Analysis of 5G Synchronous Networking Architecture and Key Technologies

1 Introduction

The 5G licenses have been issued, and the commercial deployment of 5G is underway. The 5G network is at a critical stage of industrial application cultivation, and the 5G high-precision synchronous network, as an essential foundational support network, urgently needs to be promoted in terms of technology and industrial development to effectively support the smooth rollout of 5G commercial services.

This article first analyzes the time synchronization requirements of the 5G system, elaborates on the necessity of employing ground synchronous networking to solve the synchronization issues of the 5G system, proposes a general model for high-precision synchronous networking, and focuses on analyzing key technologies such as high-precision sources, high-precision synchronous transmission, and high-precision synchronous monitoring aimed at 5G system synchronization. This provides references for subsequent 5G synchronization technology scheme selection and networking strategy formulation, promotion of international and domestic standards, and smooth evolution of synchronous networks in China.

2 5G High-Precision Time Synchronization Requirements

5G synchronization is used to support 5G networks and services, including frequency synchronization and time synchronization. The frequency synchronization requirement is similar to that of other wireless communication systems, which is better than ±0.05ppm[1], while the time synchronization requirement is even stricter, with this article focusing on 5G time synchronization. Depending on the application scenarios and synchronization precision, the time synchronization requirements of the 5G system include basic synchronization requirements, inter-station collaborative enhanced synchronization requirements, and high-precision synchronization requirements proposed by new services supported by 5G.

The basic synchronization requirement of the 5G system is a common requirement for all time-division duplex (TDD) wireless communication systems, primarily to avoid uplink and downlink slot interference, thus necessitating strict limits on the time deviation of the base station’s air interface. For 4G TDD systems, a fixed subcarrier spacing of 15kHz is used, with the guard period (GP) configured to one symbol, requiring that the time deviation between base stations be less than 3μs within a certain coverage area. The 5G system also adopts a TDD mode, which features flexible expansion of subcarrier spacing, and by flexibly configuring multiple symbols in the GP, the time deviation requirement between base stations remains less than 3μs, consistent with 4G TDD.

Inter-station collaborative enhancement in the 5G system primarily includes multi-antenna MIMO, multi-point coordination, and carrier aggregation. To ensure effective collaboration, the time difference of signals from different collaboration points must not exceed the cyclic prefix (CP), thus imposing stringent time synchronization requirements on the time deviation between collaboration points, on the order of 100ns or even higher.

Among the various new services supported by the 5G network, a typical one is base station positioning services. With the explosive growth in demand for high-precision positioning services, the potential for base station positioning based on the 5G system is significant. Generally, positioning accuracy is directly related to time synchronization accuracy; for example, to achieve a positioning accuracy of 3m, the air interface signal synchronization deviation between base stations must be ±10ns.

3 5G High-Precision Time Synchronization Networking Model

3.1 Necessity Analysis of 5G High-Precision Ground Synchronization Networking

For a long time, operators mainly met the synchronization needs of wireless mobile communication systems by installing satellite receivers at base stations. During the 4G era, some operators addressed the synchronization problems of wireless base stations through ground synchronous networking, but generally as a backup, or to solve synchronization issues for base stations in areas where satellite signals are difficult to cover, such as subways, underground garages, and certain urban high-rise buildings.

Compared to 4G systems, 5G systems have the following new synchronization requirement characteristics.

(1) Higher precision synchronization requirements. The 5G system has μs-level basic service synchronization requirements, 100ns-level collaborative enhancement technology synchronization requirements, and even higher precision synchronization requirements for other new services, making it difficult for base stations to meet these requirements using ordinary satellite receivers alone.

(2) More complex synchronization application scenarios. A significant characteristic of the 5G system is the high density of base station deployment in certain application scenarios. With the continuous advancement of urbanization in China, the proportion of indoor base stations is increasing, leading to many scenarios where 5G base stations cannot obtain satellite signals.

(3) Stricter requirements for synchronization safety and reliability. Synchronization is a prerequisite for the safe and reliable operation of the 5G system. Given the importance of the 5G system itself and the services it supports, higher requirements for synchronization safety and reliability are imposed. Considering the increasing frequency of failures caused by unintentional or intentional interference with satellite signals, and the occurrence of cases where satellite signals are attacked (such as spoofing), complete reliance on satellite timing for 5G synchronization poses significant security risks.

