GB2545696A

GB2545696A - Mobile communications

Info

Publication number: GB2545696A
Application number: GB1522648.3A
Authority: GB
Inventors: Brown Ruth; Cuevas Ramirez Maria
Original assignee: British Telecommunications PLC; BT Group PLC
Current assignee: British Telecommunications PLC; BT Group PLC
Priority date: 2015-12-22
Filing date: 2015-12-22
Publication date: 2017-06-28
Also published as: GB201522648D0

Abstract

When a cell reselection update is in progress, the frequencies available locally to a mobile communications terminal 6 are identified in a prioritised neighbour list received from the currently-serving base station and each frequency is tested in turn to determine whether a re-attachment should be made to a cell other than the cell currently serving the mobile terminal. The mobile communications terminal 6 maintains a permitted frequency data store 42 of a mapping of frequencies to the geographical areas in which the frequencies are allocated to services to which the terminal is permitted to operate. Frequencies are only considered for re attachment if they are present in both the prioritised neighbour list and the permitted frequency data store 42 for the current location of the mobile terminal 6. The permitted frequency data list may be downloaded to the store over the RF interface or it may be installed as firmware on the mobile device itself or on a subscriber interface module.

Description

MOBILE COMMMUNfCATfONS

This invention relates to mobile communications and in particular to the operation of frequency searching by a mobile device when in idle mode.

When a mobile device is in idle or standby mode, that is to say when it is not engaged in a communications session but is in contact with a mobile network so that any incoming traffic can be directed to it, it periodically searches for available frequencies to which it may connect should a session or call be initiated, either from the mobile terminal or from the network. As the mobile unit moves around, the results of these searches will change over time depending on which base stations are currently in range of the mobile unit..

Each base station broadcasts a range of frequencies on which it is able to communicate. In general each network operator is allocated a range of frequencies, which are typically allocated to base stations in a “frequency plan” to minimise interference between base stations. Typically each cell (base station) broadcasts a frequency priority list, which is part of the standard list of parameters sent as broadcast messages to all subscribers according to 3GPP TS 36.331. This may include frequencies allocated to other operators, to allow roaming for specific groups of subscribers, e.g. national roaming for partner networks.

Frequencies are often allocated to mobile operators on a regional basis, so that a frequency may be available to a user connected to some base stations of the operator but not to users connected to other base stations of the same operator in a neighbouring area. Consequently, near the boundaries of these regions, mobile terminals may be searching for the wrong frequency, in this scenario, subscribers that are not allowed to roam onto a partner operators network could end up unnecessarily searching for frequencies that belong to an operator that is forbidden to that subscriber, which wastes processing time and battery life.

Figure 1 depicts a scenario where a subscriber 6 is currently located in an area where it can detect frequencies allocated to the operators of three networks (PLMN - public land mobile network} 4a, 4b and 4c, respectively allocated frequencies 1, 2 and 3. The subscriber 6 is allowed to use the networks 4a and 4b of operator A and operator B, but not the network 4c of operator C. In Figure 1 the three operators A, B, C are transmitting beacons from separate locations 5a, 5b, 5c, but this need not be the case as it is possible for operators to share facilities.

The frequency search process is depicted in Figure 2. Initially, the device is attached to cell 5a on MNO 4a (step 21) and receives a network neighbour list from that base station. This list only contains the identity for network 4b (step 22). Subsequently, ceil 5a broadcasts a SIB5 message that contains frequencies 3, 2, and 1 in descending priority order (step 23).

According to 3GPP TS 36.133 section 4.2.2. “The tJE shall search every layer of higher priority at least every T-higher_priority_search = (60 * Niayers) seconds”. (In other words, every minute, one of the higher layers Is searched). Under those circumstances, the device would search for all frequencies (steps 24, 25). The terminal selects the highest priority frequency it detects for potential ceil reseiection, so if frequency 3 is found (step 25), that frequency is evaluated first (step 26).

Therefore, in this scenario, the device is potentially unnecessarily searching for one additional frequency every N minutes. In a worst case scenario, where there is only one frequency above the current layer, the device could be constantly performing unnecessary idle mode searches for cells that are not going to be suitable for cel! reselection in that area. If there are more than one higher layer, time and resources are wasted on searching unsuitable cells.

The signalling diagram in Figure 2 shows how, under the above circumstances, an unnecessary search for frequency 3 has been performed in step 24 and 25, since the MNO to which that frequency 3 has been allocated in that particular area is not an allowed PLMN for that particular subscriber (i.e. not contained on either EHPLMN or ePLMN lists currently stored on the SIM/device).

In situations where frequencies have been allocated on a regional basis and/or national roaming has been implemented, these unnecessary searches can shorten battery life during idle mode.

