates and orthogonal codes, which determines a present channel - 5 - condition and adaptively selects the coding rate and the orthogonal code according to the determination, and a method for operating the same. It is yet another object of the present invention to provide a traffic channel transmission device for a multicarrier CDMA communication system having a plurality of coding rates and orthogonal codes, which selects the coding rate and the orthogonal code according to control information transmitted from a transmission device, and a method for operating the same. BRIEF DESCRIPTION OF THE ACCOMPANYTOR RAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which like reference numerals indicate like parts. In the drawings: FIG. 1 is a diagram illustrating a channel transmission device in a conventional CDMA communication system; FIG. 2 is a diagram illustrating a decision apparatus for changing a rate according to a channel environment in a CDMA communication system according to an embodiment of the present invention; FIG. 3 is a diagram illustrating a single carrier forward traffic channel transmission device which has plural encoders of different rates and selects the encoders according to the channel environment in the CDMA communication system; FIG. 4 is a diagram illustrating a reverse traffic channel reception device which has plural decoders of different rates and receives the variable rate input data - 6 - according to the channel environment in a CDMA communication system; FIG. 5 is a flowchart illustrating a procedure in which a mobile station receives an order from a base station to select an encoder using a paging channel and an access channel during a call setup according to an embodiment of the present invention; FIG. 6 is a flowchart illustrating a procedure in which the mobile station receives an order from the base station to change the rate during the call progressing according to an embodiment of the present invention; FIG. 7A is a flowchart illustrating a procedure in which the base station changes a rate upon reception of a rate change request message from the mobile station according to an embodiment of the present invention; FIG. 7B is a flowchart illustrating a procedure in which the base station changes a rate of the mobile station even when the rate change request message is not received from the mobile station according to an embodiment of the present invention; FIG. 8 is a flowchart illustrating a procedure in which the mobile station changes the rate upon reception of a rate change request message from the base station and analyzes a channel environment to send a rate change request message to the base station based on the analysis according to an embodiment of the present invention; FIG. 9 is a flowchart illustrating a procedure in which the base station changes an orthogonal code during the rate change according to an embodiment of the present invention; and FIG. 10 is a diagram illustrating a multicarrier forward traffic channel transmission device which has plural encoders of different rates and adaptively. selects the encoders according to the channel environment in the CDMA - 7 - communication system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A traffic channel transmission/reception device according to an embodiment of the present invention can increase a channel performance by decreasing a coding rate to increase a coding gain, when a path loss or an interference increases between a base station and a mobile station on a CDMA link channel. For example, when using a 1/6 coding rate rather than a 1/3 coding rate, it is possible to improve the performance against an increase in the signal path loss, noise and interference. Therefore, in a relatively bad channel environment, it is more efficient to use the lower coding rate of 1/6 rather than the higher coding rate of 1/3. The embodiment implements a method of improving a receiving performance at a receiver by channel encoding at two different rates and applies it to a 3G multicarrier CDMA system. In the CDMA communication system, use of an encoder having the lower coding rate increases the channel gain, improving the channel performance. In the light of this, a system having a fixed coding rate adaptively selects the lower coding rata and the fixed coding rate to improve the performance. In the embodiment, the channel transmission device comprises plural channel encoders having different coding rates and corresponding orthogonal modulators for generating orthogonal codes, and can adaptively control the coding rate and the orthogonal modulation according to the channel environment. Further, a channel reception device examines the coding rate and the orthogonal code according to control information output from the channel transmission devices and thereafter, performs orthogonal - 8 - demodulation and channel decoding for the received signal according to the examination. In the following, although the present invention will be described on the assumption that two coding rates of 1/3 and 1/6 are used by way of example, it is also possible to use other coding rates in addition to the two coding rates. Moreover, in the embodiment, a description will be made as to the traffic channel for the forward link. In this case, the transmission device becomes the base station and the reception device becomes the mobile station. Besides, in the specification, the coding rate and the FEC rate are used in the same meaning. FIG. 2 is a decision apparatus for analyzing a channel environment and selecting a rate depending on the analysis according to an embodiment of the present invention. Referring to FIG. 2, a receiver 211 is a block for processing a signal received from the other station (a base station or a mobile station). The receiver 211 extracts a power control bit (PCB) from the received signal to detect a received signal strength indicator (RSSI), and delivers input data information INFO to a decision block 213. The decision block 213 analyzes the INFO, the PCB and the RSSI output from the receiver 211 and generates, when the rate change is required, a select signal Csel for selecting a coding rate and orthogonal code number and length signals Wno and Wlength for selecting the orthogonal code corresponding to the selected coding rate. The decision block 213 compares a signal gain, the number - 9 - of power increase requests (i.e., the number of the PCBs) and an energy of the RSSI with the respective threshold values to detect the channel environment. That is, the decision block 213 generates the signals Csel, Wno and Wlength for selecting the lower FEC rate, when the input parameters have values lower than the threshold values, i.e., when the signal gain S_high_Th, (a PCB value accumulated for a particular duration) > P_low__Th, and EfRSSI] > R_high_Th. In deciding the change of the rate, the decision block 213 can use all or some of the parameters. A transmitter 215 transmits messages MSG, including a message required for the rate change, output from the decision block 213 to the other station (the other mobile station or the base station), FIG. 3 illustrates a structure of a forward link traffic channel transmission device including a rate 1/3 encoder and a rate 1/6 encoder according to an embodiment of the present invention. Referring to FIG. 3, a selector 301 has a first output end connected to a first encoder 311 and a second output end connected to a second encoder 312. The selector 301 receives input data to be transmitted and selectively outputs the input data to the first eneoder 311 or the second encoder 312 according to the select - 10 - signal Csel output from the decision block 213. The first encoder 311, upon reception of the data input from the selector 301, encodes and punctures the input data into data symbols at a first coding rate (the 1/3 coding rate). That is, the first encoder 311 encodes one input data bit into three symbols. A convolutional encoder or a turbo encoder can be used for the first encoder 311. A first symbol repetition part 321 receives the data encoded at the first coding rate, and repeats the symbols output from the first encoder 311 so as to match the symbol rates of the data having different bit rates. A first interleaver 331 interleaves first encoded data output from the first symbol repetition part 321. A block interleaver can be used for the first interleaver 331. The second encoder 312, upon reception of the data input from the selector 301, encodes and punctures the input data into data symbols at a second coding rate (the 1/6 coding rate). That is, the second encoder 312 encodes one input data bit into six symbols. A convolutional encoder or a turbo encoder can be used for the second encoder 312. A second symbol repetition part 322 receives the data encoded at the second coding rate, and repeats the symbols output from the second encoder 312 so as to match the symbol rates of the data having different bit rates. A second interleaver 332 interleaves second encoded data output from the second symbol repetition part 322. A block interleaver can be used for the second interleaver 332. A long code generator 391 generates long codes for the user identification, which are differently assigned to the respective subscribers. A decirnator 392 decimates, the long codes so as to match a rate of the long codes to a rate of the symbols output from the interleavers 331 and 332. A selector 393 selectively - 11 - outputs the decimated long code output from the decimator 392 to a mixer 341 or a mixer 342 according to the select signal Csel. The selector 393 switches the decimated long code to the first mixer 341 to select the 1/3 coding rate and to the second mixer 342 to select the /16 coding rate. The mixer 341 mixes the first encoded data output from the first interleaver 331 with the long code output from the selector 393. The second mixer 342 mixes the second encoded data output from the second interleaver 332 with the long code output from the selector 393. A first signal mapping part 351 converts levels of the binary data output from the first mixer 341 by converting data "0" to "+1" and data " 1" to "-1". A first orthogonal modulator 361 includes a first orthogonal code generator (not shown) which generates a first orthogonal code for orthogonally modulating the first encoded data according to the orthogonal code number and length signals Wno and Wlength output from the decision block 213. The first orthogonal modulator 361 multiples the first orthogonal code generated according to the orthogonal code number and length signals Wno and Wlength by the data output from the first signal mapping part 351 to generate a first orthogonal modulation signal. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 256 is used for the data encoded at the first coding rate of 1/3. A second signal mapping part 352 converts levels of the binary data output from the second mixer 342 by converting data "0" to "+1" and data "1" to "-1". A second orthogonal modulator 362 includes a second orthogonal code generator (not shown) which generates a second orthogonal code for orthogonally modulating the second encoded data according to the orthogonal code number and length signals Wno and Wlength output from the decision block 213. The second - 12 - orthogonal modulator 362 multiples the second orthogonal code generated according to the orthogonal code number and length signals Wno and Wlength by the data output from the second signal mapping part 352 to generate a second orthogonal modulation signal. