JP4812504B2 - Radio base station and transmission control method - Google Patents

Description

  The present invention relates to a transmission control technique in a radio base station, and in particular, corrects feedback information such as CQI (channel quality indicator), which is a measurement result of a downlink radio channel from a mobile station, according to an elapsed time from a reporting time point. The present invention relates to a radio base station and a transmission control method that perform transmission control such as scheduling and link adaptation appropriately.

  In recent wireless communication systems such as high speed downlink packet access (HSDPA), downlink link adaptation (transmission power control, adaptive modulation / demodulation, adaptive coding, etc.) and packet scheduling between users are performed. The result of measuring the state of the radio channel (hereinafter referred to as “CQI”) is reported to the base station.

  In HSDPA, CQI corresponds to Ec / I0 (received chip energy to interference power ratio) of the common pilot channel. However, in the present specification and claims, not only Ec / I0 but also propagation loss, received power, signal pair An index indicating the state of the radio channel, such as an interference power ratio, or feedback information is generally referred to as CQI.

  Since the propagation environment fluctuates at a high speed according to the moving speed of the user, more frequent CQI reporting is required to perform link adaptation more efficiently and increase multiuser diversity by scheduling.

  FIG. 1A shows 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 the users 1 and 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 report increases, the capacity of the uplink is reduced, and the battery of the mobile station is consumed.

  FIG. 1B shows an example of scheduling in the frequency direction (multiuser diversity). In future mobile communication systems, it is expected that the bandwidth will be increased in order to realize higher transmission rate, lower delay, and larger capacity. However, when the system is widened, frequency selective fading occurs. Therefore, in order to transmit more efficiently, it is better to use different frequency bands in different states among users as shown in FIG. Good. 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.

  There are various reporting methods, 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. There is a method of reporting a result of encoding using a cosine transform (DCT). In any case, there is a problem that the amount of information of CQI report increases when the broadband is effectively used. As in the case of scheduling in the time axis direction, the amount of CQI information increases, so that the capacity of the uplink is reduced and the battery of the mobile station is consumed.

  In addition, when scheduling in the time axis direction and the frequency direction based on feedback information from the mobile station, a method for allocating resources for each group by grouping mobile terminals based on feedback information has also been proposed. (For example, refer to Patent Document 1).

  As described above, there is a trade-off relationship between the efficiency of one link and the amount of feedback information of the opposing 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 subband, it is impractical to simply report the CQI of all subbands for each frame. In other words, from the viewpoint of uplink capacity and battery saving, the amount of information to be reported needs to be kept to the minimum amount that can maintain the transmission efficiency, and is operated at a point with a good trade-off balance. Actually, a reduction method such as thinning out to some extent in the time direction and the frequency direction and reporting CQIs of the top three sub-bands in good condition once every several frames is effective.

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. 2 (a), the reporting period becomes longer than one frame of the scheduling unit, and it is necessary to perform scheduling in the absence of CQI reporting. As shown in FIG. 2B, the scheduling priority is determined while maintaining the previous CQI value as it is during the period when the CQI report cannot be obtained. Since scheduling cannot be performed based on the latest information, an optimal user cannot be selected, and the gain of multiuser diversity is reduced. Furthermore, link adaptation cannot be appropriately performed for users assigned by the scheduler, and transmission efficiency is reduced.
JP 2005-318434 A

  Therefore, it is an object of the present invention to provide a configuration of a radio base station and a transmission control method capable of performing optimum transmission control even when the CQI report frequency is not sufficient for radio channel fluctuation. .

In order to realize the above-described problem, the report value is corrected according to the elapsed time from the feedback report such as CQI, and transmission control such as transmission scheduling and link adaptation among a plurality of users is performed based on the correction value.

  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 transmission priority of the user whose elapsed time from the feedback report is short, that is, the user with relatively high reliability, becomes high.

