EXIT chart analysis for the bit-interleaved turbo frequency domain equalization under fast fading environments

Time variation of wireless fading channels introduces interference in block-based transmissions such as orthogonal frequency division multiplexing (OFDM) or single-carrier frequency domain equalization (SC-FDE). The interference, called inter-carrier interference (ICI) in OFDM or residual inter-symbol interference (ISI) in SC-FDE, impacts the bit error rate performance resulting in an error floor. In this paper, we investigate the performance of the bit-interleaved turbo frequency domain equalization (Bi-TFDE) over frequency-selective fading channels with the residual ISI. We focus on the impact of the residual ISI on the convergence signal-to-noise ratio (SNR) threshold using the extrinsic information transfer (EXIT) chart. We first show that the Gaussian assumption of the soft information within the TFDE corrupted by the residual ISI can accurately track the flow of the extrinsic information in the iterative procedure. Then, we demonstrate the accuracy of the EXIT chart in predicting the transfer function of the equalizer. Hence, the convergence behavior of the TFDE, with the residual ISI, is parameterized by the product of the symbol block length and Doppler spread. The accuracy of our EXIT chart prediction is verified by simulation.