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 128 is used for the data encoded at the second coding rate of 1/6. A spreader 370 combines the first and second orthogonal modulation signals output from the first and second orthogonal modulators 361 and 362 with the received spreading sequence to spread a transmission signal. Here, the PN sequence can be used for the spreading sequence and the QPSK spreader can be used for the spreader 370. A gain controller 380 controls a gain of the spread signal input from the spreader 370 according to a gain control signal Gc. Now, a description will be made as to an operation of the traffic channel transmission device with reference to FIGs. 2 and 3. The decision block 213 analyzes the parameters PCR, RSSI and INFO output from the receiver 211 and determines whether to change the rate according to the analysis. Here, the parameters include the received signal strength RSSI, the accumulated value of the PCB received during a particular duration, and the message INFO representing that the other party requests the change of the rate during communication. First, the transmitter 215 determines whether the strength RSSI of the signal received during communication is lower than the threshold value. Here, that the strength RSSI of the received signal is lower than the threshold value means that the present radio sensitivity is poor. In this case, the - 13 - decision block 213 may generate the signals Csel, Wno and Wlength for decreasing the present rate. Also, the mobile station examines the signals transmitted from the base station and outputs the power control bit PCB for controlling the transmission power of the base station through the reverse link. The base station then examines the power control bit PCB from the mobile station and counts the number of the power-up PCBs and the number of the power-down PCBs. When the count value of the power-up PCBs exceeds a predetermined value, the decision block 213 may generate the control signals for increasing the present rate. On the contrary, when the count value of the power-down PCBs exceeds a predetermined value, the decision block 213 may generate the control signals for decreasing the present rate. In addition, the rate change request can be made even at the mobile station. In this case, the mobile station makes the request using the message INFO, and the decision block 213 in the base station then receives the request message INFO through the receiver 211. Although the decision block 213 can use other parameters in addition to the above parameters, it is assumed that the embodiment uses only the three parameters stated above. Furthermore, depending on how the decision block 213 is designed, it is possible to change the rate whenever the respective parameters are received or only when the parameters are received all. In addition, when the channel environment becomes poor, the decision block 213 can improve the channel environment by selecting the lower coding rate. In opposition, when the channel environment is improved, the decision block 213 can restore the coding rate to the - 14 - original higher coding rate. To change the coding rate as stated above, the decision block 213 generates the new orthogonal code number and length signals Wno and Wlength for allocating a new channel during the change of the coding rate. This is because when the coding rate is changed, the orthogonal code should be also changed. Accordingly, the decision block 213 generates the select signal Csel for selecting the encoder having the corresponding coding rate, and the orthogonal code number and length signals Wno and Wlength for generating the new orthogonal code corresponding to the selected coding rate. When the encoder having the lower coding rate is selected, the shorter orthogonal code should be generated; when the encoder having the higher coding rate is selected, the longer orthogonal code should be generated. FIG. 3 shows the transmission channel structure in which the forward link is switched to the first encoder 311 ofthe 1/3 FEC rate or the second encoder 312 of the 1/6 FEC rate according to the change of the channel environment. Referring to FIG. 3, the data input path is switched to the encoder 311 or the encoder 312 by the selector 301. Thus, in the transmission device, the transmission data undergoes the different FEC rates according to the data path switching. That is, based on the select signal Csel output from the decision block 213, the selector 301 switches the input data to the first encoder 311 when the channel environment is good, and switches the input data to the second encoder 312 when channel environment is poor. In addition, since the orthogonal code should be also changed according to - 15 - the change of the FEC rate, it is necessary to select one of the orthogonal modulators 361 and 362 according to the change of the FEC rate. That is, when the first encoder 311 is selected to use the 1/3 FEC rate, the orthogonal code generator in the first orthogonal modulator 361 generates the orthogonal code of length 256 according to the orthogonal code number and length Wno and Wlength. Therefore, the orthogonal modulator 361 multiplies the signal encoded at the 1/3 FEC rate by the orthogonal code to generate the first orthogonal modulation signal, and the spreader 3 70 spreads the first orthogonal modulation signal using the PN sequences PNI and PNQ. Furthermore, when the second encoder 312 is selected to use the 1/6 FEC rate, the orthogonal code generator in the second orthogonal modulator 362 generates the orthogonal code of length 128 according to the orthogonal code number and length Wno and Wlength. Therefore, the orthogonal modulator 362 multiplies the signal encoded at the 1/6 FEC rate by the orthogonal code to generate the second orthogonal modulation signal, and the spreader 370 spreads the second orthogonal modulation signal using the PN sequences PNI and PNQ. As can be appreciated from the foregoing description, there is no change in structure of the spreader 370 for spreading the orthogonal modulation signal using the PN sequences. Accordingly, the 1/6 FEC rate scheme is identical in structure to the 1/3 FEC rate scheme, except the encoder and the interleaver. In the 1/6 FEC rate scheme, the bit rate of the final stage is increased from 576 to 1152 bits per frame. In addition, the interleaver size is also increased twice. FIG. 4 illustrates the structure of a reception device according to an - 16 - embodiment of the present invention. The reception device is controlled by a decision block 213 having the same structure as that shown in FIG. 2. In the figure, a despreader 410 despreads a received signal by combining the received signal with the spreading sequences which are the PN sequences. A selector 420 has a first output end connected to a first orthogonal demodulator 431 and a second output end connected to a second orthogonal demodulator 432. The selector 420 switches the despread signal output from the despreader 410 to the first orthogonal demodulator 431 or the second orthogonal demodulator 432 according to a select signal Csel output from the decision block 213. The first orthogonal demodulator 431 includes a first orthogonal code generator for generating a first orthogonal code according to the orthogonal code number and length signals Wno and Wlength output from the decision block 213. When connected to the selector 420, the first orthogonal demodulator 431 generates the first orthogonal code according to the orthogonal code number and length signals Wno and Wlength and multiplies the despread data by the first orthogonal code to output a first orthogonal demodulation signal. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 256 is used for the data encoded at the 1/3 coding rate. A first signal demapping part 441 demaps the 4-level signal output from the first orthogonal demodulator 431 into binary data by converting data "+1" to "0" and data "-1" to " 1". The second orthogonal demodulator 432 includes a second orthogonal code generator for generating a second orthogonal code according to the orthogonal code number and length signals Wno and Wlength output from the decision block 213. When connected to the selector 420, the second orthogonal demodulator 432 -17 - generates the second orthogonal code according to the orthogonal code number and length signals Wno and Wlength and multiplies the despread data by the second orthogonal code to output a second orthogonal demodulation signal. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 128 is used for the data encoded at the 1/6 coding rate. A second signal demapping part 442 demaps the 4-level signal output from the second orthogonal demodulator 432 into binary data by converting data "+1" to "0" and data "-1" to "1" A long code generator 491 generates a long code identical to that generated at the transmitter. Here, the long codes are the user identification codes, and the different long codes are assigned to the respective subscribers. A decimator 492 decimates the long code so as to match a rate of the long code to a rate of the signals output from the signal demapping parts 441 and 442. A selector 493 switches the decimated long code output from the decimator 492 to a mixer 451 or a mixer 452 according to the select signal Csel. In other words, the selector 493 switches the decimated long code to the first mixer 451 to select the 1/3 coding rate, and switches the decimated long code to the second mixer 452 to select the 1 /6 coding rate. The first mixer 451 mixes an output of the signal demapping part 441 with the long code to delete the long code contained in the received signal, and the second mixer 452 mixes an output of the signal demapping part 442 with the long code to delete the long code contained in the received signal. A first deinterleaver 461 deinterleaves the received signal output from the first mixer 451 to rearrange the interleaved first encoded data into the original state. A first symbol extraction part 471 extracts the original encoded data by deleting the -18 - symbol-repeated encoded data from the output of the first deinterleaver 461. A first decoder 481 having a 1/3 decoding rate, decodes the encoded data output from the first symbol extraction part 471 into the original data. A second deinterleaver 462 deinterleaves the received signal output from the second mixer 452 to rearrange the interleaved second encoded data into the original state. A second symbol extraction part 472 extracts the original encoded data by deleting the symbol-repeated encoded data from the output of the second deinterleaver 462. A second decoder 482 having a 1/6 decoding rate, decodes the encoded data output from the second symbol extraction part 472 into the original data. As illustrated in FIG. 4, the reception device of the CDMA communication system, which changes the rate according to the channel environment, has areverse construction of the transmission device shown in FIG, 3. As described above, the embodiment proposes a method of using the 1/6 FEC rate for the communication between the base station and mobile station in the distance against the decrease in the SNR or the increase in the BER due to the bad channel environment, in order to provide the better channel environment as compared with the case where the 1/3 FEC rate is used. In this case, one base station uses both the 1/3 FEC rate and the 1/6 FEC rate; there are 256 available orthogonal codes of length 256 when only the 1/3 FEC rate is used and there are 128 available orthogonal codes of length 128 when only the 1/6 FEC rate is used. However, when the two orthogonal code sets are both used, use of one orthogonal code of length 128 makes two of the orthogonal codes of length 256 unavailable. -19- That is, use of one orthogonal code of length 256 makes one orthogonal code of length 128 unavailable. This is because there exist orthogonal codes having the correlation between the two orthogonal code sets. For this reason, use of the 1/6 FEC rate is limited. For example, it is possible to limit the number of the channels using the 1/3 FEC rate by limiting use of the 1/6 rate encoder to the communication between the mobile station and the base station including the high link channel having the signal path loss, the high signal transmission power or the high BER. In addition, since use of one orthogonal code of length 128 makes it impossible to use two orthogonal codes of length 256, the number of the link channels using the rate 1/6 encoder is limited as long as it is possible to assign sufficient orthogonal codes the mobile stations. When using this method, the base station should be so designed