Transmitted Reference Schemes for Wireless Optical Communications

On-off keying (OOK) and pulse position modulation (PPM) are widely used modulation formats in wireless optical communications. Performance of OOK depends on the photon count threshold that should be adapted to channel fading, while performance of PPM degrades with pulse distortion caused by channel. Transmitted reference scheme, widely adopted in radio frequency communications, can help acquire channel information used for demodulation. This scheme is introduced to both OOK and PPM optical systems in this paper. The average signal and noise photon counts are estimated from reference sequences for OOK from which the optimal threshold is obtained. Since PPM symbols carry information in the pulse position in a non-linear form, Fourier transform translates time shift into a linear phase shift. Based on the linear regression technique, the parameter in the phase equation that is related to the information of pulse position is estimated. A noise model in the frequency domain is also presented.