WO2000018164A1

WO2000018164A1 - Cell load sharing in a mobile-controlled cell selection environment

Info

Publication number: WO2000018164A1
Application number: PCT/SE1999/001606
Authority: WO
Inventors: Peter Oskar Olof BJÖRKÉN; Lars Erik Westerberg
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 1998-09-22
Filing date: 1999-09-14
Publication date: 2000-03-30
Also published as: BR9913997A; CN1319316A; CA2344498A1; AU6490899A; EP1116402A1

Abstract

A method and system are disclosed for implementing cell load sharing in a mobile-controlled cell selection environment, such as, for example, in a radio packet data system (100), wherein dynamic system information (e.g., SI messages) is used to enable a mobile station to 'perceive' cell borders (108) that depend on the system load. Essentially, cell selection parameters in the system information messages are associated with the distribution of the load invarious cells (BTSA, BTSB). In this way, the mobile stations (116, 118) (which are responsible for their own cell selection) in overloaded cells (BTSB) can be prompted to select alternative, less loaded cells (BTSA).

Description

CELL LOAD SHARING IN A

MOBILE-CONTROLLED CELL

SELECTION ENVIRONMENT

BACKGROUND OF THE INVENTION

Technical Field of the Invention

The present invention relates in general to the radio telecommunications field and, in particular, to a method and system for enabling cell load sharing in a mobile-controlled cell selection environment, such as, for example, in a radio packet data system.

Description of Related Art

In the new generation of wireless data communication services, such as, for example, the General Packet Radio Service (GPRS) in the Global System for Mobile Communications (GSM), data packets are transferred across a radio air interface between a base transceiver station (BTS) and a mobile station (MS). The radio system uses a known cellular structure (e.g., as in the GSM), wherein each BTS is assigned to cover a certain geographical service area (cell). As such, each BTS is responsible for communicating with all of the MSs that are active in its cell. Typically (but not always), the cell of a BTS (e.g., BTS A) includes the geographical area in which, out of all of the BTSs in the system, that BTS provides the best radio link quality. The border at which an MS changes communication from BTS A to BTS B is called the cell border between cell A and cell B.

In a cellular data communication system, the traffic load generated by the users (MSs) varies in time. This fact implies that the radio network needs to be over-dimensioned in order to compensate for traffic peaks. At any given time, there will typically be cells in which the existing load is too high to provide adequate service quality to the MSs in those cells, while at the same time, neighboring cells can have surplus capacity. In such situations, it would be advantageous to allow some of the MSs in a congested cell to connect to neighboring cells that are experiencing less load. This approach, which is commonly referred to as "cell-load sharing," is used in conventional cellular speech communication systems (e.g., in the GSM).

Essentially, there are two ways to administer cell selection in cellular radio systems: MS-controlled cell selection; and network-controlled cell selection. For MS-controlled cell selection, it is the MS's responsibility to determine to which

BTS it is most suitable to connect. As such, the MS considers whatever information it has at hand (radio conditions, received signal strengths, service capabilities, and possibly, additional information received in system information messages, etc.) as input(s) to a pre-determined cell selection algorithm. The output from the cell selection algorithm denotes the cell to which the MS should connect.

For network-controlled cell selection, the network commands the MSs to connect to a certain BTS (cell). A necessary condition for network-controlled cell selection is that every MS consistently provides the network with measurement reports. Essentially, the measurement reports inform the network about how the reporting MS perceives the candidate BTSs or cells, in terms of their signal strength and radio link quality.

Both cell selection approaches have their merits and drawbacks. For example, the main advantage of network-controlled cell selection (also referred to as "locating") is that a relatively intelligent cell selection algorithm can be used. As such, the network can use the wealth of information it has available, not only about the radio link conditions, but also about the load in each cell, buffered data packets, quality of service contracts with users, preferred home zones, etc. Network-controlled cell selection is used, for example, in the circuit-switched GSM. The biggest disadvantages of network-controlled cell selection are the increased complexity for the network involved, and that each MS has to provide measurement reports regularly to the network. Unfortunately, the measurement reports use uplink bandwidth and, therefore, actually decrease the system's capacity. The increased complexity results from the network having to administer a locating procedure for each active MS.

