IOSR Journal of Electronics and Communication Engineering (IOSR-JECE) e-ISSN: 2278-2834,p- ISSN: 2278-8735.Volume 9, Issue 3, Ver. III (May - Jun. 2014), PP 44-51 www.iosrjournals.org www.iosrjournals.org 44 | Page Implementation of Spread-Spectrum Techniques in Optical Comunication Bharat Vashistha 1 , Nishant Panwar 2 , Rebala Neel Reddy 3 , A. Jabeena 4 1,2,3,4, School of Electronics Engineering, VIT University, Vellore , Tamil Abstract: Method for applying spread-spectrum techniques to optical communication is presented. The interference suppression capability of spread-spectrum systems is shown to be enhanced by optical transform domain processing.Effects of jammer in DSSS communication system are demonstrated in thispaper.Several possible implementations of this system are suggested, and applications to fiber optics, laser radar, free space optical communications, and other systems are discussed. Keywords: additive white gaussian noise, orthogonal frequency division multiplexing, bit error rate I. Introduction SPREAD-SPECTRUM communication consists of transmitting a given signal by modulating the informationwith a large bandwidth, coded waveform such as a pn sequence. The transmitted signal occupies a bandwidth much larger than the information bandwidth. Such systems possess a number of special properties which distinguish them from narrowband communication techniques. A primary advantage of such systems is resistance to jamming and interference. A broad spectral bandwidth signal is more difficult to distinguish from ambient noise, which adds to the security of the channel.Spread-spectrum techniques have not been utilized in optical communication systems, despite their increasingpopularity and inherently wide bandwidths, due to a lackof effective modulation and coding methods available atoptical frequencies. We propose several methods for incorporating spread-spectrum techniques into optical communications. An architecture for optical spread- spectrum encoding is developed which performs pn code modulation on the Fourier transform of the data sequence. This eliminates the need for high-frequency (GHz) optical modulators, as required by time domain pn code systems. We also present a correlation receiver design for the optical spread-spectrum system, which is simpler than the corresponding time domain receiver Several possible implementations of these designs are suggested, as well as various applications including fiber optic communications, laser radar, optical code division multiple accessnetworks, and laser range finding. A typical direct sequence spread-spectrum communications system is shown in Fig. 1. An information sequence, S ( t ) , is modulated by a pn code sequence, c( f ) . The modulated signal is corrupted in the communicationschannel by interference, I ( t ) , and additive, almost whiteGaussian noise, n ( t ) . The corrupted signal is recovered by a matched filter containing the code sequence.The ability of a spread-spectrum system to resist jammingis determined by the processing gain, which in turn is given by the ratio of transmission bandwidth to data bandwidth. Large processing gains provide a high degree of jamming immunity. Since processing gain cannot be increased indefinitely, it is desirable to supplement the jamming resistance. This has led to the use of transform domain processing techniques. A transform domain receiver is shown in Fig. 2. The received signal is x ( r ) = S ( r ) c ( r ) , plus channel interference and noise. Filtering by the transfer function H(w)is performed by multiplication followed by inverse transformation. This real-time frequency domain multiplication has been demonstrated both theoretically and experimentally. An alternate receiver implementation replaces the matched filter by multiplication with the complex conjugate of the signal spectra in the transform domain. Transform domain-processing techniques effectivelysuppress narrow band jammers in a spread- spectrum system. The jammer may be removed by the system illustrated in Fig. 3. Input consists of the code and jammer on an RF carrier; a high power* narrowband jammer appears as an impulse in the transform domain. A gating function removes the portion of the spectrum containing the jammer. The gate output is the pn code;