Title of Invention

A CODE SET GENERATING METHOD IN A MOBILE COMMUNICATION SYSTEM A TRANSMITTER AND A DATA TRANSMITTING METHOD THEREOF

Abstract

This invention relates to a code set generating method in a mobile communications system having Nf frequency channels and Np phase channels. In the method, a code length Nc is obtained by NfxNp, a minimum distance Nd between codewords in a code set and frequency diversity Nfd are determined, code sets are detected according to Nc, Nd and Nfd, a code set is selected, which shows such a Hamming distance distribution among code words that minimizes demodulation errors from the detected code sets, and the code set is stored in a mapping table of a demodulator.

Full Text

BACKGROUND OF THE INVENTION 1. Field of the Invention code set generating method In a The present invention relates to a mobile communication system a transmitter and a data transmitting method thereof. 2. Description of the Related Art Voice or data is subject to channel encoding for radio transmission and reception in a mobile communications system. On a forward link, for example, voice or data to be transmitted is channel-encoded and modulated in such a method as QPSK (Quadrature Phase Shift Keying). Channel encoding is generally performed for data transmission in a single frequency, and encoded symbols are BPSK (Binary Phase Shift Keying)/QPSK modulated prior to transmission. However, if a plurality of frequencies and phase channels are available to a user for high-speed data transmission, there is a need for assigning transmission data symbols to the plurality of channels. Conventionally, data symbols are distributed to a plurality of channels for transmission. This method advantageously enables transmission of data as much as the capacity of the channels permits, but has limitations in recovering data !f against possible failures in channels or transmission data loss. The lost data may be recovered by reinforcing data encoding, but with a decrease in transmission efficiency on multiple channels. Furthermore, when a channel is or remains defective, the channel is not used or less frequently used until it is recovered, in the prior art. Therefore, communication service quality becomes poor beyond remedy. The above problems are solved by transmitting the same data on a plurality of frequencies and phase channels. Despite possible failures in some of channels, the data transmitted on the other channels are safe, thereby allowing a reliable communication link to be constructed. Yet, this method remarkably decreases channel use efficiency. Because channel encoding and assignment of frequencies and phase channels are independently performed in the conventional technology, it is impossible to consider channel use efficiency in data encoding. SUMMARY OF THE INVENTION An object of the present invention is to provide a device and method for converting channel-encoded symbols to code words in consideration of channel use efficiency and characteristics of a plurality of frequencies and phase channels and transmitting the code words via a plurality of channels in a mobile communications system. - 2 - Another object of the present invention is to provide a device and method for detecting a code set with a desired minimum distance between code words in order to convert transmission data to code words in a mobile communications system. Still another object of the present invention is to provide a device and method for selecting the best code set from a plurality of detected code sets in a mobile communications system and transmitting the selected code set. To achieve the above objects, there is provided a code set generating method in a mobile communications system having Nf frequency channels and Np phase channels. In the method, a code length Nc is obtained by NfxNp, a niinimum distance Nd between codewords in a code set and frequency diversity Nfd are determined, code sets are detected according to Nc, Nd, and Nfd, a code set is selected, which shows such a Hamming distance distribution among code words that minimizes demodulation errors from the detected code sets, and the code set is stored in a mapping table of a demodulator. BRIEF DESCRIPTION OF THE^DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiment thereof with reference to the attached drawings in which: FIG. 1 is a block diagram of a transmitter on a forward link in a mobile communications system; FIG. 2 is a block diagram of a waveform modulator shown in FIG. 1; FIG. 3 is a block diagram of the waveform modulator constituted of logic - 3 - circuits according to an embodiment of the present invention; and FIGs. 4 and 5 are views showing memories used for the waveform modulator according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is intended to offer data encoding and frequency diversity for a digital mobile communications system such as a CDMA (Code Division Multiple Access) cellular system