Title of Invention

APPARATUS AND METHOD FOR COMPENSATING GAIN OF AN AUTOMATIC GAIN CONTROLLER

Abstract

An apparatus and method for compensating the gain of an Automatic Gain Controller (AGC) for stabilizing the reception power of discontinuously transmitted packet data in a mobile communication system are disclosed. A compensation controller receives an AGC value from the AGC, sampling the AGC value by a predetermined sample number for a predetermined period, and obtains an AGC compensation gain by comparing a predetermined value with the difference between a sampled AGC value with a reference gain for the predetermined period. A compensator compensates the AGC value with the AGC compensation gain, thereby correcting errors generated in view of the nature of the AGC. (FIG.3)

Full Text

FIELD OF THE INVENTION The present invention relates generally to an automatic gain control apparatus and method, and in particular, to an apparatus and method for compensating the error of an automatic gain controller (AGC) in order to stabilize the received signal power of discontinuously transmitted high-rate packet data in a mobile communication system. BACKGROUND OF THE INVENTION Mobile communication systems for high-rate packet data transmission (hereinafter, referred to as a high-rate packet transmission mobile communication system) usually support only data channels or support both data and voice channels. The former is referred to as an International Mobile Telecommunication-2000 (IMT-2000) Evolution Data Only (lx EV-DO), while the latter is referred to as an IMT-2000 Evolution-Data and Voice (lxEV-DV). To realize high-rate data transmission, a plurality of users share the same channel for Time Division Multiplexing (TDM) in the high-rate packet transmission mobile communication systems. A base station receives feedback forward channel state information from a mobile station and if the channel state is acceptable, the base station transmits data in a high-order modulation scheme such as 8 Phase Shift Keying (PSK), 16 Quadrature Amplitude Modulation (QAM), or 64-QAM to receive a higher data rate. Since a forward packet channel is shared among a plurality of users in TDM, the base station assigns all available transmission power to one or two particular users in a time slot assigned to the users. Because packet data is usually generated discontinuously, there exist periods where packets are not transmitted e.g. non-packet-transmission periods. Hence, if the received signal level of packet data is not kept constant, the use of a high- order modulation such as 64-QAM leads to poor packet reception quality. A typical solution is to use an AGC. The structure of the AGC and the level variations of a signal received at a receiver will be described below with reference to Fig. 1 to Fig. 2C. Fig. 1 is block diagram of a typical AGC for keeping a received signal level constant. Referring to Fig. 1, the AGC comprises a gain controlled amplifier (GCA) 10 for receiving a signal s(t) from an antenna (not shown), an accumulator 20 for accumulating the output of the GCA 10 for a predetermined period, an adder 30 for adding the output of the accumulator 20 to a target reference voltage AIM_AMP, and a feedback loop filter 40 having a predetermined bandwidth, for filtering the output of the adder 30. The GCA 10 is an amplifier controlled by a feedback signal Vc. During operation, an input signal is fed to the GCA 10. The output of the GCA 10 is divided into two parts and one part of the output is fed to the accumulator 20. The adder 30 adds an accumulated signal received form the accumulator 20 to the reference voltage AIM AMP with a negative value. That is, the adder 30 computes the difference between the output of the accumulator 20 and the reference voltage AIM_AMP. The feedback loop filter 40 filters the signal of the difference and the GCA 10 amplifies the input signal with the filtered signal. The level variations of a signal received from a base station will be described in connection with the strucfure of the AGC. FIGs. 2A, 2B and 2C are timing diagrams illustrating transmitted power level variations and received power level variations for discontinuous packet transmission. More specifically, Fig. 2A illustrates base station transmission power lor, FIG. 2B illustrates the level of a received signal I0_agc is not kept constant due to the rapid variation of the transmitted signal at the transmission start point tl or the transmission end point t3 because the AGC generally operates in a loop control manner. Thus, some time is taken until the AGC loop is stabilized. It is an evitable AGC error under the assumption of an ideal AGC. Therefore, most high-rate packet receives face the same problem in relation to discontinuous packet transmission. High-rate packet data transmission involves a high-order modulation like QPSK/8-PSK or a higher-order modulation scheme such as 16-QAM / 