Chapter 5
Three-Stage Serially Concatenated Turbo Equalization1
5.1 Introduction
Turbo equalization [82] is an effective means of eliminating the channel-induced ISI imposed on the received signal. Hence, the achievable performance may approach that recorded over the non-dispersive AWGN channel, as detailed in [70] in the context of diverse turbo equalizers. When a simple rate-1 precoder is applied before the modulator, which renders the channel to appear recursive to the receiver, the attainable performance may be further improved [105, 263].
The SISO Minimum Mean Square Error (MMSE) equalizer [131], which is capable of utilizing a priori information from other SISO modules such as a SISO channel decoder, and generating extrinsic information, forms an attractive design alternative to the MAP equalizer owing to its lower computational complexity. This is particularly so for channels having long CIRs [113, 131]. The precoder can be readily integrated into the shift register model of the ISI channel [263], and may be modeled by combining its trellis with the trellis of a MAP/Soft-Output Viterbi Algorithm (SOVA)-based equalizer. However, the precoder’s trellis description cannot be directly combined with the model of a MMSE equalizer. Hence, the achievable performance of MMSE turbo equalization is potentially limited [113, 131].
EXIT [98] charts have been proposed for analyzing the convergence behavior of iterative decoding schemes, and indicate that an infinitesimally low BER may ...
