Rate adjustment technique in a CDMA receiver

A rate adjustment device allowing the burden on a decoder to be reduced when decoding a variable-rate channel is disclosed. After despreadirg a received baseband signal based on a predetermined symbol rate to produce rate-indicating data and a sequence of received correlation values, a received symbol rate is compared to the maximum symbol rate and, when the received symbol rate is lower than the maximum symbol rate, it is determined whether a receive quality is satisfactory. When the receive quality is satisfactory, at least one received correlation value is selected from the received correlation values and is used to generate a correlation value matching the received symbol rate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
 First Embodiment Referring to FIG. 2, a CDMA receiver according to a first embodiment of the present invention is provided with a demodulator (not shown), a searcher 101 , a finger circuit 102 , and a decoder 103 . The demodulator demodulates a radio received signal to produce a baseband received signal, which is output to the searcher 101 and the finger circuit 102 . The searcher 101 searches the baseband received signal to find the receive timing of a significant path and outputs the found received timing to the finger circuit 102 . The finger circuit 102 is composed or a typical RAKE receiver structure, which performs despreading of the baseband received signal at the receive timing inputted from the searcher 101 using a despreading code determined based on rate information DSR (symbol rate or spreading factor). The rate information DSR is received from an upper layer and is set to the maximum one of possible symbol rates on a variable-rate channel. The finger circuit 102 produces rate decision data 10 , correction value data 11 obtained by the RAKE combining process, and a SIR (Signal to Interference Ratio) value 12 of the received signal, which are output to the decoder 103 . In the decoder 103 , a rate decision section 104 decodes the rate decision data 10 received from the finger circuit 102 to determine an actual symbol rate ASR of the received signal and output it to a rate comparator 105 . The rate comparator 105 compares the rate information (here, the maximum symbol rate) DSR to the actual symbol rate ASR to output a rate comparison result 13 to an effective correlation value decision section 107 . The finger section 102 outputs the SIR value 12 of the received signal to a SIR comparator 106 . The SIR comparator 106 compares the SIR value 12 of the received signal to a reference SIR value SIR REF to output a SIR comparison result 14 to the effective correlation value decision section 107 . The effective correlation value decision section 107 produces an addition control signal 15 from the rate comparison result 13 and the SIR comparison result 14 according to a predetermined procedure. The addition control signal 15 is used to instruct the correlation value addition processor 108 whether the full addition, selective addition, or no addition of correlation values is performed, which will be described hereafter by referring to FIG. 3 . The addition control signal 15 is output to a correlation value addition processor 108 . The correlation value addition processor 108 receives the correction value data 11 from the finger circuit 102 and performs addition of correlation values depending on the addition control signal 15 . The correlation value addition processor 108 produce correlation value data 16 , which is output to a decoding processor 109 . Rate Adjusting Operation Hereafter, the full addition, selective addition, and no addition operations in the above decoder 103 will be described with reference to FIG. 3 . The rate adjusting operation may be performed by a program-controlled processor running a rate adjustment program composed of the following steps thereon. Referring to FIG. 3 , the effective correlation value decision section 107 determines from the rate comparison result 13 whether the actual symbol rate ASR is lower than the predetermined symbol rate (here, the maximum symbol rate) DSR and how many times the predetermined symbol rate DSR is as high as the actual symbol rate ASR (step S 201 ). When ASR is equal to DSR (NO at stop S 201 ), it is determined that no rate adjustment is necessary. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to pass through the correction value data 11 to the decoding processor 109 (no addition operation). Accordingly, the correction value data 11 is decoded as it is by the decoding processor 109 (step S 205 ). Since the predetermined symbol rate DSR is the maximum symbol rate, there is actually no case where ASR is higher than DSR at the step S 201 . When the actual symbol rate ASR is lower than the predetermined symbol rate DSR (YES at step S 201 ), the effective correlation value decision section 107 further determines from the SIR comparison result 14 whether the received signal SIR value is greater than the reference SIR value SIR REF , that is, the receive quality is satisfactory (step S 202 ). If the received signal SIR value is not greater than the reference SIR value SIR REF (NO at step S 202 ), it is determined that the receiving quality is not sufficiently high. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to perform the addition of correlation values for the predetermined number of symbols, which is the same as the prior art (full addition operation). Accordingly, the correlation value addition processor 108 performs the full addition operation and outputs who normalized sum as correction value data 16 to the decoding processor 109 (step S 204 ). The correction value data 16 is decoded by the decoding processor 109 (step S 205 ). If the received signal SIR value is greater than the reference SIR value SIR REF (YES at step S 202 ), it is determined that the receiving quality is sufficiently high. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to perform the addition of at least one selected from the correlation values for the predetermined number of symbols, which is called hereafter a selective addition operation (step S 203 ). Accordingly, the correlation value addition processor 108 performs the selective addition operation and outputs the normalized sum as correction value data 16 to the decoding processor 109 (step S 204 ). The correction value data 16 is decoded by the decoding processor 109 (step S 205 ) Hereafter, an example of the rate adjustment according to the first embodiment will be described by referring to FIG. 4 , taking as an example the case where a predetermined symbol rate DSR (spreading factor: SF&equals;4) is four times as high as the actual symbol rate ASR (SF&equals;16) and the received signal SIR value is greater than the reference SIR value SIR REF . As shown in FIG. 4 , the finger circuit 102 performs despreading of the baseband received signal at the maximum symbol rate DSR (SF−4) to produce correlation value data 301 in symbols. Since DSR is four times as high as ASR (YES at S 201 of FIG. 3 ) and the received signal SIR>SIR REF (YES at step S 202 of FIG. 3 ), the effective correlation value decision section 107 selects one symbol 302 (here, the first symbol) from four symbols 301 and outputs the addition control signal 15 indicating the selected symbol timing to the correlation value addition processor 108 . Since only one symbol is selected from four symbols, the correlation value addition processor 108 transfers a correlation value corresponding to the selected symbol to the decoding processor 109 without the need of addition operation, resulting in a reduced amount of data to be processed in the decoder 103 . It is possible select two or three symbols from the four symbols. In this case, the amount of data to be processed in the decoder 103 is smaller than in the case of full addition of the four correlation values. As described above, in the CDMA receiver according to the first embodiment of the present invention, when the receive quality is good, that is, the received signal SIR>SIR REF , the correlation value addition operation can be omitted or simplified, resulting in a reduced burden on the decoder 103 . A received signal SIR is detected by the finger circuit 102 for each slot. Accordingly, the effective correlation value decision as described above can be performed at intervals of the slot. Alternatively, it can be performed at intervals of a longer time period by averaging received signal SIR values over a plurality of slots. In contrast, the correlation value addition is performed for each symbol. Therefore, by omitting or simplifying the correlation value addition operation, an amount of data to be processed in the decoder can be dramatically reduced as a whole. As described above, the reference SIR value SIR REF is used as a criterion to determine whether the correlation value addition operation can be omitted or simplified. Such a reference SIR value SIR REF is determined by simulation or experiment so that desired receive quality can be obtained when omitting or simplifying the correlation value addition operation. The reference SIR value SIR REF is stored in a memory (not shown) in the decoder 103 . 
 Second Embodiment The receive quality can be determined by a receive-related value other than the received signal SIR value. For example, in some CDMA communications systems proposed by 3GPP (Third Generation Partnership Project), it is required that estimated BER (Bit Error Rate) is calculated at a receiving side and an upper layer is notified of the BER. More specifically, correlation value data after the rate adjustment process is subject to error-correction decoding and the decoded data is encoded again to produce re-encoded data. By comparing the re-encoded data to the correlation value data, estimated BER can be obtained. This estimated BER can be used as a criterion to determine whether the correlation value addition operation can be omitted or simplified. The details will be described hereafter. Referring to FIG. 5, a CDMA receiver according to a second embodiment of the present invention is provided with a decoder 401 that does not have the SIR comparator 106 of FIG. 2 but a decoding processor 402 having the BER estimation function and a BER comparator 403 , where blocks similar to those previously described with reference to FIG. 2 are denoted by the same reference numerals and details thereof will be omitted. The decoding processor 402 decodes the correction value data 16 inputted from the correlation value addition processor 108 according to the well-known error-correction coding scheme. The decoded data is encoded again and the re-encoded data is compared to the correlation value data 16 to produce an estimated BER value 17 . The estimated BER value 17 is output to the BER comparator 403 . The BER comparator 403 compares the estimated BER value 17 to a reference BER value BER REF to output a BER comparison result 18 to the effective correlation value decision section 107 . The reference BER value BER REF indicates the receive quality, which is determined depending on the type of service provided by a channel. The reference BER value BER REF is stored in a memory (not shown) of the decoder 401 . The full addition, selective addition, and no addition operations in the decoder 401 are the same as those in the decoder 103 as shown in FIG. 3 . A main part of the operations will be described briefly. In FIG. 3 , when ASR is equal to DSR (NO at step S 201 ), it is determined that no rate adjustment is necessary. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to pass through the correction value data 11 to the decoding processor 109 (no addition operation). Accordingly, the correction value data 11 is decoded as it is by the decoding processor 109 (step S 205 ). When the actual symbol rate ASR is lower than the predetermined symbol rate DSR (YES at step S 201 ), the effective correlation value decision section 107 further determines from the BER comparison result 18 whether the estimated BER value is smaller than the reference BER value BER REF , that is, the receive quality is satisfactory (step S 202 ). If the estimated BER value is not smaller than the reference BER value BER REF (NO at step S 202 ), it is determined that the receiving quality is not sufficiently high. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to perform the addition of correlation values for a predetermined number of symbols, which is the same as the prior art (full addition operation). Accordingly, the correlation value addition processor 108 performs the full addition operation and outputs the normalized sum as correction value data 16 to the decoding processor 109 (step S 204 ) If the estimated BER value is smaller than the reference BER value BER REF (YES at step S 202 ), it is determined that the receiving quality is sufficiently high. Therefore, the effective correlation value decision section 107 produces the addition control signal 15 instructing the correlation value addition processor 108 to perform the addition of at least selected one of the correlation values for the predetermined number of symbols (selective addition) (step S 203 ). Accordingly, the correlation value addition processor 108 performs the selective addition operation and outputs the normalized sum as correction value data 16 to the decoding processor 109 (step S 204 ). In this manner, in the CDMA receiver according to the second embodiment of the present invention, as the case of the first embodiment, when the receive quality is good, that is, the estimated BER<BER REF , the correlation value addition operation can be omitted or simplified, resulting in a reduced burden on the decoder 401 .