TRANSMITTED POWER CONTROLLER

Abstract: An error rate of received signal is measured by a received signal error measuring unit 32, and a target SIR is changed by a target SIR decision unit using the error rate. It is also possible to detect error rate of received signal in an unit of frame using a CRC signal of a frame. It is further possible to detect error rate of received signal by detecting errors of known pilot signals inserted in a constant interval.

Description: FIELD OF THE INVENTION The present invention relates to a transmission power controller used in digital telecommunications, particularly in mobile communications applied with a code division multiple access (CDMA) system. BACKGROUND ART In a CDMA system, since the same frequency band is used by a plurality of communicators, signals of other communicators become interference signals which degrade communication quality of own channel. When a mobile station near the base station and a mobile station far from the base station make communication simultaneously, transmission signal from the near mobile station is received at a high power at the base station. Therefore, a problem occurs in that communication between a far mobile station and the base station is interfered in by a near mobile station resulting in a considerable degradation of channel quality, that is, a near-far problem. As a technology for solving the near-far problem, studies have heretofore been conducted for transmission power control. Transmission power control is for controlling the transmission power so that a reception power received by the receiving station, or a signal-to-interference plus noise power ratio (SIR) obtained from the received power is constant irrespective of the location of the mobile station, thereby obtaining an uniform channel quality within the service area. In particular, for an uplink channel, transmission power control of respective mobile stations is performed at the base station reception end so that received power of transmission signal from each mobile station or SIR is constant. In a CDMA system where interference signals from other communicators are regarded as white noise, a large number of other communicators correspond equivalently to an increased noise power and, in this case, a subscriber capacity within the same cell depends on received SIR which can obtain predetermined channel quality. For a downlink, on the other hand, since signal of own channel and interfering signals of other communicators pass through the same transmission route, long-term variations, short-term variations, instantaneous variations, and the like are the same variations as the own channel signal, and always constant in received SIR except for noise power. Therefore, transmission power control is needless when handling only interference in the same cell. However, in a CDMA of whitened interference, an adjacent cell also makes communication using the same frequency band, interference from other cells must be considered. Interference power from another cell is same instantaneous variation by Rayleigh fading as interference power within the cell, however, since it is not the same variation as the desired signal of own station, transmission power control is required which follows up instantaneous variation. As a transmission power control system which follows up instantaneous variation, there is a transmission power control system by closed loop using a transmission power control bit. When a base station is communicating with a mobile station within the cell of the base station, the mobile station measures received SIR of the desired wave from the base station, and determines a transmission power control bit for controlling transmission power of the base station according to the measurement result. The mobile station inserts the transmission power control bit in the transmission signal and transmits it to the base station. The base station receives the signal transmitted from the mobile station, extracts the transmission control bit, and determines the transmission power according to instruction of the transmission power control bit. Further, the base station measures received SIR of the desired wave from the mobile station, and determines a transmission power control bit for controlling transmission power of the mobile station according to the measurement result. The base station inserts the transmission power control bit in the transmission signal and transmits it to the mobile station. The mobile station receives the signal transmitted from the base station, extracts the transmission power control bit, and determines the transmission power according to instruction of the transmission power control bit. Object of making transmission power control is for maintaining channel quality (FER: frame error rate, or BER: bit error rate) of the channel in a predetermined quality. Fig. 1 shows the relationship between the maximum Doppler frequency and received SIR required for attaining a predetermined channel quality using the number of multipaths as a parameter. Received SIR required for attaining a predetermined channel quality varies with propagation environment such as moving speed of the mobile station, that is, the maximum Doppler frequency of fading, number of multipaths, and the