On the other hand, for MS-controlled cell selection, each MS is responsible for its own cell selection procedures. In a GPRS configuration, the system can use either MS-controlled or network-controlled cell selection. However, for reasons described below, it is highly likely that MS-controlled cell selection will be the method of choice for the majority of future GPRS networks. The pertinent prior art focuses on cell load sharing in systems using network-controlled cell selection approaches. In such systems, the process whereby an active MS selects another cell is always initiated by a cell re-selection (i.e., "handover") command issued from the network. Using such a cell load sharing approach, during periods of congestion, the network can identify candidate MSs in a congested cell, and direct these MSs to connect to less congested cells by issuing re-selection commands for dedicated cells (i.e., the less congested cells). As such, the details of which MSs to move to another cell, or where and when to move them, are essentially matters of implementation.

The main shortcoming of the above-described prior art approaches to cell load sharing is that they cannot be used in systems with MS-controlled cell selection. Consequently,, in such systems as GPRS, where either network- controlled or MS-controlled cell selection can be used, the prior art cell load sharing approaches cannot be used in conjunction with those systems' MS- controlled cell selection part. This shortcoming exists because the network has no information about each MS's individual radio environment (no measurement reports are sent to the network). Furthermore, in a system with strict MS- controlled cell selection, the decision to select a new cell is the MS's responsibility, not the network's. On the other hand, the MSs know nothing about the global load situation, current congestion levels, or lengths of queues in the system. Consequently, the MSs cannot administer cell load sharing by themselves.

Additionally, this shortcoming imposes a severe limitation in such dual- mode cell selection systems as GPRS. However, because of the drawbacks of the network-controlled cell selection approaches as described earlier, the MS- controlled cell selection schemes will still likely be the methods of choice in most GPRS networks. SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a method and system are provided for implementing cell load sharing in an MS- controlled cell selection environment, such as, for example, in a radio packet data system, wherein dynamic system information (e.g., SI messages) is used to enable an MS to perceive cell borders that depend on the system load. Essentially, cell selection parameters in the system information messages are associated with the distribution of the load in various cells. In this way, the MSs (which are responsible for their own cell selection) in overloaded cells can be prompted to select alternative, less loaded cells.

An important technical advantage of the present invention is that a method is provided for using cell load sharing in a system in which MS-controlled cell selection is also being used.

Another important technical advantage of the present invention is that it enables systems with only MS-controlled cell selection to also use cell load sharing.

Still another important technical advantage of the present invention is that in systems that can choose between a network-controlled and MS-controlled cell selection mode (e.g., GPRS), cell load sharing can be used while the system is operating in the MS-controlled cell selection mode.

Yet another important technical advantage of the present invention is that it enables the use of load sharing in radio packet data systems (e.g., GPRS) to increase capacity and avoid congestion, without overloading the uplink with measurement report traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a diagram that illustrates an exemplary system that can be used to implement the present invention; FIGURE 2 is a diagram that further illustrates how the exemplary system shown in FIGURE 1 can be used to implement the present invention; and

FIGURE 3 is a diagram that illustrates how system information messages can be used to implement a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The preferred embodiment of the present invention and its advantages are best understood by referring to FIGUREs 1 -3 of the drawings, like numerals being used for like and corresponding parts of the various drawings.

Essentially, in accordance with a preferred embodiment of the present invention, a method and system are provided for implementing cell load sharing in a radio packet data system, wherein dynamic system information (e.g., in the form of SI messages) is used to enable an MS to perceive cell borders that depend on the system load. For MS-controlled cell selection, the network determines the cell borders by including pertinent cell selection parameters in the system information messages. The system information messages can be either broadcast to all MSs in the network, or sent dedicated to a selected set of one or more MSs in an overloaded cell. The cell selection parameters contain information, for example, about offsets that can be applied to the measured signal strengths, threshold values, cell priorities, etc. For the GPRS, such cell selection parameters can be found in the GSM Technical Specification GSM 05.08, version 6.0.0. As such, for this exemplary embodiment, the cell selection parameters in the system information messages are associated with the distribution of the load in various cells. In this way, the MSs (which are responsible for their own cell selection) in overloaded cells can be prompted to select alternative, less loaded cells. As mentioned above, the system information messages can be sent on the broadcast channel, or transmitted dedicated to a selected subset of MSs in the overloaded cell.

