Next generation mobile communication systems have been standardized aiming at providing integrated services and effective interoperations between a wired communication network and a wireless communication network, beyond a simple wireless communication service provided in the conventional mobile communication systems. To process a variety of information such as video and radio data as well as providing voice-oriented services, there is a need for a system for high-speed, large-volume communications.
In a system providing a high-speed mobility, information regarding a radio channel state of a mobile station is very important since the information is necessary to determine an adaptive modulation and coding (AMC) level for the mobile station. In addition, since there is a possibility of waste of resources allocated to the mobile station when the channel state information has an error, it is also important to provide reliable channel state information.
Hereinafter, a downlink denotes a communication link from a base station (BS) to a user equipment (UE) and an uplink denotes a communication link from the UE to the BS. In general, the BS allocates a radio resource to the UE. In the uplink, the radio resource is an uplink resource. In the downlink, the radio resource is a downlink resource. The radio resource allocated to the UE may be allocated in a distributed manner over a frequency domain or a time domain.
When data is transmitted using a radio resource distributed over the frequency domain, it is called frequency diversity. A data reception rate can be increased by distributing fading of a specific frequency band according to the frequency diversity. When data is transmitted using a radio resource distributed over the time domain, it is called time diversity. In the time diversity, the same data is transmitted several times with a specific time interval and thus a fading effect can be reduced over time, thereby improving the data reception rate.
In general, to provide communication services, one BS allocates radio resources to a plurality of UEs. In this case, each UE can have a different channel gain. Each UE may have a different frequency band with good quality. The BS can increase a data rate by effectively scheduling UEs having different channel gains. This is called multi-user diversity. A band AMC scheme is one of methods for obtaining a multi-user diversity gain. In the band AMC scheme, an assigned frequency band is divided into a plurality of bands, and in each band, a UE having a maximum channel gain is allocated with a corresponding band.
In a wireless communication system using the band AMC scheme, the UE has to transmit a channel quality indicator (CQI) to the BS. The CQI may be any one of a carrier to interference and noise ratio (CINR), a data rate indicator, and a received signal strength indicator.
A channel state is not constant but may change over time. Thus, the UE has to transmit to the BS continuously (periodically or non-periodically) the CQI for at least one band allocated to the UE itself. If CQIs of all bands are transmitted with limited radio resources, CQI feedback causes deterioration in system transmission performance.
Accordingly, there is a need for a method of transmitting more accurate channel information while reducing radio resources used for CQI feedback.