ZFBF-SUS Algorithm with Delayed-Feedback for Multi-User MISO Systems

Abstract

In multiuser communication systems based on time division multiple access (TDMA), to increase total data transmission rate of all users, that is, spectral efficiency space division multiple access (SDMA) methods can be used. Also, spectral efficiency represents the spectral efficiency of the system, so SDMA methods increase the spectral efficiency. Several SDMA methods exist, but especially, ZFBF method with a greedy scheduling algorithm such as semi orthogonal user selection (SUS) can achieve asymptotically optimal spectral efficiency with low complexity. Therefore, the ZFBF SUS scheme is considered as a practical SDMA method. To achieve optimal spectral efficiency in multiuser multi-input multi output (MU MIMO) downlink systems with ZFBF SUS, channel state information (CSI) consisting of channel quality information (CQI) and channel direction information (CDI) is required to be fed back. CSI of each user indicates one channel vector. A transmitter selects the users that have high channel quality and low inter user interference. However, the channel is time varying, so the channel vector at ZFBF SUS algorithm is different from the CSI at feedback. Therefore, using ZFBF SUS algorithm without considering delayed feedback degrades spectral efficiency. To solve the problem, we proposed a feedback scheme using CSI with delayed feedback. Using channel modeling which considered delayed feedback, we estimated SINR, that is, CQI, and utilized the estimated SINR for the ZFBF SUS system. We confirmed that the proposed scheme increased spectral efficiency compared to the conventional scheme.