64-QAM. Demodulation performance is greatly degraded if the AGC error caused by discontinuous packet transmission makes the power level of an input signal inconstant. Moreover, the AGC error lasting until the stabilization of the AGC during the discontinuously packet transmission changes the received power level in one slot, which significantly affects the demodulation performance of 16-QAM/64- QAM. Therefore, there is a need for an algorithm for reducing the power level variation of a signal output from the AGC in one slot. US 2001/018275 Al describes a gain control device for packet signal receiver in which a variable gain amplifier is controlled in accordance with the detected output power of a received packet signal. Patent Abstract of Japan 2002 164756 describes a Digital Receiving Device in which the degree of amplifier in an AGC Circuit is controlled in accordance with the number of used channel signals. OBJECTS OF THE INVENTION An object of the present invention is to provide an apparatus and method for keeping constant the level of received power measured during a packet transmission period in a high-rate packet transmission mobile communication system. Another object of the present invention is to provide an apparatus and method for preventing the decrease of reception quality caused by discontinuous packet transmission in a high-rate packet transmission mobile communication system. A further object of the present invention is to provide an apparatus and method for compensating for the power level changes of a distorted received signal in a high-rate packet transmission mobile communication system. Still another object of the present invention is to provide an apparatus and method compensating for the level changes of a distorted received signal without modifying the structure of a receiver in a high-rate packet transmission mobile communication system. SUMMARY OF THE INVEMTTON The above objects are achieved by an apparatus and method in accordance with the features of the invention. In the AGC error compensating apparatus, a compensation controller receives an AGC output value form the AGC, samples the AGC output value of the predetermined sample number for a predetermined period, and obtains an AGC compensation control signal by comparing a predetermined value with the difference between a sampled AGC output value with a reference gain value for the predetermined period. A compensator compensates the AGC output value with the AGC compensation control thereby correcting errors generated in view of the nature of the AGC. This apparatus further includes an offset compensator for compensating the power level of the compensated AGC output value with an AGC compensation offset calculated in the compensation controller. In the AGC error compensating method, an AGC output value from the AGC is sampled by a predetermined sample number for a predetermined period, and an AGC compensation control signal is obtained by comparing a predetermined value with the difference between a sampled AGC output value with a reference value for the predetermined period. The AGC output value is compensated with the AGC compensation compensation control signal, thereby correcting errors generated in view of the nature of the AGC. Furthermore an AGC compensation offset is obtained using the difference between the reference value for a present period and a reference value for a next period extracted in response to a next reference value clock signal when the predetermined period expires, and the power level of the compensated AGC output value is compensation offset. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 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: Fig. 1 is a block diagram of a conventional Automatic Gain Controller (AGC) for keeping the power level of a received signal constant; Figs. 2A, 2B and 2C are timing diagrams illustrates changes in transmission power, received signal controlled by AGC, and control voltage in AGC in the conventional technology; Fig. 3 is a block diagram illustrating an apparatus for compensating the error of an AGC in a mobile station receiver in a communication system according to an embodiment of the present invention; Fig. 4 is a block diagram illustrating a compensation controller in the error compensating apparatus of Fig. 3; Figs. 5A to 5D are timing diagrams illustrates the power level variations of input signals by applying the error compensation of the AGC according top an embodiment of the present invention; and Fig. 6 is a flowchart illustrating a method of compensating the error of the AGC according to an embodiment to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described herein below with reference to the accompanying drawings. It should be noted that like reference numerals denote the same components in the drawings. Many specific details, such as specific signals and signal levels, which are shown in the following