or PCS (Personal Communications System). That is, a mobile communications system using channel encoding and multicarrier transmission is designed to enable effective information transmission and reception by efficiently assigning data and code symbols to available frequencies and phase resources in a communication environment in which channels are susceptible to errors due to fading, in an embodiment of the present invention. A transmitter in a mobile communications system according to the embodiment of the present invention has an algorithm of detecting a code set under given conditions. First, how to construct codewords according to a given number of frequencies and phase channels will be described. If the numbers of the given frequency channels and phase channels are Nf and Np, and requirements for frequency- and phase-multiplexing transmission data are Mf and Mp, the number of available channels is (NfxNp). This implies that a maximum code length Nc is (NfxNp). Then, for dmin (a minimum distance between codewords) = Nd, a code set satisfying the following conditions is detected. - 4 - The number of codewords depending on the number of phase channels is 2Np. WhenNp=2 in QPSK, the four codewords can be obtained: {00, 01, 10, 11} = {a, b, c, d}. The number of codewords depending on frequency diversity is 2Nf. For Nf=3, eight codewords can be generated. If the phase channels and frequency channels are operative together, the length of a codeword is (NfxNp) and 2N&Np codewords can be generated in a code set. ForNf = 3 andNp=2, a code set includes 64 codewords. Use of the code set having dmin of 2 to reduce demodulation errors in a receiver increases the reliability of codeword modulation/demodulation, virtually bringing about channel encoding effects. The size (the number of codewords) of the code set with dmin of 2 is 2NfeNp/2. Hence, 2N6fN,YNd codewords are available in modulation with a code set having dmin=Nd. From 2N = 2NKNp/2> the channel encoding efficiency at a demodulator is N/{(NfxNp)/Nd}. There will be shown examples of code sets constructed to satisfy the above conditions. 1. Three Frequencies 1-1. Nf=3, Np=2 (I and Q), dmin=2, and frequency diversity z 2 The length of a codeword is 6 bits (=NfxNp) and the four codewords are available for the two phase channels per frequency: {00, 01, 10,11} = {a, b, c, d}. Then, codewords for the three frequencies can be designed as follows: - 5 - Codewords starting with a are aaa = {00 00 00} abb = (00 01 01} ace = {00 10 10} add = {00 11 11} Codewords starting with b are obtained by fixing the second symbols in the order of a, b, c, d, and permuting the third symbols in an order different from the order of the third symbols in the codewords starting with a. Here, the number of the third symbol arrangements is equal to a total number of permutations minus the permutation corresponding to the third symbol arrangement in the codewords starting with a. baX = {01 00 xx} bbX = {01 01 xx} bcX = {01 10 xx} bdX={01 11 xx} Similarly, codewords starting with c are produced by fixing the second symbols in the order of a, b, c, d, and permuting the third symbols in a different manner from the orders of arranging the third symbols in the codewords starting with a and b. caX={10 00xx} cbX = {10 01xx} ccX= {10 10 xx} cdX= {10 11 xx} - 6 - Codewords starting with d can be generated in the same manner, daX-{1100xx} dbX = {1101xx} dcX*{ll IGxx} ddX»{lilixK} Therefore, a plurality of code sets can Be made. For example, a code set can aaa-{O0OOOG} abb ={00 01 01} ace =-{00 10 10} add -{00 11 11} bab-{0100 01} bbc = {010HO} bed- {01 10 11} bda = {01 1100} cac-{10 00-10} ebd-{10 01 11} cca^{10 10 00} cdb-{10 1101} dad-{11 0011} dba-{110100} deb-{11 10 01} ddc={llll 10} It is most effective to apply an optimum code set taken from the code sets 4"* resulted from the above procedure to a mobile communications system, and the effectiveness criterion is Hamming distance distribution among codewords. That is, the optimum code set shows a Hamming distance distribution which minimizes demodulation errors. 1-2. Nf=3, Np=2 (I and Q), dmin=3, and frequency diversity = 3 The length of a codeword is 6 bits (=NfxNp) and the four codewords are available for the two phase channels per frequency: {00, 01, 10, 11} - {a, b, c, d}. Then, codewords for the three frequencies can be designed as follows. If a codeword starts with a, any other code in a code set does not start with a. This rule is applied to the second and third symbols. aaa = {00 00 00} bbb = (010101} ccc = {10 10 10} ddd={ll 11 11} A plurality of code sets can be derived from the above code set by permutation. aXX = {00 xx xx} bXX={01xxxx} cXX={10xxxx} dXX={llxxxx} - 8 - The second and third symbols can be produced by permuting the order of a, b, c, d. A different example of code set is abc={00 01 10} bed ={01 10 11} cda={10 1100} dab={1100 01} 1-3. Nf=3, Np=2, and dmin=3 For frequency