Employment of 2-D RLL precoder and optical hard-limiter in optical CDMA system

Abstract

In this paper, the 2-D RLL Precoder and optical hardlimiter are utilized in time-spreading wavelength-hopping optical CDMA system, which the system performance can be drastically enhanced. In optical CDMA system, the error floor effect occurs due to the adverse impact of MAI and beat noise; this will severely limit the system performance. Employing optical hardlimiter can effectively to eliminate the detrimental effect of MAI and beat noise, thus further reduce the error floor. In addition, we also utilize spatial coding scheme to construct a 2-D RLL precoding mechanism, which the free distance is larger than 1-D RLL trellis. Aside from the beat noise, some of the physical noises are also taken into consideration, which include thermal noise, shot noise, and relative intensity noise. The numerical results reveal that our proposed scheme can improve system performance significantly.Keywords: 2-D optical CDMA; carrier hopping prime code; optical hard-limiter; RLL I. 0BINTRODUCTION As the traffic load is rapidly increasing for multimedia communications, the bandwidth requirement of high-speed internetworking is drastically increasing. Since optical fiber can provide vast amount of bandwidth requirement than other transmission media, thus the access techniques to the highspeed internetworking over optical fiber are widely investigated. Optical CDMA (code division multiple access) is one of the promising technology to pave the way for high-speed internetworking, which had been extensively discussed for many applications such as local area network (LAN), fiber to the home (FTTH), optical access networking, and so on [1]- [14]. The merits of optical CDMA can be obtained with several attributes in the networks: asynchronous access capability, scalability, suitable to bursty traffic… etc [4].The 2-D time-spreading wavelength-hopping (T/W) optical CDMA systems are more preferable than conventional 1-D optical CDMA systems in terms of cardinality, code flexibility, and multiple access interference [10]- [12]. The implementation of encoder and decoder is the prerequisite to construct a 2-D optical CDMA configuration. There are different kinds of approaches to accomplish the physical implementations such as spectral slicing of supercontinuum source using thin film filtersTTFs), electro-absorption modulator (EAM) based pulse carver integrated with the continuous-wave (CW) laser, fiber Bragg grating (FBG) arrays [5]-[7]. Also, several experiments and demonstrations had been presented the performances [5]-[8].In this paper, carrier-hopping prime code (CHP) is utilized as signature sequence whose out-of-phase autocorrelation is zero and the cross-correlation function is at most one [4]. The employment of carrier hopping prime code has some advantages. First of all, increasing the code length and code weight can improve system performance but not at the expense of the cardinality. In addition, the implementation for carrierhopping prime code can be realized via optical components mentioned above; experimental results demonstrate...