description, are disclosed for the purpose of helping to form a comprehensive understanding of the embodiments of eth present invention. It should be appreciated by those skilled in the art that the embodiments of the present invention may be implemented without these details. In addition, well- known functions or constructions are omitted for conciseness. The embodiments of the present invention will be described in the context of a forward link in a high-rate packet transmission mobile communication system that supports multimedia service including voice and data services using the Code Division Access (IX CDMA) bandwidth. The IX CDMA bandwidth is a 1.25 - MHz frequency bandwidth found is a existing IS - 95 synchronous systems in North America. It is used to determine a slot boundary reference signal T125 that in turn determines a clock cycle in an embodiment of the present invention. As packet data is discontinuously transmitted, it follows that transmission power rapidly changes at the transmission start point tl or the transmission end point t3, and as a result, the Automatic Gain Controller (AGC) 110 fails to keep received power constant, as illustrated in Fig. 2B. The implementation of an AGC error compensating algorithm in a mobile station receiver according to an embodiment of the present invention will be described below. Fig. 3 is a block diagram of an apparatus for compensating the error of an AGC in a mobile station receiver according to an embodiment of the present invention. Referring to Fig. 3, the mobile station receiver includes a packet data receiver 100, the AGC 110, which is a gain controller and a symbol demodulator 130. The AGC 110 stabilizes the power level of discontinuous packet data received from the packet data receiver 100 via gain control and outputs an AGC out put value AGC_VALUE. AGC_VALUE represents the power level of a distorted signal generated during the time required fore stabilization in the loop structure of the AGC 110. It is the gain of one symbol in a slot. The error compensating apparatus is compensating apparatus is comprised of a channel compensator 121, a Walsh demodulator 122, a first multiplier 123, and a compensation unit 200. The channel compensator 121 compensates a packet data channel and outputs P(n,m). The Walsh demodulator 122 demodulates the received packet data with a Walsh code and outputs Y(n,m). The First multiplier 123 multiplies the channel compensated signal by the Walsh-demodulated signal and outputs the products as the distorted signal to be compensated, the is, an actual AGC value Z(n,m) to be compensated. The compensation unit 200 has a compensation controller 210, a compensator 220, a symbol energy estimator 230, a second multiplier 240, and an offset compensator 250. The compensation controller 210 calculates an AGC compensation control signal AGCC__GAIN and an AGC compensation offset AGCC_OFFSET with which to compensate AGC_VALUE. The compensator 220 compensates Z(n.m) with AGCCJ3AIN. The symbol energy estimator 230 estimator a reference energy for the compensated value ZAGcc(n, m) received from the compensator 220. The second multiplier 240 multiplies the estimated symbol energy _1_ by ZAGcc (n.m). The offset compensator 250 compensates the VBRE compensation value product Z agcc (n,m) with AGC_OFFSET received from the compensation controller 210. it should be noted that AGCC_OFFSET is calculated only hen needed. In other words, AGC-OFFSET can be omitted if it is unnecessary. The symbol energy estimator 230 adopts a blind estimation technique in which it estimates JL using only the symbols of received data. Radio channel fading can VBRE be tracked by estimating _!_ for each slot, and _1_ is used as a reference vBRE vBRE energy when demodulating the symbols of a received slot. The structure and operation of the compensation controller 210 in the compensation unit 200 will be described in detail with reference to Fig. 4. Fig. 4 is a block diagram of the compensation controller 210 illustrated in Fig. 3. Referring to Fig. 4, the compensation controller 210 comprises a timing controller 211, a sampler 212, a first subtracter 213, a first look-up table 214, a storage 215, a second substractor 216, and a second look-up table 217. The timing controller 211 generates a gain clock signal GAIN_CLK is synchronized to a slot boundary reference signal T125 of a predetermined period, and used as a sampling period for aqGC_VALUE. The sampler 212 samples AGC_VALUE in response to the clock signals. The storage 215 temporarily stores a signal output from the sampler 212 in response to REF_GAIN_CLK as a reference value GREf. The First subtracter 213 calculates the difference (i.e compensation value Gcomp) between Gref and an AGC value sample which is output from the sampler 2121 in response to GAIN_CLK. The first look-up table 214 outputs AGC_GAIN by comparing Gcomp with a predetermined value. Here, the storage 