WO2009113918A1

WO2009113918A1 - Improved cell reselection in an mbsfn system

Info

Publication number: WO2009113918A1
Application number: PCT/SE2008/000192
Authority: WO
Inventors: Lei Xiao; Jie Mao; Huaisong Zhu
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-03-12
Filing date: 2008-03-12
Publication date: 2009-09-17
Also published as: CN101965742A

Abstract

A method (200) for a cellular system (100) in which a first kind of transceiver (110, 115) provides a first kind of service and a second kind of transceiver (120, 125, 130; 135, 140, 145) provides a second kind of service. Each of a number of said first kind of node (110, 115) has an association with one or more of said second kind of node (120, 125, 130; 135, 140, 145), so that a user (105) who uses the first kind of service accesses the second kind of service through a node of the second kind which is associated with a node of the first kind. Users who use the first kind of service compare (205) signal strengths from transceiver nodes of the first kind (110,115) in order to decide (210) to perform reselection of said second kind of node (120, 125, 130; 135, 140, 145).

Description

TITLE

Improved cell reselection in an MBSFN System.

TECHNICAL FIELD The present invention discloses a method for cell reselection in a cellular communications system in which a first kind of transceiver node provides users with a first kind of service and a second kind of transceiver node provides users with a second kind of service.

BACKGROUND

In cellular communication systems which use the so called TD SCDMA principle, Time Division Synchronous Code Division Multiple Access, there is also the possibility of introducing so called MBMS services, Multi media Broadcast Multicast Service. A particular case of the MBMS service is the so called MBSFN, which is MBMS over a single frequency.

In systems which use MBSFN, certain cells (or rather, the controlling nodes of those cells), provide users with MBSFN services, while other cells (or rather, the controlling nodes of those cells) are used in order to provide the users with "non MBSFN" services, such as, for example, voice, or to provide the users with system information or paging.

Each MBSFN cell has an "umbrella" over a number of non-MBSFN cells in the system, so that users who use MBSFN services and wish or need to access non-MBSFN services will use a non-MBSFN cell which is associated with one of the MBSFN cells in the system.

As in other cellular systems, users in a system with MBSFN and non-MBSFN cells may need to perform cell reselection, i.e. to switch to a cell which has a stronger signal than the present one. In a system which provides users with MBSFN services, the users may have to perform "dual" reselection, since the users in the system access different cells for the different kinds of services, i.e. MBSFN and non MBSFN. Cell reselection will occur more often for the non-MBSFN cells, since those cells will typically have a smaller coverage area than the MBSFN cells.

SUMMARY

The invention aims at providing an improved method for cell reselection in a system in which there are both MBSFN cells and non-MBSFN cells, and in which at least a number of the MBSFN cell have a number of non-MBSFB cells "under its umbrella", i.e. some or all of the MBSFN cells have a number of non-MBSFN cells associated with them. Suitably, the method should be possible to use in systems where the same combination of dual services applies, although the dual services may not be MBSFN and non-MBSFN.

Such a method is provided by the present invention in that it discloses a method for use in a cellular communications system in which a first kind of transceiver node provides users with a first kind of service and a second kind of transceiver node provides users with a second kind of service.

Each of a number of the first kind of node has an association with one or more of the second kind of node, so that a user who uses the first kind of service will access the second kind of service through a node of the second kind which is associated with a node of the first kind.

According to the method of the invention, users who use the first kind of service compare signal strengths from transceiver nodes of the first kind in order to decide to perform reselection of said second kind of node.

When this invention is applied in a system which has both MBSFN cells and non-MBSFN cells, the users will thus use the signal strengths from the MSBFN cells in order to decide to perform reselection of the non-MBSFN cells. This is beneficial for a number of reasons, for example in that the MSFN cells have a larger coverage area than the non-MBSFN cells, so cell reselection will not need to be carried out as often as it would have been otherwise.

In addition, since the MSBFN and non MBSFN cells operate on different frequencies and most users are only able to listen to one frequency at a time, the MBSFN services would, without the invention, have been interrupted by the users in order to listen for signal strength from non-MBSFN cells. By means of the invention, such interruptions are reduced, which will naturally be beneficial for users in a system in which the invention is applied.

Suitably, according to the method of the invention, a user who has compared signal strengths and has decided to perform reselection and has detected which node of the first kind that has the highest signal strength will access information from one of the nodes of the first kind in order to find out which nodes of the second kind that are associated with the node of the first kind with the highest signal strength, so that the user can perform cell reselection to a node of the second kind which is associated with the node of the first kind with the highest signal strength.

