Impact of Pointing Errors on the Performance of Mixed RF/FSO Dual-Hop Transmission Systems

In this work, the performance analysis of a dual-hop relay transmission system composed of asymmetric radio-frequency (RF)/free-space optical (FSO) links with pointing errors is presented. More specifically, we build on the system model presented in [1] to derive new exact closed-form expressions for the cumulative distribution function, probability density function, moment generating function, and moments of the end-to-end signal-to-noise ratio in terms of the Meijer's G function. We then capitalize on these results to offer new exact closed-form expressions for the higher-order amount of fading, average error rate for binary and M-ary modulation schemes, and the ergodic capacity, all in terms of Meijer's G functions. Our new analytical results were also verified via computer-based Monte-Carlo simulation results.Comment: 6 pages, 3 figure

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

Multilevel Orthogonal Coded Modulation

Multilevel orthogonal coded modulation technique is a combination of orthogonal channel coding and M-ary modulation, where two or more codes are used simultaneously to detect and correct multiple errors with bandwidth efficiency. The concept, principles, and simulations of the multilevel orthogonal coding combined with M-ary modulations is presented in this thesis. In multilevel orthogonal coding, both information and orthogonal code blocks are split into multiple levels. The information data in each level are mapped to the corresponding orthogonal codes. The outputs of the encoders are then grouped to form symbols suitable for input to spectrally efficient multilevel modulations. The modulated symbols are then transmitted to the channel. At the receiver, the decoding is performed using the correlative decoder. The correlative decoding is also performed in multiple levels independently. At each level of decoding, the incoming orthogonal codes with errors are cross-correlated with the known set of orthogonal code blocks. Then the code set that gives the smallest correlation value, at each level, is chosen as the desired orthogonal code output. The implementing of multilevel encoding and decoding structure increases the error correcting capability of the system significantly, allowing multiple numbers of error corrections. Also, error correction capability of orthogonal codes improves with increasing the code lengths. Combining longer codes in multilevel structures the system shows better error performance. Additionally, the multilevel encoding outputs are suitable for performing multilevel modulation (M-ary modulation), which allows the transmissions of a large number of bits per symbol. This makes the systems to have minimum transmission bandwidths that increase the throughput. The encoding and decoding structures for three code rates: rate ݬ rate ެ and rate 1, for different orthogonal code lengths are presented and simulated. The transmission bandwidths, bandwidth efficiencies are calculated and error performance analyses of systems are conducted. M-ary Phase Shift Keying (PSK) and M-ary Quadrature Amplitude Modulation (QAM) modulation techniques are chosen as modulation techniques to simulate and analyze the performances in MATLAB. To show the application of MOCM, multilevel orthogonal coded M-ary PSK modulation transmitted in optical wireless communication systems using ambient light cancellation technology is also presented. To simulate the error performances, additive white Gaussian noise channel is used at various signal-to-noise ratios (SNR). The number of errors before decoding and after decoding is counted and tabulated. The number of bit error corrected at specified signal to noise ratio is also tabulated. Error performance curves are plotted and coding gains are observed. The results show the performance improvement of the systems compared to uncoded systems. Also, the coded systems are bandwidth efficient. Thus, the multilevel orthogonal coded modulation systems provide higher error correction capability with bandwidth efficiency

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

RECEIVER DESIGN AND PERFORMANCE ANALYSIS FOR COMMUNICATION CHANNELS WITH CARRIER PHASE NOISE AND FADING

Ph.DDOCTOR OF PHILOSOPH