Graduation Year

2025

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Stavros Vakalis, Ph.D.

Committee Member

Huseyin Arslan, Ph.D.

Committee Member

Gokhan Mumcu, Ph.D.

Committee Member

Nasir Ghani, Ph.D.

Committee Member

Yili Ren, Ph.D.

Committee Member

Ehsan Sheybani, Ph.D.

Keywords

Accuracy, Interference, Multiplexing, Range-Doppler Map, Spectral Efficiency

Abstract

In today’s interconnected world, sensing is one of the indispensable parts of wireless systems.The need to optimize and diversify the use of the radio frequency spectrum has reached unprecedented importance. Cutting-edge applications, including autonomous technologies, intelligent urban systems, and advanced monitoring, require solutions that can effectively merge communication and sensing functions within limited bandwidth. Orthogonal frequency division multiplexing (OFDM) has emerged as a leading contender in this field due to its superior ability to maximize spectral usage, resilience to multipath interference, and versatile signal configuration options. Reconsidering the use of the OFDM waveform for sensing purposes is undeniably essential due to its characteristics. This waveform has been holding its authority on contemporary communication systems and has faultlessly improved them. Nevertheless, OFDM also enables remarkable performance for different types of sensing goals in various circumstances. In this research, therefore, sensing analysis of OFDM waveform is explored from numerous aspects, including a detailed accuracy performance of OFDM ranging, dramatically increasing the spectral efficiency exploiting a novel OFDM method for range and Doppler velocity estimation, and multiple-input multiple-output (MIMO) OFDM wireless imaging and mapping. Particularly, the dissertation addresses the following:

First, the accuracy performance of OFDM ranging is thoroughly examined. The superior correlation characteristics of OFDM signals are explored, which makes them highly suitable for precise-ranging applications in real-world scenarios. The Cramer-Rao lower bound (CRLB) is analytically derived for OFDM-based ranging, demonstrating its independence from the transmitted data once lower-order modulation schemes are employed. Simulations and empirical measurements are conducted at a carrier frequency of 1.4 GHz, utilizing a bandwidth of 200 MHz. The results highlight the capability of achieving range estimation with a standard deviation as low as 1.6 mm in a practical laboratory setting, without the use of anechoic chambers.

Second, we assess the error analyses of OFDM ranging for varying distance values under different external factors such as noise and wireless channel. We discuss the theoretical underpinnings of our investigation and present simulated and experimental ranging measurements employing OFDM signals, complemented by range estimation and error analyses.

Third, the research investigates the impact of three prominent peak-to-average power ratio (PAPR) reduction methods—partial transmit sequences (PTS), selected mapping (SLM), and clipping—on the correlation performance essential for sensing applications. Simulations were conducted using OFDM signals configured with 1024 subcarriers, a bandwidth of 200 MHz, and 256-quadrature amplitude modulation (QAM). The findings reveal that PTS and SLM preserve excellent correlation properties for sensing, with SLM offering reduced computational complexity despite introducing slight signaling overhead. In contrast, clipping severely compromises correlation performance, primarily due to the substantial rise in matched filtering side lobe levels caused by nonlinear distortions. These outcomes underscore the critical trade-offs involved in choosing PAPR mitigation strategies for OFDM systems, offering significant insights for enhancing both radar and communication functionalities.

Fourth, the study proposes an innovative method for the simultaneous estimation of range and Doppler velocity for multiple users utilizing QAM-modulated OFDM signals across an overlapping 800 MHz bandwidth. These users are distinguished solely by minor carrier frequency offsets at 3.8 GHz and 3.85 GHz. Extensive simulations and experimental evaluations confirm that multiple users can effectively and simultaneously conduct environmental sensing operations without causing interference, even when sharing identical spectral and temporal resources. This approach highlights the feasibility of achieving precise sensing capabilities in multi-user scenarios with overlapping bandwidths.

Finally, the work proposes a novel imaging framework based on MIMO-OFDM technology, enabling multiple antennas to transmit simultaneously without relying on traditional time-division or frequency-division multiplexing schemes. The framework maintains the integrity of standard OFDM waveforms, eliminating the need for complex interference cancellation techniques. The system configuration includes two transmitting (Tx) and two receiving (Rx) antennas, operating within a 400 MHz bandwidth centered at 2.4 GHz and 2.415 GHz, each transmitting unique QAM-modulated signals. Comprehensive theoretical evaluations, simulation studies, and practical experiments validate the system’s ability to achieve precise object reconstruction, even when spectral and temporal resources are fully shared. This advancement demonstrates the feasibility of high-fidelity imaging in scenarios with overlapping transmission resources, offering a significant departure from conventional multiplexing approaches.

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