Time alignment of transmission in a down-link of CDMA system

• US 5,828,659 A
• Filed: 08/02/1996
• Issued: 10/27/1998
• Est. Priority Date: 06/14/1993
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Teder, Paul, Larsson, Lars Gustav, Persson, Hakan Anders
• Primary Examiner(s): VU, HUY DUY
• Application Number:
  US08/691,436
• Time in Patent Office: 816 Days
• Field of Search: 370/503, 370/504, 370/505, 370/506, 370/507, 370/508, 370/509, 370/510, 370/511, 370/512, 370/513, 370/514, 370/515, 370/516, 370/517, 370/518, 370/519, 370/520, 370/350, 370/441, 370/342, 375/354, 375/356, 375/357, 375/358, 375/359, 375/362, 375/364, 375/368, 375/371, 455/13.2, 455/18, 455/440, 455/500, 455/502, 455/524, 455/67.1, 455/67.6, 455/69
• US Class Current: 370/328
• CPC Class Codes: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
• CPC Class Codes: H04L 7/0033 Correction by delay
• CPC Class Codes: H04L 7/005 Correction by an elastic buffer
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection

Abstract

A mobile station can be connected to more than one base station in a procedure known as macro-diversity. In order to synchronize the signals for a specific cellular call connection, the time difference between the connected base station'"'"'s signal and a macro-diversity candidate base station'"'"'s signal is measured by a mobile station. This measurement is then transmitted to the network. The measurements can be used to achieve synchronization via frame staggering.

Claims

1. A method of code division multiple access for cellular communication systems, said method comprising the steps of:

broadcasting signals on communication channels;

establishing a first call link between a first base station and a mobile station;

measuring a difference in time between the reception in said mobile station of at least one signal on a common control channel of at least one neighboring base station and a signal on a common control channel from said first base station;

initiating a second call link between said at least one neighboring base station and said mobile station;

delaying transmission of at least one of a communication signal that is to be sent from said first base station and a communication signal that is to be sent from said at least one neighboring base station in accordance with said difference in time between the reception of said signals measured in said mobile station;

transmitting said communication signal that is to be sent from said first base station and said communication signal that is to be sent from said at least one neighboring base station over first and second call links, respectively; and

receiving, in said mobile station, said communication signals transmitted over said first and second call links, respectively, within a predetermined time period.

2. A method according to claim 1, wherein said communication signals are transmitted according to a frame structure.

4. A method according to claim 1, wherein said establishing step includes the step of informing said first base station about a first time offset to be applied for transmission of signals over said first call link;

and wherein said measuring step includes the steps of measuring the difference in time between reception in said mobile station of a signal on a non-time offset channel on which signals are transmitted without a time offset from said at least one neighboring base station and reception of a signal on a channel on which signals are transmitted without a time offset from said first base station with which said mobile station is currently linked;

dispatching the measured values to a network controller;

informing said at least one neighboring base station about a second time offset for transmitting signals from said at least one neighboring base station with which said second call link is to be established on an assigned channel;

and wherein said step of delaying further comprises the step of buffering at least one of said communication signals that are to be sent on said first and second call links so that the transmission of said at least one of said communication signals is offset by the respective one of said first and second time offsets.

5. A method according to claim 2, comprising the further step of transmitting signals on said communication channel of at least one neighboring base station and on a communication channel of said first base station according to a frame structure wherein said communication signals transmitted over said first and second call link respectively are transmitted in frames that are offset with a time offset relative to frames transmitted on the respective one of said communication channels of said at least one neighboring base station and of said first base station.

6. A method according to claim 4, wherein communication signals are transmitted over said second call link from at least one neighboring base station to said mobile station with an arbitrary offset relative to the transmission of signals on said non-time offset channel of said at least one neighboring base station.

7. A method according to claim 2, wherein said predetermined time period corresponds to a rake window.

8. A method according to claim 1, wherein the signal transmitted over said first call link is received in said mobile station within a first part of said predetermined time period and the signal transmitted over said second call link is received in said mobile station within a second part of said predetermined time period.

9. A method according to claim 1, wherein said at least one neighboring base station and said first base station transmit said communication signals on the same frequency.

13. A method according to claim 1, further comprising the step of transmitting said measurements to a network controller interconnecting said first base station and said at least one neighboring base station.

14. A method according to claim 8, wherein said communication signals are transmitted according to a frame structure.

16. A method according to claim 14, wherein each of said first and second parts of said predetermined time period are identical to the duration of a frame respectively.

17. A method according to claim 4 further comprising the step of:

updating at least one of said first and second time offsets in accordance with a change in said difference in reception times, said change being caused by the movements of said mobile station.

3. A method according to claim 2comprising the further step of transmitting communication signals from said first base station in a compressed mode wherein a frame of data includes an information part and an idle part, thereby allowing said mobile station to perform said step of measuring during said idle part of a compressed mode frame.

10. A system capable of code division multiple access for cellular communication systems, said system comprising:

at least two base stations, at least one of said base stations including means for delaying a signal in order to retain a time offset, by an offset value relative to transmission of a non-time offset signal and means for transmitting said signal delayed by said delay means and for transmitting said non-time offset signal;

at least one mobile station including a receiver for receiving transmissions from said at least two base stations, a delay measurement unit for measuring delay in reception of said non-time offset signal from said at least one of said base stations and means for transmitting said delay measurement; and

at least one network controller including means for receiving said delay measurements, means for generating said offset value in accordance with said delay measurements and means for transmitting said offset value to at least one of said base stations.

