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 mmWave 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.