Performance analysis of an optical CDMA communication system considering space-time diversity over free space optical channel

Abstract

The last decades have witnessed a spectacular progress on free space optical (FSO) communication, which is very much promising for future optical networking where installation of fiber is very much limited. The capacity of such wireless optical links are highly degraded due to atmospheric channel effects like rain, fog, snow, cloud, atmospheric turbulence and pointing error. As an attractive multiple access technique optical code division multiple access (OCDMA) based FSO communication will provide a high capacity FSO communication network. In this dissertation investigations are carried out to develop analytical models in order to evaluate the bit error rate (BER) performance and capacity of an OCDMA over FSO link in presence of the above channel limitations. Investigations are also carried out towards the development of a multiwavelength (MW) OCDMA wavelength division multiplexing (MW-OCDMA-WDM) communication system for free space applications. Primarily investigations are made to find the analytical approaches to evaluate the BER performances of an FSO system without optical encoding taking into considerations the impact of atmospheric scintillations due to refractive index variation of the optical channel with Q-ary optical PPM (Q-OPPM) with direct detection receiver using PIN and APD. Further, investigations are carried out for FSO communication system with optical level DS-CDMA encoder and Sequence Inverse Keying (SIK) optical decoder taking into account the effect of strong and weak atmospheric turbulence to find the expressions for the signal and multi access interference at the output of a SIK dual detector receiver in presence of atmospheric turbulence considering IM/DD. Expressions for signal to interference plus noise ratio (SINR) and system BER are also developed. Analytical developments are made to find the combined influence of atmospheric turbulence and pointing error on FSO OCDMA system. Performance results are numerically evaluated in terms of SINR and BER for different turbulence variance, link distance, data rate and number of simultaneous users etc. Penalty suffered by the system at a given BER of 10-9 due to turbulence and pointing error are also evaluated numerically. The capacity of an OCDMA FSO system in terms of allowable number of simultaneous users at a given BER, data rate and link distance are then determined. Clouds put an important limitation on the performance of an FSO link. Several research works are reported which are mostly based on experimental demonstrations. In this dissertation, analytical models are developed to find the signal spectrum at the output of an OCDMA FSO link considering the effect of cloud with a transfer function based on the cloud characteristics. Further analysis is extended to find the combined influence of cloud, pointing error and atmospheric turbulence on the system BER performance. Numerical BER performance results are evaluated for different cloud thickness and channel parameters. OCDMA FSO systems with diversity in transmitter and receiver are also analyzed with SIMO, MISO and MIMO configurations over the atmospheric turbulent channel with direct detection OOK receivers. Power penalty suffered by the system due to turbulence and pointing error and the improvements in receiver sensitivity and capacity enhancement due to diversity are also evaluated at a given BER of 10-9. Finally, a novel MW-OCDMA-WDM system is proposed over the turbulence optical channel to increase the capacity of the system in presence of the above channel limitations. Analytical model of MW-OCDMA-WDM system is developed and analyses are carried out to obtain the expression of signal to noise plus multi-access interference (MAI) and cross talk ratio at the receiver output along with the BER for different turbulence conditions. Performance results are numerically evaluated at a bit rate of 1 Gbps for different turbulent conditions, number of simultaneous user, link distance, number of wavelength, code length etc. Optimum system design parameters are then determined at a BER of 10-9. It is noticed that MW-OCDM-WDM hybrid scheme can be a potential candidate for future terrestrial wireless optical communication network to overcome the channel limitations imposed by turbulence, cloud, pointing error etc. It can provide a higher capacity optical network with application of path diversity using a Rake receiver. Analytical results are validated by simulated and experimental results reported in literature.