Extended free-space optical communications

This thesis investigates the performance of free-space optical (FSO) communication systems in a turbulent atmosphere employing optical amplifiers (OAs) to extend transmission reach and wavelength-division multiplexing (WDM) to improve capacity. This system performance is considered in the presence of amplified spontaneous emission (ASE) noise, scintillation, beam spreading, atmospheric attenuation and interchannel crosstalk. In this work, the modulation scheme used is the on-off keying non-return-to-zero and the main performance metric employed is the average bit error rate (BER). Various performance evaluation methods are used to estimate system performance. Analysis of single link, cascaded OA and WDM FSO communication systems are given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilised at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of system parameters such as the optical signal power, amplified spontaneous emission noise, optical signal to noise ratio and the average bit error rate due to scintillation. It is found that the performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. In a saturated gain preamplified system, the optimum non-adaptive decision threshold is investigated. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification although this has a cost implication. Furthermore, a saturated gain amplified WDM FSO system with a non-adaptive decision threshold is superior to a non-amplified WDM FSO system with an adaptive decision threshold

Saturation in cascaded optical amplifier free-space optical communication systems

The performance of a free-space optical (FSO) communication system in a turbulent atmosphere employing an optical amplifier (OA) cascade to extend reach is investigated. Analysis of both single and cascaded OA FSO communication links is given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilised at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of the optical signal power, noise and the average bit error rate due to scintillation for the cascade. The performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification

Performance of cascaded gain saturated and fixed gain optical amplifier FSO communication systems limited by scintillation and pointing error

A major limitation to achieving acceptable system performance in free space optical (FSO) communication systems is the presence of atmospheric turbulence (AT), which results in received signal power fluctuations. In addition to the signal power fluctuations at the receiving end, pointing errors (PEs) can also significantly impair system performance. While achieving reliable communication over long distances is desirable, it is challenging because the AT effects and PEs are more pronounced at longer communication distances. Using Monte Carlo simulation methods, the impact of amplified spontaneous emission noise, AT, PE and geometric spread on cascaded gain saturated and fixed gain optical amplifier (OA) FSO communication systems is investigated in this paper. Results are obtained for different AT regimes, normalised PE standard deviations and normalised beam widths employing non-adaptive and adaptive decision thresholding schemes at the receiver. Results obtained show that increasing the size of the receiver aperture does not have any impact on the average bit error rate (BER) when the PEs are minimal. Also, the BER performances obtained with systems using a smaller receiver aperture are better than those obtained with systems using a larger receiver aperture when PEs are significant. Results obtained also show that even though unacceptable BER performances were obtained under moderate and strong atmospheric regimes when the decision threshold of the receiver is non-adaptive, the advantage of using gain saturated OAs is still evident as gain saturated OAs performed better than fixed gain OAs when the decision threshold of the receiver is non-adaptive

Extended free-space optical communications

This thesis investigates the performance of free-space optical (FSO) communication systems in a turbulent atmosphere employing optical amplifiers (OAs) to extend transmission reach and wavelength-division multiplexing (WDM) to improve capacity. This system performance is considered in the presence of amplified spontaneous emission (ASE) noise, scintillation, beam spreading, atmospheric attenuation and interchannel crosstalk. In this work, the modulation scheme used is the on-off keying non-return-to-zero and the main performance metric employed is the average bit error rate (BER). Various performance evaluation methods are used to estimate system performance. Analysis of single link, cascaded OA and WDM FSO communication systems are given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilised at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of system parameters such as the optical signal power, amplified spontaneous emission noise, optical signal to noise ratio and the average bit error rate due to scintillation. It is found that the performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. In a saturated gain preamplified system, the optimum non-adaptive decision threshold is investigated. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification although this has a cost implication. Furthermore, a saturated gain amplified WDM FSO system with a non-adaptive decision threshold is superior to a non-amplified WDM FSO system with an adaptive decision threshold