Abstract:
A code division multiple access system includes the steps of sequentially generating reception symbols by de-spreading a reception signal, generating a threshold on the basis of an average amplitude obtained by averaging amplitudes of a predetermined number of reception symbols, generating a weighted reception symbol by multiplying an amplitude of the reception symbol by a weighting coefficient, and sequentially selecting one of the reception symbol and the weighted reception symbol as a reception symbol of the reception signal in accordance with a result obtained by comparing the amplitude of the reception symbol and the threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a CDMA (Code Division Multiple Access) system and, more particularly, to a reception method, a reception quality estimation method, a transmission power control method, and transmitting/receiving apparatus in a CDMA system. 
     2. Description of the Related Art 
     RAKE/diversity reception techniques for high-quality reception under a multipath fading environment, transmission power control (TPC) techniques for lessening the problems associated with distance and suppressing interference, and the like are indispensable techniques for a digital mobile/portable telephone system (cellular system) using the CDMA scheme. 
     As a technique for performing transmission power control with high precision, a mobile unit for measuring the reception power, of a known pilot signal and controlling the transmission power on the basis of the measured value is disclosed in Japanese Unexamined Patent Publication No. 7-221700. 
     A receiving apparatus that allows stable transmission power control even with a low signal-to-interference ratio is disclosed in Japanese Unexamined Patent Publication No. 9-135193. In this receiving apparatus, a desired signal power is obtained by performing in-phase vector addition of the pilot signals (unique words) contained in the de-spread reception signals. Likewise, an interference signal power is obtained by averaging the error powers between the pilot signals and the desired signal power. The value obtained by multiplying the interference signal power by two different coefficients is then compared with the desired signal power to determine the signal-to-interference power ratio. The resultant data is used for operation speed control or transmission power control on a chip synchronization circuit. 
     In addition, a receiving apparatus capable of reducing errors in demodulated data even in the presence of interference due to variations or fluctuations in transmission power level from a transmission station is disclosed in Japanese Unexamined Patent Publication No. 7-321702. This receiving apparatus uses a reception method of correcting an estimated current symbol value by using the average of a plurality of estimated symbol values obtained from a correlator in the past. 
     In either of the above conventional techniques, however, when the reception level varies due to fading or the like, and the reception state deteriorates, the average of the amplitudes of received symbols cannot be accurately obtained owing to reception errors. For this reason, an error occurs in measurement of a desired reception power, and a signal-to-interference power ratio cannot be accurately estimated. As a result, transmission power control and the like cannot be properly performed. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above situation in the prior art and, has as its object to provide a method and apparatus which can improve reception quality estimation precision and stabilize communication even when a reception state deteriorates and a reception error occurs. 
     In order to achieve the above object, according to the principal aspect of the present invention, there are provided a method and apparatus for sequentially generating reception symbols by de-spreading a reception signal, generating a threshold on the basis of an average amplitude obtained by averaging amplitudes of a predetermined number of reception symbols, generating a weighted reception symbol by multiplying an amplitude of the reception symbol by a weighting coefficient, and sequentially selecting one of the reception symbol and the weighted reception symbol as a reception symbol of the reception signal in accordance with a result obtained by comparing the amplitude of the reception symbol and the threshold. According to the method and apparatus of the present invention, it is determined, on the basis of the threshold obtained from reception symbols, whether to weight a reception symbol, and reception quality estimation and transmission power control are performed by using reception symbols selected in this manner. 
     According to the present invention, even when reception state deteriorates and a reception error occurs, reception quality can be accurately estimated, and proper transmission power control can be stably performed. 
     The above and many other subjects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematically showing a receiving apparatus according to an embodiment of the present invention; 
     FIG. 2 is a block diagram schematically showing a reception symbol converter used in the receiving apparatus of the present invention; and 
     FIG. 3 is a flow chart showing a reception quality estimation method according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1 is a block diagram showing a mobile unit in a digital mobile/portable telephone CDMA system according to an embodiment of the present invention. The mobile unit of this embodiment is constituted by an RF amplifying section  10 , a radio section  11 , a baseband signal processing section  12 , a terminal interface  13 , and a control processor  14 . The control processor  14  controls the overall operation of the mobile unit in accordance with the programs stored in a memory (not shown). 
