Microwave Journal
www.microwavejournal.com/articles/30271-precise-frequency-sources-meeting-the-5g-holdover-time-interval-error-requirement

Precise Frequency Sources Meeting the 5G Holdover Time Interval Error Requirement

May 13, 2018

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 REQUIREMENTS

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.



Figure 1

Figure 1 TIE estimation algorithm.

TIE MEASUREMENT PROCEDURE

TIE measurements are done for 3 to 7 days with periodic temperature changes. A measurement duration of 3 to 7 days is necessary to count and compensate for frequency drift due to aging. In general, it may be possible to compensate for aging in holdover mode in case there is a long term record of frequency output of a precise frequency source obtained while synchronized to an external reference. It is possible to create learning systems capable of aging compensation basing on data from the last 2 to 3 days of operation.

TIE estimation, which takes into account compensation for aging, is carried out as follows (see Figure 1):

  • Choose the beginning of TIE estimation (start of the “sliding” time window). The sliding time window, moving with some step (1 to 4 hours), is applied to the data. This window consists of two parts: Fit range and TIE estimate range.
  • Approximate aging. The frequency aging approximation φ(t) is built basing on readings situated inside the fit range. The fit range lasts 24 hours. According to our research, this is most optimal for the aging approximation.
  • TIE estimation. Readings situated inside of the TIE estimate range are used for determining the subject time error. The time error in this range is determined by the difference between the frequency readings and the aging approximation:

Math 1

Figure 2

Figure 2 Temperature profiles for TIE estimation: symmetrical (a) and asymmetrical (b).

Figure 3

Figure 3 24-hour TIE for a DOCXO: temp- erature profile during the test (a), measured frequency (b) and estimated TIE (c).

The TIE estimate range is 4 to 24 hours.

A TIE of 100 to 400 ns in holdover mode for telecom and mobile networks is used primarily for grand masters, which are installed in environmentally conditioned rooms. This means that the temperature change during the day usually does not exceed 5 Centigrade degrees.

Different temperature profiles can be used for TIE estimation. Two are presented in Figure 2. It should be mentioned that the profile of Figure 2a is symmetrical with respect to the average temperature change. Thus, the time error accumulated over 24 hours along this profile should be equal to 0 (under ideal conditions). The profile shown in Figure 2b does not have symmetry, so even under ideal conditions there is a net time error accumulated over 24 hours.

For TIE estimation we use the temperature profile from Figure 2b because it models the worst case operation of a precise frequency source. An example of TIE estimation for a double oven controlled crystal oscillator (DOCXO) using the measurement procedure described above is shown in Figure 3. TIE estimation results are obtained as outlined below:

  • The initial “sliding” window position (A) for the calculated approximation line is based on frequency counts situated in the fit range.
  • Data inside the TIE estimation range is used for determining the time error, TIEA, per Equation 1.
  • The calculated TIEA value is shown in Figure 3c.
  • The sliding time window is stepped by 1 hour and all calculations are repeated.
  • The procedure continues while the TIE estimation range is within the measurement length.

TIE MEASUREMENTS

Even negligible frequency changes influence TIE estimation results. Sources of errors should be taken into account in order to obtain reliable values of TIE. These include mutual synchronization of the frequency of individual oscillators and frequency measurement instability.

Figure 4

Figure 4 Frequency and TIE estimation for a rubidium oscillator before (a) and after (b) measures to prevent mutual syntonization of frequency.

Mutual Synchronization

Mutual synchronization of oscillators at close frequencies is one of the most important sources of errors for frequency measurement. This effect may be easily seen in volume production when, simultaneously, a large number of oscillators are measured. To prevent this effect, it is necessary to minimize all possible ways oscillators can influence on each other, e.g., on the common grounds of power circuits and circuits of frequency switchers, through electromagnetic coupling and through reverse signal transmission through the open channels of the switcher. As an example, Figure 4 shows the results of rubidium oscillator TIE measurements before and after the implementation of the above measures.

Figure 5

Figure 5 Aging curve meeting the 100…400 ns TIE requirement (a) vs. “standard” aging curve (b).

Frequency Measurement Instability

For precision frequency sources to meet the TIE 100 to 400 ns requirement, it is extremely important to have aging curve monotonicity of about 1 to 2E-11/day. In other words, there should be no jumps or any other irregular frequency changes. Figure 5 compares aging that meets the TIE 100 to 400 ns requirement with one that does not. The reasons for “short-term” frequency changes may be explained by either contact phenomena, stability of the reference source or errors caused by internal issues in the precision frequency source.

Figure 6

Figure 6 Frequency and TIE for quartz oscillator without (a) and with (b) “short-term” frequency changes.

Figure 7

Figure 7 TIE measurements at 24, 16, 8 and 4 hours.

To separate internal issues from the other phenomena, good quality connectors and precision reference sources should be used. During initial measurements we found that some precision rubidium oscillators, regardless of the manufacturer, dramatically changed frequency in increments ranging from 5E-12 to 5E-11. Knowing this, we now use a hydrogen frequency standard for 100 to 400 ns TIE measurements. A TIE measurement for a quartz oscillator with and without “short-term” frequency changes is shown in Figure 6.

TIE measurements will be reliable if all factors listed above are taken into account. Examples of TIE measurements over 4, 8, 16 and 24 hours are shown in Figure 7.

References

  1. H. Li, L. Han, R. Duan and G. M. Garner, “Synchronization Requirements of 5G and Corresponding Solutions,” IEEE Communications Standards Magazine, Vol. 1, No. 1, March 2017, pp. 52–58.
  2. “NR; Base Station (BS) Radio Transmission and Reception,” 3GPP TS 38.104 specification, 3GPP Portal.