like. Therefore, in prior art transmission power control based on received SIR measurement, there has been a problem in that it is necessary to set a received SIR when giving a propagation environment of worst channel quality, and transmission is made at an excessively high transmission power even in a propagation environment of good characteristics. Further, there has also been a problem in that, when measurement accuracy of received SIR is low, since transmission power control is made according to a wrong measurement result, transmission cannot be made at a proper transmission power. EP-A-0715423 discloses a power control loop for use in a mobile communication system in which the symbol error rate in a received signal is compared with a target rate of the symbol error rate to decide the target SIR, the transmission power in the mobile station being controlled such that the SIR of the received signal reaches the target SIR. EP-A-0709973 discloses a transmission power control scheme for a mobile communication system in which the transmission power is controlled so that the carrier interference ratio (CIR) of received signals approaches a target CIR, the target CIR being changed based upon the channel quality. JP-A-07 030482 discloses a transmission control equipment for use in a base station or mobile station of a mobile communication network in which frame errors are measured by monitoring a unique word or error correcting code of a radio frame in an input signal, the transmission power then being based upon the error rate. The transmission path is determined directly upon the frame error rate. DISCLOSURE OF THE INVENTION With the aim of solving the above problems, an object of the present invention is to achieve transmission power control which provides consistent channel quality irrespective of propagation environment or received SIR measurement accuracy. According to a first aspect of the present invention, there is provided a transmitter/receiver for use in a base station or a mobile station in a mobile communication system, the transmitter/receiver having: a transmission power control bit decision means for generating transmission power control information according to a comparison result; means for outputting said transmission power control information; a transmission power control bit extractor for extracting said transmission power control information in a received signal; and a transmission power controller for controlling transmission power of transmission signal according to said transmission power control information extracted by said transmission power control bit extractor, the transmitter/receiver being characterized by: a SIR measuring means for measuring the signal-to-interference plus noise power ratio, that is the SIR, of a received signal; a SIR comparator for comparing said measurement result of said received signal with a predetermined target value of said SIR to provide said comparison result; a CRC detector for using cyclic redundancy check bits in each frame in a received signal to output a frame error detection signal which indicates the presence of a frame error; an FER measuring means for counting the number of said frame errors in a predetermined measuring time which is longer than the measuring time in said SIR measuring means to measure and output a frame error rate wherein said FER measuring means determines a moving average of said frame error rate in a predetermined number of frames and outputs said moving average; an FER comparator for comparing said measured frame error rate output from said FER measuring means with a predetermined frame error rate setting value; and a target SIR decision means for deciding said SIR target value according to said comparison result in said FER comparator. As described above, since FER is measured to change a target value of received SIR, transmission power control can be performed without influence of measurement accuracy of received SIR. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between maximum Doppler frequency and received SIR required for attaining a predetermined channel quality using the number of multipaths as a parameter; Fig. 2 is a diagram for explaining the principle of the transmission power controller according to the present invention; Fig. 3 is a diagram showing an embodiment of the transmission power controller. BEST MODE FOR PRACTICING THE INVENTION Embodiments of the present invention will be described with reference to the drawings. The principle of the present invention will be described using Fig. 2. Fig. 2 is a block diagram of part of a transmitter/receiver related to the present invention in a base station or a mobile station in a mobile communications system. In Fig. 2, received signal is caused to be a baseband signal in an RF unit 3, and then A/D converted to a digital signal. The digital signal is subjected to processing such as despreading or the like in a demodulator/decoder 31 to be decoded into digital information. Using the signal from the demodulator/decoder 31, a Signal-to Interference plus Noise power Ratio (SIR) is measured in a SIR measuring unit 6. Received SIR as a measured value from the SIR measuring unit 6 is compared in a comparator 7 with a target SIR from a target SIR decision unit. As a