as to switchably use the rate 1/3 encoder and the rate 1/6 encoder. The base station should order the mobile station in the predetermined condition to switch from the 1/3 FEC rate to the 1/6 FEC rate, and order the other mobile station not in the predetermined condition to switch from the 1/6 FEC rate to the 1/3 FEC rate. Moreover, in some cases, it is also possible to initially select one of the 1/3 FEC rate and the 1/6 FEC rate. Also, the base station may allow the mobile station requesting the high traffic channel transmission power to preferentially use the 1/6 FEC rate according to the remainder of the orthogonal codes, without fixing the setting condition for determining whether to use the 1/3 FEC rate or the 1/6 FEC rate. Other possible setting conditions can be determined depending on the receiving power of the forward pilot channel, and the signal path loss, fading and signal transmission power of the forward link or the reverse link. -20- As for distribution of the orthogonal codes, since the orthogonal codes are generated through the Hadamard transform, there exist non-orthogonal codes between a 2N*2N orthogonal code set and a 2(N+1)*2(N+1) orthogonal code set. Therefore, when assigning the orthogonal codes among the 2N*2N orthogonal code set in a base station using both the two orthogonal code sets, a base station managing the 2(N+1)*2(N+1) orthogonal code set can assign the orthogonal codes which maintain the orthogonality with the existing assigned orthogonal code of length 2(N+1). This means that the base station should examine the correlation between every new orthogonal code of length 2N and all the existing assigned orthogonal codes of length 2(N-H). Accordingly, from the Hadamard transform, it is possible to understand the following characteristics. That is, when the orthogonal codes of length 2N are A, B, C,..., the orthogonal codes of length 2(N+1) can be represented as AA, BB, CC, ..., AA', BB', CC, .... (where A', B' and C are complements of A, B and C, respectively), and the orthogonal code of length 2N can be represented as A, B, C, D.... Therefore, the orthogonal codes should be assigned in such a order that the orthogonal codes of length 2(N+1) can be used as many as possible, thereby maximally preserving the orthogonal resources of length 2(N+1). This is a method for maintaining 2N-M/2 available orthogonal codes of length 2N, when, for example, M orthogonal codes of length 2(N+1) are assigned as AA, AA', BB, BB', ... (or AA', AA, BB', BB,...). The number of available ones among the orthogonal codes of length 2N can be maximized by managing the resources such that when an arbitrary orthogonal code of length 2N is defined as X and the orthogonal code of length 2(N+1) is assigned as XX, the orthogonal code XX' is necessarily used. - 21 - The structure shown in FIGs. 2 through 4 can be so designed as to change the FEC rate at both the forward link and the reverse link according to the order transmitted from the base station by allowing the mobile station to be able to change the FEC rate, or as to change the FEC rate only at the forward link. For the forward link of the 3G CDMA system having this structure, the procedures for switching the coding rate to the 1/3 FEC rate or the 1/6 FEC rate are shown in FIGs. 5 and 6. FIG. 5 illustrates the procedure in which the mobile station is allowed by the base station to use the second encoder 312 of the 1/6 FEC rate through the paging and access channels during the call setup. FIG. 6 illustrates the procedure in which the mobile station is allowed by the base station to change the coding rate. With reference to FIG. 5, a description will be made as to an operation of selecting the 1/6 FEC rate for the forward link from the start of the call through the access channel and the paging channel at the call setup stage, and with reference to FIG. 6, a description will be made to an operation of switching from the 1/3 FEC rate to the 1/6 FEC rate during the call processing in the IS-95B system. Referring to FIG. 5, for the call setup, the mobile station sends an origination message shown in the following Table 1 to the base station, in step 511. In the origination message of Table 1, a new MOB-P-REV value (which is different from the existing value) is assigned to the mobile station which can change the coding rate, and the mobile station sends the origination message by putting its own MOB_P_REV value therein. Thereafter, upon reception of the origination message, the base station sends to the mobile station a channel assignment message shown in the following Tables 2A to 2G, in step 515. Table - 22 - 2B shows the channel assignment message for ASSIGN_MODE="000", Table 2C shows the channel assignment message for ASSIGN_MODE="001", Table 2D shows the channel assignment message for ASSIGN_MODE="010", Table 2E shows the channel assignment message for ASSIGN_MODE="011", Table 2F shows the channel assignment message for ASSIGN_MODE=" 100", and Table 2G shows the channel assignment message for ASSIGN _MODE=" 10 l'Mn step 513, the base station may first send a BS_ACK_Order in acknowledgment of the origination message. In the channel assignment message, anew ENCODER_RATE field is assigned for the coding rate to send the designated coding rate. The mobile station then fixes the coding rate according to the received channel assignment message and searches the forward link channel using the given frequency band and orthogonal code. TABLE 1 Field Length [bits] MSG_TYPE ("00000100") 8 ACK_SEQ 3 MSG_SEQ 3 ACK_REQ 1 VALID_ACK 1 ACK_TYPE 3 MSID_TYPE 3 - 23 - - 24 - - 25 - - 26 - - 21 - - 28 - Next, referring to FJG. 6, during an active state where the call is connected between the base station and the-mobile station, the base station examines the channel environment with the mobile station by estimating, for example, the RSSL In step 611, the base station estimates the RSSI, selects a coding rate lower than the present coding rate when the RSSI is lower than a threshold value R_low, and selects a coding rate higher than the present coding rate when the RSSI is higher than a threshold value R_high. In the active state, since the base station and the mobile station exchanges messages through the traffic channels, a new field for the encoder rate and the orthogonal code is added to a service configuration shown in the following Table 3 in order to switch the coding rate of the mobile station, 16 bits are assigned for the new field of the service configuration; first 2 bits are assigned for the encoder rate, next 8 bits are assigned for the orthogonal code, and the last 6 bits are reserved bits. Although a RECORD_LEN value of a service request message shown in the following Table 4 is 12 in the existing IS-95B Standard, it is 14 in the embodiment since two octets are added. This is changed in the same manner even in a service response message shown in Table 5 and a service connect message shown in Table - 29 - 6. The contents of the service configuration are input to the type-specific fields of the messages (i.e., the service request message, the service response message and the service connect message). By way of example, Table 3 represents the service configuration for the case where the two coding rates of 1/3 and 1/6 are used. In this example, if the mobile station includes at least two encoders having the different coding rates and the orthogonal code length is changed according to the coding rates, the lengths of the ENCODER_RATE field and the CODE_CHAN field of Table 3 are also changed to accommodate all the cases, and the RECORDLEN values of Tables 4, 5 and 6 are also adjusted. After the service configuration is corrected, the base station sends the service request message and selects the new coding rate and orthogonal code to change the coding rate, in step 613. In response to the service request message, the mobile station then outputs the service response message through the reverse traffic channel, in step 615. Here, if the mobile station does not respond to the service request message, the base station repeats the step 613 to send again the service request message for changing the coding rate until the mobile station sends the service response message responding to the request message. In step 617, if the service configuration of the mobile station coincides with that of the base station, the base station sends the service connect message and sets a rate change action time of the ACTION_TIME field, or implements the service comment message by default a predetermined time after receiving the message. In step 619, the mobile station sends a service connect completion message through the reverse link to acknowledge the service connect message. In step 621, the mobile station and the - 30 - base station both change the rate at the set action time. - 31 - - 32 - - 33 - It is possible to differently change the coding rate for the voice service and the packet data service. That is, during the packet data service, to change the coding rate of the supplemental channel for the packet service can be implemented through a dedicated control channel (DCCH). In addition, when the message is received through the traffic channel, not through the DCCH, changing the coding rate can be implemented in the same manner as done by the fundamental channel. For - 34 - example, when 2 bits are used for the coding rate (providing four available cases), the two cases are used in changing the coding rate for the fundamental channel and other two cases are used in changing the coding rate for the supplemental channel. As described above, 256 orthogonal codes of length 256 bits are used for the 1/3 coding rate and 128 orthogonal codes of length 128 bits are used for the 1/6 coding rate. Here, since the orthogonal codes of length 256 are created by applying the Hadamard transform to the orthogonal codes of length 128, one orthogonal code of length 128 has a correlation with two orthogonal codes of length 256, losing the orthogonality between the channels. Therefore, assignment of one orthogonal code of length 128 decreases the available orthogonal codes of length 256 by two. In opposition, assignment of one orthogonal code of length 256 makes one orthogonal code of length 128 unusable. The base station continuously monitors the assigned orthogonal codes of length 128 and 256 to assign the new orthogonal codes so as to avoid the correlation with the previously assigned orthogonal codes. In this manner, the embodiment maintains the good channel condition by changing the coding rate and the orthogonal code according to the channel environment. Here, the transmission power is also related to a tolerance for the channel environment. Further, the orthogonal code is so assigned as to avoid the correlation between the base station and the different mobile stations. Accordingly, the embodiment changes the coding rate according to the channel environment, taking into consideration the transmission power. Further, if the orthogonal code is changed while the coding rate is changed by the base station or the mobile station in the same cell, it is determined whether there is an orthogonal code causing the - 35 - correlation between the different orthogonal code sets. In this way, it is possible to solve the interference and non-orthogonality problems of the CDMA communication system. FIGs. 7A and 7B are flowcharts illustrating a rate change operation performed in the decision block 213 of the base station. More specifically, FIG. 7A illustrates a rate change operation performed in the base station upon reception of the rate change request message from a particular mobile station, and FIG. 7B illustrates the procedure where the base station analyzes the channel environment of the mobile station to determine whether to change the rate when the mobile station does not generate the rate change request message. Accordingly, when a particular mobile station generates the rate change request message, the base station performs the rate change procedure of FIG. 7A with the particular mobile station while performing the rate change procedure of FIG. 7B with