GB2417164A

GB2417164A - Method of minimising uplink interference

Info

Publication number: GB2417164A
Application number: GB0417872A
Authority: GB
Inventors: Volker Breuer; Toby Kier Proctor; Thomas Ulrich
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2004-08-11
Filing date: 2004-08-11
Publication date: 2006-02-15
Anticipated expiration: 2024-08-11
Also published as: GB0417872D0; GB2417164B

Abstract

A method of minimising uplink interference in a predetermined cell (1, 11) of a communication system providing broadcast or multicast services comprises broadcasting to a plurality of mobile terminals a notification of a broadcast or multicast service in the predetermined cell (1, 11). Each mobile terminal changes to an alternative state, if the terminal requires the broadcast service. Then an offset is applied to the mobile terminal cell reselection criteria and the mobile terminal moves to the predetermined cell in response to the application of the offset.

Description

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A METHOD OF MINIMISING UPLINK INTERFERENCE

This invention relates to a method of reducing uplink interference in a predetermined cell of a communication system, in particular for second and third generation mobile phone systems, such as Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN), offering multicast broadcast multimedia services (MBMS) or other broadcast services.

Broadcast services, such as MBMS which is standardised in radio access network (RAN), are becoming more and more important for mobile communication networks. Studies of MBMS have shown that MBMS may require up to 30% to 40% of NodeB available power, depending on the scenario, which means most operators may only provide MBMS on one wideband code division multiple access (WCDMA) carrier. Limiting to a single carrier is also necessary for mobility purposes and for combining gains at cell edges. As the traffic in the network increases, operators may have to use more than one of their frequencies. In particular, a second carrier would be required for micro and pica NodeBs to cover hot spot areas with high traffic volumes.

Thus, the concept of frequency layer convergence (FLC) for MBMS is of interest to the operators. This is where an MBMS notification is sent in all cells on all frequencies indicating a final service providing layer, or macro cell. All user equipments (UEs) interested in MBMS then reselect to this layer, which is brought about by applying certain backoff, or offset values. However, one problem in FLC is that if many UEs select a single layer at the same time and send corresponding notification messages in the uplink (UL) indicating that they have moved, then the UL interference in macro NodeBs of the MBMS service layer increases significantly and there is a consequent impact on available capacity. Another difficulty is that there are radio access channel (RACH) problems in the umbrella cells, which cover an area equivalent to several pica or micro hot spot cells. In addition, the network must be able to monitor UE's to ensure that they can be reached for other purposes e.g. to allow a UE to be paged for an incoming call.

In accordance with a first aspect of the present invention, a method of minimising uplink interference in a predetermined cell of a communication system providing broadcast or multicast services; the method comprising broadcasting to a plurality of mobile terminals a notification of a broadcast or multicast service in the . a predetermined cell; changing each mobile terminal to an alternative state, if the terminal requires the broadcast service; applying an offset to the mobile terminal cell reselection criteria; and moving to the predetermined cell in response to the application of the offset.

The present invention deals with the problem of uplink interference caused by multiple terminals moving to the same preferred frequency layer or macro cell for a broadcast by changing the state of the UK, so that the change signalling can be carried out in the original frequency layer or micro cell and then moving the UK, so that the change notifications are spread out amongst the different layers, rather than the predetermined cell in which the broadcast or multicast takes place having to bear the full burden of uplink signalling.

Preferably, the alternative state is one in which no notification is required as the mobile terminal moves from its current cell to the predetermined cell.

The alternative state could be idle mode, which requires no signalling, but this has the problem that a large amount of downlink resource is required for the network to communicate with any terminal in idle mode, so preferably, the alternative state comprises one of UTRAN registration area (URA) paging channel (URA-PCH) mode and a customised radio resource control (RRC) state defined for MBMS broadcasts or multicasts.

The benefit of URA-PCH is that no change of location within a URA needs to be notified to the network and the UE location in the network is known more accurately than a UE in idle mode. Alternatively, a specific RRC state can be defined for MBMS if desired.

Preferably, the notification of the broadcast service comprises notification of the identity of the service on offer; the frequency on which the broadcast service will be broadcast; an offset to be applied to the mobile terminal; and the RRC state to which the terminal must change.

Preferably, the network analyses the number of terminals changing state and derives whether to transmit the broadcast as point to point or point to multipoint, therefrom.

Preferably, the plurality of cells comprises a combination of micro cells and at least one macro cell.

eec te* Preferably, the URA comprises all micro cells covering the same area as the predetermined cell.

Preferably, the predetermined cell comprises a macro cell.

