Holdover in synchronization applications

Holdover in Synchronization Applications

Introduction

“Synchronization is as important as power at the cell site.”

The quote above suggests that we can think of holdover in synchronization applications as analogous to running on backup power.

Modern wireless communication systems require at least knowledge of frequency and often knowledge of phase as well in order to work correctly. Base stations need to know what time it is, and they usually get this knowledge from the outside world somehow (from a GPS Time and Frequency receiver, or from a synchronization source somewhere in the network they are connected to).

But if the connection to the reference is lost then the base station will be on its own to establish what time it is. The base station needs a way to establish accurate frequency and phase (to know what time it is) using internal (or local) resources, and that’s where the function of holdover becomes important.

The Importance of GPS Derived Timing

GPS as a source of timing is a key component in not just Synchronization in telecommunications but to critical infrastructure in general. Of the 18 Critical Resource and Key infrastructure (CIKR)sectors, 15 use GPS derived timing to function correctly.

A key application for GPS in Synchronization in telecommunications is to provide synchronization in wireless basestations. Base stations depend on timing to operate correctly, particularly for the handoff that occurs when a user moves from one cell to another. In these applications holdover is used in base stations to ensure continued operation while GPS is unavailable and to reduce the costs associated with emergency repairs, since holdover allows the site to continue to function correctly until maintenance can be performed at a convenient time.

Some of the most stringent requirements come from the newer generation of wireless base stations, where phase accuracy targets as low as 1μs need to be maintained for correct operation. However the need for accurate timing has been an integral part of the history of wireless communication systems, and it has been suggested that the search for reliable and cost effective timing soluctions was spurred on by the need for CDMA to compete with lower cost solutions.

Within the base station, besides standard functions, accurate timing and the means to maintain it through holdover is vitally important for services such as E911

As mentioned above there are key applications for GPS derived timing in other critical infrastructure applications. One notable application where timing accuracy (and the means to maintain it through holdover) is of importance is in the use of Synchrophasors in the power industry to detect line faults.

How GPS Derived Timing Can Fail

GPS is sensitive to jamming and interferance because the signal levels the system levels are so low and can easily be swamped by other sources, that can be accidental or deliberate. Also since GPS depends on line of sight signals can be disrupted by Urban canyon effects, making GPS only available to some locations at certain times of the day, for example.

A GPS outage however is not initially an issue because clocks can go into holdover, allowing the interference to be alleviated as much as the stability of the oscillator providing holdover will allow. The more stable the oscillator, the longer the system can operate without GPS.

Defining Holdover

In Synchronization applications holdover can be defined as:

An operating condition of a clock which has lost its controlling input and is using stored

data, acquired while in locked operation, to control its output. The stored data are used to control phase and frequency variations, allowing the locked condition to be reproduced within specifications. Holdover begins when the clock output no longer reflects the influence of a connected external reference, or transition from it. Holdover terminates when the output of the clock reverts to locked mode condition.

We can regard Holdover then as a measure of accuracy or error acquired by a clock when there is no controlling external reference to correct for any errors.

MIL-PRF-55310 defines Clock Accuracy as:

$T(t)=T_{0}+\int _{0}^{t}R(t)\,dt\ +\epsilon (t)=T_{0}+(R_{0}t+{\frac {1}{2}}At^{2}+...)+\int _{0}^{t}E_{t}(t)\,dt+\epsilon (t)$

Where $T_{0}$ is the synchronization error at $t=0$; $R(t)$ is the fractional frequency differenct between two clocks under comparison; $\epsilon (t)$ is the error due to random noise; $R_{0}$ is $R(t)$ at $t=0$; $A$ is the linear aging rate and $E_{1}(t)$ is the frequency difference due to environmental effects.

Similarly ITU G.810 defines Time Error as:

$x(t)=x_{0}+y_{0}t+{\frac {D}{2}}t^{2}+{\frac {\phi (t)}{2\pi \nu _{nom}}}$

Where $x(t)$ is the time error; $x_{0}$ is the time error at $t=0$; $y_{0}$ is the fractional frequency error at $t=0$; $D$ is the linear fractional frequency drift rate; $\phi (t)$ is the random phase deviation component and $\nu _{nom}$ is the nominal frequency.

Implementing Holdover

Two independent clocks once synchronized will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The periods between synchronizations is referred to as Holdover and performance under Holdover relies on the quality of the reference oscillator (usually an OCXO), the PLL design and the correction mechanisms employed.

The dominant factors influencing the quality of the reference oscillator are taken to be aging and temperature stability. During Holdover the clock error caused by aging and temperature instability can be corrected by control mechanisms. A combination of quartz based reference oscillator (such as an OCXO) and modern correction algorithms can get good results in Holdover applications.

GPS Clocks are used and in this context are often known as a GPSDO or GPS TFS.

Definition of a Disciplined Oscillator

Amongst the building blocks of a GPS Time and Frequency solution the oscillator is a key component

Usually built around an Oven Controlled Crystal Oscillator (OCXO).

How a GPSDO works

GPS clock block diagram

The holdover capability is provided by either by a free running local oscillator, or a local oscillator that is steered with software that retains knowledge of its past performance.

An addition of a Microprocessor can improve temperature stability and aging

Aging can be effectively compensated for

Basic aim of a control mechanism is to improve the stability of a clock or oscillator while minimizing the number of times it needs calibration

In Holdover the learned behaviour of the OCXO is used to anticipate and correct for future behavior

Holdover problem solved by predicting current errors from past history. Prediction allows the system to remain stable in holdover. All sort of choices for algorithms and techniques to do this correction extrapolation, interpolation, predictive filters, including the Kalman filters.

Kalman filters are used to generate correction signals

Designers have found that a high quality quartz oscillator matched with a Kalman filter algorithm seems to be able to provide the best compromise between quality and reliability versus cost.

Once the barriers of aging and environmental effects are removed the only theoretical limitation to holdover performance in such a GPSO is irregularity or noise in the drift rate, which we detect using a metric like Allan deviation.

Allan variance can measure instabilities

Time deviation can measure instabilities

Stability definitions have been around for a long time

Random Walk in an oscillator mostly from outside

Complexity in trying to implement has resulted in tailor made Holdover solutions in the market

External links

• example.com