## 3 5G Fronthaul Architecture for Private Hotspots  
Figure 1a shows the conceptual representation of the proposed optical fronthaul architecture of a sports-stadium. The proposed system consists of a BBU-Box at a centralized location, e.g., at the backhaul or the metro-ring network of a converged FiWi architecture, being optically interconnected with multiple RRHs placed around the stadium (hotspot) in a bus topology and equipped with a ROADM and mm-wave antennas. The traffic exchange between the BBU and the RRHs of an optical bus relies on dedicated wavelength-specific WDM TxRxs comprising a number of integrated photonic EMLs and photo-receivers equal to the number of the WDM channels. It should be noted that the proposed fronthaul architecture supports frequency aggregation per wavelength, allowing in this way to load several user channels on different IF bands all carried by the same wavelength. Each RRH employs an integrated photonic ROADM for dropping any of the available WDM channels to the specific RRH or forwarding it to the next RRH of the bus, achieving in this way allocation of the available optical downlink capacity to different regions of the stadium. The ROADM is also responsible for adding any of the available WDM channels to the respective RRH, when uplink traffic is transmitted from the wireless-users to the BBU. Moreover, each RRH comprises a number of mm-wave antennas, each of them responsible for the optical-to-electrical (o/e) conversion of a specific WDM channel dropped by the ROADM and its up-conversion to the mm-wave band for wireless transmission to the end-users. Similarly, during the uplink traffic, each RRH's antenna is responsible for the reception, IF-conversion and electrical-to-optical (e/o) conversion of the mm-wave signal to a wavelength-specific optical signal. Figure 1: (**a**) Fronthaul architecture for serving a typical stadium area, where a number of buses is deployed. Each bus consists of a number of remote radio heads (RRHs), each one interconnected with a reconfigurable optical add-drop multiplexer (ROADM) through single mode fiber (SMF) wavelength division multiplexing (WDM) is also exploited. (**b**) Configuration of the baseband unit’s (BBU’s) and the RRHs’ WDM wavelength-specific TxRxs employed for the downlink communication between the BBU and the RRHs, and the uplink communication of the RRHs with the BBU. Figure 1b depicts the configuration of the BBU's and RRHs' WDM TxRxs enabling the simultaneous transmission of the n downlink and n uplink signals through two independent and different direction SMF links. Considering that this fronthaul architecture targets small-scale private networks, wavelength-specific WDM TxRxs are preferred to be employed over wavelength-tunable solutions, since for this scale, they allow for simpler and more cost efficient implementations. The same WDM channel grid can be used for both uplink and downlink traffic, since the uplink and the downlink are independent WDM links. Focusing on the downlink connection, the BBU's Tx contains an array of n wavelength-specific EMLs, each one modulating an individual multi-IF signal on a different WDM wavelength and an n:1 multiplexer (MUX) that multiplexes the n WDM channels and provides the single optical interface to the downlink SMF. At the hotspot site, n RRHs are employed, where the receiver (Rx) of each RRH comprises the drop part of the ROADM that is capable to either drop any of the available n WDM channels at one of its n output drop ports (if the data are intended for the specific RRH) or to forward it into the bus of the other RRHs (if the data are intended for another RRH). The RRH's WDM Rx is also equipped by an array of n photodiodes (PDs) and transimpedance amplifiers (TIAs) that are connected with the n drop ports of the ROADMs in order to perform the o/e conversion and the amplification of any of the n WDM channels.