Approximating EXIT Characteristics of Soft Demodulators in OFDM BICM-ID Systems for Time Variant Frequency Selective Fading Channels

Orthogonal Frequency Division Multiplexing (OFDM) in combination with Bit-Interleaved Coded Modulation with Iterative Decoding (BICM-ID) is known to provide very good BER performance: While OFDM decomposes a frequency selective broadband transmission channel causing intersymbol interference (ISI) into narrow band flat fading subchannels, the iterative BICM-ID scheme enables demodulation and decoding of the transmitted data at nearly optimum Bit Error Rates (BERs). To provide a stopping criterion for the iterative process, the Extrinsic Information Transfer (EXIT) chart is an efficient tool. For the use in time-varying ISI environments however it is impossible to provide EXIT characteristics of the Soft Demodulator (SDM) as these change with the channel. In this paper we describe a simple but effective and accurate method for constructing SDM characteristics for ISI environments based on a limited set of SDM characteristics prerecorded under AWGN conditions.