Specifically, FIGURE 1 is a diagram that illustrates an exemplary system 100 that can be used to implement the present invention. System 100 is preferably a system that employs MS-controlled cell selection, and can be a radio packet data system or packet- switched system such as the GPRS . The exemplary radio packet data system 100 can be operating in conjunction with a circuit-switched system, such as the GSM, the Digital- Advanced Mobile Phone System (D-AMPS), or any other system capable of supporting a radio packet data system or service. System 100 includes a plurality of base stations or BTSs, such as, for example, BTS A ( 102) and neighboring BTS B (104). BTS A defines one cell (A), and BTS B defines a second cell (B). The dotted line 106 represents a cell border between cells A and B where (to a mobile terminal or MS) the signal strength of cell A is perceived to be equal to the signal strength of cell B (i.e., SS_A=SS_B). For the GSM and similar systems, it can be assumed that a base station controller (BSC) is connected to, and controls the functions of, the two BTSs A 102 and B 104 shown.

For a relatively low traffic load scenario, the cell selection parameters contained in the system information messages (e.g., SI messages in the GSM and GPRS) broadcast or transmitted to the mobile terminals 110-118 shown, define an offset of 2 dB which is added to the MS-measured signal strength from BTS A 102 (i.e., SS_A+2dB=SS_B) to form the (solid line) cell border 108. As shown for this exemplary scenario, three of the mobile terminals (110, 112, 114) have initially connected to BTS A 102, while two mobile terminals (116, 118) have connected to BTS B 104.

FIGURE 2 is illustratively similar to FIGURE 1, except that the scenario depicted in FIGURE 2 has more mobile terminals connected to BTS B 104 than to

BTS A 102. For example, three mobile terminals (110, 112, 114) have connected to BTS A 102, while seven mobile terminals (116-128) have connected to BTS B 104. As such, it can be assumed for this scenario, that the load in cell B is such that a sufficient quality of service (e.g., QoS in GPRS) can no longer be offered to the packet data system users in cell B. However, at the same time, it can be assumed that there are more than enough radio resources available for adequate service in neighboring cell A.

FIGURE 3 is a diagram that illustrates how system information messages can be used to implement a preferred embodiment of the present invention. For the scenario depicted in FIGURE 3 , the network of system 100 sends system information messages (via a BSC and one or more of the BTSs A and B) which contain appropriate cell parameters such that the affected mobile terminals add an offset with a 4 dB increase to the perceived signal strength of cell B (i.e., SS_A+4dB=SS_B). Effectively, as a result of the offset, for the affected mobile terminals, the cell border (108) between cells A and B appears to be shifted towards cell B, and thereby, cell A effectively covers a larger number of the mobile terminals than before. For example, because of the 4 dB offset received in a system information message, five of the mobile terminals (110-118) now appear (to the mobile terminals) to be covered by cell A, and the five other mobile terminals (120-128) appear to be covered by cell B.

An example of a system information message that can be used to convey the above-described offset to one or more mobile terminals in an MS-controlled cell selection environment is a GPRS cell selection parameters information element as defined at Section 11.2.19 of the GSM Technical Specification GSM 04.60, version 6.0.0, June 1998. The information element can be delivered to all, or a subset of, the mobile terminals in cell A, either on the Packet Broadcast Control Channel (PBCCH) in cell A, or on one or more Packet Associated Control Channels (P ACCHs) in cell A.

For each cell defined as a neighboring cell to cell A, the information element can contain the 5 bit information element GPRS_RESELECT_OFFSET that defines a bias, or offset, to be used in the "C31/C32" criterion which is used in the GPRS for MS-controlled cell reselection. This bias or offset value can be different for different cells and thus can be used to shift the MS-perceived boundary between two cells (e.g., between A and B). For example, for mobile terminals camping on cell A, an offset value of 4 dB can be added to the ranking criterion, C32, of cell B, by setting the five corresponding bits in the system information element described above to "01100" (+4 dB). When the system information element is transmitted on the Broadcast channel in cell A (or when transmitted on the one or more Associated control channels in cell

A), the receiving mobile terminals perceive the cell border between cell A and cell B (108 in this example) as shifted by 4 dB relative to the equal-signal-strength border (106). In order to avoid a situation in which the mobile terminals that have selected cell B from cell A immediately return to cell A, the corresponding system information element in cell B (i.e., the element controlling the cell border between cell A and cell

B, as viewed from cell B) can be set accordingly. Yet another example of a system information message that can be used to convey the above-described offset is found in a GSM/GPRS system using no PBCCH. In such systems, the MS-controlled cell selection of GSM mobile terminals in the Idle mode, and the GPRS mobile terminals functioning in any mode, can use the GSM SI message Cell Selection Parameters information element as defined in Section

