MIMO free-space optical communication employing subcarrier intensity modulation in atmospheric turbulence channels

In this paper, we analyse the error performance of transmitter/receiver array free-space optical (FSO) communication system employing binary phase shift keying (BPSK) subcarrier intensity modulation (SIM) in clear but turbulent atmospheric channel. Subcarrier modulation is employed to eliminate the need for adaptive threshold detector. Direct detection is employed at the receiver and each subcarrier is subsequently demodulated coherently. The effect of irradiance fading is mitigated with an array of lasers and photodetectors. The received signals are linearly combined using the optimal maximum ratio combining (MRC), the equal gain combining (EGC) and the selection combining (SelC). The bit error rate (BER) equations are derived considering additive white Gaussian noise and log normal intensity fluctuations. This work is part of the EU COST actions and EU projects

Analysis of visibility effects on free space earth-to-satellite optical link based on measurement data in Nigeria

The effect of atmospheric variables on signals propagating from earth to space are of concern in the design and performance of wireless  communications systems. Since atmospheric effects which led to atmospheric losses (attenuations, fading and scintillation) are location-dependent, climatological phenomena such as fog, snow, rain, and haze has much effect on Free Space Optical (FSO) and can cause reduction in the link availability. This paper analysed the effect of fog on FSO communication links from earth to satellite links over five selected locations (Ikeja, Akure, Enugu, Jos and Port-Harcourt) in Nigeria. One year (2012) visibility data obtained at Nigeria Meteorological Agency (NIMET) have been used to estimate the specific attenuation caused by fog on FSO communication links in the selected locations. Link margin was deduced in all the five locations. The estimated specific attenuation caused by fog was based on operating frequencies associated with a typical commercial FSO. The result shows that at 352 THz frequency, Ikeja recorded the highest value of about 38.60 dB/km for the specific attenuation due to fog on FSO link followed by Akure with value of about 30.33 dB/km. Enugu recorded the least estimated attenuation of about 14.19 dB/km . The implication of the result is that Ikeja and Akure will suffer more signal loss, while Enugu will suffer less signal loss. Result on link margin performed on FSO link also showed that Ikeja has the highest link margin of -109.79 dB, Port-Harcourt with link margin value of -91.99 dB while Jos experienced lowest link  margin of about -37.48 dB. Keywords: Free space optical systems; Fog induced specific attenuation; Earth-space links; Link margi

Error mitigation using RaptorQ codes in an experimental indoor free space optical link under the influence of turbulence