Applied to a system with MBSFN as well as non-MBSFBN cells, this means that if a user in the system has detected a ratio between signal strengths from MBSFN cells which is such that the user should perform cell reselection of the non-MBSFN cell, the user will access the non-MBSFN neighbour list of the MBSFN cell with the highest signal strength in order to find out which non-MBSFN cells the user should choose among when carrying out the reselection.

However, other embodiments for reselection are also disclosed by the invention, and will be described in the following.

The invention also discloses a transceiver for use as a user terminal in a system of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows a system in which the invention may be applied, and

Fig 2 shows a flow chart of a method of the invention, and

Fig 3 shows a block diagram of a user terminal of the invention.

DETAILED DESCRIPTION

Fig 1 shows an overview of a system 100 in which the invention may be applied. The system is a TD SCDMA system, Time Division Synchronous Code Division Multiple Access, in which users are provided with the possibility to use so called MBSFN services, i.e. MBMS, Multimedia Broadcast Multicast Service, over a single frequency, as well as with the possibility of using "ordinary" TD SCDMA services, i.e. "non-MBSFN" services.

One user 105, shown as a UE, User Equipment, is shown symbolically in the system 100, although the number of users can of course vary greatly.

In the system 100, the MBSFN services to the UE 105 are provided by the controlling nodes of two cells, indicated as cell 7 and cell 8, also referenced as 110 and 115 in fig 1. As also indicated in fig 1 , each MBSFN cell (or a number such cells) may have a number of non-MBSFN cells associated with it, which is shown in fig 1 by means of two groups of cells, with each group being under the "umbrella" of one of the MBSFN cells, i.e. cells 7 and 8. Cells 1 , 2 and 3 are associated with cell 7, and cells 4, 5, and 6 are associated with cell 8.

The UE 105 can receive and combine the MBSFN transmissions from cell 7 and cell 8, by means of the transmissions from the two cells having one and the same MBMS code, although the transmissions from the two cells use different so called scrambling codes and midamble codes. However, the invention can also be used if the user is not combining signals from the MBSFN cells, but is "merely" listening to a plurality of MBSFN cells in order to be able to access the MBSFN cell with the highest signal strength.

The cells 1-3 and 4-6, which are "under the umbrella" of one of the MBSFN cells, are, as mentioned previously, non-MBSFN cells, and the relationship between an MBSFN cell and its associated non-MBSFN cells is as follows: the MBSFN services are downlink only, i.e. the controlling nodes of the cells 7 and 8 cannot receive transmissions from the UE 105.

Thus, if the UE 105 wishes to use a service other than MBSFN, for example to make a call, the call will have to be routed via one of the non-MBSFN cells. In addition, if the UE needs system information such as, for example, so called neighbour cell lists, or if it needs to listen to paging from the system, this is only transmitted from the non-MBSFN cells, i.e. cells 1-3 and 4-6.

Accordingly, there needs to be a "link" between a UE 105 which is using MBSFN services and a non-MBSFN cell if the UE 105 should need to use non-MBSFN services, the "link" being that the UE 105 is seen as "belonging" to one of the non-MBSFN cells, as well as to one of the MBSFN cells.

In a system without the invention, the following would happen: assume that the UE is combining MBSFN signals from cells 7 and 8, and that the received signal strength from cell 7 is stronger than that of cell 8. The UE will then establish a link with the controlling node of one of the non-MBSFN cells 1-3 under the umbrella of cell 7. In this example, it will be assumed that the non- MBSFN cell to which the UE has a link is cell 1.

Under the procedure used in TD SCDMA systems without MBSFN services, the UE 105 will periodically listen to signals from the surrounding cells in order to see if cell reselection should be carried out. However, if the system offers MBSFN services, and the UE 105 utilizes these services, the MBSFN services are transmitted on a separate frequency, and since the UE can only receive transmissions on one frequency at a time, the UE 105 will have to interrupt the use of the MBSFN services in order to perform the measurements for reselection, at the "normal" TD SCDMA ("non-MBSFN") intervals. This will cause undesired interruptions in the MBSFN services.

The invention proposes a solution by means of which these interruptions can be greatly reduced: return again to the example in which the UE 105 uses MBSFN services by means of combining transmissions from the MBSFN cells, i.e. cells 7 and 8.

