Riding the Waves: Decoding Asynchronous Multi-User MISO via Time-Variant Zero-Forcing

Future wireless communication systems are envisioned to meet stringent latency requirements to support control applications and machine interactions. One important step in this direction is to eliminate waiting times in the protocol stack, which can be achieved by letting multiple nodes transmit the same data concurrently just when the data becomes available. This communication paradigm essentially enables wide-range broadcasting and low-latency flooding. However, imperfect transmitter synchronization may arise in such schemes, which causes signal incoherence, which in turn prohibits reception with conventional decoders. In this paper, we introduce time-variant zero-forcing (TVZF), a novel equalization technique that deals with non-stationary interferences. In doing so, TVZF relaxes the requirements for time and frequency synchronization in concurrent multi-user transmissions. We present a receiver design for asynchronous MU-MISO with IEEE 802.11g compliant signals and evaluate it with over-the-air transmissions in a WARP SDR testbed in LOS and NLOS scenarios. The relative timing and frequency offsets between concurrent transmitters are systematically tuned to assess our receiver's performance in comparison to commodity hardware. For timing offsets up to the length of the cyclic prefix and frequency offsets up to 5 kHz, our receiver achieves average frame reception rates of 97 % and 92 % for LOS and NLOS, respectively.