Comparative Performance of Three DSSS/Rake Modems Over Mobile UWB Dense Multipath Channels

Modems built and analyzed for use on Ultra Wideband (UWB) channels generally use short pulses. Such techniques have been called "impulse radio". Another suitable class of modems employs Direct Sequence Spread Spectrum (DSSS)/Rake techniques. The latter techniques have the advantage of causing far less time domain interference with other users. This report presents a comparative analysis of the performance of three UWB DSSS/Rake modems used to communicate with mobile platforms over postulated outdoor and indoor dense multipath UWB channel models. The Wide-Sense-Stationary-Uncorrelated-Scattering (WSSUS) channel model has provided a useful tool for modeling random time variant linear channels in the four decades since its introduction. However, with the advent of UWB communications, it is necessary to replace this model which is valid for Narrowband (NB) channels. This paper develops such a replacement. The development is based upon the tapped delay line model of a linear time-variant channel, which is valid for any ratio of bandwidth-to-carrier frequency. For simplicity, the analysis assumes a dense multipath environment such as may be experienced in outdoor and indoor urban environments. The thrust of this study is to derive an UWB channel model for dense multipath that will allow a meaningful comparative modem performance evaluation rather than to develop a new precise characterization of UWB channels. Such a model should be useful in preparation of UWB communication standards.