(1) Field of the Invention
The present invention relates to a scheduler, a radio base station apparatus including the scheduler, and a scheduling method. The invention relates to, for example, technology suitable for use in a radio base station apparatus in a radio mobile communication system.
(2) Description of the Related Art
In a radio mobile communication system using packet communication, communication quality is monitored, and control is performed so that the transmission speed and the modulation method suitable for communication quality are adaptively selected. The reason of change in communication quality is variation in radio propagation environment, and it is often occurred that radio quality is instantly becomes lower than the required radio quality in the mobile radio system.
When radio propagation environment is deteriorated, an error rate in a radio segment becomes high. Thus, in the radio layer, methods of lowering the error rate are performed such as decreasing transfer information amount, increasing the error correction information, or using a modulation scheme high in error resilience. In addition, when the error rate in a radio segment becomes high, opportunities of transmission of NACK which indicates abnormal reception on the reception end are increased, and the turn-around of the packet is lengthened. Therefore, in the upper layer, measurement of ACK/NACK and measurement of turn around time of a packet are performed. In this manner, communication quality is monitored, thereby performing controlling of allocation of radio resources (sub-carrier frequency, etc.) to satisfy the required quality.
Further, in MAC (Media Access Control) layer or a physical layer, using a scheduler which performs allocation of radio resources, the priority in consideration of radio quality is calculated. In accordance with the priority, controlling to allocate radio resources to users is performed, so that the system use efficiency (use efficiency of radio resources) is improved. Hereinafter, a function for calculating the priority will be called an evaluation formula.
As scheduling for allocating radio resources, there is a representative algorism of Maximum Carrier-to-Interference-Ratio (MaxCIR) (see paragraph [0006] of the following patent document 1). The MaxCIR method is a method in which radio resources are always allocated to a user who is the best in instantaneous radio quality [CIR or SINR (Signal-to-Interference-plus-Noise-power-Ratio) at the time scheduling is performed. According to the MaxCIR method, since radio resources are allocated to users good in radio quality, the error rate in a radio segment is low and the system throughput becomes the maximum. As to the use efficiency of the system, the MaxCIR method realizes the highest system throughput. Hence, it is the method realizing the outstanding merit for an operator of the radio system.
However, the MaxCIR method causes a blind sector in which communication is not available, because radio resources are not allocated to users located in an area, such as a cell edge, in which radio quality is poor. Hence, in a case where plane-like service such as a cellular system is required, additional base stations for compensating for the blind sector are required.
Further, the Proportional Fairness (PF method) is also a well-known scheduling algorism (see paragraph [0007] of patent document 1 and the following non-patent document 1). According to the PF method, radio resources are allocated to users with good instantaneous radio quality with respect to average radio quality. Thus, since radio resources are allocated to all the users in a similar manner regardless of the instantaneous radio quality, fairness among the users is high. Hence, since radio resources are allocated to users located at cell edges, the number of blind sector becomes small, so that plane-like service is realized with base stations lower in number than MaxCIR.
Further, an algorism called the Generalized PF (GPF) method (see the following non-patent document 2) is also well-known. According to the GPF method, using the following formula (1), radio resources are allocated to a user having the highest result of the evaluation formula.
                    value        =                              S            ins            α                                S            ave            β                                              (        1        )            
Here, Sins indicates instantaneous radio quality; Save indicates average radio quality. The case of α=1, β=0 corresponds to the MaxCIR method, and the case of α=β=1 corresponds to the PF method. That is, the GPF method is a method in which balance between improvement and fairness of the system user efficiency can be changed by means of parameters α and β on the basis of the PF method. For example, if β is greater than a, users with lower average radio quality are allocated at high probability, so that even users whose quality is poor are allocated with high probability, and fairness of throughput among users is improved.
Here, when such fairness is assured, the priority of users poorer in radio quality are higher, and thus, the fairness is assured with low throughput, so that the system use efficiency becomes poor.
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-152630
[Non-patent Document 1] A. Jalai, R. Padovani, R. Pankaj, “Data Throughput of CDMA-HDR a High Efficiency-High Data Rate Personal Communication Wireless System”, VTC2000 Spring, May 2000
[Non-patent Document 2] Nortel Networks, “Nortel Networks' reference simulation methodology for the performance evaluation of OFDM/WCDMA in UTRAN”, R1-03-0785, 3GPP TSG RAN WG1#33, August 2003
To use radio resources effectively, although it is important to monitor communication quality so as to control transmission using appropriate transmission speed and modulation method, instantaneous radio quality exceeding the predetermined error rate can appear due to instantaneous variation. According to the previous methods, even if the instantaneous variation becomes lower than the required radio quality, radio resources are allocated when the priority (hereinafter will be called the evaluation value) of radio resource allocation calculated by the scheduler is high. Even in such a case, since the upper layer monitors and controls communication quality, the radio quality becomes stable in average. However, since the upper layer is in charge of controlling, the control delay is large. When the control delay is large, instantaneous variation of radio quality cannot be followed up. Thus, radio resources are used in vain during control delay. In addition, since NACK occurs a lot, the traffic is suppressed.
On the other hand, to realize plane-like service, a scheduling method with a coverage as large as possible needs to be employed. Although in the PF method, the system throughput is lowered since radio resources are allocated to users low in radio quality, an operator is forced to use the PF method in order to ensure the coverage of a cell.