In the last chapter, baseband adaptive signal processing techniques were investigated for the performance improvement of a Fi-Wi system assuming an AWGN wireless channel. In high-speed wireless links, however, multipath characteristics of the wireless channel are significant. Therefore, in this and subsequent chapters, a multipath wireless channel is considered in series with the nonlinear optical channel and the combined system is investigated in a joint manner.
Noteworthy issues of the wireless channel are reviewed in Chapter 2. Multipath propagation is, of course, the major issue. The multipath propagation will result in multiple, delayed arrivals of the transmitted signal. When the symbol duration is shorter than the delay spread, this will cause intersymbol interference. Hence, the memory of the wireless channel can be defined as the number of symbols (or number of sampling intervals) the delay will spread across.1 In the Fi-Wi system, the wireless channel memory will be mixed with ROF link nonlinearity, which is difficult to handle.
The combined Fi-Wi channel is a nonlinear system with memory. This can also be modeled as a Volterra system. However, there are a few noteworthy drawbacks with the Volterra series representation of a nonlinear system with memory that needs to be characterized with input/output measurements :