diversity = 2, a code set size is 4 and given as aaa = {00 0Q 00} adb = {00 11 01} dda={ll 1100} dab-{11 00 01} For frequency diversity ^ 2, a code set size is 8 and six code sets can be produced. Code Set #1 aaa ={00 00 10} adb = {00 11 01} bdc={01 11 10} bad = {01 00 11} cbc = {10 01 10} ccd={10 10 11} _ 9 _ dbb= {11 01 01} dca={ll 10 00} Code Set #2 aaa={00 00 00} adb = {00 1101} bdc-{01 11 10} bad={0100 11} cbd = {10 01 11} ccc={10 10 10} dba={110100} deb -{11 10 01} Code Set #3 aaa={00 00 00} adb={00 1101} bbc = {0101 10} bed ={01 10 11} cad={10 00 11} cdc={10 11 10} dbb={110101} dca={ll 10 00} Code Set #4 aaa={00 00 00} adb = {00 1101} - 10 - bbc={0101 10} bed ={01 10 11} cbd={1001 11} ccc = {10 10 10} dab- {11 00 01} dda={ll 1100} Code Set #5 aaa={00 00 00} adb = {00 1101} bbd={0101 11} bcc = {01 10 10} cad ={10 00 11} cdc={10 11 10} dba={110100} dcb={ll 1001} Code Set #6 aaa= {00 00 00} adb={00 1101} bbd={0101 11} bcc = {01 10 10} cbc={10 01 10} ccd-{10 10 11} dab = {11 00 01} dda={ll 1100} - 11 - 2. Six Frequencies The number of available frequencies is not limited to three and can be six on a multicarrier forward link. Thus, code sets for six frequencies can be designed in consideration of frequency diversity, as follows. {a, b, cd, } = {00, 01, 10, 11} is also used in building code sets for six frequencies. 2-1. Frequency diversity exists, and dmin=2 A total number of codewords is 45. A codeword is expressed as el e2 e3 e4 e5 e6 and e4 e5 e6 are determined from el e2 e3. Then, the following three-frequency sets are made. aaa bab cac dad SA={ abb bbc cbd dba} ace bed cca deb add bda cdb ddc aba bbb cbc dbd SB={ acb bec ccd dca} adc bdd cda ddb aad baa cab dac aca beb ccc ded SC={ adb bde edd dda} aac bad caa dab - 12 - abd bba ebb dbc ada bdb ede ddd SD = { aab bac cad daa} abc bbd cba dbb acd bca ccb dec If the first three symbols of el e2 e3 e4 e5 e6 belong to SA, the last three symbols are selected from SA. A code set size is 4s when codewords are generated in this manner. 2-2. Frequency diversity=6 Since a codeword has different code symbols in every symbol position, a code set size is 4. To maximize the distance between codewords, symbol distance should be considered. For the four codewords, a codeword distance is six symbols and 01, 10, 11 occur twice respectively regardless of symbol positions in the six symbols. Since four of the six symbols have weight Is and the other two symbols have weight 2s, the six symbols are composed of four symbols having weight Is and two symbols having weight 2s to maximize the codeword distance. That is, cl + c2=>{0101 10 10 11 11} c2 + c3=>{10 10 11 11 0101} cl + c3=>{ll 110101 10 10} cl + c4->{10 10 11 110101} C2.+ C4=>{11 110101 10 10} c3 + c4->{0101 10 10 11 11} - 13 - For example, when cl = 00 00 00 00 00 00, the code set is cl = 00 00 00 00 00 00 C2-0101 10 10 11 11 c3 = ll 110101 10 10 c4=10 10 11 110101 where the code set size is 4 and dmin=8. 2-3. Frequency diversity ^ 5 A total of 12 bits are grouped into 4 bits. That is, if symbols are aO, al, a2, , al5, each symbol of a codeword (ai aj ak) is different from any other symbol in view of the given frequency diversity. Though a code size is 16, it cannot be said that the frequency diversity s 5 with all codewords each having different symbols. Therefore, an actual code set size is 8 and a code set is aaaaaa abbbbb b a c c c c bbdddd c c ab c d c db c d a d c c d ab d d dab c 2-4. Frequency diversity ^ 4 The first three symbols in a codeword are different, and the number of - 14 - codewords in a code set is smaller than 43. A code set having 32 (=2x 42) codewords is constructed in the following procedure. First, 16 codewords are generated by SA+SA, that iss repeating the 16 codewords in SA once, aaaaaa b abb ab c a c c a c d add a d abb ab b b b c b b c c b d cb d db adb a a c c ac c b c db c d c c ac c a d c b d c b ad dadd b d ab d a c db c db d d c d dc Then, the other 16 codewords are generated by SB+SB*. SB* is achieved - 15 - by changing the order of symbols in SB. ccd dca acb bcc SB*-{cda ddb adc bdd} cab dac aad baa cbc dbd aba bbb The other 16 codewords are ab a c c d b b b d c a cb'cac b db db c c acb c da b c c d db c c d a d c d c ab d d a d c c ab b d d d ac c daaad d db b aa aadcb c b aa db d c ab ab a dacb bb - 16 - Thus, a total of 32 codewords are produced. There will be given a description of an exemplary application of the above code set generating method to a waveform modulator on a multicarrier forward link. FIG. 1 is a block diagram of a transmitter on a forward link in a mobile communications system. Referring to FIG. 1, a source coder 111 encodes input data, and a channel coder 112 channel-encodes the data received from the source coder 111 and spreads the channel-encoded data. A waveform modulator 113 converts the spread data received from the channel coder 112 to codewords taken from a predetermined code set. The present invention is