Hardware-constrained signal processing for mm-Wave LoS MIMO

At high carrier frequencies, spatial multiplexing gains can be obtained even in line of sight (LoS) environments with reasonable node form factors. We investigate design of a LoS MIMO link operating at millimeter (mm) wave frequencies beyond 100 GHz, with 4-fold spatial multiplexing, and bandwidth of 10-20 GHz, with data rates even with relatively small constellations (QPSK) reaching 80-160 Gbps. Even small misalignments lead to frequency selectivity at these bandwidths, and we show that this leads to performance floors for conventional linear space-time equalizers. An alternative approach, which closely couples hardware and signal processing design in utilizing analog delays with subsymbol precision, is shown to eliminate such error floors. By using analog techniques to address frequency selectivity and to perform spatial demultiplexing, this proposed architecture also alleviates the difficulty of analog-to-digital conversion at high sampling rates.