Subset-codebook precoding for 1-bit massive multiuser MIMO

We consider a massive multiuser MIMO downlink scenario with 1-bit digital-to-analog converters. QPSK information symbols are transmitted to the users by a channel-dependent mapping of the symbols to 1-bit DAC signals that are transmitted from each base station (BS) antenna. The mapping is selected to emulate the desired information bearing QPSK symbols at the users. Non-linear precoding is considered, where the mapping from the information symbols to the transmit antenna signals is selected based on the combination of information symbols. First, a linear quantized precoder is used to create tentative transmit symbols. For each combination of information symbols, a subset of antennas is selected, and an exhaustive search over a limited size codebook of precoding alternatives is performed. The resulting method significantly lowers the error floor caused by quantization, with a complexity that is linear in the number of transmit antennas.