Scintillation reduction using multi-beam propagating technique in atmospheric WOCDMA system

Abstract

We propose employing multi-beam propagating technology to mitigate the influence of atmospheric scintillation to the wireless optical code division multiple access (WOCDMA) system and then deduce the bit error rate (BER) formulas of systems in weak and strong scintillations, respectively. According to simulation experiment results, multi-beam propagation can improve the system performance very well compared with single-beam propagating technique. Moreover, the more beams we use, the better the performance we get. When the received optical power is -30 dBm, the BER of the system employing four beams is 5 and 1 dB lower than that of using single-beam propagating technique in weak and strong scintillations, respectively.OCIS , multiuser detection, and analysis of system performance [5] , among others. As an effort to weaken the influence of scintillation and improve system performance, Ohba et al.[6] proposed a symbol decision scheme, while Kozawa et al. [7] proposed a new system structure using modified pseudo orthogonal M-sequence sets. Moreover, using turbo as channel codes to improve system performance was reported [8] . Adaptive optics [9] , largeaperture receiving [10] , and multi-beam propagating [11] were proposed to mitigate the effects of scintillation and improve system performance in atmospheric laser communication. This letter employs multi-beam propagating technique to the WOCDMA system and then deduces the bit error rate (BER) formulas of systems using multi-beam propagation in weak and strong scintillations, respectively. We simulate and compare the BER of the systems using one, two, four, and eight beams, and then analyze the improvement of using multi-beam propagation to the WOCDMA system. WOCDMA system employs pulse position modulation (PPM), which does not need dynamic estimation of the receiver threshold and has higher energy information efficiency than on-off keying (OOK) modulation. This is because in atmospheric communication, systems are easily influenced by background light and scintillation. The system is composed of laser, PPM modulator, encoder, transmitting/receiving antenna, decoder, avalanche photo diode (APD) detector, and PPM demodulator. Figure 1 shows the model of the system.At the transmitter, the user's data are passed to the PPM modulator and changed into PPM symbols. The laser is then modulated by the PPM symbols to emit the optical pulses placed in one of the M time slots of the PPM frame. The pulses are encoded into their desired code sequences and sent to the atmospheric channel. In the receiver, the received signal is decoded by the decoder and converted into electrical signal by the APD. The signal is then passed to the PPM demodulator, which can decode the PPM symbol into the user's data. In the atmospheric channel, laser beam propagation is subject to the following effects: beam spread, scintillation, angular spread, absorption-induced attenuation, and depolarization. Scintillation is one of the main problems. It is generally divided into weak scintillation (logarithm ...

Mitigation of Scintillation Effects in WDM FSO System using Multibeam Technique

“…Each beam travels a different path, and thus its attenuation is different. This technique uses spatially diverse transmitters and so it is also called Spatial Diversity Technique [8], [9]. At the receiver, the beam that has undergone least attenuation is selected and processed for data extraction.…”