The Optical CDMA System With Encoder/Decoder Based On Fiber Bragg Gratings

Optical code division multiple access (OCDMA) is a communication mode which introduces the code division multiple access (CDMA) technology into optical fiber communication system. Besides providing asynchronous communications access, OCDMA system has many advantages, such as high capacity, security against interception, suppression of narrow band disturbances, capability in forming variable bit rate networks and multi-rate networks. It implies a great challenge for the development of future communication networks, especially all optical networks.In comparison with radio CDMA system, OCDMA system can also be divided into many versions, such asdirect sequence (DS), frequency encoded (FE), frequency hopping (FH), hybrid direct sequence/frequencyï¿¡ hopping (DSIFH), and so on. But OCDMA system uses optical signal processing and fiber transmission, thedesign method of spread spectrum codes and the technique to implement will be distinct from radio CDMA. Based on the analyzing, concluding and summarizing the publications concerned, my dissertation mainly focuses on three aspects of the OCDMA system with fiber Bragg gratings (FBGs) based encoder/decoder, namely, spread spectrum codes, FBGs based encoder/decoder and performance evaluation.Firstly, the spread spectrum codes of OCDMA system are discussed. According to the characteristics of FH-OCDMA system, an upper bound on the size of spread spectrum codes with good performance in autocorrelation and cross-correlation is deduced. Then, the quadratic congruence-hop codes that achieve this bound are presented in conformity to the quadratic congruence codes. At the same time, according to the prime-hop codes for asynchronous DS/FH-OCDMA system, the modified prime-hop codes with good performance in autocorrelation and cross-correlation for synchronous DSIFH-OCDMA system are presented. The examples of the two new codes are also given.Secondly, the FBGs based encoders/decoders for all versions of OCDMA system are investigated. On the basis of recent researches on the ladder encoder/decoder based on FBGs, the possibility of this equipment used to DS, FE, FH and DS/FH versions of OCDMA system is discussed. And the FBGs based parallel encoder/decoder is put forward for the first time. Then, the principles and the optimized parameters of the systems are deduced, the first-rank scheme of FBGs based encoder/decoder is discussed, and the example of potential application is given. Compared with other encoders/decoders for OCDMA system, the FBGs based encoder/decoder is addressable and low cost, as well as it can be designed and integrated conveniently.Finally, according to the configuration of network, spread spectrum codes and FBGs based encoder/decoder for OCDMA system, the mathematical models for DS, FE, FM and DS/FH versions of OCDMA system are investigated respectively. Then, the system is simulated and analyzed. The results reveal that the OCDMA systems with FBGs based encoder/decoder are not only easy to plan, but also have large number of simultaneous users, high power efficiency and low bit error rate (B ER).Further more, the double optical hard-limiters and balance receiver with FBGs based encoder/decoder is introduced into OCDMIA system to reduce the BER of system. And a novel method is applied to calculate the precise BER caused by multiple user interference.