215 is a D-flipflop connected to the timing controller 211 and operated in response to REF_GAIN-CLK. The second subtracter 216 calculates the difference (i.e compensation offset Goffset) between the reference value for the present slot and a reference value for the next slot. The second look-up table 217 outputs AGC_OFFSET by comparing GoffsEt with the predetermined value. Returning to Fig. 3, the actual AGC value Z(n,m) to be compensated with AGC- GAIN from the compensation controller 210 is determined by where m is the index of a symbol in one slot, and n is the index of the slot. Z (n, m) is the product of the output P(n,m) of the channel compensator 121 and the output Y (n,m) of the Walsh-demodulator 212. In Eq. (1), P is the strength of a pilot signal g(n,m) is again reflected in a received signal by the AGC loop, h (n,m) is the product of x(n,m) to be multiplied by a carrier c, and the strength s(n,m) of a received signal s(t) added to P, and n(n,m) is added to h(n,m). these computations are performed in the packet data receiver 100 prior to input to the AGC 110. Figs. 5A to 5D are timing diagrams illustrates the power level variations of input signals by applying the error compensation of the AGC 110 according to the present invention. Fig. 5A illustrates the operation timing of the compensation controller 210 illustrated in Fig. 4 and Fig. 5B illustrates the level change of AGC_GAIN computed in the compensation controller 210. Figs. 5C and 5D will be described later in connection with a discussion of FIG. 7. Since the AGC-controlled power level of a discontinuously received signal is variable, errors are generated in the output from the AGC 110 during a stabilization period. That is, the compensation controller 210 receives the distorted received signal on a symbol basis for one slot. Therefore, correction of the AGC errors is equivalent to compensation of the output from the SGC 110, that is, AGC-VALUE. The timing controller 211 outputs GAIN_CLK and REF_GAIN_CLK to the sampler 212 is response to T125. At the same time, the timing controller 211 outputs REF_GAIN_CLK to the storage 215. Referring to Fig. 5A. AGC_VALUE is synchronized to T125 and frequency-divided into a predetermined number of sample according to GAIN_CLK in the sampler 212. The frequency-division numbers of AGC-VALUE are used as symbol indexes and the clock pulses of T Represent slot indexes. The sampler 212 outputs AGC_VALUE samples by sampling AGC_VALUE according to GAINJXK and REF.GAIN _CLK. AGC_VALUE sample output from the sampler 212 in response to REF-GAIN-CLK is stored as GREf for one in the storage 215. The first subtracter 213 subtracts an AGC_VALUE sample generated in response to GAIN_CLK from GREf. The first look-up table 214 obtains AGC_GAIN by comparing Gcomp received from the first subtracter 213 a stored value. As illustrated in Fig. 5B, since AGC_GAIN is calculated in correspondence with AGC_VALUE, its level changes in the opposite to the level change illustrated in Fig. 2. Here, the AGCC_GAIN is calculated by Eq. (2). The compensator 220 illustrated in Fig. 3 compensates Z(n,m) with AGCC_GAIN. AGCC_GAIN = (gREF(n)/g(n,m))2 Meanwhile, the second subtracter 216 calculates the compensation offset Goffset subtracting the reference value for the present slot from that for the next slot, upon generation of REF_GAIN_CLK. The second look-up table 217 then obtains AGCC-OFFSET for the present slot by comparing Goffset with a corresponding table value. AGC_OFFSET is calculated by EQ. (3). Returning to Fig. 3, AGCC- OFFSET is reflected in Z" agcc (n,m) to thereby keep constant the power of a packet data signal input to the symbol demodulator 130. AGCC.OFFSET = (gREF (n+1) / gREF (n))2 .........3 Hereinafter, a described will be made of a method of compensating the AGC error-caused distorted signal, AGC_VALUE using AGCC_GAIN and AGC-OFFSET. Fig. 6 is a flowchart illustrating a method of correcting AGC errors according to an embodiment of the present invention. Referring to FIG. 6, the compensation controller 210 sets variables to their initial values in step 300. The variables will be described first. AGC.VALUE is a control signal for a GCA. A sampling period AGC_SAMP_DUR for AGC_VALUE and the number of samples per slot AGC_SAM_NUM are determined. REF_GAIN_CLK is synchronized to T125, the slot boundary reference signal indicating the start of a slot. GAIN_CLK is used as a sampling clock signal. The compensation controller 210 initializes counters by setting the symbol index m and the slot index n to 0s in step 310 and compares m with AGC_SAM_NUM in step 320. If m is equal to AGC_SAM_NUM, the compensation controller 210 sets m to the initial value, 0 and increases n by 1 (n = n+1) in step 330. The compensation controller 210 generates REF_GAIN_CLK in step 335 and proceeds to step 340. If m is not equal to AGC_SAM_NUM is step 320, the compensation controller 210 performs step 340. After