Implicit Pilot-Borne Interference Feedback for Multiuser MIMO TDD Systems

In closed-loop future time division duplex (TDD) wireless systems, where channel reciprocity is maintained, uplink (UL) channel sounding method is considered as one of the most promising feedback methods due to its bandwidth and delay efficiency. However, in TDD interference-limited systems, conventional channel sounding pilots only provide instantaneous channel state information at the transmitter (CSIT), but knowledge about the level of the interference at the receiver cannot be obtained. Therefore, a scheduler cannot optimally exploit the available degree of freedom (DoF) in order to maximize the system capacity (e.g. the downlink (DL) sum capacity). In this paper, a novel interference feedback scheme is proposed to provide the base station (BS) with implicit knowledge about the interference level received by each mobile station (MS). It is proposed to weight the uplink channel sounding pilots by the level of the received interference at each MS. The weighted uplink channel sounding pilots act as a bandwidth-efficient and delay-efficient means for providing the BS with both, CSIT and the level of interference received by each active user. The performance of a system using weighted sounding pilots is compared against a system using non- weighted sounding pilots for both greedy (maximum capacity) as well as fair scheduling criteria such as proportional fair and score based scheduling policies. It is shown that the throughput is increased while fairness is not compromised.