81 research outputs found
Improving Channel Estimation Performance for Uplink OTFS Transmissions: Pilot Design based on A Posteriori Cramer-Rao Bound
Orthogonal time frequency space (OTFS) has been widely acknowledged as a
promising wireless technology for challenging transmission scenarios, including
high-mobility channels. In this paper, we investigate the pilot design for the
multi-user OTFS system based on the a priori statistical channel state
information (CSI), where the practical threshold-based estimation scheme is
adopted. Specifically, we first derive the a posteriori Cramer-Rao bound (PCRB)
based on a priori channel information for each user. According to our
derivation, the PCRB only relates to the user's pilot signal-to-noise ratio
(SNR) and the range of delay and Doppler shifts under the practical power-delay
and power-Doppler profiles. Then, a pilot scheme is proposed to minimize the
average PCRB of different users, where a closed-form global optimal pilot power
allocation is derived. Our numerical results verify the multi-user PCRB
analysis. Also, we demonstrate an around 3 dB improvement in the average
normalized-mean-square error (NMSE) by using the proposed pilot design in
comparison to the conventional embedded pilot design under the same total pilot
power
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
On the Pulse Shaping for Delay-Doppler Communications
In this paper, we study the pulse shaping for delay-Doppler (DD)
communications. We start with constructing a basis function in the DD domain
following the properties of the Zak transform. Particularly, we show that the
constructed basis functions are globally quasi-periodic while locally
twisted-shifted, and their significance in time and frequency domains are then
revealed. We further analyze the ambiguity function of the basis function, and
show that fully localized ambiguity function can be achieved by constructing
the basis function using periodic signals. More importantly, we prove that time
and frequency truncating such basis functions naturally leads to approximate
delay and Doppler orthogonalities, if the truncating windows are periodic
within the support. Motivated by this, we propose a DD Nyquist pulse shaping
scheme considering signals with periodicity. Finally, our conclusions are
verified by using various strictly or approximately periodic pulses
Predictive Precoder Design for OTFS-Enabled URLLC: A Deep Learning Approach
This paper investigates the orthogonal time frequency space (OTFS)
transmission for enabling ultra-reliable low-latency communications (URLLC). To
guarantee excellent reliability performance, pragmatic precoder design is an
effective and indispensable solution. However, the design requires accurate
instantaneous channel state information at the transmitter (ICSIT) which is not
always available in practice. Motivated by this, we adopt a deep learning (DL)
approach to exploit implicit features from estimated historical delay-Doppler
domain channels (DDCs) to directly predict the precoder to be adopted in the
next time frame for minimizing the frame error rate (FER), that can further
improve the system reliability without the acquisition of ICSIT. To this end,
we first establish a predictive transmission protocol and formulate a general
problem for the precoder design where a closed-form theoretical FER expression
is derived serving as the objective function to characterize the system
reliability. Then, we propose a DL-based predictive precoder design framework
which exploits an unsupervised learning mechanism to improve the practicability
of the proposed scheme. As a realization of the proposed framework, we design a
DDCs-aware convolutional long short-term memory (CLSTM) network for the
precoder design, where both the convolutional neural network and LSTM modules
are adopted to facilitate the spatial-temporal feature extraction from the
estimated historical DDCs to further enhance the precoder performance.
Simulation results demonstrate that the proposed scheme facilitates a flexible
reliability-latency tradeoff and achieves an excellent FER performance that
approaches the lower bound obtained by a genie-aided benchmark requiring
perfect ICSI at both the transmitter and receiver.Comment: 31 pages, 12 figure
Deep Learning-empowered Predictive Precoder Design for OTFS Transmission in URLLC
To guarantee excellent reliability performance in ultra-reliable low-latency
communications (URLLC), pragmatic precoder design is an effective approach.
However, an efficient precoder design highly depends on the accurate
instantaneous channel state information at the transmitter (ICSIT), which
however, is not always available in practice. To overcome this problem, in this
paper, we focus on the orthogonal time frequency space (OTFS)-based URLLC
system and adopt a deep learning (DL) approach to directly predict the precoder
for the next time frame to minimize the frame error rate (FER) via implicitly
exploiting the features from estimated historical channels in the delay-Doppler
domain. By doing this, we can guarantee the system reliability even without the
knowledge of ICSIT. To this end, a general precoder design problem is
formulated where a closed-form theoretical FER expression is specifically
derived to characterize the system reliability. Then, a delay-Doppler domain
channels-aware convolutional long short-term memory (CLSTM) network (DDCL-Net)
is proposed for predictive precoder design. In particular, both the
convolutional neural network and LSTM modules are adopted in the proposed
neural network to exploit the spatial-temporal features of wireless channels
for improving the learning performance. Finally, simulation results
demonstrated that the FER performance of the proposed method approaches that of
the perfect ICSI-aided scheme.Comment: 8 pages, 6 figure
Radar Sensing via OTFS Signaling: A Delay Doppler Signal Processing Perspective
The recently proposed orthogonal time frequency space (OTFS) modulation
multiplexes data symbols in the delay-Doppler (DD) domain. Since the range and
velocity, which can be derived from the delay and Doppler shifts, are the
parameters of interest for radar sensing, it is natural to consider
implementing DD signal processing for radar sensing. In this paper, we
investigate the potential connections between the OTFS and DD domain radar
signal processing. Our analysis shows that the range-Doppler matrix computing
process in radar sensing is exactly the demodulation of OTFS with a rectangular
pulse shaping filter. Furthermore, we propose a two-dimensional (2D)
correlation-based algorithm to estimate the fractional delay and Doppler
parameters for radar sensing. Simulation results show that the proposed
algorithm can efficiently obtain the delay and Doppler shifts associated with
multiple targets.Comment: ICC-2023 Accepte
Extended Target Parameter Estimation and Tracking with an HDA Setup for ISAC Applications
We investigate radar parameter estimation and beam tracking with a hybrid
digital-analog (HDA) architecture in a multi-block measurement framework using
an extended target model. In the considered setup, the backscattered data
signal is utilized to predict the user position in the next time slots.
Specifically, a simplified maximum likelihood framework is adopted for
parameter estimation, based on which a simple tracking scheme is also
developed. Furthermore, the proposed framework supports adaptive transmitter
beamwidth selection, whose effects on the communication performance are also
studied. Finally, we verify the effectiveness of the proposed framework via
numerical simulations over complex motion patterns that emulate a realistic
integrated sensing and communication (ISAC) scenario.Comment: 6 page
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