Mixed Coherent and Non-Coherent Transmission for Multi-CPU Cell-Free Systems

TL;DR

This paper introduces a mixed transmission scheme for multi-CPU cell-free systems that combines coherent and non-coherent transmissions based on synchronization capabilities, improving scalability and performance.

Contribution

It proposes a novel mixed transmission strategy and extends clustering algorithms to enhance multi-CPU cell-free system performance under realistic synchronization constraints.

Findings

Mixed transmission performs close to ideal coherent transmission.

Extensions of clustering algorithms improve system scalability.

Simulation results validate the effectiveness of the proposed approach.

Abstract

Existing works on cell-free systems consider either coherent or non-coherent downlink data transmission and a network deployment with a single central processing unit (CPU). While it is known that coherent transmission outperforms noncoherent transmission when assuming unlimited fronthaul links, the former requires a perfect timing synchronization, which is practically not viable over a large network. Furthermore, relying on a single CPU for geographically large cell-free networks is not scalable. Thus, to realize the expected gains of cellfree systems in practice, alternative transmission strategies for realistic multi-CPU cell-free systems are required. Therefore, this paper proposes a novel downlink data transmission scheme that combines and generalizes the existing coherent and non-coherent transmissions. The proposed transmission scheme, named mixed transmission, works based on the realistic assumption that only the access points (APs) controlled by a same CPU are synchronized, and thus transmit in a coherent fashion, while APs from different CPUs require no synchronism and transmit in a non-coherent manner. We also propose extensions of existing clustering algorithms for multi-CPU cell-free systems with mixed transmission. Simulation results show that the combination of the proposed clustering algorithms with mixed transmission have the potential to perform close to the ideal coherent transmission.