WO2016198825A1

WO2016198825A1 - A method in a terminal, a method in a communications system, a terminal and communications system for assisting a terminal to a select a serving cell

Info

Publication number: WO2016198825A1
Application number: PCT/GB2016/050188
Authority: WO
Inventors: Paul Bucknell; Yiwei Fang; Timothy Moulsley
Original assignee: Fujitsu Limited
Priority date: 2015-06-11
Filing date: 2016-01-28
Publication date: 2016-12-15
Also published as: GB2539243A; GB201510187D0

Abstract

Invention embodiments provide a method in a terminal of a wireless communications system, the wireless communications system comprising the terminal and base stations providing cells which can be selected to serve the terminal, the method comprising: the terminal accessing cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and the terminal applying one or more criteria to choose whether to monitor broadcast messages from the potential serving, the criteria including at least one criterion based on the cell combination information.

Description

A METHOD IN A TERMINAL, A METHOD IN A COMMUNICATIONS SYSTEM, A TERMINAL AND COMMUNICATIONS SYSTEM FOR ASSISTING A TERMINAL TO A SELECT A SERVING CELL

Field of the Invention

The present invention relates to a method in a terminal of a wireless communication system. The present invention further relates to the terminal itself, the wireless communications system, a method in the wireless communications system, and computer programs for use in said methods.

Particularly, but not exclusively, the present invention relates to techniques for assisting a terminal to select a serving cell in a wireless communication system which may be compliant with the LTE (Long Term Evolution) and LTE-Advanced radio technology standards, for examples as described in Release 12 (REL 12) and subsequent of the 3GPP specification series.

Background of the Invention Wireless communication systems are widely known in which terminals (also called user equipments or UEs, subscriber or mobile stations) communicate with base stations (BSs) within range of the terminals.

The geographical areas served by one or more base stations are generally referred to as cells, and typically many BSs are provided in appropriate locations so as to form a network covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms "system" and "network" are used synonymously). Each BS may support one or more cells (including cells formed by Remote Radio Heads (RRHs) which are linked to the BS via a fixed link such as a fibre optic cable). In each cell, the BS divides its available bandwidth, i.e. frequency and time resources, into individual resource allocations for the user equipments which it serves. The terminals are generally mobile and therefore may move among the cells, prompting a need for handovers between the base stations of adjacent cells. A terminal may be in range of (i.e. able to detect signals from and/or communicate with) several cells at the same time, but in the simplest case it communicates with one "serving" cell. One type of cellular wireless network is based upon the set of standards referred to as Long-Term Evolution (LIE). The current version of the standard, Release 11 (Rel 11), is also referred to as LTE-A (LTE-Advanced), and the specifications for Release 12 are currently being finalised. The network topology in LTE is illustrated in Figure 1. As can be seen, each terminal 10, called a UE in LTE, connects wirelessly over an air interface (labelled Uu in Figure 1) to a base station in the form of an enhanced node-B or eNB 20. It should be noted that various types of eNB are possible. An eNB may support one or more cells at different carrier frequencies, each cell having differing transmit powers and different antenna configurations, and therefore providing coverage areas (cells) of differing sizes. Multiple eNBs deployed in a given geographical area constitute a wireless network called the E-UTRAN (and henceforth generally referred to simply as "the network"). An LTE network can operate in a Time Division Duplex, TDD, mode in which the uplink and downlink are separated in time but use the same carrier frequency, or Frequency Division Duplex, FDD, in which the uplink and downlink occur simultaneously at different carrier frequencies. Radio Resource Control (RRC) is a protocol layer in the UE and eNB to control various aspects of the air interface, including establishing, maintaining and releasing a RRC connection between the UE and eNB. Thus, for a UE to be served by a cell implies a RRC connection with the eNB providing or controlling that cell. Each eNB 20 in turn is connected by a (usually) wired link (S1 in Figure 1) to higher- level or "core network" entities 101, including a Serving Gateway (S-GW), and a Mobility Management Entity (MME) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition (not shown), a Packet Data Network (PDN) Gateway (P-GW) is present, separately or combined with the S-GW, to exchange data packets with any packet data network including the Internet. Thus, communication is possible between the LTE network and other networks. Meanwhile, the eNBs can communicate among themselves via a wired or wireless X2 interface as indicated in the Figure. Figure 1 shows what is sometimes called a "homogeneous network"; that is, a network of base stations in a planned layout and which have similar transmit power levels, antenna patterns, receiver noise floors and similar backhaul connectivity to the core network. Current wireless cellular networks are typically deployed as homogeneous networks using a macro-centric planned process. The locations of the base stations are carefully decided by network planning, and the base station settings are properly configured to maximise the coverage and control the interference between base stations. However, future cellular wireless networks will adopt a "heterogeneous network" structure composed of two or more different kinds of cell, also referred to as a Small Cell Network or SCN.

