Comparative study of modulation and coding schemes for coherent indoor optical wireless systems

Coherent optical wireless systems can be used to realize ultra high data rate links in home/office or data center networks. When designing such a system, it is important to keep the laser power as low as possible due to both eye-safety concerns and energy efficiency requirements. In this paper, we show how the choice of both modulation and coding schemes affects the required laser power and the spectral efficiency of the system. Common multilevel modulation schemes are examined, including M-ary amplitude and phase shift keying, differential phase shift keying and quadrature amplitude modulation. We also consider convolutional and low-density parity check coding schemes. This paper shows that a spectrally efficient multi-gigabit-per-second connection can be sustained over typical indoor distances and a wide coverage area, without exceeding Class-1 laser power specifications, provided that an appropriate modulation and coding scheme is selected. Coherent optical wireless systems therefore constitute a technology of choice for future ultra-broadband green local area network implementations.