Method and apparatus for evaluating a received signal in a wireless communication utilizing long and short term values

• US 6,049,715 A
• Filed: 02/20/1997
• Issued: 04/11/2000
• Est. Priority Date: 06/01/1994
• Status: Expired due to Fees

• Current Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Original Assignee: Nortel Networks Corporation
• Inventors: Mayes, John S., LaBauve, Lance Miguel, Willhoff, Steven J., Pyle, Mark Bruce, Meche, Paul S.
• Primary Examiner(s): Chin, Wellington
• Assistant Examiner(s): SOBUTKA, PHILIP
• Application Number:
  US08/803,408
• Time in Patent Office: 1,146 Days
• Field of Search: 455/436, 455/437, 455/438, 455/439, 455/442, 455/443, 455/444, 455/517, 455/67.1, 455/226.1, 455/226.2, 455/525
• US Class Current: 455/436
• CPC Class Codes: H04W 24/00 Supervisory, monitoring or testing arrangements

Abstract

A long term average and a short term value of the strength of a received signal is used to determine a trigger condition in a mobile communications system to perform such functions as control channel or traffic channel reselect. The long term average removes the effects of short term fades in the short term signal that otherwise may cause a premature trigger decision. The short term value offsets historical bias that may be built into long term average. The actual trigger is only realized when both the long term average and the short term value exceed predetermined thresholds in a positive or negative direction.

Claims

1. A method, in a wireless communications system, of evaluating a received signal, comprising the steps of:

obtaining a long term average value and a short term value indicating the strength of a received signal;

determining a characteristic of the received signal according to a separate consideration of both the long term average value and the short term value;

determining that a signal strength based trigger condition exists when both the long term average value and the short term average value exceed a predetermined threshold; and

if the short term average value is not greater than said predetermined threshold, continuing checking said short term average value until one of both said short and long term average values is greater than said predetermined threshold.

2. The method of claim 1 further comprising the steps of:

maintaining a list of control channels that are adjacent to a current control channel;

maintaining a long term average and short term value for each control channel on the table; and

sampling each control channel at a predetermined interval to determine a received signal strength.

3. The method of claim 1 further comprising the steps of:

sampling a serving control channel at a predetermined rate to determine the received signal strength; and

maintaining a long term and short term average of the received signal strength for a serving control channel.

4. The method a s recited in claim 1 further comprising the step of determining a traffic channel signal strength based reselect trigger condition exists when both the long term average value and the short term value of a serving traffic channel are below a pre determined threshold value.

5. The method as recited in claim 1 further comprising the step of determining a traffic channel signal strength based reselect trigger condition exists when both the long term average value and the short term value of a neighboring reference channel are above a predetermined threshold value.

6. The method of claim 1 further comprising the steps of:

maintaining a list of reference channels that are adjacent to a current traffic channel;

maintaining a long term average and short term value for each reference channel on the table; and

sampling each reference channel at a predetermined interval to determine a received signal strength.

7. A method of correctly determining in a wireless unit in a wireless system, when a wireless unit should reselect a control channel, the method comprising the steps of:

comparing a first signal value indicative of a long term averaged signal strength of a received signal to a predetermined first threshold value;

comparing a second signal value indicative of a short term value of the received signal strength to a predetermined second threshold value if the first signal value is below the first threshold value;

determining that a signal strength based reselect condition exists if both the first and second signal values are below the first and second threshold values, respectively; and

if the second signal value is not less than said second threshold value, continuing checking said second signal value until one of both said first and second signal values are less than said first and second threshold values, respectively.

8. The method as recited in claim 7 wherein said first and second threshold values are equal.

9. The method as recited in claim 7 wherein said first and second threshold values are different.

10. The method of claim 7 wherein the long term average is comprised of an average of a plurality of the short term values.

11. A method of correctly determining in a mobile unit in a wireless system, when a mobile unit should reselect a control channel, the method comprising the steps of:

comparing a first signal value indicative of a short term value of a received signal strength to a predetermined first threshold value;

comparing a second signal value indicative of a long term average value of the received signal strength to a predetermined second threshold value if the first signal value is below the first threshold value;

determining that a signal strength based reselect condition exists if both the first and second signal values are below the first and second threshold values, respectively, and

if the first signal value is not less than said first threshold value, continuing checking said first signal value until one of both said first and second signal values are less than said first and second threshold values, respectively.

12. A mobile station in a mobile communications system, comprising:

means for determining the strength of a received signal;

a memory containing a long term average value and a short term value indicating the strength of the received signal;

means for determining that a trigger condition is present when both the long term average value and the short term value are below a predetermined threshold; and

means for continuing checking said short term value if said trigger condition is not met until one of both said short term value and long term average value is less than said predetermined threshold.