The motivation for SCNs is the idea of network densification: increasing the number of network nodes, and thereby bringing them physically closer to the terminals, in order to improve traffic capacity and extending the achievable user-data rates of a wireless communication system. SCNs achieve network densification by the deployment of complementary low-power nodes under the coverage of an existing macro-node layer. In such a heterogeneous deployment, the low-power nodes provide very high traffic capacity and very high user throughput locally, for example in indoor and outdoor hotspot positions. Meanwhile, the macro layer ensures service availability and Quality of Experience (QoE) over the entire coverage area. In other words, the layer containing the low-power nodes can also be referred to as providing local-area access, in contrast to the wide-area-covering macro layer. Figure 2 depicts a simple SCN. The large ellipse represents the coverage area or footprint of a Macro cell provided by a base station (Macro BS) 20. The smaller ellipses represent small cells (such as Pico or Femto cells) within the coverage area of the Macro cell, each having a respective low-power base station 21 - 26 (exemplified by Pico BS 21). Here, the Macro cell is a cell providing a "macro layer" of basic coverage in the network of a certain area, and the small cells are overlaid over the Macro cell, using the same or different carrier frequencies, providing a "low-power layer" for capacity boosting purposes particularly within so-called hot spot zones. A UE 10 is able to communicate both with Macro BS 20 and Pico BS 21 as indicated by the arrows in the Figure.

Carrier aggregation (CA) is a technique applicable to the SCN. Using LTE Advanced carrier aggregation, it is possible to utilise more than one carrier (frequency band) and in this way increase the overall transmission bandwidth. These channels or carriers may be in contiguous elements of the spectrum, or they may be in different bands. Thus, a UE in an SCN may be simultaneously connected to one cell (often, but not necessarily, a Macro cell) having a first carrier frequency and referred to as the "primary cell" (PCell) for that UE: and to one or more other cells (usually but not necessarily small cells) using different carrier frequencies and possibly different frequency bands, and referred to as "secondary cells" (SCells) of that UE. The SCells are provided from the same (or a nearby) location as the PCell. The PCell and the SCell(s) typically operate on different carrier frequencies and possibly on different frequency bands. The UE can receive and decode the data from all the bands simultaneously, and from a number of carriers limited by the UE capability. In carrier aggregation, the two eNBs do not act independently: the PCell controls the SCells.

When such communication occurs simultaneously with two independent base stations (whether they are of the same or a different type, or indeed operating as part of different networks), this is called "dual connectivity" (DC) of the UE. For the DC case the UE can be served by radio bearers which are either from one or both the Primary eNB or the Secondary eNB (connected to the Primary eNB by the X2 logical interface). A Master eNB can provide a Primary Cell (a PCell) and one or more Secondary eNBs in a Secondary Cell Group (SCG) can provide at least a Primary SCell (PSCell) and possibly further secondary cells.

In SCN scenarios, the Macro BS can be a mobility anchor point for the UE, providing the control signalling for handovers of the UE between small cells. Whilst a Pico BS 21 is shown by way of example, it should be noted that various types of station can provide the small cells, including Home eNBs (HeNBs) providing Femto cells, or even other UEs if these can operate in a device-to-device (D2D) mode. Thus, other base stations 22-26 shown in Figure 2 could form other Pico cells or alternatively, Femto cells which are even lower power and smaller range than the Pico cell around pico base station 21. Any such cells are henceforth referred to simply as "small cells".

It should be noted that the presence of a Macro cell is not essential and the SCN may consist only of small cells. In any case, however, coordination among the cells at the same or nearby location is required and this is conveniently provided by a primary eNB such as the Macro BS 20 even for UEs not directly connected to the Macro BS.

To assist in understanding the invention to be described, some explanation will now be given of the E-UTRAN layers for data and signalling, which are defined at various levels of abstraction within an LTE network.

Figure 3 shows some of the protocol layers defined in LTE at each of a physical channel level (Layer 1); transport channel level, logical channel level, radio bearer level and control traffic level (together forming Layer 2); and the Layer 3 levels of radio resource control and Non-Access Stratum (NAS) for the control plane and Internet Protocol (IP) for the user traffic.

