Air-to-Ground NOMA Systems for the "Internet-Above-the-Clouds"

The provision of high-speed Internet access in an aircraft is mainly supported by satellite links at the time of writing, aided by links between the aircraft and the ground stations. It is anticipated that Air-To-Ground (A2G) communications between en-route aircraft and the ground stations will have a major role in providing the required quality of service, while complying with the low latency requirements of next-generation of communications networks. Non-orthogonal multiple access (NOMA) systems will increase the system throughput by allowing multiple aircraft to simultaneously communicate with the ground station, while requiring fewer resource slots. Given the limited number of orthogonal resource "slots" and the high number of aircraft to be supported, a potentially high level of interference is expected. In this contribution, we employ beamforming based on the angle of arrival of the signals and antenna arrays having multiple antenna elements, as well as a novel interference-exploiting sphere decoder, which detects the signals of the supported users, while beneficially exploiting those of the interfering users. We show that an improved performance may be achieved in both hard-input hard-output scenarios, as well as in iterative soft-input soft-output scenarios, when compared with the conventional sphere decoder, the maximum likelihood detector and the maximum a posteriori probability detector. We also characterize the complexity of the proposed receiver and evaluate its performance with the aid of bit error ratio simulations and extrinsic information transfer charts.