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The fifth generation mobile communication system (short for 5G) is an extension of the 4G system with the fifth generation mobile communication technology. Ultra-high precision time synchronization is one of the key technologies in 5G network. It needs to synchronize the fronthaul, midhaul and backhaul devices with ultra-high precision. As with current time server and performance of 4G supporting network it is not more able to meet the new requirements in 5G network, China Mobile, the leading operator in China, is positively striving to put forward the requirements and schemes for ultra-high precision time synchronization performance with ±50ns accuracy in time server, ±200ns in transmission network, and ±50ns in base station.

With the development of IP-based 3G network and services, packet transport network (PTN) technology has been developing rapidly since it was proposed in 2008. PTN needs to synchronize the time, and transmit the time synchronous information to each node in the network. Currently PTN mainly transports high-precision time information between different sites which are accord with IEEE 1588v2 protocol. However, in practical situation, the transport networks mostly are bi-directional optical fiber network. As the fiber lengths in transmit end and receive end are quite different, it means the transport link will be asymmetric. And this would surely affect the accuracy of time synchronization as per IEEE1588 V2. To solve this problem, we have put forward corresponding solutions to carry out on-site measurements according to the actual network situation.

Communication modes such as TD-SCDMA, TD-LTE, CDMA2000, LTE-A, and MBMS need to synchronize base station air interfaces before working properly. IEEE1588 V2,  the mainstream high precision timing and synchronization protocol, illustrates that synchronization information can be terrestrially transmitted to solve the potential security problems of GPS signals which are difficult to be obtained in application scenarios like in the subway.

Generally speaking, the implementation of time and frequency synchronization networks is divided into several stages including installation of various time/frequency synchronous equipment such as time/clock source, transmission/bearer devices and wireless base station, service commissioning and opening, performance validation and acceptance of the networks, service cutover and network maintenance & management. Here we draw an overall test flow of engineering maintenance when implementing such networks, as shown in Fig 1.