WO2003009628A2

WO2003009628A2 - Telecommunications system and method

Info

Publication number: WO2003009628A2
Application number: PCT/EP2002/007852
Authority: WO
Inventors: Gunnar Bergquist
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2001-07-18
Filing date: 2002-07-15
Publication date: 2003-01-30
Also published as: GB2377855A; GB0117524D0; WO2003009628A3

Abstract

A cellular telecommunications system (1) comprises a radio access network controller (2) and at least one mobile station (7). When the radio access network controller (2) determines that a cell re-selection is required by a mobile station to a cell which is one of a plurality of co-located cells, the mobile access network controller (2) determines the loading of co-located cells, and allocates a new cell identity to the mobile station (7) selected from the co-located cells on the basis of the determined loading.

Description

TELECOMMUNICATIONS SYSTEM AND METHOD

Field of the invention

The invention relates to a telecommunications system and, in particular, to a cell re-selection procedure in a telecommunications system.

Background of the invention

Figure 1 shows a typical cellular communications system 1 comprising a radio access network controller 2 and a plurality of cells 3 . Each cell 3 has a certain coverage area, and comprises a base station 5. Based on the knowledge of the coverage areas of each cell in the system, it is possible to define the neighboring cells of each cell in the system.

When a mobile station (MS) 7 is within a particular cell 3, the base station for that particular cell will broadcast the radio identifications of the neighboring cells. The mobile station 7 monitors the quality of transmissions of the neighboring cells, allowing the mobile station 7 to perform cell re- selection based on these received signals.

In a CDMA based Radio Access Network (RAN) , several cells may be co-located using different frequencies.

Figure 2 shows a typical CDMA based UTRAN network

(UMTS Terrestrial Radio Access Network) . In such a network, the UTRAN network 9 resides at a lower level to the core network 11. Thus, when a mobile station 7 is in a "connected mode" within a cell in a UTRAN network 9, the mobile station 7 is assigned a UTRAN Radio Network Temporary Identity (U-RNTI) , which is used to identify the mobile station 7 on the common UTRAN channels.

In addition, such a mobile station 7 in a radio resource state, known as CELL_FACH, is assigned a Cell Radio Network Temporary Identity (C-RNTI) . The C-RNTI is used to identify the mobile station within a particular cell. The cell itself is identified by a Cell Id (C-ID) .

When a mobile station 7 is in the "connected mode" in a UTRAN network, (in either substates CELL_PCH or

CELL_FACH) , the mobile station will carry out cell re- selection procedures using a standardised radio resource control procedure known as "Cell Update" .

The mobile station 7 can therefore choose to reselect a new cell if it is within an area in which the system is configured with several co-located cells. The re-selection may be explicit in the message content, or, alternatively, the radio access network can determine a cell re-selection when it receives a

Cell Update procedure in a cell other than the cell in which the mobile station is identified by the C-RNTI.

The above mentioned cell re-selection procedure among co-located cells is based on a static broadcast system. The mobile station acts on this static information using standardised internal procedures for determining which cell it should relocate to. These standardised procedures are defined in 3GPP standards TS 25.304, version 3.3.0, section 5.3. In summary, cell re-selection is determined based on:

(i) connected mode radio measurements, and

(ii cell re-selection criteria which are either programmed in the mobile station or broadcast from the UTRAN network to the mobile station.

Therefore, the known cell re-selection procedures mentioned above do not take into account any other factors, such as the dynamic load of co-located cells. This can result in a mobile station being re-allocated to a cell which is heavily loaded, while other co- located cells may be lightly loaded. Thus, the conventional cell re-selection method does not make efficient use of the limited radio resource that is available .

The aim of the present invention is to provide a telecommunications system having an improved cell re- selection procedure, resulting in a more efficient use of the limited radio resource.

Summary of the invention

According to a first aspect of the present invention, there is provided a cell re-selection method for a cellular telecommunications system comprising a radio access network controller and at least one mobile station, whereby the radio access network controller carries out the steps of: determining that a cell re-selection is required by a mobile station; and when a new cell is one of a plurality of co- located cells: determining the loading of the co-located cells; and allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading.

According to another aspect of the present invention, there is provided a cellular telecommunications system comprising a radio access network controller and at least one mobile station, whereby the radio access network controller comprises: means for determining that a cell re-selection is required by a mobile station; means for determining the loading of co-located cells when a new cell is one of a plurality of co- located cells; and means for allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading.

