Phase noise scaling and tracking in OFDM multi-user beamforming arrays

Many-element antenna arrays, used for multi-user MIMO, are expected to be one of the cornerstone technologies for 5G wireless systems. Large arrays also offer the opportunity to average out some of the transceivers' analog imperfections, potentially enabling a lower-power implementation. In this paper we study the effect of local oscillator phase noise on beamforming MU-MIMO-OFDM systems. We show that the array does average out uncorrelated phase noise at each element. Exploiting this, we propose scaling the per-element phase noise specification proportionally to the array size, thereby maintaining constant array-level performance with lower power consumption. However, if the phase noise is entirely uncorrelated, this scaling causes a substantial degradation in the recovered signal energy. If, instead, some correlated low-frequency phase noise is introduced at each element, we show that phase noise scaling incurs no performance loss. In fact, under these conditions, a single, global pilot tracking loop can replace carrier recovery at each element. Additionally, this level of phase noise correlation eliminates the phase noise-induced channel aging effect. This type of correlation can be achieved by distributing a common reference and optimizing the bandwidth of the PLL.