This paper is a postprint of a paper submitted to and accepted for publication in [journal] and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at IET Digital LibraryIn free space optical (FSO) communications, several factors can strongly affect the link quality. Among them, one of the most important impairments that can degrade the FSO link quality and its reliability even under the clear sky conditions consists of optical turbulence. In this work, the authors investigate the generation of both weak and moderate turbulence regimes in an indoor environment to assess the FSO link quality. In particular, they show that, due to the presence of the turbulence, the link experiences both erasure errors and packet losses during transmission, and also compare the experimental statistical distribution of samples with the predicted Gamma Gamma model. Furthermore, the authors demonstrate that the application of the RaptorQ codes noticeably improves the link quality decreasing the packet error rate (PER) by about an order of magnitude, also offering in certain cases an error-free transmission with a PER of ∼10−2 at Rytov variance value of 0.5. The results show that the recovery rate increases with the redundancy, the packet length and the number of source packets, and it decreases with increasing data rates.This work was supported by the European Space Agency under grant no. 5401001020. We are very grateful to Dr. E. Armandillo for enlightening discussions. This research project also falls within the frame of COST ICT Action IC1101 - Optical Wireless Communications - An Emerging Technology (OPTICWISE). J. Perez's work is supported by Spanish MINECO Juan de la Cierva JCI-2012-14805.Pernice, R.; Parisi, A.; Ando, A.; Mangione, S.; Garbo, G.; Busacca, AC.; Perez, J.... (2015). Error mitigation using RaptorQ codes in an experimental indoor free space optical link under the influence of turbulence. IET Communications. 9(14):1800-1806. https://doi.org/10.1049/iet-com.2015.0235S18001806914Tsukamoto, K., Hashimoto, A., Aburakawa, Y., & Matsumoto, M. (2009). The case for free space. IEEE Microwave Magazine, 10(5), 84-92. doi:10.1109/mmm.2009.933086Paraskevopoulos, A., Vučić, J., Voss, S.-H., Swoboda, R., & Langer, K.-D. (2010). Optical Wireless Communication Systems in the Mb/s to Gb/s Range, Suitable for Industrial Applications. IEEE/ASME Transactions on Mechatronics, 15(4), 541-547. doi:10.1109/tmech.2010.2051814Ghassemlooy, Z., Le Minh, H., Rajbhandari, S., Perez, J., & Ijaz, M. (2012). Performance Analysis of Ethernet/Fast-Ethernet Free Space Optical Communications in a Controlled Weak Turbulence Condition. Journal of Lightwave Technology, 30(13), 2188-2194. doi:10.1109/jlt.2012.2194271Ciaramella, E., Arimoto, Y., Contestabile, G., Presi, M., D’Errico, A., Guarino, V., & Matsumoto, M. (2009). 1.28-Tb/s (32 $\times$ 40 Gb/s) Free-Space Optical WDM Transmission System. IEEE Photonics Technology Letters, 21(16), 1121-1123. doi:10.1109/lpt.2009.2021149Parca, G. (2013). Optical wireless transmission at 1.6-Tbit/s (16×100  Gbit/s) for next-generation convergent urban infrastructures. Optical Engineering, 52(11), 116102. doi:10.1117/1.oe.52.11.116102Hulea, M., Ghassemlooy, Z., Rajbhandari, S., & Tang, X. (2014). Compensating for Optical Beam Scattering and Wandering in FSO Communications. Journal of Lightwave Technology, 32(7), 1323-1328. doi:10.1109/jlt.2014.2304182Ghassemlooy, Z., Popoola, W. O., Ahmadi, V., & Leitgeb, E. (2009). MIMO Free-Space Optical Communication Employing Subcarrier Intensity Modulation in Atmospheric Turbulence Channels. Communications Infrastructure. Systems and Applications in Europe, 61-73. doi:10.1007/978-3-642-11284-3_7Garcia-Zambrana, A. (2007). Error rate performance for STBC in free-space optical communications through strong atmospheric turbulence. IEEE Communications Letters, 11(5), 390-392. doi:10.1109/lcomm.2007.061980Abou-Rjeily, C. (2011). On the Optimality of the Selection Transmit Diversity for MIMO-FSO Links with Feedback. IEEE Communications Letters, 15(6), 641-643. doi:10.1109/lcomm.2011.041411.110312García-Zambrana, A., Castillo-Vázquez, C., & Castillo-Vázquez, B. (2010). Rate-adaptive FSO links over atmospheric turbulence channels by jointly using repetition coding and silence periods. Optics Express, 18(24), 25422. doi:10.1364/oe.18.025422Andò, A., Mangione, S., Curcio, L., Stivala, S., Garbo, G., Pernice, R., & Busacca, A. C. (2013). Recovery Capabilities of Rateless Codes on Simulated Turbulent Terrestrial Free Space Optics Channel Model. International Journal of Antennas and Propagation, 2013, 1-8. doi:10.1155/2013/692915MacKay, D. J. C. (2005). Fountain codes. IEE Proceedings - Communications, 152(6), 1062. doi:10.1049/ip-com:20050237Shokrollahi, A. (2006). Raptor codes. IEEE Transactions on Information Theory, 52(6), 2551-2567. doi:10.1109/tit.2006.874390Anguita, J. A., Neifeld, M. A., Hildner, B., & Vasic, B. (2010). Rateless Coding on Experimental Temporally Correlated FSO Channels. Journal of Lightwave Technology, 28(7), 990-1002. doi:10.1109/jlt.2010.2040136Wang, N., & Cheng, J. (2010). Moment-based estimation for the shape parameters of the Gamma-Gamma atmospheric turbulence model. Optics Express, 18(12), 12824. doi:10.1364/oe.18.012824Zvanovec, S., Perez, J., Ghassemlooy, Z., Rajbhandari, S., & Libich, J. (2013). Route diversity analyses for free-space optical wireless links within turbulent scenarios. Optics Express, 21(6), 7641. doi:10.1364/oe.21.007641Pernice, R., Perez, J., Ghassemlooy, Z., Stivala, S., Cardinale, M., Curcio, L., … Parisi, A. (2015). Indoor free space optics link under the weak turbulence regime: measurements and model validation. IET Communications, 9(1), 62-70. doi:10.1049/iet-com.2014.043

