Hybrid Beamforming with Fixed Phase Shifters for Uplink Cell-Free Millimetre-Wave Massive MIMO Systems

Several innovative ideas are being proposed by researchers to lay a foundation for future generations of wireless communications due to anticipated explosive demand for throughput, ultra-low latency, ultra-high reliability and ubiquitous coverage. However, these demands will consume huge amount of resources, especially for cell-free (CF) millimetre-wave (mm-Wave) massive multiple input multiple output systems (MIMO), which is the promising direction for the coming wireless generations. In this paper, we present hybrid beamforming scheme based on alternating minimization and a few number of fixed phase shifters to maximize energy efficiency in the uplink CF mm-Wave massive MIMO systems. Simulation results illustrate that our proposed scheme with much fewer fixed phase shifters, e.g., 10 phase shifters, achieves up to approximately 20% and 50% energy efficiency improvement compared to adaptive radio frequency (RF) chains activation/deactivation and antenna selection schemes.