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.
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.
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.
There is provided a transmission power controller of at least one of a base station and a mobile station in a mobile communication system including the base station and the mobile station, characterized by comprising:
means for measuring a received SIR (signal-to-interference plus noise power ratio);
means for comparing a measurement result of the received SIR with a predetermined target value of SIR;
means for outputting transmission power control information to an opposing station according to the comparison result;
means for receiving and demodulating the transmission power control information transmitted from opposing station;
means for controlling transmission power of own station according to the demodulated transmission power control information;
error/rate measuring means for measuring an error rate of received signal; and
means for changing the SIR target value according to measurement of the received signal error rate.
Here, the error rate measuring means may comprise:
means for detecting presence of a frame error by detecting a CRC (cyclic redundancy check) bit added to wireless frame signal;
means for counting the number of the frame errors in an optional measuring time; and
means for comparing the number of measured frame errors with a predetermined frame error number setting value.
The error rate measuring means may comprise:
means for detecting presence of a frame error by detecting a CRC bit added to wireless frame signal;
means for determining a moving average of errors in an optional number of frames using the frame errors; and
means for comparing moving average of the measured errors with a predetermined setting value of frame errors independently of period of the optional number of frames.
The means for changing the received SIR target value:
may make an instruction to decrease the SIR target value by a predetermined step width when number of measured frame errors or a moving average thereof is smaller than a setting value, and may make an instruction to increase the SIR target value by a predetermined step width when the number of frame errors or a moving average thereof is greater than a setting value.
The error rate measuring means may comprise:
means for detecting bit errors of known pilot signals inserted at a predetermined interval in wireless frame;
means for counting the number of bit errors in an optional measuring time; and
means for comparing the measured number of bit errors with a predetermined bit error number setting value.
The error rate measuring means may comprise:
means for detecting bit errors of pilot signals inserted at a predetermined interval in wireless frame signal;
means for determining a moving average of bit errors in an optional period using the bit errors; and
means for comparing moving average of the measured errors with a predetermined setting value of bit errors independently of the optional period.
The means for changing the received SIR target value:
may make an instruction to decrease the SIR target value by a predetermined step width when number of bit errors in measured pilot signals or a moving average thereof is smaller than a setting value, and may make an instruction to increase the SIR target value by a predetermined step width when the number of bit errors or a moving average thereof is greater than a setting value.
As described above, since FER or BER is measured to change a target value of received SIR, transmission power control can be performed without influence of measurement accuracy of received SIR.