It is also possible to control link adaptation parameters in accordance with the elapsed time from the feedback report. As a control example,
(i) Increasing the coding rate as the elapsed time from the feedback report becomes longer,
(ii) Use a modulation method that reduces the number of multi-values as the elapsed time from the feed report increases.
(iii) The transmission power is increased as the elapsed time from the feedback report becomes longer.

  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, you may correct | amend so that the elapsed time of the user whose movement speed is fast may become longer than actual.

In the first aspect of the present invention, the radio base station comprises:
(A) a feedback report receiving unit for receiving a feedback report from the mobile station;
(B) a correction unit that corrects the received feedback report value so as to decrease according to the elapsed time from the report;
(C) a scheduler that performs transmission assignment based on the corrected feedback report value.

  Based on the corrected feedback report value, the scheduler controls at least one of transmission priority among multiple users and a link adaptation parameter.

  More specifically, the scheduler controls transmission based on the corrected feedback report so that the coding rate is increased as the elapsed time from the feedback report becomes longer. Alternatively, transmission control is performed so that the modulation scheme with a smaller multi-value number is used as the elapsed time from the feedback report becomes longer. Alternatively, transmission control is performed so that the transmission power is increased as the elapsed time from the feedback report becomes longer. 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) receiving a downlink feedback report from the mobile station;
(B) correcting the received feedback report value so as to decrease according to the elapsed time from the report;
(C) including a step of performing transmission assignment control to the mobile station based on the corrected feedback report.

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

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 3 is a conceptual diagram of the operation of link adaptation to which the present invention is applied. Similar to FIG. 2, in order to maintain transmission efficiency, the CQI reporting period is set longer than one frame length. In this example, it is assumed that a CQI report is made from the mobile station every four frames.

  In the conventional method of FIG. 3A, in a frame in which a CQI report cannot be obtained, link adaptation is applied using the previous CQI report value as it is, which causes a problem that an error rate increases in some frames. In other words, as shown by the circle in the figure, although the actual propagation has deteriorated, the result of link adaptation using the previous value indicating a better propagation state is the result of optimal encoding. It becomes impossible to select a modulation method, and it is easy to make an error.

  In order to solve this inconvenience, in the embodiment of FIG. 3B, the CQI value is corrected according to the elapsed time from the report. In this example, the CQI value received last time is decreased stepwise for each frame until the next CQI report is received. This is because the reliability of the CQI value decreases with time. By correcting in this way, it is possible to perform link adaptation with a value that more closely matches the actual propagation state, as shown by the circle in FIG.

  The correction need not be performed for each frame, and may be reduced with a finer step size 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.

Examples of parameters controlled based on feedback reports such as CQI in link adaptation include coding rate, multi-value 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 as time elapses. For example,
(1) The coding rate is increased as the elapsed time from the CQI report becomes longer.
(2) Use a modulation scheme with a smaller number of multi-values as the elapsed time from CQI report becomes longer. (3) To reduce errors by controlling to increase the transmission power as the elapsed time from CQI report becomes longer. Can do.

  Such correction of the CQI value can also be applied to adjustment of simultaneous access from a plurality of users.

  FIG. 4 shows an example of applying feedback report correction to multi-user scheduling. As shown in FIG. 4 (a), user 1 and user 2 transmit CQI reports to the base station at different timings every four frames. Until the next CQI value is reported to each of user 1 and user 2, correction is performed to reduce the previously received CQI report value stepwise. As a result, as shown in FIG. 4B, the parameter value for determining the priority to be considered in the scheduling among a plurality of users is set to be lower with the passage of time. In other words, control is performed such that the transmission priority of the user whose elapsed time from the CQI report is short is high.

  As a result of such correction, a user with a good propagation state is appropriately selected in the frame indicated by the thick frame in FIG. That is, in the frame sequence A, in the conventional method, the user 2 having a high CQI value at the time of reporting is selected even when time elapses, and the error increases, but by correcting the CQI value as in the embodiment, The user 1 can be correctly selected. Also, in the frame sequence B, if it is based only on the CQI value at the time of reporting, the user 1 is selected against the actual propagation situation. However, in the method of the embodiment, the user 2 is correctly selected based on the correction value. Is selected.

