CN1128513C - Reverse external-loop power control method - Google Patents

Abstract

The invention discloses a method for reverse outer loop power control. The receiver receives the rate indication and symbol error rate of the current frame signal; judges the operating state according to the received frame rate indication; enters the operating state after the judgment and counts the frame error rate; uses the frame error rate and the target frame error rate in the corresponding state to adjust the target symbol error rate; compare the difference between the current frame symbol error rate obtained by the receiver and the target symbol error rate; adjust the outer loop threshold with the obtained difference; control the outer loop threshold by the limiter at within the specified range. The present invention comprehensively considers two factors of realization speed and control precision, and is easy to realize and fast in speed.

Description

A Method of Reverse Outer Loop Power Control

The invention relates to a power control method of a wireless communication system, in particular to a reverse power control technology in a code division multiple access communication system.

Code Division Multiple Access (CDMA) is one of the multiple access modulation techniques commonly used in mobile communications. Figure 1 shows a typical CDMA cellular system structure diagram. A mobile station (MS) 12A or 12B communicates with a base station controller (BSC) 14A via a base transceiver station (BTS) 16A or 16B, and the mobile station may be in contact with both base stations simultaneously for some period of time. The base station controller communicates with PSTN or ISDN through the mobile switching center (MSC). At the same time, one MSC can also connect to other base station controllers, such as BSC14B.

Actually, there are multiple mobile users in a certain cell. If all the users in the cell transmit with the same power, the signals from the mobile stations close to the base station to the base station are strong (such as MS12B to BTS16B), and the signals from the mobile stations far away from the base station to the base station are weak (such as MS12A to BTS16B), resulting in a strong signal Covering up weak signals is the problem of "near-far effect" in mobile communications. In addition, the CDMA system is a self-interference system, and all users share the same frequency, so the "near-far effect" problem is more prominent. In the CDMA system, the power of a certain user's signal is strong, which is beneficial to the correct reception of the user, but it will increase the interference to other users in the shared frequency band, and even submerge the useful signals of other users, resulting in poor communication quality of other users. degradation, leading to a decrease in system capacity. In order to overcome the near-far effect, it is necessary to adjust the power required by the transmitter in real time according to the difference in the communication distance, which requires "power control". Due to the influence of urban buildings and complex terrain on the ground, the signal received by the receiver is the result of multiple reflections or scattering paths. Its amplitude and phase change randomly, which is called "multipath signal". The fading is usually very severe, and power control is also required to compensate for the fading for a certain period of time. In addition, due to obstacles such as tall buildings, mountains, and woods on the radio wave propagation path, the shadow of electromagnetic wave propagation is formed, and power control is also required to overcome this shadow effect. Therefore, power control can be said to be the core of all CDMA key technologies. Otherwise, many advantages of the CDMA system cannot be realized, and a high-capacity, high-quality CDMA system cannot be realized.

In order to realize precise power control, the CDMA system usually adopts a combination of forward power control and reverse power control. Power control is introduced in the forward link of the CDMA system. By adjusting the power of the base station transmitter of the traffic channel, the transmission power of the forward traffic channel is made as small as possible while meeting the minimum required signal-to-noise ratio for demodulation of the mobile station.

The power control of the reverse link generally includes three parts: open loop power control, closed loop power control and outer loop power control. Each mobile station in the CDMA system is always calculating the path loss from the base station to the mobile station. When the mobile station receives a strong signal from the base station, it indicates that the mobile station is either very close to the base station or has a particularly good propagation path. , at this time, the mobile station can reduce its transmission power, and the base station can still receive normally; on the contrary, when the signal received by the mobile station is weak, it increases the transmission power to offset the attenuation, which is open-loop power control. The open-loop power control is simple and direct, without exchanging control information between the mobile station and the base station, and at the same time, the control speed is fast and the cost can be saved. However, in the CDMA system, the frequencies used for forward and reverse transmission are different (such as the frequency difference specified by IS-95 is 45MHz), and the frequency difference far exceeds the coherent bandwidth of the channel, so it cannot be considered that the fading characteristics on the forward channel are equal to the reverse The fading characteristics on the channel, which is the limitation of open-loop power control.

In order to overcome the uncorrelated Rayleigh fading on the forward and reverse links, the signal-to-noise ratio of the signal from the mobile station can be detected by the base station and compared with a threshold value, and the forward channel is moved according to the comparison result The station sends an instruction of power up or power down, and the mobile station adjusts its transmit power according to the received instruction, which is closed-loop power control. The key to realizing closed-loop power control is to generate, transmit, process and execute power control commands quickly, so as to track the Rayleigh fading of the reverse link as much as possible.

