The Feasibility of Interference Alignment for Reverse TDD Systems in MIMO Cellular Networks

TL;DR

This paper investigates the feasibility of interference alignment in reverse TDD MIMO cellular networks, establishing conditions for linear IA and proposing a beamforming design that enhances DoF and sum rate.

Contribution

It derives necessary and sufficient conditions for one-shot linear interference alignment in reverse TDD MIMO systems and proposes a beamforming method to achieve these conditions.

Findings

Optimal sum degrees of freedom characterized by the conditions.

Proposed beamforming achieves larger DoF and improves sum rate.

Simulation confirms effectiveness in practical SNR regimes.

Abstract

The feasibility conditions of interference alignment (IA) are analyzed for reverse TDD systems, i.e., one cell operates as downlink (DL) but the other cell operates as uplink (UL). Under general multiple-input and multiple-output (MIMO) antenna configurations, a necessary condition and a sufficient condition for one-shot linear IA are established, i.e., linear IA without symbol or time extension. In several example networks, optimal sum degrees of freedom (DoF) is characterized by the derived necessary condition and sufficient condition. For symmetric DoF within each cell, a sufficient condition is established in a more compact expression, which yields the necessary and sufficient condition for a class of symmetric DoF. An iterative construction of transmit and received beamforming vectors is further proposed, which provides a specific beamforming design satisfying one-shot IA. Simulation results demonstrate that the proposed IA not only achieve lager DoF but also significantly improve the sum rate in the practical signal-to-noise ratio (SNR) regime.