Minimum bandwidth Nyquist and root-Nyquist pulses for optical intensity channels

Indoor diffuse wireless optical intensity channels are bandwidth constrained channels in which all transmitted signals must be non-negative. In order to control the impact of inter-symbol interference (ISI) on this channel, pulse shaping is required. This paper derives the minimum bandwidth, ISI-free Nyquist pulse that satisfies the amplitude non-negativity constraint. The minimum bandwidth required is twice that of conventional electrical channels. Further, it is shown that there are no bandlimited root-Nyquist pulses satisfying the amplitude non-negativity constraint. In fact, all optical intensity root-Nyquist pulses are time-limited. Examples of optical intensity root-Nyquist pulses which have minimum fractional energy bandwidth are presented.