Due to different environmental conditions, the signal-to-noise ratio threshold value required to maintain a given communication quality is different. Therefore, in the closed-loop power control, the threshold value of the signal-to-noise ratio is not constant, but can be changed dynamically under the outer-loop power control. The so-called outer loop power control is actually a power control process that takes place in the base station or between the base station and the mobile switching center. It uses the received voice quality information as the control input to adjust the SNR threshold, thereby Changes the basis for power control command discrimination.

In an actual system, the reverse power control is accomplished jointly by the above three power controls. That is to say, the open-loop estimation is made to the transmit power of the mobile station first, and then the open-loop estimation is further corrected by the closed-loop power control and the outer-loop power control, trying to achieve accurate power control. In this reverse power control method, the outer loop method has great flexibility, and the adjustment of the signal-to-noise ratio threshold determines the control performance to a large extent. Therefore, an effective outer loop power control method is needed to automatically correct the SNR threshold value when the external conditions change, so as to change the transmit power of the mobile station, so that the quality of the received reverse link is always maintained at a certain target value nearby.

There are many methods for outer loop power control at present. Generally, they can be classified into two categories: one is based on the principle of path compensation, which keeps the received signal strength constant; the other is based on the received signal quality. The biggest advantage of the former is that it is simple, but this method can hardly increase the capacity while compensating the path loss completely. Some improvement schemes based on this idea only make up for part of the path loss, and some capacity improvements can be obtained, but divergence in the control process may often occur. The latter makes all users have the same quality, basically no divergence problem, and it realizes the improvement of capacity. US Patent US5396516, "Method and System for the Dynamic Modification of ControlParameters in A Transmitter Power Control System", its basic technical solution is to divide the communication process into several states (such as full-rate operation, variable-rate operation, delete state, etc.) , the threshold value decreases by one step when a good frame is received, and the threshold value increases by one step when a bad frame is received, but different values are used for the increase and decrease step in different states. Such a method will make the adjustment of the threshold value drastic, that is, the variance of the threshold value is large, and the system capacity cannot be well utilized.

Another U.S. patent US5745520, "Method and Apparatus for Power Control in A Spread Spectrum Communication System Using Threshold Step-down SizeAdjustment", the method it proposes is to directly use the difference between the frame error rate FER and the target frame error rate to adjust the rising step If the outer loop threshold value is adjusted by using this step length, the variance of the threshold value adjusted by this method is still relatively large, which affects the full utilization of the system capacity. In addition, US Patent US5732328 "Method for Power Control Threshold in Wireless Networks for Communication Multiple Information Classes", the idea is to use the difference between the symbol error rate and the target symbol error rate to control the adjustment of the threshold value, and the target symbol error rate is based on the error rate The frame rate is adjusted. This method is relatively simple to implement, but it is difficult to compromise between the frame error rate (FER) and the symbol error rate (SER) directly according to the two parameters, and it is not easy to achieve good performance in the end.

The purpose of the present invention is to overcome the contradiction between the implementation difficulty, control speed and system capacity in the existing outer loop control method based on the received signal quality, and propose an outer loop control method that can improve the system capacity under the premise of ensuring the target control quality. ring power control method.

For realizing the purpose of the invention, the method proposed by the present invention mainly includes the following steps:

(1) The receiver obtains the rate indication and symbol error rate of the current frame signal;

(2) Judging the running state according to the received frame rate indication;

(3) Enter the running state obtained after the judgment and count the frame error rate;

(4) Using the difference between the frame error rate and the target frame error rate in the corresponding state, adjust the target symbol error rate;

(5) compare the difference between the current frame symbol error rate obtained by the receiver and the target symbol error rate;

(6) adjust the outer ring threshold with the difference obtained in (5);

(7) Control the threshold value of the outer ring within the specified range through the limiter.

In the described outer loop power control method, the following steps may also be included between step (4) and step (5): after obtaining the adjustment value of the target symbol error rate, the sampling module 250 performs a buffering operation according to the selected sampling rate. The state is sampled, and then the sampling result is sent to the accumulator 252 to control the adjustment rate of the target SER value.

Adopting the method of the present invention, compared with the existing method, comprehensively considers the realization speed and control precision of the outer loop control method, the variance of the control is relatively small, the power of the transmitter is saved, and the quality of the received signal is balanced , Improved system capacity and battery life. The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a structural diagram of the CDMA cellular mobile communication system. FIG. 2 is a schematic diagram of reverse link closed-loop power control. Fig. 3 is a flow chart of the reverse outer loop power control method proposed by the present invention. Fig. 4 is an embodiment of the method of the present invention.

