“T-shape” Pilot Pattern and CFO-induced BEM Receiver for OTFS Systems with Low Pilot Overhead

Carrier frequency offset (CFO) breaks the channel sparsity, and introduces channel response spreading in delay-Doppler (DD) domain in orthogonal time frequency space (OTFS) systems. Previous pilot patterns and basis expansion model (BEM) are vulnerable to channel response spreading caused by CFO, leading to large channel estimation errors. Therefore, we design a “T-shape” pilot pattern with low pilot overhead, and propose a CFO-induced BEM (C-BEM) receiver, where the effect of CFO is integrated into BEM, robust against CFO. To the best of our knowledge, this is the first work to investigate pilot pattern and BEM channel estimation for OTFS with CFO. First, the proposed “T-shape” pilot pattern places pilots along a Doppler axis and a delay axis, to simultaneously enhance channel response spreading tracking and data interference cancellation capability. In contrast, the previous “Tail-shape” pilot pattern places pilots along a delay axis only, and cannot well handle channel response spreading and data interference cancellation, due to insufficient pilots along a Doppler axis, impairing channel estimation. Also, the proposed “T-shape” pilot pattern reduces pilot overhead by shrinking guard area, thereby achieving a lower pilot overhead than the previous “Tail-shape” pilot pattern with a large guard area. Second, the proposed C-BEM receiver introduces a CFO-induced term into BEM coefficients to track channel response spreading. This is different from the previous BEM, incapable of tracking channel response spreading. We derive an input-output relationship of the proposed C-BEM, and propose a C-BEM channel estimation method, by formulating an objective function between the received pilots and the proposed C-BEM. The CFO-induced term and BEM coefficients are estimated jointly by minimizing the formulated objective function.