Demonstrating distributed transmit beamforming with software-defined radios

We present a fully wireless implementation of distributed transmit beamforming using software-defined radios. Distributed beamforming is a cooperative transmission scheme whereby a number of nodes in a wireless network organize themselves into a virtual antenna array and focus their transmission in the direction of the intended receiver, potentially achieving order of magnitude improvements in energy efficiency. The main technical challenge in realizing these gains is in precisely synchronizing the radio frequency signals of the cooperating nodes. This idea has been studied extensively over the past decade, and several techniques and architectures for its practical implementation have been developed. In this work, we demonstrate our recent implementation of distributed beamforming on a standard, open-source, software-defined radio platform, where the low quality oscillators make synchronization particularly challenging. Our demonstration will consist of three cooperating transmitters sending signals that add up constructively at the receiver. Low-rate feedback packets broadcast from the receiver are employed for frequency and phase synchronization at each transmitter in completely distributed fashion.