WO2002032157A2

WO2002032157A2 - Mobile station driven cell switching and handoff with load balancing for wireless systems

Info

Publication number: WO2002032157A2
Application number: PCT/CA2001/001407
Authority: WO
Inventors: Mo-Han Fong; Wei Huang; Hang Zhang
Original assignee: Nortel Networks Ltd
Current assignee: Nortel Networks Ltd
Priority date: 2000-10-13
Filing date: 2001-10-04
Publication date: 2002-04-18
Anticipated expiration: 2003-04-13
Also published as: WO2002032157A3; AU2001295320A1

Abstract

A remote station selects one of a plurality of base stations for communicating with the remote station. Quality information associated with at least two of the plurality of base stations is determined at predefined intervals. Loading information associated with the base stations is received at further predefined intervals. A respective one of the base stations is selected as a function of the quality information and the loading information. Communication with the selected base station is initiated.

Description

MOBILE STATION DRIVEN CELL SWITCHING AND HANDOFF WITH LOAD BALANCING FOR WIRELESS SYSTEMS

BACKGROUND OF THE INVENTION

The present invention is directed to communication via a wireless connection and, more particularly, to wireless communication capable of carrying voice and/or data signals at high data rates. Recent wireless communication systems, such as cellular systems that use code division multiple access (CDMA) technology, often employ a "soft handoff ' where, at any given time, a mobile station communicates with the network via more than one base transceiver station (BTS). The soft handoff ensures that the mobile station communicates continuously with the network while the mobile station moves across cell regions covered by respective BTSs.

Soft handoff systems, however, are prone to having the transmissions from one BTS interfere with those from another BTS. Also, when such systems are used to transfer data packets, complex scheduling of the routing of the packets is required which often delays the movement of the packets. To reduce interference and improve the packet transfer speed, cell switching may be employed instead of a soft handoff. In cell switching, only the BTS having the best transmission channel quality is selected to transmit to the mobile station. The mobile station periodically measures the channel quality of each BTS in its active set, typically by measuring the carrier to interference ratio (C/I) of each BTS. When a mobile station that is receiving forward link transmissions from one BTS enters a region where it can receive transmissions from more than one BTS, or when the mobile station that is receiving forward link

transmissions from the BTS experiences a reduction in forward link transmission quality because of fading, shadowing or signal path loss, the mobile station determines which BTS is transmitting at the highest channel quality and sends an indication of the desired BTS, namely the BTS transmitting at the highest channel quality, to each BTS in the active set.

Because the switching from one BTS to another is based solely on the channel quality, a mobile station may be switched from one BTS to another BTS in its active set that is already heavily loaded. As a result, the traffic load is unevenly distributed among the BTSs which results in traffic congestion and transmission delays that degrade the quality of service for some or all of the mobile stations that are served by the heavily loaded BTS.

It is therefore desirable that the mobile stations be switched from one BTS to another in a manner that more evenly distributes the loading of the BTSs.

Further, when the network-side entity drives the switching from one BTS to another, signals representing the channel quality measurements are sent from the mobile stations to the network-side entity via the BTSs, and signals informing the mobile station of the new BTS are sent from the network-side entity to the mobile stations via the BTSs. Thus, the cell switching operation increases the amount of access network signaling and the amount of backhaul signaling. It is thus further desirable that the switching be driven by the mobile stations so that the amount of access network signaling and the amount of backhaul signaling needed to switch the mobile station from one BTS to' another be minimized.

SUMMARY OF THE INVENTION

The present invention provides for fast switching from one BTS to another that is

mobile station driven and that is based on the loading conditions of the BTSs that are in the active set as well as being based on the transmission channel quality of the BTSs.

In accordance with an aspect of the invention, a remote station selects one of a plurality of base stations for communicating with the remote station. Quality information associated with at least two of the plurality of base stations is determined at predefined intervals. Loading information associated with the base stations is received at further predefined intervals. A respective one of the base stations is selected as a function of the quality information and the loading information. Communication with the selected base station is initiated.