     In the antenna diversity reception system of the mobile unit, antennas  101  for two systems are connected to a reception amplifying section  103  through an antenna switch  102  of the RF amplifying section  10 . The reception amplifying section  103  comprises a low-noise amplifier (LNA) for amplifying a received RF signal. The amplified received RF signal is output to a receiver  104  of the radio section  11  to be subjected to quasi-synchronous detection. The baseband reception signal obtained by quasi-synchronous detection is converted into a digital signal by an A/D converter  105 . The digital signal is then output to the baseband signal processing section  12 . 
     The baseband reception signal is de-spread by a de-spread spectrum unit  106  of the baseband signal processing section  12 , and estimated values S RI  of the reception symbols are sequentially generated by a RAKE diversity synthesizer  107  for implementing a diversity handover synthesis function. Each estimated value S RI  of the reception signal obtained in this manner is converted by a received symbol converter  100  such that the calculation precision for the average of the amplitudes of reception symbols improves. Each converted reception symbol S RO  is output to a baseband signal processor  109 . 
     As will be described later, the baseband signal processor  109  receives the estimated reception symbol values S RI  and the converted reception symbols S RO , and calculates the average of the amplitudes of the reception symbols. The baseband signal processor  109  then calculates a signal-to-interference power ratio SIR as a reception quality on the basis of the amplitude average, and outputs it to the control processor  14 . The control processor  14  performs transmission power control on the RF amplifying section  10  on the basis of the magnitude of the signal-to-interference power ratio SIR. In addition, the baseband signal processor  109  performs signal processing such as error correction decoding and data separation with respect to each reception symbol received from the received symbol converter  108 . 
     The reception data obtained in this manner is output as speech from, for example, the receiver of a telephone handset through the terminal interface  13 , or output to an information terminal connected to the terminal interface  13 . 
     Transmission data is input to the baseband signal processor  109  through the terminal interface  13  to be subjected to error correction coding and framing. The resultant data is spread-modulated by a spread spectrum unit  110  and is output to the radio section  11 . The spread-modulated baseband transmission signal is converted into an analog signal by a D/A converter  111  of the radio section  11 . The analog signal is quadrature-modulated by a transmitter  112  to be output as an RF transmission signal to a transmission amplifying section  113  of the RF amplifying section  10 . The RF transmission signal power-amplified by the transmission amplifying section  113  is sent from the antennas  101  through the antenna switch  102 . The gain of the transmission amplifying section  113  is controlled by the control processor  14  in accordance with the signal-to-interference power ratio SIR of the reception signal. 
     FIG. 2 is a block diagram showing an example of the reception symbol converter in FIG.  1 . Although the received symbol converter  108  is implemented by a program in the baseband signal processor  109 , the functional arrangement of the converter will be described below with reference to FIG.  2 . 
     In the received symbol converter  108  in FIG. 2, the reception symbols S RI  sequentially input from the RAKE diversity synthesizer  107  are stored in a buffer memory  201  by N (N is a predetermined integer) symbols. The identical reception symbols S RI  are sequentially accumulated by an accumulator  202 . The resultant value is divided by a 1/N divider  203  to obtain the average of the amplitudes of the reception symbols. A multiplier  204  multiplies the average amplitude by a given coefficient a to generate a threshold D TH , and outputs it to a comparator  205 . If, for example, α=¼, a level corresponding to ¼ the average of the amplitudes of the reception symbols is set as a threshold. 
     The reception symbols S RI  stored in the buffer memory  201  are sequentially read out, and each symbol is output as a reception symbol DATA 1  to the comparator  205  and a selector  207  and multiplied by a given coefficient β by a multiplier  206 . The resultant data is output as a weighted reception symbol DATA 2  to the selector  207 . For example, weighting coefficient β=−{fraction (1/2 )} is set. The comparator  205  compares the reception symbol DATA 1  with the threshold D TH , and outputs the comparison result as a selection control signal to the selector  207 . The selector  207  selects either the reception symbol DATA 1  or the weighted reception symbol DATA 2  in accordance with the comparison result, and outputs the selected symbol as the converted reception symbol S RO  to the baseband signal processor  109 . More specifically, if the amplitude of the reception symbol DATA 1  is smaller than the threshold D TH , the selector  207  selects the weighted reception symbol DATA 2 . If the amplitude of the reception symbol DATA 1  is equal to or larger than the threshold D TH , the selector  207  selects the reception symbol DATA 1 . 
