1. Field of the Invention
The present invention relates to a communication system and a communication method for the same and a mobile station and a base station used for the same, and more specifically, to a communication channel quality informing system for informing a downlink communication channel quality of a plurality of different bands in downlink packet communication using a shared channel in a mobile communication system.
2. Description of the Prior Art
As a packet communication system for performing a communication in which a plurality of mobile stations share a radio band, a HSDPA (High Speed Downlink Packet Access) or a LTE (Long Term Evolution) that are standardized with the 3GPP (3rd Generation Partnership) are known. In such a packet communication system, the mobile station measures the receiving quality of a common pilot channel transmitted in a downlink (CPICH) and informs a base station of the measurement via an uplink as a communication channel quality (CQI: Channel Quality Indicator).
Correspondence between the receiving quality and the CQI is decided in advance and a plurality levels of CQIs are defined in the HSDPA. The base station uses the CQI informed from the mobile station for packet scheduling for performing allocation of occasion of transmitting packets and adaptive modulation for changing a modulating method and a coding rate for the communication channel coding. By using the communication channel quality in such a manner, a communication can be provided according to the communication channel for each mobile station; thus, a radio communication band can be efficiently used.
For the access system in the downlink of the LTE, the OFDM (Orthogonal Frequency Division Multiplexing) method is examined. In the OFDM method, a scheduler in the base station performs scheduling for allocating frequencies and time resources to mobile stations that are users in a cell to make the most of wireless capacity of the cell of the base station.
Specifically, a frequency division multiplex is realized as a wireless communication band allocated to the system is divided into a plurality of small wireless bands (PRB: Physical Resource Block) and mobile stations are allocated to the PRBs. It is assumed that relationship between the communication band of the system and the PRB is such as shown in FIG. 13. It is also assumed that the communication band of the system is divided into PRBs by the number of 24 in total of 0L to 11L and 0R to 11R.
As an example of a state of allocating frequency resources at a certain time by the above-mentioned scheduling at the base station, a mobile station (UE: User Equipment) 1 is allocated to PRBs 5L to 11L, a mobile station 2 is allocated to PRBs 0L to 4L, 0R and 1R, a mobile station 3 is allocated to PRBs 2R to 5R, and a mobile station 4 is allocated to PRBs 6R to 11R, respectively, as shown in FIG. 13.
The base station needs to know a channel state, i.e., a communication channel quality (CQI) of each PRB in order to select and allocate the PRB suitable for a mobile station. Thus, the mobile station needs to measure the CQIs relating to all the PRBs as frequently as possible and inform the measurements to the base station.
The resource allocation and the notification of CQI relating to PRB are disclosed in 3GPP TSG RAN, TR25.814 ver. 1.2.0, “Physical Layer Aspects for Evolved UTRA”, February 2006 and 3GPP TSG RAN, TR25.814 ver. 1.2.3, “Physical Layer Measurements”, May 2006.
When each mobile station informs a base station of a CQI, it needs to receive a pilot signal of each PRB, perform channel estimation on each PRB and calculate the CQI from each of the estimated channel. Thus, there is a problem in that the mobile station is required high throughput resulting in a high-loaded state.
There is another problem in that the information causes significant overhead in uplink. If there are 24 PRBs requiring 100 times of information for each second and eight bits are required for informing a CQI of one PRB, the required total bit rate in uplink is 24×100×8=19200 bits/second (bps). If there are 100 mobile stations in a cell, 100×19.2 kbps=1.92 Mbps is wasted on only informing a CQI for scheduling.
An object of the present invention is to provide a communication system, a communication method for the same and a mobile station and a base station used for the same, that can reduce a load on a mobile station or uplink overhead in informing the measurement of CQI of PRB required for scheduling or adaptive modulation.