## 2 Wireless Signals Transport Strategies for Fiber-Wireless Link  
To understand how wireless signals are distributed in a fiber-wireless link, Figure 2 shows the different link configuration for a simple point-to-point transmission connecting a CO and an antenna BS. Regardless of the wireless carrier frequencies, the wireless signals can be distributed based on five transport schemes namely RF-over-fiber (RFoF), IF-over-fiber (IFoF), Baseband-over-fiber (BBoF), Digitized RF-over-fiber (DRFoF) and Digitized IF-over-fiber (DIFoF) as shown in Figure 2.  
The simplest scheme for transporting wireless signals via an optical fiber backhaul network is to directly transport the wireless signals at the wireless carrier frequency without any further frequency processing at the base station via RF-over-fiber transport scheme (Figure 2a). In this configuration, the wireless signals are modulated onto an optical carrier and the optically modulated wireless signals are transported in an analog format over the optical link. RF-over-fiber has the advantage of simple base station design with added benefits of centralized control, wireless signal transparency and multi-band wireless support. However one of the major drawbacks is the needs for optical devices with speeds matching that of the wireless carrier frequency which becomes more stringent for wireless signals beyond the microwave bands. It is also well-established that optically modulated wireless signals suffers from fiber chromatic dispersion which degrades the received RF power and limits the fiber transmission distance [18,19]. In contrast, IF-over-fiber transport scheme transports the wireless signals at a lower intermediate frequency (IF) over the optical platform (Figure 2b) which in turn relaxes the requirements for high-speed optoelectronic hardware. The optical distribution of the IF signals also has a much reduced  
Figure 2: Schematic showing the transport scheme for **(a)** RF-over-Fiber (RFoF) **(b)** IF-over-Fiber (IFoF) **(c)** Baseband-over-Fiber (BBoF) **(d)** Digitized RF-over-Fiber (DRFoF) **(e)** Digitized IF-over-Fiber (DIFoF). fiber chromatic dispersion impact. However this is at the expense of a more complex antenna BS design where it necessitates a stable local oscillator (LO) and mixers for frequency translation purposes. Figure 2c shows the architecture for baseband-over-fiber transport where the wireless signal is transported as baseband data optically and the wireless signal is processed entirely at the antenna BS. This transport scheme is compatible to existing optical networks where data are transmitted in a predominantly digital environment. Despite the simplicity in the optical domain, the antenna BS for baseband-over-transport has to house additional hardware to electronically process the wireless signals which increases the BS complexity while decreases the link transparency. In the recent years, a new transport scheme based on digitization of the wireless signals have been proposed and demonstrated [16] for fiber-wireless application that exploits the superior digital optical links while maintaining a relatively simple BS design. As opposed to baseband-over-fiber transport where the wireless signals are transported at the information level and all signal processing are carried out in the antenna BS, digitized wireless transport essentially digitized the wireless signals before the optical transport. Digitized wireless transport can be further classified into Digitized RF-over-fiber (DRFoF) transport and Digitized IF-over-fiber (DIFoF) transport. Digitized RF-over-fiber scheme as illustrated in Figure 2d digitizes the wireless signal to produce a sampled digital data stream in a serial format that can directly modulate the optical source. This enables the use of digital photonic link to transport the wireless signals overcoming the inherent issues related to analog photonic links.