Methods and apparatus for reference regeneration in real time location systems

1. A method comprising:

obtaining reference phase offsets corresponding to a difference between clock values of a plurality of radio frequency identification (RFID) receivers;

transmitting a first synchronization signal from a central hub to the plurality of RFID receivers via a wireline link;

obtaining differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the plurality of RFID receivers receives the first synchronization signal;

determining a residual offset table based at least in part on the differential wireline coarse sync measurements and the reference phase offsets;

responsive to determining that the reference phase offsets are invalid:

transmitting a second synchronization signal from the central hub to the plurality of RFID receivers via the wireline link; and

obtaining revised differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the RFID receivers receives the second synchronization signal;

generating revised reference phase offsets by combining the revised differential wireline coarse sync measurements with the residual offset table; and

determining a physical location of a RFID tag based at least in part on i) the revised reference phase offsets and ii) RFID receiver clock values corresponding to a time-of-arrival of over-the-air data transmitted from the RFID tag.

2. The method of claim 1 , wherein each reference phase offset has a coarse reference component and a fine reference component, and the coarse reference component comprises an integral number of cycles of a receiver clock signal.

3. The method of claim 2 , wherein the frequency of the receiver clock signal is 10 MHz.

4. The method of claim 2 , wherein the frequency of the fine reference component is 1 GHz.

5. The method of claim 1 , wherein determining the residual offset table based at least in part on the differential wireline coarse sync measurements and the reference phase offsets comprises subtracting the differential wireline coarse sync measurements from the reference phase offsets.

6. The method of claim 1 , wherein the transmitting of the second synchronization signal is done in response to a loss of electrical power to at least one of the plurality of RFID receivers.

7. The method of claim 1 , wherein the obtaining of the reference phase offsets from the plurality of RFID receivers is based reference tags placed on a target area at known reference locations.

8. The method of claim 1 , further comprising distributing a clock signal to the plurality of RFID receivers via the wireline link.

9. A location system comprising:

a reference phase offset generator configured to obtain reference phase offsets for a plurality of radio frequency identification (RFID) receivers;

a synchronization signal transmitter to transmit a first synchronization signal from a central hub to the plurality of RFID receivers via a wireline link and, responsive to determining that the reference phase offsets are invalid, to transmit a second synchronization signal from the central hub to the plurality of RFID receivers via the wireline link;

a differential wireline coarse sync measurements generator to:

obtain differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the plurality of RFID receivers receives the first synchronization signal; and

obtain revised differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the RFID receivers receives the second synchronization signal;

a residual offset table generator configured to determine residual offsets based at least in part on the differential wireline coarse sync measurements and the reference phase offsets;

a revised reference phase offset generator configured to generate revised reference phase offsets by combing the revised differential wireline coarse sync measurements with the residual offset table; and

a physical location calculator configured to determine a physical location of an RFID tag based at least in part on i) the revised reference phase offsets and ii) RFID receiver clock values corresponding to a time-of-arrival of over-the-air transmitted data from the RFID tag.

10. The location system of claim 9 , wherein each reference phase offset has a coarse reference component and a fine reference component, and the reference measurements generator is configured to obtain the reference measurements based on the coarse reference component comprising an integral number of cycles of a receiver clock signal.

11. The location system of claim 10 , wherein the frequency of the receiver clock signal is 10 MHz.

12. The location system of claim 10 , wherein the frequency of the fine reference component is 1 GHz.

13. The location system of claim 9 , wherein the residual offset table generator is configured to subtract the differential wireline coarse sync measurements from the reference phase offsets to determine the residual offsets.

14. The location system of claim 9 , wherein the synchronization signal transmitter is configured to transmit the second synchronization signal responsively to a loss of electrical power to at least one of the plurality of RFID receivers.

15. The location system of claim 9 , wherein the reference phase offset generator is configured to obtain the reference phase offsets based on reference tags placed on a target area at known reference locations.

16. The location system of claim 9 , wherein a clock signal is distributed to the plurality of RFID receivers via the wireline link.

17. A non-transitory computer readable storage medium including a set of instructions for execution by a processor, the set of instructions, when executed, cause a controller to:

obtain reference phase offsets corresponding to a difference between clock values of a plurality of radio frequency identification (RFID) receivers;

transmit a first synchronization signal from a central hub to the plurality of RFID receivers via a wireline link;

obtain differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the plurality of RFID receivers receives the first synchronization signal;

determine a residual offset table based at least in part on the differential wireline coarse sync measurements and the reference phase offsets;

responsive to determining that the reference phase offsets are invalid:

transmit a second synchronization signal from a central hub to the plurality of RFID receivers via the wireline link; and

obtain revised differential wireline coarse sync measurements corresponding to a difference between pairs of RFID receiver clock coarse values when each of the plurality of RFID receivers receives the second synchronization signal;

generate revised reference phase offsets by combining the revised differential wireline coarse sync measurements with the residual offset table; and

determine a physical location of a RFID tag based at least in part on i) the revised reference phase offsets and ii) RFID receiver clock values corresponding to a time-of-arrival of over-the-air data transmitted from the RFID tag.

18. The non-transitory computer readable storage medium of claim 17 , wherein each reference phase offset has a coarse reference component and a fine reference component, and the coarse reference component comprises an integral number of cycles of a receiver clock signal.

19. The non-transitory computer readable storage medium of claim 17 , the instructions, when executed, cause the controller to determine the residual offset table by subtracting the differential wireline sync measurements from the reference phase offsets.

20. The non-transitory computer readable storage medium of claim 17 , the instructions, when executed, cause the controller to distribute a clock signal to the plurality of RFID receivers via the wireline link.