Methods and arrangements in a radio communications system

• US 6,449,290 B1
• Filed: 06/12/1998
• Issued: 09/10/2002
• Est. Priority Date: 06/13/1997
• Status: Expired

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Näsman, Karl Anders, Willars, Per Hans Åke
• Primary Examiner(s): Olms, Douglas
• Assistant Examiner(s): HOM, SHICK C
• Application Number:
  US09/096,171
• Time in Patent Office: 1,551 Days
• Field of Search: 370/312, 370/320, 370/324, 370/331, 370/335, 370/342, 370/441, 370/479, 370/503, 370/507, 370/509, 370/512, 370/516, 370/517, 370/350, 370/337, 370/508, 375/356, 375/357, 375/354, 375/355, 455/13.2, 455/436
• US Class Current: 370/507
• CPC Class Codes: H04B 7/022 Site diversity; Macro-diversity using two or more spaced independent antennas H04B7/04
• CPC Class Codes: H04B 7/2668 Arrangements for Wireless Code-Division Multiple Access [CDMA] System Synchronisation, for code acquisition H04B1/7075, for code tracking H04B1/7085
• CPC Class Codes: H04B 7/2684 Synchronisation of a mobile station with more than one base station
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 56/00 Synchronisation arrangements
• CPC Class Codes: H04W 56/0015 one node acting as a reference for the others

Abstract

Methods and an arrangement for synchronizing communication of framed data via asynchronous base stations (BS1, BS2) in a cellular communication system are presented. The synchronization methods are performed continuously by sending out certain system frame counter states from a central node in the system to all its connected base stations (BS1, BS2). Each base station (BS1, BS2) includes a local frame counter (LFC_BS1, LFC_BS2), which generates local frame counter states (t1(1)-t1(4), t2(1)-t2(4)) correlated to the system frame counter states. Transmission of information via the base stations (BS1, BS2) is synchronized by assigning each data frame (DR(1)-DR(4)) a particular frame number, which is given by the local frame counter states (t1(1)-t1(4), t2(1)-t2(4)), so that data framed (DF(1)-DF(4)) having identical numbers contain copies of a certain data packet. Correct frame numbers are derived from common downlink channel offset measurements (CCO1, CCO2) carried out in the base stations (BS1, BS2), and timing advance values (TA2) and downlink channel offsets (DCO1, DCO2) calculated in the central node.

Claims

1. A method in a cellular radio communications system, which is arranged for communicating information in data frames (DF) of a predetermined duration (T_f) the system comprising at least one central node (RNC1, RNC2) and at least one asynchronous base station (BS1-BS5), for synchronizing all base stations (BS1, BS2) that are connected to one (RNC1) of the at least one central node (RNC1, RNC2) independently of any global time reference for the system, the method comprising the steps of:

sending a system frame counter state (SFC) from the central node (RNC1) to all its connected base stations (BS1, BS2), the frame counter state being incremented one step for each of the data frames (DF); and

aligning in each of the connected base stations (BS1, BS2) a state of a respective local frame counter (LFC_BS1, LFC_BS2) with the system frame counter state (SFC);

wherein each of the data frames (DF) is associated with a specific frame number (t1(1)-t1(4), t2(1)-t2(4)) which is derived from the respective local frame counter (LFC_BS1, LFC_BS2).

2. The method according to claim 1, wherein the system frame counter state (SFC) is incremented one step after each lapse of one of the data frames (DF) and wherein each of the base stations (BS1-BS5) has at its disposal at least one common downlink control channel (CDCH1, CDCH2), the method further comprising the steps of:

in each of the connected base stations (BS1, BS2), measuring a common downlink control channel offset value (CCO1, CCO2) between the local frame counter states (LFC_BS1, LFC_BS2) and a corresponding common downlink control channel (CDCH1, CDCH2), where the local frame counter state (LFC_BS1, LFC_BS2) is incremented at a rate correlated to the system frame counter (SFC) step rate; and

reporting the common downlink control channel offset values (CCO1, CCO2) to the central node (RNC1).

3. The method according to claim 1, wherein an update of the system frame counter state (SFC) is sent out at regular time intervals (T).

4. The method according to claim 1, wherein a one-way delay (D₁, D₂) is determined for each connection between the central node (RNC1) and all its connected base stations (BS1, BS2), and that the one-way delay (D₁, D₂) is compensated for.

