Differential amplify-and-forward cooperative relaying in a software-defined radio testbed with an aerial relay node

In this work, we implement and demonstrate differential amplify-and-forward (DAF) cooperative relaying as a novel diversity technique to combat bit errors and outages common in airborne environments. By using differential modulation and demodulation, we reduce the system complexity by avoiding channel estimation which is often unreliable or computationally costly in dynamic environments with mobile nodes. We further simplify the requirements of the relay node by employing amplify-and-forward relaying where the relay amplifies the incoming signal and retransmits at a different frequency assignment. We implemented DAF relaying using the GNU Radio software platform and Universal Software Radio Peripherals (USRPs). We designed a pilot sequence specifically for differential modulation and implemented frame level synchronization which can combine frames properly at the destination node for joint decoding even when some frames are dropped either from the direct link or relay link. We verified simulation results of DAF relaying in a controlled test in an anechoic chamber and showed that cooperative relaying achieves significantly lower bit error rate compared to direct link transmission and two-hop relaying schemes. We then validated our system outdoors with an airborne relay node and demonstrated our results.