Overview
SISO and MIMO refer to arrangements of optical transmitters and receivers. SISO systems are generally simpler but have lower data rates while MIMO systems are more complex and robust against varying atmospheric conditions. These concepts are analogous to SISO and MIMO RF systems, where multiple antennas are used instead of optical transmitters. There are also variants where spatial diversity is applied only at the transmitter or receiver, MISO and SIMO respectively.
SISO
In a SISO FSO system, one optical transmitter and one optical receiver is used for transmitting data through free space. This is generally simpler than a MIMO system, less costly and less difficult to manage, but can be more prone to atmospheric conditions such as fog or rain. This may in turn result in higher bit-error rates or communication outages.
MIMO
In a MIMO system, multiple optical transmitters and receivers are used in order to improve performance, reliability, and data rates. Multiple laser beams are sent through different spatial paths in the atmosphere, and multiple receivers are used to capture the arriving beams. These data streams are then recombined using signal processing techniques. As atmospheric conditions may affect different spatial paths in a different manner, this arrangement can be more reliable than a SISO system when atmospheric conditions like rain occur. In one simulated evaluation the Bit Error Rate (BER) was reduced from .1 with a SISO systme to 10^-16 in moderate turbulent conditions and 10^-14 in strong atmospheric turbulence conditions at a SNR of 20 dB.[2] In another simulated study it was determined that a 2×2 MIMO system can effectively compensate for heavy fog, while a 4×4 system can then provide superior performance in worse weather conditions such as rain.[3]
MIMO systems also offer higher data rates. For example from a comparabile SISO to MIMO 2×2 system a data rate increase of 128% was recorded, and a 4×4 MIMO system a 185% increase as well as a increase of recieved light intensity of .75dB.
References
[1]Dubey, Arjun, and Davinder Prakash. “Performance improvement of FSO Communication system using MIMO technique.” FOG 4.6.78209 (2019): x10-6.
https://www.researchgate.net/profile/Davinder-Chechi/publication/335260701_Performance_Improvement_of_FSO_Communication_System_using_MIMO_Technique/links/5d5bd8b192851c37636c0144/Performance-Improvement-of-FSO-Communication-System-using-MIMO-Technique.pdf
[2]Prabu, K., D. Sriram Kumar, and Reza Malekian. “BER analysis of BPSK-SIM-based SISO and MIMO FSO systems in strong turbulence with pointing errors.” Optik 125.21 (2014): 6413-6417.
https://repository.up.ac.za/bitstream/handle/2263/49347/Prabu_BER_2014.pdf?sequence=1
[3]Sangeetha, A., Nalini Sharma, and Ipsita Deb. “Feasibility Evaluation of MIMO Based FSO Links.” J. Commun. 14.3 (2019): 187-194.
https://d1wqtxts1xzle7.cloudfront.net/73125397/20190225114720463-libre.pdf
[4]Abaza, Mohamed R., et al. “MIMO techniques for high data rate free space optical communication system in log-normal channel.” 2013 The International Conference on Technological Advances in Electrical, Electronics and Computer Engineering (TAEECE). IEEE, 2013.
https://hal.univ-brest.fr/file/index/docid/823258/filename/ABAZA_TAEECE.pdf