related to the waveform modulator 113 of FIG. 1. FIG. 2 is a block diagram of the waveform modulator 113 shown in FIG. 1. Referring to FIG. 2, a serial-to-parallel converter 211 converts the data received from the channel coder 112 to parallel data, and a codeword modulator 212 maps the parallel data to corresponding codewords in a predetermined method. The codeword modulator 212 is shown in FIG. 2 to modulate input three bits to six codeword symbols F1I, F1Q, F2I, F2Q, F3I, and F3Q. The waveform modulator 113 constituted as shown in FIG. 2 can map N-bit input data to an M-symbol output codeword. In this case, the codeword modulator 212 should convert the N-bit input data to the M-symbol codeword. FIG. 3 is a detailed block diagram of the codeword modulator 212 of FIG. 2 realized in hardware. In the codeword modulator 212, three-bit input data is - 17 - converted to a codeword with six symbols selected from eight codewords. Referring to FIG. 3, the serial-to-parallel converter 211 converts input data to three-bit data. The codeword modulator 212 includes eight codewords #0-#7, and multiplexers MUX1-MIJX7 for selecting a corresponding codeword from the eight codewords according to the three bits A0-A2 received from the serial-to-parallel converter 211. The above method can be adapted to output an M-symbolt codeword for the N-bit input data. FIG. 4 is a view of a ROM used as the waveform modulator 113, for outputting a six-symbol codeword for three-bit input data A0-A2. If a codeword mapping table is stored in the ROM of FIG. 4, the N-bit data can be converted to the M-symbol codeword. Various code sets can be generated depending on dmin and frequency diversity requirements. These code sets can be applied to a waveform modulator on a multicarrier forward link, such as a forward link modulator in a CDMA cellular system, PCS, or IMT-2000, thereby achieving coding gains and frequency diversity at the same time. Use of codesets designed by the method of the present invention in transmitting data on a multicarrier forward link with three frequency channels and I and Q phase channels enables frequency diversity to be efficiently achieved. Since transmission channels can be used without the need for distinguishing - 18 - frequencies and phase channels in the frequency diversity scheme, data transmission efficiency is increased. Furthermore, it is possible to set an appropriate ratio of a data transmission rate to frequency diversity. Therefore, channel coding gain requirements in other parts of a system can be reduced by means of a modulator using the above codesets, and a waveform modulator can be realized, which has a high coding rate with maximum frequency diversity and dmin. The concept of the above method for generating codesets regardless of frequencies and phases can be applied to generation of multi-symbol block codes. When there are three frequency channels and two phase channels, the six channels are basically assigned to one symbol. The multi-symbol block code scheme refers to assigning symbols to a total of 12 channels derived from three frequency channels, two phase channels, and two time slots. For 12 channels, code sets can be designed in which the distance between codewords is set to be large in consideration of frequency diversity. With these multi-symbol block codes, frequency multiplexing, time multiplexing, and coding gains can be achieved all together. In the method of generating codeword symbols using frequency, phase, and time, the entire channels are [fl-I fl-Q f2-I f2-Q f3-I f3-Q fl-I fl-Q fl-I f2-Q f3-I f3-Q] Codewords are constructed for the above channels in consideration of frequency multiplexing and dmin. For 12 channels, dmin=2, and frequency - 19 - diversity ^ 2, the number of available codwords is 210. If dmin is increased according to coding gain requirement, 10 or less symbol bits can be transmitted but the distance between codewords increases, facilitating demodulation. A description of generating a code set for the 12 channels will be omitted because the code set generating method can be inferred from the afore-described examples. FIG. 5 is a view showing a ROM used as a modulator in which data is block-encoded for three frequency channels, two phase channels, and two time channels which are used together. The codeword modulator 212 of FIG. 5 generates 12 output codeword symbols. Here, f ltl-I represents a codeword symbol transmitted in frequency f 1, time slot tl, and phase channel I, and f3t2-Q represents a codeword symbol transmitted in frequency f3, time slot t2, and phase channel Q. The codeword modulator of FIG. 5 is applicable to a communications system using a plurality of frequency channels, phase channels, or time channels, like a CDMA cellular system, PCS, and IMT-2000. Application of data encoding and modulation together to a transmission system in the embodiment of the present invention increases the effects of frequency