Time-Asynchronous Robust Cooperative Transmission for the Downlink of C-RAN

TL;DR

This paper proposes a robust downlink precoding scheme for C-RAN systems that mitigates unknown time and phase offsets caused by fronthaul delays, improving transmission reliability in asynchronous environments.

Contribution

It introduces a novel robust precoding method that correlates signals with delayed versions to counteract unknown asynchronism in C-RAN downlink transmission.

Findings

Robust scheme outperforms conventional non-robust designs.

Significant improvement over non-cooperative transmission.

Effective in handling unknown time and phase offsets.

Abstract

This work studies the robust design of downlink precoding for cloud radio access network (C-RAN) in the presence of asynchronism among remote radio heads (RRHs). Specifically, a C-RAN downlink system is considered in which non-ideal fronthaul links connecting two RRHs to a Baseband Unit (BBU) may cause a time offset, as well as a phase offset, between the transmissions of the two RRHs. The offsets are a priori not known to the BBU. With the aim of counteracting the unknown time offset, a robust precoding scheme is considered that is based on the idea of correlating the signal transmitted by one RRH with a number of delayed versions of the signal transmitted by the other RRH. For this transmission strategy, the problem of maximizing the worst-case minimum rate is tackled while satisfying per-RRH transmit power constraints. Numerical results are reported that verify the advantages of the proposed robust scheme as compared to conventional non-robust design criteria as well as non-cooperative transmission.