According to another aspect of the present invention, there is provided a radio access network controller for use in a cellular telecommunications system, the radio access network controller comprising: means for determining that a cell re-selection is required by a mobile station; means for determining the loading of co-located cells when a new cell is one of a plurality of co- located cells; and means for allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading. Brief description of the drawings.

For a better understanding of the present invention, and to show more clearly how the invention may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 shows a typical cellular telecommunications system;

Figure 2 shows how a UTRAN network architecture is located between a core network and the mobile stations; and,

Figure 3 shows the method of cell re-selection according to the present invention.

Detailed description of a preferred embodiment of the present invention. Figure 3 shows the steps involved in the cell re- selection procedure according to the present invention.

As is conventional, a mobile station operating in a Universal mobile Telecommunications System (UMTS) has a connection to a cell, and performs carrier measurements on alternative cells. The UTRAN controls which carrier measurements are to be made, for example signal strength measurements, received signal strength indicator (RSSI) measurements, or other measurements such as carrier to noise measurements (i.e. SIR or Ec/IO measurements) .

When the mobile station determines that it would be appropriate to connect to a different cell, it reports to the network using the Cell Update procedure. It is noted that the Cell Update procedure is used by the mobile station not only on cell re-selection, but also for other reasons, ranging from periodically reporting the whereabouts of the mobile station, to reporting re-entry into a UMTS service area.

The mobile station determines whether a cell re- selection is necessary or desirable, based on its programmed criteria. When the mobile station has determined that a cell re-selection is required, it sends a message initiating the Cell Update procedure in the new cell.

The mobile station does not necessarily have any knowledge regarding which cells are co-located with one another and which ones are not. This is because the information relating to which cells are co-located does not necessarily need to be in the measurement object list of the mobile station. In this respect, the invention is not dependant on the mobile station knowing the identity of the co-located cells. Thus, if the mobile station is not aware of the identity of co- located cells, the cell re-selection can be made on the basis of a measurement in only one of a group of co- located cells. On the other hand, if the mobile station is aware of the identity of co-located cells, the mobile station could be ordered to perform a measurement on a candidate for cell re-selection.

The signal strength measured on one cell selected by the mobile station could be used to predict the signal strength on another, intended to be co-located cell. Alternatively, the mobile station can be additionally controlled to make a specific measurement on the co-located cell. As shown at step 31 of the procedure of Figure 3, the network can determine that a cell re-selection is required, even though this may not be explicit in the message initiating the Cell Update procedure. Specifically, each mobile station has a Cell Radio

Network Temporary Identity (C-RNTI) , which is used to identify the mobile station within the cell. When the network receives the message initiating the Cell Update procedure in a cell other than the cell in which the mobile station is identified by the C-RNTI, it deduces that a cell re-selection is required.

As is known, a UMTS Terrestrial Radio Access Network (UTRAN) can include a plurality of co-located cells in a given area. For example, the co-located cells may use different operating frequencies. In step 32 of the procedure shown in Figure 3, it is assumed that the new cell, in which the message initiating the Cell Update procedure is received, is co-located with at least one other cell. Step 32 then involves determining the load in one or more of those co-located cells.

The load may be determined, for example, by counting the number of C-RNTIs allocated in respect of each of the co-located cells. Alternatively, the load of a cell may be determined by measuring the load on radio link control buffers per cell (e.g. DTCH RLC-TM) or the consumed power per cell .

Once the loading of the co-located cells have been determined, the network allocates a cell identity, C- RNTI, to the mobile station which had sent the message initiating the cell re-selection procedure, step 33. This C-RNTI can be determined on the basis that it causes the traffic load distribution between the co- located cells to approximate more closely to a desired load distribution. Thus, when the mobile station determines that it is appropriate to select a new cell, the network may instead allocate a cell which is co- located with that cell.

Once the new cell identity has been allocated based on the cell loading, the network then sends a message from the radio resource part to the mobile station with the newly allocated C-RNTI, using a UTRAN Radio Network Temporary Identity (U-RNTI) , allowing identification of the mobile station by lower layers of the network.

There is therefore provided a method and a system which allows the allocation of traffic amongst co- located cells to meet desired criteria in a UTRAN system.