diversity in an environment of fading related with frequencies, and allows dynamic data encoding and modulation in any case between the extremes of no multiplexing with a high data transmission rate and multiplexing to every frequency channel with a low data transmission rate. In addition, since a plurality of phase channels and frequency channels are used together, a demodulation efficiency in a - 20 - receiver is increased. In the preferred embodiment of the present invention as described above, system capacity and power use efficiency as well as encoding and modulation efficiency are increased by dynamically applying data encoding and modulation to a transmission/reception system using a plurality of frequencies and phase channels in accordance with characteristics of the frequencies and phase channels. While the present invention has been described in detail with reference to the specific embodiment, it is a mere exemplary application. Thus, it is to be clearly understood that many variations can be made by anyone skilled in the art within the scope and spirit of the present invention. i»r*"~ - 21 - We CLAIM 1. A code set generating method in a mobile communications system having Nf frequency channels and Np phase channels, comprising the steps of: calculating a code length Nc by NfxNp; setting a minimum distance Nd between codewords in a code set and frequency diversity Nfd, and detecting code sets according to Nc, Nd and Nfd; and selecting a code set having a hamming distance distribution among code words which minimizes demodulation errors from the detected code set, and storing the code set in a mapping table of a demodulator. 2. A transmitter in a mobile communication system comprising : a source coder for encoding transmission data; a channel coder for channel-encoding the output of the source coder; a waveform modulator having a code set for mapping the channel encoded data to corresponding code words, referring to the code set table; and a transmitting portion for transmitting a codeword received from the waveform modulator on a plurality of channels. 3. The transmitter as claimed in claim 2, wherein the waveform modulator comprises: a seriai-to-paralie! converter for converting the channel-encoded data to parallel data; and a codeword modulator having the code set with a code length being the product of the numbers of frequencies and phase channels, for address-mapping the parallel data and selecting corresponding codewords. 4. The transmitter as claimed in claim 2, wherein the waveform modulator comprises: a seriai-to-parallei converter for converting the channel-encoded data to parallel data; and a codeword modulator having the code set with a code length being the product of the numbers of frequencies, phase channels, and time slots, for address-mapping the parallel data and selecting corresponding codewords, 5. A data transmitting method in a mobile communication system having a code set for storing codewords as long as the product of the numbers af frequency channels and phase channels, comprising the steps of: encoding source data to be transmitted; channel-encoding the encoded source data; converting the channel-encoded data to parallel data; address-mapping the parallel data in the code set, selecting a corresponding codeword, and modulating the waveform of the selected codeword; and transmitting the waveform-modulated codeword on channels as many as the product of the numbers of frequency channels and phase channels. 6. A data transmitting method in a mobile communication system having a code set for storing codewords as long as the product of the numbers of frequency channels, phase channels, and designated time slots, comprising the steps of: encoding source data to be transmitted; channel-encoding the encoded source data; converting the channel-encoding data to parallel data; address-mapping the parallel data in the code set, selecting a corresponding codeword, and modulating the waveform of the selected codeword; and transmitting the waveform-modulated codeword on channels as many as the product of the numbers of frequency channels, phase channels and designated time slots. Dated this 11th day of September, 1998. 0FL.S.DAVAR8TC0. APPLICANTS'AGENT 2-3 This invention relates to a code set generating method in a mobile communications system having Nf frequency channels and Np phase channels. In the method, a code length Nc is obtained by NfxNp, a minimum distance Nd between codewords in a code set and frequency diversity Nfd are determined, code sets are detected according to Nc, Nd and Nfd, a code set is selected, which shows such a Hamming distance distribution among code words that minimizes demodulation errors from the detected code sets, and the code set is stored in a mapping table of a demodulator.

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01623-cal-1998-abstract.pdf

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