JP4274106B2 - Transmission power control method for wireless communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transmission power control method capable of attaining a desired reception quality while preventing increase in average transmission power even if a propagation path gain fluctuation in relatively short cycle occurs. <P>SOLUTION: A transmission power is so controlled that a propagation path gain increases a communication path capacity as the transmission power for fluctuation. In other words, the transmission power is determined so that the sum of a noise power reduced to a transmission end (= reception noise power/propagation path gain) and a transmission power is constant. As a result, contrary to a conventional manner, the transmission power is so controlled that it is reduced when the propagation path gain drops while it is increased when the propagation path gain increases. <P>COPYRIGHT: (C)2005,JPO&amp;NCIPI

Description

  The present invention relates to a method for controlling radio transmission power of a radio communication system, and is particularly suitable for application to a mobile communication system.

In a wireless communication system, a technique for controlling transmission power of a wireless communication device in order to obtain desired reception quality is known. For example, USP 5,267,262, Qualcomm Inc. , "Transmitter Power Control System" In the CDMA mobile communication system, the signal reception power from the terminal is measured at the base station in the CDMA mobile communication system, and the transmission power increase instruction is moved when it is smaller than the desired value, and the transmission power decrease instruction is moved when it is larger A technique is shown in which the transmission power is transmitted to a station, and the mobile station controls the transmission power in accordance with the transmission power control instruction, thereby maintaining the reception power at the base station substantially constant.
Also, USP 5,559,790, Hitachi Ltd. , "Spread Spectrum Communication System and Transmission Power Control Method thefor", the mobile station measures the reception quality of the pilot signal transmitted by the base station with a known power, and if the reception quality is poor based on the measurement result The base station transmits a transmission power control signal that requires a larger transmission power than when the base station is good, and the base station controls the transmission power of the signal directed to the mobile station based on the transmission power control signal. A technique for maintaining the signal reception quality from the base station in the station almost constant is shown.
Each of these techniques aims to control the reception power and quality at the reception side to be constant. That is, according to the above-described conventional transmission power control method, reception quality is made constant, and degradation of reception quality due to propagation path gain fluctuations and unnecessary interference in the system due to excessive transmission power are prevented. be able to.

USP5,267,262

USP5,559,790

However, when there is fading that is a relatively short period propagation path gain fluctuation that occurs with the movement of a mobile station, using the conventional technique, when the propagation path gain decreases instantaneously, a very large transmission As a result, the average transmission power increases. An increase in average transmission power increases the mutual interference given to the entire system, which causes a problem of reducing the communication throughput of the entire system. Further, in the terminal, there is a problem that an increase in average transmission power increases power consumption and shortens a callable time.
Accordingly, a first object of the present invention is to provide a transmission power control method that achieves desired reception quality while preventing an increase in average transmission power even when a propagation gain variation of a relatively short period occurs. That is.
Further, when the average transmission power is not increased, the average reception power is decreased, and the capacity of the communication path is reduced due to the deterioration of the reception quality (SN ratio, SNR). That is, the maximum data rate that can be communicated is reduced. Therefore, the second object of the present invention is to keep the communication channel capacity as large as possible even when a propagation gain variation with a relatively short period occurs.

  Means for solving the above-mentioned problem has means for measuring propagation path gain and means for transmitting transmission power control information to the first wireless communication device, and receives the transmission power information to the second wireless communication device. Control means for controlling the transmission power of the second wireless communication device to increase when the channel gain increases and to decrease when the channel gain decreases. It is characterized by providing.

  According to the present invention, required transmission power is reduced and mutual interference is reduced. Further, according to the present invention, the channel capacity is increased, and the bit rate capable of communication can be improved.

