Multiuser diversity gain enhancement by guard time reduction

Multiuser diversity (MUD) underlies much of the recent work on scheduling design in wireless networks. This form of diversity can be exploited by choosing the receiver with the best quality in order to obtain a higher system spectral efficiency. In systems having MUD based scheduling algorithms the sender will query receivers for their channel conditions. The process of obtaining channel state information will be performed within a guard time. The length of the guard time will depend on what feedback mechanism is implemented. In this context, it has already been shown that the number of users giving feedback can be significantly decreased by applying multiple carrier-to-noise ratio thresholds. However, publications on MUD have not yet taken into account guard time reduction and possibility for contention between users that want to feed back channel state information. In this paper we reduce the guard time and resolve the feedback contention problem by proposing two different solutions: (i) a ranked feedback algorithm and (ii) a tailored version of the exponential backoff algorithm. Closed-form expressions are found for the guard time duration and the spectral efficiency for both feedback schemes. Plots show that the ranked protocol yields a better performance than the exponential backoff protocol when we have a small number of thresholds and long packet duration.