     The amplitude of a reception symbol used to calculate the reception quality SIR changes depending on the magnitude of the amplitude of the reception symbol DATA 1 . In other words, if the amplitude of the reception symbol DATA 1  is smaller than the threshold D TH , i.e, the reception state has deteriorated, the reception symbol DATA 2  obtained by weighting the reception symbol DATA 1  by the coefficient β is used to calculate the reception quality SIR. If the reception state has not deteriorated much, the reception symbol DATA 1  is used without any change to calculate the reception quality SIR. With this operation, proper transmission power control can be performed regardless of the reception state, thereby realizing stable communication. 
     FIG. 3 is a flow chart showing a reception quality estimation method according to an embodiment of the present invention. Reception quality estimation in this embodiment is executed by the baseband signal processor  109  including the received symbol converter  108 . 
     The baseband signal processor  109  receives the reception symbols S RI  sequentially output from the RAKE diversity synthesizer  107 , calculates the power of each symbol, and obtains the average of the powers, thereby calculating a total reception power value P O  (step S 301 ). The baseband signal processor  109  then obtains an average amplitude A AV1 , of the reception symbols S RI  (step S 302 ), and sets the value obtained by multiplying the average amplitude A AV1 , by a (e.g., α=¼) as the threshold D TH  (step S 303 ). 
     The baseband signal processor  109  compares each reception symbol with the set threshold D TH  (step S 304 ). If the amplitude of the reception symbol is equal to or larger than the set threshold D TH , the baseband signal processor  109  uses the symbol without any change (step S 305 ). If the amplitude of the reception symbol is smaller than the threshold D TH , the baseband signal processor  109  uses the weighted reception symbol obtained by multiplying the symbol by β(e.g., β=−½) as a reception symbol (step S 306 ). Steps S 304  to S 306  are repeated for all the reception symbols to obtain the converted reception symbols S RO  (YES in step S 307 ). 
     The baseband signal processor  109  calculates an average amplitude A AV2  of the reception symbols S RO  obtained in this manner, and obtains the square sum of the average amplitude, thereby obtaining a desired wave reception power value P S  (step S 308 ). The baseband signal processor  109  then calculates an interference wave reception power value P I  by subtracting the desired wave reception power value P S  from the total reception power value P O  that has already been obtained (step S 309 ). The baseband signal processor  109  can obtain the signal-to-interference power ratio SIR estimating reception quality from the desired wave reception power value P S  and the interference wave reception power value P I . This reception quality SIR is output from the baseband signal processor  109  to the control processor  14  to determine control information for transmission power control (step S 310 ). 
     As described above, since the average amplitude A AV2 is calculated by using the reception symbols S RO  converted by the received symbol converter  108 , and the desired wave reception power value P S  is obtained, the influences of a deterioration in reception state can be reduced, and proper transmission power control can be stably performed. 
     Note that the coefficients α and β used in the received symbol converter  108  can be variable parameters that are properly set through the control processor  14  or the terminal interface  13 . By using such variable parameters, changes in hardware and software can be minimized, and the average of the amplitudes of reception symbols can be obtained more accurately. This allow more accurate reception quality estimation. In addition, by setting the weighting coefficient β to a negative value, e.g., β=−½, the influences of reception symbols whose amplitudes are smaller than the threshold on the variance of reception symbols can be further suppressed. For example, errors in calculation of interference wave reception power can be further suppressed. 
     The above embodiment has exemplified the digital mobile/portable telephone CDMA system. Obviously, however, the received symbol converter  108  is used for a reception apparatus in a CDMA system, and can be applied to a receiving apparatus in a base station as well as the receiving apparatus of a mobile unit.