Open/Closed Loop Synchronization for Radio Transmitters

1 . A method, comprising:

determining a first propagation delay estimate to a first node;

determining a second propagation delay estimate to a second node;

receiving a first synchronization message from the first node; and

transmitting a second synchronization message to the second node wherein the second synchronization message depends on the first synchronization message, the first propagation delay estimate, and the second propagation delay estimate.

2 . The method of claim 1 , wherein the receiving the first synchronization message from the first node comprises receiving an estimate of accumulated movement of the first node.

3 . The method of claim 1 , further comprising:

receiving a third synchronization message from the second node; and

transmitting a fourth synchronization message to the first node, wherein the fourth synchronization message depends on the third synchronization message, the first propagation delay estimate, and the second propagation delay estimate.

4 . The method of claim 3 , wherein the receiving the third synchronization message from the second node comprises receiving an estimate of accumulated movement of the second node.

5 . The method of claim 4 , further comprising:

estimating a change in a length of a radio path in a time interval; and

discarding a propagation delay estimate when the estimated change in path length exceeds a threshold.

6 . The method of claim 5 , wherein the estimating the change comprises using the time interval starting with the estimation of the propagation delay.

7 . The method of claim 5 , wherein the estimating the change in the length of the radio path comprises

determining a first movement estimate of the first radio node in a time interval;

determining a second movement estimate of a receiving radio node in a time interval; and

summing first and second movement estimates.

8 . The method of claim 5 , wherein the estimating the change in the length of the radio path further comprises

receiving a first accumulated movement from the first radio node;

receiving a second accumulated movement from the first radio node; and

calculating a difference between the second accumulated movement and the first accumulated movement.

9 . A method, comprising:

estimating the accumulated movement of a radio transmitter device;

encoding the accumulated movement into a motion estimation message; and

transmitting the motion estimation message to a device.

10 . The method of claim 9 , further comprising:

including, with the motion estimation message, a synchronization message requesting synchronization via the device with a second radio transmitter device.

11 . An apparatus, comprising:

at least one memory including computer program code; and

at least one processor,

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to

determine a first propagation delay estimate to a first node;

determine a second propagation delay estimate to a second node;

process a received first synchronization message from the first node; and

transmit a second synchronization message to the second node wherein the second synchronization message depends on the first synchronization message, the first propagation delay estimate, and the second propagation delay estimate.

12 . The apparatus of claim 11 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to process an estimate of accumulated movement of the first node.

13 . The apparatus of claim 11 , wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus at least to

process a received third synchronization message from the second node; and

transmit a fourth synchronization message to the first node, wherein the fourth synchronization message depends on the third synchronization message, the first propagation delay estimate, and the second propagation delay estimate.

14 . The apparatus of claim 13 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to process an estimate of accumulated movement of the second node.

15 . The apparatus of claim 11 , wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus at least to

estimate a change in a length of a radio path in a time interval; and

discard a propagation delay estimate when the estimated change in path length exceeds a threshold.

16 . The apparatus of claim 15 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to use, as the time interval, a time interval starting with the estimation of the propagation delay.

17 . The apparatus of claim 15 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to

determine a first movement estimate of the first radio node in a time interval;

determine a second movement estimate of a receiving radio node in a time interval; and

sum the first movement estimate and second movement estimate to obtain an estimate of the change in the length of the radio path.

18 . The apparatus of claim 15 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to

process a received first accumulated movement from the first radio node;

process a received second accumulated movement from the first radio node; and

calculate a difference between the second accumulated movement and the first accumulated movement to obtain an estimate of the change in the length of the radio path.

19 . An apparatus, comprising:

at least one memory including computer program code; and

at least one processor,

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to

estimate the accumulated movement of a radio transmitter device;

encode the accumulated movement into a motion estimation message; and

transmit the motion estimation message to a device.

20 . The apparatus of claim 19 , wherein the at least one memory and the computer program code are also configured to, with the at least one processor, cause the apparatus at least to include, with the motion estimation message, a synchronization message requesting synchronization via the device with a second radio transmitter device.

21 . (canceled)