Optical Code Division Multiple Access Systems

The optical code division multiple access (optical CDMA) systems is a versatile multiple access technique as compared to time division multiple access (TDMA) and wavelength division multiple access (WDMA). In optical CDMA, the users are assigned unique codes whereas the WDMA and TDMA systems users are assigned different wavelengths or time slots so as to distinguish the data recovered at the receiver end. The user codes in optical CDMA are unique in order to reduce the multiple access interference that appears in the user receiver due to many users those may simultaneously access the network. Hence the MAI depends on the code properties. In this paper, the optical CDMA technology is reviewed and some important codes and the results are discussed using these codes. 1. INTRODUCTION There has been an increasing demand for the higher data rates and the number of users has been increasing day-by-day. To cope up with the situation, a number of solutions have been proposed. For example, the number of users has been increased by multiplexing the data streams from many users. Further to provide variable data rates, the TDMA uses techniques such as bit-stuffing and assigning multiple time-slots. The WDMA systems assign multiple wavelengths to the nodes with larger traffic. These schemes require a strict system and network control executed through very strict protocols. Comparatively, optical CDMA systems do not require cumbersome protocols hence simplifying the networks and reducing the resources required. The optical CDMA systems are limited only by the multiple access interference (MAI) among the users that originates due to the code correlation properties which are defined and fixed for any designed code. Another advantage offered by optical CDMA in addition to requiring very simple protocols is that the network performance is not devastated if the number of users increases above the number of supported users with a guaranteed error probability which rather will operate only with a reduced system performance. The gain is that since each user has a unique code for transmission and reception, the data meant for a receiver can not be received by the other listener thus providing an inherently secure media of multiple access. Figure 1 shows a generalized optical CDMA system. 2. Code Families and Implementation of Encoders/Decoders Many codes have been proposed for implementing the encoders/decoders for the transmitters and receivers. The earlier codes were one-dimensional and were used for time-domain coding [1-2] or spectral coding …