As mentioned earlier, the network may have a desired load distribution for the co-located cells. For example, in the case where there are three co- located cells, the desired load distribution may be 5% of the traffic in a first co-located cell, 45% of the traffic in a second co-located cell, and 50% of the traffic in a third co-located cell. It is noted that the load is closely related to coverage, whereby the greater the load, the less the coverage. When developing a system, a network operator may find reasons for having a certain coverage requirement for one carrier and other requirements for other carriers. For example, an operator may not want to deploy all carriers everywhere since it can be deemed to be costly in terms of infrastructure cost. Therefore, to best serve the load/coverage scenario, a network operator may wish to have greater load margins on one carrier than on another. Thus, a 5%, 45%, 50% load distribution in a certain cell may correspond to an equal load distribution on a system level.

The desired load distribution mentioned above can be varied or amended on the basis of measurements such as the load on radio link control buffers per cell (e.g. DTCH RLC-TM) and/or the consumed power per cell.

This allows the traffic load to be distributed amongst the co-located cells on the basis of any desired criteria. For example, a desired load distribution may be obtained in terms of the amount of traffic on each cell. Alternatively, or additionally, different traffic types, such as circuit-switched or packet-switched traffic may preferentially or exclusively be allocated to specific cells. By way of example only, with a circuit-switched service such as speech, the mobility of speech users could be improved by arranging handovers from a cell covering one geographical area to a cell covering an adjacent geographical area, rather than handing over from a cell covering one geographical area to a cell intended to cover the same geographical area. Thus, if desired, the network could be configured so that users are maintained in one layer in order to improve the mobility of users.

Although the preferred embodiment has been described in relation to the case where the radio resource of a cell is one carrier, the invention could equally apply to the case where a base station provides resources for one or more cells. In the case of more than one cell, such cells would include both cells intended to cover separate areas and cells intended to cover the same areas, i.e. co-located cells.

Claims

CLAIMS .

1. A cell re-selection method for a cellular telecommunications system comprising a radio access network controller and at least one mobile station, whereby the radio access network controller carries out the steps of: determining that a cell re-selection is required by a mobile station; and when a new cell is one of a plurality of co- located cells: determining the loading of the co-located cells; and allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading.

2. A method as claimed in claim 1, wherein the step of determining that a cell re-selection is required comprises the step of receiving a cell update procedure from a cell which is different to the cell in which the mobile station is currently identified.

3. A method as claimed in claim 1, wherein the step of determining that a cell re-selection is required comprises the step of detecting a cell re-selection request in a cell update message.

4. A method as claimed in any one of claims 1 to 3, wherein the step of determining the loading of co- located cells comprises the step of determining the number of cell radio network temporary identities (C- RNTIs) currently allocated for each co-located cell.

5. A method as claimed in any one of claims 1 to 3 , wherein the step of determining the loading of co- located cells comprises the step of measuring the buffer load of each co-located cell.

6. A method as claimed in any one of claims 1 to 3, wherein the step of determining the loading of co- located cells comprises the step of measuring the buffer load of each co-located cell and the type of service .

7. A method as claimed in any one of claims 1 to 3, wherein the step of determining the loading of co- located cells comprises the step of measuring the power consumed by each co-located cell.

8. A method as claimed in any one of claims 1 to 3, wherein the step of determining the loading of co- located cells comprises the step of measuring the load according to buffer load of each co-located cell, the type of service and the power consumption of each co- located cell.

9. A method as claimed in any one of the preceding claims, wherein the allocating step comprises the step of signalling the new cell identity to the mobile station using a UTRAN Radio Network Temporary Identity (U-RNTI) .

10. A method as claimed in any preceding claim, comprising setting predetermined load parameters, and allocating the new cell identity so that the loading more closely approximates the predetermined load parameters .

11. A method as claimed in claim 10, wherein the predetermined load parameters for a group of co-located cells are set such that equal loading is provided at a system level .

12. A method as claimed in claim 10 or 11, wherein the predetermined load parameters for a group of co-located cells are set such that at least two of the co-located cells have a different target loading.

13. A method as claimed in claim 10, wherein the predetermined load parameters for a group of co-located cells are not related to the capacity of any particular cell.

14. A cellular telecommunications system comprising a radio access network controller and at least one mobile station, whereby the radio access network controller comprises : means for determining that a cell re-selection is required by a mobile station; means for determining the loading of co-located cells when a new cell is one of a plurality of co- located cells; and means for allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading.

15. A system as claimed in claim 14, wherein the means for determining that a cell re-selection is required comprises means for detecting that a cell update procedure is from a cell which is different to the cell in which the mobile station is currently identified.

16. A system as claimed in claim 14, wherein the means for determining that a cell re-selection is required comprises means for detecting a cell re-selection request in a cell update message.

