Optical Spatial Modulation

In this paper, a power and bandwidth efficient pulsed modulation technique for optical wireless (OW) communication is proposed. The scheme is called optical spatial modulation (OSM). In OSM, multiple transmit units exist where only one transmitter is active at any given time instance. The spatially separated transmit units are considered as spatial constellation points. Each unique sequence of incoming data bits is mapped to one of the spatial constellation points, i.e., activating one of the transmit units. This is the fundamental concept of the spatial modulation (SM) technique. In OW communication systems, the active transmitter radiates a certain intensity level at a particular time instance. At the receiver side, the optimal SM detector is used to estimate the active transmitter index. An overall increase in the data rate by the base 2 logarithm of the number of transmit units is achieved. The optical MIMO (multiple-input multiple-output) channel and the channel impulse response are obtained via Monte Carlo simulations by applying ray tracing techniques. It will be shown in this paper that the optical MIMO channel is highly correlated if transmitter and receiver locations are not optimized, which results in a significant power penalty. The power efficiency can be improved by increasing the number of receive units to enhance receive diversity and/or by using soft and hard channel coding techniques. Conversely, it is shown that aligning transmit and receive units creates nearly uncorrelated channel paths and results in substantial enhancements in system performance even as compared to the diversity or coding gain. The resultant aligned scheme is shown to be very efficient in terms of power and bandwidth as compared to on-off keying, pulse position modulation, and pulse amplitude modulation. In this paper also, the upper bound bit error ratios of coded and uncoded OSM are analyzed. The analytical results are validated via Monte Carlo simulations and the results demonstrate a close match.