Figure 2 clearly shows the role of the outer loop adjustment method in the reverse power control process. In the reverse link closed-loop control, the base station first receives the fading signal transmitted by the antenna of the mobile station, and completes the demodulation and decoding of the received signal in module 200, and can obtain the normalization of the received signal within a period of time (such as a power control group time). The energy or the signal-to-noise ratio of the received signal can also be used to determine the quality information of the current received signal frame (such as frame rate indication, frame check word, and symbol error rate SER, etc.) from the decoded data field. The signal-to-noise ratio value received in each period of time is sent to the comparator 270 for comparison with the threshold value. At the same time, the module 220 receives the quality information of the current frame as the input of the outer loop control adjustment module 240, where the adjustment of the signal-to-noise ratio threshold value is completed, and the adjustment result is provided to the comparator 270 to dynamically change the comparison benchmark of the comparator . According to the result of the comparator, corresponding power control bits are produced. If the currently received signal-to-noise ratio is greater than the threshold value, a control bit representing power decline is generated in module 285, otherwise, a control bit representing power rise is generated in module 280. . Then in module 290, it is inserted into the power control sub-channel of the corresponding forward traffic channel according to the rules stipulated in the air interface protocol, and the power control command is sent to the mobile station through the forward link. The mobile station receives the signal of the forward link in module 110 and detects an effective power control command (corresponding to the power control group of effective transmission), and adjusts the transmission power by a certain step in the transmission power adjustment module 100; If the station is in the soft handoff state and receives multiple power control commands at the same time, it will try to execute the command to reduce its power.

The outer loop power control method proposed by the present invention is shown in FIG. 3 . First, in 300, the receiver demodulates and decodes the received signal to obtain the current frame rate indication and the number of symbol errors; in 310, the operating state is judged according to the received frame quality indication; in 320, the frame error rate is counted under the corresponding state; Then by comparing the difference between the frame error rate and the target frame error rate in 330, the target symbol error rate is adjusted; in 340, the symbol error rate of the received frame is compared with the target symbol error rate, and the difference is used to modify the outer loop threshold value; in 350, use the limiter to control the threshold value within a certain range, so as to avoid setting the outer ring threshold value of a certain user too high or too low; finally output the adjusted outer ring setting value in 360 to The comparator performs power control bit discrimination.

FIG. 4 shows the detailed working process of an embodiment of the outer loop power control method.

In this embodiment, the mobile station has a variable rate vocoder which can operate at full rate, 1/2 rate, 1/4 rate or 1/8 rate. The data information obtained by the base station through the demodulation and decoding process can provide the judgment of the current frame rate or generate the information of the frame rate by adopting a certain rate judgment method.

First, the input of the frame quality information is received, here mainly the frame rate obtained by the rate judgment and the symbol error rate SER of each frame. In the state judgment module 242, according to the rate judgment information (including four kinds of correct frame indication information and several wrong frame indication information), determine the state that the current frame statistics should belong to. The division of this state is carried out according to the different quality requirements at different rates, and the rate with the same quality requirements can be classified into one state. Different statistical branches a, b and c are selected according to the state of discrimination. Taking the statistical branch a as an example, first count the frame error rate in a certain period of time (such as 360 frames) in a ring buffer 244a, and the ring buffer follows the principle of first-in-first-out (FIFO) to count the frame error rate in the latest period of time. FER to reflect the quality of the current received signal. In the comparator 246a, the quality of the current received signal is compared with the target communication quality, that is, the statistical FER is compared with the target FER1 to obtain a difference in communication quality, where the difference is reflected in the difference in the frame error rate FER.

After the multiplier 248 utilizes the conversion constant C1 to convert the difference of the communication quality into an adjustment value to the target symbol error rate, the sampling module 250 samples the state of the buffer area at regular intervals according to the selected sampling rate, and then samples the The result is fed into the accumulator 252 to control the adjustment rate of the target SER value. The sampling rate of the sampling module 250 is a compromise between the rate of adjusting the target SER value and the control accuracy. The reason for this extraction operation is that the buffer area works in the form of a circular buffer, and it takes a period of time for an error frame to be covered by a new frame, which depends on the length of the buffer area set. Obviously, in the process of maintaining the same state, it is meaningless to sample once every frame. Therefore, the sampling module 250 needs to sample the "state" in the buffer area at a certain rate. The size of the sampling rate can be determined according to the adjustment requirements of the target SER according to the specific situation. The acceleration of the sampling rate can increase the adjustment of the target SER each time. If the sampling rate is reduced, the range of each adjustment of the target SER value can be reduced, so that the change of the target SER can be slowed down.

The adjusted target SER value serves as one input terminal of the comparator 254 , and the other input terminal of the comparator 254 is the output of the receive filter 256 . The filter 256 performs filtering processing on the symbol error rate SER of the currently received frame, and the filtering process can be realized by using a certain order finite impulse response filter. In the comparator 254, the filter output is compared with the adjusted target SER, the comparison result is sent to the multiplier 258, multiplied by the conversion constant C2, and the conversion from the SER error to the SNR threshold value correction is realized. conversion, and the converted result is sent to the SNR threshold value accumulator 260 to generate the adjusted threshold value.

