Interference Management in 5G Reverse TDD HetNets With Wireless Backhaul: A Large System Analysis

TLDR

The study examines a heterogeneous network comprising a macro base station with many antennas and a dense tier of small cell access points that use wireless backhaul, where static or low‑mobility user equipment is served by the small cells and medium‑to‑high‑mobility users by the macro base station. The authors propose geographically separating co‑channel small cells to reduce uplink interference from macro‑UE transmissions. Using a reverse TDD protocol, the macro base station estimates intra‑ and inter‑tier channels and applies concatenated linear precoding—zero‑forcing or regularized zero‑forcing—to simultaneously serve macro‑UEs and small cells in the downlink while nulling uplink interference, and evaluates power consumption under target rates and imperfect CSI in the asymptotic large‑antenna regime. Large‑system analysis yields closed‑form expressions for asymptotic uplink and downlink transmit powers and precoding vectors, and numerical simulations confirm the theory and compare favorably with alternative network architectures.

Abstract

This work analyzes a heterogeneous network (HetNet), which comprises a macro base station (BS) equipped with a large number of antennas and an overlaid dense tier of small cell access points (SCAs) using a wireless backhaul for data traffic. The static and low mobility user equipment terminals (UEs) are associated with the SCAs while those with medium-to-high mobility are served by the macro BS. A reverse time division duplexing (TDD) protocol is used by the two tiers, which allows the BS to locally estimate both the intra-tier and inter-tier channels. This knowledge is then used at the BS either in the uplink (UL) or in the downlink (DL) to simultaneously serve the macro UEs (MUEs) and to provide the wireless backhaul to SCAs. A geographical separation of co-channel SCAs is proposed to limit the interference coming from the UL signals of MUEs. A concatenated linear precoding technique employing either zero-forcing (ZF) or regularized ZF is used at the BS to simultaneously serve MUEs and SCAs in DL while nulling interference toward those SCAs in UL. We evaluate and characterize the performance of the system through the power consumption of UL and DL transmissions under the assumption that target rates must be satisfied and imperfect channel state information is available for MUEs. The analysis is conducted in the asymptotic regime where the number of BS antennas and the network size (MUEs and SCAs) grow large with fixed ratios. Results from large system analysis are used to provide concise formulae for the asymptotic UL and DL transmit powers and precoding vectors under the above assumptions. Numerical results are used to validate the analysis in different settings and to make comparisons with alternative network architectures.