Fiber-optic links have the potential of terabit per second transmission. Long-haul links, such as transoceanic and transcontinental data-trunks, have demonstrated a need for such high data throughput. System designers are able to provide the high data throughput using a technique known as dense wavelength division multiplexing (WDM), where multiple lower rate signals are combined unto a single fiber by using several wavelengths of light. Long-haul WDM fiber channels are performance limited by linear dispersion and nonlinear crosstalk. The nonlinear effects create a correlation between the signals received at different wavelength channels. In this research multiuser detection schemes to exploit this dependence and jointly demodulate multiple channels are developed. Multiuser detection has attracted enormous attention in RF multiuser communications in the last twenty years. They are powerful at combating the harmful effects of interference from other users in the system. In so doing they allow the channel capacity per user to increase from that possible in single-user communications. The same advantages are suspected to persist in applying these detectors to WDM fiber systems. Many aspects of the physical model of the fiber system differ from the RF model. Because of the fiber nonlinearities and the photodetector processing, the interference and dominant noise are signal dependent, giving a very different statistical description of the received signal. Moreover, the modulation and the signal detection act only on the intensity of the signal, prohibiting the use of the signal phase at either end. Therefore, multiuser detectors developed for RF communications cannot be used directly for the fiber problem.
In this research, optimal and suboptimal detectors such as maximum likelihood, linear, quadratic, and decision feedback detectors are designed to suit fiber-optic systems. Special attention is placed on the hardware requirements of each design to ensure that it can be implemented at 10 or even 40 gigabits per second, as required for these types of systems. A system of small simultaneous detectors over small groups of adjacent channels allows the most significant nonlinear crosstalk to be cancelled without requiring a prohibitive computational complexity. Detectors must also operate on data with very coarse resolution. The performance of these detectors can be measured using variants of minimum distance, bounds on the error probability, calculations of the asymptotic multiuser efficiency, and simulation. By using these techniques, system designers will be able to increase the span length between fiber amplifiers to more than the 100 km currently possible, increase the channel density by decreasing the channel spacing to below the current 20 GHz limit, and improve the quality of service of each channel to below an error-rate of 10- 12 .
