Embedded Pilot-Aided Channel Estimation for OTFS in Delay-Doppler Channels (1808.08360v1)

Abstract: Orthogonal time frequency space (OTFS) modulation was shown to provide significant error performance advantages over orthogonal frequency division multiplexing (OFDM) in delay--Doppler channels. In order to detect OTFS modulated data, the channel impulse response needs to be known at the receiver. In this paper, we propose embedded pilot-aided channel estimation schemes for OTFS. In each OTFS frame, we arrange pilot, guard, and data symbols in the delay--Doppler plane to suitably avoid interference between pilot and data symbols at the receiver. We develop such symbol arrangements for OTFS over multipath channels with integer and fractional Doppler shifts, respectively. At the receiver, channel estimation is performed based on a threshold method and the estimated channel information is used for data detection via a message passing (MP) algorithm. Thanks to our specific embedded symbol arrangements, both channel estimation and data detection are performed within the same OTFS frame with a minimum overhead. We compare by simulations the error performance of OTFS using the proposed channel estimation and OTFS with ideally known channel information and observe only a marginal performance loss. We also demonstrate that the proposed channel estimation in OTFS significantly outperforms OFDM with known channel information. Finally, we present extensions of the proposed schemes to MIMO and multi-user uplink/downlink.

• The paper presents an embedded pilot scheme that organizes pilot, guard, and data symbols in the delay-Doppler plane to minimize interference.
• The channel estimation uses a threshold-based method and a Message Passing algorithm, achieving overhead as low as 1% for integer and 8% for fractional Doppler conditions.
• Numerical results show that OTFS with embedded estimation slightly lags ideal performance yet significantly outperforms OFDM in high-mobility, delay-Doppler environments.

Embedded Pilot-Aided Channel Estimation for OTFS in Delay-Doppler Channels

The paper presents a focused examination of channel estimation techniques specifically tailored for Orthogonal Time Frequency Space (OTFS) modulation, aiming to enhance its applicability in delay-Doppler channels. OTFS has gained attention for its robustness over traditional modulation schemes like OFDM, particularly in environments characterized by high mobility and significant Doppler spreads.

Key Contributions

The authors propose embedded pilot-aided channel estimation schemes that organize pilot, guard, and data symbols within the delay-Doppler plane, effectively minimizing interference at the receiver. This symbol arrangement is crucial for maintaining performance while ensuring low overhead. The research develops these arrangements for channels with both integer and fractional Doppler shifts.

Channel estimation is performed using a threshold-based method, followed by data detection via a Message Passing (MP) algorithm. Notably, the proposed methods allow channel estimation and data detection to occur within the same OTFS frame, reducing the overhead to 1% for integer Doppler conditions and 8% for fractional Doppler situations.

Numerical Results and Performance

Simulation results indicate that OTFS with the proposed channel estimation slightly drifts from the ideal OTFS with perfectly known channel information, exhibiting only marginal performance degradation. More impressively, OTFS with embedded channel estimation significantly outperforms OFDM even when OFDM benefits from complete channel knowledge. This outcome underscores the potential of OTFS in handling multipath effects and Doppler shifts more efficiently than existing alternatives.

Extensions and Implications

The paper also extends the proposed estimation schemes to Multiple-Input Multiple-Output (MIMO) systems and multi-user scenarios for both uplink and downlink configurations. This broadens the applicability of the research, providing a pathway for OTFS adaptation in complex communication systems.

Theoretical and Practical Implications

Theoretically, the work enhances the understanding of OTFS modulation's capability to leverage delay-Doppler domain characteristics. The introduction of efficient channel estimation techniques bolsters the feasibility of OTFS in real-world applications where channel conditions are unpredictable and rapidly changing.

Practically, the reduced overhead in channel estimation while retaining performance enhances OTFS's suitability for integration into next-generation communication systems, which demand both high reliability and adaptability.

Speculation on Future Developments

Looking ahead, the exploration of adaptive methods to further optimize the threshold-based estimation and improve the resilience of OTFS against extreme channel conditions could be a valuable direction. Additionally, developing techniques to reduce computational complexity without sacrificing performance will be essential as OTFS moves towards widespread deployment.

Conclusion

This paper methodically advances the field of OTFS modulation by marrying theoretical insight with practical channel estimation solutions in the delay-Doppler domain. The proposed pilot-aided schemes not only demonstrate significant performance improvements over traditional methods but also extend OTFS’s applicability across various advanced communication scenarios. As a result, it serves a foundational role in the continued evolution of robust, high-efficiency modulation techniques.