Coherence Time Estimation and Mobility Detection for Wireless Channel

• US 20110009071A1
• Filed: 06/02/2010
• Published: 01/13/2011
• Est. Priority Date: 07/09/2009
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Silverman, Matthew A., Kuppa, Srikant, Stager, Paul J.
• Granted Patent: US 8,559,887 B2
• Time in Patent Office: Days
• US Class Current: 455/67.16
• CPC Class Codes: H04B 17/309 Measuring or estimating channel quality parameters
• CPC Class Codes: H04B 7/0617 for beam forming
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/0222 Estimation of channel variability, e.g. coherence bandwidth, coherence time, fading frequency

Abstract

Techniques are provided to measure how fast a wireless channel is changing between a first wireless communication device (e.g., a wireless base station or access point) and a second wireless device (e.g., a wireless client device) based on signals that the first communication wireless device receives at a plurality of antennas from a second wireless communication device. Techniques are also provided to measure a degree of mobility of the second wireless communication device based on signals received at the first communication wireless device from the second communication wireless device.

Claims

1. A method comprising:

at a first wireless communication device, receiving at a plurality of antennas transmissions in a channel from a second wireless communication device to produce a corresponding plurality of antenna-specific receive signals;

computing from the plurality of antenna-specific receive signals a channel estimate for a channel between the first wireless communication device and the second wireless communication device;

computing a channel observation parameter from the channel estimate;

storing the channel observation parameter computed for each of a plurality of transmissions received over time at the first wireless communication device from the second wireless communication device;

computing data representing differences between the channel observation parameters stored for combinations of pairs of the plurality of transmissions received over time;

storing the data representing the differences in bins sorted by time interval between time of reception of the pairs of transmissions used to compute the respective differences; and

generating an estimate of the coherence time of the channel based on a distribution of the data representing the differences stored in the bins.

2. The method of claim 1, and further comprising generating data defining the bins such that each bin is dedicated to a different time interval length.

3. The method of claim 2, wherein generating the estimate of the coherence time of the channel comprises:

computing a cumulative distribution function of the data representing the differences in each bin;

identifying one or more bins having a cumulative distribution value that is less than a probability threshold and which stores difference data that is less than a channel observation threshold; and

of the bins whose difference data are less than the threshold, selecting the bin associated with the longest time interval and setting the coherence time for the channel based on the time interval for the selected bin.

4. The method of claim 2, wherein computing the channel observation parameter comprises computing the channel observation value at each of a plurality of subcarriers, and wherein generating the estimate of the channel coherence time comprises:

comparing the channel observation parameter computed for the plurality of transmissions at each subcarrier to accumulate a count of subcarriers at which the channel observation parameter differs by more than (or less than) a difference threshold;

identifying the bins for which the count of subcarriers is less than (or greater than) a count threshold; and

of the bins for which the count of subcarriers is less than (or greater than) the count threshold, selecting the bin associated with the longest time interval and using the time interval of that bin as the coherence time for the channel.

5. The method of claim 1, wherein computing the channel observation parameter comprises computing a co-phase angle at each of a plurality of subcarriers for each received transmission from the channel estimate computed for the received transmissions at each of the plurality of subcarriers, and computing data representing differences comprising computing data representing differences between co-phase angles computed for each of the plurality of transmissions.

6. The method of claim 1, wherein computing the channel observation parameter comprises computing an eigenvector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers, and computing data representing differences comprises computing data representing Euclidean distances between eigenvectors computed from reception of each of the plurality of transmissions.

7. The method of claim 1, wherein computing the channel observation parameter comprises computing a transmit beamforming vector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers, and computing data representing differences comprises computing data representing Euclidean distances between the transmit beamforming vectors computed from reception of each of the plurality of transmissions.

8. The method of claim 1, wherein receiving comprises receiving transmissions that contain timing information used by the first wireless communication device to synchronize a timing reference of the first wireless communication device with a timing reference of the second wireless communication device, and further comprising ignoring variations in the timing information when they are determined to occur between transmissions made within the coherence time of the channel.

9. The method of claim 8, and further comprising comparing the data representing the differences for reception of the transmissions containing timing information greater than a threshold, and when the data representing the differences for reception of the transmissions containing timing information is greater than the threshold, supplying adjustment information to a synchronization filter in the first wireless communication device.

10. An apparatus comprising.

a plurality of antennas;

a radio receiver configured to be coupled to the plurality of antennas and to generate a plurality of antenna-specific receive signals derived from a transmission sent by an other wireless communication device and detected at the plurality of antennas;

a controller coupled to the radio receiver and configured to;

generate from the plurality of antenna-specific receive signals a channel estimate for a channel with respect to the other wireless communication device;

compute a channel observation parameter from the channel estimate;

store the channel observation parameter computed for each of a plurality of transmissions received over time from the other wireless communication device;

compute data representing differences between the channel observation parameters stored for combinations of pairs of the plurality of transmissions received over time;

store the data representing the differences in bins sorted by time interval between time of reception of the pairs of transmissions used to compute the respective differences; and

generate an estimate of the coherence time of the channel based on a distribution of the data representing the differences stored in the bins.

11. The apparatus of claim 10, wherein the controller is further configured to generate data defining the bins such that each bin is dedicated to a different time interval length.

12. The apparatus of claim 11, wherein the controller is configured to compute the channel observation parameter at each of a plurality of subcarriers, and to generate the estimate of the channel coherence time by comparing the channel observation parameter computed for the plurality of transmissions at each subcarrier to accumulate a count of subcarriers at which the channel observation parameter differs by more than (or less than) a difference threshold, identifying the bins for which the count of subcarriers is less than (or greater than) a count threshold, and of the bins for which the count of subcarriers is less than (or greater than) the count threshold, selecting the bin associated with the longest time interval and using the time interval of that bin as the coherence time for the channel.

