Hybrid Massive MIMO Unlicensed Transmission with 1-Bit Quantization

We study the downlink beamforming schemes for the unlicensed wireless transmission in the cognitive radio (CR) Z-channel, where a massive multiple-input-multiple-output (MIMO) secondary base station (SBS) is considered with limited radio frequency (RF) chains and low-cost 1-bit digital-to-analog converters (DACs). In this paper, we design the analog beamformer based on the singular value decomposition (SVD) of the channel, and for the digital beamforming, instead of designing a linear transmit beamforming matrix, we propose to directly design the transmit signals by exploiting the concept of constructive interference, where the interference from the SBS to the primary users (PUs) is also taken into consideration. The optimization problem is then formulated based on the geometry of the modulation constellations and is shown to be non-convex. We then propose to solve the problem in two steps, where we firstly relax the constraint of 1-bit DACs, followed by the normalization in each entry of the baseband signals to satisfy the 1-bit DAC transmission. Numerical results show that the proposed scheme achieves an improved performance over the conventional downlink CR beamforming for the massive MIMO SBS with the hybrid structure and 1-bit DACs.