10.5.2.34 of the GSM Technical Specification GSM 04.08, version 6.1.1, August 1998. The SI 3 Rest Octets information element is a type 5 information element that contains a "CELL-RESELECT-OFFSET" information field. The CELL-RESELECT- OFFSET is a bias or offset value (in dB) that is included in the so-called "C2" criterion for a cell. This bias or offset value can be different for different cells and thus can be used to shift the MS-perceived boundary between two cells (e.g., between A and B). For example, a CELL-RESELECT-OFFSET value of 4 dB can be added as an offset in the C2 criterion for cell A, by transmitting the corresponding bit pattern in the CELL_RESELECT_OFFSET information element in the System Information 3 Rest Octets transmitted on the BCCH in cell A. However, this exemplary scenario assumes that the CELL_RESELECT_OFFSET field in the System Information 3 Rest Octets transmitted on the BCCH in cell B corresponds to a 0 dB bias or offset in cell B.

Consequently, in both examples above, and in accordance with the preferred embodiment of the present invention, the offset caused by the cell parameters in the transmitted system information messages causes the mobile terminals in cell B which are close to the border of cell A (e.g., mobile terminals 116 and 118) to rank cell A as a "better" cell to be connected to than cell B. Therefore, these two mobile terminals effect a change (e.g., via a conventional handover procedure) from cell B to cell A. The result is that the load (mobile terminal traffic) is moved from cell B to cell A in such a way that a sufficient quality of service can be offered to the users in both cell

A and cell B (as illustrated in FIGURE 3). If the load in cell A and/or cell B changes, the cell parameters in the system information messages can be altered and the offset changed accordingly, in order to compensate for the change in load. By using the contents of the system information messages in such a dynamic fashion, the network can move neighboring cell borders in order to adjust to changes in the distribution of the load. As such, the present invention increases the cellular system's ability to maintain a sufficient quality of service during periods of local congestion in the network.

As such, there are a number of load conditions that can "trigger" a change in transmitted system information to cause an MS-perceived shift of a cell border, in accordance with the teachings of the present invention. For example, a pertinent load can be related to (1) the total number of users in a cell, (2) the total number of active users in a cell, (3) the ratio of the total number of active users in a cell to the number of channels in the cell, (4) the ratio of the total number of queued data packets in a cell to the number of channels in the cell, (5) a measure of the QoS that can be offered in a cell (e.g., bandwidth per user, fraction of QoS contracts to users that can be fulfilled, etc.), and so on. Furthermore, the cell boundary shifts that can result are not limited to boundaries between so-called macro-cells, but could include boundaries between disparate types of cells, such as, for example, micro-cells and macro-cells, pico-cells and micro-cells, etc. In a different aspect of the present invention, the system information in all cells in a defined network area is dynamic information. Using this dynamic information at any given time, the network can (with any given temporary load distribution) optimize the quality of service offered to the packet data service users in the whole area. In order to achieve this capability, in accordance with the present invention, the system information (to be broadcast or transmitted from the network) can be made dependent on the number of mobile terminals in each cell, the number of scheduled data packets in each cell, by the users' perceived quality of service in each cell, the packet delays in each cell, and so on. As such, a network algorithm (e.g., being executed by a processor at a base station controller, BSC, or a mobile services switching center, MSC) can be used to derive an appropriate system information message with an offset value included, and direct the message to dedicated MSs or broadcast it to all pertinent MSs.

Although the present invention has been described above in the exemplary context of a GSM packet data service (e.g., GPRS), the present invention can be applied to any cellular radio system that can use system information as input to an MS- controlled cell selection algorithm. Furthermore, the present invention can be applied to any cellular radio system that can operate in an MS-controlled cell selection mode. Such a system can include, for example, GPRS in GSM, GPRS in D-AMPS, an IS-95 system, cellular radio satellite systems, as well as the Universal Mobile Telephone System (UMTS), and both present and future spread spectrum and Wideband Code Division Multiple Access (WCDMA) systems.

Although a preferred embodiment of the method and apparatus of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method for sharing cell load in a mobile-controlled cell selection environment, comprising the steps of: determining a first load in a first cell and a second load in a neighboring cell; if said first load is greater than a predetermined value, generating a system information message including at least one cell selection parameter; conveying said system information message to at least one mobile terminal in said first cell; and responsive to said at least one cell selection parameter, said at least one mobile terminal connecting to said neighboring cell.

