PhD Oral Exam - Khan Zeb, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The ever-increasing proliferation of mobile users and new technologies with different applications and features, and the demand for reliable high-speed high capacity, pervasive connectivity and low latency have initiated a roadmap for the next generation wireless networks, fifth generation (5G), which is set to revolutionize the existing wireless communications. 5G will use heterogeneous higher carrier frequencies from the plentifully available spectra in higher microwave and millimeter-wave (MMW) bands, including licensed and unlicensed spectra, for achieving multi-Gb/s wireless connectivity and overcoming the wireless spectrum crunch in the sub-6 GHz bands of the current wireless systems. This is complemented by multiple-input-multiple-output (MIMO) technology, which can significantly increase data capacity through spatial multiplexing and improve coverage and system reliability through spatial diversity. However, high-frequency RF MMW signals are prone to extreme propagation path loss and are challenging to generate with conventional bandwidth-limiting electronics. In addition, the existing digitized fronthaul with centralized radio access network (C-RAN) architecture is considered inefficient for 5G and beyond. Thus, to fully exploit this promising 5G new radio (NR) resource and to alleviate the electronics and fronthaul bottleneck, microwave photonics with analog radio-over-fiber (A-RoF) technology becomes instrumental for optically synthesizing and processing broadband RF MMW wireless signals over optical links. This facilitates seamless integration of high capacity, reliable and transparent optical networks with flexible, mobile and pervasive wireless networks extending the reach and coverage of high-speed broadband MMW wireless communications. Consequently, this not only overcomes the problem of electronic bottleneck, high bandwidth requirements, transmission capacity and span limitation but also significantly reduces system complexity considering the deployment of ultra-dense small cells with large numbers of 5G remote radio units (RRUs) having massive antennas with MIMO and beamforming capabilities connected to the baseband units (BBU) in a C-RAN environment through an optical fiber-based fronthaul network. Nevertheless, photonic RF MMW signal generation suffers from frequency fluctuation and phase noise due to uncorrelated optical sources, which can degrade system performance. Thus highly correlated, low-noise, simple and cost-efficient optical sources are desirable for next-generation MMW RoF wireless communication systems.

More recently, well-designed quantum confined nanostructures such as semiconductor quantum dash multi-wavelength lasers (QD-MWLs) have attracted more interest in the photonic generation of RF MMW signals due to the inherent properties of QD materials and their simple and compact design with highly coherent and correlated optical signals having a very low phase and intensity noise. The main theme of this thesis revolves around the experimental investigation of such nanostructures on the device and system level for applications in high-speed high, high-capacity broadband RoF-based fronthaul and wireless access networks. Several photonic-aided multi-Gb/s MMW RoF system designs are proposed and experimentally demonstrated using QD-MWLs with the photonic generation, wireless transmission and detection of broadband multi-Gb/s MMW wireless signals at 5G NR (FR2) in the K-band, Ka-band and V-band in simplex, duplex and MIMO configurations. These QD-MWL-based MMW RoF wireless systems’ designs and experimental demonstrations could usher in a new era of ultra-high-speed broadband multi-Gb/s wireless communications at the MMW frequency bands for next-generation wireless networks.

The QD-MWLs investigated in this thesis include a compact, monolithic and low-noise single-section InAs/InP QD buried heterostructure passively mode-locked (PML) laser-based optical coherent frequency comb (CFC) and a novel highly correlated, monolithic and low noise InAs/InP buried heterostructure common cavity QD dual-wavelength distributed feedback laser (QD-DW-DFBL). The performance of each device is experimentally characterized in terms of optical phase noise, relative intensity noise (RIN), timing jitter and RF phase noise exhibiting promising results. Based on these devices, different RoF wireless systems, including simplex single-input-single-output (SISO) and multiple-input-multiple-output (MIMO), and bidirectional configurations, are proposed and experimentally demonstrated with real-time remote RF synthesizer-free photonic generation, wireless transmission and detection of wide-bandwidth RF MMW multi-level quadrature amplitude modulated (M-QAM) wireless signals having bit rates ranging from 8 Gb/s to 36 Gb/s over different hybrid fiber-wireless links comprising of standard single mode fiber (SSMF) and free-space wireless channel. The end-to-end links are thoroughly investigated in terms of error-vector-magnitude (EVM), bit-error-rat (BER), constellations and eye diagrams, realizing successful error-free transmission. Finally, novel system designs for achieving high-speed and high-capacity spectrally efficient MIMO and optical beamforming enabled photonic MMW RoF wireless transceivers based on QD-MWLs with wavelength division multiplexing (WDM) and space division multiplexing (SDM) are proposed and discussed.