Optimal performance of coherent optical pulse CDMA systems based on code and phase synchronisation and interference cancellation

Abstract

An optimal multi-user receiver is proposed for asynchronous coherent optical pulse code division multiple access (CDMA) systems. The optical code sequences of all transmitted users are locally generated and correlated to the received signal via a proper code and phase synchronisation process. The resultant correlation signals are integrated over the bit duration and then discriminated to recover the information data in the electrical domain. Since each data bit of any intended user is interfered with by two bits from all other users, multi-stage interference cancellation is employed. By estimation of the random ingredients of all user signals, the multi-user interference is rebuilt and removed from the desired bit. The key advantage within the context of a narrow pulse optical implementation is that the proposed interference cancellation can be performed in the electrical domain. The system implementation is described and the validity of the cancellation technique is theoretically proved. A numerical evaluation for the system performance is provided, and issues with respect to the system application are discussed. The predicted performance of the system offers a theoretical basis for the consideration of optical pulse CDMA systems for use in high-capacity optical fibre networks.