MIMO-OTFS in High-Doppler Fading Channels: Signal Detection and Channel Estimation (1805.02209v1)

Abstract: Orthogonal time frequency space (OTFS) modulation is a recently introduced multiplexing technique designed in the 2-dimensional (2D) delay-Doppler domain suited for high-Doppler fading channels. OTFS converts a doubly-dispersive channel into an almost non-fading channel in the delay-Doppler domain through a series of 2D transformations. In this paper, we focus on MIMO-OTFS which brings in the high spectral and energy efficiency benefits of MIMO and the robustness of OTFS in high-Doppler fading channels. The OTFS channel-symbol coupling and the sparse delay-Doppler channel impulse response enable efficient MIMO channel estimation in high Doppler environments. We present an iterative algorithm for signal detection based on message passing and a channel estimation scheme in the delay-Doppler domain suited for MIMO-OTFS. The proposed channel estimation scheme uses impulses in the delay-Doppler domain as pilots for estimation. We also compare the performance of MIMO-OTFS with that of MIMO-OFDM under high Doppler scenarios.

• The paper introduces a low-complexity iterative message passing algorithm for MIMO-OTFS signal detection, achieving a BER of 10⁻⁵ at 14 dB SNR under high-Doppler conditions.
• It proposes a tailored channel estimation method using delay-Doppler impulses as pilots, reliably estimating multiple MIMO paths even at high mobility.
• The study demonstrates that MIMO-OTFS significantly outperforms MIMO-OFDM in high-Doppler environments, highlighting its potential for next-generation high-speed wireless communications.

MIMO-OTFS in High-Doppler Fading Channels: Signal Detection and Channel Estimation

The paper addresses the challenge of achieving reliable communication in high mobility scenarios, specifically in high-Doppler fading channels, utilizing a novel modulation approach called Orthogonal Time Frequency Space (OTFS). The contribution is specifically focused on the combination of Multiple-Input Multiple-Output (MIMO) techniques with OTFS, referred to as MIMO-OTFS, and covers detailed aspects of signal detection and channel estimation.

Overview of MIMO-OTFS

OTFS is designed to operate in the delay-Doppler domain, converting doubly-dispersive channels typical of high-mobility environments into almost time-invariant channels. Unlike conventional modulation techniques that operate in the time-frequency domain, OTFS effectively transforms the channel characteristics, facilitating improved resilience to delay and Doppler effects. This paper exploits the advantages of OTFS combined with the spectral efficiency of MIMO systems, offering an innovative solution for wireless communications in dynamic environments.

Signal Detection and Algorithmic Implementation

A key focus of the paper is on signal detection where an iterative message passing algorithm is developed. This low complexity detection mechanism capitalizes on the sparse delay-Doppler impulse response and OTFS channel-symbol coupling, achieving high-performance levels. For instance, in a $2 \times 2$ MIMO-OTFS setup, a bit error rate (BER) of $10^{-5}$ is attained at 14 dB SNR with a Doppler shift of 1880 Hz. In contrast, MIMO-OFDM exhibits significantly higher BER floors under similar conditions, demonstrating the robustness of MIMO-OTFS.

Channel Estimation at High-Doppler

The paper presents a tailored channel estimation approach using delay-Doppler impulses as pilots. This method leverages the time-invariance of the sparse channel representation in the delay-Doppler domain, facilitating simultaneous estimation for multiple MIMO paths in a single OTFS frame. The proposed estimation algorithm maintains close performance to scenarios with perfect channel knowledge, with negligible performance degradation observed even at high Doppler frequencies.

Implications and Future Directions

The introduction of MIMO-OTFS opens up new avenues for high-mobility communications, especially in systems like 5G, where efficient support for high-speed mobility and millimeter-wave communication are crucial. By demonstrating significant performance benefits over traditional OFDM techniques in high-Doppler settings, this paper underscores the potential for MIMO-OTFS to become integral in future wireless standards. Enhanced spectral and energy efficiency, combined with the robustness of delay-Doppler domain processing, suggest that MIMO-OTFS can significantly enhance the reliability and efficiency of communication in challenging environments.

Speculations on Future Developments

Future developments may focus on refining the message passing algorithm for further complexity reduction and extension of MIMO-OTFS to even more complex scenarios involving larger MIMO configurations and diverse environmental conditions. Moreover, integration with advanced coding techniques and exploration of real-time adaptation to channel dynamics could further enhance the practical deployment capabilities of MIMO-OTFS in wireless networks.

In conclusion, this paper not only contributed to the theoretical understanding of MIMO-OTFS in high-Doppler environments but also laid the groundwork for practical implementations, paving the way for robust, next-generation wireless communications.