Detecting a presence of near field communications (NFC) devices

1. A first near field communication (NFC) device, comprising:

a modulator configured to modulate a detection signal onto a carrier wave to provide a modulated detection sequence, the detection signal having a first power level for a first duration to provide a first interval, a second power level for a second duration to provide a second interval, and a third power level for a third duration to provide a third interval, the second power level being greater than the first power level;

a demodulator configured to demodulate the modulated detection sequence to provide a recovered detection sequence; and

a controller configured to:

determine a first signal metric of the recovered detection sequence corresponding to the first interval and a second signal metric of the recovered detection sequence corresponding to the second interval,

estimate a signal metric estimate of the recovered detection sequence based upon a relationship between the first and the second signal metrics,

determine a third signal metric of the recovered detection sequence corresponding to the third interval, and

determine, based upon a difference between the third signal metric and the signal metric estimate, whether the third signal metric is non-linearly related to the first and the second signal metrics indicating that a second NFC device is present within a magnetic field of the first NFC device.

2. The first NFC device of claim 1 , wherein the first, the second, and the third signal metrics each represent at least one selected from a group consisting of an indication of a mean voltage or current of the recovered detection sequence;

an indication of a total energy of the recovered detection sequence;

an indication of an average power of the recovered detection sequence;

an indication of a mean square voltage or current of the recovered detection sequence;

an indication of an instantaneous power of the recovered detection sequence;

an indication of a root mean square power of the recovered detection sequence;

an indication of a voltage or current variance of the recovered detection sequence;

an indication of a norm voltage or current of the recovered detection sequence;

an indication of a coupling factor between the first NFC device and the second NFC device; and

an indication of a voltage level of the recovered detection sequence.

3. The first NFC device of claim 1 , wherein the detection signal comprises:

a plurality of detection signals being arranged to form a monotonically increasing step function.

4. The first NFC device of claim 1 , wherein the detection signal comprises:

a first detection signal being at the first power level for the first duration to provide the first interval;

a second detection signal being at the second power level for the second duration to provide the second interval; and

a third detection signal being at the third power level for the third duration to provide the third interval.

5. The first NFC device of claim 1 , wherein the second power level is less than the third power level.

6. The first NFC device of claim 1 , wherein the relationship between the first and the second signal metrics comprises:

a statistical relationship.

7. The first NFC device of claim 6 , wherein the statistical relationship comprises:

a line of best fit between the first and the second signal metrics.

8. The first NFC device of claim 1 , wherein the controller is further configured to compare the signal metric estimate and the third signal metric to determine the difference.

9. The first NFC device of claim 1 , wherein the controller is further configured to determine the third signal metric is non-linearly related to the first and the second signal metrics in response to the difference being greater than or equal to a threshold.

10. The first NFC device of claim 1 , wherein the controller is further configured to determine the third signal metric is linearly related to the first and the second signal metrics in response to the difference being less than a threshold.

11. The first NFC device of claim 1 , wherein the controller is further configured to determine, based upon the difference, whether the third signal metric is linearly related to the first and the second signal metrics indicating that the second NFC device is not present within the magnetic field of the first NFC device.

12. The first NFC device of claim 1 , wherein the first signal metric is less than the second signal metric, and

wherein the second signal metric is less than the third signal metric.

13. A method for detecting a presence of a first near field communication (NFC) device, comprising:

modulating, by a second NFC device, a detection signal onto a carrier wave to provide a modulated detection sequence, the detection signal having a first power level for a first duration to provide a first interval, a second power level for a second duration to provide a second interval, and a third power level for a third duration to provide a third interval, the second power level being greater than the first power level;

applying, by the second NFC device, the modulated detection sequence to an inductive coupling element to generate a magnetic field;

demodulating, by the second NFC device, the modulated detection sequence to provide a recovered detection sequence;

determining, by the second NFC device, a first signal metric of the recovered detection sequence corresponding to the first interval and a second signal metric of the recovered detection sequence corresponding to the second interval;

estimating, by the second NFC device, a signal metric estimate of the recovered detection sequence based upon a relationship between the first and the second signal metrics;

determining, by the second NFC device, a third signal metric of the recovered detection sequence corresponding to the third interval; and

determining, by the second NFC device based upon a difference between the third signal metric and the signal metric estimate, whether the third signal metric is non-linearly related to the first and the second signal metrics indicating that the first NFC device is present within the magnetic field.

14. The method of claim 13 , wherein the modulating comprises:

modulating a first detection signal at the first power level for the first duration to provide the first interval;

modulating a second detection signal at the second power level for the second duration to provide the second interval; and

modulating a third detection signal at the third power level for the third duration to provide the third interval.

15. The method of claim 13 , wherein the estimating comprises:

estimating the signal metric estimate based upon a statistical relationship between the first and the second signal metrics.

16. The method of claim 15 , wherein the estimating further comprises:

estimating the signal metric estimate based upon line of best fit between the first and the second signal metrics.

17. The method of claim 13 , wherein the determining whether the third signal metric is non-linearly related comprises:

determining the difference.

18. The method of claim 13 , wherein the determining whether the third signal metric is non-linearly related comprises:

determining the third signal metric is non-linearly related to the first and the second signal metrics in response to the difference being greater than or equal to a threshold.

19. The method of claim 13 , wherein the determining whether the third signal metric is non-linearly related comprises:

determining the third signal metric is linearly related to the first and the second signal metrics in response to the difference being less than a threshold.

20. The method of claim 13 , wherein the determining the first signal metric and the second signal metric comprises:

determining, for each of the first and the second metrics, at least one selected from a group consisting of:

an indication of a mean voltage or current of the recovered detection sequence;

an indication of a total energy of the recovered detection sequence;

an indication of an average power of the recovered detection sequence;

an indication of a mean square voltage or current of the recovered detection sequence;

an indication of an instantaneous power of the recovered detection sequence;

an indication of a root mean square power of the recovered detection sequence;

an indication of a voltage or current variance of the recovered detection sequence;

an indication of a norm voltage or current of the recovered detection sequence;

an indication of a coupling factor between the first NFC device and the second NFC device; and

an indication of a voltage level of the recovered detection sequence.

21. The method of claim 13 , wherein the modulating comprises:

modulating a monotonically increasing step function onto the carrier wave.

22. The method of claim 13 , further comprising:

determining, by the second NFC device based upon the difference, whether the third signal metric is linearly related to the first and the second signal metrics indicating that the second NFC device is not present within the magnetic field.

23. The method of claim 13 , wherein the first signal metric is less than the second signal metric, and

wherein the second signal metric is less than the third signal metric.

24. The first NFC device of claim 1 , further comprising:

an antenna module configured to apply the modulated detection signal to an inductive coupling element to generate the magnetic a field.

25. The method of claim 13 , wherein the third power level is greater than the second power level.