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This work explores clock synchronization algorithms used to process timestamps from the IEEE 1588 precision time protocol (PTP). It focuses on the PTP-unaware network scenario, where the network nodes do not actively contribute to PTP's operation. This scenario typically imposes a harsh environment for accurate clock distribution, primarily due to the packet delay variation experienced by PTP packets. In this context, it is essential to process the noisy PTP measurements using algorithms and strategies that consider the underlying clock and packet delay models. This work surveys some attractive algorithms and introduces an open-source analysis library that combines several of them for better performance. It also provides an unprecedented comparison of the algorithms based on datasets acquired from a sophisticated testbed composed of field-programmable gate arrays (FPGAs). The investigation provides insights regarding the synchronization performance under various scenarios of background traffic and oscillator stability. INDEX TERMS Clock synchronization, IEEE 1588, partial timing support, precision time protocol.

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.

This work investigates a mechanism for alignment of the timing on which spatially distributed and cooperative radio units transmit in radio-frequency (RF) when served over a packet-based fronthaul. It analyzes the problem by considering the imperfect clock synchronization of the radio units and the packet delay variation that fronthaul packets are subject to. Following the analysis, this paper proposes an implementation architecture for distributed RF transmission timing alignment based on synchronized triggering among radio units and centralized processing units. Throughout this discussion, special attention is given to the scheme's impact on the overall achievable fronthaul latency. Subsequently, this work discusses both hardware and software aspects of a prototype that was developed based on field-programmable gate arrays (FPGAs). In the end, it presents results obtained on an Ethernet fronthaul testbed where the referred FPGA-based prototypes implement radio units that are synchronized using the IEEE 1588 precision time protocol or by pulse-per-second references. Results validate the functionality of the proposed architecture and illustrate various relevant choices concerning system parameters. INDEX TERMS 5G, clock synchronization, Ethernet, fronthaul, precision time protocol.

The evolving broadband access systems using copper networks are currently deployed in a frequency band that goes up to 30 MHz, as specified in VDSL2. As hybrid fibercopper architectures become more important in the industry and academia, using shorter loop lengths (i.e. up to 250 meters) from the last distribution point to users enables adopting even higher frequencies to achieve very high data rates of 500 Mbps and beyond, as is the case with the G.fast standard under development by ITU-T. In this work, a time-domain simulator has been developed to evaluate G.fast system performance. System capacity is evaluated with different cyclic extension lengths and different reference loop topologies specified by ITU-T. The simulation results show that G.fast systems are robust to bridgetaps and capable of providing very high data rates for all simulated loop topologies to support next generation ultra high speed broadband services.