One distinction in these protocol layers is between the Access Stratum (AS) which is for dialogue explicitly between the UE and the network and the Non-Access Stratum (NAS) which is for dialogue between the UE and core network nodes (MME).

On the downlink, at the physical layer level, each cell conventionally broadcasts a number of channels and signals to all UEs within range, whether or not the UE is currently being served by that cell. Of particular interest for present purposes, these include a Physical Broadcast Channel PBCH. PBCH carries a so-called Master Information Block (MIB), which gives, to any UEs within range of the signal, basic information as described below. Primary and Secondary Synchronization Signals (PSS/SSS) are also broadcast to all devices within range. In addition to establishing a timing reference for a cell, these carry a physical layer cell identity and physical layer cell identity group for identifying the cell. These kinds of information are referred to below as "synchronization information".

In LTE specifications, a UE can be considered . as either synchronised or unsynchronised with respect to a cell. Successfully decoding the PSS and SSS allows a UE to obtain the synchronization information, including downlink timing and cell ID for a cell; in other words the UE becomes "synchronized" with the cell. In the synchronized state, the UE can transmit signals in the uplink (assuming resources are made available by the network, with a defined timing (the uplink timing is obtained by subtracting a "timing advance" TA from the downlink timing).

Once a UE has decoded a cell's PSS and SSS it is aware of the cell's existence and may decode the MIB in the PBCH referred to earlier. The PBCH is transmitted every frame, thereby conveying the MIB over four frames. The MIB includes some of the basic information which the UE needs to join the network, including system bandwidth, number of transmit antenna ports, and system frame number (SFN). Reading the MIB enables the UE to receive and decode the System Information Blocks (SIBs). The first System Information Block (SIB1) is relevant when evaluating if a UE is allowed to access a cell. When a UE-to-cell association is formed, the UE can begin to receive user data (packets) from the cell (or serving cell), and/or transmit user data to the cell.

A UE can be in RRC Idle state (or Idle Mode) in which it is not known to the eNB, or in RRC Connected State in which it is connected. For a UE operating in Idle Mode in cellular systems such as GSM, LTE, UMTS and CDMA2000, there are defined procedures for cell connection that typically have to be performed.

The first procedure is PLMN (Public Land Mobile Network) selection in which a selected PLMN is associated with a particular radio access technology and an operator. PLMNs are deployed by operators and are made up of the MCC (Mobile Country Code) and a MNC (Mobile Network Code). The PLMN information is broadcasted by a cell in the information element PLMN-ldentlty which is part of the SIB1 broadcast information. PLMN Selection

For PLMN selection the following PLMNs may be stored and known and used by a UE: • Home PLMN (HPLMN): the PLMN whose MCC and MNC match the MNC which is stored in the IMSI (International mobile subscriber identity). For GSM, UMTS and LTE networks, this number is provisioned in the SIM card and for CDMA2000 in the phone directly or in the R-UIM card. The first 3 digits of the IMSI are the mobile country code (MCC), which are followed by the mobile network code (MNC), either 2 digits (European standard) or 3 digits (North American standard).

• Equivalent HPLMN (EHPLMN): any PLMN that is equivalent to the HPLMN. The

EHPLMN(s) defines a set of PLMN(s) which are treated as equivalent to the PLMN with which UE is registering. This list stored in the UE can be updated or deleted by network during attach & TAU update. EPLMN(s) are ordered in terms of their priority.

• Registered PLMN (RPLMN): The PLMN for which registration is successful

• Equivalent PLMN (EPLMN) Any PLMN which is equivalent to the RPLMN · Visited PLMN (VPLMN) When service is provided to the UE when roaming, the PLMN is called the Visited PLMN (VPLMN)

• Primary PLMN The first listed PLMN-ldentity is the primary PLMN

Currently, in the E-UTRA case, the UE scans all RF channels in the E-UTRA bands according to its capabilities to find available PLMNs. On each carrier, the UE searches for the strongest cell and reads its system information, in order to find out which PLMN(s) the cell belongs to. If the UE can read one or several PLMN identities in the strongest cell, each found PLMN is reported to the NAS as a high quality PLMN (but without the basic Physical layer measurement of RSRP value which is discussed in more detail later), provided that the following high quality criterion is fulfilled:

For an E-UTRAN cell, the measured RSRP value shall be greater than or equal to -110 dBm.