  FIG. 5 is a diagram showing variations of the correction function. In the conventional system, as shown in FIG. 5 (a), the received CQI report value is used as it is during the CQI report cycle over a plurality of 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 reduced stepwise with a linear inclination as shown in FIG. 5B, or may be reduced exponentially and stepwise as shown in FIG. 5C.

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

  FIG. 7 is a schematic block diagram of a radio base station according to an embodiment of the present invention. In the radio base station 10, the CQI receiver 13 receives the CQI reported from the mobile station 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 information on the synchronization timing is supplied to the CQI correction unit 20 together with the CQI (CQI report value).

The CQI correction unit 20 corrects the CQI report value according to the elapsed time from the report timing. The correction may be performed by the methods shown in FIGS. 5B and 5C, 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.
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 assignment in consideration of the corrected CQI and the monitoring result from the monitoring unit 24.
The transmission signal generation unit 22 performs encoding, modulation, and the like of data to be transmitted in accordance with an instruction from the scheduler 21, and adjusts and outputs transmission power. At this time, link adaptation according to an instruction from the scheduler 21 is applied. The transmission RF unit 25 converts the transmission signal into an RF signal and outputs it. The transmission RF signal is transmitted from the antenna 11 via the circulator 12.
The CQI may be corrected taking into account the moving speed of the mobile station. That is, the radio base station 10 includes a speed detection unit 16, detects the movement speed of the mobile station from the received signal, 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.
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, it correct | amends so that the elapsed time of the user whose movement speed is fast may become longer than actual. As a specific example, it is conceivable to increase the slope of the correction function.
Signal reception, CQI reception, CQI report timing detection, mobile station moving speed detection, CQI correction, transmission data buffering, transmission signal generation, and conversion to an RF signal are performed for each user. The scheduler 21 performs transmission scheduling among a plurality of users and determination of parameters for link adaptation based on the CQI correction value determined for each user.
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.

It is a figure which shows the example of the scheduling of a time direction and a frequency direction. It is a figure for demonstrating the conventional transmission control using a CQI report value. It is a figure for demonstrating the operation | movement concept and effect of this invention. It is a figure explaining the example of application to the multiuser scheduling of this invention. It is a figure which shows the variation of the correction function used by embodiment. It is a figure which shows the adjustment example of the correction function according to a CQI report value. It is a block diagram which shows the structural example of the wireless base station which concerns on embodiment.

Explanation of symbols

10 radio base station 11 antenna 13 reception RF unit 14 CQI reception unit 15 synchronization unit 16 speed detection unit 20 CQI correction unit 21 scheduler 22 transmission signal generation unit 23 transmission buffer 24 monitoring unit 25 transmission RF unit

Claims (11)

A feedback report receiver for receiving a feedback report from the mobile station;
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 scheduler that performs transmission allocation control based on the corrected feedback report value;
A radio base station comprising:   The radio base station according to claim 1, wherein the correction unit corrects an elapsed time from the feedback report with a moving speed of the mobile station. The radio base station according to claim 1 , wherein the correction unit corrects an elapsed time of a user whose movement speed is fast to be longer than an actual time. The radio base station according to claim 3, further comprising a speed detector that detects a moving speed of the mobile station.   The radio base station according to claim 3, wherein the radio base station receives movement speed information from the mobile station.   The radio base station 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.   The radio base station according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so that the coding rate is increased as the elapsed time from the feedback report becomes longer.   2. The radio base station according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so as to use a modulation scheme that reduces the number of multi-values as the elapsed time from the feedback report becomes longer. .   The radio base station according to claim 1, wherein the scheduler performs transmission control based on the corrected feedback report so that transmission power is increased as an elapsed time from the feedback report becomes longer.   The radio base station according to claim 1, wherein the scheduler performs control so as to increase a transmission priority of a user whose elapsed time from the feedback report is short based on the corrected feedback report. Receive a downlink feedback report from the mobile station,
Correcting the received feedback report value to decrease 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.

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