13. The apparatus of claim 10, wherein the controller is configured to compute the channel observation parameter by computing a co-phase angle at each of a plurality of subcarriers for each received transmission from the channel estimate computed for the received transmissions at each of the plurality of subcarriers, and the controller is configured to compute the data representing differences by computing data representing differences between co-phase angles computed for each of the plurality of transmissions.

14. The apparatus of claim 10, wherein the controller is configured to compute the channel observation parameter by computing an eigenvector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers, and the controller is configured to compute the data representing differences by computing data representing Euclidean distances between eigenvectors computed from reception of each of the plurality of transmissions.

15. The apparatus of claim 10, wherein the controller is configured to compute the channel observation parameter by computing a transmit beamforming vector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers, and the controller is configured to compute the data representing differences by computing data representing Euclidean distances between the transmit beamforming vectors computed from reception of each of the plurality of transmissions.

16. A computer-readable medium encoded with instructions that, when executed by a processor, cause the processor to:

generate from a plurality of antenna-specific receive signals derived from a transmission received at a plurality of antennas of a first wireless communication from a second wireless communication device, a channel estimate for a channel between the first wireless communication device and the second wireless communication device;

compute a channel observation parameter from the channel estimate;

store the channel observation parameter computed for each of a plurality of transmissions received over time at the first wireless communication device from the second wireless communication device;

compute data representing differences between the channel observation parameters stored for combinations of pairs of the plurality of transmissions;

store the data representing the differences in bins sorted by time interval between time of reception of the pairs of transmissions used to compute the respective differences; and

generate an estimate of the coherence time of the channel based on a distribution of the data representing the differences stored in the bins.

17. The computer-readable medium of claim 16, and further comprising instructions, that when executed by the processor, cause the processor to generate data defining the bins such that each bin is dedicated to a different time interval length, and wherein the instructions that cause the processor to compute the channel observation parameter comprise instructions that cause the processor to compute the channel observation parameter at each of a plurality of subcarriers, and wherein the instructions that cause the processor to generate the estimate of the coherence time of the channel comprise instructions that cause the processor to compare the channel observation parameter computed for the plurality of transmissions at each subcarrier to accumulate a count of subcarriers at which the channel observation parameter differs by more than (or less than) a difference threshold, identify the bins for which the count of subcarriers is less than (or greater than) a count threshold, and of the bins for which the count of subcarriers is less than (or greater than) the count threshold, select the bin associated with the longest time interval and using the time interval of that bin as the coherence time for the channel.

18. The computer-readable medium of claim 16, wherein the instructions that cause the processor to compute the channel observation parameter comprise instructions that cause the processor to compute a co-phase angle at each of a plurality of subcarriers for each received transmission from the channel estimate computed for the received transmissions at each of the plurality of subcarriers, and the instructions that cause the processor to compute the data representing differences comprise instructions that cause the processor to compute data representing differences between co-phase angles computed for each of the plurality of transmissions.

19. The computer-readable medium of claim 16, wherein instructions that cause the processor to compute the channel observation parameter comprise instructions that cause the processor to compute an eigenvector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers, and the instructions that cause the processor to compute the data representing differences comprise instructions that cause the processor to compute data representing Euclidean distances between eigenvectors computed from reception of each of the plurality of transmissions.

20. A method comprising:

at a first wireless communication device, receiving at a plurality of antennas transmissions in a channel from a second wireless communication device to produce a corresponding plurality of antenna-specific receive signals;

computing from the plurality of antenna-specific receive signals a channel estimate for a channel between the first wireless communication device and the second wireless communication device;

computing a channel observation parameter from the channel estimate, the channel observation parameter representing conditions of the channel at a plurality of subcarriers;

comparing corresponding subcarriers of the channel observation parameter computed for successive transmissions; and

generating an indication as to whether the second wireless communication device is moving or stationary with respect to the first wireless communication device based on the number of subcarriers determined to exhibit a difference.

21. The method of claim 20, and further comprising comparing the number of subcarriers determined to exhibit a difference greater than (or less than) a difference threshold with a count threshold, and wherein generating comprises generating an indication that the second wireless communication device is stationary with respect to the first wireless communication device when the number of subcarriers that exhibit a difference greater than (or less than) the difference threshold is less than (or greater than) the count threshold, and generating an indication that the second wireless communication device is moving with respect to the first wireless communication device when the number of subcarriers that exhibit a difference greater than (or less than) the difference threshold is greater than (or less than) the count threshold.

22. The method of claim 21, wherein when a relatively small number of transmissions have been received, determining comprises first determining whether the number of subcarriers determined to exhibit a difference greater than (or less than) the difference threshold is greater than (or less than) a first count threshold, and when a relatively large number of transmissions have been received, determining comprises second determining whether a number of subcarriers, which are different than the subcarriers previously determined to exhibit a difference greater than (or less than) the difference threshold when a relatively small number transmissions have been received, that exhibit a difference greater than (or less than) the difference threshold is greater than (or less than) a second count threshold.

23. The method of claim 20, wherein computing the channel observation parameter comprises computing a co-phase angle at each of the plurality of subcarriers for each received transmission from the channel estimate computed for the received transmissions at each of the plurality of subcarriers.

24. The method of claim 20, wherein computing the channel observation parameter comprises computing an eigenvector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers.

25. The method of claim 20, wherein computing the channel observation parameter comprises computing a transmit beamforming vector at each of a plurality of subcarriers for each received transmission from the channel estimate computed from the received transmissions at each of the plurality of subcarriers.