Research On Several Techniques Of Optical Code Division Multiple Access (OCDMA) Systems

Optical code division multiple access (OCDMA) communication technique has many virtues and advantages, and becomes a hotspot in optical communication fields recently. There are some technique difficulties for OCDMA technique to achieve practicality. For the studies of OCDMA technique, it's flourishing overseas and beginning inland. In this thesis, we are working on some main techniques used in OCDMA system based on some of the precedents' research works.According to the background of OCDMA communication system, in this thesis, some main technique problems are studied from four aspects. Firstly, constructions for users' addressing codes with good correlation properties are studied. Secondly, relationship between decision threshold of optical correlation receiver and characteristic parameters of users' addressing code is investigated. Thirdly, influence of transmission characteristics of fiber channel, loss of optical elements and chirp effect of laser on system bit error rate (BER) performance is analyzed. Fourthly, design, achievement and analysis of OCDMA experimental system is carried out.The main works and innovations of this thesis are as follows:(1) Definitions of quasi optimal and strict optimal optical orthogonal codes (OOCs) are introduced. The constraint conditions of quasi optimal and strict optimal OOCs are analyzed from set theory. Based on these constraint conditions, we proposed one direct construction. With assistance design of computer, some quasi optimal and strict optimal OOCs with different code length and weight are obtained.(2) The constraint conditions of quasi optimal and strict optimal OOCs constructed by recursive method are theoretic analyzed. Based on these constraint conditions, as the initial conditions of OOCs obtained by direct construction, some quasi optimal and strict optimal OOCs with larger code length and sizes are obtained by recursive method.(3) Based on Wilson's lemma on evenly distributed differences, elementary number theory and fundamental theory of finite field, the finite field construction for strict optimal OOCs is proposed and the basic idea of this construction is investigated. The main steps and flow chart of assistance design by computer are obtained. Under Matlab compile environment, the design software of finite field construction is achieved by Matlab program languages. The detail process of constructing strict optimal OOCs is given. The correlation properties of OOCs obtained by finite field construction are simulated. The results show that the code correlation requirements are satisfied.(4) Making use of the characteristics of one-dimensional time-spreading codes, OOCs and quadratic congruent code (QCC), one construction for two-dimensional frequency-hop time-spreading (FH-TS) codes is proposed. The basic idea of this construction is investigated. Through certain instance, the process of detail design for FH-TS code is provided. The code sizes and correlation properties are analyzed and calculated.(5) Combining users' addressing codes with different correlation properties, the decision threshold of optical correlation receiver used in OCDMA system is further studied. The relationship between decision threshold and characteristic parameters of users' addressing codes is obtained. The effect of decision threshold on OCDMA system BER performance is analyzed. The simulation results indicate that the decision threshold is closely related to the correlation properties of users' addressing code. The more exact evaluation of OCDMA system BER performance can be achieved only if appropriate decision threshold is chosen.(6) According to configuration of noncoherent OCDMA system centered by star coupler, the losses included by fiber channel, optical elements (optical encoder/decoder, star coupler and fiber active connector) and the chirp effect of laser are studied theoretically. The influence of transmission characteristic of fiber (dispersion and attenuation), loss of optical elements and chirp effect of laser on OCDMA system BER performance is also analyzed. Based on the on-line compensating technique by dispersion compensator fiber (DCF), the dispersion compensate plan of OCDMA system is discussed. Combining the practical system input parameters, the numerical calculation of system BER performance is achieved. The results show that the system performance is limited by fiber dispersion when the duration of optical pulse is under picosecond level, and must take measures against fiber dispersion. In addition, the inherent loss of optical elements is also large under the condition of more active users, and must be considered in total power budget of OCDMA system.(7) OCDMA experimental system is established, the fundamental parameters of experimental system is designed. The design and physical realization of OCDMA optical encoder/decoder based on parallel fiber delay lines are given. The principle experiment of OCDMA communication techniques is carried out. The experimental results of spreading and despreading frequency of baseband signal are obtained and analyzed.