According to another aspect of the invention, a remote station selects one of a plurality of base stations for communicating with the remote station. Quality information associated with a respective one of the base stations that is currently communicating with the remote station and quality information associated with at least another of the base stations is determined at predefined intervals. Loading information associated with the currently communicating base station and loading information associated with the at least another base station is received at further predefined intervals. An associated value that is a function of the associated quality information and the associated loading information is determined for the currently communicating base station and for the at least another base station. Communication with the currently communicating base station is terminated and communication with the at least another base station is initiated when the associated value for the at least another base station exceeds the associated value for the currently communicating base station by a predetermined quantity.

The at least another base station may be selected from an active set of the remote station.

Other features and advantages, of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described n greater detail in the following detailed description with reference to the drawings in which:

Figure 1 is a block diagram showing an example of a wireless communication arrangement in which a remote station communicates with one of a plurality of base transceiver stations under the control of a mobile control point.

Figure 2 is a flow chart illustrating a sequence of operations for selecting one of a plurality of base transceiver stations in accordance with the invention.

Figure 3 is a flow chart illustrating a sequence of operations for switching a remote station from one of a plurality of base transceiver stations to another in accordance with the

invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates an example of a cellular, wireless communication arrangement for providing voice and/or data services, such as packet data services, to a remote station. A plurality of base transceiver stations (BTS) 110, 120, 130, ... are each assigned to respective geographic regions, known as cells.

A remote station 140 communicates with one of the BTS 110 of its active set, i.e. the set of nearby BTSs whose forward link transmissions could be received by the remote station. The remote station may be a mobile station (MS), such as cellular telephone or other wireless

telephone, a personal digital assistant (PDA) or other handheld or pocket device, or another wireless device. Alternatively, the remote station may be a fixed location wireless device. A mobile control point (MCP) 100 or other network-side control device directs each voice and/or data connection to a respective BTS via a cable connection. The BTS, in turn, provides a wireless link for the voice and/or data connection to the remote station. The MCP may also send control information to the remote station and receive control information from the remote station via the BTS using, for example, inBand signaling.

When a remote station is located within the transmission range of more than one BTS, namely when the remote station is situated in a region where two or more cells overlap, it must determine the BTS with which it will communicate.

Figure 2 illustrates an example of a method, according to the invention, by which a remote station that is presently not communicating with a BTS selects one of the BTSs in its active set. As step 200 shows, the remote station periodically measures the channel quality of

BTSs in its active set. Typically, the transmission channel quality of each BTS is determined by the carrier to interference ratio (C/I) of the forward link transmission channel. The C/I is defined as the ratio of the energy of the modulation carrier signal received from the BTS, integrated over time, to the energy of all other received signals that are on the same frequency band, integrated over time. However, other indications of the transmission channel quality may alternatively be used.

The C/I of the BTS may be based on a single value determined by integrating over the duration of one or more time slots that are each of a duration of, typically, 1.25 ms. Alternatively, the C/I may be based on plural values that are each determined by integrating

over the duration of one or more time slots and then averaging the values and/or eliminating anomalous values. The choice of method may be based on the conditions at the locations of the BTSs.

Each BTS also periodically determines its forward link loading condition L and delivers the loading condition information to the remote station, as step 202 shows. The forward link loading condition L of the BTS is typically determined by the number of packets that are stored in its buffer and that are awaiting transmission, though other loading indicators may be used. The forward link loading condition L may be determined with every time slot or at intervals of two or more time slots. Then, as shown at step 204, the remote station periodically determines a value K for each BTS that is a function of both the C/I and the forward link loading condition L. The value K for each BTS is typically based on the relation:

where i is the index of a respective BTS, and/ andf₂ are functions. The remote station then selects the BTS having the greatest K value, as step 206 shows, and notifies the selected BTS, such as by using a reverse link common channel or a dedicated signaling channel. The remote station also sends the C/I associated with the BTS or sends a desired forward link data transmission rate that is determined from the C/I associated with the BTS . The selected BTS then notified the MCP.

Figure 3 shows an example of a method, according to the invention, by which a remote station that is presently communicating with a BTS determines whether to switch to another BTS in its active set. The remote station periodically determines the channel quality of BTSs in its active set in the manner described above, as step 300 shows, and each BTS periodically determines its forward link loading condition L and delivers the loading condition information to the remote station in the above-described manner, as step 302 shows. Then, as shown at step 304, the remote station periodically determines a value K_t for each BTS which is a function of both the C/I and the forward link loading condition L, also as described above.