17. A system as claimed in any one of claims 14 to 16, wherein the means for determining the loading of co- located cells comprises means for determining the number of cell radio network temporary identities (C- RNTIs) currently allocated for each co-located cell.

18. A system as claimed in any one of claims 14 to 16, wherein the means for determining the loading of co- located cells comprises means for measuring the buffer load of each co-located cell.

19. A system as claimed in any one of claims 14 to 16, wherein the means for determining the loading of co- located cells comprises means for measuring the buffer load of each co-located cell and the type of service.

20. A system as claimed in any one of claims 14 to 16, wherein the means for determining the loading of co- located cells comprises means for measuring the power consumed by each co-located cell.

21. A system as claimed in any one of claims 14 to 16, wherein the means for determining the loading of co- located cells comprises means for measuring the load according to buffer load of each co-located cell, the type of service and the power consumption of each co- located cell.

22. A system as claimed in any one of claims 14 to 21, wherein the means for allocating a new cell identity comprises means for signalling the new cell identity to the mobile station using a UTRAN Radio Network Temporary Identity (U-RNTI) .

23. A system as claimed in any one of claims 14 to 22, comprising means for setting predetermined load parameters, and allocating the new cell identity so that the loading more closely approximates the predetermined load parameters .

24. A system as claimed in claim 23, wherein the means for setting the predetermined load parameters for a group of co-located cells is configured such that it provides equal loading at a system level .

25. A system as claimed in claim 23 or 24, wherein the means for setting predetermined load parameters for a group of co-located cells is configured such that at least two of the co-located cells have a different target loadings .

26. A system as claimed in claim 23, wherein the means for setting predetermined load parameters for a group of co-located cells is configured such that the load parameters are not set according to the capacity of any particular cell.

27. A radio access network controller for use in a cellular telecommunications system, the radio access network controller comprising: means for determining that a cell re-selection is required by a mobile station; means for determining the loading of co-located cells when a new cell is one of a plurality of co- located cells; and means for allocating a new cell identity to the mobile station selected from the co-located cells on the basis of the determined loading.

28. A radio access network controller as claimed in claim 27, wherein the means for determining that a cell re-selection is required comprises means for detecting that a cell update procedure is from a cell which is different to the cell in which the mobile station is currently identified.

29. A radio access network controller as claimed in claim 27, wherein the means for determining that a cell re-selection is required comprises means for detecting a cell re-selection request in a cell update message.

30. A radio access network controller as claimed in any one of claims 27 to 29, wherein the means for determining the loading of co-located cells comprises means for determining the number of cell radio network temporary identities (C-RNTIs) currently allocated for each co-located cell.

31. A radio access network controller as claimed in any one of claims 27 to 29, wherein the means for determining the loading of co-located cells comprises means for measuring the buffer load of each co-located cell.

32. A radio access network controller as claimed in any one of claims 27 to 29, wherein the means for determining the loading of co-located cells comprises means for measuring the buffer load of each co-located cell and the type of service.

33. A radio access network controller as claimed in any one of claims 27 to 29, wherein the means for determining the loading of co-located cells comprises means for measuring the power consumed by each co- located cell.

34. A radio access network controller as claimed in any one of claims 27 to 29, wherein the means for determining the loading of co-located cells comprises means for measuring the load according to buffer load of each co-located cell, the type of service and the power consumption of each co-located cell.

35. A radio access network controller as claimed in any one of claims 27 to 34, wherein the means for allocating a new cell identity comprises means for signalling the new cell identity to the mobile station using a UTRAN Radio Network Temporary Identity (U- RNTI) .

36. A radio access network controller as claimed in any one of claims 27 to 35, comprising means for setting predetermined load parameters, and allocating the new cell identity so that the loading more closely approximates the predetermined load parameters .

37. A radio access network controller as claimed in claim 36, wherein the means for setting the predetermined load parameters for a group of co-located cells is configured such that it provides equal loading at a system level .

38. A radio access network controller as claimed in claim 36 or 37, wherein the means for setting predetermined load parameters for a group of co-located cells is configured such that at least two of the co- located cells have a different target loadings.

39. A radio access network controller as claimed in claim 36, wherein the means for setting predetermined load parameters for a group of co-located cells is configured such that the load parameters are not set according to the capacity of any particular cell.

40. A mobile station suitable for use in a method as claimed in any one of claims 1 to 13.

41. A method of cell re-selection in a cellular telecommunications system, the method being substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.

42. A cellular telecommunications system substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.

43. A radio access network controller substantially as hereinbefore described, with reference to, and as illustrated in the accompanying drawings.