(4) More sensitivity to costs. With the large scale of 5G base station deployment, if each base station were to be equipped with a satellite receiver, the equipment investment and operational maintenance costs would be enormous. However, achieving ground high-precision synchronization networking through the carrying network is relatively low in construction and operational maintenance costs.

In light of the above analysis, to meet the synchronization requirements of the 5G system, solve satellite coverage blind spots, enhance safety and reliability, and save construction and operational maintenance costs, the research and construction of an autonomous, controllable, and secure high-precision time synchronization network is a trend. It should be noted that the establishment of a high-precision ground time synchronization network will not completely replace the base station satellite timing solution in one step; the two will coexist and complement each other for a long time.

3.2 5G High-Precision Synchronous General Networking Model

Domestic CCSA and international organizations such as ITU-T, 3GPP, CPRI, IEEE, and ORAN are conducting research on 5G synchronization solutions. Currently, compared to fiber timing and Network Time Protocol (NTP), the networking based on Precision Time Protocol (PTP) is the primary implementation scheme for 5G high-precision time synchronization.

The general networking model for 5G high-precision time synchronization based on PTP is shown in Figure 1. The high-precision time server (PRTC/ePRTC) as the source device can adopt satellite timing key technologies. In cases where satellites are unavailable, ultra-high precision time synchronization signals can be obtained from the ground (e.g., through fiber timing traced back to national timekeeping units), ensuring that the 5G time synchronization network is autonomous and controllable. The PRTC/ePRTC typically also implements the GrandMaster (GM) function, so reference point A in Figure 1 is generally located inside the device, and in this case, there is no need to standardize its performance requirements.

Analysis of 5G Synchronous Networking Architecture and Key Technologies

Figure 1 5G High-Precision Time Synchronization General Networking Model

The performance indicators of the high-precision time server should meet the requirements of ITU-T G.8272.1 standard, with time accuracy better than ±30ns. The core part of the 5G time synchronization network, between reference points B and C in Figure 1, can use high-precision synchronous transmission technology to achieve high-precision synchronous carrying, consisting of multiple telecom boundary clocks (T-BC). It is important to emphasize that the time synchronization performance of a single node and the network scale (the number of hops in the time synchronization chain) are two important limiting parameters for the synchronization indicators of the carrying portion between B and C. To enhance end-to-end synchronization performance and expand network scale, the time synchronization accuracy of transmission devices at single nodes should be better than a certain threshold (e.g., ITU-T G.8273.2 specifies that the time error for type C and type D T-BC is on the order of 10ns or even smaller).

Reference points C or D in Figure 1 represent the connection points between the 5G time synchronization network and wireless end devices (such as 5G base stations). High-precision synchronization interfaces (such as in-band 10GE/25GE) can be considered for connection to reduce time errors introduced by internal interconnections. In 5G networking, through the reallocation of functions on the wireless access network (RAN) side and the use of Ethernet-based eCPRI interfaces in front-haul, reference point D in Figure 1 may be located within the wireless device, as the clock (Slave) and end applications (such as AAU) could be integrated into the same device.

The synchronization requirements of 5G are generally measured by the relative time deviation between wireless air interfaces (reference point E in Figure 1), while the synchronization network generally meets the relative time precision requirements on the wireless side by achieving absolute time precision relative to Coordinated Universal Time (UTC). For example, to meet the relative time deviation between two AAU’s wireless air interface reference points E (such as 3μs), the absolute time deviation of each AAU’s wireless air interface output relative to UTC must meet certain limits (e.g., ±1.5μs).

It is important to emphasize that high-precision time synchronization networks typically adopt a hierarchical master-slave networking approach, which is a classic networking model successfully applied in traditional frequency synchronization network construction. A prominent feature of the hierarchical master-slave networking approach is that due to noise accumulation effects, the longer the transmission link and the more nodes it passes through, the more significant the signal degradation. Therefore, when actually constructing a high-precision time synchronization network, consideration should be given to reducing the networking range, such as regional networking based on counties; another networking approach is to sink the source device as close to the network’s end base stations as possible, reducing the impact of the network carrying portion on synchronization performance and achieving a flatter networking structure.

4 5G High-Precision Synchronization Key Technology Analysis

The synchronization network generally consists of synchronization node devices and timing links connecting the node devices, and the 5G high-precision ground time synchronization network is no exception. Building a 5G high-precision ground time synchronization network involves several key technologies, including high-precision source technology as node devices, 1588 technology for constructing ground timing links, and monitoring technology for network-wide management.

