Spatially Sparse Precoding in Millimeter Wave MIMO Systems

TLDR

Millimeter‑wave links suffer extreme pathloss, so large antenna arrays and beamforming/precoding are required, yet digital baseband processing is costly, making low‑cost RF analog solutions attractive and limiting the set of feasible precoders. The study investigates transmit precoding and receiver combining strategies for mmWave systems equipped with large antenna arrays. By modeling the spatial sparsity of mmWave channels, the authors cast precoding/combining as a sparse reconstruction problem and use basis‑pursuit algorithms to approximate optimal unconstrained precoders and combiners suitable for low‑cost RF hardware. Numerical simulations demonstrate that the proposed algorithms enable mmWave systems to approach their unconstrained performance limits despite practical transceiver hardware constraints.

Abstract

Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications and all cellular systems. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.