Synchronization is an essential prerequisite for all mobile networks to operate. It is fundamental to data integrity; without it, data will suffer errors and networks can suffer outages. Radio base stations rely on having access to reliable and accurate reference timing signals in order to generate radio signals and maintain frame alignment. Effective synchronization also permits hitless handover of subscriber connections between adjacent radio base stations. The measurement of time interval error (TIE) is a method for evaluating reference timing signals. This article describes the process.
Historically, frequency synchronization has been provided either by a Global Navigation Satellite System (GNSS) or derived from the transport network to which the network device requiring synchronization was connected. Public GNSS provides an accurate and stable synchronization source, but the financial cost to equip every site in a network with a GNSS-derived synchronization source may be prohibitive because of the requirement to install and manage additional equipment. Cost concerns for GNSS synchronization are more prevalent for small cell sites where the number of sites is increased compared with macro sites.
Telecommunication networks rely on the use of highly accurate primary reference clocks which are distributed network-wide using synchronization links and synchronization supply units. Primary reference clocks (PRC) or primary master clocks must meet the international standards requirement for long term frequency accuracy. To achieve this performance, atomic clocks or GPS disciplined oscillators are normally used.
Synchronization supply units (SSU) are used to ensure reliable synchronization distribution. They have a number of key functions:
- Filter the synchronization signal they receive to remove the higher frequency phase noise.
- Provide distribution with a scalable number of outputs to synchronize other local equipment.
- Provide a capability to carry on producing a high quality output even when their input reference is lost. This is referred to as holdover mode.
5G backhaul networks have higher requirements for frequency and time synchronization when compared to all previous generations. As mobile networks eventually migrate from LTE Advanced (LTE-A) to 5G, there are three fundamental changes that will have the most significant upstream impact:
- 10- to 15-fold increase in capacity (from LTE/LTE-A capacity of ~100 Mbps to ~10 Gbps in 5G).
- Ultra-low latency of ~1 ms (round trip).
- Ultra-dense nature of the network setting unprecedented requirements for the synchronization of the cell sites as small and overlapping cell sites proliferate.
For 5G, higher accuracy time synchronization requirements are increased due to new services, technologies and the network architecture:
- New services
- High accuracy positioning service; high accuracy location capability of less than 3 m on 80 percent of occasions in traffic roads and tunnels, underground car parks and indoor environments.
- New technologies
- Carrier aggregation; carrier aggregation enables the use of multiple carriers in the same or different frequency bands, to increase mobile data throughput.
- Coordinated multi-point technologies.
- 5G frame structure.
- New network architecture
- Back-haul and front-haul.
Carrier aggregation technologies require the time error between the base stations to be less than 260 ns. The 5G new frame structure under study may require as high as ±390 ns accuracy for the air interface to avoid interference. The 5G network will combine centralized radio access networks (C-RAN) and distributed radio access networks (D-RAN). The time synchronization should be achieved in both the back-haul and front-haul transport network.1
Time interval error (TIE) is the metric to specify clock accuracy/stability requirements in telecommunication standards. Of specific interest is the TIE of a network clock in holdover mode (not locked) for mobile networks. The key requirement for 5G communication networks is a TIE of 100 to 400 ns in holdover mode for 4 to 24 hours.2
Frequency stability versus temperature and long-term stability (aging) are the key parameters of precision frequency sources that have the greatest influence on TIE in holdover mode. This article covers measurements and some results obtained for precision frequency sources ensuring a TIE of 100 to 400 ns for 4 to 24 hours.