Optimal transmission decisions for airborne relay communications

This paper presents the optimal transmission scheme for a subscriber communicating to/through an airborne relay. This scheme provides situational awareness to each communication subscriber by considering channel variations (predicted offline and estimated in real time from relay signals), data traffic and energy consumption (subject to energy constraints for battery-operated units). The optimal decision on when and at what rate to transmit is made individually by each subscriber to maximize the expected throughput. Both cases of uplink communications (from subscriber to the relay) and relay communications (from a subscriber to the relay and back to another subscriber) are considered. For a given airborne relay orbit/trajectory, a high fidelity channel model is built (using real terrain information) to predict future channel conditions. These predicted channels are combined with instantaneous channel estimation from received signals to calculate signal-to-noise-ratio (SNR) and link rate (using analytical waveform models). By using different relay orbits, the optimal transmission decision is compared with SNR threshold-based and channel-independent random transmission schemes, and significant performance gains are demonstrated in terms of sustained data rates for communication subscribers.