Average symbol error rate analysis of reconfigurable intelligent surfaces-assisted free-space optical link over log-normal turbulence channels

Optical wireless communication (OWC) has attracted significant interest recently in academia and industry. Free-space optical (FSO) communication systems are where free space acts as a communication channel between transceivers that are line of sight (LOS) for the successful transmission of optical signals. The FSO transmissions through the atmosphere, nevertheless, bring significant challenges, besides the uncertainty of atmospheric channels, especially the signal fading due to the atmospheric turbulence, attenuation and pointing errors caused by the random beam misalignments between transceivers, signal obstruction due to buildings or trees can pre-vent the transmitted message to reach the destination. This study theoretically investigates the average symbol error rate (ASER) of reconfigurable intelligent surfaces (RIS) assisted FSO link over log-normal turbulence channels. The RIS effect is examined by considering the influence of link distance, transmitted optical power, and quadrature amplitude modulation (QAM) scheme on the ASER

Investigation on iterative multiuser detection physical layer network coding in two-way relay free-space optical links with turbulences and pointing errors

Physical layer network coding (PNC) improves the throughput in wireless networks by enabling two nodes to exchange information using a minimum number of time slots. The PNC technique is proposed for two-way relay channel free space optical (TWR-FSO) communications with the aim of maximizing the utilization of network resources. The multipair TWR-FSO is considered in this paper, where a single antenna on each pair seeks to communicate via a common receiver aperture at the relay. Therefore, chip interleaving is adopted as a technique to separate the different transmitted signals at the relay node to perform PNC mapping. Accordingly, this scheme relies on the iterative multiuser technique for detection of users at the receiver. The bit error rate (BER) performance of the proposed system is examined under the combined influences of atmospheric loss, turbulence-induced channel fading, and pointing errors (PEs). By adopting the joint PNC mapping with interleaving and multiuser detection techniques, the BER results show that the proposed scheme can achieve a significant performance improvement against the degrading effects of turbulences and PEs. It is also demonstrated that a larger number of simultaneous users can be supported with this new scheme in establishing a communication link between multiple pairs of nodes in two time slots, thereby improving the channel capacity

Performance analysis of free space optical links over turbulence and misalignment induced fading channels

The performance of free-space optical (FSO) communication system is investigated from the information theory perspective, under the influence of atmospheric loss and channel fading due to the turbulence and pointing error. The outage probability and average channel capacity of the FSO link are evaluated using a combined slow-fading channel model and a Gaussian beam wave model, unlike the limiting Rytov-based scintillation model considered in earlier work. Taking into account various link design criteria, our study reveals that lower outage probability can be achieved with larger receiver aperture due to the aperture averaging effect. Optimization of the capacity metrics is best performed through proper selection of the aperture size and beam width for known laser wavelength, particularly for long-distance links and strong turbulence scenarios. Hence, findings and results from our analysis can be treated as the design benchmark for optimal planning and deployment of optical links

Analiza i načini poboljšanja performansi bežičnih optičkih telekomunikacionih sistema u uslovima atmosferske turbulencije

Since contemporary radio-frequency (RF) communication systems are characterized by overcrowded and licensed spectrum, limited bandwidth and relatively low transmission data rates, the use of these technologies does not provide a series of new demands that future generations of telecommunications systems have to realize. Although optical fiber systems allow more bandwidth and higher transmission rates, they are characterized by complicated and expensive implementation. As one possible solution for the "last mile" problem, Free Space Optics (FSO) technology has gained importance since it provides simultaneously the optical fibre systems data rates and the flexibility of wireless communications. The optical signal at the reception is converted into an electrical one by photodiode after transmission via the atmospheric channel. Commercial FSO systems generally use intensity modulation (IM) with OOK scheme at the transmitting part of the system, while direct detection (DD) is performed at the reception. The FSO system employing IM/DD with OOK scheme and PIN photodiode at the receiver is analyzed in the thesis. The outage probability and error rate expressions are presented, which are used to examine the effect of atmospheric turbulence and pointing errors. The FSO systems with subcarrier intensity modulation (SIM) employing various modulations in electrical domain (phase shift keying (PSK), differential phase shift keying (DPSK) and quadrature amplitude modulation (QAM)) are analyzed in the continuation of the thesis. The error rate expressions are derived. The SIM-PSK and SIM- DPSK FSO system analysis is performed considering more general case when the hardware imperfections of the receiver electrical part are taken into account. The ergodic and outage capacity analysis is also presented, taking into consideration the probability of the optical signal blockage due to random obstacles. Based on the derived results, the FSO link optimization is done in order to achieve better system performance. Furthermore, the analysis of FSO systems with avalanche (APD) photodiode at the reception is presented. The bit error rate expressions for the FSO systems employing IM/DD with OOK scheme, SIM-PSK and SIM-BDPSK are derived. Based on the presented results, the APD gain optimization is performed in order to achieve minimal values of the error rate. The use of relaying technology is investigated as a method to improve system performance. The mixed RF/FSO systems with fixed and variable AF (Amplify and Forward) relays are analyzed. Beside the noise-limited scenario, the more general case, when the relay is affected by noise and interference, is also considered. The outage probability and error rate expressions are presented, which are utilized to determine the effects of the RF and FSO link conditions on system performance. The determination of a variable gain is performed based on instantaneous channel state information (CSI), which is happened to be outdated in practical scenario. Therefore, the analytical expressions for the outage probability and error rate are derived, considering the RF/FSO system with variable AF relay based on outdated CSI. Further improvement of the system performance is achieved by using diversity combining techniques in the RF domain by multiple parallel relaying. The analytical expressions for the system performance are derived for the RF/FSO system with partial relay selection based on outdated CSI. The RF/FSO system with fixed AF relays is considered, as well as the RF/FSO system with variable AF relays when the amplification gain is determined by the same outdated CSI used for relay selection. The effect of RF channel state is examined, as well as the influence of atmospheric conditions, pointing errors and FSO aperture design on the system performance. A developed simulation model is utilized to confirm the numerical results obtained by the analytical expressions presented in the dissertation thesis