In order to limit the abnormal transmission power of a certain link caused by the excessive badness of the link, which will cause huge interference to other users of the entire system and affect the performance of the entire system, through the function of the limiter 262, a limit signal-to-noise ratio gate is used The maximum threshold value (MAX_threshold) and the minimum threshold value (MIN_threshold) of the limit value adjustment are limited, so that the threshold value of each output is controlled between the upper and lower limits. After the action of the limiter, the adjustment result of the final output signal-to-noise ratio threshold value is obtained.

The control process initialization state may select a state into which a frame rate with a low probability of occurrence falls. At the same time, the three statistical FER ring buffers need to be initialized. This process needs to calculate the number of errored frames according to the target communication quality and the total time of statistics, and fill them into the buffers evenly, so that the quality of statistics in the buffers at the initial moment is just right. Consistent with target communication quality.

What has been presented above is only one exemplary embodiment of the present invention. There are many other obvious variations of the methods described in the present invention.

As in another embodiment, the state of statistical FER can be divided into full rate state (i.e. receive full rate frame) and sub rate state (receive low rate frame, including 1/2, 1/4, 1/8 etc. rates) The status is counted separately. Similarly, the statistical state can also be divided into four kinds of statistical states in detail, so as to correspond to the four rates accordingly, which does not depart from the description of the technical solution of the present invention.

The present invention can also adopt other quality information as the input information for adjustment. For example, not only the two types of received frame quality indication information in the above embodiments, namely the rate judgment result and the symbol error rate SER, but also the frame quality check word, and even other additional information about the current link quality can be used. Such as moving speed, communication environment level, etc.

The present invention can be used in links using vocoders such as 8K, 13K and EVRC without any modification, as long as the receiving link can provide certain receiving quality indication information.

Further expanded, the present invention can be applied in all closed-loop power control methods, not just in reverse closed-loop power control methods. For example, in the case that the power control sub-channel is also added to the reverse link, the present invention can be used in the mobile station to adjust the SNR threshold value according to the received communication quality of the forward link, and compare the SNR of the received signal. The ratio and the threshold value generate an appropriate power control command, which is sent to the base station on the reverse power control sub-channel, and the base station receives and demodulates the power control command, thereby controlling the transmit power of the corresponding forward link.

In a word, the present invention adopts linear operation, which is easy to implement, fast, and the control variance is relatively small. On the premise of ensuring the target control quality, the capacity of the self-interference system can be improved.

Claims (5)

1, a kind of method of backward external ring power control is characterized in that mainly may further comprise the steps: (1) receiver receives the speed indication and the error sign ratio of current frame signal; (2) according to the frame rate indication that receives, carry out the running status judgement; (3) enter the running status that draws according to judgement and add up frame error rate; (4) utilize the poor of target frame error rate under frame error rate and the corresponding state, adjust the target error sign ratio; (5) the present frame error sign ratio that obtains of receiver and target error sign ratio poor relatively; (6) adjust the outer loop threshold value with the difference that obtains in the step (5); (7) by the amplitude limiter adjustment outer loop threshold value is controlled within the scope of appointment.

2, the method for a kind of backward external ring power control as claimed in claim 1, it is characterized in that to comprise between step (4) and the step (5): after obtaining the adjusted value of target, decimation blocks (250) is according to the sampling rate size of selecting, state to buffer area is sampled, and then sampling results is sent into the adjustment speed of accumulator (252) in order to the controlled target error sign ratio.

3, the method for a kind of backward external ring power control as claimed in claim 1 or 2 is characterized in that the running status in the step (2) is to divide according to the difference to quality requirement under the different rates, and the speed that quality requirement is identical is included into a kind of state.

4, the method for a kind of backward external ring power control as claimed in claim 1 or 2, it is characterized in that step (4) specifically comprises: frame error rate and target frame error rate that statistics is obtained compare, obtain the difference of communication quality, in multiplier (248), utilize conversion constant C1 to convert the difference of communication quality to adjusted value then the target error sign ratio.

5, the method for a kind of backward external ring power control as claimed in claim 1 or 2, it is characterized in that step (6) comprising: result and adjusted target error sign ratio that the error sign ratio of current received frame is carried out after the Filtering Processing compare, comparative result is delivered to multiplier (258), multiply by conversion constant C2, realization is by the conversion to signal-noise ratio threshold value correction of the error of error sign ratio, and the result of conversion delivers to signal-noise ratio threshold value accumulator (260) again and produces adjusted outer loop threshold value.

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