First, the power control algorithm of the present invention will be described.
Consider the case where the channel gain fluctuates as shown in FIG. That is, consider a propagation path in which the average gain becomes 1 at times t1, t2, t3, and t4 and gains of 2, 1, 1/3, and 2/3, respectively. Assuming that constant noise is added at power 1 on the receiving side as shown in FIG. 2, this is equivalent to power 1/2, 1, 2, and t4 at times t1, t2, t3, and t4 as shown in FIG. Equivalent to the addition of 3, 3/2 noise. That is, the fluctuation of the propagation path gain can be equivalently regarded as the fluctuation of the noise power.
On the other hand, it is known that the capacity C of the communication path is theoretically C = W log2 (1 + S / N). Here, C is the number of bits that can be transmitted per second, W is the frequency bandwidth, S is the signal power, N is the noise power, and log2 (x) is the logarithm of x with 2 as the base. Therefore, the channel capacity in the time-varying propagation path as described above is C = Ave (W log2 (1 + S (t)), where signal power S (t) at time t and noise power N (t) / N (t))). Here, Ave (x) represents the time average of x. Therefore, when S (t) is changed with time by power control, the channel capacity changes. In the present invention, transmission power is controlled so as to increase the channel capacity as much as possible. Specifically, it is as follows.
Consider S (t) that maximizes the channel capacity C when the average transmission power, that is, the time average Ave (S (t)) of S (t) is constant. Since Ave (S (t)) is constant, if the transmission power at a certain time is increased, the transmission power at another time must be decreased. Here, since the rate of increase of C with respect to the slight increase of S is dC / dS = W / log (2) / (N + S) from the definition equation of the channel capacity, constant power is distributed in the time direction. Sometimes distributing transmission power where N + S is the smallest will increase the channel capacity. In this way, when the transmission power is sequentially distributed to the place where N + S is the smallest, when all the power is finally distributed, N + S is constant and N is higher than S + N achieved. S is not distributed at all in the time zone when is large, and this state has the largest channel capacity.
Here, if the noise power received by the receiver is a function of time Nr (t) and the propagation path gain is a function of time g (t), the equivalent noise power N (t) seen on the transmission side is
N (t) = Nr (t) / g (t)
It becomes. Therefore, the transmission power S (t) that maximizes the channel capacity is
N (t) + S (t) = Nr (t) / g (t) + S (t) = P_const.
This condition is satisfied. That is,
S (t) = P_const−Nr (t) / g (t)
Control may be performed so that However, when S (t) <0, the actual transmission power is 0 (that is, transmission is stopped). If P_const is increased, average transmission power and channel capacity increase. Conversely, if P_const is reduced, the average transmission power and the channel capacity are reduced. Therefore, it is only necessary to determine P_const to a value that provides a desired communication path capacity.
For example, when the average transmission power is 1 under the propagation path gain fluctuation shown in FIG. 1, the transmission power control result is as shown in FIG. In the figure, a portion surrounded by a thick line is signal power, and a portion surrounded by a thin line is noise power. That is, the transmission powers at times t1, t2, t3, and t4 are 11/6, 4/3, 0, and 5/6, respectively. Average transmission power is
(11/6 + 4/3 + 0 + 5/6) / 4 = 1
Also, the received power when the result of the transmission power control in FIG. 4 is viewed on the receiving side is 11/3, 4/3, and t3 at time t1, t2, t3, t4, respectively, as shown in FIG. 0, 5/9.
On the other hand, in the power control according to the conventional technique, in order to keep the reception power or the reception quality constant, the transmission power is controlled to be proportional to the noise power as shown in FIG. That is, the transmission powers at times t1, t2, t3, and t4 are 1/3, 2/3, 2, and 1, respectively. Average transmission power is
(1/3 + 2/3 + 2 + 1) / 4 = 1
In addition, the received power when the result of power distribution (transmission power control) in FIG. 6 is viewed on the receiving side is 2/3 at time t1, t2, t3, t4, respectively, as shown in FIG. 2/3, 2/3, 2/3.
FIG. 8 compares transmission power control with respect to fluctuations in propagation path gain. The horizontal axis represents propagation path gain, and the vertical axis represents transmission power as a transmission power control result. In the figure, circles indicate the present invention, and diamonds indicate the prior art. In other words, in the conventional transmission power control, the channel gain and the transmission power are in an inversely proportional relationship, and when the channel gain decreases, the transmission power increases, whereas when the channel gain increases, the transmission power decreases. In the present invention, conversely, when the channel gain decreases, the transmission power is reduced, and when the channel gain increases, the transmission power is increased.
The channel capacity achieved by the transmission power control according to the present invention is
C = W (log2 (1 + 11/3) + log2 (1 + 4/3) + log2 (1 + 0) + log2 (1 + 5/9)) / 4
= 1.02W
It becomes. On the other hand, the channel capacity achieved by the conventional transmission power control is
C = W log2 (1 + 2/3) = 0.737W
