In this paper, Multiple-Input Multiple-Output (MIMO) technology is applied to diffuse free-space optical (DFSO)
links. We compare the theoretical BER performance of simulated MIMO and Single-Input Single-output (SISO)
optical links in an indoor office environment. An iterative site-based simulation tool is used to determine the
impulse response of wireless infrared (IR) channels for specified locations within a room. For our purposes, we
use a MIMO 4x4 orthogonal space-time block code. Using this scheme a BER calculation is done based on
received signal power and the corresponding channel gains. By setting a BER threshold within which the system
can operate, we are able to see the coverage area provided by MIMO and SISO DFSO system architecture.
We simulate a stationary transmitter while the receiver is moved through 735 different locations in the room,
resulting in a BER contour plot of the system for a specified room. Simulation results show that by using 4-element arrays at both ends of the link, along with space-time block coding techniques, allows the effective coverage area to be increased by approximately 4 times. Also, when operating with a BER threshold of 10<sup>-3</sup>, the MIMO architecture requires up to 15dB less signal power than the SISO architecture to remain below the threshold. An optical testbed is used to begin hardware validation of our theory, both with and without optical orthogonal frequency division multiplexing (OFDM) techniques. We provide initial measurement results for the proposed optical system.
Currently, free space optical interconnect systems can be severely limited by optical crosstalk that can arise due
to unfocused systems and misalignments between transmitter and receiver elements. To address this limitation,
space-time codes, largely developed for radio frequency channels, are adapted for use in a free space optical
interconnect system. We have extended space-time coding for a 4x4 optical channel based on on-off keying
that uses real intensity-based signals. These codes improve system performance by taking advantage of the
optical crosstalk in a system composed of multiple transmitters and receivers. Data is encoded by space-time
codes based on orthogonal designs and is split into four streams that are simultaneously transmitted using four
transmitters with the same wavelength. The received signal at each of the four optical receivers is a superposition
of the transmitted signals with the addition of noise. Decision metrics are calculated making use of the received
signals and the optical path gains which are determined using channel training. These metrics, in conjunction
with maximum likelihood decoding, decouple the individual signals transmitted from different transmitters. Use
of the modified codes based on orthogonal designs allows for simple maximum likelihood decoding based on
minimum Euclidean distance. Simulated results show that our system can achieve a low BER on the order of
10<sup>-6</sup> even in case of a substantial misalignment between the transmitter and receiver.