New results on the capacity of M-ary PPM ultra-wideband systems

In this paper, some new results on the capacity of a typical M-ary pulse position modulation (M-PPM) time hopping (TH) ultra-wideband (UWB) systems are presented. Previous results using a "pure PPM model" are proven to exaggerate the real capacity of a UWB system. Based on an extended model containing correlator and soft decision decoding, the capacity is evaluated in the single-user case and in the case of a system with asynchronous multiple user interference (MUI) when the inputs are equiprobable. It is found that only when bit- signal-to-noise ratio (bit-SNR) is high enough, larger M leads to higher capacity; and for a specific M, the optimal values of PPM time offset parameter T_d, which maximizes the capacity, are independent of bit-SNR. The influence of MUI on capacity is detrimental, especially in the case of high bit-SNR.