SMOOTHING FILTER FOR IRREGULAR UPDATE INTERVALS

• US 20090247103A1
• Filed: 03/28/2008
• Published: 10/01/2009
• Est. Priority Date: 03/28/2008
• Status: Active Grant

• Current Assignee: Trapeze Networks Incorporated (Juniper Networks Incorporated)
• Original Assignee: Trapeze Networks Incorporated (Juniper Networks Incorporated)
• Inventors: Aragon, David B.
• Granted Patent: US 8,150,357 B2
• Time in Patent Office: Days
• US Class Current: 455/226.200
• CPC Class Codes: H04B 17/318 Received signal strength

Abstract

A technique for determining a received signal strength from multiple messages filters noise from the received signal to provide an accurate signal strength value. Advantageously, the more accurate output signal strength value can be used to identify movement of a station as well as estimate locations and direction of movement.

Claims

1. A system comprising:

a radio;

a dynamically windowed filter;

wherein, in operation,

the radio receives a second sample associated with a second sample signal strength value;

the dynamically windowed filter obtains a weighted input signal strength value from the second sample signal strength value;

the dynamically windowed filter obtains a decayed previous output value from a previous output value;

the dynamically windowed filter adds the decayed previous filter output value to the weighted input signal strength value to obtain a filter output.

2. The system of claim 1 wherein the radio receives a first sample associated with a first sample signal strength value and initializes the previous output filter value.

3. The system of claim 1 further comprising:

a dynamic time window for the filter, wherein the dynamic time window is dynamically allocated as a factor of a weight applied to an input signal and a factor of a weight applied to a previous output signal.

4. The system of claim 1 wherein the dynamically windowed filter obtains the decayed previous filter output value by multiplying the previous filter output value by e⁻

t/τ

wherein t is a time elapsed value of a difference in time between receipt of the first sample and receipt of the second sample and τ

represents a dynamic time window.

5. The system of claim 1 wherein the dynamically windowed filter obtains the weighted input signal strength value by 1-e⁻

t/τ

, wherein t is a time elapsed value of a difference in time between receipt of the first sample and receipt of the second sample and τ

represents a dynamic time window.

6. The system of claim 1 wherein the weighted input signal strength is obtained by calculating a value of a function expressed as a Taylor polynomial approximation.

7. The system of claim 1 wherein the system receives a third signal associated with a third signal strength value and calculates a subsequent output signal strength value using the second output signal strength value and the third signal strength value.

8. The system of claim 1 further comprising:

a clock coupled to the processor, wherein the processor determines the elapsed time value from by comparing a time value from the clock when the signal is received with a second time value stored when the first sample was received.

9. A method comprising:

filtering an irregularly received signal through a first dynamic time window at a first network portal (NP) to obtain a first received signal strength value of the irregularly received signal at a first location;

filtering the irregularly received signal through a second dynamic time window at a second NP to find a second received signal strength value of the irregularly received signal at a second location;

comparing the first received signal strength and the second received signal strength to identify a direction of movement of a source of the irregularly received signal.

10. The method of claim 9 further comprising:

filtering the irregularly received signal at a third NP to find a third signal strength at a third location; and

using location of the third NP along with the location of the first NP and the location of the second NP to find and a location of the source of the signal.

11. The method of claim 9 wherein the first NP and the second NP use different dynamic time windows, τ

₁ and τ

₂.

12. The method of claim 9 wherein the first dynamic time window and the second dynamic time window are of equal size.

13. The method of claim 9 wherein filtering comprises adding a weighted input signal strength value of the irregularly received signal to a decayed previous output signal strength value of the irregularly received signal to obtain a filter output.

14. A method comprising:

providing a dynamic time window, a previous output signal strength value and a time elapsed value;

receiving, at a radio, a signal having a signal strength value;

decaying the previous output signal strength value with a first function of the time elapsed value and the dynamic time window to obtain a decayed previous output signal strength value;

weighting the signal strength value with a second function of the time elapsed value and the dynamic time window to obtain a weighted current signal strength value;

adding the decayed previous output signal strength value to the weighted signal strength value to find a current output signal strength value.

15. The method of claim 14 wherein the first function and the second function include a ratio of the time elapsed value to the dynamically allocated time constant value.

16. The method of claim 14 wherein the first function is 1-e⁻

t/τ

wherein t is the time elapsed value and τ

represents a dynamic time window.

17. The method of claim 14 wherein the time elapsed value is determined as a difference between a first time when the current signal strength value is recorded and a second time when the second sample signal strength value is recorded.

18. The method of claim 14 further comprising:

receiving a third signal associated with a third signal strength value, and determining a subsequent output signal strength value by adding a weighted third signal strength value to a decayed current output signal strength value.

19. A device comprising:

a radio;

a trend detector including;

a first dynamically windowed filter having a first dynamic time window, and

a second dynamically windowed filter having a second dynamic time window;

a comparison module;

wherein, in operation,

the radio receives an irregular signal and provides the signal to the trend detector,

the trend detector filters the signal through the first dynamically windowed filter and the second dynamically windowed filter and compares the output of the first dynamically windowed filter with the output of the second dynamically windowed filter using the comparison module.

20. The device of claim 19 wherein a report is generated including a description of motion of a transmitter transmitting the irregular signal.