Device-to-device assisted cooperative communications.

摘要

In a cooperative communications scenario, wireless devices assist each other in their transmissions or receptions. This cooperation has been shown to improve the communication reliability while increasing the efficiency of the spectrum usage.;In this thesis, we study how a device, which may be idle otherwise, can be leveraged to increase the spectral efficiency of the wireless system. First, we consider a two-user case where a secondary user perceives the environment and adapts its behaviour in a way that it does not degrade the performance of a primary user (also known as the "cognitive radio"). Moreover, we consider a multi-user case where single antenna devices can be clustered together to virtually form a more sophisticated device that can perform beamforming. .;In the two-user cognitive radio, under the half-duplex assumption and collision avoidance model, the primary and secondary instantaneous data rates are derived as a function of the power allocation factor by the secondary user. Also, the performance of various cooperative schemes including decode-and-forward and amplify-and-forward is investigated.;The two-user case is further extended to the multi-user case where the problems of clustering (of the source device with a subset of perhaps idle devices) and beamforming are separated and studied. For the clustering problem, a simple and agnostic greedy algorithm is shown to perform optimal. Also, two beamforming techniques are proposed: firstly, a selfish beam- forming using the feedback channel, and secondly a coordinated beamforming (also known as interference alignment) using the reciprocal channel.