The synchronization accuracy achieved via the IEEE 1588 Precision Time Protocol (PTP) in packet-based fronthaul networks is substantially impaired by packet delay variation (PDV). Nevertheless, in the particular case of deployment over tree topologies, it is known that PDV can be avoided by controlling the departure of PTP packets such that they experience close to constant delays over the fronthaul. This paper analyzes controlled PTP departure under constraints that are peculiar to a fronthaul scenario of interest and considering that radio traffic itself behaves as background traffic relative to PTP. Since the method involves buffering of radio traffic prior to controlled PTP transmissions, its impact on buffer sizes at the baseband and radio units, and the corresponding increase in fronthaul latency are also analyzed. In the end, results collected through a self-developed FPGA-based testbed are presented.

Provisioning of cost-effective Ethernet-based fronthaul by reusing the LAN infrastructure available in most commercial buildings is challenging predominantly in terms of the required bandwidth and synchronization. In contrast to a synchronous fronthaul, a PTP-based Ethernet network must cope with estimation noise introduced by packet delay variation (PDV) for synchronization recovery. The SYNC packet used for PTP on such networks is expected to suffer from significant PDV due to the fronthaul traffic and other background traffic. Further challenge is when the involved network switches do not support PTP and therefore synchronization can only be done by end-devices. Focusing on this scenario, this paper analyzes the problems that may affect the time-offset estimation accuracy and presents schemes to mitigate these problems. The performance is evaluated through a self-developed FPGA-based testbed and the results suggest that the end-to-end PTP approach can fulfill the less strict time alignment requirements of 3GPP standards if PDV is handled properly.

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Cloud radio access network (C-RAN) is a recent trend of RAN architecture positioned to help the operators to address challenges of new wireless services, such as emerging 4G and 5G mobile networks. C-RAN uses baseband processing units in a central server which connects to the radio front-ends at cell sites via the so-called fronthaul network. The fronthaul infrastructure is currently provided by CPRI (Common Public Radio Interface) and OBSAI (Open Basestation Architecture Initiative) industry standards which use dedicated optical links with high deployment costs. An alternative is to use Ethernet technology aiming to reuse of network infrastructure available in many commercial buildings. However, in contrast to the traditional synchronous fronthaul, Ethernet suffers with packet delay variation (PDV) and challenging synchronization recovery. This work presents a complete and flexible testbed to evaluate Ethernet-based fronthaul. The system is validated via extensive measurements that show the effects of synchronization procedures and network impairments on regenerated clock phase noise.

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