Limited Feedback Study In MIMO Systems

As MIMO can increase the transmission rate and make a substantial improvement in link reliability, MIMO is recognized as a key technology which can improve the spectrum efficiency, ease the tension for spectrum and provide support for high-speed services. In recent years, research focuses on the multi-user MIMO system, especially how to achieve the multi-user diversity gain. In this paper, we will study the problem of limited feedback in multi-user MIMO systems. Since the channel state information consists of the channel quality information and the channel direction information, the paper studies limited feedback of CQI and CDI respectively.For CQI , we mainly introduce 1 bit feedback scheme.1 bit feedback scheme is a scheme that each user feedback a bit information for one subchannel in a MIMO-OFDMA system to indicate whether its CQI is above the predetermined threshold. Although 1 bit feedback scheme can reduce the feedback load considerably, it will cause non-negligible system capacity loss.For CDI, the feedback load decreases mainly by using a quantified codebook.That is, a set of precoding matrices is predetermined which is known both at the transmitter and at the receiver.After the optimal matrix has been selected at the receiver, the index in the codebook of the optimal matrix is sent to the transmitter.In order to reduce the capacity loss caused by 1 bit feedback scheme, we study a kind of feedback algorithm based on multiple thresholds, including two schemes.Obviously, the feedback load of the first scheme is smaller than the feedback load of the second scheme, while the first scheme will incur delay.The numerical results show that the capacity loss caused by the multiple-thresholds feedback scheme is obviously smaller than 1 bit feedback scheme.