Introduction

All-optical networks have been of immense interest to the persons involved in communication networking. Optical fiber has become the choice for the medium of transmission in most of the point-to-point links. This is evident by the fact that most of the trunk line and inter-exchange carrier lines use optical fiber. But such point-to-point links are having limitation when a multiaccess environment is considered. In that case a switch need to be present at central location and all the users are connected to this switch using point-to-point links. Since fiber has tremendous bandwidth (around 40 THz) [1], one should be able to support a large number of users in a passive optical broadcast network without any switch. Only requirement is a method to access and utilize the available bandwidth. Simplest of the broadcast optical network is star toplogy (Fig.).

Passive optical broadcast networks suffer from split loss problem. In these, a transmitted optical signal is split and directed to every user. The receiver depending on the address in the packets either drops or uses the packets. The splitting of optical power reduces the amount of optical power recieved. Since receivers need to receive a minimum amount of power (receiver sensitivity), the amount of split and hence the number of users that can be supported are limited. In order to alleviate the network from this problem, use of optical amplifier has been studied in past [2,3]. Further to utilize the large available bandwidth in the fiber, all-optical networks use TDMA, WDMA or CDMA techniques. Among these the impact of using OAs in WDMA systems has been extensively studied. But the impact of using OAs in CDMA is relatively unexplored.

  

Figure: Star topology for CDMA network

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Salehi and Brackett [4] have given that the use of hard limiter before CDMA decoders improves the performance. OA is also a nonlinear device due to gain saturation. Consequently, it will be interesting to explore the impact of OA and its non linear characterstics on the CDMA receiver performance. In this paper we have given a methodology to investigate this. In next section, we have described basic CDMA network structure. CDMA decoder and receiver for all-optical CDMA network are also discussed. The concept and model of OA is also given. Section gives cases of optical amplifier and their placement options which are considered alongwith the computational model. Section gives computed results for a typical network. Thereafter the conclusions are drawn Section .