According to the invention, in order to decide to perform cell reselection of its non-MBSFN cell, the UE 105 will measure the received signal strength of a signal from cells 7 and 8, suitably the so called pilot signals from those cells, although other signals could also be used for this purpose. Since the UE is already listening to signals from the MBFSN cells 7 and 8, this listening will not interrupt the MBSFN services.

Assume further that the UE 105 combines MBSFN signals from the two MBSFN cells 7 and 8, but has also established a link to one of the non- MBSFN cells which are associated with cell 7. When the UE carries out the signal strength measurements from cells 7 and 8, no further reselection action will be taken as long as the received signal strength from cell 7 exceeds the corresponding signal from the other MBSFN cell, i.e. cell 8, by a certain predefined margin.

However, according to the invention, if the difference in signal strengths between the MBSFN cells, i.e. cells 7 and 8, changes, so that the signal from cell 8 becomes stronger than that from cell 7, the UE will carry out measurements to see if a non-MBSFN cell reselection is necessary, i.e. if the UE should change the non-MBSFN cell to which it has a link. Suitably, the signal from cell 8 should exceed that from cell 7 for a certain period of time, and by a certain margin or threshold. The difference in signal strength from the MBSFN cells can be expressed either as a difference or as a ratio.

Since, as mentioned previously, the MBSFN transmissions from cells 7 and 8 take place on the same carrier frequency, the UE's use of the MBSFN services will not be interrupted by the UE's measurements of signal strength in order to see if non-MBSFN cell reselection should take place, as opposed to the prior art solution. The need for the UE to listen to non-MBSFN cells will only occur when the relationship between signal strengths from the MBSFN cells changes, which will be less often than the corresponding signal strengths from the non-MBSFN cells, since the MBSFN cells typically cover a larger area than the non-MBSFN cells.

Thus, in accordance with the invention, if the UE's measurements indicate that the received signal strength from cell 8 exceeds that of the signal from cell 7, the UE will carry out a cell reselection procedure for the non-MBSFN cell. This reselection procedure can take place in different ways, which will be described below, but will suitably involve the UE obtaining a so called "neighbour list" from the "new" stronger MBSFN cell, i.e. cell 8.

The invention provides a way for the UE to get the neighbour list from the MBSFN cell, i.e. cell 8, by means of this list being transmitted by cell 8 as so called system information in a separate channel on the carrier frequency used for the MBSFN service by cell 8. In addition, in one embodiment of the invention, so called paging is also transmitted by the MBSFN cell, thus additionally reducing the need for the UEs in the system to interrupt listening to the MBSFN services.

The neighbour list is, in this system, a list which comprises information about the non-MBSFN cells of cell 8, i.e. those cells which are "under the umbrella" of cell 8, and of data such as, for example, the frequencies on which those cells (or, to be more exact, the controlling nodes of those cells) transmit. The neighbour list of cell 8 is used by the UE 105 in its reselection procedure. This is where different alternatives can be discerned, both of which are - encompassed by the invention:

a) The UE 105 only attempts to reselect to a non-MBSFN cells of cell 8, i.e. to the stronger MBSFN cell,

b) The UE 105 attempts reselection among the non-MBSFN cells of both the "stronger" and the "weaker MBSFN cell, i.e. of both cells 7 and 8.

Thus, in embodiment a) above, the UE 105 will invariably have a new non- MBSFN cell if there is a change in which MBSFN cell has the higher signal strength, while in embodiment b), the UE 105 may stay with the same non- MBSFN cell as previously, if the same non-MBSFN cell as before, i.e. one of those which are under the umbrella of cell 7, has the stronger signal.

In addition, a number of alternative ways of designing the neighbour lists are disclosed by the present invention. Which of the alternatives that is chosen, will, as can be realized, to some extent depend on which of the reselection alternatives a) or b) above that is chosen.

The alternative ways of designing the neighbour lists are as follows:

1. The neighbour list of an MBSFN cell comprises information about its MBSFN neighbours, as well as information about the non-MBSFN cells which are under the umbrella of the MBSFN cell. In the example shown in fig 1 and described above, the neighbour list of cell 7 would thus have information about cell 8 (MBSFN neighbour) as well as about cells 1 , 2 and 3 (non MBSFN cells "under the umbrella" of cell n 2. The neighbour list of an MBSFN cell comprises information about its MBSFN neighbours, as well as information about the non-MBSFN cells which are under the umbrella of the MBSFN cell, and information about the non-MBSFN cells which are under the umbrella of the neighbouring MBSFN cells. In the example shown in fig 1 and described above, the neighbour list of cell 7 would thus comprise information about cell 8 as well as information about cells 1 , 2 and 3 as non-MBSFN cells of cell 7 and information about cells 4, 5 and 6 as non-MBSFN cells of cell 8.