result, when the received SIR is smaller than the target SIR, a control bit for instructing an increase of transmission power is generated from a transmission power control bit decision unit 13, and when the received SIR is greater than the target SIR, a transmission power control bit for instructing a decrease of transmission power is generated. The generated transmission power control bit is transmitted to the object station to control transmission power of the object station. This control is the same as prior art transmission power control. In the present invention, an error rate of received signal is measured by a received signal error rate measuring unit 32, and the target SIR is changed by the error rate in the target SIR decision unit 12. Error of the received signal can be detected for error rate of received signal in an unit of frame using a CRC signal which is an error detection code in an unit of frame being sent. Fig. 3 shows an embodiment of transmitter/receiver apparatus incorporating the transmission power controller of the present invention. In Fig. 3, reference numeral 1 indicates an antenna, 2 is a TX RX duplexer, 3 is an RX RF unit, 4 is a despreader, 5 is a coherent detector/RAKE combiner, 6 is an SIR measuring unit, 7 is an SIR comparator, 8 is a Viterbi decoder, 9 is a CRC detector, 10 is an FER measuring unit, 11 is an FER comparator, 12 is a target SIR decision unit, 13 is a transmission power control bit decision unit, 14 is a signal generator, 15 is an encoder, 16 is a modulator, 17 is a spreader, 18 is a TX RF unit, 19 is a transmission power control bit extractor, and 20 is a transmission power controller. Next, operation when the present apparatus is used as a mobile station will be described. Spectrum spread signal transmitted from the base station is received by the antenna 1. The received signal passes through the TX/RX duplexer 2 and is inputted in the RX RF unit 3. In the RX RF unit 3, the received signal is passed through a bandpass filter (BPF) to remove out-of-band components, and then frequency converted to an intermediate frequency band (IF band) by a clock generated by a local oscillator. After the received signal which is frequency converted to IF band is passed through a BPF, the received signal is corrected to an appropriate signal level by an automatic gain controller (AGC), quadrature detected, and then frequency converted to a baseband. The received signal which is frequency converted into baseband, after being passed through a low pass filter (LPF), is analog/digital (A/D) converted into a digital signal which is outputted. The digital signal outputted from the RX RF unit 3 is despread in the despreader 4, and outputted as a narrow band modulated signal. Signal outputted from the despreader 4 is demodulated in the coherent detector/RAKE combiner 5 to be RAKE combined, and in the SIR measuring unit 6, measured for received SIR at every predetermined measurement period. Further, transmission power control bit is extracted in the transmission power control bit extractor 19 and outputted to the transmission power controller 20. In the transmission power controller 20, transmission power is determined according to the transmission power control bit, and control information is outputted to the TX RF unit 18. Still further, the received signal which is demodulated in the coherent detector/RAKE combiner 5, the RAKE combined received signal is Viterbi decoded by the Viterbi decoder, and outputted as an information signal. A CRC bit is detected by the CRC detector 9 from the Viterbi decoded information signal. When CRC bit is not detected, a frame error detection signal is outputted to the FER measuring unit 10. In the FER measuring unit 10, the number of frame errors is counted in an optional period (much longer than SIR measuring period in the SIR measuring unit 6), and FER measurement result is outputted to the FER comparator 11. In the FER measuring unit 10, an average of frame error rate in a certain period is measured. In the FER comparator 11, a comparison is made with a predetermined target FER. When the measured FER value is smaller than the target FER, the FER comparator makes an instruction to the target SIR decision unit 12 to decrease the target SIR, and when the measured FER value is greater than the target FER, the FER comparator 11 instructs the target SIR decision unit 12 to increase the target SIR. Measurements in the FER measuring unit 10 are averaged at every constant period to obtain a measurement result. However, by using moving average in FER measurement, averaging time to determine FER and target SIR control period can be made asynchronous with one another. The moving average of FER is given by FER(n) = α × FER (n-1) + (1-α)ERR. wherein n is a frame number, α is a forgetting factor, and ERR is a CRC measurement result, that is, it is 0 when the present n'th frame is normally detected, and 1 for error. Therefore, in the above FER moving average equation, when it is assumed as α = 0.9, CRC measurement of the present frame is added with an weight of 0.1, thereby obtaining the moving averaged FER. This corresponds to determination of moving average of 10 frames moved by every 1 frame. As described above, by making measurement of the FER measuring unit 10 