It becomes.
Thus, in the example shown here, the channel capacity increases by 1.38 (= 1.02 / 0.737) times as compared with the conventional power control method according to the power control of the present invention. On the other hand, using the conventional transmission power control method, in order to achieve the same channel capacity as the channel capacity when the present invention is applied,
Since 1.02 = log2 (1 + 1.028), S / N = 1.028 is necessary, and S / N = 2/3 1.54 (= 1.028 / (2/3)) achieved by the conventional transmission power control. Double the average transmission power is required. Therefore, according to the present invention, the transmission power for achieving the same channel capacity is reduced to 0.649 times that in the case of using the conventional technique.
The transmission power control algorithm that theoretically maximizes the channel capacity has been described above, but substantially the same effect can be obtained without strictly following the above algorithm. That is, it is possible to perform transmission power using a function that approximates the relationship between the propagation path gain and transmission power shown in FIG. It is desirable that the function has a positive slope as a whole. For example, even a simple function that makes the transmission power proportional to the channel gain can obtain substantially the same effect.
The algorithm for determining the transmission power
S (t) = P_const−Nr (t) / g (t)
According to FIG. 9, when the propagation path gain increases stepwise at time t0 as shown in FIG. 9, the transmission power also changes stepwise as shown in FIG. 10 (a). Further, when a control delay occurs, it changes with a certain rise time as shown in FIG. 10 (b).
In the control of FIGS. 10 (a) and 10 (b), the channel capacity is large when the mobile station is located near the base station where the channel gain is large, and conversely the mobile station is located far from the base station. Sometimes the channel capacity becomes smaller. If this difference is undesirable in system design, for example
P_const = C0 Ave (Nr (t)) / Ave (g (t))
Thus, it is practical to control P_const relatively slowly using the average gain and noise power of the current channel condition. Here, C0 is a constant. As a result, the power control is applied to short-term fluctuations in the communication path while obtaining a substantially constant communication path capacity regardless of the distance from the base station.
In this case, with respect to the propagation path gain fluctuation shown in FIG. 9, as shown in FIGS. 10 (c) and 10 (d), transmission power similar to that in FIGS. 10 (a) and 10 (b) is obtained in a short time. After that, the response gradually approaches the transmission power that cancels the propagation path gain variation as in the conventional power control.
According to the power control described above, the channel capacity varies with time. For this reason, when the channel capacity is above the average over a certain period of time, the bit rate is controlled to perform communication at a high bit rate. Conversely, when the channel capacity is below the average, communication is performed at a low bit rate. Preferably it is done.
In addition, it is not necessary to explicitly control the bit rate by making the average time of Ave (Nr (t)) and Ave (g (t)) used for calculating P_const coincide with the unit for performing substantially channel coding. It is possible to improve the average bit rate, which is suitable for a system that requires a constant bit rate.
Hereinafter, a system and apparatus configuration for executing the above algorithm will be described.
FIG. 30 shows the system configuration of the present invention. A plurality of mobile stations 3, 4, 5 communicate with the base stations 1, 2 via radio, and the base stations 1, 2 are controlled by the base station control station 6 between the mobile stations or in a fixed network Establish communication with the communication device to which it belongs.
FIG. 11 shows the configuration of the transmission side wireless communication device of the present invention, and FIG. 13 shows the configuration of the reception side wireless communication device of the present invention. Here, the wireless communication device whose transmission power is controlled by the transmission power control of the present invention is a transmission-side wireless communication device, and the other is a reception-side wireless communication device. In the system configuration shown in FIG. 30, either the mobile station or the base station may be either radio communication device, and if the base station is a transmission side radio communication device, the transmission power control of the downlink signal is performed. On the contrary, if the mobile station is a transmitting side wireless communication device, the transmission power control of the uplink signal is performed.
The signal received from the antenna in FIG. 11 is converted into a baseband signal by the radio frequency circuit 101. The baseband signal is subjected to demodulation processing such as detection by the demodulator 102 and error-corrected by the channel decoder 121. On the other hand, the baseband signal is input to the power signal generation unit 105 to generate a transmission power control signal according to the power control algorithm. The transmission power control signal includes the third pilot signal generated by the third pilot signal generator 130, the data signal subjected to channel coding by the error correction encoder 106 and the interleaver 107, and the multiplexer 109. Is multiplexed. The multiplexed signal has a format as shown in FIG. 14, for example. 303 is a data signal, 304 is a power control signal, and 305 is a third pilot signal. In the figure, the horizontal direction represents time and the vertical direction represents a code used for code division. Multiplexed in the way. The multiplexed signal is modulated by the modulator 110 and sent to the radio propagation path via the radio frequency circuit 101.
