WO2007123121A1

WO2007123121A1 - Radio base station and transmission control method

Info

Publication number: WO2007123121A1
Application number: PCT/JP2007/058340
Authority: WO
Inventors: Mikio Iwamura; Minami Ishii
Original assignee: Ntt Docomo, Inc.
Priority date: 2006-04-19
Filing date: 2007-04-17
Publication date: 2007-11-01
Also published as: JP2007288676A; TW200803244A; JP4812504B2

Abstract

Even when a CQI report cycle covers a plurality of frames, it is possible to perform transmission control appropriate for actual propagation condition and maintain transmission efficiency. A radio base station (10) includes: a feedback report reception unit (14) for receiving a feedback report from a mobile station; a correction unit (20) for correcting the received feedback report in accordance with the elapse time from the report; and a scheduler (21) for controlling the transmission allocation in accordance with the corrected feedback report value. The correction unit makes a correction so that the feedback report value decreases as the time elapses from the feedback report.

Description

Specification

Radio base station and transmission control method

Technical field

TECHNICAL FIELD [0001] The present invention relates to a transmission control technique in a radio base station, and in particular, feedback information from a mobile station, for example, CQI (channel quality indica tor) information, which is a measurement result of a downlink radio channel, from the time of reporting. The present invention relates to a radio base station and a transmission control method for correcting transmission according to elapsed time and appropriately performing transmission control such as scheduling and link adaptation. Background art

[0002] Recent wireless communication systems such as HSDPA (high speed downlink packet access) perform downlink link adaptation (transmission power control, adaptive modulation / demodulation, adaptive coding, etc.) and packet scheduling between users. Therefore, the result of measuring the state of the downlink radio channel from the mobile station (hereinafter referred to as “CQI”) is reported to the base station!

[0003] In HSDPA, CQI is the common pilot channel EcZl

0

Power ratio), but not limited to EcZl in this specification and claims.

0

Indicators or feedback information representing the state of the radio channel, such as carrier loss, received power, and signal-to-interference power ratio, are collectively referred to as CQI.

[0004] The propagation environment fluctuates at a high speed in accordance with the moving speed of the user. Therefore, in order to perform link adaptation more efficiently or to increase multi-user diversity through scheduling, Frequent CQI reporting is required.

FIG. 1A is a diagram showing an example of scheduling in the time direction (multiuser diversity). As indicated by the dotted line and the solid line, the propagation status of each of user 1 and user 2 changes every moment, and radio resources are allocated according to the propagation status of each user on the time axis. However, when the amount of CQI reports increases, the uplink capacity is under pressure and the mobile station battery is exhausted.

[0006] FIG. 1B shows an example of scheduling in the frequency direction (multiuser diversity). In future mobile communication systems, broadband transmission is expected to achieve higher transmission rates, lower delay, and higher capacity. However, if the system is broadband Because frequency selective fading occurs, in order to transmit more efficiently, as shown in Fig. 1B, it is better to use different frequency bands for each user. For this purpose, it is necessary to divide the entire frequency band of the system into several subbands and measure and report CQI for each subband.

[0007] There are various ways of reporting, for example, the number of the top three subbands with the highest CQI and the CQI value are reported. The average value of all bands and the difference from the average value of each subband are reported. For example, there is a method of reporting the result of encoding using a discrete cosine transform (DCT). In any case, there is a problem that the amount of information in CQI reports increases when the broadband is used effectively. As the amount of CQI information increases, the capacity of the uplink is reduced and the mobile station's battery is exhausted, similar to scheduling in the time axis direction.

[0008] When scheduling is performed in the time axis direction and the frequency direction based on feedback information from the mobile station, mobile terminals are divided into groups based on the feedback information, and resource allocation is determined for each group. A method has also been proposed (for example, see Patent Document 1).

[0009] As described above, there is a trade-off relationship between the efficiency of one link and the amount of feedback information of the opposite link. If the minimum unit in the time direction of scheduling and link adaptation is defined as a frame, and the minimum unit in the frequency direction is defined as a sub-band, it is impractical to simply report the CQI of all sub-bands for each frame. In other words, from the viewpoint of uplink capacity and battery saving, it is necessary to keep the amount of information to be reported to the minimum that can maintain transmission efficiency, and to operate at a point with a good trade-off balance. In practice, reduction methods such as thinning out to some extent in the time direction and frequency direction and reporting the CQI of the top three subbands in good condition once every few frames are effective.