3. The neighbour list of an MBSFN cell comprises information about the cell's MBSFN neighbours, and the non-MBSFN cells transmit neighbour lists with information about their non-MBSFN cells, as well as information about which MBSFN cell that the cell is currently associated with. In the example shown in fig 1 and described above, the neighbour list of cell 7 would thus comprise information about the neighbouring MBSFN cell, i.e. cell 8, and cell 1 (as an example of a non-MBSFN cell) would transmit a neighbour list with information about cells 2-6, as well as the information that cell 1 is currently under the MBSFN umbrella of cell 7.

Fig 2 is a rough flow chart of steps of a method 200 of the invention. Steps which are options or alternatives are shown with dashed lines in fig 2.

As has emerged from the description above, the inventive method 200 is intended for use in a cellular communications system in which a first kind of transceiver node provides users with a first kind of service and a second kind of transceiver node provides users with a second kind of service.

Each of a number of the first kind of node has an association with one or more of said second kind of node, so that a user who uses the first kind of service will access the second kind of service through a node of the second kind which is associated with a node of the first kind. As indicated in step 205, users who use the first kind of service compare signal strengths from transceiver nodes of the first kind in order to decide, as shown in step 210, to perform reselection of the second kind of node.

In one embodiment of the invention, a user who has compared signal strengths and decided to perform reselection and has detected which node of the first kind that has the highest signal strength accesses, step 215, information from one of the nodes of the first kind in order to find out which nodes of the second kind that are associated with the node of the first kind with the highest signal strength, so that the user can perform cell reselection to a node of the second kind which is associated with the node of the first kind with the highest signal strength.

Alternatively, step 215 can signify that a user who has compared signal strengths and decided to perform reselection accesses information from one of the nodes of the first kind in order to find out which nodes of the second kind that are associated with the nodes of the first kind, so that the user can perform cell reselection to a node of the second kind which is associated with one of the nodes of the first kind.

In another alternative, step 215 can signify that a user who has compared signal strengths and decided to perform reselection accesses information from one of the nodes of the first kind and one of the nodes of the second kind in order to find out which nodes of the second kind that are associated with the nodes of the first kind, so that the user can perform cell reselection to a node of the second kind which is associated with one of the nodes of the first kind.

As show in step 220, a user may in one embodiment of the invention listen to transmissions from more than one node of the first kind simultaneously in order to combine those transmissions in order to improve the first kind of service. In this embodiment, as shown in step 225, the user compares the signal strengths from the nodes of the first kind whose signals it is combining in order to, as shown in step 230, to perform reselection of a node of the second kind among nodes of the second kind which are associated with one of the nodes of the first kind which the user is listening to.

Fig 3 shows a rough block diagram of components of an inventive transceiver 300 for use as a user terminal, a UE, in a system in which the invention is used.

As indicated in fig 3, the transceiver 300 comprises an antenna 310, a receiver 320, a control means such as, for example, a microprocessor 330, as well as a memory 340. The components 310, 320, 330, 340, are used by the transceiver 300 in order to access a first kind of service from at least one transceiver node of a first kind and a second kind of service from at least one transceiver node of a second kind. The components 330 and 340 are used by the transceiver for, when accessing the first kind of service, comparing a signal strength from transceiver nodes of the first kind to decide to perform reselection of said second kind of transceiver node.

In one embodiment of the invention, the components 310, 320, 330 are used for accessing information from a node of the first kind with the highest signal strength in order to find out which nodes of the second kind that are associated with that node of the first kind, and the components 330, 340 are used for carrying out cell reselection to a node of the second kind which is associated with that node of the first kind.

In a further embodiment of the invention, the components 310, 320, 330, and 340 are used for listening to transmissions from more than one node of the first kind simultaneously, and the components 330 and 340 are used for combining those transmissions in order to improve the first kind of service. In this embodiment, the components 330 and 340 are also used for comparing the signal strengths from the nodes of the first kind whose signals are combined in order to decide to perform reselection of nodes of the second kind between nodes of the second kind which are associated with one of the nodes of the first kind which the terminal is listening to.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.

