Frequency synchronization using first and second frequency error estimators

An apparatus comprising:

• a communication device comprising a clock recovery module;
• the communication device being operative as a slave device relative to another communication device that is operative as a master device;
• the clock recovery module comprising a clock recovery loop configured to control a slave clock frequency of the slave device so as to synchronize the slave clock frequency with a master clock frequency of the master device;
• the clock recovery loop comprising:
• a primary loop comprising a loop filter and a first frequency error estimator configured to generate a first estimate of error between the master clock frequency and the slave clock frequency;
• a second frequency error estimator arranged outside of the primary loop and configured to generate a second estimate of error between the master clock frequency and the slave clock frequency;
• and
• an accumulator coupled between the second frequency error estimator and the primary loop;
• wherein the second estimate is controllably injected into the primary loop via the accumulator;
• wherein the second frequency error estimator comprises a linear time-variant system of two state variables given by time t and clock period T and wherein the linear time-variant system generates the second estimate as an estimate {circumflex over (t)} of the time t and an estimate {circumflex over (T)} of the clock period T;
• wherein the linear time-variant system comprises a Kalman filter;
• wherein the second frequency error estimator is configured to initialize the estimate {circumflex over (t)} of the time t to an arrival timestamp generated in the slave device for a corresponding timing message received from the master device and to initialize the estimate {circumflex over (T)} of the clock period T to a nominal master clock period;
• wherein for a given iteration of the second frequency error estimator associated with a particular one of a plurality of timing messages received from the master device, the second frequency error estimator calculates a predicted value t of a corresponding arrival timestamp as a sum of the time estimate and the master clock period estimate:
• t={circumflex over (t)}+{circumflex over (T)}
• where {circumflex over (t)} and {circumflex over (T)} are estimates from a previous iteration;
• wherein for the given iteration a prediction error αp is calculated based on a corresponding arrival timestamp Ts as follows:
• αP=Ts− t
• and wherein new estimates of {circumflex over (t)} and {circumflex over (T)} are determined based on the prediction error and current values of Kalman filter gains;
• and
• wherein the new estimates of {circumflex over (t)} and {circumflex over (T)} are determined based on the prediction error and current values of Kalman filter gains as follows:
• {circumflex over (t)}= t+αp·{tilde over (P)}11
• {circumflex over (T)}={circumflex over (T)}+αp·{tilde over (P)}12
• where {tilde over (P)}11 and {tilde over (P)}12 denote particular ones of the current values of the Kalman filter gains.

An apparatus comprising:

• a communication device comprising a clock recovery module;
• the communication device being operative as a slave device relative to another communication device that is operative as a master device;
• the clock recovery module comprising a clock recovery loop configured to control a slave clock frequency of the slave device so as to synchronize the slave clock frequency with a master clock frequency of the master device;
• the clock recovery loop comprising:
• a primary loop comprising a loop filter and a first frequency error estimator configured to generate a first estimate of error between the master clock frequency and the slave clock frequency;
• a second frequency error estimator arranged outside of the primary loop and configured to generate a second estimate of error between the master clock frequency and the slave clock frequency;
• and
• an accumulator coupled between the second frequency error estimator and the primary loop;
• wherein the second estimate is controllably injected into the primary loop via the accumulator;
• wherein the second frequency error estimator comprises a linear time-variant system of two state variables given by time t and clock period T and wherein the linear time-variant system generates the second estimate as an estimate {circumflex over (t)} of the time t and an estimate {circumflex over (T)} of the clock period T;
• wherein the second frequency error estimator is configured to initialize the estimate of the time t to an arrival timestamp generated in the slave device for a corresponding timing message received from the master device and to initialize the estimate {circumflex over (T)} of the clock period T to a nominal master clock period;
• and
• wherein the second estimate to be injected into the primary loop is calculated from a converged estimated period {circumflex over (T)} as follows:
• 1−{circumflex over (T)}/Tnominal
• where Tnominal denotes the nominal master clock period.

A method for use in a communication device, the communication device being operative as a slave device relative to another communication device that is operative as a master device, the method comprising the steps of:

• generating first and second estimates of error between a master clock frequency of the master device and a slave clock frequency of the slave device;
• and
• controlling the slave clock frequency of the slave device based on the first and second error estimates to synchronize the slave clock frequency with the master clock frequency of the master device;
• wherein the first estimate is generated in a primary clock recovery loop comprising a loop filter;
• wherein the second estimate is generated external to the primary loop;
• wherein the controlling step further comprises controllably injecting the second estimate into the primary loop via an accumulator by combining an output of the accumulator with an output of the loop filter;
• and
• wherein the first and second estimates of error are independently generated by processing arrival timestamps generated in the slave device for respective timing messages received from the master device.

A communication system, comprising:

• at least first and second communication devices configured to communicate with one another over a packet network;
• a first one of the communication devices being operative as a slave device relative to another one of the communication devices operative as a master device;
• the first communication device comprising a clock recovery module;
• the clock recovery module comprising a clock recovery loop configured to control a slave clock frequency of the slave device so as to synchronize the slave clock frequency with a master clock frequency of the master device;
• the clock recovery loop comprising:
• a primary loop comprising a loop filter and a first frequency error estimator configured to generate a first estimate of error between the master clock frequency and the slave clock frequency;
• a second frequency error estimator arranged outside of the primary loop and configured to generate a second estimate of error between the master clock frequency and the slave clock frequency;
• and
• an accumulator coupled between the second frequency error estimator and the primary loop;
• wherein the second estimate is controllably injected into the primary loop via the accumulator by combining an output of the accumulator with an output of the loop filter;
• and
• wherein the first and second frequency error estimators independently generate the respective first and second estimates by processing arrival timestamps generated in the slave device for respective timing messages received from the master device.