[0010] However, when the CQI reporting frequency decreases, there is a problem that the performance of scheduling and link adaptation deteriorates. As a result of reducing the frequency of CQI reporting, as shown in Fig. 2A, the reporting period is longer than one frame of the scheduling unit, and scheduling without CQI reporting is required. During the period when CQI reports cannot be obtained, the scheduling priority is determined by maintaining the previous CQI value as shown in Figure 2B. Yes. Since scheduling cannot be performed based on the latest information, the optimum user cannot be selected, and the gain of multiuser diversity is reduced. Furthermore, link adaptation cannot be performed properly for users assigned by the scheduler, and transmission efficiency decreases.

Patent Document 1: Japanese Patent Laid-Open No. 2005-318434

Disclosure of the invention

Problems to be solved by the invention

[0011] Therefore, the present invention provides a configuration of a radio base station and a transmission control method that can perform optimum transmission control even when the CQI reporting frequency is not sufficient for radio channel fluctuation. Is an issue.

Means for solving the problem

[0012] In order to realize the above problem, the report value is corrected according to the elapsed time of feedback reporting ability such as CQI, and based on the correction value, transmission control such as transmission scheduling between multiple users and link adaptation is performed. I do.

[0013] For example, the transmission priority can be controlled according to the elapsed time from the feedback report. In this case, the report value is corrected so that the user having a short elapsed time with feedback reporting power, that is, the reliability is relatively high and the transmission priority of the user is high.

[0014] The parameter of the link adaptation can be controlled according to the elapsed time from the feedback report. As a control example,

(0 Increase the sign rate as the elapsed time of feedback reporting power increases,

(ii) Use a modulation scheme that reduces the number of multi-values as the elapsed time of feed reporting power increases. (m) Increase the transmission power as the elapsed time of feedback reporting power increases.

[0015] Furthermore, it is possible to estimate the moving speed of the mobile station and correct the elapsed time with the estimated speed. In this case, it may be corrected so that the elapsed time force of the user whose movement speed is fast becomes longer than the actual time.

[0016] In the first aspect of the present invention, the radio base station comprises:

(a) a feedback report receiving unit that receives a feedback report of a mobile station, and (b) a correction unit that corrects the received feedback report value so as to decrease according to the elapsed time of the reporting power;

(C) a scheduler that performs transmission assignment based on the corrected feedback report value.

[0017] Preferably, in the example, the scheduler controls at least one of a transmission priority among multiple users and a link adaptation parameter based on the corrected feedback report value.

[0018] More specifically, the scheduler controls transmission based on the corrected feedback report so that the code rate increases as the elapsed time of the feedback reporting power increases. Alternatively, transmission control is performed so as to use a modulation scheme that reduces the number of multiple values as the elapsed time from the feedback report becomes longer. Alternatively, transmission control is performed so that the transmission power increases as the elapsed time of the feedback reporting power increases. Alternatively, control is performed so as to increase the transmission priority of the user whose elapsed time from the feedback report is short.

In the second aspect of the present invention, the transmission control method includes:

(a) Receive a downlink feedback report from the mobile station,

(b) Correct the received feedback report value so as to decrease according to the elapsed time from the report,

(c) Control transmission allocation to the mobile station based on the corrected feedback report!

Process.

The invention's effect

[0020] Even when the CQI reporting frequency is low, appropriate link adaptation and scheduling control can be performed.

Brief Description of Drawings

FIG. 1A is a diagram showing an example of scheduling in the time direction.

FIG. 1B is a diagram showing an example of scheduling in the frequency direction.

FIG. 2A is a diagram for explaining conventional transmission control using CQI report values.

FIG. 2B is a diagram for explaining conventional transmission control using CQI report values. [3] The operation concept and effect of the present invention will be described in comparison with a conventional method.

FIG. 4A is a diagram for explaining an application example of the present invention to multi-user scheduling.

FIG. 4B is a diagram illustrating an application example of the present invention to multi-user scheduling.

FIG. 5 is a diagram showing a correction function nomination used in the embodiment.

[6] It is a diagram showing an example of adjustment of the correction function according to the CQI report value.

[7] FIG. 7 is a block diagram showing a configuration example of a radio base station according to the embodiment.

