Slewing detector system and method for the introduction of hysteresis into a hard handoff decision

• US 6,826,161 B1
• Filed: 07/20/2000
• Issued: 11/30/2004
• Est. Priority Date: 07/20/2000
• Status: Expired due to Fees

• Current Assignee: Unwired Planet LLC (JP Morgan Chase & Co)
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Shahidi, Reza, Chen, Jen M., Duan, Long L., Lima, Octavio Jose De Franca, Sesnic, Tom T.
• Primary Examiner(s): NGUYEN, HANH N
• Application Number:
  US09/620,374
• Time in Patent Office: 1,594 Days
• Field of Search: 370/331-335, 370/341-342, 370/441, 370/350, 370/503, 370/504, 370/507, 370/515, 370/518, 370/520, 370/508, 370/337, 370/338, 455/436-444, 455/12.1, 455/13.2
• US Class Current: 370/331
• CPC Class Codes: H04W 36/008375 based on historical data

Abstract

A method for introducing hysteresis into a hard handoff decision in code division multiple access (CDMA) communications following an initial hard handoff is provided. The method introduces hysteresis when it is determined that the communicating mobile station system clock is slewing. The slewing determination follows from a calculation of the rate of change in the round trip delay (RTD) in communications with the mobile station. A calculated RTD rate of change is compared with a RTD slewing threshold, where the RTD threshold is the assumed slew rate of the mobile station. Hard handoffs are suspended as long as the calculated RTD rate of change exceeds the RTD slewing threshold. A system and apparatus to implement the above-described method are also provided.

Claims

1. In a code division multiple access (CDMA) base station subsystem, a method for introducing hysteresis in a call handoff, the method comprising:

detecting instability in a mobile station (MS) system clock by measuring a slew rate associated with the mobile station system timing; and

in response to detecting the mobile station clock instability, suspending hard handoffs of the mobile station.

2. The method of claim 1 further comprising:

preceding the detecting of a mobile station system clock instability, handing off the mobile station from a first cell to a second cell; and

wherein detecting the mobile station system clock instability includes a base station in the second cell detecting the mobile station system clock instability.

3. The method of claim 2 wherein measuring the slew rate of the mobile station system clock includes:

measuring the round trip delay (RTD) of a first message at a first time, from the second cell base station to the mobile station;

measuring the RTD of a second message at a second time, from the second cell base station to the mobile station;

calculating the difference in RTD measurements; and

calculating the RTD rate of change.

4. The method of claim 3 wherein suspending handoffs of the mobile station in response to detecting the mobile station clock instability includes:

accepting a predetermined RTD rate of change threshold; and

suspending handoffs if the calculated RTD rate of change exceeds the RTD rate of change threshold.

5. The method of claim 4 further comprising:

preceding the handing off of the mobile station from the first cell to the second cell, communicating messages from the mobile station to a base station in the first cell at a first system time;

following the handing off of the mobile station from the first cell to the second cell, changing the mobile station system clock from the first system time to a second system time for communications to the second cell base station; and

wherein detecting instability in the mobile station system clock includes measuring the change in the mobile station system clock from the first system time to the second system time.

6. The method of claim 5 wherein changing the mobile station system time includes changing the system time at a first predetermined slew rate;

wherein accepting the RTD rate of change threshold includes determining a first slew rate estimate for the first predetermined slew rate;

wherein detecting instability in the mobile station system clock includes;

in response to calculating the RTD difference, calculating the mobile station measured slew; and

wherein suspending the handoff of the mobile station includes suspending the handoff if the measured slew rate exceeds the first slew rate estimate.

7. The method of claim 6 in which communications to the mobile station are spread with a PN chips at a PN chip rate;

wherein the mobile station first slew rate is estimated to be in the range between 3/8 and 10/8 chips per second;

wherein calculating the RTD difference includes expressing the difference in RTD measurements as a second number of chips;

wherein calculating the RTD rate of change includes expressing the RTD rate of change as the second number of chips occurring in a first time period, where the first time period is defined between the first communication at the first time and the second communication at the second time; and

wherein suspending handoffs to the mobile station includes suspending handoffs if the calculated RTD rate of change exceeds the first slew rate estimate.

8. The method of claim 7 wherein the mobile station first slew rate estimate is multiplied by the first time period to calculate a first number of chips;

and

wherein suspending handoffs to the mobile station includes suspending handoffs if the second number of chips exceeds the first number of chips.

9. The method of claim 7 wherein the mobile station first slew rate is estimated to be 5/8 chips per second.

10. The method of claim 1 further comprising:

following the handing off the mobile station to the second cell, starting a timer with a timer limit; and

regardless of the detected mobile station system clock instabilities, permitting mobile station handoffs after the timer has reached the timer limit.

11. The method of claim 1 further comprising:

permitting mobile station handoffs if no mobile station system clock instabilities are measured.

12. In code division multiple access (CDMA) communications, a system for introducing hysteresis into call handoff, the system comprising:

a mobile station (MS) having a port connected to a receiver and a transmitter for the communication of messages, the mobile station having a system clock connected to the transmitter and receiver for initiating message transmission in response to receiving messages; and

a base station (BS) located in a second cell having a port connected to a receiver and transmitter to communicate with the mobile station, the second cell base station having a clock connected to the receiver and transmitter to measure message timing, a handoff circuit to negotiate mobile station handoffs with other cells and a detector connected to the clock to detect instabilities in the mobile station system clock, the detector having an output connected to the handoff circuit to suspend handing off the mobile station in response to detecting instabilities in the mobile station system clock.