according to the concept of moving average, control of the SIR target value can be determined independent of averaging period. Therefore, for example, it is possible to determine FER by setting the average of 1000 frames as α = 0.999, and make SIR target value setting control at every 100 frames. When the target SIR is changed, in addition to a dynamic changing method of the target value, it is also possible to increase or decrease by only a predetermined step width. In this case, when measured value of FER in the FER comparator 11 is smaller than the target FER, an instruction is made to the target SIR decision unit 12 to decrease the target SIR by a predetermined value (step width), and when the FER measured value is greater than the target FER, an instruction is made to the target SIR decision unit 12 to increase the target SIR by the predetermined value (step width). In the SIR comparator 7, the target SIR updated at every FER period is compared with received SIR measured in the SIR measuring unit 6 and, according to the comparison result, the transmission power control bit decision unit 13 generates a control bit to instruct an increase of transmission power when the received SIR is smaller than the target SIR, or a control bit to instruct a decrease of the transmission power when the received SIR is greater than the target SIR, and outputs the control bit to the signal generator 14. In the signal generator 14, transmission frame is constructed including the transmission power control bit sent from the transmission power control bit decision unit 13, and outputted to the encoder 15. Encoded transmission signal is modulated in the modulator 16, spread in the spreader 17, and then outputted to the TX RF unit 18. In the TX RF unit 18, transmission signal which is frequency converted to IF and RF bands is transmitted at a transmission power based on control information outputted from the transmission power controller 20. UTILIZABILITY IN INDUSTRY As described above in detail, it is possible to achieve transmission power control which provides the same channel quality irrespective of propagation environment and received SIR measurement accuracy.

Claims: A transmitter/receiver for use in a base station or a mobile station in a mobile communication system, the transmitter/receiver having: a SIR measuring means (6) for measuring the signal-to-interference plus noise power ratio, that is the SIR, of a received signal; a SIR comparator (7) for comparing said measurement result of said received signal with a predetermined target value of said SIR to provide a comparison result; a transmission power control bit decision means (13) for generating transmission power control information according to said comparison result; means (14, 15, 16, 17, 18) for outputting said transmission power control information; a transmission power control bit extractor (19) for extracting said transmission power control information in a received signal; and a transmission power controller (20) for controlling transmission power of transmission signal according to said transmission power control information extracted by said transmission power control bit extractor (19); a CRC detector (9) for using cyclic redundancy check bits in each frame in a received signal to output a frame error detection signal which indicates the presence of a frame error; an FER measuring means (10) for counting the number of said frame errors in a predetermined measuring time which is longer than the measuring time in said SIR measuring means (6) to measure and output a frame error rate wherein said FER measuring means (10) determines a moving average of said frame error rate in a predetermined number of frames and outputs said moving average; an FER comparator (11) for comparing said measured frame error rate output from said FER measuring means (10) with a predetermined frame error rate setting value; and a target SIR decision means (12) for deciding said SIR target value according to said comparison result in said FER comparator (11). A transmitter/receiver as claimed in claim 1, wherein said target SIR decision means (12) renews said SIR target value to decrease said SIR target value by a predetermined step width when said measured frame error rate or a moving average thereof is smaller than a setting value, and renews said SIR target value to increase said SIR target value by a predetermined step width when said measured frame error rate or said moving average thereof is greater than a setting value. A transmitter/receiver as claimed in claim 1, wherein said moving average of the frame error rate is calculated as follows: FER(n) = α x FER (n-1) + (1-α) ERR where FER(n) is the frame error rate in the nth frame; α is a forgetting factor; and ERR is a value indicating a CRC measurement result. A base station including a transmitter/receiver according to any one of the preceding claims. A mobile station including a transmitter/receiver according to any one of claims 1 to 3.

Inventors: DOHI TOMOHIRO, OKUMURA YUKIHIKO, SAWAHASHI MAMORU, SEO SYUNSUKE, DOHI, TOMOHIRO, OKUMURA, YUKIHIKO, SAWAHASHI, MAMORU, SEO, SYUNSUKE, DOHI, TOMOHIRO, NTT TOMIOKA DAIICHI
Applicants: NTT DOCOMO INC, NTT DOCOMO, INC.
Application year: 1997
Main classification: H04B7
IPC codes: H04L1, H04B7