Explanation of symbols

[0022] 10 radio base stations

11 Antenna

13 Reception RF section

14 CQI receiver

15 Synchronization section

16 Speed detector

20 CQI correction section

21 Scheduler

22 Transmission signal generator

23 Transmission notifier

24 Monitoring unit

25 Transmit RF section

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 3 (b) is a conceptual diagram of the operation of the link adaptation to which the present invention is applied. As a comparative example, Fig. 3 (a) shows the concept of operation of a conventional link adaptation. In Fig. 3, in order to maintain transmission efficiency, the CQI reporting period is set longer than one frame length, as in Fig. 2A. In this example, it is assumed that the mobile station power is also reported every 4 frames.

[0024] In the conventional method of Fig. 3 (a), CQI report cannot be obtained! /, And the frame adaption is applied using the previous CQI report value as it is. When the rate increases, problems arise. In other words, as shown by the circle in the figure, Even though the transport is degraded, link adaptation is performed using the previous value that indicates a better propagation state, so that the optimum coding modulation method can be selected. Disappears and is easy to make mistakes.

[0025] In order to solve this inconvenience, in the embodiment of FIG. 3 (b), the CQI value is corrected according to the elapsed time of the reporting ability. In this example, until the next CQI report is received, the previously received CQI value is decreased step by step for each frame. It is also the power that reduces the reliability of CQI values over time. By making corrections in this way, link adaptation can be performed with values that more closely match the actual propagation state, as shown by the circle in FIG. 3B.

It should be noted that the correction may be further reduced by the step size as well as needing to be performed for each frame, or may be reduced as a continuous function in an analog manner. In either case, the increase in error rate that was a problem in the conventional method can be solved.

[0027] Examples of parameters controlled by link adaptation based on feedback reports such as CQI include coding rate, multi-level number of modulation, and transmission power. As a result of the CQI report value (feedback report value) being corrected so as to decrease as time elapses from the reception time, the values of these parameters are also controlled over time. For example,

(1) Increase the coding rate as the elapsed time of CQI reporting power increases.

(2) Use a modulation method with fewer multi-values as the elapsed time of CQI reporting power increases.

(3) Increase the transmission power as the elapsed time of CQI reporting power increases.

By controlling as described above, errors can be reduced.

[0028] Such correction of the CQI value can also be applied to adjustment of simultaneous access by a plurality of users.

[0029] FIGS. 4A and 4B show examples of applying feedback report correction to multi-user scheduling. As shown in Fig. 4A, user 1 and user 2 send CQI reports to the base station at different timings every 4 frames. For each of User 1 and User 2, correction is performed to reduce the previously received CQI report value in a stepwise manner until the next CQI value is reported. As a result, as shown in Fig. 4B, the parameter value power for determining the priority to be considered in the scheduling among multiple users is set lower with the passage of time. In other words, the transmission of users with a short elapsed time with CQI reporting power The priority is controlled to be higher.

[0030] As a result of such correction, a user with a good propagation condition is appropriately selected in the frame indicated by the thick frame in FIG. 4A. That is, in the frame sequence A, in the conventional method, even if time elapses, the user 2 having a high CQI value at the time of reporting is selected and the error increases, and the error is increased by correcting the CQI value as in the embodiment. User 1 can be selected. In frame column B, in the method based only on the CQI value at the time of reporting, user 1 is selected against the actual propagation situation. User 2 is selected correctly based on this.

FIG. 5 is a diagram showing the correction function nomination. In the conventional method, as shown in Fig. 5 (a), the received CQI report value is used as it is during the CQI report cycle over multiple frames. On the other hand, in the embodiment, the CQI report value is decreased with the passage of time in consideration of propagation path degradation after reception. The decrease may be stepped with a linear slope as shown in Fig. 5 (b), or it may be reduced exponentially and stepwise as shown in Fig. 5 (c).

FIG. 6 shows an example of further appropriately adjusting the correction function. For example, if the profile correction function shown in Fig. 6 (a) is the basic form, the correction function is stretched (change rate of change) according to the size of the received report value. Specifically, when the received CQI report value is large as shown in Fig. 6 (b), the rate of change of the correction function is increased, and when the received CCQI value is small as shown in Fig. 6 (c), Make the rate of change gentle. Of course, the same scale may be used regardless of the size of the received CQI value.

