[Paper Review] Integrated Sensing and Communication-assisted Orthogonal Time Frequency Space Transmission for Vehicular Networks

Weijie Yuan, Zhiqiang Wei+3 more|arXiv (Cornell University)|May 7, 2021

PAPR reduction in OFDMEngineering49 references338 citations

TL;DR

This paper proposes an integrated sensing and communication (ISAC)-assisted orthogonal time frequency space (OTFS) transmission scheme for vehicular networks, leveraging vehicle echo reflections to estimate and predict kinematic parameters for beamforming and channel estimation. The scheme eliminates pilot overhead in both uplink and downlink by using predicted delay-Doppler parameters, achieving near-perfect BER performance with zero training overhead and significant SNR gains over conventional methods.

ABSTRACT

Orthogonal time frequency space (OTFS) modulation is a promising candidate for supporting reliable information transmission in high-mobility vehicular networks. In this paper, we consider the employment of the integrated (radar) sensing and communication (ISAC) technique for assisting OTFS transmission in both uplink and downlink vehicular communication systems. Benefiting from the OTFS-ISAC signals, the roadside unit (RSU) is capable of simultaneously transmitting downlink information to the vehicles and estimating the sensing parameters of vehicles, e.g., locations and speeds, based on the reflected echoes. Then, relying on the estimated kinematic parameters of vehicles, the RSU can construct the topology of the vehicular network that enables the prediction of the vehicle states in the following time instant. Consequently, the RSU can effectively formulate the transmit downlink beamformers according to the predicted parameters to counteract the channel adversity such that the vehicles can directly detect the information without the need of performing channel estimation. As for the uplink transmission, the RSU can infer the delays and Dopplers associated with different channel paths based on the aforementioned dynamic topology of the vehicular network. Thus, inserting guard space as in conventional methods are not needed for uplink channel estimation which removes the required training overhead. Finally, an efficient uplink detector is proposed by taking into account the channel estimation uncertainty. Through numerical simulations, we demonstrate the benefits of the proposed ISAC-assisted OTFS transmission scheme.

Motivation & Objective

• Address the high training overhead and latency in conventional pilot-based beam tracking for high-mobility vehicular networks.
• Leverage ISAC signals to estimate vehicle motion parameters (position, speed) from reflected echoes at the roadside unit (RSU).
• Predict future vehicle states using estimated kinematic parameters to enable beamforming without feedback or pilot signals.
• Design a guard space-free uplink symbol placement scheme based on predicted channel parameters to minimize training overhead.
• Achieve reliable communication with minimal training overhead by exploiting ISAC-OTFS signal structure and channel prediction.

Proposed method

• Use ISAC signals transmitted from the RSU to simultaneously convey downlink data and enable sensing of vehicle motion parameters via echo reflection analysis.
• Estimate vehicle delays and Doppler shifts from reflected echoes using a maximum likelihood (ML) estimator to infer location and speed.
• Predict future vehicle states (position, velocity) using a dynamic network topology derived from estimated kinematic parameters.
• Formulate downlink beamformers based on predicted parameters to pre-compensate for channel effects, eliminating the need for downlink pilots.
• Design a superimposed pilot-data symbol placement scheme in the delay-Doppler domain that enables channel estimation without guard spaces.
• Develop an efficient uplink detector that accounts for channel estimation uncertainty, improving robustness in high-mobility environments.

Experimental results

Research questions

• RQ1 Can ISAC-OTFS signals be used to estimate and predict vehicle motion parameters (position, speed) from echo reflections to enable beamforming without feedback?
• RQ2 To what extent can the training overhead for channel estimation be reduced in uplink OTFS systems by leveraging predicted delay-Doppler parameters?
• RQ3 How does the proposed guard space-free symbol placement scheme compare to conventional methods in terms of channel estimation accuracy and BER performance?
• RQ4 Can the proposed ISAC-assisted OTFS scheme achieve BER performance close to the ideal case with perfect CSI while eliminating pilot overhead?
• RQ5 What is the impact of channel estimation uncertainty on uplink detection performance, and how can it be effectively mitigated in high-mobility scenarios?

Key findings

• The proposed ISAC-assisted OTFS scheme achieves near-perfect BER performance, closely approaching the ideal case with perfect channel state information (CSI), even without downlink pilots.
• The training overhead for channel estimation is reduced to 0% by eliminating guard spaces and using a superimposed pilot-data symbol placement scheme, compared to 12.5% in conventional schemes.
• The normalized mean square error (NMSE) of channel estimation is bounded by approximately 10−2 due to estimation uncertainty, which has negligible impact on BER performance.
• The proposed uplink detector, which accounts for channel estimation uncertainty, outperforms a baseline SPA detector that neglects uncertainty, showing measurable BER improvement.
• The proposed beam alignment scheme outperforms both feedback-based and beam search-based (ABP) benchmarks in terms of receive SNR, especially in high-mobility scenarios with rapid angular changes.
• The scheme enables reliable uplink communication with a BER performance close to symbol-wise ML detection with perfect CSI, while achieving zero training overhead and robustness to time-varying channels.