TVZF: Decoding Concurrent Cooperative Transmissions with Imperfect Transmitter Synchronization

Flooding messages in wireless networks enables communication from one to many devices over multiple hops. Applications include route discovery in routing protocols, distributing media content to many devices, or sending control messages to perform coordinated transitions. To enable low-latency floods with efficient channel use, we use an approach where multiple devices are allowed to transmit simultaneously. Here, for broadband signals, the problem arises that simultaneous transmitters cause interference at the receiver, which continuously changes due to their frequency offsets, leading to reception errors with conventional decoders. We propose Time-Variant Zero-Forcing (TVZF) as a novel channel equalisation technique that resolves the strict time and frequency synchronisation requirements for simultaneous transmissions from multiple transmitters (MU-MISO), thus enabling low-latency flooding even with high signal bandwidths. To this end, we have developed a decoder for IEEE 802.11g that successfully decodes asynchronous MU-MISO transmissions even in practical testbed experiments with WARP software-defined radios at receive rates above 90%. In the video, we demonstrate the reconstruction of apparently time-variant channels at the receiver, which result from the respective specified combinations of time and frequency offsets of several simultaneous transmitters and their respective channels. In practice, TVZF-capable receivers can perform a transition to TVZF-based decoding as soon as the frequency offsets of the simultaneous transmitters exceed a certain value.

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