FIG. 7 is a schematic block diagram of a radio base station according to one embodiment of the present invention. In the radio base station 10, the CQI receiver 13 receives the CQI for which the mobile station power is also reported via the antenna 11, the circulator 12, and the reception RF unit 13. The synchronization unit 15 detects the CQI report timing and synchronizes the timing. The synchronization timing information is supplied to the CQI correction unit 20 together with CQI (CQI report value).

[0034] The CQI correction unit 20 corrects the CQI report value according to the elapsed time of the report timing power. The correction may be performed by the method shown in FIGS. 5 (b) and 5 (c), or may be combined with the method shown in FIG. 6 according to the CQI value. The corrected CQI is sent to the scheduler 21. [0035] The transmission buffer 23 buffers transmission user data and control data. The monitoring unit 24 monitors the data staying in the transmission buffer and transmits the monitoring result to the scheduler 21. The scheduler 21 performs transmission allocation based on the corrected CQI and the monitoring result from the monitoring unit 24.

[0036] In response to an instruction from the scheduler 21, the transmission signal generation unit 22 performs encoding and modulation of data to be transmitted, and adjusts and outputs transmission power. At this time, the link adaptation according to the instruction from the scheduler 21 is applied. The transmission RF unit 25 converts the transmission signal into an RF signal and outputs it. The transmitted RF signal is transmitted from the antenna 11 via the circulator 12.

[0037] The CQI may be corrected in consideration of the moving speed of the mobile station! That is, the radio base station 10 has a speed detection unit 16, detects the moving speed of the mobile station also with the received signal power, and inputs the measurement result to the CQI correction unit 20. Alternatively, the mobile station may detect the moving speed and report the measurement result using a wireless protocol.

[0038] In this case, the CQI correction unit 20 corrects the elapsed time from the feedback report with the moving speed of the mobile station. For example, correction is performed so that the elapsed time of a user whose movement speed is fast is longer than the actual time. As a specific example, it is conceivable to increase the slope of the correction function.

[0039] Signal reception, CQI reception, CQI report timing detection, mobile station movement speed detection, CQI correction, transmission data buffering, transmission signal generation, and conversion to RF signals are performed for each user. Is called. The scheduler 21 performs transmission scheduling among a plurality of users and parameter determination for link adaptation based on the CQI correction value determined for each user.

[0040] With such a configuration, even when the frequency of CQI reporting is relatively low, appropriate transmission control is performed by performing correction in consideration of propagation degradation.

[0041] This international application claims priority based on Japanese Patent Application No. 2006-115805 filed on April 19, 2006, the entire contents of which are incorporated herein by reference.

Claims

The scope of the claims

[1] A feedback report receiver for receiving feedback reports of mobile stations,

A correction unit that corrects the received feedback report value so that the report value decreases according to the elapsed time from the report;

A radio base station, comprising: a scheduler that performs transmission assignment control based on the corrected feedback report value.

[2] The radio base station according to claim 1, wherein the correction unit corrects an elapsed time from a feedback report with a moving speed of the mobile station.

[3] The radio base station according to claim 3, wherein the correction unit corrects the elapsed time of a user whose movement speed is fast to be longer than the actual time.

[4] A speed detector for detecting a moving speed of the mobile station,

The radio base station according to claim 3, further comprising:

[5] The radio base station according to claim 3, wherein the radio base station receives movement speed information from the mobile station.

6. The scheduler according to claim 1, wherein the scheduler controls at least one of a transmission priority among a plurality of users and a link adaptation parameter based on the corrected feedback report value. Wireless base station.

7. The scheduler according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so that the code rate increases as the elapsed time of the feedback report increases. Radio base station.

8. The scheduler according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so that the modulation method uses a lower number of multi-values as the elapsed time from the feedback report becomes longer. Wireless base station.

[9] The radio base station according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so that the transmission power is increased as the elapsed time of the feedback report becomes longer. Bureau.

[10] The scheduler determines a feedback report based on the corrected feedback report. 2. The radio base station according to claim 1, wherein control is performed so as to increase a transmission priority of a user whose elapsed time has been short since notification.

Receive a downlink feedback report from the mobile station,

Correct the received feedback report value so that it decreases with the elapsed time since the report,

A transmission control method comprising a step of performing transmission control to the mobile station based on the corrected feedback report.