the other mobile stations which do not generate the rate change request message. Therefore, the base station can perform the procedures of FIGs. 7A and 7B in parallel. FIG. 8 illustrates the procedure where the mobile station performs the rate change operation with the base station, when received the rate change request message from the base station or when a rate change condition occurs as the channel environment is changed. FIG. 9 is a flowchart illustrating a procedure for assigning, when a coding rate is changed, an orthogonal code corresponding to the changed coding rate. That is, when assigning the forward channel to the mobile station, the base station assigns the orthogonal codes in such a manner that the number of available - 36 - orthogonal codes is as large as possible. FIG. 9 shows the procedure in which the base station assigns the orthogonal codes to the mobile station according to an embodiment of the present invention. It is assumed that the embodiment changes the coding rate and the orthogonal code according to the channel environment, and in particular, simultaneously changes the coding rate and the length of the corresponding orthogonal code. However, it is also possible to independently change the coding rate and the length of the orthogonal code. Furthermore, the embodiment assigns the longer orthogonal code when the coding rate is increased (e.g., from 1/6 to 1/3), and assigns the shorter orthogonal code when the coding rate is decreased (e.g., from 1/3 to 1/6), thereby maintaining the same chip rate irrespective of the change in the rate. However, it is also possible to change the coding rate and the orthogonal code without maintaining the same chip rate during the channel communication between the base station and the mobile station. In the following description, the procedure for assigning the orthogonal code will be first mentioned with reference to FIG. 9, and then the rate change procedure between the base station and the mobile station will be described with reference to FIGs. 7A, 7B and 8. Referring to FIG. 9, when the mobile station requests assignment of the orthogonal code of length N (where N=2K) according to the change in the channel assignment or the coding rate, a undepicted rate controller searches for the available orthogonal codes in step 911. Here, the orthogonal codes should be assigned such that the available orthogonal codes are as many as possible. To this end, in step - 37 - 913, the rate controller searches an orthogonal code table to determine whether there are unused orthogonal codes of length N. When all the orthogonal codes of length N are used (i.e., assigned to the channel), the procedure goes to step 929 to indicate unusablility of the orthogonal codes and then terminates. However, when there exist the available orthogonal codes of length N, the corresponding orthogonal codes are written in a search list W(k), in step 915. The search list W(k) stores information about the unused orthogonal codes in the form of w(k,i) as follows: where k is an integer representing the length of the Walsh codes and i is a Walsh code number, where i=0,1,2, ...,N-1. Accordingly, if it is assumed that 11th, 12th, 15th, 21th and 30th orthogonal codes among the orthogonal codes of length 2k are not in use, the search list W(k) consists of the orthogonal codes w(k,l 1), w(k,12), w(k,15), w(k,21) and w(k,30). After that, in step 917, a search procedure 1 is performed to delete the orthogonal codes being non-orthogonal with the orthogonal codes of length longer - 38 - than 2k, among the orthogonal codes stored in the search list W(k). That is, in the search procedure 1, the orthogonal codes unsatisfying the orthogonality with the orthogonal codes presently in use among the orthogonal codes of length longer than 2k are deleted from the search list W(k). More specifically, the orthogonal codes unsatisfying the orthogonality with the orthogonal codes w(k+j,i) (where j > 1, i=0, 1,2,..., 2k+i-l), are deleted from the search list W(k). As j increases one by one to increase the length of the orthogonal code, the search and extraction procedure is repeated for all the orthogonal codes. The search procedure 1 performed in step 917 is defined as: After performing the search procedure 1, it is determined in step 919 whether there is any remaining orthogonal code in the search list W(k) (i.e., the number of w(k,i)>0). When the search list W(k) does not have any orthogonal code, it is indicated in step 929 that there is no available orthogonal code. - 39 - However, when there exist orthogonal codes in the search list W(k), it is determined in step 921 whether the orthogonal code having the length 2k and the number (i+N/2 )mod N is presently in use or not with respect to all the orthogonal codes w(k,i) in the search list W(k). If such orthogonal codes exist in the search list W(k), the corresponding orthogonal codes are assigned as available orthogonal codes in step 927. - 40 - However, when the search list W(k) does not have the corresponding orthogonal codes, a search procedure 2 is performed in step 923 to delete the orthogonal codes unsatisfying the orthogonality with the orthogonal codes presently in use among the orthogonal codes of length shorter than 2k. More specifically, among the presently used orthogonal codes w(k-j,i) (where j^ 1, i=0, 1, 2, ..., 2k-1-l), the orthogonal codes unsatisfying the orthogonality with the orthogonal codes stored in the search list W(k) are deleted from the search list W(k). As j decreases one by one to decrease the length of the orthogonal code, the search and extraction procedure is repeated for all the orthogonal codes. The search procedure 2 performed in step 923 is defined as: After performing the search procedure 2, it is determined in step 925 whether there is any remaining orthogonal code in the search list W(k) (i.e., the number of w(k,i)>0). When the search list W(k) does not have any orthogonal code, it is indicated in step 929 that there is no available orthogonal code. However, when there exist the orthogonal codes in the search list W(k), the orthogonal codes remaining in the search list W(k) are assigned as the available orthogonal codes in step 927. - 41 - An operation of assigning the orthogonal codes will be described more specifically. When the length of the orthogonal code is N=2k, the orthogonal codes w(k,i) of length N are written in the search list W(k) in step 915. Here, i is an orthogonal code number Wno which is a Haramard matrix element number. The orthogonal codes presently not in use are represented as follows: Here, the orthogonal codes w(6,28) and w(4,11) are in use exceptionally. That is, if w(4,10)=B, w(4,ll)=C and w(4,12)=Ds then w(5,10)=BB, w(5,ll)=CC, w(5,I2)=DD, w(5,26)=BB, w(5,27)-CCs w(5,28)=DD, w(6,ll)-CCCC, w(6,26)= BBBB, w(6,27)= CCCC, w(6,28)= DDDD, w(6,43)= CCCC, w(6,58)=BBBB,w(6,59)=CCCC andw(6,60)= DDDD .Here, the barred codes represent complementary codes. - 42 - Combinations of the orthogonal codes are shown in the following Table 8, in which it is assumed that the orthogonal codes w(6,28), w(5,10), w(5,12) and' w(4,ll) are in use. In Table 8, the orthogonal codes of length k=5 have a relationship to the search list W(k-5). Table 8 shows the orthogonal codes, when the orthogonal code w(6,28) is in use and the orthogonal code w(4,11) is not in use. Further, the underlined orthogonal codes in Table 8 are the orthogonal codes in the search list W(k). Referring to Table 8 and FIG. 9, in the search procedure 1 of step 917, among the orthogonal codes w(5,l 1), w(5,26), w(5,27) and w(5,28) written in the search list W(k), the orthogonal codes unsatisfying the orthogonality with the orthogonal codes of length 2k+I presently in use are searched for to be deleted from the search list W(k). As a result, in the search procedure 1, the orthogonal code w(5,28) unsatisfying the orthogonality with the orthogonal code w(6,28) presently in use is deleted from the search list W(k). Thus, after the search procedure 1 is performed, W(k)={w(5,ll), w(5,26), w5,27)}. Here, since the number of the orthogonal codes in the search list W(k) is 3, the condition of step 919 (the number of W(k) > 0) is satisfied. Further, since the orthogonal codes w(5,26) and w(5,(26+16)mod 32)=w(5,10) are already in use, the condition of step 921 is also satisfied. Accordingly, the orthogonal code w(5,26) is assigned as an available orthogonal code. If the orthogonal code w(5,10) is not in use and the elements of the search list W(k) after performing the search procedure 1 include the orthogonal codes - 43 - w(5,10),w(5,l 1), w(5,26) and w(5,27), the search list W(k) has no orthogonal code satisfying the step 921. Then, the search procedure 2 is performed in step 923. In the search procedure 2, among the orthogonal codes of length 2k-1 (i.e., k-l=4) in the search list W(k), the orthogonal codes in the search list W(k) unsatisfying the orthogonality with the orthogonal codes presently in use are searched for and deleted from the search list W(k). Since the orthogonal code w(4,l 1)=C is in use, the orthogonal codes w(5,11)=CC and w(5,27)=CC are deleted from the search list W(k). As a result, the orthogonal codes stored in the search list W(k) are W(k=5)={w(5,10), w(5,26)}, which satisfies the condition of step 925. Thus, in step 927, the orthogonal codes w(5,10) and w(5,26) are assigned as the available orthogonal codes. Assignment of the orthogonal codes is performed by the decision block 213 of FIG. 2. In assigning the new orthogonal codes, the decision block 213 first determines whether there are available orthogonal codes among the orthogonal codes of length N to be used. When there are the available orthogonal codes of the corresponding length N, the decision block 213 examines, in assigning the orthogonal codes, whether there are the orthogonal codes having the correlation with the orthogonal codes for the forward channel assigned to the other mobile stations, and avoids assigning the corresponding orthogonal codes, if any. Here, when there exist the orthogonal codes satisfying the corresponding condition, the corresponding orthogonal code length and number information is output to assign the orthogonal codes. Accordingly, in the CDMA communication system, when the channel data is transmitted at a variable data rate, the base station can effectively assign the orthogonal codes to the mobile station such that the orthogonal codes assigned to the different mobile stations and channels have no correlation with one - 44- another. Therefore, the communication system supporting the variable data rate can efficiently use the orthogonal code resources and quickly assign the orthogonal codes. The term "rate" used in connection with FIGs. 7 A, 7B and 8 refers to the coding rate and/or the length of the orthogonal code. A "first rate change condition" means a condition for switching from the higher rate to the lower rate, and a "second rate change condition" means a condition for switching from the lower rate to the higher rate. For example, the first rate change condition for changing the higher rate to the lower rate means that the channel environment is changed from a state where the 1/3 coding rate and the orthogonal code of length 256 are used to a state where the 1/6 coding rate and the orthogonal code of length 128 are used. On the contrary, the second rate change condition for changing the lower rate to the higher rate means that the channel environment is changed from a state where the 1/6 coding rate and the orthogonal code of length 128 are used to a state where the 1/3 coding rate and the orthogonal code of length 256 are used. In the embodiment, when the higher coding rate is used, the longer orthogonal code is assigned, and when the lower coding rate is used, the shorter orthogonal code is assigned, thereby to maintain the constant data rate. Referring to FIGs. 7A and 7B, the decision block 213 of the base station analyzes the received signal in step 711, to determine whether the rate change request message is received from a mobile station. When the rate change request message is received from