Spatial Pulse Position Modulation for Optical Communications

This paper proposes a low complexity spatial modulation (SM) scheme that combines spatial shift keying (SSK) with pulse position modulation (PPM) for optical wireless communication systems. SM is a multi-transmitter technique for achieving increased data rate over the traditional on-off keying (OOK) and PPM signalling methods. Analysis of the error performance of the system in the presence of noise is presented and validated via simulations. There is a perfect agreement between the simulation and the theoretical analysis for the case of M = 2 bits/symbol and other values of M at symbol error rate (SER) of less than 10-2. At higher SER values the analytical prediction is about 1.2 dB more than that of the simulation. We also show the energy efficiency/bandwidth requirement trade-off involved when determining the system parameters such as the number of transmitters and the number of bits per symbol M. Using fewer transmitters improves the energy efficiency but requires more bandwidth. Moreover, the error performance of SPPM is dictated by both the individual channel gains of the multiple transmitters and the difference between these channel gains or path losses. Hence, distinct channel gains are a prerequisite in spatial modulation. An experimental set up to measure and show the dependence of the channel gains on the relative position of the transmitter to the receiver is also presented. These measured channel parameters are then used to evaluate the system error performance. The performance of the SPPM is also compared, in terms of energy and spectral efficiencies, with the classical SSK and repetition coded (RC) schemes in which the multiple transmitters are used to transmit the same data simultaneously. The results show the SPPM as a multi-transmitter signalling scheme that combines the energy efficiency of the PPM with the high spectral efficiency of the SSK.