5. The method according to claim 4, wherein the one-way delay (D₁, D₂) is calculated through a procedure comprising the consecutive steps of:

sending a round-trip-delay message (RTD₁) back and forth between the central node (RNC1) and a given base station (BS1);

calculating a difference between an arrival time (t_a) and a corresponding sending time (t_s) of the round-trip-delay message (RTD₁) and dividing the calculated difference by two;

repeating the steps of sending the round-trip-delay message (RTD₁) and calculating the difference a predetermined number of times to produce (p) differences; and

averaging over the (p) differences.

6. The method according to claim 5, wherein the round-trip-delay message (RTD₁) is originated from a base station (BS1, BS2).

7. The method according to claim 6, wherein the one-way delay (D₁, D₂) is compensated for in each of the connected base stations (BS1, BS2) by adjusting the local frame counter state according to the equation:

8. The method according to claim 5, wherein the round-trip-delay message (RTD₁) originates from the central node (RNC1).

9. The method according to claim 8, wherein the one-way delay (D₁, D₂) is compensated for in the central node (RNC1) by bringing forward in time the transmission of each system frame counter state (SFC_x) message to the connected base stations (BS1, BS2) according to the equation:

10. The method according to claim 1, wherein the at least one central node (RNC1, RNC2) is a radio network control node.

11. The method according to claim 2, wherein the at least one common downlink control channel (CDCH1, CDCH2), downlink channels (DCH1, DCH2) and uplink channel(s) (UCH2) are distinguished from each other through either

(A) a code division of a radio spectrum,

(B) a code and frequency division of the radio spectrum,

(C) a code and time division of the radio spectrum, or

(D) a combination of code, frequency and time division of the radio spectrum.

12. A method in a cellular radio communications system comprising at least one central node (RNC1, RNC2), which is connected to at least one asynchronous base station (BS1-BS5), each of which serves at least one geographical sector (s11-s56), which in turn are each associated with a common downlink control channel (CDCH1, CDCH2),

where the base stations (BS1-BS5) communicate information with mobile stations (MS1-MS4), the information being divided into data packets (DP), which are transmitted in data frames (DF)

on downlink channels (DCH1, DCH2) via one or more sectors (s23, s24) to the mobile stations (MS1-MS4), and

on uplink channels (UCH2) from the mobile stations (MS1-MS4) via one or more sectors (s23, s24),

for establishing a connection between one of the mobile stations (MS2) and at least one base station (BS1), the method comprising the steps of:

sending a system frame counter state (SFC) from one central node (RNC1) of the at least one central node (RNC1, RNC2) to all its connected base stations (BS1, BS2), the frame counter state being incremented one step for each of the data frames (DF) and being incremented one step after each lapse of one of the data frames (DF);

aligning in each of the connected base stations (BS1, BS2) a state of a respective local frame counter (LFC_BS1-LFC_BS2) with the system frame counter state (SFC), each of the data frames (DF) being associated with a specific frame number (t1(1)-t1(4), t2(1)-t2(4)) which is derived from the respective local frame counter (LFC_BS1-LFC_BS2);

in each of the connected base stations (BS1, BS2), measuring a common downlink control channel offset value (CCO1, CCO2) between the local frame counter states (LFC_BS1-LFC_BS2) and a corresponding common downlink control channel (CDCH1, CDCH2), where the local frame counter state (LFC_BS1-LFC_BS2) is incremented at a rate correlated to the system frame counter (SFC) step rate;

reporting the common downlink control channel offset values (CCO1, CCO2) to the at least one central node (RNC1, RNC2);

defining for the mobile station (MS2) an active set (AS) in which at least one of the downlink channels (DCH1) and one of the uplink channels (UCH2) is specified;

for each of the at least one downlink channels (DCH1) in the active set (AS), setting a timing advance value (TA1), which indicates an offset between the common downlink control channel (CDCH1) and the downlink channel (DCH1);

for each of the at least one downlink channels (DCH1) in the active set (AS), calculating a downlink channel offset (DCO1) as a sum (CCO1+TA1) of a common downlink control channel offset (CCO1) and the timing advance value (TA1);

assigning a specific frame number (t1(1)-t1(4)) to each data frame (DF(1)-DF(4)) on each of the at least one downlink channels (DCH1), by giving an initial data frame (DF(1)) a first number (t1(1)) and each following data frame (DF(2)-DF(4)) an integer incrementation (t1(2), t1(3), t1(4)) of this number (t1(1)) equal to an order of each respective data frame (DF(2)-DF(4)) in relation to the initial data frame (DF(1)).