13. The system of claim 12 further comprising:

a second cell base station switch having a switch control input connected to the handoff circuit for selectively engaging the detector circuit measurement of mobile station system clock instabilities in response to the second cell base station receiving the mobile station handoff from a first cell.

14. The system of claim 13 wherein the second cell base station detector measures a slew rate of the mobile station system clock.

15. The system of claim 14 wherein the second cell base station clock measures the round trip delay (RTD) between messages from the second cell base station to the mobile station;

and

wherein the second cell base station detector calculates the difference between RTD measurements and the RTD rate of change.

16. The system of claim 15 wherein the second cell base station detector has an input to accept a predetermined RTD rate of change threshold;

and

wherein the second call base station detector signals the handoff circuit to suspend a handoff of the mobile station if the RTD rate of change calculation exceeds the RTD rate of change threshold.

17. The method of claim 16 further comprising:

a base station located In the first cell having a port connected to a receiver and a transmitter to communicate with the mobile station;

wherein the mobile station system clock changes system time in response to being handed off from the first cell base station to the second cell base station; and

wherein the second cell base station detector measures the change in the mobile station system clock.

18. The system of claim 17 wherein the mobile station system clock changes the system time at a first predetermined slew rate;

and

wherein the second cell base station detector acceptance of the RTD difference threshold includes accepting a first slew rate estimate for the first predetermined slew rate, the detector measuring the mobile station slew rate in response to calculating the RTD rate of change, and signaling the handoff circuit to suspend a mobile station handoff if the measured slew rate exceeds the first slew rate estimate.

19. The system of claim 18 wherein base station and mobile station communications are spread with pseudo-noise (PN) chips at a PN chip rate;

wherein the second cell base station detector accepts a first slew rate estimate in the range between 3/8 and 10/8 chips per second;

wherein the second cell base station expresses RTD difference measurements as a second number of chips;

wherein the second cell base station calculates the RTD rate of change by expressing the RTD rate of change as the second number of chips occurring in a first time period where the first time period is defined between the first communication at the first time and the second communication at the second time; and

wherein the second cell base station suspends handoffs to the mobile station if the calculated RTD rate of change exceeds the first slew rate estimate.

20. The system of claim 19 wherein the second cell base station multiplies the mobile station first slow rate estimate by the first time period to calculate a first number of chips;

and

wherein the second cell base station suspends handoffs to the mobile station if the second number of chips exceeds the first number of chips.

21. The system of claim 19 wherein the first slew rate is estimated to be 5/8 chips per second.

22. The system of claim 13 further comprising:

a second cell base station timer having an Input to accept a timer start signal once the second call base station accepts the mobile station handoff from the first cell, and having an output connected to the switch to disengage the detector circuit; and

wherein the handoff circuit is permitted to handoff the mobile station after the time has reached the timer limit.

23. In a code division multiple access (CDMA) base station including a receiver and a transmitter to communicate messages associated with a call, circuitry for introducing hysteresis in a handoff of the call between cells, the circuitry comprising:

a clock connected to the receiver and the transmitter to measure message timing;

a handoff circuit to negotiate call handoffs with other cells; and

a detector connected to the clock to detect an instability of system timing associated with the messages received from a mobile station, the detector having an output connected to the handoff circuit to suspend handing off the call in response to detecting the system timing instability, wherein the detector detects the system clock instability by measuring a slew rate associated with the received message system timing.

24. The circuitry of claim 23 further comprising:

a switch having a control input connect to the handoff circuit to selectively engage the detector circuit, the handoff circuit signaling the switch to engage the detector in response to the station receiving a call handoff.

25. The circuitry of claim 24 further comprising:

a timer having an input to accept a timer start signal following the acceptance of a call handoff from another cell, the timer having an output connected to the switch to disengage the detector circuit; and

wherein the handoff circuit is permitted to handoff the mobile station after the timer has reached the timer limit.

26. The circuitry of claim 23 wherein the clock measures the round trip delay (RTD) between transmitted and received messages;

and

wherein the detector calculates the difference between RTD measurements and the RTD rate of change.

27. The circuitry of claim 26 wherein the detector has an input to accept a predetermined RTD rate of change threshold;

and

wherein the detector signals the handoff circuit to suspend a call handoff if the RTD rate of change calculation exceeds the RTD rate of change threshold.

28. The circuitry of claim 27 in which the system timing of received messages changes in response to being handed off from another cell;

and

wherein the detector measures the change in the system timing.

29. The circuitry of claim 28 in which the system timing of the received messages changes at a first predetermined slew rate;

and

wherein the detector'"'"'s acceptance of the RTD difference threshold includes accepting a first slew rate estimate for the first predetermined slew rate, measuring the slew rate associated with the received messages in response to calculating the RTD rate of change, and signaling the handoff circuit to suspend a call handoff if the measured slew rate exceeds the first slew rate estimate.

30. The circuitry of claim 29 in which the transmitted and received messages are spread with pseudo-noise (PN) chips at a PN chip rate;

wherein the detector has an input to accepts a first slew rate estimate in the range between 3/8 and 10/8 chips per second;

wherein the detector expresses the difference in RTD measurements as a second number of chips;

wherein the detector calculates the RTD rate of change by expressing the RTD rate of change as the second number of chips occurring in a first time period, where the first time period is defined between the first communication at the first time and the second communication at the second time; and

wherein the detector suspends handoffs if the calculated RTD rate of change exceeds the first slew rate estimate.

31. The circuitry of claim 30 wherein the detector multiplies the first slew rate estimate by the first time period to calculate a first number of chips;

and

wherein the detector suspends handoffs if the second number of chips exceeds the first number of chips.

32. The circuitry of claim 30 wherein the detector accepts a first slew rate estimate of 5/8 chips per second.