Oscillator calibration
A basestation for a cellular communication system has an interface, for connection to a computer network, and also includes an oscillator, for generating wireless transmit and receive frequencies. A controller receives timestamped response messages from a time server over the computer network, each response message being subject to a network propagation delay, which is a sum of a minimum network propagation delay and a jitter component. For each received response message an apparent network propagation delay is determined as a function of a difference between a first timestamp applied by the time server and a second timestamp based on a clock derived from said oscillator. A subset of the received response messages are selected, whose network propagation delays include minimal jitter components. The frequency accuracy of the oscillator is then determined based on changes over time in the apparent network propagation delays of the selected received response messages. The oscillator can then be adjusted based on this frequency accuracy.
1. A method of determining an accuracy of an oscillator used to generate a local clock signal, the method comprising:
sending bursts of requests to a time server over a computer network;
receiving a plurality of bursts each of a plurality of timestamped response messages from the time server over the computer network, each of said response messages being subject to a network propagation delay, said network propagation delay being a sum of a minimum network propagation delay and a jitter component;
for each received response message, determining an apparent network propagation delay as a function of a difference between a first timestamp applied by the time server and a second timestamp based on a clock derived from said oscillator;
preselecting one response message from each burst of response messages having a minimum apparent network propagation delay;
defining a time interval, over which the accuracy of the oscillator is to be determined;
dividing said time interval into time windows;
within each time window, selecting one of the preselected response messages as the minimum network propagation delay response message, based on the determined network propagation delay of each preselected response message and each preselected response message's corresponding time position within said time window; and
determining the accuracy of the oscillator based on changes over time in the apparent network propagation delays of the selected received response messages.
2. A method as claimed in claim 1 , comprising:
defining a first time window;
selecting a response from the first time window;
defining a further time window starting at the selected response from the first time window; and
selecting a response from the further time window.
3. A basestation for a cellular communication system, comprising:
an interface, for connection to a computer network;
an oscillator, for generating a clock signal;
a controller, for adjusting the oscillator, wherein the controller is adapted to:
send bursts of requests to a time server over a computer network;
receive a plurality of bursts each of a plurality of timestamped response messages from the time server over the computer network, each of said response messages being subject to a network propagation delay, said network propagation delay being a sum of a minimum network propagation delay and a jitter component;
for each received response message, determine an apparent network propagation delay as a function of a difference between a first timestamped applied by the time server and a second timestamp based on a clock derived from said oscillator;
preselect one response message from each burst of response messages having a minimum apparent network propagation delay;
define a time interval, over which the accuracy of the oscillator is to be determined;
divide said time interval into time windows;
within each time window, select one of the preselected response messages as the minimum network propagation delay response message, based on the determined network propagation delay of each preselected response message and each preselected response message's corresponding time position within said time window; and
determine the accuracy of the oscillator based on changes over time in the apparent network propagation delays of the selected received response messages; and
adjust the oscillator based on said determined accuracy.
4. A basestation as claimed in claim 3 , further comprising:
transceiver circuitry, for generating transmit and/or receive frequency signals from an output of said oscillator.