[Paper Review] Interference Management in 5G Reverse TDD HetNets with Wireless Backhaul: A Large System Analysis

Luca Sanguinetti, Aris L. Moustakas+1 more|arXiv (Cornell University)|Jul 24, 2014

Advanced MIMO Systems Optimization25 references10 citations

TL;DR

This paper proposes a reverse TDD protocol in a 5G heterogeneous network with a massive MIMO macro base station and small cells using wireless backhaul. By leveraging channel reciprocity and large-system analysis, it designs concatenated RZF precoding to manage inter-tier and intra-tier interference, achieving low UL/DL transmit power (around 6 W) and high spectral efficiency (4.8 Gb/s/km²) under imperfect CSI, with a critical CSI quality threshold beyond which performance collapses.

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.

Motivation & Objective

• To address the challenge of interference and energy efficiency in 5G heterogeneous networks with dense small cells and wireless backhaul.
• To enable simultaneous uplink and downlink transmission in macro and small cell tiers using reverse TDD for channel reciprocity.
• To design a concatenated RZF precoding scheme that satisfies rate constraints and nulls interference toward small cells under imperfect CSI for mobile users.
• To characterize the asymptotic power consumption of uplink and downlink transmissions in large-scale networks with fixed ratios of antennas and users.

Proposed method

• Employs reverse TDD between macro and small cell tiers, enabling channel reciprocity and eliminating need for CSI feedback over backhaul.
• Uses a massive MIMO base station with large-antenna array to estimate both intra-tier and inter-tier channels via reciprocity.
• Applies concatenated linear precoding with zero-forcing (ZF) or regularized ZF (RZF) at the base station to serve macro UEs and provide wireless backhaul to small cells.
• Implements MMSE receivers at the base station for uplink interference mitigation and RZF precoding for downlink rate constraints.
• Conducts large-system analysis using random matrix theory to derive deterministic equivalents for transmit powers and beamforming vectors.
• Derives closed-form expressions for asymptotic UL and DL transmit powers and beamformers under imperfect CSI for mobile users.

Experimental results

Research questions

• RQ1 What is the impact of imperfect CSI on the power consumption of a 5G HetNet with massive MIMO and wireless backhauled small cells?
• RQ2 How does reverse TDD enable interference management without explicit CSI exchange in a heterogeneous network?
• RQ3 What is the critical threshold of CSI quality beyond which the system becomes infeasible due to diverging transmit power?
• RQ4 How does the proposed RZF precoding scheme balance rate constraints and interference nulling in a large-scale network?
• RQ5 What are the asymptotic power and spectral efficiency limits of the proposed architecture under fixed ratios of antennas to users?

Key findings

• The system achieves an aggregate area spectral efficiency of 4.8 Gb/s/km² on a 10 MHz bandwidth with only 6 W of transmit power in both uplink and downlink.
• A critical threshold of CSI quality exists beyond which all transmit powers diverge, rendering the network infeasible for high-mobility users.
• The proposed concatenated RZF precoding scheme enables low-power operation for static and low-mobility users while satisfying rate constraints.
• The large-system analysis yields closed-form expressions for asymptotic transmit powers and beamforming vectors, enabling precise system design.
• The performance is robust under moderate CSI estimation errors, but degrades sharply beyond a critical error level.
• The use of reverse TDD enables implicit coordination between tiers without backhaul signaling, reducing system complexity.