Evaluation of mobile ultra wideband modems in dense multipath-part 2: performance of DSSS/Rake modems

This paper presents a comparative analysis of the performance of three UWB (ultra wideband) DSSS (direct sequence spread spectrum)/Rake modems used to communicate with mobile platforms over new mobile outdoor and indoor dense multipath UWB channel models developed in a companion paper. The Rake modems analyzed differ in the techniques used to estimate the channel and are called "decision-directed" "parallel- probe" and "serial-probe" modems. The performance analysis includes the effects of time-selective distortion and noise on the estimated tap weights wherein the time constant of the tap weight measurements and the number of Rake taps are optimized. Numerical results are presented for 2 GHz bandwidth transmitted signals, a 7.5 GHz carrier frequency, and channel BPSK symbol rates of .15, 1.5, and 15 Megasymbols/sec. The well known rate 1/2, constraint length 7 convolutional code using 3 bit soft decision Viterbi decoding is assumed. One moving and one stationary platform are postulated, with a velocity of 100 ft/sec for the outdoor channel and 5 ft/sec for the indoor channel. Required values of E_s/N_o to achieve 10^-6 decoded error rate are computed as a function of multipath spread.