4.1 High-Precision Synchronization Source Technology

The realization of high-precision synchronization sources is closely related to satellite timing technology. The accuracy of satellite timing depends on multiple factors, including the satellite system, atmosphere, receiving system, local clock source, phase-locked loop, and distribution interface. Among various satellite timing technologies, single-frequency satellite timing is the most widely used. However, due to the influence of various atmospheric environmental factors, the timing accuracy is limited, only achieving synchronization precision on the order of hundreds of nanoseconds, which cannot meet the 30ns level requirement of high-precision synchronization devices[2]. Dual-frequency satellite technology can eliminate ionospheric delay errors and significantly improve timing accuracy, making it a primary implementation technology for future high-precision synchronization source devices. Satellite co-viewing technology can further eliminate related errors of satellite clocks and propagation paths, achieving long-distance high-precision tracing, and can be considered as a measurement technology option for high-precision synchronization.

It should be noted that in recent years, China’s independently developed BeiDou satellite navigation system has been continuously improved and deployed. The BeiDou II system was officially commercialized in 2012, and the BeiDou III system is expected to be completed and put into commercial use by the end of 2020. The application scale of the BeiDou system in communication networks will further expand, helping to reduce dependence on satellite navigation systems from other countries and enhancing the safety and reliability of communication networks. The use of dual-frequency technology based on the BeiDou system will become the mainstream implementation technology for future high-precision synchronization source devices.

4.2 High-Precision Synchronization Transmission Technology

High-precision synchronization transmission is used to organize timing links and is a key aspect of 5G high-precision synchronization networking. Currently, 1588v2 technology has a large application scale in telecom networks, high maturity, and good interoperability. It is recommended to optimize existing configurations to enhance precision, including placing the stamping position as close to the physical interface as possible, improving stamping resolution, enhancing the synchronization accuracy of the real-time clock (RTC), strengthening cooperation between modules, and selecting high-quality oscillators. This will facilitate the rapid deployment and commercial maturity of the 5G high-precision time synchronization network. Additionally, the industry is also paying attention to other high-precision synchronization transmission technologies such as White Rabbit (WR) and 1588v2.1. However, both WR technology and the new version of the 1588 standard are new high-precision transmission implementation solutions, which are currently not considered as high-precision synchronization transmission technologies due to their high implementation difficulty compared to the optimized 1588v2 solution.

Considering the susceptibility of 1588v2 technology to fiber asymmetry in practical applications, it is recommended that the 5G time synchronization network adopt a single-fiber bidirectional deployment of 1588v2 whenever conditions permit. Furthermore, for the opening and operation of 1588v2, it is recommended to introduce software-defined synchronization network functionalities[3] to enhance the security and reliability of the synchronization network and improve operational management efficiency.

4.3 High-Precision Synchronization Monitoring Technology

For the 5G high-precision synchronization network, safe and reliable operation is crucial, and it is necessary to establish a monitoring system to conduct real-time monitoring and management of the synchronization operation quality across the entire network. In terms of monitoring methods, this includes relative performance monitoring based on the self-monitoring capabilities of network devices and absolute performance monitoring using external devices such as probes.

For relative performance monitoring based on the self-monitoring capabilities of network devices, specific monitoring items and requirements have already been standardized in relevant standards[4]. For absolute performance monitoring using external devices, suitable locations in the network can deploy external probe devices that support satellite co-viewing functions as monitoring points. At the same time, the entire network can be configured with a public reference benchmark station and data processing center. Based on the satellite co-viewing method, the time deviation between each monitoring point and the public reference benchmark station can be obtained. Through big data analysis and processing in the data processing center, the operational status of the entire network synchronization can be accurately grasped.

Relative monitoring based on built-in functions of network devices is relatively simple to implement, requiring no additional devices. However, this monitoring method only conducts relative monitoring of network segments and cannot monitor the entire network. Moreover, all involved devices must support specific monitoring functions, which poses some drawbacks in practical applications. Monitoring through external devices based on co-viewing can achieve real-time monitoring of network synchronization performance. In the long run, this is a better monitoring method, but initial construction costs are required, and specific monitoring plans need further research, including the selection of the number and location of monitoring points, the choice or construction of authoritative public reference benchmark stations, how to achieve data transmission between monitoring points and the benchmark station and data processing center, reasonable data processing algorithms, and the determination of monitoring evaluation indicators.

