Throughput calculation of an optical CDMA system using coherent modulation

Optical code division multiple access (CDMA) maps low-information-rate electrical or optical signals into high-rate optical sequences to achieve random, asynchronous multi-access. In optical CDMA, many nodes transmit simultaneously in the same frequency band. Accordingly, signals must be designed to reduce interference. In order to reduce interference among signals to acceptable levels, their bandwidth would be several orders of magnitude greater than the information bandwidth 1/T of the digital modulation; T being a bit duration. In order to do this, each bit is sub-divided into a number of binary 'chips'. The chip sequence constitutes a code that permits a bit stream broadcast on a network to be selected by means of a correlation process at the receiver destination. A large number of chip codes can be assigned to different users. The set of optical sequences essentially becomes a set of address codes or signature sequences for the network. The advantage of CDMA is that, it is a true tell-and-go protocol. When one node wants to send traffic to another node, prior coordination is required only with that node. Multiple users can simultaneously access the channel with no waiting time, in contrast to other asynchronous protocols like CSMA or CSMA/CD where each user must wait for the channel to become idle before gaining access. A CDMA network is also highly scalable and modular.