13. The method according to claim 12, wherein during downlink communication of information, data packets (DP) are buffered (B) in each of the connected base stations (BS1, BS2) until the frame number for the data frames transmitting each specific data packet (DP(1)-DP(4)) matches the frame number (t1(1)-t1(4), t2(1)-t2(4)) on the respective downlink channel (DCH1, DCH2).

14. The method according to claim 13, wherein the buffering (B) holds data packets (DP) up to a maximum number and that a data packet (DP) is discarded if it arrives at one of the connected base stations (BS1, BS2) too late to meet the frame number on the respective downlink channel (DCH1;

DCH2) indicated by the central node (RNC1).

15. The method according to claim 12, wherein during uplink communication of information, data packets (DP) are received in the connected base stations (BS1, BS2) in data frames (DF), being numbered (t1(1)-t1(4), t2(1)-t2(4)) in relation to the frame numbering of the downlink channels (DCH1, DCH2) indicated by the central node (RNC1) and that a diversity procedure is performed in the central node (RNC1) on data packets (DP), which are sent in data frames (DF) having identical numbers.

16. The method according to claim 15, wherein the diversity procedure is performed when all copies of a given data packet (DP) have arrived at the central node (RNC1), but no later than a time (τ

) after arrival of a first copy of the data packet (DP).

17. The method according to claim 16, wherein, in the diversity procedure, one data packet (DP) having a highest quality is selected to represent the communication of information.

18. The method according to claim 16, wherein, in the diversity procedure, contents of all data packets (DP) are combined to form a representation of the communication of information.

19. A method in a cellular radio communications system comprising at least one central node (RNC1, RNC2), which is connected to at least one asynchronous base station (BS1-BS5), each of which serves at least one geographical sector (s11-s56), which in turn are each associated with a common downlink control channel (CDCH1, CDCH2),

where the base stations (BS1-BS5) communicate information with mobile stations (MS1-MS4), the information being divided into data packets (DP), which are transmitted in data frames (DF)

on downlink channels (DCH1, DCH2) via one or more sectors (s23, s24) to the mobile stations (MS1-MS4), and

on uplink channels (UCH2) from the mobile stations (MS1-MS4) via one or more sectors (s23, s24),

for commencing communication via at least one second sector (s21) with one of the mobile stations (MS2) which is already communicating information via at least one first sector (s14) specified in an active set (AS) for the mobile station (MS2), the method comprising the steps of:

sending a system frame counter state (SFC) from one central node (RNC1) of the at least one central node (RNC1, RNC2) to all its connected base stations (BS1, BS2), the frame counter state being incremented one step for each of the data frames (DF) and being incremented one step after each lapse of one of the data frames (DF);

aligning in each of the connected base stations (BS1, BS2) a state of a respective local frame counter (LFC_BS1-LFC_BS2) with the system frame counter state (SFC), each of the data frames (DF) being associated with a specific frame number (t1(1)-t1(4), t2(1)-t2(4)) which is derived from the respective local frame counter (LFC_BS1-LFC_BS2);

in each of the connected base stations (BS1, BS2), measuring a common downlink control channel offset value (CCO1, CCO2) between the local frame counter states (LFC_BS1-LFC_BS2) and a corresponding common downlink control channel (CDCH1, CDCH2), where the local frame counter state (LFC_BS1-LFC_BS2) is incremented at a rate correlated to the system frame counter (SFC) step rate;

reporting the common downlink control channel offset values (CCO1, CCO2) to the at least one central node (RNC1, RNC2);

measuring at least one frame offset value (O_f12) between a downlink channel (DCH1) in the active set (AS) and a second common downlink control channel (CDCH2), associated with the second sector (s21), not included in the active set (AS),

reporting the frame offset value (O_f12) to a central node (RNC1),

updating the active set (AS) by adding thereto the second sector (s21),

calculating a timing advance value (TA2) and a downlink channel offset value (DCO2) for at least one second downlink channel (DCH2) in the second sector (s21),

