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

AVERAGE BER PERFORMANCE OF SIM-DPSK FSO SYSTEM WITH APD RECEIVER

In this paper, average bit error rate (BER) analysis of the free-space optical (FSO) system employing subcarrier intensity modulation (SIM) with differential phase-shift keying (DPSK) and avalanche photodiode (APD) receiver is presented. The atmospheric turbulence is described by the Gamma-Gamma statistical model taking the pointing errors into account. Numerical results are presented and confirmed by Monte Carlo simulations. The effects of atmospheric turbulence, pointing errors and receiver parameters on the average BER performance are observed and discussed. Based on the presented results, it is concluded that the minimum of the average BER exists for an optimal value of APD gain, which is heavily dependent on receiver noise temperature, bit rate and atmospheric conditions

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

Performance analysis of coherent FSO system with SSC receiver

This paper analyzes the performance of coherent free-space optical (FSO) system employing the switch-and-stay (SSC) dual diversity receiver. The intensity fluctuations of the optical signal are modeled by Gamma-Gamma distribution, being caused by atmospheric turbulence. In addition, pointing errors are taken into account. Novel analytical expressions for the outage probability are derived. The general scenario of unbalanced average signal-to-noise ratios (SNRs) of independent and identically distributed SSC branches is considered, which is further simplified to the balanced SNR case. The effects of various system and channel parameters are investigated and discussed. [Projekat Ministarstva nauke Republike Srbije, br. TR-32028

BER analysis of IM/DD FSO system with APD receiver over gamma-gamma turbulence

In this paper, the bit-error rate (BER) performance of intensity modulated with direct detection (IM/DD) free space optical (FSO) system using the on-off keying (OOK) and avalanche photodiode (APD) receiver is analyzed. The intensity fluctuations of the received optical signal are modeled by gamma gamma distribution, while both zero and nonzero inner scale models are observed. The total receiver noise includes APD shot noise and thermal noise. The BER expression is theoretically derived and numerical results are presented. The results illustrate the BER dependence on the turbulence strength, propagation path length, APD gain and noise temperature. [Projekat Ministarstva nauke Republike Srbije, br. TR-32028 i br. III-44006