One approach to solving this issue would be for the eNB to send a dedicated frequency priority list that does not contain the frequencies allocated to non-al!owed PLMNs for that particular subscriber. However that solution requires quite complex SPID allocations to different subscribers and different Registration Areas to ensure that the right frequency priority list is sent to the right subscriber group in each geographical area, which may not be supported by all RAN and EPC core implementations.

According to a first aspect of the present invention, there is provided a mobile communications device comprising: a wireless interface for receiving and transmitting data a permitted-frequency data store for storing a plurality of mappings, each mapping having an associated frequency allocation band, a network operator identifier and a geographical area locator; a location identifier; a processor for receiving a list of potential reseiection frequencies from a first base station whilst the mobile communications device is in idle mode and prioritising the possible reseletion frequencies against a set of predetermined selection criteria, and selecting a second base station in accordance with the prioritised list, wherein: the processor is further operable to form a filtered subset of base stations, by identifying correspondence between at least one of the possible reselection frequencies in the list and an entry in the permitted-frequency data store corresponding to the present location of the mobile device; and the selecting means is operable to select a base station in accordance with the filtered subset.

According to a second aspect of the present invention, there is provided a method of operating a mobile communications device comprising the steps of: storing data in a permitted-frequency data store relating to a plurality of mappings, each mapping having an associated frequency allocation band, a network operator identifier and a geographical area locator; and, whilst the mobile communications device is in idle mode, identifying the current location of the mobile device; receiving from a first base station a set of potential reselection frequencies comparing the set of potential reselection frequencies received, against the mappings in the permitted-frequency data store corresponding to the current location of the mobile device, selecting a second base station in accordance with the received set of reselection frequencies, wherein: a filtered subset of base stations is generated by identifying correspondence between at least one of the possible reselection frequencies in the list and an entry in the permitted-frequency data store corresponding to the present location of the mobile device; and selecting the second base station in accordance with the filtered subset.

In the preferred embodiment, a priority level is allocated to each frequency, and a frequency is selected having the highest priority level available in the filtered subset.

The mapping data may be downloaded to the permitted-frequency data store over the wireless interface from a broadcast message transmitted over a communications network. Alternatively it may be encoded in firmware in the mobile device itself or a data carrier connected to the device.

The current location of the mobile communications device may be determined by reference to an identification code received over the wireless interface from a currently serving base station. Alternatively, a location-finding system independent of the fixed infrastructure of the nmoble network may be used. .By configuring a mapping of permitted frequencies on the mobile device to identify the applicability of different frequencies to different operators’ networks in different geographical areas, it is possible to avoid unnecessary idle mode frequency searches, so that devices from one particular operator only search for frequencies that they are allowed to use, as opposed to searching for all frequencies in the frequency priority list broadcast by the serving ceil.

An illustrative embodiment of the Invention will now be described with reference to the drawings, in which:

Figure 1 is a schematic depiction of an illustrative situation in which a mobile device is in a region served by multiple base stations coupled to different networks

Figure 2 illustrates a typical frequency search process performed by such a terminal

Figure 3 is a flow chart illustrating an embodiment of a process performed by the mobile terminal as part of the frequency search process; and

Figure 4 is a schematic representation of the mobile device depicting the functional components which co-operate to perform the process of the invention

The embodiment avoids unnecessary idle mode searches for frequencies belonging to forbidden network operators by creating a mapping table, which can be stored on either the device itself or a SIM (subscriber interface module) card. The list may be installed in the device or its SIM permanently, but in one embodiment updates to the priority iist can be broadcast to all subscribers at suitable intervals.

As shown in figure 4, the mobile device or “user equipment” (UE) 6 has a radio interface 40 for communicating wirelessly with other wireless devices, and in particular with base stations such those depicted at 5a, 5b, 5c in Figure 1. The UE 6 also has a means 41 of identifying its location. As shown this may be by reference to the identities of local base stations, as transmitted by the base stations in the vicinity. However, an independent location-finding system such as the Global Positioning System may be used as well or instead. A permitted-frequency data store 42 maintains a look-up table of permitted frequencies by location. The store may be part of the device itself, or the data may be stored on a SIM card. In either case the relevant values may be pre-provisioned on the SIM card or a carrier-file on the device. Alternatively, a more flexible, dynamic and accurate mapping may be provided by Over-The-Air (OTA) updates, downloaded via the RF interface 40 and a download link 420 to the permitted-irequency data store 42. These updates may be provided for example as broadcast Information regarding frequencies used by neighbouring cells to all subscribers (known as SIB5 messages in the LTE standards)

The look-up table maintained by the permitted-irequency data store 42 stored on the mobile device or the associated SIM contains the frequency bands allocated to each MNO in a particular geographical area. An example of mapping table containing this information is shown below:

The UE 6 also comprises a ceil reseiection system 43, a frequency monitor 44, and associated prioritised frequency store 444, and a processor 45. The processor comprises a data filter function 46, and a selection function 49,

Figure 3 shows the steps performed by the UE to determine which frequencies to search for whilst on idle mode.