Claims

1. A method (200) for use in a cellular communications system (100) in which a first kind of transceiver node (110, 115) provides users (105) with a first kind of service and a second kind of transceiver node (120, 125, 130; 135, 140, 145) provides users with a second kind of service, with each of a number of said first kind of node (110, 115) having an association with one or more of said second kind of node (120, 125, 130; 135, 140, 145), so that a user (105) who uses the first kind of service will access the second kind of service through a node of the second kind which is associated with a node of the first kind, the method (200) being characterized in that users who use the first kind of service compare (205) signal strengths from transceiver nodes of the first kind (110,115) in order to decide (210) to perform reselection of said second kind of node (120, 125, 130; 135, 140, 145).

2. The method (200) of claim 1 , according to which a user (105) who has compared said signal strengths and decided to perform reselection and has detected which node of the first kind (110, 115) that has the highest signal strength accesses (215) information from one of the nodes of the first kind in order to find out which nodes of the second kind (120, 125, 130; 135, 140, 145) that are associated with the node of the first kind with the highest signal strength, so that the user (105) can perform cell reselection to a node of the second kind which is associated with the node of the first kind with the highest signal strength.

3. The method (200) of claim 1 , according to which a user (105) who has compared said signal strengths and decided to perform reselection accesses (215) information from one of the nodes of the first kind in order to find out which nodes of the second kind (120, 125, 130; 135, 140, 145) that are associated with the nodes of the first kind, so that the user (105) can perform cell reselection to a node of the second kind which is associated with one of the nodes of the first kind.

4. The method (200) of claim 1 , according to which a user (105) who has compared said signal strengths and decided to perform reselection accesses (215) information from one of the nodes of the first kind and one of the nodes of the second kind in order to find out which nodes of the second kind (120, 125, 130; 135, 140, 145) that are associated with the nodes of the first kind, so that the user (105) can perform cell reselection to a node of the second kind which is associated with one of the nodes of the first kind.

5. The method (200) of any of claims 1-4, according to which a user (105) listens to transmissions from more than one node of the first kind (110, 115) simultaneously and combines (220) those transmissions in order to improve the first kind of service, according to which method the user compares (225) the signal strengths from the nodes of the first kind whose signals it is combining in order to decide (230) to perform reselection of a node of the second kind (120, 125, 130; 135, 140, 145) between nodes of the second kind which are associated with one of the nodes of the first kind which the user is listening to.

6. The method (200) of any of claims 1-5, according to which the signal strength from the node of the first kind which is used on order to perform cell reselection is the signal strength of a special signal, a "pilot" signal.

7. The method (200) of any of claims 2-6, according to which said information is a neighbour list which includes information about neighbouring cells of the first and/or the second kind.

8. The method (200) of any of claims 1-7, applied to a system (100) in which the first kind of service is MBSFN, Multi Media Broadcast Multicast Service Single Carrier.

9. The method (200) of any claims 1-8, applied to a system (100) in which the second kind of service is TD SCDMA, Time Division Synchronous Code Division Multiple Access.

10. A user terminal (300) for use in a cellular communications system (100), said terminal (300) being equipped with means (310, 320, 330, 340) for accessing a first kind of service from at least one transceiver node of a first kind (110, 115) and a second kind of service from at least one transceiver node of a second kind (120, 125, 130; 135, 140, 145), the user terminal being characterized in that it comprises means (330, 340) for, when accessing the first kind of service, comparing a signal strength from transceiver nodes of said first kind to decide to perform reselection of said second kind of transceiver node.

11. The terminal (300) of claim 10, being equipped with means (310, 320, 330) for accessing information from a node of the first kind with the highest signal strength in order to find out which nodes of the second kind that are associated with that node of the first kind, and with means (330, 340) for carrying out cell reselection to a node of the second kind which is associated with that node of the first kind.

12. The terminal (300) of claim 10 or 11 , being equipped with means (310, 320, 330, 340) for listening to transmissions from more than one node of the first kind simultaneously, and with means (330, 340) for combining those transmissions in order to improve the first kind of service, the terminal (300) also being equipped with means (330, 340) for comparing the signal strengths from the nodes of the first kind whose signals it is combining in order to decide to perform reselection of nodes of the second kind between nodes of the second kind which are associated with one of the nodes of the first kind which the terminal is listening to.

13. The terminal (300) of any of claims 10-12, in which the signal strength from the node of the first kind is used to decide to perform cell reselection is the signal strength of a special signal, a "pilot" signal.

14. The terminal (300) of any of claims 10-13, in which the first kind of service is MBSFN, Multi Media Broadcast Multicast Service Single Carrier.

15. The terminal (300) of any of claims 10-14, in which the second kind of service is TD SCDMA, Time Division Synchronous Code Division Multiple Access.