Efficient Multi-User Quantize-Forward Relaying in Massive MIMO HetNets

Ahmad Abu Al Haija, Ben Liang, Min Dong, Gary Boudreau

Abstract: We utilize the orthogonality and channel hardening properties of massive multiple-input multiple- output (MIMO) systems to propose an efficient uplink transmission scheme for a heterogeneous network (HetNet). Such a network consists of multiple user-equipments (UEs) communicating with a macro-cell base station (MCBS) through a small-cell BS (SCBS) where both BSs have a large number of antennas and deploy zero-forcing (ZF) detection. The SCBS helps relay UEs' information using quantize-forward (QF) relaying with Wyner-Ziv (WZ) binning and multiple-timeslot transmission for the binning indices to the MCBS. The MCBS then deploys separate and sequential decoding for each UE's message. To maximize the rate region, we optimize the quantization levels through geometric programming and further obtain the optimal transmission timeslot durations in terms of the optimal quantization. We show that the proposed scheme has linear codebook size and decoding complexity in the number of UEs, while it achieves the same rate region of other QF schemes that employ joint transmission at the SCBS and/or joint decoding at the MCBS, all of which have exponential complexity. Furthermore, simulation results show that the SCBS should employ finer quantization for UE signals that have strong UE-SCBS links compared with the UE-MCBS links, and the proposed scheme can substantially outperform several existing alternatives under a wide range of parameter settings.