The signal transmitted from the receiving wireless communication device is received by the transmitting wireless communication device shown in FIG. Operations of 101, 102, 103, and 104 are the same as those of the reception-side wireless communication device. The transmission power control unit 111 extracts the power control signal 304, and calculates transmission power according to the extracted transmission power control signal 304. On the other hand, the transmission data encoded by the channel encoder 122 is multiplexed by the multiplexer 112 with the second pilot signal generated by the second pilot signal generator 108 and input to the transmission power variable means 113. . The transmission power varying means 113 varies the signal amplitude so that the transmission power specified by the transmission power control unit 111 is obtained. The output of the transmission power varying means 113 is multiplexed by the multiplexer 115 with the first pilot signal set to a predetermined power by the first pilot signal generating means 114, and has the form as shown in FIG. Signal. In FIG. 12, 301 is a first pilot signal, 302 is a second pilot signal, and 303 is a data signal.
As shown in FIG. 12, various multiplexing formats are possible. The first pilot signal 301 (P0) is transmitted with a predetermined power without being subjected to power control by the transmission power control unit 111. On the other hand, the second pilot signal 302 is transmitted together with the data signal 303 under the power control. The signal multiplexed in the format of FIG. 12 is modulated by the modulator 110 and sent to the radio propagation path via the radio frequency circuit 101.
The transmission power signal generation unit 105 in the reception-side wireless communication device and the transmission power generation unit 111 in the transmission-side wireless communication device are configured, for example, as shown in FIGS. 15 and 16, respectively. The transmission power signal generation unit in FIG. 15 separates the first pilot signal and the second pilot signal by the first pilot signal separation unit 201 and the second pilot signal separation unit 205, respectively.
S (t) = P_const−Nr (t) / g (t)
In
P_const = C0 Ave (Nr (t)) / Ave (g (t))
The comparator 211 determines whether the current transmission power is large or small with respect to the transmission power to be, and when the transmission power is large, a transmission power control signal 304 instructing a decrease in transmission power and a transmission power increase when the transmission power is small. Generate. Accordingly, the transmission power control unit in FIG. 16 extracts the previous transmission power control signal 304, and increases or decreases the current transmission power according to the transmission power control signal. In FIG. 15, the noise power is obtained from the second pilot signal, but can also be obtained from the first pilot signal (dotted line).
In the above embodiment, when the channel capacity is above the average over a certain period of time as described above, the bit rate is controlled to perform communication at a high bit rate, and conversely, when the channel capacity is below the average, It is preferable to perform communication at a low bit rate. For this purpose, instead of the channel encoder 122 in FIG. 13 and the channel decoder 121 in FIG. 11, the data rate is changed in response to the data rate instruction as shown in FIGS. 17 and 18, respectively. It is only necessary to multiplex and transmit the data rate information specifying the data rate to the data signal, and to perform the channel decoding process in accordance with the rate information in the channel decoder 121.
The transmission power control unit 105 can also be configured as shown in FIG. In the figure, a function calculation unit 214 calculates a function f (x) whose output increases as the input signal increases. Thereby, when the propagation path gain increases from the average value, a transmission power control signal instructing an increase in transmission power is generated. Further, when it can be assumed that the noise power is constant regardless of time, simplification is possible as shown in FIG.
Furthermore, as shown in FIG. 21, even when the second pilot signal 302 is not included in the signal transmitted by the transmission-side wireless communication device, for example, the normalized transmission power S (t) / It is possible to obtain P0, use this as a transmission power control signal, and obtain S (t) by the transmission power control unit shown in FIG. More simply, the configuration of FIG. 24 can be used instead of FIG. 22, and the configuration of FIG. 25 can be used instead of FIG.
Also, as shown in FIG. 26, even when the first pilot signal 301 is not included in the signal transmitted by the transmission-side wireless communication device, for example, the transmission power control signal generation unit shown in FIG. The transmission power control unit can determine S (t). More simply, the configuration of FIG. 28 can be used instead of FIG. 27, and the configuration of FIG. 29 can be used instead of FIG.