Preferably, the mobile terminal notifies the network that it is changing to the alternative state, before the offset is applied and the terminal moves to the alternative state.

Preferably, the broadcast is an MBMS broadcast or multicast.

Preferably, the network comprises UTRAN An example of a method of minimising uplink interference in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic arrangement of cells for carrying out a method in accordance with the present invention; and, Figure 2 illustrates cell arrangements for operating the method of the present invention in a built up area.

When using a FLC function in a network supporting MBMS, multiple UEs may reselect to the preferred frequency layer (PFL) - an umbrella macro cell at the same time, when a service for which they are registered is indicated on an MBMS Control Channel (MCCH).

The FLC layer is a macro cell layer and all UEs from pica or micro cells which want to receive the MBMS will reselect to the particular carrier or umbrella cell which holds the FLC layer. As the FLC layer consists of macro NodeBs these cells are sensitive to uplink interference.

Usually, any UEs moving to a cell in the preferred frequency which are in cell paging channel (Cell_PCH) or cell frequency access channel (Cell_FACH) state perform a cell update according to the procedure set out in standard RRC protocol 25.331. UEs in many cells can be notified of an ongoing service for which they are registered at the same time. However, this gives rise to a significant amount of UL traffic in the form of CELL UPDATE messages, used for mobility purposes, in the cell in the PFL which, because of the power offset applied for the FLC function, may not be the best cell to use in terms of legacy procedures. The cell update messages depend c. :, Be. *. ts ie.

upon the set-up for a particular UE in a network, so if the network knows a UE location at cell level, then it must be informed of a move to a new cell. This cell update procedure also causes significant interference in all nearby cells.

The effect of the power offset is that although the UE sees the new cell or PFL as being the optimum one to move to, it actually requires more power to send its update message from that cell than it would have done from its current cell, because the macro cell is not really optimum and in fact suffers from worse interference.

Within UTRAN it is possible to set up sub-groups, called URA's, which consist of a number of cells in a common area. The URA can take any shape and contain any number of cells, so the definition of the URA is chosen to enable the method of the present invention to operate effectively. Within these URA's, a move from one cell to another, when the UE is in URA_PCH state, does not need to be notified to the network. If the network needs to contact the UK, then it will page all cells within the URA.

The present invention is able to reduce the overhead involved in the update procedure. An example of the present invention is illustrated with reference to Fig. 1.

A network (not shown) sends a notification message to all cells within its coverage area to inform all mobile terminals or user equipment (UK) in each cell that an MBMS service is available and about to start in another frequency layer. This coverage area includes macro cells 1, 10 and micro cells 2 to 8 and 12 to 18. The notification in those cells m which the service is not going to be distributed includes a service identity, the frequency on which service will be delivered (i.e. the PFL) and a power offset in dBs which needs to be applied to legacy cell reselection procedures in order to trigger UE cell reselection to the PFL.

Each UE in a cell which wishes to subscribe to the offered service is told to move into another state, typically either an RRC state or idle mode. If the other state is an RRC state, then this can be an existing one e.g. URA_PCH state, or a new one. The move to another state takes place after the UE has received the indication of the target state in the notification message on the MCCH. The new state has the characteristic that no further update needs to be sent when subsequently changing the frequency to the FLC layer. In the following example, the alternative state to which the UE must change is URA-PCH state. An advantage of moving into URA_PCH state, is that if the cell into which the UE moves is in the same URA as the cell from which the UE rte t j # moves, then no UL signalling is required to indicate a change of location when camping on the FLC layer at which the MBMS service is being provided.

As can be seen in Fig. I, there are two URA's l O. 20 containing cells I to 8 and cells 11 to 18 respectively. For example, if UE's (not shown) in cells 2, 5, 12 and 16 indicate that the UE's require the offered MBMS service, they will indicate to the network that they are changing their state to URA-PCH state, make the change of state and apply to the cell reselection criteria, the power offset specified in the notification message from the network. This offset when applied, triggers the move into the new cell, but because the UE is now in a state which does not require any notification of change of position to the network, because it is within the same URA still, then no uplink messages need to be sent from the new cell.

A benefit of the present invention derives from fact that upon detection of information on the MCCH which relates to a service for which a UE is registered and which it wishes to take up, the UE moves to URA_PCH state and confirms this state change to the UTRAN network without changing to a new frequency layer, thereby ensuring that confirmation of the change is performed in the micro or pica cells, so that uplink requests are distributed across many cells. Furthermore, these cells are not that sensitive to interference, as compared with the macro cells, i.e. the cell to which each UE is moving to receive the MBMS service.