The process starts when the cell reselection system 43 generates a radio resource control (RRC) connection establishment request for transmission by the RF interface 40 (step 31) .

The UE 6 receives, through the RF interface 40, data from the local base station 5a relating to public land mobile networks (PLMN) A, B, C in the area, together with a frequency mapping (step 32). The frequencies are monitored by a frequency monitor 44 to establish a list of frequencies prioritised for example by signal quality (step 33). It then identifies which network is associated with each frequency and stores the results (step 34) in a ranked list in the prioritised frequency data store 444.

An example of such a ranked list is shown in the table below

Before performing any idle-mode searches for higher priority frequencies, the filter function 46 in the processor 45 compares the data in the permitted-frequency data store 42 to the local network/frequency prioritisation mapping in the prioritised frequency store 444 to determine which frequencies in the prioritisation mapping in the prioritised frequency store 444 belong to an MNO to which the subscriber is allowed to attach. These will typically be its home network (HPLMN), or one of the PLMN IDs in the Extended HPLMN list or the ePLMN list sent by the network to which the UE 6 is currently attached).

The filter 46 thus checks the rankings in the prioritised frequency store 444 against the list of permitted frequencies in the permitted-frequency data store 42 to determine whether any of the highest-priority frequencies selected by the filter 46 relate to the user’s home network or any other special user group (step 35). As frequencies may be allocated to different operators in different regions, this filtering also takes account of the current location of the UE 6, as determined by the location function 41.

Having identified the highest-priority network (or group of networks) for which a frequency can be detected, the base station selection function 49 then limits its search to frequencies associated with that (or those) same networks (step 36). If one of the frequencies is allocated to a forbidden network then the UE 6 will not search for that frequency unnecessarily since that would result in a waste of battery life and radio resources.

If a suitable frequency to which to connect is identified, ihe base station selection system instructs the cel! reselection system 43 to initiate a change to cause the Rf interface 40 to switch to operation on that frequency.

The invention may be used to allow an MNO to offer roaming capabilities to other MNOs in the same country, but can prevent its own customers from unnecessarily searching for other MNO frequencies by ensuring that they prioritise its own frequencies.

Claims

1. A mobile communications device comprising: a wireless interface for receiving and transmitting data a permitted-frequency data store for storing a plurality of mappings, each mapping having an associated frequency allocation band, a network operator identifier and a geographical area locator; a location identifier; a processor for receiving a list of potential reseiection frequencies from a first base station whilst the mobile communications device is in idle mode and prioritising the possible reselection frequencies against a set of predetermined selection criteria, and selecting a second base station in accordance with the prioritised list, wherein: the processor is further operable to form a filtered subset of base stations, by identifying correspondence between at least one of the possible reselection frequencies in the list and an entry in the permitted-frequency data store corresponding to the present location of the mobile device; and the selecting means is operable to select a base station in accordance with the filtered subset.

2. A mobile communications device according to claim 1, wherein the processor is arranged to receive a set of reselection parameters in which each frequency is allocated a priority level, and the selecting means is arranged to select a frequency from the highest priority level available in the filtered subset.

3. A mobile communications device according to claim 1 or Claim 2, wherein the permitted-frequency data store is arranged to download information received over the wireless interface tor creating and updating mappings to be stored therein.

4. A mobile communications device according to claim 1 or Claim 2, having a location identification system for identifying the present location of the mobile communications device by reference to an identification code received over the wireless interface from a currently serving base station.

5. A method of operating a mobile communications device comprising the steps of: storing data in a permitted-frequency data store relating to a plurality of mappings, each mapping having an associated frequency allocation band, a network operator identifier and a geographical area locator; and, whilst the mobile communications device is in idle mode, identifying the current location of the mobile device; receiving from a first base station a set of potential reselection frequencies comparing the set of potential reselection frequencies received, against the mappings in the permitted-frequency data store corresponding to the current location of the mobile device, selecting a second base station in accordance with the received set of reselection frequencies, wherein: a filtered subset of base stations is generated by identifying correspondence between at least one of the possible reselection frequencies in the list and an entry in the permitted-frequency data store corresponding to the present location of the mobile device; and selecting the second base station in accordance with the filtered subset.

8. A method according to claim 5, allocates a priority level is allocated to each frequency in the set of reselection frequencies, and the a frequency is selected having the highest priority level available in the filtered subset.

7. A method according to claim 5 or Claim 6, wherein the mapping data is downloaded to the permitted-frequency data store over the wireless interface from a broadcast message transmitted over a communications network.

8. A method according to claim 5 or Claim 8 or Claim 7, wherein the present location of the mobile communications device is determined by reference to an identification code received over the wireless interface from a currently serving base station.