This paper investigates multiple input multiple output (MIMO) transmission techniques based on realistic assumptions on feedback of channel state information. We consider three conventional techniques as the baseline: 3GPP long-term evolution (LTE) single user MIMO (SU-MIMO) based on implicit channel feedback, zero-forcing multiuser MIMO (ZF MU-MIMO) based on explicit channel feedback, and ZF MU-MIMO based on implicit channel feedback. SU-MIMO may not be able to exploit the full spatial dimension of the downlink MIMO channel. ZF MU-MIMO has the potential to improve the spectral efficiency, but the explicit channel feedback is not compatible with implicit feedback whereas implicit based ZF MU-MIMO is limited by performance and also the commonly assumed rank restriction makes it impossible to perform dynamic switching of SU/MU MIMO transmission. We propose a new hybrid scheme which enables such dynamic switching of SU/MU MIMO transmission by allowing UE to feed back the implicit channel information without any rank restriction. Computer simulation results show the benefits of the new hybrid scheme, which can properly switch to the better transmission mode in various correlation scenarios.
['Katsutoshi Kusume', 'Karim Khashaba', 'Guido Dietl', 'Wolfgang Utschick']
Hybrid Single/Multi-User MIMO Transmission Based on Implicit Channel Feedback