13. An apparatus for correctly determining in a wireless unit in a wireless system, when a wireless unit should reselect a control channel, the apparatus comprising:

an antenna receiving a transmitted signal;

a receiver connected to the antenna, the receiver providing a signal indicative of a signal strength of the received signal;

a memory storing a predetermined threshold value;

a processor connected to receive the signal from the receiver indicative of the signal strength of the received signal, the received signal being sampled at predetermined intervals of time, the processor being configured to determine a first signal value, the first signal value being determined from a plurality of samples of the strength of the received signal and being indicative of a long term averaged signal strength of a received signal, the processor further being configured to determine a second signal value indicative of a short term value of the received signal, the short term value being comprised of at least the most recent sample of the strength of the received signal, the processor further being configured to store the first and second signal values in the memory, the processor further being configured to compare the first and second signal values to the predetermined threshold value, the processor having an output indicating when both the first and second signal values exceed the threshold value, thereby indicating that a signal strength based reselect condition exists; and

means for continuing checking said second signal value if said reselect condition is not met until one of both said first and second signal values is greater than said predetermined threshold.

14. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

comparing a first signal value indicative of a long term averaged signal strength of a received signal from said mobile station, to a predetermined threshold value;

comparing a second signal value indicative of a short term value of said received signal strength to said threshold value if said first signal value is below said threshold value;

determining that a handoff condition exists if and only if both said first and second signal values are below said threshold;

if the handoff condition is not met, continuing checking said second signal value until one of both said first and second signal values are less than said threshold value; and

initiating handoff of said call to said voice channel in another cell site when said handoff condition is met.

15. The method of claim 14 wherein a time period over which said first signal value is averaged is longer than a time period over which said second signal is averaged.

16. The method of claim 14 wherein a time period over which said second signal value is averaged is correlated to a time period over which a signal strength value is measured by locate/receive devices of adjacent cells.

17. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

(a) determining if a long term sample rate window average of a received signal strength indicator (RSSI) is less than a handoff threshold;

(b) if said long term sample rate window average is less than said handoff threshold, determining if a short term average of said RSSI is less than said handoff threshold;

(c) if both said long term sample rate window average and said short term average is less than said handoff threshold, sending a handoff request to a controller; and

(d) if said short term average is not less than said handoff threshold, continuing checking with every sample taken until one of both said short and long term averages are less than said handoff threshold, or call releases.

18. The method as recited in claim 17 further comprising the steps of:

setting a handoff guard timer after sending said handoff request; and

returning to step (a) if said handoff guard timer expires before a handoff command is received from said controller.

19. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

comparing a first signal value indicative of a long term averaged signal strength of a received signal from a mobile station, to a predetermined threshold;

comparing a second signal value indicative of a short term value of said received signal to said threshold; and

determining that said handoff condition exists if both said first and second signal values are below said threshold;

if said second signal value is not less than said threshold, continuing checking said second signal value until one of both said first and second signal values indicate that a handoff condition exists; and

initiating handoff of said call to said voice channel in another cell site when said handoff condition is met.

20. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

sampling signals received from said mobile station to determine a signal strength of said received signals at predetermined intervals and outputting a received signal strength indication (RSSI) sample at said predetermined intervals;

calculating a long term average of said samples at said predetermined intervals once a predetermined number of said samples have been received;

calculating a short term average of said samples;

comparing said long term average to a predetermined threshold value;

comparing said short term average to said threshold value if said long term average is below said threshold value;

determining that said handoff condition exists if both said long term and short term averages are below said threshold value;

if said short term average is not less than said threshold value, continuing checking said short term average until one of both said short and long term averages indicate that a handoff condition is met; and

initiating handoff of said call to said voice channel in another cell site when said handoff condition is met.

21. A voice channel apparatus for determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, said apparatus comprising:

a receive channel for receiving signals from said mobile station;

a cell controller interface for receiving a threshold value from a cell controller;

a controller, receiving samples representing a signal strength of said received signals from said receive channel, said controller maintaining a long and short term buffer and storage locations for a long term average of said long term buffer and a short term average of said short term buffer, said controller including a processor which determines a handoff condition when both said long term average and said short term average are less than said threshold value, said controller continuing checking said short term average if said handoff condition is not met until one of both said long and short term averages is less than said threshold value, said controller sending a handoff request, including said short term average, to said cell controller through said cell controller interface upon determination of said handoff condition.

22. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

comparing a first signal value indicative of a long term averaged signal strength of a received signal from said mobile station, to a predetermined threshold value;

comparing a second signal value indicative of a short term value of said received signal strength to said threshold value only if said first signal value is below said threshold value;

determining that a handoff condition exists if both said first and second signal values are below said threshold; and

initiating handoff of said call to said voice channel in another cell site when said handoff condition is met.

23. A method of determining, in a cellular system, a time for handing off a phone call on a mobile station on a particular voice channel in one cell site to a voice channel in another cell site, the method comprising the steps of:

comparing a first signal value indicative of a long term averaged signal strength of a received signal from said mobile station, to a predetermined fixed threshold value;

comparing a second signal value indicative of a short term value of said received signal strength to said threshold value if said first signal value is below said threshold value;

determining that a handoff condition exists if both said first and second signal values are below said threshold; and

initiating handoff of said call to said voice channel in another cell site when said handoff condition is met.