the mobile station in step 711, the decision block 213 of the base station determines in step 713 whether the received rate change request message represents a change to the higher rate or to the lower rate. - 45 - If the received rate change request message represents the change to the lower rate in step 713, the decision block 213 of the base station examines in step 715 whether the first rate change condition is satisfied. Here, the first rate change condition where the base station decreases the coding rate, represents the conditions shown in the following Table 9. In the embodiment, it is assumed that the first rate change condition is satisfied in the case where at least three conditions including a condition 1 and a condition 4 in Table 9 are satisfied. In Table 9, the condition 1 is satisfied when the transmission power to the mobile station is higher than or equal to a value obtained by dividing a value, [(total available power at the base station for all forward link in the same FA) - (a power margin)], by the number of the mobile stations in service in the same area. The condition 2 is satisfied when an average reverse link RSSI for a particular duration is lower than or equal to a value obtained by subtracting a standard deviation of the RSSI, Orssi from a threshold RSSI, Thrss. The condition 3 is satisfied when an - 46 - average reverse link SNR for a particular duration is lower than or equal to a value obtained by subtracting a standard deviation of the SNR, Osnr from a threshold SNR, Thsnr The condition 4 is satisfied when there exist the available orthogonal codes among the orthogonal codes of the requested length. Here, the orthogonal codes are searched for and extracted based on the procedure of FIG. 9. That is, as the result of the search, even though there exist the available orthogonal codes, those are considered as unavailable orthogonal codes when they have the correlation with the other orthogonal codes in used. That is, the orthogonal codes satisfying the condition 4 should have the length corresponding to the requested coding rate and have no correlation with the forward channel for the other mobile stations. To satisfy the first rate change condition, the conditions 1 and 4 in Table 9 should be satisfied. Accordingly, when the conditions 1 and 4 are satisfied, the present rate can be changed to the lower rate. However, when the conditions 2 and 3 are satisfied while the conditions 1 and 4 are unsatisfied, the rate change is not required. That is, only when the conditions 1 and 4 are both satisfied, the present rate can be changed to the lower rate. Here, it is assumed that even when one or both of the conditions 2 and 4 are satisfied while the conditions 1 and 4 are both satisfied, the first rate change condition is satisfied. Accordingly, when the first rate change condition is satisfied in step 715, the base station sends to the mobile station the information about the requested coding rate and the assigned orthogonal code together with the response message, in step 717. For example, when the 1/3 coding rate is presently used, it can be changed to the 1/6 coding rate, and when the 1/2 coding rate is presently used, it can be change to 1/4 coding rate. In this case, the shorter orthogonal codes are assigned - 47 - which do not have the correlation with the orthogonal codes used for the other forward link channels. The decision block 213 includes a table for storing the orthogonal codes previously set by the Hadamard transform, and assigns the orthogonal codes by selecting from the table the orthogonal codes having no correlation with one another based on the procedure of FIG. 9. After sending the changed coding rate and the orthogonal information, the decision block 213 of the base station outputs the coding select signal Csel and the orthogonal code number and length signals Wno and Wlength for changing the present rate to the requested lower rate in step 719, thereby to change the coding rate and the orthogonal code of the channel encoder in the base station. Then, as illustrated in FIG. 3, in the base station, the selector 301 outputs the input data to the second encoder 312 and the selector 393 outputs the decimated long code from the decimator 392 to the second mixer 342 according to the coding select signal Csel. Further, the second orthogonal modulator 362 multiplies the symbol data output from the second encoder 352 by the newly assigned orthogonal code. Therefore, the rate of the orthogonal spread signal applied to the spreader 370 is changed to the lower rate. In addition, the decision block 213 of the mobile station also outputs the received Csel, Wno and Wlength. Thus, as illustrated in FIG. 4, the selector 420 applies the received signal output from the despreader 410 to the second orthogonal demodulator 432, which multiplies the despread signal by the newly assigned orthogonal code. Furthermore, the selector 493 outputs the decimated long code from the decimator 492 to the second mixer 452 according to the coding select signal Csel, thereby outputting the data decoded in the second decoder 482 as the received data. - 48 - However, when the rate change request message represents the change to the higher rate in step 713, the decision block 213 of the base station determines in step 721 whether the second rate change condition is satisfied. Here, the second rate change condition where the base station increases the rate, represents the conditions shown in the following Table 10. In the embodiment, it is assumed that the second rate change condition is satisfied in the case where at least two conditions including a condition 1 in Table 10 are satisfied. In Table 10, the condition 1 is satisfied when a transmission power to the mobile station is lower than or equal to a value obtained by subtracting a standard deviation, opwr' of the average transmission power for the respective forward traffic channels from an average transmission power to all mobile stations. The condition 2 is satisfied when an average reverse link RSSI for a particular duration is higher than or equal to a value obtained by adding the standard deviation of the RSSI, orssi, to the threshold RSSI, Thrssi, The condition 3 is satisfied when an average reverse link SNR for a particular duration is higher than or equal to a value obtained by adding the standard deviation of the SNR, osnr, to the threshold SNR, Thsnr - 49 - To satisfy the second rate change condition, the condition 1 in Table 10 should be satisfied. Accordingly, when the condition 1 is satisfied, the present coding rate can be changed to the higher coding rate and the length of the orthogonal code can also be changed. However, when the conditions 2 and 3 are satisfied while the condition 1 is unsatisfied, the coding rate and the orthogonal code are not changed. That is, only when the condition 1 is satisfied, the present rate can be changed to the higher rate. Here, it is assumed that even when one or both of the conditions 2 and 3 are satisfied while the condition 1 is satisfied, the second rate change condition is satisfied. Accordingly, when the second rate change condition is satisfied in step 721, the base station sends to the mobile station the information about the requested coding rate and the assigned orthogonal code together with the response message, in step 717. For example, when the present coding rate is 1/6, it can be changed to the 1/3, and when the present coding rate 1/4, it can be change to 1/2. In this case, as the coding rate is increased, the longer orthogonal codes can be assigned which do not have the correlation with the orthogonal codes used for the other forward link channels. After sending the changed coding rate and orthogonal code, the decision block 213 of the base station outputs the coding select signal Csel and the orthogonal code number and length signals Wno and Wlength for changing the present rate to the requested higher rate in step 719, thereby to change the coding rate and the orthogonal code of the channel encoder in the base station. Then, as illustrated in FIG. 3 the selector 301 outputs the input data to the first encoder 311 and the selector 393 outputs the decimated long code from the declinator 392 to the first mixer 341 according to the coding select signal Csel. - 50 - Further, the first orthogonal modulator 361 multiplies the symbol data output from the first encoder 351 by the newly assigned orthogonal code. Therefore, the rate of the orthogonal spread signal applied to the spreader 370 is changed to the higher rate. In addition, the decision block 213 of the mobile station also outputs the received Csel, Wno and Wlength. Thus, as illustrated in FIG. 4, the selector 420 applies the received signal output from the despreader 410 to the first orthogonal demodulator 431, which multiplies the despread signal by the newly assigned orthogonal code. Furthermore, the selector 493 outputs the decimated long code from the decimator 492 to the first mixer 451 according to the coding select signal Csel, so that the data decoded in the first decoder 481 is applied to the receiver as the received data. However, when the rate change request message from the mobile station does not satisfy both the first and second rate change conditions, the decision block 213 of the base station perceives this in step 715 or 721, and sends to the mobile station a response message representing the impossibility of changing the coding rate and the orthogonal code, in step 723. When the rate change request message is not received from the mobile station in step 711, the procedure of FIG. 7B is performed to determine whether or hot to change the rate. Also, even when the rate change request message is received from a particular mobile station, the base station can perform the procedures of FIGs. 7A and 7B in parallel to determine whether or not to change the rates of the other mobile stations which have not requested the rate change. In FIG. 7B, the decision block 213 of the base station detects the power consumption of the forward traffic channel for the mobile stations and changes the rates according to - 51 - the detection. That is, the decision block 213 selects the lower rate for the mobile station which consumes the high power, and selects the higher rate for the mobile station which consumes the low power. First, a description will be given as to the rate change operation of the mobile station which consumes the high power. The decision block 213 of the base station searches for the forward link and the mobile station which consume the highest power among the forward links using the higher rate encoder, in step 751. The decision block 213 determines in step 753 whether the searched mobile station can change the rate or not by consulting the internal search list. When the searched mobile station can change the rate, the decision block 213 checks in step 755 whether the first rate change condition is satisfied or not. Here, it is assumed that the first rate change condition represents the case where at least three conditions including the conditions 1 and 4 in Table 9 are satisfied. When the first rate change condition is unsatisfied, the decision block 213 returns to step 711 to repeat the procedure of FIG. 7B. However, when the first rate change condition is satisfied in step 755, the decision block 213 of the base station sends to the mobile station a request message for selecting the lower rate and performs a procedure for decreasing the coding rate of the forward channel, in step 757. Next, a description will be given as to the rate change operation of the mobile station which consumes the low power. The decision block 213 of the base station searches for the forward link and the mobile station which consume the lowest power among the forward links using the lower rate encoder, in step 759. The decision block 213 determines in step 761 whether the searched mobile station can change the rate or not by consulting the internal search list. When the searched - 52 - mobile station can change the rate, the decision block 213 checks in step 763 whether the second rate