This document studies the Power Spectral Density (PSD) of time-hopping impulse radio (TH-IR) signals. The "conventional" TH-IR system with pulse-position modulation and time-hopping multiple access gives rise to spectral lines that either violate the regulations, or require a significant power back-off. To remedy this situation, we propose the use of polarity randomization, which eliminates the spectral lines and also leads to a smoothing of the continuous part of the spectrum. We analyze the effect of symbol-based or pulse-based polarity randomization sequences on memoryloss modulation formats such as PPM, OOK and PAM. We provide a general demonstration of the spectral smoothing characteristics of these techniques without restriction on the definition of the TH sequences, and we discuss how finite sequences impact these results. |