Found PLMNs that do not satisfy the above high quality criterion, but for which the UE has been able to read the PLMN identities, are reported to the NAS together with the RSRP value. The quality measure reported by the UE to NAS shall be the same for each PLMN found in one cell. The search for PLMNs may be stopped on request of the NAS. The UE may optimise PLMN search by using stored information e.g. carrier frequencies and optionally also information on cell parameters from previously received measurement control information elements.

Once the UE has selected a PLMN, the cell selection procedure is performed in order to select a suitable cell of that PLMN to camp on. A suitable; cell is where the measured cell attributes satisfy the cell selection criteria, for example the cell PLMN is the selected PLMN, RPLMN or an EPLMN and the cell is not barred or reserved and the cell is not part of a tracking area which is in the list of "forbidden tracking areas for roaming"

If a Closed Subscriber Group ID (CSG ID) is provided by NAS as part of PLMN selection, the UE shall search for an acceptable or suitable cell belonging to the provided CSG ID to camp on. When the UE is no longer camped on a cell with the provided CSG ID, AS shall inform NAS.

When a UE Access Stratum (AS) is searching for available PLMNs, the UE usually uses the last RPLMN. If this search fails then the PLMN selection mode described above is entered. This can also be triggered by a user with manual intervention.

During the PLMN selection the UE AS scans for available PLMNs by scanning all E- UTRAN bands supported by the UE. During this scan the UE should synchronise (PSS/SSS) with a cell and read SIB1 to enable the PLMN of each identified cell to be decoded.

SIB1 includes a:

plmn-ldentityList

List of PLMN identities. As mentioned above, the first listed PLMN-ldentity is the primary PLMN

PLMN-ldentityList ::= SEQUENCE (SIZE (1..maxPLMN-r11)) OF PLMN-ldentitylnfo

Suitable Cell, Acceptable Cell, Barred Cell and Reserved Cell

Some definitions are provided below to aid understanding of the cell selection process.

A suitable cell is where the measured cell attributes satisfy the cell selection criteria, for example, the cell PLMN is the selected PLMN, RPLMN or an EPLMN and the cell is not barred or reserved and the cell is not part of a tracking area which is in the list of "forbidden tracking areas for roaming"

An acceptable cell is one for which the measured cell attributes satisfy the cell selection criteria and the cell is not barred.

A reserved cell is a cell where normal services are provided to a UE with special access rights. Typically this is a cell on which camping is not allowed, except for particular UEs, if so indicated in the system information. When a UE is camped on a cell this means that a UE has completed the cell selection/reselection process and has chosen a cell. Figure 4 is a flow diagram illustrating idle mode state transitions in cell selection. Initially the UE is in idle mode. It then starts the cell selection process and camps on to a suitable cell. The UE then monitors system information and (in most cases) paging information. The cell reselection process takes place while the UE is camped on. If a new (better) suitable cell is found, the UE camps on to that cell. If not (or if there was no suitable cell available in the first place or a suitable cell is no longer available), the UE carries out AnyCellSelection to find an acceptable cell. If one is found the UE camps on to the cell and starts reselection. If no acceptable cell is found AnyCellSelection is re-started. Cell Selection

Cell selection is normally triggered by PLMN selection to define a suitable cell for the selected PLMN.

Cell selection is also triggered when the UE leaves RRC_Connected mode. Usually Cell selection occurs either as part of initial cell selection or with the assistance of stored information of the carrier frequencies and other cell parameters which allow a faster cell selection procedure.

In this specification, any general mention of cell selection is also taken to include cell re-selection, in which the UE finds a "better cell", re-selects it and camps onto it.

More details of the Layerl (physical layer) Measurement procedure are provided here:

Reference Signal Receive Power (RSRP)

RSRP is the most basic of the Physical layer measurements. It is a linear average of the downlink Reference Signals, in watts, across the channel bandwidth. RSRP gives no indication of the signal quality.

