Maximum Likelihood Channel Path Detection and MMSE Channel Estimation in OTFS Systems
Vibhutesh Kumar Singh, Mark F. Flanagan, Barry Cardiff
Abstract
In orthogonal time frequency space (OTFS) systems, channel estimation (CE) is often performed using a pilot based approach. This is usually done in two steps: first, valid channel paths are detected by comparing the magnitude of received symbols against a threshold, and then the associated channel coefficient is estimated. In an attempt to avoid channel path misdetection, existing approaches often deploy large guard bands surrounding a relatively high-power pilot symbol. Furthermore, it is generally assumed that this guard region is sufficiently large to ensure that the time/frequency spreading of the channel does not result in any interference between the pilot and data symbols, thus facilitating simple channel estimation schemes. In this paper, we propose a channel estimation scheme which works even when this assumption is no longer true, i.e., when the channel results in pilot-data interference as may occur in scenarios involving high mobility and/or large delay-spread. The ability of a receiver to operate in such scenarios allows system designers the freedom to use smaller guard bands based on typical (not worst-case) channel spread parameters, thereby yielding higher spectral efficiency. In this work, we derive a maximum likelihood channel path detection scheme followed by a minimum mean-square error channel estimator. The performance advantage of the proposed receiver is verified using extensive bit error rate simulations.