# A QoS-Enabled Medium-Transparent MAC Protocol for Fiber-Wireless 5G RAN Transport Networks
###### Abstract  
In order to meet the ever-increasing 5G and beyond Radio Access Network (RAN) densification demands, Fiber-Wireless transport networks are expected to play a key role in accelerating 5G deployment by providing the essential RAN flexibility, while at the same time avoiding costly fiber-trenching. Due to the inefficiency of the Radio-and-Fiber (R&F) networks for application in dense RAN topologies, Analog-Radio-over-Fiber (A-RoF) technology is regarded as a key enabling solution, since it greatly simplifies the remote antenna while offering very high spectral efficiency. For this type of dense A-RoF network, new and efficient Medium-Transparent-Medium Access Control (MT-MAC) protocols are required that can concurrently arbitrate optical and wireless resources, while at the same time offering the necessary Quality-of-Service (QoS) for correct operation of the combined Fronthaul/Midhaul/Backhaul segments present in 5G disaggregated RANs. In this paper, we propose a QoS-enabled MT-MAC (qMT-MAC) protocol that can combine Fronthaul/Midhaul/Backhaul flows under the same framework, while satisfying the strict delay and jitter requirements set by the relevant standards. Results show that qMT-MAC concurrently achieves the delay and jitter requirements for combined Fronthaul/Midhaul/Backhaul traffic even when loads approach the network's capacity, while attested enhanced prioritization policies can offer up to a 64% delay reduction over State-of-the-Art MT-MAC protocols. analog Radio-over-Fiber (a-RoF) 5G; fronthaul; midhaul; backhaul; Disaggregated Radio Access Networks (D-RAN); Fiber-Wireless (FiWi); Medium-Transparent MAC (MT-MAC); Ultra-Reliable and Low-Latency Communications (URLLC); Quality-of-Service (QoS) +
Footnote †: journal: Accepted: 26 August 2022  
(i) the interconnection of the numerous AP locations and (ii) the equipment expenditure associated with the acquisition and operation of the mmWave 5G NR APs. Regarding the first issue, fiber carries the advantage of immense capacity and, as such, has been established as the main sub-6 GHz RAN enabler. However, the RAN densification goals set by the mobile operators are being hampered by the high fiber trenching costs, as well as license fees to rights-of-way and regulatory requirements to access public or utility-owned sites [3]. All these immense costs lead to increased Capital Expenditures (CAPEX) and a longer Time-to-Market [4], prompting the industry to aggressively develop combined Fiber-Wireless (FiWi) RAN transport solutions to speed up 5G deployment and provide the essential RAN flexibility [5]. Regarding the second issue, 5G NR proposed RAN disaggregation, which splits the monolithic Base Station to Central/Distributed/Remote Units (CU/DU/RUs). As a consequence of the above, densification costs are driven down since the numerous RUs become functionally simple while computationally demanding functionalities are passed on to the more centralized CU and DU modules, which offer pooling advantages while providing enhanced flexibility to the MNOs [6]. However, the produced Fronthaul, i.e., the network between the CU and DU modules, and Midhaul, i.e., the network between the DU and CU modules, aggravate the RAN resource provisioning problem, as such connections must simultaneously support tremendous bandwidth, ultra-low latencies, and zero jitter [7]. In addition to the densification and adequate capacity provisioning issues, the nature of emerging 5G and industrial Internet applications that are classified under the Ultra-Reliable and Low-Latency Communications (URLLC) category pose severe latency constraints, which can be as low as 250 \(\upmu\)s for factory automation applications [8]. From the above, it is evident that highly efficient converged FiWi RAN solutions that can support ultra-low latencies can play a critical role in the formation of the next-generation 5G RAN transport network. Layer-2 protocol efficiency for converged FiWi networks has received much attention in recent years.