# Integrated Photonics for IoT, RoF, and Distributed Fog-Cloud Computing: A Comprehensive Review
## 3 Radio-over-Fiber (RoF) for IoT and Fog Computing
The use of remote radio heads and centralized signal processing reduces the power and processing burden at edge nodes, making the architecture more energy-efficient and suitable for constrained environments [10]. Additionally, RoF allows for the dynamic allocation of wireless resources and centralized control of modulation and frequency schemes, enhancing the flexibility of Fog-based networks [19]. An illustrative example is a smart factory equipped with thousands of wireless sensors measuring variables such as temperature and vibration. Instead of embedding local processing in each sensor, lightweight RF-capable transceivers transmit signals to distributed access points. These RF signals are then carried over optical fiber using Radio-over-Fiber (RoF) links to a centralized fog node. This intermediate node performs real-time analytics on the incoming data and forwards summarized insights to the cloud for long-term storage or machine learning model updates [20]. Another illustrative example is a smart city environment equipped with thousands of distributed wireless sensors monitoring environmental conditions such as air quality, traffic flow, noise levels, energy consumption, and public safety parameters. Instead of relying on local processing at each sensor node--which would significantly increase power consumption and system complexity--lightweight transceivers transmit RF signals to strategically placed access points. These RF signals are then carried over optical fiber using RoF links to geographically distributed Fog nodes located in traffic management centers, utility hubs, or public safety stations. The Fog nodes perform real-time analytics to support immediate decision-making, such as dynamic traffic light control, air quality alerts, or emergency response coordination. Processed data and long-term trends are subsequently forwarded to Cloud servers for historical analysis, city-wide optimization, and predictive modeling. This RoF-enabled architecture ensures high-bandwidth, low latency communication across the smart city while maintaining centralized control and scalability [21]. Overall, the integration of RoF with photonic technologies provides a scalable and energy-efficient foundation for future distributed computing networks, enabling seamless communication between IoT devices, Fog nodes, and Cloud infrastructures. Table 2 outlines key research opportunities for enhancing RoF systems through the integration of PICs [22; 23]. It highlights how PICs can address specific figures of merit across cost, performance, scalability, and reliability in various RoF configurations. Figure 3 presents a schematic of the experimental setup for generating multi-channel 28 GHz millimeter-wave signals using a silicon photonic integrated circuit. The system employs two tunable laser sources (TLSs) with orthogonal polarization states, which are aligned using a polarization controller (PC) and separated by a polarization beam-splitter (PBS). Each optical carrier is modulated independently by a Mach-Zehnder modulator (MZM) before being combined on-chip by a polarization beam combiner (PBC) implemented in silicon photonics. The polarization multiplexed signal is then converted into an electrical millimeter-wave output via a photodetector (PD). This configuration enables the compact and efficient integration of polarization control and high-frequency signal generation on a single chip [24]. Figure 3: Schematic diagram of the experimental setup for multi-channel 28 GHz millimeter-wave signal generation on a silicon photonic chip. The system includes two tunable laser sources (TLSs), a polarization controller (PC), a polarization beam-splitter (PBS), and silicon photonic integrated circuit components such as a 2 \(\times\) 2 multimode interference (MMI) coupler, Mach–Zehnder modulators (MZMs), and an on-chip polarization beam combiner (PBC). A photodetector (PD) is used to convert the optical signal into an electrical millimeter-wave output. Figure taken from [24]. Figure 2: RoF network in urban areas.