Spatially redundant, precision-constrained transmit precoding for mmWave LoS MIMO

Line of sight (LoS) multi-input multi-output (MIMO) systems have attractive scaling properties with increase in carrier frequency, with the potential for 100+Gbps links over 10s to 100s of meters with the reasonable form using the millimeter wave (mmWave) and terahertz (THz) bands. We propose and investigate an approach to all-digital LoS MIMO which addresses the difficulty of limited available precision for digital-to-analog converters (DACs) and analog-to-digital converters (ADCs). In order to reduce dynamic range requirements at the receiver, we consider spatially redundant transmit precoding (more transmit antennas than number of spatially multiplexed data streams) with 1-bit DACs. We introduce a novel approach for attaining the higher dynamic range required for precoding over the air (OTA), which we term OTA-DAC: the 1-bit DACs for clusters of transmit elements synthesize approximations to zero-forcing precoding across clusters. We illustrate our ideas for 64×4 and 16×4 LoS MIMO systems, comparing with a benchmark approach of 1-bit quantized ZF precoding for transmit elements evenly spaced across the aperture. Our OTA-DAC approach substantially outperforms the benchmark, and does not exhibit the error floors incurred by the latter.