The example of the time fluctuation of propagation path gain. An example of time variation of noise power. An example of time variation of equivalent noise power at the transmitting end. The transmission power control example by this invention. The example of a time change of the reception power by this invention. The example of transmission power control by a prior art. The example of a time change of the reception power by a prior art. Comparison of transmission power by transmission power control according to the present invention and the prior art. 2nd example of propagation path gain fluctuation. 6 shows a second example of transmission power control according to the present invention. 1 is a configuration example of a receiving side wireless communication device according to the present invention. 1 is a first example of a transmission signal multiplexing format of a transmitting side wireless communication device according to the present invention. The structure of the transmission side radio | wireless communication apparatus by this invention. The example of the transmission signal multiplexing format of the receiving side radio | wireless communication apparatus by this invention. 1 is a first configuration example of a transmission power control signal generation unit according to the present invention. 1 is a first configuration example of a transmission power control unit according to the present invention. 2 shows a second configuration example of an encoder with a data rate control function according to the present invention. 2 shows a second configuration example of a decoder with a data rate control function according to the present invention. 6 shows a second configuration example of a transmission power control signal generation unit according to the present invention. FIG. 9 is a third configuration example of a transmission power control signal generation unit according to the present invention. FIG. The 2nd example of the transmission signal multiplexing format of the transmission side radio | wireless communication apparatus by this invention. 4 shows a fourth configuration example of a transmission power control signal generation unit according to the present invention. 2 shows a second configuration example of a transmission power control unit according to the present invention. 7 shows a fifth configuration example of a transmission power control signal generation unit according to the present invention. 3 shows a third configuration example of a transmission power control unit according to the present invention. The 3rd example of the transmission signal multiplexing format of the transmitting side radio | wireless communication apparatus by this invention. 6 shows a sixth configuration example of a transmission power control signal generation unit according to the present invention. 7 shows a seventh configuration example of a transmission power control signal generation unit according to the present invention. 4 shows a fourth configuration example of a transmission power control unit according to the present invention. The system block diagram of this invention.

