Global Reference Time Clock Recovery for Wireless Fronthaul
Solution Overview
Problem
Conventional Differential Clock Recovery (DCR) implementations in wireless fronthaul systems face limitations in synchronizing both frequency and phase across bidirectional paths, particularly in packet-based networks, due to unknown phase offsets and the inability to carry timestamps efficiently using existing standards like IEEE 1914.3 for Radio over Ethernet (RoE).
Innovation Solution
The use of a Global Reference Time for Differential Clock Recovery allows for constant time delay and phase preservation across packet networks, enabling frequency and phase synchronization. This is achieved by embedding timestamps in a subtype of control packets with specific operational codes, such as Vendor Specific Data (VSD), separate from the 'orderInfo' field, and including flags to indicate frequency or phase-aligned Time of Day (ToD) timestamps, maintaining compliance with the RoE standard.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If SyncE is used to syntonize clocks on each node, then frequency synchronization is achieved, but phase offset remains unknown and cannot be recovered
Solution Approach 1:
The patent introduces a Global Reference Time (GRT) as an intermediary reference that both transmitting and receiving nodes use to measure time stamps. This GRT mediator enables phase measurement by providing a common reference frame, allowing the receiving node to calculate phase offset from the time stamp difference between transmission and reception events.
Solution Approach 2:
The patent applies preliminary action by having the transmitting node record the time stamp of the client signal relative to the GRT before transmission occurs. This pre-recorded time stamp is embedded in the packet and transmitted to the receiving node, enabling phase calculation without requiring real-time interaction or additional signaling during the actual transmission.
2Ease of operation
If the 'orderInfo' field in RoE is used for sequence numbering, then packet ordering is maintained, but timestamp carrying capability is lost
Solution Approach 1:
The patent segments the time synchronization function from the packet sequencing function. Instead of using the 'orderInfo' field for both purposes, it extracts a dedicated timestamp field specifically for carrying time information. This segmentation allows the 'orderInfo' field to maintain packet sequencing while the extracted timestamp field handles time synchronization independently.
Solution Approach 2:
The patent adds another dimension to the RoE header structure by extracting a separate timestamp field from the existing 'orderInfo' field. This dimensional separation creates independent functionality: the 'orderInfo' field continues to handle sequence numbering while the new timestamp field handles time synchronization, allowing both functions to operate simultaneously without interference.
3Adaptability or versatility
If packet networks are used for wireless fronthaul, then flexibility and cost-effectiveness improve, but phase synchronization and delay control become difficult
Solution Approach 1:
The patent implements feedback by having the receiving node measure the actual reception time relative to the GRT and compare it with the transmitted time stamp. This feedback mechanism allows the system to calculate the actual propagation delay and phase offset, enabling compensation and precise phase synchronization even in the variable delay environment of packet networks.
Solution Approach 2:
The patent changes the parameter reference frame from node-specific local clocks to a universal Global Reference Time. This parameter change allows time measurements to be expressed in a common reference frame, making phase and delay measurements consistent and comparable across different packet network paths, thereby enabling precise synchronization in flexible packet-based architectures.
Data Source
AI summary
A method, at an egress node, includes synchronizing with a global reference time; receiving a signal including a presentation time for when a specific part of the signal is to be transmitted, wherein the presentation time was determined by the ingress node with reference to the global reference time; determining an actual transmission time when the specific part of the signal is transmitted; and causing adjustment of a clock based on a time error derived from a difference between the presentation time and the actual transmission time.