Performance Analysis Of Wireless Optical Communication Systems Based On PPM And RS

Wireless Optical Communication (Free Space Optical Communication-FSO) has many advantages, such as good security, effective anti-interference, high data transfer rate, flexibility and convenience, and so on. It has the potential of very high data rate, with potential applications ranging from deep-space/intersatellite communication and redundant links to search-and-rescue operations and solution for"last mile"problem. FSO has attracted considerable attention recently for a variety of applications.When the signal is transmitted through the atmosphere, the atmosphere fading and atmospheric turbulence have a great influence on it, especially atmospheric turbulence. The atmospheric turbulence leads to variations of the refractive index along the transmission path. These index variations can deteriorate the quality of the received signal and can cause fluctuations in both the intensity and the phase of the received signal. These fluctuations can lead to an increase in the link error probability, limiting the performance of the communication system. Current FSO systems typically use OOK intensity modulation with direct detection (IM/DD). Although this kind of modulation realized simply, its anti-interference ability is worse. For improving the ability of anti-interference, we adopt the PPM in FSO systems. Another four modulation schemes are analyzed, including OOK modulation, DPPM modulation, DPIM modulation and MPPM modulation. Compared with the other four modulations, the two outstanding advantages of PPM are presented: higher power efficiency and extremely lower symbol error rate. The performance of these modulation schemes are compared and simulated on their signal pulse waveform, power requirement, bandwidth requirement, channel capacity and packet error rate. So the PPM modulation is more propitious to the FSO systems.RS code has the ability of correcting burst error and random error, especially the burst error, so we choose RS code as channel code method according to the characteristic of PPM channel of FSO. Using M elements RS code in M elements PPM communication systems can match properly and it can decrease the BER of the systems. Coding for background-limited direct-detection optical PPM systems is studied in this paper. The performance of RS codes using hard-decision decoding with different code rates is simulated. The relationship curve of bit error probability and signal-to-noise ratio per information bit for the uncoded and coded systems is given. The simulation results show that coding dose not necessarily improve the performances of background-limited direct-detection optical PPM systems with decreasing code rates. The optimal code rate is about 0.8-0.9.