[Paper Review] Position Based Compressed Channel Estimation and Pilot Design for High Mobility OFDM Systems

Xiang Ren, Wen Chen+1 more|arXiv (Cornell University)|Mar 1, 2020

Advanced Wireless Communication Techniques32 references51 citations

TL;DR

This paper introduces a position-based high-mobility channel model and a joint pilot placement and pilot symbol design method to enable compressed sensing-based channel estimation for OFDM in high-speed scenarios, improving estimation accuracy under Doppler-induced ICI.

ABSTRACT

With the development of high speed trains (HST) in many countries, providing broadband wireless services in HSTs is becoming crucial. Orthogonal frequency-division multiplexing (OFDM) has been widely adopted for broadband wireless communications due to its high spectral efficiency. However, OFDM is sensitive to the time selectivity caused by high-mobility channels, which costs large spectrum or time resources to obtain the accurate channel state information (CSI). Therefore, the channel estimation in high-mobility OFDM systems has been a long-standing challenge. In this paper, we first propose a new position-based high-mobility channel model,in which the HST's position information and Doppler shift are utilized to determine the positions of the dominant channel coefficients. %In this way, we can reduce the estimation complexity and to design the transmitted pilot.Then, we propose a joint pilot placement and pilot symbol design algorithm for compressed channel estimation. It aims to reduce the coherence between the pilot signal and the proposed channel model, and hence can improve the channel estimation accuracy. Simulation results demonstrate that the proposed method achieves better performances than existing channel estimation methods over high-mobility channels. Furthermore, we give an example of the designed pilot codebook to show the practical applicability of the proposed scheme.

Motivation & Objective

• Motivate reliable CSI acquisition for OFDM in high-mobility environments such as high-speed trains.
• Propose a position-based high-mobility channel model that leverages GPS position and Doppler shift to locate dominant channel coefficients.
• Develop a joint pilot placement and pilot symbol design algorithm to minimize CS coherence and improve estimation accuracy.
• Demonstrate feasibility and performance gains via simulations and provide a practical pilot codebook example.

Proposed method

• Define a delay-Doppler domain channel model with sparsity for high-mobility channels.
• Introduce a position-based channel structure where Doppler shifts map to dominant coefficient positions.
• Formulate a compressed sensing based estimator using a comb-type pilot and a dominant channel dictionary Phi_x.
• Mitigate ICI by iteratively subtracting data-induced ICI using estimated channels.
• Develop a coherence-optimized pilot design problem that jointly selects pilot locations and symbol powers to minimize mu_delta{X_d(p) Phi_x}.

Experimental results

Research questions

• RQ1 How can GPS position and speed information be exploited to predict dominant channel coefficients in high-mobility OFDM channels?
• RQ2 Can joint optimization of pilot placement and pilot symbols reduce CS coherence and improve channel estimation accuracy in high-mobility scenarios?
• RQ3 What is the impact of ICI on pilot-based CS channel estimation, and how can it be mitigated iteratively?
• RQ4 How does the proposed position-based model compare with existing high-mobility channel models in terms of estimation performance?

Key findings

• The proposed position-based model constrains dominant channel coefficients to a small set tied to position and Doppler, yielding S-sparse channels in delay-Doppler space.
• A joint pilot placement and pilot symbol design method minimizes coherence mu_delta between the measurement matrix and the dominant channel dictionary, improving CS recovery.
• ICI mitigation is employed to further enhance estimation by removing data-induced ICI using estimated channels.
• Simulation results indicate the proposed method achieves better performance than existing methods in high-mobility channels.
• An example pilot codebook is provided to illustrate practical applicability of the scheme.