Another feature which is illustrated in Fig. I is that there may be circumstances in which a micro cell within a URA deems the optimum macro cell to move to as being one in a different URA. This is most likely to occur close to boundaries of the URA, where external factors can influence the cell to which a UE intends to move. In this example, a UE in cell 12 is within the same URA as cell 11 and would be expected to move to this cell. However, it chooses to move to cell I and in so doing needs to inform the network of its new location. Thus, although the majority of uplink notifications from the macro cells are prevented, they are not eliminated entirely.

Fig. 2 shows a specific example of cells in a built up area, such as a number of buildings on the crossroads of a city street. Each building is covered by a micro cell 21,22, 23, 24 and an elongated cell 25, 26 covers each of the intersecting streets 27, 28.

The effect of the building structure means that the shape of each cell in the streets is non-ideal. For the purpose of applying the method of the present invention, a macro cell 29 incorporates all of the cells 21 to 26.

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The need for a confirmation of a state change occurs for a number of reasons.

Firstly, the UTRAN needs to know the state of the UE for legacy procedures such as paging. Secondly, the UTRAN may also use this information in a counting procedure which is performed in the MBMS function. The assumption can be made that all UEs in micro and pico cells, which change state on receipt of a transmission of the MICH can be assumed to be highly likely to participate in the MBMS session. This provides a good indication as to whether to run the MBMS as point to point or as point to multipoint in the corresponding umbrella cell on the FLC layer. Normally, the network gives individual UE's permission to change mode, so it is unlikely that a mode change would occur coincidentally, unrelated to the desire to receive, the offered service and so blurring the results, because the network would not give the individual UE permission to change state at that time.

The confirmation can be sent as an existing message (e.g. a URA Update) or as a new message which is required to indicate to the UTRAN that the state change has taken place. In the case of an URA_Update, this procedure is completed by an URA_Update confirm message from UTRAN to UK. Although, moving to URA_PCH state is preferred over idle mode, since the UE is required to indicate to the network that a state change has taken place for purposes mentioned above, it may be possible to create a new state to fulfil the counting needs e.g. idle-m.

Another possible way to move the UE into either URA_PCH state or idle mode, is for the network to use information in the MBMS notification information to trigger a Cell Update in the existing cell and then use the confirmation message (Cell update confirm) to move the UEs into either URA_PCH state or Idle mode. In this case the network would have full knowledge of UE location for paging etc. and would be using only legacy procedures.

Although, the invention has been described with respect to a change of mode to the URA_PCH state, it is not limited to this example and a change to idle mode or a customised RRC state is also possible. : ec

Claims

: .. : be: :. e. :. . : CLAIMS

1. A method of minimising uplink interference in a predetermined cell of a communication system providing broadcast or multicast services; the method comprising broadcasting to a plurality of mobile terminals a notification of a broadcast or multicast service in the predetermined cell; changing each mobile terminal to an alternative state, if the terminal requires the broadcast service; applying an offset to the mobile terminal cell reselection criteria; and moving to the predetermined cell in response to the application of the offset.

2. A method according to claim 1, wherein the alternative state is one in which no notification is required as the mobile terminal moves from its current cell to the predetermined cell.

3. A method according to claim 1 or claim 2, wherein the alternative state is one of idle mode, URA-PCH mode and a customised radio resource control (RRC) state defined for MOMS broadcasts or multicasts.

4. A method according to any of claims I to 3, wherein the notification of the broadcast service comprises notification of the identity of the service on offer; the frequency on which the broadcast service will be broadcast; an offset to be applied to the mobile terminal; and the RRC state to which the terminal must change.

5. A method according to any preceding claim, wherein the network analyses the number of terminals changing state and derives whether to transmit the broadcast as point to point or point to multipoint, therefrom.

6. A method according to any preceding claim, wherein the plurality of cells comprises a combination of micro cells and at least one macro cell.

7. A method according to at least claim 3, wherein the URA comprises all micro cells covering the same area as the predetermined cell.

:: . . :.: . .

8. A method according to any preceding claim, wherein the predetermined cell comprises a macro cell.

9. A method according to any preceding claim, wherein the mobile terminal notifies the network that it is changing to the alternative state, before the offset is applied and the terminal moves to the alternative state.

10. A method according to any preceding claim, wherein the broadcast is an MBMS broadcast or multicast.

11. A method according to any preceding claim wherein the network comprises

UTRAN

12. A method of minimising uplink interference as hcreinbefore described with reference to the accompanying drawings.