The Gaussian free space optical MIMO channel with Q-ary pulse position modulation

• Published in: IEEE transactions on wireless communications Vol. 7; no. 5; pp. 1744 - 1753
• Main Authors: Letzepis, N., Holland, I., Cowley, W.
• Format: Journal Article
• Language: English
• Published: Piscataway, NJ IEEE 01.05.2008; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Applied sciences; Atmosphere; Bit error rate; Channels; Codes; Ergodic processes; Exact sciences and technology; Fluctuation; Fluctuations; Irradiance; Laser beams; Lasers; MIMO; Optical beams; Optical modulation; Optical propagation; Optical pulses; Optical telecommunications; Pulse modulation; Pulse position modulation; Scintillation; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Wideband communications; Wireless optical communications; Bit error rate; Optical telecommunication; Wireless telecommunication; random media; Scintillation; Irradiance; Pulse position modulation; Laser range finder; Coding; Optical communication; Random medium; Free space optics; Gaussian channels; MIMO system; Channel capacity; Logarithmic function; Gaussian channel; Likelihood ratio; Ergodicity; MIMO systems; Optical beam; Free space propagation; Atmospheric turbulence; Laser beam; Information theory
• ISSN: 1536-1276; 1558-2248
• DOI: 10.1109/TWC.2008.061002

The main drawback in communicating via the free space optical (FSO) channel is the detrimental effect the atmosphere has on a propagating laser beam. Atmospheric turbulence causes random fluctuations in the irradiance of the received optical laser beam, commonly referred to as scintillation. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of scintillation. In particular, the FSO multiple-input multiple-output (MIMO) channel with Q-ary pulse position modulation (QPPM) and transmit repetition under the assumption of non-ideal photodetection is analyzed in terms of its uncoded bit error rate (BER) and ergodic channel capacity. This analysis is based, in part, on the use of irradiance fluctuation samples that were obtained from a laser range experiment that was conducted in the presence of moderate turbulence conditions. Expressions are derived for the log-likelihood ratio (LLR) of a received bit, uncoded BER and ergodic capacity. Using these results it is shown that large gains are possible with the use of MIMO combined with strong coding techniques.