Switching Rate and Dwell Time in M-of-N Selection Diversity

Many investigations of hybrid selection (HS) diversity assume maximal ratio combining (MRC) of the selected branches. However, a coherent detector needs to dwell on the received signal for some time before it can produce accurate channel estimates for fading compensation, a requirement that appears inconsistent with the branch switching that occurs in a selection diversity receiver. Motivated by this observation, we derive in this letter analytical results on the switching rate and average dwell time of a selection diversity receiver where M out of a total of N independent branches are selected for combining. We show that the switching rate can be many times the Doppler frequency, while the average dwell time can be a small fraction of the reciprocal Doppler frequency. The brevity of the dwell times suggests difficulty in obtaining channel state information, which in turn calls into question performance analyses of idealized HS/MRC structures. Our results also suggest that HS/MRC should be frame-based, rather than continuously acting in time