Receiver techniques for direct sequence spread spectrum ISM band radio LANs

This paper presents the results of an investigation into the performance of DS spread spectrum ISM band radio LANs using BPSK modulation and matched filter receivers with short spreading sequences (one sequence per symbol). Differential and quasi-coherent systems were considered. The BER against normalised delay spread was estimated using Monte-Carlo techniques for the two receiver types. The results show that the use of quasi-coherent rather than differential techniques gives a significant increase in the maximum tolerable delay spread from 0.7 to 0.9 for combining diversity. However, for selection diversity the increase is insignificant, the maximum tolerable delay spread being around 0.4. The results also show that optimum system performance in noise is achieved when the normalised delay spread is between 0.05 and 0.3. For a transmission rate of 1 Mbit/s this is equivalent to a range in delay spread from 50 ns to 300 ns. This means that these systems will work well in all indoor environments