change condition is satisfied or not. Here, it is assumed that the first rate change condition represents the case where at least two conditions including the condition 1 in Table 10 are satisfied. When the second rate change condition is unsatisfied, the decision block 213 returns to step 711 to repeat the procedure of FIG. 7B. However, when the second rate change condition is satisfied in step 763, the decision block 213 of the base station sends to the mobile station a request message for selecting the higher rate and performs a procedure for increasing the coding rate of the forward channel, in step 765. However, when the mobile station having the highest power consumption or the lowest power consumption can not change the rate during the call, such as the conventional IS-95 mobile station, the decision block 213 of the base station perceives this in step 753 or 761, and goes to step 767 to delete from the search list the mobile station which cannot change the rate. After deletion, the decision block 213 returns to step 711 to repeat the procedure of FIG. 7B. Referring to FIG. 8, in step 811, the decision block 213 of the mobile station analyzes the received signal to determine whether the rate change request message is received from the base station. When the rate change request message is received from the base station in step 811, the decision block 213 of the mobile station checks in step 813 whether the received rate change request message represents the change to the higher rate or to the lower rate. When the received rate change request message represents the change to the lower rate in step 813, the decision block 213 of the mobile station determines in - 53 - step 815 whether a first rate change condition is satisfied or not. Here, the first rate change condition where the mobile station selects the lower rate, represents a case in which at least two of the conditions in the following Table 11 are satisfied. In Table 11, the condition 1 is satisfied when an average reverse transmission power for a particular duration is higher than or equal to a value obtained by adding a standard deviation Opwr to a threshold power Thpwr A condition 2 is satisfied when an average received forward link RSSI for a particular duration is lower than or equal to a value obtained by subtracting a standard deviation orssi from a threshold RSSI Thrssi. A condition 3 is satisfied when an average received forward link SNR for a particular duration is lower than or equal to a value obtained by subtracting a standard deviation osnr from a threshold SNR Thsnr. Here, it is assumed that at least two conditions out of the conditions 1 to 3 in Table 11 should be satisfied in order to satisfy the first rate change condition. When at least two of the conditions in Table 11 are satisfied, the present coding rate can be changed to the lower rate and the length of the orthogonal code can also - 54 - be changed accordingly. Accordingly, when the first rate change condition is satisfied in step 815, the mobile station sends to the base station the requested coding rate and the assigned orthogonal code together with a response message in step 817, and changes the coding rate and the orthogonal code according to the change rate in step 819 to perform the communication service at the changed rate. However, when the received rate change request message represents the change to the higher rate in step 813, the decision block 213 of the mobile station examines in step 821 whether a second rate change condition is satisfied or not. Here, the second rate change condition where the mobile station changes the present rate to the higher rate, represents a condition in which at least two of the conditions in the following Table 12 are satisfied. In Table 12, the condition 1 is satisfied when an average reverse transmission power for a particular duration is lower than or equal to a value obtained by subtracting a standard deviation, Opwr, of the reverse link from a -55- threshold power Thpwr A condition 2 is satisfied when an average received forward link RSSI for a particular duration is higher than or equal to a value obtained by adding a standard deviation of the RSSI, orssi' to a threshold RSSI Thrssi. A condition 3 is satisfied when an average received forward link SNR for a particular duration is higher than or equal to a value obtained by adding a standard deviation of the SNR, Osnr, to a threshold SNR Thsnr. Here, it is assumed that at least two of the conditions 1 to 3 in Table 12 should be satisfied in order to satisfy the second rate change condition. When at least two of the conditions in Table 12 are satisfied, the present FEC rate can be changed to the higher rate and the length of the orthogonal code can also be changed accordingly. Accordingly, when the second rate change condition is satisfied in step 821, the mobile station sends to the base station the requested coding rate and the assigned orthogonal code together with a response message in step 817. For example, when the present coding rate is 1/6, it can be changed to 1/3, and when the present coding rate is 1/4, it can be changed to 1/2. Further, the longer orthogonal code can be assigned. After sending the changed coding rate and orthogonal code, the decision block 213 of the mobile station outputs the coding select signal Csel and the orthogonal code number and length signals Wno and Wlength for selecting the requested higher rate to change the coding rate of the encoder and the orthogonal code in step 819, thereby to perform the communication service at the changed rate. However, when the rate change request message received from the base - 56 - station does not satisfies both the first and second rate change conditions, the decision block 213 of the mobile station perceives this in step 815 or 821, and sends to the mobile station a response message representing impossibility of changing the coding rate and the orthogonal code in step 823, terminating the procedure. As stated above, the base station and the mobile station change the coding rate and the orthogonal code according to the rate change request message or the states of the signal received from the other party, so that they can adaptively maintain the good rate according to the channel environment. Although FIGs. 7 A, 7B and 8 show an embodiment which changes both the coding rate and the orthogonal code to change the rate, it is also possible to change the rate by selectively changing the coding rate or the orthogonal code according to the channel environment. The forward traffic channel transmission device of FIG. 3 has the structure of using a single carrier. However, with the progress of the communication technology and service, the subscribers to the communication service are increasing in number. Also, there have been proposed many methods for meeting the subscribers' demands for the services. As one of the methods, the TIA/EIA TR45.5 conference has proposed the fundamental channel forward link structure for the multicarrier CDMA system. A method using the multicarrier overlays three forward link carriers for the multicarrier system on three 1.25MHz bandwidths used in the IS-95 CDMA system, or selects the three 1.25MHz bands with one forward channel. In this case, all the three carriers used in the multicarrier system have independent transmission powers. - 57 - Accordingly, when the transmission device of the invention is applied to the multicarrier system, the decision block 213 of FIG. 2 should generate the orthogonal code number and length signals Wno and Wlength for generating the orthogonal codes for the multicarrier system together with the coding select signal Csel for selecting the coding rate. Since the respective carriers are independent of one another, the Walsh code number signal Wno output from the decision block 213 should also be able to assign the orthogonal codes as many as the number of the carriers. FIG. 10 illustrates a multicarrier transmission device according to another embodiment of the present invention. It is assumed that the forward traffic channel transmission device uses 3 carriers, and includes a rate 1/3 encoder, a rate 16 encoder and a plurality of orthogonal modulators for independently modulating the signals according to the three carriers. Referring to FIG. 10, a selector 301 has a first output end connected to a first encoder 311 and a second output end connected to a second encoder 312. The selector 301 receives input data to be transmitted and selectively outputs the input data to the first encoder 311 or the second encoder 312 according to the select signal Csel output from the decision block 213. The first encoder 311, upon reception of the data input from the selector 301, encodes and punctures the input data into data symbols at the 1/3 coding rate (the first coding rate). That is, the first encoder 311 encodes one input data bit into three symbols. A convolutional encoder or a turbo encoder can be used for the first encoder 311, A first symbol repetition part 321 receives the data encoded at the first - 58 - coding rate, and repeats the symbols output from the first encoder 311 so as to match the symbol rates of the data having different bit rates. A first interleaver 331 interleaves first encoded data output from the first symbol repetition part 321. A block interleaver can be used for the first interleaver 331. The second encoder 312, upon reception of the data input from the selector 301, encodes and punctures the input data into data symbols at the coding rate 1/6 (the second coding rate). That is, the second encoder 312 encodes one input data bit into six symbols. A convolutional encoder or a turbo encoder can be used for the second encoder 312. A second symbol repetition part 322 receives the data encoded at the second coding rate, and repeats the symbols output from the second encoder 312 so as to match the symbol rates of the data having different bit rates. A second interleaver 332 interleaves second encoded data output from the second symbol repetition part 322. A block interleaver can be used for the second interleaver 332. A long code generator 391 generates long codes for the user identification, which are differently assigned to the respective subscribers. A decimator 392 decimates the long codes so as to match a rate of the long codes to a rate of the symbols output from the interleavers 331 and 332. A selector 393 selectively outputs the decimated long code output from the decimator 392 to a mixer 341 or a mixer 342 according to the encoder select signal Csel. The selector 393 switches the decimated long code to the first mixer 341 to select the 1/3 coding rate and to the second mixer 342 to select the 1/6 coding rate. The mixer 341 mixes the first encoded data output from the first interleaver 331 with the long code output from the selector 393. The second mixer 342 mixes the second encoded data output from - 59 - the second interleaver 332 with the long code output from the selector 393. A first demultiplexer 1011 demultiplexes data output from the first mixer 341 to the respective carriers in sequence. Signal mapping parts 1021-1023 map levels of the binary data output from the first demultiplexer 1011 by converting data "0" to "+1" and data "1" to "-1". Orthogonal modulators 1031-1033, in the same number as that of the carriers, each include a first orthogonal code generator (not shown) which generate a first orthogonal code for orthogonally modulating the first encoded data according to the orthogonal code number and length Wno and Wlength output from the decision block 213. The orthogonal modulators 1031-1033 multiply the first orthogonal code generated according to the orthogonal code number and length Wno and Wlength by the data output from the signal mapping parts 1021-1023, respectively, to generate a first orthogonal modulation signal. