Sparse Superimposed Pilot Based Channel Estimation in OTFS Systems

Traditional orthogonal time frequency space (OTFS) channel estimation schemes dedicate an entire frame or a part of the frame for accommodating pilot and guard symbols to avoid pilot-data interference, which compromises spectral efficiency. This spectral efficiency loss can be avoided using superimposed pilots, where delay-Doppler (DD) bins in the OTFS frame carries both data and pilot symbols. In this paper, we propose a sparse superimposed pilot scheme for channel estimation, where all the DD bins in a frame carry data symbols and pilot symbols are superimposed over some of them, sparsely. The proposed scheme does not suffer spectral efficiency loss due to pilot/guard symbols. It also has the advantage of more localized pilot-data interference profile that leads to better performance. We derive the minimum mean square error (MMSE) channel estimator for the proposed scheme. We obtain optimum number of pilot symbols per frame and power distribution among data and pilot symbols through simulations. Simulation results show that the proposed scheme achieves better performance at a lesser complexity compared to existing superimposed pilot scheme. An iterative scheme that further improves performance is also proposed.