setting an offset between data frames (DFs) transmitted on the at least one second downlink channel (DCH2) and the second common downlink control channel (CDCH2) equal to the timing advance value (TA2),

assigning a specific frame number (t2(1)-t2(4)) to each data frame (DF(1)-DF(₄)) on the second downlink channel (DCH2), by giving an initial data frame (DF(1)) after a current local frame counter state from the second series of local frame counter states (LFC_BS2(n)) a frame number (t2(1)) equal to a following local frame counter state from this series and each subsequent data frame (DF(2)-DF(4)) an integer incrementation (t2(2)-t2(4)) of this frame number (t2(1)) equal to the order of each respective data frame (DF(2)-DF(4)) in relation to the initial data frame (DF(1)).

20. The method according to claim 19, wherein the step of calculating is carried out according to the equation:

21. The method according to claim 20, wherein the downlink channel offset (DCO1) for the first downlink channel (DCH1) is re-calculated according to the equation:

22. An arrangement for communicating framed information in a cellular radio communications system comprising at least one central node (RNC1), which is connected to at least one asynchronous base station (BS1, BS2) through which data packets (DPs) are communicated on downlink channels (DCH1, DCH2) and uplink channels (UCH1, UCH2) with mobile stations (MS2) and where control signals are transmitted on common downlink control channels (CDCH1, CDCH2) to the mobile stations (MS2), wherein the at least one central node (RNCL ) comprises a master timing unit for generating system frame counter states (SFC) to be sent to the at least one base station (BS1, BS2), a master control unit for calculating timing advance values (TA) and downlink channel offset values (DCO1, DCO2) to be used while communicating data packets (DPs) in numbered data frames (DF) on the downlink channels (DCH1, DCH2), a diversity handover unit for executing simultaneous communication via more than one of the at least one base station (BS1, BS2) with a particular mobile station (MS2).

23. The arrangement according to claim 22, wherein the at least one central node (RNC1) further comprises a clock generator for synchronizing all other units included in the at least one central node (RNC1), a reference time generator providing an absolute time reference (T_R) to be used by the master timing unit and a switching unit for alternately connecting the diversity handover unit to one specific base station of the at least one base station (BS1, BS2).

24. The arrangement according to claim 23, wherein the reference time generator is a GPS receiver.

25. The arrangement according to claim 22, wherein each of the base stations (BS1, BS2) comprises:

a clock generator for synchronizing all other units in the base station (BS1, BS2);

a timing unit for receiving the system frame counter states (SFC) and generating local frame counter states (LFC_BS1, LFC_BS2);

a transceiver unit for communicating data packets (DPs) in numbered data frames (DF) and for measuring offset values (CCO1, CCO2) between the local frame counter states (LFC_BS1, LFC_BS2) and the common downlink control channels (CDCH1, CDCH2); and

a timing control unit for receiving the timing advance values (TA1, TA2) and the downlink channel offset values (DCO1, DCO2) and for controlling (I₁, I₂) the transceiver unit.

26. The arrangement according to claim 22, wherein at least one particular and one separate connection is dedicated for transmitting the system frame counter states (SFC) from the at least one central node (RNC1) to each of the base stations (BS1, BS2).

27. The arrangement according to claim 26, wherein each of the particular and separate connections is compensated for a one-way delay (D₁, D₂) between the at least one central node (RNC1) and each respective base station (BS1, BS2).

28. The arrangement according to claim 25, wherein each of the base stations (BS1, BS2) further comprises a first buffer unit for buffering data packets (DPs) which have been transmitted from the at least one central node (RNC1).

29. The arrangement according to claim 28, wherein an output of the first buffer unit is connected to the transceiver unit.

30. The arrangement according to claim 22, wherein the at least one central node (RNC1) further comprises a second buffer unit for buffering data packets (DPs), which have been transmitted from the base stations (BS1, BS2).

31. The arrangement according to claim 30, wherein an output of the second buffer unit is connected to the diversity handover unit.

32. The arrangement according to claim 22, herein each base station includes a timing unit to receive the system frame counter states (SFC) and to create local frame counter states (LFC_BS1, LFC_BS2), the system frame counter states (SFC) being incremented one step for each data frame (DF) and where each of said data frames (DF) being associated with a specific frame number (t1(1)-t1(4), t2(1)-2(4)) is derived from a respective local frame counter (LFC_BS1, LFC_BS2).