Two-Dimensional ICI-ISI Turbo Equalization for Doubly-Dispersive High-Spread Channels

We present an iterative equalization scheme that accounts for both inter-carrier interference (ICI) and inter-symbol interference (ISI) on multicarrier communication systems without a cyclic prefix (CP). We employ two equalizers based on the BCJR algorithm that process data in the two-dimensional (2D) time-frequency space; the equalizers exchange soft-bit information with each other and with a channel decoder. We generalize an existing iterative maximum likelihood equalizer (IMLE) to account for ICI-ISI and compare against it. We also use a 1D ICI-only BCJR equalizer with full CP as a baseline. We show that, in presence of moderate amounts of ICI-ISI, our equalizers improve bit error rate (BER) by exploiting Doppler and time diversity. In fact, our CP-free ICI-ISI equalizers achieve the same performance as the full CP ICI-only equalizer on some channels, which results in a data throughput gain. Furthermore, BERs as low as 10
^-6
are achieved in high-spread channels that surpass the orthogonal time-frequency space (OTFS) crystallization condition for interference-free transmission. Even when perfect channel state information (CSI) is not available, our equalizers are more robust, outperforming the IMLE despite higher CSI estimation error variance.