Povećanje informacionog kapaciteta i pouzdanosti u bežičnim optičkim komunikacijama primenom algoritama adaptivnog prenosa

This dissertation considers adaptive transmission algorithms and their influence towards increasing information capacity and system reliability. Theses algorithms are applied to optical wireless communications, as well as to radio-frequency system with diversity receiver. System of adaptive transmission is described in detail, and different algorithms of adaptation are applied: ORA, OPRA, CIFR, and TIFR. Derived probability density function combines influence of atmospheric turbulence, atmospheric losses and positioning errors that consist of laser pointing precision and jitter. FSO system without adaptation is also considered. For this case, bit-error rate and outage probability are determined when signal is exposed to different strengths of atmospheric turbulence, when there is atmospheric attenuation and positioning errors due to both main causes, when IM/DD and heterodyne detection are used. Power penalty is also determined for such system. Bit-error rate is also determined for the case when SIM is used in combination with binary modulation and quadrature amplitude modulation. Capacity analysis is given for the use of ORA, OPRA, CIFR and TIFR adaptive transmission algorithms, and for IM/DD and heterodyne detection cases. Algorithms used for transmitter power adaptation are additionally analyzed and improved with an aim to obtain realistic and functional systems. Based on analytical expression, simulation model is developed for adaptive transmission systems, which is used to validate numerical results. Approximate expressions are also given for used adaptive algorithms. Finally, adaptive transmission is considered also for a class of wireless RF systems. Fading model used is based on α-κ-μ distribution. Outage probability and amount of fading are given as representative performance measures. Besides adaptive transmission algorithms used in FSO systems, analysis of space diversity reception is also presented for the general case of using L antennas

Robust Optical Wireless Links over Turbulent Media using Diversity Solutions

Free-space optic (FSO) technology, i.e., optical wireless communication (OWC), is widely recognized as superior to radio frequency (RF) in many aspects. Visible and invisible optical wireless links solve first/last mile connectivity problems and provide secure, jam-free communication. FSO is license-free and delivers high-speed data rates in the order of Gigabits. Its advantages have fostered significant research efforts aimed at utilizing optical wireless communication, e.g. visible light communication (VLC), for high-speed, secure, indoor communication under the IEEE 802.15.7 standard. However, conventional optical wireless links demand precise optical alignment and suffer from atmospheric turbulence. When compared with RF, they suffer a low degree of reliability and lack robustness. Pointing errors cause optical transceiver misalignment, adversely affecting system reliability. Furthermore, atmospheric turbulence causes irradiance fluctuations and beam broadening of transmitted light. Innovative solutions to overcome limitations on the exploitation of high-speed optical wireless links are greatly needed.Spatial diversity is known to improve RF wireless communication systems. Similar diversity approaches can be adapted for FSO systems to improve its reliability and robustness; however, careful diversity design is needed since FSO apertures typically remain unbalanced as a result of FSO system sensitivity to misalignment. Conventional diversity combining schemes require persistent aperture monitoring and repetitive switching, thus increasing FSO implementation complexities. Furthermore, current RF diversity combining schemes may not be optimized to address the issue of unbalanced FSO receiving apertures.This dissertation investigates two efficient diversity combining schemes for multi-receiving FSO systems: switched diversity combining and generalized selection combining. Both can be exploited to reduce complexity and improve combining efficiency. Unlike maximum ratio combing, equal gain combining, and selective combining, switched diversity simplifies receiver design by avoiding unnecessary switching among receiving apertures. The most significant advantage of generalized combining is its ability to exclude apertures with low quality that could potentially affect the resultant output signal performance.This dissertation also investigates mobile FSO by considering a multi-receiving system in which all receiving FSO apertures are circularly placed on a platform. System mobility and performance are analyzed. Performance results confirm improvements when using angular diversity and generalized selection combining.The précis of this dissertation establishes the foundation of reliable FSO communications using efficient diversity-based solutions. Performance parameters are analyzed mathematically, and then evaluated using computer simulations. A testbed prototype is developed to facilitate the evaluation of optical wireless links via lab experiments

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