On Managing Interferences under Heterogeneous CSIT Feedback for Multi-User Transmission

Multi-user transmission, enabled through spatial multiplexing, is a promising technique in wireless systems by sending information to more than one user on a common resource block. Although efficient, precise channel information is needed in order to minimize the possible interferences. Motivated by its importance, the study investigates interference management in multi-user downlink systems under heterogeneous feedback of channel state information to the transmitter (CSIT). In the considered scenario, the base station (BS) can obtain different levels of channel realization information from the users under multi-user transmission. In particular we focus on a generic scenario with two user equipments (UEs), one (UE-1) being able to provide a perfect CSIT to the BS and the other (UE-2) unable to, where an effective transmission strategy at the BS is proposed with the premise of mitigating interference at UE-1. Unfortunately the absence of CSIT for UE-2 pushes it in interference-limited regime. To this end, two approaches are considered in the study: one by developing additional spatial diversity, and the other via advanced receiver design at UE-2. We investigate both of the options and show that the rate of UE-2 gets significantly improved in both cases. However hardware and RF design constraints may make the advanced receiver design solution preferable over the spatial diversity solution. We then put this heterogeneous CSIT feedback system in the context of long term evolution (LTE) of third generation partnership project (3GPP) where interference cannot be nulled out even for UE-1 due to the feedback of low level quantized CSIT and the restriction of the use of a very limited set of beamformers. The paper demonstrates that the advanced interference-aware receiver design can bring significant gains in the considered scenario, thereby underlining the necessity of intelligent receiver-processing in modern wireless systems.