This paper details the current and future work on a Hybrid channel access protocol being undertaken at DRA Malvern. The protocol is based on both Carrier Sense Multiple Access and Adaptive Reservation Time Division Multiple Access channel access protocols. It is to be implemented on a Packet Radio Demonstrator System currently employing a contention based CSMA protocol to access a single 25 kHz VHF channel. At high traffic Iloadings, system performance degrades due to an increase in clashes as more stations try to access the channel. The implementation of a protocol based on a time slot principle with the addition of carrier sensing within unused slots is shown to increase the channel efficiency under all traffic loading conditions and thus improve overall system performance.
Results of experiments made at 62.4 GHz in an urban mobile radio environment to characterize the coherence bandwidth are presented. The correlation coefficients between signal envelopes separated in frequency are measured and expressed as functions of distance from the base station. Due to the high variation of this coefficient with distance and in order to provide accurate estimates of the coherence bandwidth in the microcell, the correlation coefficient has to be measured over large sections. Two methods to calculate the coherence bandwidth are presented and compared. It is shown that estimating this parameter from the frequency correlation function obtained at each position may yield incorrect results. The coherence bandwidths for correlation levels of 0.5, 0.7, and 0.9 are given. A ray-tracing tool has been used to assist in interpreting experimental results.
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