Optical impulse modulation for indoor diffuse wireless communications

Current lasers and LEDs have far higher pulse rates than can be supported by the lowpass indoor diffuse optical wireless channel. Although high-frequency emissions are attenuated by the channel and are not detected by the receiver, a key insight of this paper is that these bands can be used to satisfy the channel non-negativity constraint. We define optical impulse modulation (OIM) in which data are confined to the lowpass region while the highpass region, which is attenuated by the channel, is used to satisfy the channel amplitude constraints. A mathematical framework for OIM is presented, and a simple suboptimal receiver filter is designed which is channel independent. Using a well-known exponential model for indoor diffuse optical channels, at a normalized delay spread of 0.2, the gain in optical average power of OIM with a simple lowpass receiver is shown to be 4.9 dBo which exceeds the gain of rectangular on-off keying (Rect-OOK) with a complex decision feedback equalizer. From an information theory point of view, at the same normalized delay spread of 0.2, the information rate of OIM with a lowpass receiver is shown to be 11.5% higher than that of Rect-OOK with a more complex whitened matched filter receiver.