the compensation controller 210 samples AGC_VALUE according to GAIN- CLK in step 340, it determines whether m is the initial value, 0 in step 350. If m is 0, the compensation controller 210 sets AGC_VALUE for the present symbol as a reference for an nth slot, GRef(n) (GREF(n) = AGC-VALUE) in step 360. AGC_VALUE being GREF(n) is extracted at each slot start point and stored as GREF(n) for the slot in the storage 215. Using GREF(n), AGC_GAIN is extracted in relation to the AGC error of each symbol. Since the control signal for the GCA and the gain of the GCA in the AGC loop is in the relationship of an exponential function, the relationship between AGC_VALUE and the gain of the GCA is expressed as Eq. (4). For reference, upon input of T125 at each slot start point (m=0), the timing controller 211 generates REF_GAIN_CLK and thus the storage 215 extracts GREF each time it receives REF_GAIN_CLK, in steps 330 and 335. and then step 380 is performed. On the other hand, if M is not 0 is step 350, the procedure proceeds to step 380. The compensation controller 210 obtains an compensation offset GoFFSET(n-l) for the previous slot by calculating the difference between the reference value GREF(n) for the present AGC_VALUE and the reference value GREF(n-l) for the previous slot by Eq. (5). By Eq. (6), the compensation controller 210 sets the AGC compensation offset of the previous compensation offset G0FFSET(n-l) as the previous AGC compensation offset AGCC_OFFSET(n-l) using the second look-up table 217. The compensator 220 reflects AGC_OFFSET(n-l) in the distorted signal. In step 380, the compensation controller 210 calculates AGC_GAIN by The compensation controller 210 obtains he compensation value GcoMp(m) for the present symbol by calculating the difference between the reference value GREF(n) for the present slot and the AGC output value AGC_VALUE(m) for the present symbol by Eq. (7). It then obtains a value expressed as an exponential function in the first look-up table 214 in Eq. (8), corresponding to GcoMp(m), as AGCC-GAIN. AGC_LUT(x) = 105xagc_gain_step10 By applying Eq. (7) to Ep. (8), the AGC compensation control signal for the present symbol in the present slot, AGCC.GAIN (n,m) is expressed as AGCC_GAIN(n,m) = AGCC_LUT(GC0Mp(m)) ........(9) After calculating AGC_GAIN, the compensation controller 210 increase m by 1 in step 390 and returns to step 320. It should be noted that step 370 for calculating AGC_OFFSET and offset compensation with AGCJDFFSET is optional. The offset compensator 250 obtains a compensation offset GOFFSET by multiplying AGC_OFFSET by a compensation value product Z" AGcc(n-l, m) For the previous slot according to Eq. (10). Z" AGcc(n-l, m) is the product of the compensated AGC value and the estimated symbol energy, ZAGcc_oFFSET(n-l, m) for its power offset, thereby making the power of Z" AGC(n-l,m) constant. ZAGcc_oFFSET(n-l, m) = AGCC_OFFSET(n-l)* Z"AGcc(n-l),m) ..............10 The compensator 220 multiplies AGCC_GAIN by the distorted received signal, thereby compensating the signal for AGC errors. The compensated signal ZAGcc(n,m) is expressed as = g2 REFpx [ I hm l2x (n,m) + h (n,m) n(n,m)] ...........(11) By Eq. (11), ZAGCC(n,m) is calculated by multiplying AGCC_GAIN (=(gREF(n)/g(n,m))2) by Z(n,m) from the first multiplier 123 having a output from the loop of the AGC 110. Thus, ZAGC(n,m) renders the variations of the output of the AGC 110 for one slot fixed to the present reference value gREF(n) is set for each slot, AGC is performed with respect to channel variations on a slot basis. Signal level variations exhibited during this process are shown in Fig. 5C and 5D. Fig. 5C illustrates the level of an AGC error-caused distorted signal after reflecting AGC_GAIN . A dotted line denotes the distorted signal and a solid line denotes the level-controlled signal. The signal distortion is compensated for with AGC-GAIN and thus the signal power level is kept constant in a corresponding slot. Fig. 5D illustrates the power level of the final received signal after AGCC_OFFSET compensates the signal for the compensation offsets GOFFSET of its power level involved with the AGC compensation process in each slot. AGCC_OFFSET(n) is calculated at the boundary t3 of the text slot (i.e., n+l)th slot), at which the next REF_GAIN_CLK is generated, that is, when the next T125 is generated. In the same manner, AGCC_OFFSET(n+l) is calculated when T125 is generated in an (n+2)th slot. Meanwhile, fading-incurred channel variations are very slow relative to AGC ERROR-CAUSED CHANNEL VARIATIONS. Hence, the slot-based AGC has little influence on the AGC"s original function (i.e. keeping constant the power level of a received signal against radio channel changes). While the first AGC output value in a slot is used to calculate AGCC-OFFSET in an embodiment of the present invention, it can be further contemplated as another embodiment of the present invention that the