Multiuser Detection for WDM Long-Haul Fiber-Optic Communication Systems

光纤链路具有每秒传输太比特的潜力。长途链路，如跨洋和横贯大陆的数据干线，已表明需要如此高的数据吞吐量。系统设计者能够使用一种称为密集波分复用（WDM）的技术提供高数据吞吐量，其中多个低速率信号通过使用几个波长的光组合到单个光纤中。长距离wdm光纤信道的性能受到线性色散和非线性串扰的限制。非线性效应在不同波长通道接收到的信号之间产生了相关性。在本研究中，开发了利用这种依赖性的多用户检测方案，并对多个信道进行联合解调。近二十年来，多用户检测在射频多用户通信领域引起了广泛的关注。它们在对抗系统中其他用户干扰的有害影响方面非常强大。通过这样做，它们允许每个用户的信道容量从单用户通信中可能增加。将这些检测器应用于WDM光纤系统，可能会有同样的优势。光纤系统的物理模型在许多方面与射频模型不同。由于光纤的非线性和光电探测器的处理，干扰和显性噪声是信号相关的，给出了一个非常不同的接收信号的统计描述。此外，调制和信号检测仅对信号的强度起作用，禁止在任何一端使用信号相位。因此，针对射频通信开发的多用户检测器不能直接用于光纤问题。在本研究中，最优和次优检测器如最大似然、线性、二次和决策反馈检测器被设计为适合光纤系统。特别注意每个设计的硬件要求，以确保它可以实现每秒10甚至40千兆比特，这是这些类型的系统所需要的。在小组相邻信道上的小型同时检测器系统允许消除最重要的非线性串扰，而不需要令人生畏的计算复杂性。探测器还必须对分辨率非常低的数据进行操作。这些检测器的性能可以使用最小距离的变量、误差概率的界限、渐近多用户效率的计算和仿真来测量。通过使用这些技术，系统设计人员将能够将光纤放大器之间的跨度长度增加到目前可能的100公里以上，通过将信道间距降低到目前20ghz的限制来增加信道密度，并将每个信道的服务质量提高到低于10- 12的错误率。

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 WDMfiber 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 toincrease 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 alsooperate 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 current20 GHz limit, and improve the quality of service of each channel to below an error-rate of 10- 12 .