## 2 Key Technologies for Analog-Radio-over-Fiber (A-RoF) Fronthaul Links  
In order to prepare mobile fronthaul networks for the emerging 5G era, worldwide research efforts have been directed towards upgrading the underlying key technologies, which currently appear to follow some widely adopted deployment directions:  
(1) High capacity optical links between the BBU and the RRH for efficient mobile fronthauling. Current CPRI-based D-RoF links may not meet the high bandwidth and low cost requirements that will be imposed by 5G mobile traffic, due to the digitization of samples from the radio domain [20]. For instance, for a potential 5G implementation with carrier aggregated 200 MHz signals, \(64\times 64\) MIMO and three sector antennas, a fronthaul link of 2.4 Tb/s would be required by using CPRI [33], necessitating the prohibiting number of \(100\times 24\) Gb/s optical TxRxs. Moreover, digital interfaces of CPRI introduce excess delay and jitter, rendering them incompatible with the high speed services that require strict synchronization. To solve these issues, A-RoF technique has been proposed as a promising candidate for the realization of a bandwidth- and cost-efficient fronthaul links [22]. A-RoF allows the transportation of native radio waveforms over the fiber via analog optics, enabling a remarkable reduction of the required bandwidth [22; 23]. The most common A-RoF transport methods are considered to be the Radio Frequency-over-Fiber (RFoF) and the IFoF techniques [34]. In RFoF, the BBU directly transmits the mm-wave signal via the fiber to the RRH, without the need for frequency up-conversion at the RRH side. Although this technique allows for simple RRH designs, it is not preferred for the transport of mm-wave signals over fiber, since it requires the equipment of the RRH with expensive high-speed TxRxs and at the same time the mm-wave signals are significantly affected by the fiber chromatic dispersion [35]. In this context, IFoF has emerged as a more appealing A-RoF technique that allows transportation of multiple aggregated IF bands, modulated by cost-effective, low-bandwidth linear EMLs [24]. In IFoF, the signals are transmitted via a fiber in different IFs and the up-conversion at the mm-wave band takes place just before the wireless transmission. In this way the effect of the fiber chromatic dispersion is significantly reduced and the use of cost effective low-speed optical TxRxs is enabled. So far, the IFoF-based fronthaul architectures supporting multiple frequency bands, evaluated only for the fiber part of the network have achieved CPRI equivalent rates up to 1 Tb/s per wavelength [25; 36; 37]. Moreover, converged FiWi IFoF demonstrations have been shown to reach capacities up to 45 Gb/s per wavelength when using simple quadrature amplitude modulation (QAM) formats and sub-carrier (SC) multiplexing of multiple IF bands [38; 39; 40; 41]. Considering FiWi links using OFDM formats, the maximum capacity has been reported to be equal to 24.08 Gb/s, using a single RF band with a bandwidth of 6.02 GHz [42]. On the other hand, FiWi IFoF links that satisfy at the same time both requirements for increased number of IF bands and the use of OFDM numerology, have been demonstrated with lower aggregate bit rates [43; 44]. Particularly, the maximum reported capacity for this type of demonstrations reached up to 4.56 Gb/s using multiple IF bands of 152 MHz bandwidth [43]. (2) OFDM-based waveforms, relying on narrow-band SCs, formed the cornerstone and shaping technology for the success of 4G LTE networks [45]. The employment of OFDM allows for simple wireless channel estimation and efficient hardware utilization. Therefore, the OFDM waveform numerology in both uplink and downlink communication has already been approved by 3 GPP as part of the NR physical layer design [26; 46]. This has stimulated also the demonstration of various OFDM-based 5G optical fronthaul links, with the majority of the high capacity demonstrations based on costly, bulky Mach Zehnder modulators (MZMs) [42; 43; 47].