Some considerations of DS-SS BPSK spectral density and error rate of a high bit rate rake

• Authors: Richard Framjee;Vasant K. Prabhu
• Venue: MILCOM'09 Proceedings of the 28th IEEE conference on Military communications
• Year: 2009

As cellular technologies evolve, evaluation of modulation schemes with frequency selective fading is an important consideration. We present methods to evaluate a high data rate DS-SS BPSK reverse link, over a wideband channel, with filters and a modified delayed signature dual finger RAKE. For several years the standard Gaussian approximation has been used to estimate error rate of DS-SS BPSK. We derive simple upper and lower bounds on the conditional error rates. The upper bound is in terms of the error rate obtained with two intersymbol interference terms and bounds on the marginal distribution of the smaller terms. The Gaussian approximation error rate is pessimistic for SNR's greater than 8dB and 12dB when compared to that obtained by the total probability theorem and upper bound respectively. Intersymbol interference is not Gaussian and at high front end signal-to-noise ratio's (SNR's) our error rate bound is a better estimate. We have also derived the bandwidth occupancy in a rigorous way for the first time by developing a method to compute the spectral density. The fractional containment bandwidth with two Gold codes is smaller than that with a PN sequence. The spectral density has no discrete lines while it is a function of the signature coefficients and chip Fourier transform. Within the bandwidth the spectral density of a set of frequencies is 15dB greater than at other frequencies and this set varies from signature to signature. Finally, we show that a modified delayed signature RAKE combats multipath interference. These methods can be used to evaluate a dual space diversity four finger RAKE and to select signatures that minimize adjacent and co-channel interference.