When the greatest K, value, K_URGEST, exceeds the K_t value associated with the current BTS, K_CURRENT, by a threshold value Δ, namely when

^LARGEST " KcURRENT^> Δ, as shown in step 306, the remote station selects the BTS associated with £^__?GJ__S_ as step 308 shows. The remote station notifies the selected BTS and sends the C/I associated with the BTS, or sends a desired forward link data transmission rate that is determined from the C/I associated with the BTS_? in the manner described above. The selected BTS then notifies the

MCP. Alternatively, when the greatest K_t value, -S^_ΛG£S7 does not exceed the K, value associated with the current BTS, K_CURRENT, by the threshold value Δ, namely when

^^■LARGEST ~ ^-CURRENT ^≤ Δ, or when the greatest K, value is associated with the current BTS, namely when K_URGEST = K_CURRENT, the remote station selects the current BTS, as shown at step 310, and need not send any signals to the BTS.

The remote stations does not switch from the current BTS to the BTS associated with

K-_LARGEST unless K_URGEST - K_CURRENT exceeds Δ to prevent excessive switching from one BTS to another which could itself introduce transmission delays.

Thus, the remote station determines which BTS is to communicate with the remote station not only based on the channel quality of the BTS but also based on the loading condition of the BTS. As a result, the remote station is less likely to be switched to a heavily loaded cell, thereby reducing network congestion and signal delay and avoiding degradation of the quality of service.

Further, the signals needed to cany out the selection of a BTS or the switching from one BTS to another are sent in only one direction, namely from the remote station to the BTS and from the BTS to the MCP. Because there is no additional inter-BTS signaling and no additional BTS-to-remote station signaling, there is no increase in backhaul signaling in the access network. The reduction in the amount of signaling also reduces the latency period between the time the remote station selects a BTS and the time BTS begins communication with the remote station.

Although the present invention has been described in relation to particular

embodiments thereof, many other variations and modifications and other uses may become apparent to those skilled in the art. It is preferred,; therefore, that the present invention be limited not by this specific disclosure herein, but only by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method, carried out by a remote station, of selecting one of a plurality of base stations for communicating with said remote station, said method comprising the steps of:

determining, at predefined intervals, quality information associated with at least two of said plurality of base stations;

receiving, at further predefined intervals, loading information associated with said at least two base stations; selecting a respective one of said at least two base stations as a function of said quality information and said loading information; and initiating communication with said selected one of said at least two base stations.

2. The method of claim 1 wherein said remote station comprises at least one of: a mobile station (MS) and a stationary wireless station.

3. The method of claim 2 wherein said mobile station comprises at least one of: a cellular telephone, a wireless telephone, a personal digital assistant (PDA), a handheld wireless communication device, and a pocket sized wireless communication device.

4. The method of claim 1 wherein said predefined intervals each comprise at least one time slot interval.

5. The method of claim 1 wherein said quality information associated with a particular one of said at least two base stations comprises a carrier to interference ratio (C/I) of a forward link transmission channel for said particular base station.

6. The method of claim 1 wherein said receiving step includes receiving said

loading information from at least two base transceiver stations (BTS).

7. The method of claim 1 wherein said further predefined intervals each comprise at least one time slot interval.

8. The method of claim 1 wherein said loading information associated with a particular one of said at least two base stations is a function of a number of packets that are stored in a buffer of said particular base station.

9. The method of claim 1 wherein said respective one of said at least two base

stations has a greatest ratio of transmission channel quality information associated therewith to loading information associated therewith.

10. The method of claim 1 wherein said selecting step includes selecting said respective one of said at least two base stations having a greatest K value, wherein said K value is defined by the relation:

K oc MCi T),

Λ(A)

wherein i is a respective index associated with a particular one of said at least two base stations,/_; and ^ are functions, (C/I), is a carrier to interference ratio (C/I) of a forward link transmission channel associated with said particular base station, and L, is a function of number of packets that are stored in a buffer of said particular base station.