2. The method of Claim 1, wherein said at least one cell selection parameter comprises a cell selection offset value.

3. The method of Claim 1, wherein said at least one cell selection parameter comprises a cell selection signal strength threshold value.

4. The method of Claim 1, wherein said at least one cell selection parameter comprises a cell selection cell priority value.

5. The method of Claim 1, wherein said at least one cell selection parameter comprises a parameter that affects a decision by said at least one mobile terminal about which cell to select.

6. The method of Claim 1 , wherein said mobile-controlled cell selection environment comprises a radio packet data network.

7. The method of Claim 1 , wherein said mobile-controlled cell selection environment comprises a GPRS network.

8. The method of Claim 1, wherein said system information message includes a CELL-RESELECT-OFFSET information field.

9. The method of Claim 1, wherein said system information message comprises a GPRS SI message.

10. The method of Claim 1, wherein the conveying step comprises broadcasting a system information message over a broadcast control channel.

11. The method of Claim 1, wherein said conveying step comprises transmitting said system information message over a dedicated channel to said at least one mobile terminal.

12. The method of Claim 1 , wherein said first load comprises a ratio of a total number of users in a cell to a number of channels in said cell.

13. The method of Claim 1, wherein said first load comprises a total number of users in a cell.

14. The method of Claim 1, wherein said first load comprises a total number of active users in a cell.

15. The method of Claim 1 , wherein said first load comprises a ratio of a total number of active users in a cell to a number of channels in said cell.

16. The method of Claim 1 , wherein said first load comprises a ratio of a total number of queued data packets in a cell to a number of channels in said cell.

17. The method of Claim 1, wherein said first load comprises a measure of QoS that can be offered in a cell.

18. The method of Claim 17, wherein said QoS comprises a bandwidth per user.

19. The method of Claim 17, wherein said QoS comprises at least one QoS contract to a user that can be fulfilled.

20. A system for sharing cell load in a mobile-controlled cell selection environment, comprising: a first base station for defining a first cell; a second base station for defining a second cell, said second cell adjacent to said first cell; at least one mobile terminal connected to said first base station; means for determining a first load in said first cell and a second load in said second cell; means for determining if said first load is greater than a predetermined value, and if so, generating a system information message including at least one cell selection parameter; means for conveying said system information message to said at least one mobile terminal; and means, responsive to said at least one cell selection parameter, for connecting said at least one mobile terminal to said second cell.

21. The system of Claim 20, wherein said at least one cell selection parameter comprises a cell selection offset value.

22. The system of Claim 20, wherein said at least one cell selection parameter comprises a cell selection signal strength threshold value.

23. The system of Claim 20, wherein said at least one cell selection parameter comprises a cell selection cell priority value.

24. The system of Claim 20, wherein said at least one cell selection parameter comprises a parameter that affects a decision by said at least one mobile terminal about which cell to select.

25. The system of Claim 20, wherein said mobile-controlled cell selection environment comprises a radio packet data network.

26. The system of Claim 20, wherein said mobile-controlled cell selection environment comprises a GPRS network.

27. The system of Claim 20, wherein said system information message includes a CELL-RESELECT-OFFSET information field.

28. The system of Claim 20, wherein said system information message comprises a GPRS SI message.

29. The system of Claim 20, wherein said means for conveying comprises a transmitter for broadcasting a system information message over a broadcast control channel.

30. The system of Claim 20, wherein said means for conveying comprises a transmitter for transmitting said system information message over a dedicated channel to said at least one mobile terminal.

31. The system of Claim 20, wherein said first load comprises a ratio of a total number of users in a cell to a number of channels in said cell.

32. The system of Claim 20, wherein said first load comprises a total number of users in a cell.

33. The system of Claim 20, wherein said first load comprises a total number of active users in a cell.

34. The system of Claim 20, wherein said first load comprises a ratio of a total number of active users in a cell to a number of channels in said cell.

35. The system of Claim 20, wherein said first load comprises a ratio of a total number of queued data packets in a cell to a number of channels in said cell.

36. The system of Claim 20, wherein said first load comprises a measure of QoS that can be offered in a cell.

37. The system of Claim 36, wherein said QoS comprises a bandwidth per user.

38. The system of Claim 36, wherein said QoS comprises at least one QoS contract to a user that can be fulfilled.