Explanation of symbols

  1,2 base station, 3, 4, 5 mobile station, 6 base station control station, 7 fixed network, 101 radio frequency circuit, 102 demodulator, 103 deinterleaver, 104 error correction decoder, 121 channel decoder, 105 Transmission power control signal generator, 106 Error correction encoder, 107 Interleaver, 109, 112, 115, 124 Signal multiplexer, 110 modulator, 111 Transmission power controller, 122 Channel encoder, 108 Second pilot Signal generator, 113 Transmission power variable means, 114 First pilot signal generator, 130 Third pilot signal generator, 301 First pilot signal, 302 Second pilot signal, 303 Data signal, 304 Power control signal, 305 Third Pilot signal, 201 First pilot signal separation means, 202, 210 Signal power measurement means, 203, 207, 223 Signal averaging means, 204, 212, 216, 217, 228 Divider, 205 Second pilot signal separation means, 206 Noise Power measurement means, 208, 213, 215, 218, 222, 224, 226 multiplier, 209, 219, 225 adder, 2 11 comparison means, 220 power control signal separation means, 221 transmission power calculation means, 123 data rate information generation means, 125 data rate information separation means, 214 function calculation means, 227 signal delay means.

Claims (11)

A signal transmission method in a wireless communication system, comprising:
A first step of performing error correction coding on a transmission signal for transmission from a transmitter to a receiver;
A second step of setting transmission power of the transmission signal subjected to the error correction coding,
In the second step, when the propagation path state between the transmitter and the receiver does not satisfy a predetermined state, the transmission power of the transmission signal subjected to the error correction coding is set to zero. ,
A reference P_const for determining the predetermined state is set according to an average propagation path state ,
A signal transmission method characterized in that an average time of the average propagation path state is substantially matched with a unit for encoding .   2. The signal transmission method according to claim 1, wherein, in the second step, a propagation path gain between the transmitter and the receiver is evaluated as the propagation path state.   The signal transmission method according to claim 1, wherein in the second step, the propagation path state is determined based on reception power of a signal transmitted and received between the transmitter and the receiver. Signal transmission method. The signal transmission method according to claim 1, comprising:
In the second step, the receiver determines the propagation path state using a signal transmitted from the transmitter, generates a control signal based on the determined result, and transmits the control signal to the transmitter And the transmitter controls the transmission power based on the notified control signal. A transmission device in a wireless communication system,
An encoding unit that performs error correction encoding on a transmission signal;
A transmission power setting unit that sets transmission power of a transmission signal input from the encoding unit;
A transmission unit for transmitting the transmission signal with the set transmission power;
A receiving unit for receiving a signal from a receiving device that is a transmission destination of the transmission signal,
The transmission power setting unit sets the transmission power of the transmission signal subjected to the error correction coding to zero when the propagation path state with the receiver does not satisfy a predetermined state,
The reference P_const for determining the predetermined state is set according to an average propagation path state ,
The transmission apparatus according to claim 1, wherein an average time of the average propagation path state is substantially matched with a unit for encoding .   6. The transmission apparatus according to claim 5, wherein the propagation path state is a propagation path gain between the transmitter and the receiver.   6. The transmission apparatus according to claim 5, wherein the propagation path state is determined using reception power of a signal transmitted / received between the receiver and the transmitter.   6. The transmission apparatus according to claim 5, wherein the radio unit receives a control signal generated based on the propagation path state from the reception apparatus that has measured the propagation path state. . A receiving device in a wireless communication system,
A radio unit for receiving a signal from the transmission device and transmitting a control signal to the transmission signal;
A control signal generator that generates the control signal based on the received signal;
The control signal generation unit determines a propagation path state with the transmission device based on the received signal,
If the propagation path state does not satisfy a predetermined state, generate a control signal instructing to set the transmission power of the transmission signal from the transmission device to zero,
A reference P_const for determining the predetermined state is set according to an average propagation path state ,
The receiving apparatus according to claim 1, wherein an average time of the average channel state is substantially matched with a unit for encoding .   The receiving apparatus according to claim 9, wherein a propagation path gain between the transmitter and the receiver is evaluated as the propagation path state.   The receiving apparatus according to claim 9, wherein the propagation path state is determined based on received power of the received signal.

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