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 256 is used for the data encoded at the first coding rate of 1/3. A second demultiplexer 1012 demultiplexes data output from the second mixer 342 to the respective carriers in sequence. Signal mapping parts 1026-1028 map levels of the binary data output from the second demultiplexer 1012 by converting data "0" to "+l" and data "1" to "-1". Orthogonal modulators 1036-1038, in the same number as that of the carriers, each include a second orthogonal code generators (not shown) which generate a second orthogonal code for orthogonally modulating the second encoded data accordingto the orthogonal code number and length Wno and Wlength output from the decision block 213. The orthogonal modulators 1036-1038 multiply the second orthogonal code generated according to the orthogonal code number and length Wno and Wlength by the data - 60 - output from the signal mapping parts 1021-1023, respectively, to generate second orthogonal modulation signals. Here, it is assumed that the Walsh code is used for the orthogonal code and a Walsh code of length 128 is used for the data encoded at the second coding rate of 1/6. Spreaders 1041-1043 combine the first and second orthogonal modulation signals output from the orthogonal modulators 1031-1033 and second orthogonal modulators 1036-1038 with the received spreading sequence to spread transmission signals. Here, the PN sequence can be used for the spreading sequence and the QPSK spreaders can be used for the spreaders. Gain controllers 1051-1053 control gains of the spread signals input from the spreaders 1041-1043 according to gain control signals G1-G3. The respective gain controllers 1051-1053 output different carriers. As described above, during the call setup or call processing, the base station and the mobile station change the coding rate and the orthogonal code according to the channel environment, in order to provide the communication service in the good channel environment. By changing the FEC rate for all the link channels of the CDMA communication system, it is possible to improve the performance of the reception device and save the transmission power of the transmission device. In addition, it is possible to simply change the rate using the message. While the invention has been shown and described with reference to a certainpreferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. - 61 - WE CLAIM 1. A channel communication apparatus for a wireless communication system comprising: - a channel receiver for receiving a channel signal; - a controller for analyzing the received channel signal to assess a channel environment, said assessment being used by said channel receiver to generate a coding rate select signal and orthogonal code information; and - a channel transmitter including: a channel encoder for adaptively encoding transmission data at a coding rate selected according to the coding rate select signal, and an orthogonal modulator for generating an orthogonal code according to the orthogonal code information to spread the encoded transmission data with the generated orthogonal code; wherein said controller performs said channel assessment by: -62- (i) measuring at least one of a receiving power interference, a bit error rate (BER) and a signal-to-noise ratio (SNR) from the received channel signal, (ii) comparing the measured values with corresponding upper threshold values to generate the coding rate select signal for decreasing the coding rate and the orthogonal code information by reducing a length of the orthogonal code when the measured values exceed the upper threshold values, and (m) comparing the measured values with corresponding lower threshold values to generate the coding rate select signal for increasing the coding rate and the orthogonal code information by increasing the length of the orthogonal code when the measured values are below the lower threshold values. -63- 2. The channef communication apparatus as claimed in claim 1, wherein said channel transmitter further comprises: - at (east two channel encoders having different coding rates, for encoding an input transmission signal at a corresponding coding rate; - at feast two interleaves for interleaving the corresponding encoded data by a frame unit; - selectors for selectively connecting the Input transmission signal to the channel encoders according to the coding rate select signal and selectively outputting an output of the corresponding interleave^ - an orthogonal modulator for generating the orthogonal code corresponding to the orthogonal code information and spreading the encoded data output from the selected interleaver with the generated orthogonal code; and - a pseudo-random noise (PN) spreader for PN spreading the orthogonal spread signal. -64- 3. The channel communication device as claimed in claim 2, wherein said orthogonal code information includes a number and a length-of the orthogonal code. 4. The channel communication apparatus as claimed in claim 1, wherein said channel receiver further comprises: - a PN despreader for FN dispreadmg the received signal; - an orthogonal demodulator for generating an orthogonal code corresponding to the orthogonaf code information and orthogonally dispreading the PN despread signal; - at least two deinterleavers for deinterfeaving the orthogonaHy despread signal; - at least two channel encoders having different coding rates for encoding the deinterleaved signal at the corresponding encoding rate; and -65- - channel decoders for selectively connecting the orthogonally despread signal to the corresponding deinterleaver according to the coding rate select signal and selectively outputting an output of the channel encoder having the corresponding coding rate. 5. A channel communication apparatus for a wireless communication system using multiple carriers, comprising: - a channel receiver for receiving a channel signaf; - a controller for analyzing the received channel signal to assess a channel environment, said assessment being used by said channel receiver to generate a coding rate select signal and orthogonal code information; and - a channel transmitter including: a channel encoder for adaptiveiy encoding transmission data at a coding rate selected according to the coding rate select signal, and an orthogonal modulator for generating an orthogonal code according to the orthogonal code Information to spread the encoded transmission data with the generated orthogonal code; -66- wherein said controller performs said channel assessment by: (i) measuring at least one of a receiving power, interference, a bit error rate (BER) and a signal-to-noise ratio (SNR) from the received channel signal, (ii) comparing the measured values with corresponding upper threshold values to generate the coding rate select signal for decreasing the coding rate and the orthogonal code information by reducing a length of the orthogonal code when the measured values exceed the upper threshold values, and (iii) comparing the measure values with corresponding lower threshold values to generate the coding rate select signal for increasing the coding rate and the orthogonal code information by increasing the length of the orthogonal code when the measured vaiues are below the lower threshold values. 6. The channel communication device as claimed in claim 5, wherein said channel transmitter-comprises: - at least two channel encoders each having different coding rates, for encoding an input transmission signal at the corresponding coding rate; - interleavers for interleaving the encoded data output from the respective channel encoders, respectively; - selectors for selectively connecting the input transmission signal to the channel encoder having the corresponding coding rate according to the coding rate select signal and selectively outputting an output of the corresponding interleaver; - a demultiplexer for demultiplexing the encoded data output from the selectors to the respective carriers; -68 - orthogonal modulators for generating the orthogonal code corresponding to the orthogonal code information and spreading the encoded data output from the demultiplexer with the generated orthogonal code; and - transmitters for PN spreading the orthogonal spread signals and transmitting the PN spread signals by carrying them on the corresponding carriers, respectively. 7. The channel communication device as claimed in claim 6, wherein said orthogonal code information includes a number and a length of the orthogonal code. 8. The channel communication device as claimed in claim 6, wherein said demultiplexer uniformly distributes the encoded data to the respective carriers. -69- 9. The channel communication device as* claimed in claim 5, wherein said channel receiver comprises: - PN despreaders for frequency shifting the received multicarrier signal using the corresponding carriers and PN dispreading the frequency-shifted signals; - orthogonal demodulators for generating orthogonal codes corresponding to the orthogonal code information and dispreading the PN despread signals with the corresponding orthogonal codes; - a demultiplexer for demultiplexing outputs of the orthogonal demodulators; - deinterleavers in the same numbers as that of the coding rates, for deinterleaving the despread signals; -70- - at least two channel encoders each having different coding rates, for encoding the deinterleaved signal at the corresponding encoding rate; and - channel decoders for selectively connecting the orthogonally despread signal to the corresponding deinterleaver according to the coding rate select signal and selectively outputting an output of the channel encoder having the corresponding coding rate. 10. A channel communication method for a wireless communication system, comprising the steps of: (i) analyzing an environment of a channel in service, and selecting a coding rate and orthogonal code when the selecting a coding rate and orthogonal code when the channel environment satisfies a rate change condition; (ii) generating a message including the selected coding rate and the orthogonal code; -71- (iii) sending the message to a mobile station; and (iv) upon reception of a response message from the mobile station responsive to said message; (v) switching from a presently selected coding rate and presently selected orthogonal code to the selected coding rate and orthogonal code in a channel transmitter; wherein said step of selecting a coding rate comprises the steps of: - examining a call condition associated with the mobile station to determine whether the rate change condition is satisfied; - when a first rate change condition is satisfied, selecting a coding rate lower than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate; and -72- - when a second rate change condition is satisfied, selecting a coding rate higher than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate. XL The channel communication method as claimed in claim 10, wherein the first rate change condition is satisfied when a transmission power to the mobile station is higher than an average transmission power of all mobile stations presently in service and orthogonal codes corresponding to the selected coding rate are available. 12. The channel communication method as claimed in claim IX, wherein said average transmission power is obtained by subtracting a power margin from a maximum transmission power of the base station, and dividing the result by the number of mobile stations presently in service. -73- 13. The channel communication method as claimed in claim 10, wherein said step of selecting an orthogonal code comprises the steps of: - selecting an orthogonal code length corresponding to the selected coding rate, and writing unused orthogonal codes having the selected length in a search list; - examining a correlation between the orthogonal codes in the search list and orthogonal codes longer than the orthogonal codes in the search list and deleting those orthogonal codes from the search list not satisfying orthogonality therebetween; - determining whether complementary orthogonal codes of those • undeleted orthogonal codes from said search list in use; - selecting one of the orthogonal codes from said list whose complementary orthogonal code is in use; -74- - examining a correlation between the orthogonal codes in the search list and orthogonal codes shorter than the orthogonal codes in the search list and deleting the orthogonal codes not unsatisfying the orthogonality therebetween, when there is no orthogonal code whose complementary orthogonal code is in use; and - selecting one of the orthogonal codes remaining in the search list after deletion. 