Abstract
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.
Highlights
Centralized and cloud radio access networks (C-RAN) have been widely investigated as key enablers for fifth-generation (5G) mobile communications [1]
Since the precision time protocol (PTP) real-time clocks (RTCs) is continuously rate-adjusted, eventual discrepancies can occur. Due to this hardware limitation, we achieve the best timing alignment performance in our setup with the radio frame synchronization (RFS) controller still based on the PTP RTC but over the PTP-aware FH and with the RTC driven by the oven-controlled XO (OCXO), rather than the XO adopted in all other experiments
This work investigated the problem of timing alignment of RF transmissions carried out by spatially distributed radio units that are served over an Ethernet FH
Summary
Centralized and cloud radio access networks (C-RAN) have been widely investigated as key enablers for fifth-generation (5G) mobile communications [1]. By aligning the start of the 10 ms frames of LTE or 5G new radio (5G-NR) [14] among the radio-frequency (RF) transmissions carried out by spatially distributed RRUs. the process is termed radio frame synchronization (RFS) in this text. Most testbed-based FH studies such as [28]–[30] focus on metrics like FH latency and PDV, rather than the RF timing alignment among cooperative RRUs. To the best of our knowledge, there are no publications focusing on practical evaluations of eCPRI’s or RoE’s mechanisms for timing alignment of distributed transmissions. The work is organized as follows: Section II formulates the problem of distributed radio timing alignment; Section III describes the adopted RFS architecture; Section IV discusses the hardware and software of a prototype implementation; Section V gives an overview of our testbed; Section VI presents experimental results.
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