Signal Alignment for Independently Distributed Precoding in CoMP and Cell-Free MIMO

Abstract

Independently distributed precoding in coordinated multi point transmission (CoMP) and cell-free multiple-input multiple-output systems–where each access point computes its precoder independently–faces the challenge of ensuring constructive superposition of signals at the receivers. While this issue is straightforward to resolve in single-antenna receiver setups, simply by compensating for the propagation phase shifts, the problem becomes non–trivial with multi-antenna receivers, as phase alignment cannot be achieved across multiple transmit–receive antenna pairs simultaneously. In this paper, we propose post-alignment methods for distributed precoding that can be applied to any standard independent precoding strategy to ensure constructive superposition of distributed transmit signals. We consider both fully independent and coordinated alignment strategies and evaluate their performance through simulations, highlighting the trade-offs between performance and overhead.