last AGC output value in the slot is used as a reference value for stabilizing the loop of the AGC 110. The AGC error compensation algorithm according to the second embodiment of the present invention is different from the of the first embodiment in that a storage is added to store AGC output values because the last AGC output value in a slot is used as a reference value for the slot. Except for the reference value, offset compensation is performed in the same manner as described above. While it has been described that offset compensation is performed when the dynamic area of a received signal is to be kept constant, the offset compensation apparatus and operation can be omitted because the offset compensation has little influence on performance. As described above, the present invention compensates a received signal for signal distortion caused by AGC errors due to discontinuous transmission using an AGC compensation control signal and an AGC compensation offset calculated from an AGC error compensating algorithm. Therefore, the degradation of reception quality of a packet channel due to signal distortion is prevented. While the invention has been shown and described with reference to certain embodiments 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 sprit and scope of the invention as defined by the appended claims. WE CLAIM 1. An apparatus for compensating the error of an automatic gain controller (110) for controlling the gain of received packet data in a receiver in a mobile communication system where packet data is discontinuously transmitted, the apparatus comprising: a compensation controller (200) for receiving an AGC output value AGC_VALUE from the AGC, sampling the AGC output value by a predetermined sample number for a predetermined period, and obtaining an AGC compensation control signal AGCC_GAIN by calculating the difference between a sampled AGC output value with a reference value (Gref) stored for the predetermined period; and a compensator 220 for compensating the AGC output value with the AGC compensation control signal, thereby correction errors generated in view of the nature of the AGC. 2. The apparatus as claimed in claim 1, wherein the reference value comprises the AGC output value extracted at the start of the predetermined period and temporarily stored. 3. The apparatus as claimed in claim 2, wherein the compensation controller comprises : a timing controller (211) for generating a reference value clock signal in the predetermined period, and generating a gain clock signal by dividing the frequency of the reference value clock signal by the predetermined sample number; a sampler (212) for sampling the AGC output value for the predetermined period in response to the gain clock signal; a storage (215) for temporarily storing the AGC output value and outputting the AGC output value as the reference value for the predetermined period in response to the reference value clock signal; a first subtracter (213) for subtracting the AGC output value sampled in response to the gain clock signal from the reference value and outputting the difference as a compensation value (Gcomp); and a first look-up table (214) for obtaining the AGC compensation control signal by outputting a stored value corresponding to the compensation value. 4. The apparatus as claimed in claim 3, wherein the predetermined period comprises one slot including a transmission unit of packet data. 5. The apparatus as claimed in claim 2, comprising an offset compensator (250) for compensating the power level of the compensated AGC output value with an AGC compensation offset calculated in the compensation controller. 6. The apparatus as claimed in claim 5, wherein the compensation controller (200) comprises : a timing controller (211) for generating a reference value clock signal in the predetermined period and generating a gain clock signal by dividing the frequency of the reference value clock signal by the predetermined sample number; a sampler (212) for sampling the AGC output value for the predetermined period in response to the gain clock signal; a storage (215) for temporarily storing the AGC output value and outputting the AGC output value as the reference value for the predetermined period in response to the reference value clock signal; a first subtracter (213) for subtracting the AGC output value sampled in response to the reference value clock signal from the reference value and outputting the difference as a compensation value (Gcomp); and a first look-up table for obtaining the AGC compensation control signal by outputting a stored value corresponding to the compensation value. 