Method and system for frequency scan using a differential power metric

1. A method comprising:

for each of a plurality of channel hypotheses, each channel hypothesis having a hypothetical occupied portion and a hypothetical guard band portion, calculating a respective metric based on a power value for the hypothetical occupied portion and a power value for the hypothetical guard band portion; and

based on the metrics determined for the plurality of channel hypotheses, selecting at least one channel hypothesis for further processing for cell selection or re-selection.

2. The method of claim 1 further comprising for each of the at least one channel hypothesis selected for further processing, attempting to perform at least one of:

detecting a reference signal within a bandwidth defining the channel hypothesis;

detecting a synchronization signal within a bandwidth defining the channel hypothesis; and

identifying a cell operating within a bandwidth defining the channel hypothesis.

3. The method of claim 1 wherein calculating a respective metric comprises:

determining a power value in each of a plurality of frequency bins defining an operating band that contains all of the hypothetical occupied portions and hypothetical guard band portions.

4. The method of claim 3 wherein calculating a respective metric comprises:

for each hypothetical occupied portion, determining the power value for the hypothetical occupied portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical occupied portion; and

for each hypothetical guard band portion, determining the power value for the hypothetical guard band portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical guard band portion.

5. The method of claim 4 wherein determining the respective metric is based on a difference between the power value for the hypothetical occupied portion and the power value for the hypothetical guard band portion.

6. The method of claim 5 wherein the difference comprises:

a sum of power values for the subset of the plurality of frequency bins for the hypothetical occupied portion divided by a number of frequency bins in the hypothetical occupied portion minus a sum of power values for the subset of the plurality of frequency bins for the hypothetical guard band portion divided by a number of frequency bins in the hypothetical guard band portion.

7. The method of claim 5 wherein the difference comprises:

a logarithm of the ratio of a sum of power values for the subset of the plurality of frequency bins for the hypothetical occupied portion to a number of frequency bins in the hypothetical occupied portion minus a logarithm of the ratio of a sum of power values for the subset of the plurality of frequency bins for the hypothetical guard band portion to a number of frequency bins in the hypothetical guard band portion.

8. The method of claim 3 wherein determining power in each frequency bin comprises:

filtering a received signal with a narrowband filter having a bandwidth substantially equal to a bandwidth of the frequency bin; and

measuring the power of the frequency bin.

9. The method of claim 1 wherein calculating a respective metric comprises:

performing at least one discrete Fourier transform (DFT) to produce a power value for each of a plurality of frequency bins.

10. The method of claim 9 , wherein calculating a respective metric comprises:

for each hypothetical occupied portion, determining the power value for the hypothetical occupied portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical occupied portion; and

for each hypothetical guard band portion, determining the power value for the hypothetical guard band portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical guard band portion.

11. The method of claim 9 , wherein performing at least one DFT comprises performing a respective DFT for each of at least two bandwidths that collectively form an operating band that contains all of the hypothetical occupied portions and hypothetical guard band portions.

12. The method of claim 1 wherein selecting the at least one channel hypothesis for further processing comprises:

arranging the metrics and selecting N f metrics, where N f ≧1, each associated with a channel hypothesis, in order of magnitude of the metrics; and

selecting at least one of the N f channel hypotheses based on a largest magnitude metric.

13. The method of claim 1 wherein the plurality of channel hypotheses comprise channel hypotheses for two or more different sized bandwidths located at each of a plurality of carrier frequencies in an operating band that contains all of the hypothetical occupied portions and hypothetical guard band portions.

14. The method of claim 13 wherein the two or more different sized bandwidths comprise bandwidths equal to at least two of: 1.4 MHz; 3 MHz; 5 MHz; 10 MHz; 15 MHz; and 20 MHz.

15. The method of claim 3 wherein determining a power value in each of a plurality of frequency bins comprises performing multiple power measuring passes of the frequency bins in the operating band.

16. The method of claim 3 wherein determining a power value in each of a plurality of frequency bins comprises performing a received signal strength indicator (RSSI) measurement for each of the frequency bins.

17. The method of claim 15 , wherein a delay is included between consecutive passes of the multiple power measuring passes.

18. The method of claim 17 , wherein the delay time is increased with each consecutive pass.

19. The method of claim 1 , wherein a start of a hypothetical guard band portion may be adjusted by moving it away from a nominal transmission band edge by a margin corresponding to a maximum expected frequency offset.

20. The method of claim 5 , further comprising, upon determining the respective metric:

stopping evaluation of candidate carrier frequencies early if the metric exceeds a threshold; and

proceeding directly to searching for synchronization signals.

21. A wireless device comprising:

a processor;

a Power Difference Calculation function configured to:

for each of a plurality of channel hypotheses, each channel hypothesis having a hypothetical occupied portion and a hypothetical guard band portion, calculate a respective metric based on a power value for the hypothetical occupied portion and a power value for the hypothetical guard band portion;

based on the metrics determined for the plurality of channel hypothesis, select at least one channel hypothesis for further processing.

22. The wireless device of claim 21 wherein the Power Difference Calculation function configured to calculate a respective metric is configured to:

determine a power value in each of a plurality of frequency bins defining an operating band that contains all of the hypothetical occupied portions and hypothetical guard band portions.

23. The wireless device of claim 22 wherein the Power Difference Calculation function configured to calculate a respective metric is configured to:

for each hypothetical occupied portion, determine the power value for the hypothetical occupied portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical occupied portion; and

for each hypothetical guard band portion, determine the power value for the hypothetical guard band portion based on the power values of a subset of the plurality of frequency bins, the subset collectively forming the hypothetical guard band portion.

24. The wireless device of claim 22 wherein the Power Difference Calculation function configured to calculate a respective metric is configured to:

determine the respective metric based on a difference between the power value for the hypothetical occupied portion and the power value for the hypothetical guard band portion.