Modulation and Equalization

In this chapter we examine the performance of a number of modulation schemes for wireless infrared communication systems. At first, we neglect multipath dispersion. We review the unique characteristics of the intensity-modulation channel, which differs from a conventional linear Gaussian-noise channel in two ways: the input cannot be negative, and the average amplitude — not power — of the input is limited. We then compare the power efficiency and bandwidth efficiency of a number of modulation schemes for the intensity-modulation channel. We find that the intensity-modulation channel favors baseband modulation schemes over subcarrier and multiplesubcarrier modulation schemes, particularly those baseband modulation schemes with low duty cycles like pulse-position modulation (PPM). Unfortunately, at high data rates, the performance of PPM is severely degraded by multipath dispersion. Therefore, we also investigate several detection and equalization strategies for PPM in the presence of multipath dispersion.