11. The method of claim 1 wherein said at least two of said plurality of base stations are members of an active set of base stations that is associated with said remote

station.

12. The method of claim 1 wherein said initiating step includes sending, to a mobile control point (MCP), an indication that said selected base station is to commence communication with said remote station.

13. An apparatus, associated with a remote station, for selecting one of a plurality of base stations for communicating with said remote station, said apparatus comprising: means for determining, at predefined intervals, quality information associated with at least two of said plurality of base stations; means for receiving, at further predefined intervals, loading information associated with said at least two base stations;

means for selecting a respective one of said at least two base stations as a function of said quality information and said loading information; and means for initiating communication with said selected one of said at least two base stations.

14. The apparatus of claim 13 wherein said remote station comprises at least one of: a mobile station (MS) and a stationary wireless station.

15. The apparatus of claim 14 wherein said mobile station comprises at least one of: a cellular telephone, a wireless telephone, a personal digital assistant (PDA), a handheld wireless communication device, and a pocket sized wireless communication device.

16. The apparatus of claim 13 wherein said predefined intervals each comprise at least one time slot interval.

17. The apparatus of claim 13 wherein said quality information associated with a particular one of said at least two base stations comprises a carrier to interference ratio (C/I) of a forward link transmission channel for said particular base station.

18. The apparatus of claim 13 wherein said receiving means includes means for receiving said loading information from said at least two base transceiver stations (BTS).

19. The apparatus of claim 13 wherein said further predefined intervals each comprise at least one time slot interval.

20. The apparatus of claim 13 wherein said loading information associated with a particular one of said at least two base stations is a function of a number of packets that are stored in a buffer of said particular base station.

21. The apparatus of claim 13 wherein said respective one of said at least two base stations has a greatest ratio of transmission channel quality information associated therewith to loading information associated therewith.

22. The apparatus of claim 13 wherein said selecting means includes means for selecting said respective one of said at least two base stations having a greatest K value, wherein said K value is defined by the relation:

/ι(C7 /),

K, ∞

Λ( )

wherein i is a respective index associated with a particular one of said at least two base stations,/ and/ are functions, (C/I), is a carrier to interference ratio (C/I) of a forward link transmission channel associated with said particular base station, and K_f is a function of number of packets that are stored in a buffer of said particular base station.

23. The apparatus of claim 13 wherein said at least two of said plurality of base stations are members of an active set of base stations that is associated with said remote station.

24. The apparatus of claim 13 wherein said initiating means includes means for sending, to a mobile control point (MCP), an indication that said selected base station is to commence communication with said selected base remote station.

25. A method, carried out by a remote station, of selecting one of a plurality of base stations for communicating with said remote station, said method comprising the steps of: determining, at predefined intervals, quality information associated with a respective of said plurality of base stations that is currently communicating with said remote station and quality information associated with at least another of said plurality of base stations;

receiving, at further predefined intervals, loading information associated with said currently communicating one of said base stations and loading information associated with said at least another base station; determining, for each of said currently communicating one of said base stations and for said at least another base station, an associated value that is a function of said associated quality information and said associated loading information; and terminating communication with said currently communicating one of said base stations and initiating communication with said at least another base station when said associated value for said at least another base station exceeds said associated value for said currently communicating one of said base stations by a predetermined quantity.

26. The method of claim 25 further comprising the step of maintaining communication with said currently communicating one of said base stations when said associated value for said at least another base station does not exceed said associated value for said currently communicating one of said base stations by a predetermined quantity.

27. The method of claim 25 wherein said remote station comprises at least one of:

a mobile station (MS) and a stationary wireless station.

28. The method of claim 27 wherein said mobile station comprises at least one of: a cellular telephone, a wireless telephone, a personal digital assistant (PDA), a handheld wireless communication device, and a pocket sized wireless communication device.

29. The method of claim 25 wherein said predefined intervals each comprise at least one time slot interval.

30. The method of claim 25 wherein said quality information associated with a

particular one of said at least two base stations comprises a carrier to interference ratio (C I) of a forward link transmission channel for said particular base station.

31. The method of claim 25 wherein said receiving step includes receiving said loading information from at least two base transceiver stations (BTS).

