Two-Level Spatial Multiplexing Using Hybrid Beamforming for Millimeter-Wave Backhaul

Spatial multiplexing is an important factor usable for improving the throughput of future millimeter-wave (mm-wave) backhaul links. One conventional strategy in mm-wave multiple-input multiple-output (MIMO) systems uses densely packed antennas and exploits the spatial signature of multiple paths. Meanwhile, spatial multiplexing over a single line-of-sight (LoS) path, known as LoS MIMO communication, offers an alternative option with widely spaced antennas exploiting the phases of spherical waves. In this paper, we first show that those two conventional approaches exploit two different degrees in the channel matrices, which we denote as inter- and intra-path multiplexing, respectively. Then, we show that the two kinds of spatial multiplexing can be jointly exploited and identify their different requirements on system design. Fulfilling all requirements simultaneously, we propose a system with multiple widely spaced subarrays. With the help of analog beamforming, the intra-path multiplexing of conventional LoS MIMO systems can be introduced to other non-LoS paths owing to its robustness. Merging the two kinds of spatial multiplexing in a combined channel model and connecting it with a hybrid beamforming architecture, the proposed system achieves spatial multiplexing of a higher order than conventional ones. Simulation results for a backhaul scenario illustrate that the channel of the proposed method has higher ranks than that of conventional approaches.