## 4 QoS Provisioning Techniques for Future FiWi Networks
### FiWi Architectures  
Recently, various FiWi network architectures were surveyed in [42]. In this section, we briefly review the state-of-the-art of FiWi network architectures, including FiWi network design proposals not covered in [42]. Typically, cellular networks (such as LTE) are used for fast moving users, e.g., train passengers. They suffer from frequent hand-overs when hopping from one BS to another one. The frequent hand-overs may cause numerous packet losses, resulting in a significantly decreased network throughput. An interesting approach to avoid hand-overs in cellular networks for train passengers is the use of an optical fiber WDM ring-based FiWi network installed along the rail tracks in combination with the moving cell concept, as recently proposed in [43]. Fig. 4 depicts the moving cell-based FiWi network architecture using an optical fiber WDM ring to interconnect the RAUs with the CO, where all processing is performed. Each RAU deploys an optical add-drop multiplexer (OADM) fixed tuned to a separate wavelength channel. At the CO, a WDM laser generates the desired wavelengths, which are optically switched and passed to an array of RF modulators, one for each RAU. The modulated wavelengths are multiplexed onto the optical fiber WDM ring and received by each addressed RAU on its assigned wavelength. An RAU retrieves the RF signal and transmits it to the antennas of a passing train. In the upstream direction, the RAU receives all RF signals and sends them to the CO for processing. By processing the received RF signals, the CO is able to keep track of the train location and identifying the RAU closest to the moving train. It then assigns downstream RF signals to the corresponding RAU such that the train and moving cells move along in a synchronous fashion. To provide connectivity for movement of end-users in any possible direction, the _moving extended cell_ concept was proposed in [44]. A hybrid frequency division multiplexing (FDM)/WDM network architecture was used to support the delivery of multiple RF channels in the 60 GHz frequency band over the same wavelength. The extended cell involves the current user's cell and the surrounding cells ensuring connectivity for any random direction. It is adaptively restructured when the user enters a new cell. It was shown that the proposed concept can provide zero packet loss and call dropping probability in high-rate wireless services for a wide range of mobile speeds of up to 40 m/sec, independently of the fiber link distances. As mentioned above, both EPON (and PON in general) and WiMAX networks have a PMP topology with a central control station (OLT in EPON and BS in WiMAX) performing DBA by means of centralized polling and scheduling. These similarities give rise to interesting convergence problems whose optimization is expected to lead to an improved FiWi network performance. According to [45], the integration of EPON and WiMAX access networks can be done in several ways, ranging from independent and unified connection-oriented to microwave-over-fiber architectures. The FiWi network proposed in [46] consists of an optical WDM backhaul ring with multiple single-channel or multi-channel PONs attached to it, as shown in Fig. 5. More precisely, an OADM is used to connect the OLT of each PON (located at the CO) to the WDM ring. In this figure, MPPs are used to bridge PONs and WMN. In the downstream direction, data packets are routed from the CO to the MPPs through the optical backhaul and are then forwarded to the STAs by MAPs. In the upstream direction, MAPs forward data packets to one of the MPPs, where they are subsequently transmitted to the CO on one of the wavelength channels of the optical backhaul WDM ring, as each PON operates on a separate dynamically allocated wavelength channel. Since the optical backhaul and WMN use different technologies, an interface is defined between each ONU and the corresponding MPP in order to monitor  
Figure 4: Moving cell-based RoF network architecture for train passengers [43].