Low Complexity Channel Estimation for OTFS Modulation with Fractional Delay and Doppler

Abstract

We consider the problem of accurate channel estimation for OTFS based systems with few transmit/receive antennas, where additional sparsity due to large number of antennas is not a possibility. For such systems the sparsity of the effective delay-Doppler (DD) domain channel is adversely affected in the presence of channel path delay and Doppler shifts which are noninteger multiples of the delay and Doppler domain resolution. The sparsity is also adversely affected when practical transmit and receive pulses are used. In this paper we propose a Modified Maximum Likelihood Channel Estimation (M-MLE) method for OTFS based systems which exploits the fine delay and Doppler domain resolution of the OTFS modulated signal to decouple the joint estimation of the channel parameters (i.e., channel gain, delay and Doppler shift) of all channel paths into separate estimation of the channel parameters for each path. We further observe that with fine delay and Doppler domain resolution, the received DD domain signal along a particular channel path can be written as a product of a delay domain term and a Doppler domain term where the delay domain term is primarily dependent on the delay of this path and the Doppler domain term is primarily dependent on the Doppler shift of this path. This allows us to propose another method termed as the twostep method (TSE), where the joint two-dimensional estimation of the delay and Doppler shift of a particular path in the M-MLE method is further decoupled into two separate one-dimensional estimation for the delay and for the Doppler shift of that path. Simulations reveal that the proposed methods (M-MLE and TSE) achieve better channel estimation accuracy at lower complexity when compared to other known methods for accurate OTFS channel estimation.

Alternative threshold-based channel estimation and message-passing-based symbol detection in MIMO-OTFS systems using superimposed pilots

“…This group utilizes methods that allocate an entire frame for pilot transmission, which involves using the initial OTFS frame to estimate the channel and the following frames for transmitting data [14,[17][18][19][20][21]. In [18], a method is proposed for compressed sensing in 2D Turbo that models the support matrix of the DD channel using a Markov random field and the Bernoulli Gaussian distribution.…”

“…Reference [19] presents an uplink DD-CE algorithm for OTFS multiple access systems, formulating the CE problem as a sparse recovery problem and solving it using OMP and modified subspace pursuit algorithms. Reference [17] proposes two low-complexity algorithms for CE in OTFS with fractional delay and Doppler. The first (Modified Maximum Likelihood CE) is based on joint 2D estimation of the DD shift of a path.…”

Joint Channel Estimation and Symbol Detection for OTFS System Using a Two-Stage Algorithm

“…Several channel estimation schemes for OTFS in the DD domain have been proposed, and they can be divided into three categories. The first one concerns conventional pilot-aided (CPA) schemes in which a superframe architecture is used: the first frame is used for channel estimation, and the subsequent frames are used for data detection [4], [5]. The second category of algorithms, called embedded pilots (EP), consists of channel estimation and data detection in the same frame [1], [6]- [15].…”