7. The apparatus as claimed in claim 6, wherein the predetermined period comprises one slot including a transmission unit of packet data. 8. The apparatus as claimed in claim 7, wherein the compensation controller further comprises: a second subtracter (216) for subtracting the reference value for a present period from a reference value for a next period extracted in response to a next reference value clock signal when the predetermined period expires and outputting the difference as a compensation offset (Goffset); and a second look-up table (217) for obtaining the AGC compensation offset (AGCC_OFFSET) by outputting a stored value corresponding to the compensation offset. 9. The apparatus as claimed in claim 1, comprising a symbol energy estimator for estimating the energy of the compensated AGC value received from the compensator and normalizing the estimated energy. 10.The apparatus as claimed in claims 1 to 9, wherein the compensator (220) receives as input a modified AGC value from signal processing units (121, 122). 11. A method of compensating the error of an automatic gain controller (AGC) for controlling the gain of received packet data in a receiver in a mobile communication system where packet data is discontinuously transmitted, the method comprising the steps of: (1) receiving an AGC output value from the AGC, sampling the AGC output value by a predetermined sample number for a predetermined period, and obtaining an AGC compensation control signal by comparing a predetermined value with the difference between a sampled AGC output value with a reference value for the predetermined period; and (2) compensating the AGC output value with the AGC compensation control signal, thereby correcting errors generated in view of the nature of the AGC. 12.The method as claimed in claim 11, wherein the reference value comprises the AGC output value extracted at the start of the predetermined period and temporarily stored. 13.The method as claimed in claim 12, wherein the step of (1) comprises the steps of: generating a reference value clock signal in the predetermined period, and generating a gain clock signal by dividing the frequency of the reference value clock signal by the predetermined sample number; sampling the AGC output for the predetermined period in response to the gain clock signal; temporarily storing the AGC output value and out putting the AGC output value as the reference value for the predetermined period in response to the reference value clock signal; subtracting the AGC output value sampled in response to the gain clock signal from the reference value and outputting the difference as a compensation value; and obtaining the AGC compensation control signal by outputting a stored value corresponding to the compensation value. 14. The method as claimed in claim 13, wherein the predetermined period comprises one slot including a transmission unit of packet data. 15.The method as claimed in claim 12, comprising the steps of: obtaining an AGC compensation offset using the difference between the reference value for a present period and a reference value for a next period extracted in response to a next reference value clock signal when the predetermined period expires; and compensation the power level of the compensated AGC output value with the AGC compensation offset. 16. The method as claimed in claim 11, wherein the AGC compensation offset obtaining step comprises the steps of: generating a reference value clock signal in the predetermined period and generating a gain clock by dividing the frequency of the reference value clock signal by the predetermined sample number; sampling the AGC output value for the predetermined period in response to the gain clock signal; temporarily storing the AGC output value and outputting the AGC output value as the reference value for the predetermined period in response to the reference value clock signal; subtracting the AGC output value sampled in response to the gain clock signal from the reference value and outputting the difference as a compensation value; and obtaining the AGC compensation control signal by outputting a stored value corresponding to the compensation value. 17.The method as claimed in claim 16, wherein the predetermined period comprises one slot including a transmission unit of packet data. 18.The method as claimed in claim 11 to 17, wherein the AGC output value is further processed before being compensated. An apparatus and method for compensating the error of an Automatic Gain* Controller (AGC) for stabilizing the reception power of discontinuously transmitted packet data in a mobile communication system are disclosed. A compensation controller receives an AGC output value from the AGC, sampling the AGC output value by a predetermined sample number for a predetermined period, and obtains an AGC compensation control signal by comparing a predetermined value with the difference between a sampled AGC output value with a reference value for the predetermined period. A compensator compensates the AGC output value with the AGC compensation control signal, thereby correcting errors generated in view of the nature of the AGC.

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