Multi-RF Beamforming-Based Cellular Communication Over Wideband mmWaves

The existing literature on cellular multi-user mmWave communication focus on joint baseband and RF precoding designs to enable spatial multiple access with minimum interference. These studies either assume the number of users <tex-math notation="LaTeX">M </tex-math> is less than the number of RF units <tex-math notation="LaTeX">NRF </tex-math> or schedule the users in time domain by dedicating one RF unit to each user if <tex-math notation="LaTeX">M >NRF </tex-math> . It is expected that serving multiple users over OFDMA in each analog beam will offer better utilization of the wideband channel. To this end, for the scenario with <tex-math notation="LaTeX">M NRF </tex-math> we propose a sectored-cell model that is supported by multi-RF chains over the wideband mmWave channel, with each beam serving multiple users within a sector and the sectors being scheduled in round-robin fashion. We also propose a variable time frame structure that conducts sector-wise initial access, with simultaneous access to all users within a sector. It provides improved spectrum efficiency and decreased initial access delay as compared to the initial access using exhaustive beam search method. We then jointly estimate the optimum beamwidth and optimum <tex-math notation="LaTeX">NRF </tex-math> that offer maximum average long-run user rate. Further, we introduce a reduced-complexity sector sojourn time optimization for non-homogeneously distributed users, that improves fairness of long-run user rates leveraging the variable time frame structure. The numerical results show that, while a high value of <tex-math notation="LaTeX">NRF </tex-math> causes more interference to peak data rate, the average long-run user rate improves. Additionally, using a very narrow beam is also not optimal for providing maximum rate support. The proposed beamforming method offers a higher average long-run user rate over the competitive beamforming schemes while the complexity of user scheduling is independent of <tex-math notation="LaTeX">M </tex-math> .