14. The channel communication method as claimed in claim 10, wherein the first rate change condition is satisfied when a transmission power to the mobile station is higher than an average transmission power of all mobile stations presently in service, orthogonal codes corresponding to the selected coding rate are available, a receiving strength of a reverse link is lower than a reference value, and a signal-to-noise ratio of the reverse link is lower than a reference signal-to-noise ratio. -75- 15. The channel communication method as claimed in claim 10, wherein the second rate change condition is satisfied when a transmission power to the corresponding mobile station is lower than a reference average transmission power to other mobile stations. 16. The channel communication method as claimed in claim 10, wherein the second rate change condition is satisfied when a transmission power to the corresponding mobile station is lower than a reference average transmission power to other mobile stations, a receiving strength of a reverse link is higher than a reference strength value, and a signal-to-noise ratio of the reverse link is higher than a reference signal-to-noise ratio. 17. A channel communication method for a wireless communication system, comprising the steps of: - upon reception of a rate change request message from a mobile station, selecting a coding rate and orthogonal code according to the rate change request message and determining whether there exist available orthogonal codes corresponding to the selected coding rate; -76- - generating a response message including information about the selected coding rate and orthogonal code and sending the generated response message to the mobile station initiating said rate change request message; and - switching a present coding rate and orthogonal code of a channel transmitter to the selected coding rate and orthogonal code; wherein said step of selecting a coding rate comprises the steps of: - examining a call condition with the mobile station to determine whether a rate change condition is satisfied; - when a first rate change condition is satisfied, selecting a coding rate lower than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate; and -77- - when a second rate change condition is satisfied, selecting a coding rate higher than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate. 18. The channel communication method as claimed in claim 17, wherein the first rate change condition is satisfied when a transmission power to the mobile station is higher than an average transmission power of all mobile stations presently in service, and orthogonal codes corresponding to the selected coding rate are available. 19. The channel communication method as claimed in claim 18, wherein said average transmission power is obtained by subtracting a power margin from a maximum transmission power of the base station and then dividing the result by the number of the mobile stations presently in service. -78- 20. The channel communication method as claimed in claim 17, wherein said orthogonal code selection step comprises the steps of: - selecting an orthogonal code length corresponding to the selected coding rate, and writing unused orthogonal codes having the selected length in a search list; - examining a correlation between the orthogonal codes in the search list and orthogonal codes longer than the orthogonal codes in the search list and deleting those orthogonal codes from said search list not satisfying orthogonality therebetween; - determining whether complementary orthogonal codes of those undeleted orthogonal codes from said search list are in use; selecting one of the orthogonal codes from said list whose complementary orthogonal codes is in use; -79- - examining a correlation between the orthogonal codes in the search list and orthogonal codes shorter than the orthogonal codes in search list and deleting the orthogonal codes in the search list and deleting the orthogonal codes not unsatisfying the orthogonality therebetween, when there is no orthogonal code whose complementary orthogonal code is in use; and - selecting one of the orthogonal codes remaining in the search list after deletion. 21. The channel communication method as claimed in claim 17, wherein the first rate change condition is satisfied when a transmission power to the mobile station is higher than an average, transmission power of all mobile stations presently in service, orthogonal codes corresponding to the selected coding rate are available, a receiving strength of a reverse link is lower than a reference strength value, and a signai-to-noise ratio of the reverse link is lower than a reference signal-to-noise ratio. -80- 22. The channel communication method as claimed in claim 17, wherein the second rate change condition is satisfied when a transmission power to the corresponding mobile station is lower than a reference average transmission power to the other mobile stations. 23. The channel communication method as claimed in claim 17, wherein the second rate change condition is satisfied when a transmission power to the corresponding mobile station is lower than a reference average transmission power to the other mobile stations, a receiving strength of a reverse link is higher than a reference strength value, and a signal-to-noise ratio of the reverse link is higher than a reference signal-to-noise ratio. 24. A channel communication method for a wireless communication system, comprising the steps of: - analyzing an environment of a channel in service to determined whether a rate change condition is satisfied, and sending a rate change request message to a base station when the rate change condition is satisfied; and -81- - upon reception of a response message from a mobile station in response to said rate change request message, switching a presently used coding rate and a presently used orthogonal code, of a channel receiver to a coding rate and an orthogonal code corresponding to information included in the response message from the mobile station; wherein the step of determining whether a rate change condition is satisfied comprises the steps of; - examining an environment of a channel in communication selecting a coding rate lower than a present coding rate, when a first rate change rate condition is satisfied; and - selecting a coding rate higher than a present coding rate, when a second rate change condition is satisfied; and wherein the first rate change condition is satisfied when a condition selected from the group consisting of: -82- - condition 1: an average reverse link transmission power is higher than an upper threshold transmission power; - condition 2: an average forward link receiving strength is lower than a lower threshold receiving strength; and - condition 3: an average forward link signal-to-noise ratio is lower than a lower threshold signal-to-noise ratio; - is satisfied. 25. The channel communication method as claimed in claim 24, wherein the second rate change condition is satisfied when a condition selected from the group consisting of: - condition 1: an average reverse link transmission power is lower than a lower threshold transmission power; - condition 2: an average forward link receiver strength is higher than an upper threshold receiving strength; and -83- - condition 3; an average forward link signaf-to-noise ratio is higher than an uperthreshold signal-to-noise ratio; - is satisfied. 26. A channel communication method for a wireless communication system, comprising the step of: - upon reception of a rate change request message from a base station, selecting a coding rate and an orthogonal code according to information included in the request message; - sending a response message to the base station; and - changing a presently used coding rate and orthogonal code of a channel receiver to the selected coding rate and orthogonal code; wherein said step of selecting a coding rate comprising the steps of; -84- - examining a call condition associated with the mobile station to determine whether the rate change condition is satisfied; - when a first rate change condition is satisfied, selecting a coding rate lower than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate; and - when a second rate change condition is satisfied, selecting a coding rate higher than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate. 27. A channel communication method for a CDMA communication system, comprising the steps of: (i) upon reception of a rate change request message from a base station, determining whether a present coding rate and orthogonal code can be changed to a different coding rate and orthogonal code based on information in the request message; -85- (ii) when the present coding rate and orthogonal code can be changed, changing the present* coding rate and orthogonal code to the different coding rate and orthogonal code, and sending a response message to the base station indicating said changes and (iii) when the present coding rate and orthogonal code cannot be changed, generating and sending a message to the base station indicating that a change cannot occur; wherein said step of determining whether the present coding rate and orthogonal code can be changed to the different coding rate and orthogonal code based on the information in the request message comprises the steps of: - examining a call condition associated with the mobile station to determine whether the rate change condition is satisfied; -86- - when a first rate change condition is satisfied, selecting a coding rate lower than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate; and - when a second rate change condition is satisfied, selecting a coding rate higher than a present coding rate of the mobile station and selecting an orthogonal code having a length corresponding to the selected coding rate. Dated this 3rd day of March 1999. -87- A traffic channel transmission device for a COMA cmmunication system using a plurality of coding rates and orthogonal codes, determines a present channel condition and adactively selects a coding rate and an orthogonal code according to the determination. In the device, a channel receiver receives a channel signal and a controller analyzes the received signal to decide an environment of a channel in service and generates a coding rate select signal and orthogonal code information according to the decision result- A channel transmitter include* a channel encoder for encoding transmission data at a coding rate selected according to the coding rate select signal and an orthogonal modulator for generating an orthogonal code according to the orthogonal code information to spread the encoded data with the generated orthogonal code, whereoy the channel transmitter adaptively encodes and spreads the transmission data according to the channel environment- The orthogonal code information includes a number and a length of the orthogonal code. |
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| Patent Number | 209752 | |||||||||||||||
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| Indian Patent Application Number | 00169/CAL/1999 | |||||||||||||||
| PG Journal Number | 36/2007 | |||||||||||||||
| Publication Date | 07-Sep-2007 | |||||||||||||||
| Grant Date | 06-Sep-2007 | |||||||||||||||
| Date of Filing | 03-Mar-1999 | |||||||||||||||
| Name of Patentee | SAMSUNG ELECTRONICS CO. LTD. | |||||||||||||||
| Applicant Address | 416, MAETAN-DONG, PALDAL-GU, SUWON-CITY, KYUNGKI-DO, KOREA. | |||||||||||||||
Inventors:
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| PCT International Classification Number | H04 B 7/00 | |||||||||||||||
| PCT International Application Number | N/A | |||||||||||||||
| PCT International Filing date | ||||||||||||||||
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