Atmospheric channel effects on terrestrial free space optical communication links

Abstract. This paper illustrates the challenges imposed by the atmospheric channel on the design of a terrestrial laser communication link. The power loss due to scattering effect is described using the Kim/Kruse scattering model while the effect and the penalty imposed by atmospheric turbulence is highlighted by considering the bit error rate (BER) of an On-Off Keying modulated link in an optical Poisson channel. The power loss due to thick fog can measure over 100 dB/km while snow and rain result in much lower attenuation. We show that non-uniformity in the atmospheric temperature also contributes to performance deterioration due to scintillation effect. At a BER of 10-4, for a channel with a turbulence strength of>0.1, the penalty imposed by turbulence induced fading is over 20 photoelectron counts in order to achieve the same level of performance as a channel with no fading. The work reported here is part of the EU COST actions and EU projects.

Estimation of Outage Capacity for Free Space Optical Links Over I-K and K Turbulent Channels

The free space optical communication systems are attracting great research and commercial interest due to their capability of transferring data, over short distances, with high rate and security, low cost demands and without licensing fees. However, their performance depends strongly on the atmospheric conditions in the link’s area. In this work, we investigate the influence of the turbulence on the outage capacity of such a system for weak to strong turbulence channels modeled by the I-K and the K-distribution and we derive closed-form expressions for its estimation. Finally, using these expressions we present numerical results for various link cases with different turbulence conditions

Fading pdf of free-space optical communication system with pointing error

A free-space optical (FSO) laser communication system with perfect fast-tracking experiences random power fading due to atmospheric turbulence. For a FSO communication system without fast-tracking or with imperfect fast-tracking, the fading probability density function (pdf) is also affected by the pointing error. In this thesis, the overall fading pdfs of FSO communication system with pointing errors are calculated using an analytical method based on the fast-tracked on-axis and off-axis fading pdfs and the fast-tracked beam profile of a turbulence channel. The overall fading pdf is firstly studied for the FSO communication system with collimated laser beam. Large-scale numerical wave-optics simulations are performed to verify the analytically calculated fading pdf with collimated beam under various turbulence channels and pointing errors. The calculated overall fading pdfs are almost identical to the directly simulated fading pdfs. The calculated overall fading pdfs are also compared with the gamma-gamma (GG) and the log-normal (LN) fading pdf models. They fit better than both the GG and LN fading pdf models under different receiver aperture sizes in all the studied cases. Further, the analytical method is expanded to the FSO communication system with beam diverging angle case. It is shown that the gamma pdf model is still valid for the fast-tracked on-axis and off-axis fading pdfs with point-like receiver aperture when the laser beam is propagated with beam diverging angle. Large-scale numerical wave-optics simulations prove that the analytically calculated fading pdfs perfectly fit the overall fading pdfs for both focused and diverged beam cases. The influence of the fast-tracked on-axis and off-axis fading pdfs, the fast-tracked beam profile, and the pointing error on the overall fading pdf is also discussed. At last, the analytical method is compared with the previous heuristic fading pdf models proposed since 1970s. Although some of previously proposed fading pdf models provide close fit to the experiment and simulation data, these close fits only exist under particular conditions. Only analytical method shows accurate fit to the directly simulated fading pdfs under different turbulence strength, propagation distances, receiver aperture sizes and pointing errors

Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel

Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Relay-Assisted Free-Space Optical Communications

The atmospheric lightwave propagation is considerably influenced by the random variations in the refractive index of air pockets due to turbulence. This undesired effect significantly degrades the performance of free-space optical (FSO) communication systems. Interestingly, the severity of such random degradations is highly related to the range of atmospheric propagation. In this thesis, we introduce relay-assisted FSO communications as a very promising technique to combat the degradation effects of atmospheric turbulence. Considering different configurations of the relays, we quantify the outage behavior of the relay-assisted system and identify the optimum relaying scheme. We further optimize the performance of the relay-assisted FSO system subject to some power constraints and provide optimal power control strategies for different scenarios under consideration. Moreover, an application of FSO relaying technique in quantum communications is investigated. The results demonstrate impressive performance improvements for the proposed relay-assisted FSO systems with respect to the conventional direct transmission whether applied in a classical or a quantum communication channel

Capacity of and Coding for Multiple-Aperture, Wireless, Optical Communications

Thesis Supervisor: Jeffrey H. Shapiro Title: Julius A. Stratton Professor of Electrical EngineeringRefractive index turbulence causes random power fluctuations in optical communication systems, making communication through the atmosphere diffcult. This same phenomenon makes the stars twinkle at night, and pavement shimmer on a hot sum- mer day. True to the old adage, "don't put all your eggs in one basket," we examine laser communication systems that use multiple transmit and receive apertures. These apertures provide redundant replicas of the transmitted message to the receiver, each corrupted separately by the atmosphere. Reliable communication occurs when not all of these paths are deeply faded. We quantify the maximum rate of reliable communication, or capacity, and study space-time coding techniques for both direct- and coherent-detection receivers. We also experimentally verify the performance of some simple techniques for optically-preamplified, direct-detection receivers.Defense Advanced Research Projects Agenc