EP detection with banded approximation for unitary-spread OFDM in doubly selective channels

Unitary-spread OFDM, such as DFT-spread OFDM, improves robustness to frequency selectivity by spreading each data symbol across all active subcarriers. However, in doubly selective channels with fractional Doppler, inter-carrier interference (ICI) makes the effective channel difficult to diagonalize, so high-performance detection becomes computationally demanding. This paper develops a low-complexity expectation propagation (EP) detector that performs cross-domain iterative message passing between the modulation domain and the frequency domain. Exploiting the fact that fractional-Dopplerinduced ICI is largely concentrated around the main diagonal of the effective frequency-domain channel, we introduce a banded approximation and incorporate it into the EP update via banded LDU factorization. Simulation results show that the proposed banded EP substantially outperforms linear minimum meansquare error (LMMSE) and closely approaches a dense full-EP reference with a small half-bandwidth, e.g., B=2 in the considered setup, providing an attractive performance-complexity tradeoff.