24 research outputs found
AFDM vs OTFS: A Comparative Study of Promising Waveforms for ISAC in Doubly-Dispersive Channels
This white paper aims to briefly describe a proposed article that will
provide a thorough comparative study of waveforms designed to exploit the
features of doubly-dispersive channels arising in heterogeneous high-mobility
scenarios as expected in the beyond fifth generation (B5G) and sixth generation
(6G), in relation to their suitability to integrated sensing and communications
(ISAC) systems. In particular, the full article will compare the
well-established delay-Doppler domain-based orthognal time frequency space
(OTFS) and the recently proposed chirp domain-based affine frequency division
multiplexing (AFDM) waveforms. Both these waveforms are designed based on a
full delay- Doppler representation of the time variant (TV) multipath channel,
yielding not only robustness and orthogonality of information symbols in
high-mobility scenarios, but also a beneficial implication for environment
target detection through the inherent capability of estimating the path delay
and Doppler shifts, which are standard radar parameters. These modulation
schemes are distinct candidates for ISAC in B5G/6G systems, such that a
thorough study of their advantages, shortcomings, implications to signal
processing, and performance of communication and sensing functions are well in
order. In light of the above, a sample of the intended contribution (Special
Issue paper) is provided below
Cyclic Delay-Doppler Shift: A Simple Transmit Diversity Technique for Delay-Doppler Waveforms in Doubly Selective Channels
Delay-Doppler waveform design has been considered as a promising solution to
achieve reliable communication under high-mobility channels for the
space-air-ground-integrated networks (SAGIN). In this paper, we introduce the
cyclic delay-Doppler shift (CDDS) technique for delay-Doppler waveforms to
extract transmit diversity in doubly selective channels. Two simple CDDS
schemes, named time-domain CDDS (TD-CDDS) and modulation-domain CDDS (MD-CDDS),
are proposed in the setting of multiple-input multiple-output (MIMO). We
demonstrate the applications of CDDS on two representative delay-Doppler
waveforms, namely orthogonal time frequency space (OTFS) and affine frequency
division multiplexing (AFDM), by deriving their corresponding CDDS matrices.
Furthermore, we prove theoretically and experimentally that CDDS can provide
OTFS and AFDM with full transmit diversity gain on most occasions
A Proof of Concept for OTFS Resilience in Doubly-Selective Channels by GPU-Enabled Real-Time SDR
Orthogonal time frequency space (OTFS) is a modulation technique which is
robust against the disruptive effects of doubly-selective channels. In this
paper, we perform an experimental study of OTFS by a real-time software defined
radio (SDR) setup. Our SDR consists of a Graphical Processing Unit (GPU) for
signal processing programmed using Sionna and TensorFlow, and Universal
Software Radio Peripheral (USRP) devices for air interface. We implement a
low-latency transceiver structure for OTFS and investigate its performance
under various Doppler values. By comparing the performance of OTFS with
Orthogonal Frequency Division Multiplexing (OFDM), we demonstrate that OTFS is
highly robust against the disruptive effects of doubly-selective channels in a
real-time experimental setup.Comment: ACCEPTED for 2023 IEEE Global Communications Conference: Wireless
Communication
Orthogonal Time Frequency Space (OTFS) Modulation for Wireless Communications
The orthogonal time frequency space (OTFS) modulation is a recently proposed multi-carrier transmission scheme, which innovatively multiplexes the information symbols in the delay-Doppler (DD) domain instead of the conventional time-frequency (TF) domain. The DD domain symbol multiplexing gives rise to a direct interaction between the DD domain information symbols and DD domain channel responses, which are usually quasi-static, compact, separable, and potentially sparse. Therefore, OTFS modulation enjoys appealing advantages over the conventional orthogonal frequency-division multiplexing (OFDM) modulation for wireless communications.
In this thesis, we investigate the related subjects of OTFS modulation for wireless communications, specifically focusing on its signal detection, performance analysis, and applications. In specific, we first offer a literature review on the OTFS modulation in Chapter~1. Furthermore, a summary of wireless channels is given in Chapter 2. In particular, we discuss the characteristics of wireless channels in different domains and compare their properties.
In Chapter 3, we present a detailed derivation of the OTFS concept based on the theory of Zak transform (ZT) and discrete Zak transform (DZT). We unveil the connections between OTFS modulation and DZT, where the DD domain interpretations of key components for modulation, such as pulse shaping, and matched-filtering, are highlighted.
The main research contributions of this thesis appear in Chapter 4 to Chapter 7. In Chapter 4, we introduce the hybrid maximum a posteriori (MAP) and parallel interference cancellation (PIC) detection. This detection approach exploits the power discrepancy among different resolvable paths and can obtain near-optimal error performance with a reduced complexity.
In Chapter 5, we propose the cross domain iterative detection for OTFS modulation by leveraging the unitary transformations among different domains. After presenting the key concepts of the cross domain iterative detection, we study its performance via state evolution. We show that the cross domain iterative detection can approach the optimal error performance theoretically. Our numerical results agree with our theoretical analysis and demonstrate a significant performance improvement compared to conventional OTFS detection methods.
In Chapter 6, we investigate the error performance for coded OTFS systems based on the pairwise-error probability (PEP) analysis. We show that there exists a fundamental trade-off between the coding gain and the diversity gain for coded OTFS systems. According to this trade-off, we further provide some rule-of-thumb guidelines for code design in OTFS systems.
In Chapter 7, we study the potential of OTFS modulation in integrated sensing and communication (ISAC) transmissions. We propose the concept of spatial-spreading to facilitate the ISAC design, which is able to discretize the angular domain, resulting in simple and insightful input-output relationships for both radar sensing and communication. Based on spatial-spreading, we verify the effectiveness of OTFS modulation in ISAC transmissions and demonstrate the performance improvements in comparison to the OFDM counterpart.
A summary of this thesis is presented in Chapter 8, where we also discuss some potential research directions on OTFS modulation. The concept of OTFS modulation and the elegant theory of DD domain communication may have opened a new gate for the development of wireless communications, which is worthy to be further explored
Jamming Suppression Via Resource Hopping in High-Mobility OTFS-SCMA Systems
This letter studies the mechanism of uplink multiple access and jamming
suppression in an OTFS system. Specifically, we propose a novel resource
hopping mechanism for orthogonal time frequency space (OTFS) systems with delay
or Doppler partitioned sparse code multiple access (SCMA) to mitigate the
effect of jamming in controlled multiuser uplink. We analyze the non-uniform
impact of classic jamming signals such as narrowband interference (NBI) and
periodic impulse noise (PIN) in delay-Doppler (DD) domain on OTFS systems.
Leveraging turbo equalization, our proposed hopping method demonstrates
consistent BER performance improvement under jamming over conventional
OTFS-SCMA systems compared to static resource allocation schemes