32. - The method of claim 25 wherein said further predefined intervals each comprise at least one time slot interval.

33. The method of claim 25 wherein said loading information associated with a particular one of said at least two base stations is a function of a number of packets that are stored in a buffer of said particular base station.

34. The method of claim 25 wherein said associated value of a particular one of said at least two base stations is a function of a ratio of transmission channel quality information associated with said particular base station to loading information associated with said particular base station.

35. The method of claim 25 wherein said associated value is defined by the

relation:

MC/i),

K, oc (A) wherein i is a respective index associated with a particular one of said at least two base stations,/ and/ are functions, (C/I), is a carrier to interference ratio (C/I) of a forward link transmission channel associated with said particular base station, and K, is a function of number of packets that are stored in a buffer of said particular base station.

36. The method of claim 25 wherein said at least two of said plurality of base stations are members of an active set of base stations that is associated with said remote

station.

37. The method of claim 25 wherein said terminating step includes sending, to a mobile control point (MCP), an indication that said at least another base station is to commence communication with said remote station.

38. The method of claim 25 wherein said terminating step includes sending, to a mobile control point (MCP), an indication that said currently communicating base station is to terminate communication with said remote station.

39. An apparatus, associated with a remote station, for selecting one of a plurality of base stations for communicating with said remote station, said apparatus comprising: means for determining, at predefined intervals, quality information associated with a respective of said plurality of base stations that is currently communicating with said remote station and quality information associated with at least another of said plurality of base stations; means for receiving, at further predefined intervals, loading information associated with said currently communicating one of said base stations and loading information associated with said at least another base station; means for determining, for each of said currently communicating one of said base stations and for said at least another base station, an associated value that is a function of said associated quality information and said associated loading information; and means for terminating communication with said currently communicating one of said base stations and initiating communication with said at least another base station when said associated value for said at least another base station exceeds said associated value for said

currently communicating one of said base stations by a predetermined quantity.

40. The apparatus of claim 39 further comprising means for maintaining communication with said currently communicating one of said base stations when said associated value for said at least another base station does not exceed said associated value for said currently communicating one of said base stations by a predetermined quantity.

41. The apparatus of claim 39 wherein said remote station comprises at least one of: a mobile station (MS) and a stationary wireless station.

42. The apparatus of claim 41 wherein said mobile station comprises at least one of: a cellular telephone, a wireless telephone, a personal digital assistant (PDA), a handheld wireless communication device, and a pocket sized wireless communication device.

43. The apparatus of claim 39 wherein said predefined intervals each comprise at least one time slot interval.

44. The apparatus of claim 39 wherein said quality information associated with a particular one of said at least two base stations comprises a carrier to interference ratio (C/I) of a forward link transmission channel for said particular base station.

45. The apparatus of claim 39 wherein said receiving means includes means for receiving said loading information from at least two base transceiver stations (BTS).

46. The apparatus of claim 39 wherein said further predefined intervals each comprise at least one time slot interval.

47. The apparatus of claim 39 wherein said loading information associated with a particular one of said at least two base stations is a function of a number of packets that are stored in a buffer of said particular base station.

48. The apparatus of claim 39 wherein said associated value of a particular one of

said at least two base stations is a function of a ratio of transmission channel quality information associated with said particular base station to loading information associated with said particular base station.

49. The apparatus of claim 39 wherein said associated value is defined by the relation:

/ι(C/ /),

K, oc

ΛW

wherein i is a respective index associated with a particular one of said at least two base stations,/ and/ are functions, (C/I), is a carrier to interference ratio (C/I) of a forward link transmission channel associated with said particular base station, and K, is a function of number of packets that are stored in a buffer of said particular base station.

50. The apparatus of claim 39 wherein said at least two of said plurality of base stations are members of an active set of base stations that is associated with said remote

station.

51. The apparatus of claim 39 wherein said terminating means includes means for sending, to a mobile control point (MCP), an indication that said at least another base station is to commence communication with said remote station.

52. The apparatus of claim 39 wherein said terminating means includes means for sending, to a mobile control point (MCP), an indication that said currently communicating base station is to terminate communication with said remote station.