Advanced Detection Techniques and Compensation of Channel Impairments

This chapter is devoted to advanced detection and channel impairments compensation techniques applicable to both wireless and optical communication systems. To better understand the principles behind advanced detection and channel impairments compensation, we start the chapter with fundamentals of detection and estimation theory including optimum receiver design in symbol error probability sense and symbol error probability derivations. In section on wireless communication systems performance, we describe different scenarios including outage probability, average error probability, and combined outage-average error probability scenarios. We also describe the moment-generating function-based approach to average error probability calculation. We also describe how to evaluate performance in the presence of Doppler spread and fading effects. In section on channel equalization techniques, after short introduction, we first describe how zero-forcing equalizers can be used to compensate for ISI, chromatic dispersion, and polarization-mode dispersion (PMD) in fiber-optics communication, ISI and multipath fading effects in wireless communications, and ISI and multipath effects in indoor optical wireless communications. After that we describe the optimum linear equalizer design in the minimum mean-square error sense as well as Wiener filtering concepts. The most relevant post-compensation techniques, applicable to both wireless and optical communications, are described next including feedforward equalizer, decision-feedback equalizer, adaptive equalizer, blind equalizers, and maximum likelihood sequence detector (MLSD) also known as Viterbi equalizer. The turbo equalization technique is postponed for Chap. 9. The next section is devoted to the relevant synchronization techniques. In section on adaptive modulation techniques, we describe how to adapt the transmitter to time-varying wireless/optical channel conditions to enable reliable and high spectral-efficient transmission. Various scenarios are described including data rate, power, code rate, error probability adaptation scenarios, as well as a combination of various adaptation strategies. In particular, variable-power variable-rate modulation techniques and adaptive coded modulation techniques are described in detail. Next, the Volterra series-based equalization to deal with fiber nonlinearities and nonlinear effects in wireless communications is described. In section on digital backpropagation, we describe how this method can be applied to deal with fiber nonlinearities, chromatic dispersion, and PMD in a simultaneous manner. In section on coherent optical detection, we describe various balanced coherent optical detection schemes for two-dimensional modulation schemes, polarization diversity and polarization demultiplexing schemes, homodyne coherent optical detection based on phase-locked loops, phase diversity receivers, as well as the dominant coherent optical detection sources including laser phase noise, polarization noise, transimpedance amplifier noise, and amplified spontaneous emission (ASE) noise. In section on compensation of atmospheric turbulence effects, we describe various techniques to deal with atmospheric turbulence including adaptive coding, adaptive coded modulation, diversity approaches, MIMO signal processing, hybrid free-space optical (FSO)-RF communication approach, adaptive optics, and spatial light modulators (SLM)-based backpropagation method. In particular, linear adaptive optics techniques are described in detail.