5 Conclusion

The characteristics of synchronization requirements for 5G are clear, with both microsecond-level basic synchronization requirements similar to 4G and nanosecond-level synchronization requirements represented by positioning needs. Through high-precision synchronization networking for 5G, various synchronization requirements of the 5G system can be met, the synchronization issues of complex 5G network deployment scenarios can be solved, and mutual backup between ground and satellite systems can be achieved, avoiding the security risks associated with complete reliance on satellite timing, further enhancing the safety and reliability of 5G applications.

To meet the high-precision synchronization requirements of the 5G system and support various applications with high-precision synchronization needs, the industry is actively conducting research on synchronization source technologies, synchronization transmission technologies, synchronization monitoring technologies, and more. From the perspective of source technology, dual-frequency technology is more suitable for the construction and deployment of high-precision time synchronization networks, especially dual-frequency technology based on the independent BeiDou system will become the mainstream application; from the perspective of synchronization transmission technology, 1588v2 technology remains the basic transmission technology for high-precision synchronization, which can be further improved and enhanced to meet multi-scenario high-precision synchronization transmission needs; from the perspective of high-precision synchronization monitoring technology, there are absolute monitoring based on satellite signals and relative monitoring based on device functions, which can be comprehensively selected based on business requirements, network scale, cost budget, and other factors. It is expected that the final direction of application will be based on absolute performance monitoring methods using satellite co-viewing.

Overall, with the continuous advancement of 5G system commercialization, as a foundational support network, the research and deployment of 5G synchronization networks need to be accelerated. In conjunction with operators’ 5G carrying technologies and networking architectures, further research on 5G synchronization network evolution strategies, high-precision synchronization testing technologies, synchronization network security, and other topics should be conducted to promote the formulation and improvement of 5G high-precision synchronization solutions, fully supporting the commercial deployment of 5G systems.

References

[1] 3GPP TS 38.104, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; NR; Base Station (BS) Radio Transmission and Reception; (Release 15)[S], 2019.

[2] ITU-T, G.8272.1/Y.1367.1, Timing Characteristics of Enhanced Primary Reference Time Clocks[S], 2016.

[3] 2017-0952T-YD. Software Defined Synchronization Network Technical Requirements (Draft for Approval)[S], 2019.

[4] YD/T 2879-2015. Synchronization Network Operation Management and Maintenance (OAM) Technical Requirements Based on Packet Networks[S], 2015.

Author Introduction

Hu Changjun: Chief Engineer of the Technical and Standard Research Institute of the China Academy of Information and Communications Technology, Senior Engineer, with research fields including time-frequency synchronization, satellite timing, optical communication, etc., mainly engaged in standard research, technical consulting, national projects, testing, and evaluation.

Lv Bo:Senior Engineer at the Technical and Standard Research Institute of the China Academy of Information and Communications Technology, Ph.D., mainly engaged in technical research, standardization, and testing in the fields of synchronization and artificial intelligence.

Miao Xinyu: Engineer at the Technical and Standard Research Institute of the China Academy of Information and Communications Technology, mainly engaged in technical research, standardization, and testing in the fields of synchronization and satellite timing.

Paper Citation Format:

Hu Changjun, Lv Bo, Miao Xinyu. Analysis of 5G Synchronous Networking Architecture and Key Technologies[J]. Information Communication Technology and Policy, 2020(4):36-40.

∗ Fund Project: National Key Research and Development Program Project (No. 2016YFF0200205) and National Natural Science Foundation Project (No. 61671159) funded

This article was published in “Information Communication Technology and Policy”, 2020, Issue 4

Organizer: China Academy of Information and Communications Technology

“Information Communication Technology and Policy” is a professional academic journal supervised by the Ministry of Industry and Information Technology and hosted by the China Academy of Information and Communications Technology. The journal is positioned as a “barometer of cutting-edge information and communication technology, a think tank for exploring information society policy,” focusing on technology trends, public policy, national/industry/enterprise strategies in the information and communication field, publishing cutting-edge research results, analysis of focal issues, interpretation of hot policies, etc., promoting innovation and development of technologies and industries such as 5G, industrial internet, digital economy, artificial intelligence, blockchain, big data, and cloud computing, guiding national technology strategy choices and industrial policy formulation, and building a high-end academic exchange platform for industry, academia, research, and application.

Reviewers | Chen Li, Shan Shan

Editors | Ling Xiao

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