Testing zero-feedback distributed beamforming with a low-cost SDR testbed

Collaborative beamforming from distributed wireless transmitters has been envisioned as a means for range extension/connectivity enhancement if the signals transmitted from distributed terminals constructively add (i.e. in phase) at the destination receiver. The research community has mainly relied on feedback schemes from the final receiver towards the distributed transmitters, either in the form of pilot signals or explicit messages that coordinate the distributed transmissions and alleviate, to an extent, the frequency and time synchronization problems. This work assumes zero-feedback from destination towards the distributed transmitters, in sharp contrast to prior art. The frequency offsets among the distributed transmitters, typically undesired in classic beamforming schemes, are turned into an advantage and offer beamforming gains through signal alignment, i.e. when the transmitted signal-phasors align in time. The idea is validated in a custom, low-cost testbed, consisting of three embedded commodity radio transmitters with highly inaccurate, internal oscillators and a software-defined radio (SDR) receiver. This collaborative beamforming demonstration, perhaps the first of its kind on zero-feedback from the final destination, could potentially spark interest on relevant critical applications with ultra low complexity portable radios such as in emergency radio.