Analysis of Clock Distribution in User-centric Radio Access Network for Cell-Free Massive MIMO

We analyze the impact on the wireless quality of time division duplex Cell-Free massive multiple-input multiple-output (TDD CF-mMIMO) caused by phase variation in reference signals that modulate RF signals due to clock distribution over a radio access network (RAN). Phase variation in reference signals causes temporal phase variation due to temporal difference in transmission and reception in TDD. In addition, since APs are distributed among a RAN, phase variation also occurs due to clock distribution through fronthaul depending on the placement of reference clocks (RCs), which is a challenge for practical wide-area deployment. We propose RC distribution for practical user-centric RAN, considering use of IEEE 1588 or radio over fiber (RoF) technology. We analyze phase variation of reference signals and evaluate SINR of TDD CF-mMIMO by numerical simulations, revealing that clock distribution from distributed RCs with RoF secures high SINR.