## 1 Introduction
Currently, the centralized BBU is connected with the RRHs via digital radio-over-fiber (D-RoF) links using standardized protocols such as the Common Public Radio Interface (CPRI) [20], which may not be capable of covering the traffic demands of the next-generation mobile networks [14]. This is because CPRI imposes a very high penalty in terms of bandwidth overhead since it requires a high number of samples in order to digitally transport and reconstruct the wireless signal at the edges of the fronthaul. To overcome these issues, enhanced CPRI (eCPRI) has been proposed as the next generation interface for D-RoF fronhauling [21], providing flexibility in the splitting of the functions performed in the BBU and RRH. However, the different levels of functional split introduce a trade-off between the bit rate and centralization of the RAN. While D-RoF technology tries to overcome the bandwidth explosion with a network layer approach, analog-radio-over-fiber (A-RoF) fronthaul solutions focusing on the physical-layer have been recently proposed in order to further improve the optical spectrum efficiency by the transmission of native analog waveforms over the fiber [22; 23]. Towards bringing A-RoF fronthaul networks into realization, research efforts have been currently directed in the development of some key-enabling technologies comprising the use of: intermediate frequency (IF) channel aggregation [24; 25], orthogonal frequency division multiplexing (OFDM) waveforms [26], wavelength division multiplexing (WDM) [27] and photonic integrated circuits (PICs) [28]. Drawing from these latest achievements, we recently proposed an analog optical fronthaul 5G architecture that relies on PIC-based transceivers (TxRxs) and reconfigurable add/drop optical multiplexers (ROADMs) to synthesize a privately owned hotspot fiber-wireless (FiWi) network for stadiums [29], evaluated in terms of network throughput and latency [30]. Preliminary physical-layer simulation results of only the downlink fronthaul link have been also reported in [31]. In this paper, we extend our previous work [31] by providing for the first time the complete physical-layer evaluation analysis of the proposed 5G fronthaul architecture for both downlink and uplink OFDM-based analog IF-over-fiber (A-IFoF) links. The proposed links employ photonic indium phosphide (InP) externally modulated lasers (EMLs) at the transmitter (Tx) site and optical Mach-Zehnder Interferometer (MZI)-based ROADMs at the hotspot area. Our analysis relies on an experimentally-verified simulation models of the optical TxRxs as well as the signal degradation induced by the fiber. The WDM functionalities are leveraged by a novel design of a ROADM configuration supporting up to four WDM channels with 100 GHz channel spacing. The proposed ROADM device is integrated on the ultra-low loss Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) TriPleX platform, with propagation loss of 0.1 dB/cm [32]. The successful transmission of 4-IF band OFDM 64-QAM signals with bit rates up to 12 Gb/s per wavelength was confirmed by carrying out error vector magnitude (EVM) measurements. The successful operation revealed that the proposed architecture can be a cost-effective, energy-efficient and low-latency 5G candidate network solution for high-density hotspots. The paper is organized as follows: Section 2 presents the key technologies of the A-RoF fronthaul links. Section 3 describes the concept of the proposed fronthaul. Section 4 presents the experimentally-matched simulation models, while Section 5 presents the PHY-layer setup and results. Conclusions are addressed in Section 6.