On the Block-Wise Feedback of Channel Adaptive Multi-Carrier Systems

Channel state information (CSI) is an important part of the adaptation process in orthogonal frequency division modulation (OFDM) where the modulation and coding schemes are adapted according to the channel state of each subcarrier. However, reporting the CSI for each subcarrier can result in considerable overhead. Besides coding and modulation, another scheme that affects the overall throughput of the system is the utilized automatic repeat request (ARQ) process. If hybrid ARQ (HARQ), such as chase combining, is utilized, all the transmitted bit energy can be harnessed by the receiver by combining the erroneously received code block with the consecutive copies transmitted by the ARQ process. The feedback information can be reduced by grouping the subcarriers into subbands with one feedback per subband. The subbands can further be grouped into one or several blocks with a joint CSI feedback per block. The subbands still have their own ARQ process to cope with the possible mismatch between the joint CSI and the actual subbands state. However, this raised the question of what is the best basis on which the joint CSI should be determined since there are subbands with different channel states in a block. In this paper we study the impact of different decision variables for the feedback information. We propose the use of rank ordering to find the subband state that maximizes the total throughput. Our results suggest that the use of the maximum subband CSI as the decision variable is desirable in case of fast mobility and low signal-to-interference + noise ratio (SINR) while the minimum value subband provides better performance in a high SINR region. However, in practical ranges of SINR and mobility speed, some value in between should be chosen. We also show that using block feedback achieves higher throughput than individual subband feedback when the number of feedback bits is small.