Design and Evaluation of Advanced Photonic Radio Heads for Next-Generation Wireless Networks

In recent times, communication technologies have evolved significantly. The urge for high-speed internet, live streaming, and video conferencing has drastically increased capacity and bandwidth demands, straining the radio frequency (RF) spectrum.
This necessitates exploring new frequency bands like millimeter wave (mm-Wave) or transitioning to optical carriers. Photonics plays a crucial role in meeting these needs. This thesis explores the trade-off between optical and electronic solutions.
It covers topics such as broadband radio over fiber (RoF) links, photonic frequency conversion, photonic remote radio heads (RRHs) with full-duplex RoF links and optically RF-based beamsteering, free-space optical (FSO) communications, and
light detection and ranging (LiDAR) sensing.
The fusion of broadband wireless components with fixed networks utilizing high-speed optical infrastructure is crucial for future access networks. We explore two orthogonal modulation approaches for hybrid analog radio-over-fiber (A-RoF) and
digital radio-over-fiber (D-RoF) infrastructures suited for heterogeneous networks (HetNets) with small- and macro-cells. The first approach encodes a 10 Gb/s digitized signal onto an optical carrier via phase modulation and a 1-GHz orthogonal
frequency division multiplexing (OFDM) radio signal via intensity modulation on a 33 GHz mm-Wave carrier, achieving a single-wavelength transport. The phase-modulated digitized signal is decoded with a silicon micro-ring resonator (MRR)
and the radio signal experiences RF free-space transmission between two antennas. The second approach employs optical independent sideband (OISB) modulation, encoding a 1-GHz OFDM radio signal at 28 GHz and a 16-QAM 10 Gb/s carrierless
amplitude phase (CAP) modulated signal, both using single sideband (SSB) modulation accomplished by an in-phase-and-quadrature-components (I/Q) modulator.
In this thesis, we give special attention to photonic frequency conversion methods used in our experiments. For photonic up-conversion, we experimentally demonstrate two approaches: OISB modulation, which utilizes complex-valued optical
modulation, and optical photomixing of two light tones with different frequencies. For down-conversion, we explore a cost-effective coherent detection method using electro-absorption modulated lasers (EMLs).
Distributed multiple-input multiple-output (MIMO) networks require efficient RRHs due to the large number of antennas involved. With that in mind, in this thesis, we investigate integrating photonic technology into RRHs to enhance transparency and power efficiency. First, we examine a photonic RRH with full-duplex analog transmission at mm-Wave frequencies, utilizing EMLs for optical transceiver functions and photonic down-conversion for the uplink, eliminating
dispersion-induced fading over 15 km. Second, we investigate optical RF beamsteering through a Gires-Tournois etalon cascade, providing transparent optical true time delay (TTD) cost-effectively for a 2x5 antenna setup without adding complexity to
the antenna site. We demonstrate beamsteering up to 32○ with a 2x5 phased-array antenna at a 3.5 GHz RF carrier, transmitting 64-QAM OFDM radio signals. The OFDM signal transmission performance over optical fronthaul and free-space RF
propagation is evaluated using offline error vector magnitude (EVM) estimation and real-time high-definition video transmission. Additionally, we show a 2 kHz carrier phase switching using direct frequency modulation of the involved optical sources,
validating the concept for fast-tracking mobile users.
Optical wireless communication has gained significant importance due to its high data rates, security advantages, license-free spectrum, and immunity to electromag-
netic interference. In this thesis, we approach FSO wireless communication by integrating it with LiDAR sensing using the same opto-electronic hardware by using a frequency modulation continuous wave (FMCW) LiDAR. The LiDAR sensing
utilizes an RF-based frequency modulation method with a fixed-wavelength laser and coherent detection while the FSO data channel is integrated through intensity modulation and frequency-agnostic direct-detection (DD).
In conclusion, this thesis scientifically contributes to energy-efficient 5G or upcoming 6G networks by using photonic technologies to address RF spectrum congestion
and high data demand. Our innovative approaches in hybrid RoF infrastructures, photonic frequency conversion, and optical wireless communication enhance network performance and capacity. These contributions support sustainable, high-speed
communication systems, benefiting society by enabling more reliable and efficient connectivity.