OTFS: The Promising New Modulation Scheme for 6G High Mobility Communications

The advent of 6G wireless networks is expected to bring forth many new applications and use cases that require seamless connectivity, high data rates, and ultra-reliable low-latency communications (URLLC). One of the key challenges in realising this vision is supporting high mobility scenarios, such as vehicle-to-vehicle communications, high-speed rail, and satellite networks. Conventional modulation schemes like OFDM, which have been the workhorse of 4G and 5G systems, suffer significant performance degradation in channels with high Doppler spreads.

This is where Orthogonal Time Frequency Space (OTFS) modulation emerges. OTFS is a novel modulation technique that operates in the delay-Doppler domain, as opposed to the traditional time-frequency domain used by OFDM. OTFS can effectively handle the dynamics of rapidly time-varying channels by multiplexing data symbols over delay-Doppler grids. The key idea is to transform the time-varying multipath channel into a two-dimensional (2D) time-invariant channel in the delay-Doppler domain. This results in a stable, sparse, compact channel representation resilient to Doppler spreads.

The delay-Doppler domain representation in OTFS has some unique advantages. Firstly, it directly links the information symbols and the physical propagation environment. The delay and Doppler parameters relate to the relative distance and velocity of the reflectors/scatterers in the wireless channel. This property can be leveraged for efficient channel estimation and equalisation. Secondly, OTFS enjoys full diversity by spreading each information symbol across the entire time-frequency plane. Thirdly, OTFS can achieve lower signalling overhead and latency than OFDM, as it requires a shorter cyclic prefix (CP). Finally, the OTFS waveform exhibits a lower peak-to-average power ratio (PAPR), which benefits energy-efficient transmission.

The OTFS modulation and demodulation can be efficiently implemented using inverse symplectic finite Fourier transform (ISFFT) and symplectic finite Fourier transform (SFFT) operations. The modulated symbols in the delay-Doppler domain are converted to the time-frequency domain using ISFFT, followed by conventional multicarrier modulation like OFDM. The time-frequency signal is transformed back to the delay-Doppler domain at the receiver using SFFT for symbol detection. An alternative approach is to utilise the Zak transform for direct conversion between delay-Doppler and time domain signals, bypassing the intermediate time-frequency processing.

Innovative transceiver designs are being developed to unleash the full potential of OTFS. Transmit precoding techniques such as pulse shaping, windowing, and power allocation optimise the OTFS signal for different channel conditions and performance metrics. Efficient channel estimation algorithms based on pseudo-noise (PN) pilots, guard space insertion, and compressive sensing have been proposed to learn the delay-Doppler channel parameters accurately. For symbol detection, low-complexity algorithms like message passing (MPA), expectation propagation (EP), and successive interference cancellation (SIC) are being explored as alternatives to the optimal but computationally prohibitive maximum a posteriori (MAP) detection.

Apart from point-to-point links, OTFS is also being investigated for multi-user multiple access scenarios. Multiplexing users in the delay-Doppler domain can effectively manage inter-user interference. Non-orthogonal multiple access (NOMA) schemes based on OTFS have shown promising results regarding spectral efficiency and user fairness.

The most exciting aspect of OTFS is its inherent suitability for integrated sensing and communications (ISAC). The delay-Doppler representation provides a unified framework for radar sensing and wireless communications. The same OTFS waveform can be utilised for estimating the range and velocity of targets while simultaneously transmitting information symbols. This opens up possibilities for joint waveform design, hardware reuse, and spectrum sharing between the two functionalities.

OTFS is also finding applications in various emerging wireless technologies. In visible light communication (VLC), OTFS has been shown to outperform OFDM by exploiting the multipath diversity. For underwater acoustic (UWA) channels, which experience severe delay and Doppler spreads, OTFS provides a robust and efficient modulation scheme. Combining OTFS with index modulation (IM) techniques is another promising direction to enhance the spectral efficiency. Moreover, OTFS is considered a candidate waveform for satellite and non-terrestrial networks, integral to the 6G vision.

While OTFS holds immense potential, several research challenges still need to be addressed. Reducing the latency and computational complexity of OTFS transceivers is an important issue, especially for URLLC applications. Handling fractional Doppler and delay shifts that arise due to insufficient signal bandwidth and duration is another open problem. The extension of OTFS to multiple-input multiple-output (MIMO) systems and its integration with higher-layer protocols require further investigation. Cross-layer optimisation techniques that jointly design the OTFS waveform and network topology are also being explored. Another exciting avenue is the development of predictive communication schemes that leverage the slow-varying nature of the delay-Doppler channel.

In conclusion, OTFS modulation is a promising enabler for 6G wireless networks, particularly in high mobility scenarios. By operating in the delay-Doppler domain, OTFS provides a robust, efficient, and flexible waveform that can adapt to the dynamics of time-varying channels. With its unique advantages of Doppler resilience, full diversity, and low PAPR, OTFS has the potential to revolutionise the physical layer design of future wireless systems. Moreover, the applicability of OTFS in integrated sensing and communications makes it a key technology for realising the vision of pervasive connectivity and intelligence in the 6G era. As research on OTFS continues to gather momentum, we expect significant advancements in its theoretical foundations, transceiver architectures, and practical implementations in the coming years.