The DoF of Network MIMO with Backhaul Delays

TL;DR

This paper investigates the impact of backhaul delays on the DoF of Network MIMO systems with imperfect CSI and proposes a robust beamforming strategy to mitigate performance degradation.

Contribution

It introduces a robust beamforming method that maximizes DoF under delayed CSI conditions in Network MIMO, addressing a key challenge in practical systems.

Findings

Delay significantly degrades system performance with existing precoding methods.

The proposed robust beamforming achieves maximal DoF despite backhaul delays.

Simulation results confirm finite SNR performance gains from the new strategy.

Abstract

We consider the problem of downlink precoding for Network (multi-cell) MIMO networks where Transmitters (TXs) are provided with imperfect Channel State Information (CSI). Specifically, each TX receives a delayed channel estimate with the delay being specific to each channel component. This model is particularly adapted to the scenarios where a user feeds back its CSI to its serving base only as it is envisioned in future LTE networks. We analyze the impact of the delay during the backhaul-based CSI exchange on the rate performance achieved by Network MIMO. We highlight how delay can dramatically degrade system performance if existing precoding methods are to be used. We propose an alternative robust beamforming strategy which achieves the maximal performance, in DoF sense. We verify by simulations that the theoretical DoF improvement translates into a performance increase at finite Signal-to-Noise Ratio (SNR) as well.