11. A system according to claim 10, wherein at least one of said at least two base stations includes:

means for receiving signals including said delay measurements from said at least one mobile station; and

means for transmitting said delay measurements to said network controller.

12. A system according to claim 10, wherein said generating means of said network controller includes means for generating an arbitrary offset value not dependent on said delay measurements.

15. A method according to claim, wherein said at least one neighboring base station and said first base station transmit said communication signals on different frequencies.

18. A method according to claim 15, wherein said predetermined time period is identical to the duration of a frame.

19. A method of code division multiple access for cellular communication systems wherein signals are transmitted according to a frame structure including signal frames, said method comprising the steps of:

broadcasting common control signal frames containing information intended for any mobile station on common control channels of a first base station and at least one neighboring base station;

providing a first communication link between said first base station and a mobile station;

specifying a first time offset to be used to offset a time of transmission from said first base station to said mobile station of a first signal frame on said first communication link relative to transmission time of a first common control signal frame from said first base station on a common control channel;

measuring a difference in time between the reception in said mobile station of a second common control signal frame transmitted on a common control channel from said at least one neighboring base station and the reception in said mobile station of one of a first signal frame and a first common control signal frame transmitted from said first base station;

specifying at least one second time offset to be used to offset a time of transmission from said at least one neighboring base station of a second signal frame on a new communication link between said at least one neighboring base station and said mobile station relative to transmission time of a second common control signal frame on said common control channel of said at least one neighboring base station;

initiating said new communication link between said at least one neighboring base station and said mobile station;

delaying transmission of said second signal frame on said new communication link from said at least one neighboring base station so that said second time offset is retained;

delaying transmission of a first signal frame on said first communication link from said first base station so that said first time offset is retained;

transmitting said first signal frame on said first communication link from said first base station and said second signal frame on said new communication link from said at least one neighboring base station; and

receiving said first and second signal frames in said mobile station.

20. A method according to claim 19, wherein said second time offset is set so that said first and second signal frames are received in the mobile station within a frame duration.

21. A method according to claim 20, wherein said first and said second signal frames contain identical information.

22. A method according to claim 19, wherein said mobile station performs said measurement step.

23. A method according to claim 19, wherein, in said measuring step, the difference in time is measured between the reception in said mobile station of a second common control signal frame transmitted on said common control channel from said at least one neighboring base station and the reception in said mobile station of a first common control signal frame transmitted on said common control channel from said first base station.

24. A method according to claim 19, comprising the further step of calculating, based on the result of said measuring step, said second time offset.

25. A method according to claim 19, comprising the further step of transmitting frames on said first communication link in a compressed mode, wherein information is transmitted during a first part of a compressed mode frame and information is not transmitted during a second part of said compressed mode frame;

and wherein said measuring step is performed during said second part of said compressed mode frame.

26. A method according to claim 25, comprising the further step of returning to non-compressed mode transmission, wherein information is transmitted during the whole frame.

27. A method according to claim 19, wherein said first signal frame is received within a first predetermined time period and said second signal frame is received within a second predetermined time period in said mobile station, said second predetermined time period occurring in consecutive order after said first predetermined time period.

28. A method according to claim 27, wherein said first and second predetermined time period each corresponds to a rake window.

29. A method according to claim 27, wherein said first and second signal frames are transmitted in a compressed mode on said first and second call link, respectively, said first signal frame being transmitted during a first part of a compressed mode frame and said second signal frame being transmitted during a second part of said compressed mode frame.

30. A method according to claim 29, wherein said first predetermined time period corresponds to said first part of a compressed mode frame in said mobile station and said second predetermined time period corresponds to said second predetermined time period in said mobile station.

31. A method according to claim 30, wherein said at least one neighboring base station and said first base station transmit on said communication links on different frequencies.

32. A method of code division multiple access for cellular communication systems wherein signals are transmitted according to a frame structure including signal frames, said method comprising the steps of:

broadcasting control signal frames containing information intended for any mobile station on a first control channel and on at least one second control channel;

providing a first communication link to a mobile station;

specifying a first time offset to be used to offset a time of transmission of first signal frames on said first communication link relative to transmission of first control signal frames on said first control channel;

measuring a difference in time between the reception in said mobile station of a second control signal frame transmitted on said at least one second control channel and the reception in said mobile station of a first control signal frame transmitted on said first control channel;

specifying at least one second time offset to be used to offset a time of transmission of second signal frames on a new communication link to said mobile station relative to transmission time of second control signal frames on said second control channel;

initiating said new communication link to said mobile station;

delaying transmission of a second signal frame on said new communication link so that said second time offset is retained;

delaying transmission of a first signal frame on said first communication link so that said first time offset is retained;

transmitting said first signal frame on said first communication link and said second signal frame on said new communication link; and

receiving said first and second signal frames in said mobile station.

33. The method of code division multiple access according to claim 32, comprising the further step of periodically updating at least one of said first time offset and said second time offset.