Problem
Research questions and friction points this paper is trying to address.
OTFS
channel estimation
overspread Doppler shifts
delay-Doppler domain
aliased Doppler shifts
Channel Estimation for OTFS Systems With Overspread Doppler Shifts
🤖 AI Summary
This work addresses the failure of conventional channel estimation in orthogonal time frequency space (OTFS) systems under ultra-wide Doppler shifts, where severe aliasing degrades performance. To overcome this challenge, the authors propose a novel delay–Doppler (DD) domain training frame that integrates cosine pilots with pilot symbols, along with a two-stage channel estimation method. In the first stage, coarse estimates of path delays, aliased Doppler shifts, and channel gains are obtained directly in the DD domain. The second stage leverages time–frequency transformation and cosine pilot detection to identify true Doppler peaks and performs parameter pairing via thresholding. This approach uniquely combines DD-domain and frequency-domain information to resolve ultra-wide Doppler aliasing, significantly reducing normalized mean square error (NMSE) and improving bit error rate (BER) performance.
📝 Abstract
In this paper, we consider an orthogonal time frequency space (OTFS) system in time-varying channels with overspread Doppler shifts, typically found in non-terrestrial multi-satellite links. The overspread Doppler shifts with magnitude greater than half of the subcarrier spacing, result in aliased Doppler shifts in the delay-Doppler (DD) domain due to the OTFS modulo operation. This makes channel estimation very challenging and the traditional channel estimation methods become ineffective. To address this challenge, we propose a DD training frame and a two-stage channel estimation method. The training frame comprises a cosine pilot signal and a pilot symbol. In the first stage of the channel estimation, the pilot symbol in the DD domain is utilized to estimate the delays, aliased Doppler shifts, and channel gains of the propagation paths. In the second stage, the received time domain signal is converted into the frequency domain to detect the peaks of all the Doppler shifts using the cosine pilot signal. Then, we present a threshold-based method to pair the estimated actual Doppler shifts with their corresponding delays and channel gains. The complexity of the proposed channel estimation is also discussed. Finally, the performance of the proposed channel estimation is validated in terms of the normalized mean square error (NMSE) and bit error rate (BER) in various scenarios.
Innovation
Methods, ideas, or system contributions that make the work stand out.
OTFS
overspread Doppler shifts
channel estimation
delay-Doppler domain
pilot design
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