Integrated Sensing and Communications in Time-Varying Channels

Sun, Yang

Integrated Sensing and Communications in Time-Varying Channels

Sun, Yang

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Publication Type:: Thesis
Issue Date:: 2024

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Field	Value	Language
dc.contributor.author	Sun, Yang
dc.date.accessioned	2024-06-18T02:52:54Z
dc.date.available	2024-06-18T02:52:54Z
dc.date.issued	2024
dc.identifier.uri	http://hdl.handle.net/10453/179544
dc.description	University of Technology Sydney. Faculty of Engineering and Information Technology.	en_US.UTF-8
dc.description.abstract	In the era of 5G beyond wireless communication systems, a significant advancement lies in the integration of sensing capabilities, a function traditionally associated with radar systems. This convergence, referred to as integrated sensing and communications (ISAC), empowers the system to detect moving vehicles, human activities, and environmental changes by leveraging received communication signals. However, the presence of time-varying channels, usually caused by fast-moving objects, poses significant challenges for both communication and sensing. One of the promising approaches to address time-varying scenarios is the orthogonal time frequency space (OTFS) modulation system, which remains in its early stage. Several critical issues need attention within the context of integrated sensing and communication in time-varying channels: Flexible Data Allocation in OTFS: In OTFS systems, the allocation of different user data to the same delay-Doppler grid in one OTFS frame lacks flexibility and adaptability to different varying channel conditions. Mitigating Large Doppler Frequencies: The presence of large Doppler frequencies in integrated sensing and communication scenarios has not been considered, potentially leading to significant intercarrier interference. Sensing Performance Bound: The sensing performance bounds for ISAC in time-varying channels remains unknown. Addressing these challenges, in this thesis, I propose the following solutions: 1. Mixed-stage OTFS System: To address the first issue, I first show that channel parameters play a role in determining the optimal choice of delay-Doppler grid in OTFS systems. Consequently, I propose a mixed-stage OTFS system capable of accommodating users with different channel conditions in one OTFS frame. 2. Frequency-Domain Sensing Framework: To address the second issue, I propose a frequency-domain sensing framework for OFDM ISAC systems. I first derive a frequency-domain closed-form expression of the received signals, to characterise the delay and Doppler frequency impact. I then propose a sensing framework, taking advantages of both intra-block and inter-block sensing methods. The framework is further completed with exemplified pilot design and periodogram sensing algorithm. 3. Sensing Performance Bound Establishment: To address the third issue, I aim to establish the sensing performance bound for ISAC in time-varying channels. Firstly, I establish input-output relationships in such channels. Then, I derive the delay and Doppler Cramér-Rao lower bound (CRLB) in time-varying channels. Finally, I optimize preambles via CRLB minimization.	en_US.UTF-8
dc.format	Thesis (PhD)
dc.language.iso	en_US	en_US.UTF-8
dc.relation	https://opus.lib.uts.edu.au/bitstream/10453/179544/1/thesis.pdf
dc.rights	info:eu-repo/semantics/openAccess
dc.rights	The author owns the copyright in this thesis including all reproduction and reuse rights for the work. The work may not be altered without the permission of the copyright owner. Attribution is essential when quoting or paraphrasing from this thesis.
dc.rights	© 2024 Yang Sun
dc.rights	au.edu.uts.lib/nph
dc.title	Integrated Sensing and Communications in Time-Varying Channels	en_US.UTF-8
dc.type	Thesis
utslib.copyright.status	open_access	*

Abstract:

In the era of 5G beyond wireless communication systems, a significant advancement lies in the integration of sensing capabilities, a function traditionally associated with radar systems. This convergence, referred to as integrated sensing and communications (ISAC), empowers the system to detect moving vehicles, human activities, and environmental changes by leveraging received communication signals. However, the presence of time-varying channels, usually caused by fast-moving objects, poses significant challenges for both communication and sensing. One of the promising approaches to address time-varying scenarios is the orthogonal time frequency space (OTFS) modulation system, which remains in its early stage. Several critical issues need attention within the context of integrated sensing and communication in time-varying channels: Flexible Data Allocation in OTFS: In OTFS systems, the allocation of different user data to the same delay-Doppler grid in one OTFS frame lacks flexibility and adaptability to different varying channel conditions. Mitigating Large Doppler Frequencies: The presence of large Doppler frequencies in integrated sensing and communication scenarios has not been considered, potentially leading to significant intercarrier interference. Sensing Performance Bound: The sensing performance bounds for ISAC in time-varying channels remains unknown. Addressing these challenges, in this thesis, I propose the following solutions: 1. Mixed-stage OTFS System: To address the first issue, I first show that channel parameters play a role in determining the optimal choice of delay-Doppler grid in OTFS systems. Consequently, I propose a mixed-stage OTFS system capable of accommodating users with different channel conditions in one OTFS frame. 2. Frequency-Domain Sensing Framework: To address the second issue, I propose a frequency-domain sensing framework for OFDM ISAC systems. I first derive a frequency-domain closed-form expression of the received signals, to characterise the delay and Doppler frequency impact. I then propose a sensing framework, taking advantages of both intra-block and inter-block sensing methods. The framework is further completed with exemplified pilot design and periodogram sensing algorithm. 3. Sensing Performance Bound Establishment: To address the third issue, I aim to establish the sensing performance bound for ISAC in time-varying channels. Firstly, I establish input-output relationships in such channels. Then, I derive the delay and Doppler Cramér-Rao lower bound (CRLB) in time-varying channels. Finally, I optimize preambles via CRLB minimization.

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