Received Signal Strength Indicator (RSSI)

RSSI is the entire UE received power, including the wanted power from the serving cell, as well as all other (unwanted) co-channel power (interference) and noise. Reference Signal Receive Quality (RSRQ)

Given RSRP and RSSI, the RSRQ is defined as a ratio between RSRP and RSSI. RSRQ does not accurately measure the quality of the measured cell as the RSSI also includes the co-channel interference. Cell Selection Criterion

The cell selection criterion, referred to as the S criterion, is the name of the method for determining whether a measured signal from a particular cell exceeds a given threshold. Cell selection

For cell selection in REL-8 only RSRP is used in the "S Criteria", but for REL-9 and above, both RSRP and RSRQ are used to help a UE to select a cell that shows a high level of RSRP and RSRQ. So when RSRQ is available the UE will select a cell if both Srxiev and Squai exceed OdB for the measured cell, where both S_rX!ev and S_quai are defined as:

RSRP metric: Srxlev ^— Qrxlevmeas ~" (Qrxlevmin Qrxlevminoffset) " P compensation

RSRQ metric. Squai ⁼ Qqualmeas^— (Qqualmin Qualminoffset)

Srxiev = Cell Selection Quality Value

qual^— Cell Selection RX Level Value

Q[rxlev,qual]meas ⁼ measurement

Q[rxlev,qual]min = required minimum level for cell selection

Q[rxlev,qual]offset = offset to avoid Ping-Pong during periodic searching at PLMN boundaries Pcompensation ⁼ ΠΊ3Χ(ΡΕΜΑΧ ^— PpowerClass, 0), PE AX = max Tx power at which UE is allowed to transmit in the cell, and Ppowerciass is the max UE power output according to UE power class, this offset avoids the UE selecting the cell where the transmission power of the UE may not be enough to reach the eNB.

The parameters related to cell selection are broadcast by the eNB in CellSelectionlnfo which is included in the SystemlnformationBlockTypel broadcast message. Measurement Rules

The UE perform measurements according to measurement requirements defined in 3GPP TS 36.133 "Requirements for support of radio resource management" as follows: · Intra-frequency measurement requirements:

o Serving Cell: The UE measures RSRP and RSRQ level of the serving cell at least once per discontinuous receive (DRX) cycle o Intra-frequency neighbouring cell: When the UE is required to perform intra-frequency measurements, it measures at least every

Tmeasure.EUTRANJntra · Inter-frequency measurement requirements:

o Neighbouring cell of higher frequency priority at lease every T igh_prioirity_searoh = (60 * iayers) seconds, Niayers = total number of configured higher priority carrier frequencies including all Radio Access Technologies (RATs). For higher layers that are identified then the UE performs measurements at least every T_measure,EUT ANjnter o For neighbouring cells if equal or lower priority the UE performs measurements at least every Kcamer * Tmeasure, EUTRAN Inter Where Kcarrier = number of E-UTRAN carrier frequencies indicated by the serving cell in SIB5

In order to avoid wasteful measurements there are additional rules defined in the current 3GPP specifications, for example if the measured level of the serving cell is good then the measurement of neighbouring cells can be skipped. Parameters to allow efficient measurements include: o For intra frequency measurements: the UE can skip measurements of intra- frequency neighbour cells if Srxlev SintraSearchP and S qual SlntraSearchQ

o For inter frequency measurements: the UE can skip measurements of intra- frequency neighbour Cells if Srxlev > SnonlntraSearchP and Squal > SnonlntraSearchQ

The optional parameters S IntraSearchP and Snonintrasearcnp are received by the UE in SIB3 and are set to infinity if not present and zero for SintraSearcnQ and S_n0nintrasearchQ

E-UTRAN Inter-frequency and inter-RAT Cell Reselection criteria

If threshServingLowQ is provided in SystemlnformationBlockType3 and more than 1 second has elapsed since the UE camped on the current serving cell, cell reselection to a cell on a higher priority E-UTRAN frequency or inter-RAT frequency than the serving frequency shall be performed if:

- A cell of a higher priority EUTRAN or UTRAN FDD RAT/ frequency fulfils Squal > Threshx, Higho during a time interval TreselectionRAT; or

- A cell of a higher priority UTRAN TDD, GERAN or CDMA2000 RAT/ frequency fulfils Srxlev > Threshx, Highp during a time interval TreselectionRAT.

Otherwise, cell reselection to a cell on a higher priority E-UTRAN frequency or inter- RAT frequency than the serving frequency shall be performed if:

- A cell of a higher priority RAT/ frequency fulfils Srxlev > Threshx, _Highp during a time interval TreselectionRAT; and

- More than 1 second has elapsed since the UE camped on the current serving cell.

Cell reselection to another RAT, for which Squal based cell reselection parameters are broadcast in system information, shall be performed based on the Squal criteria if the UE supports Squal (RSRQ) based cell reselection to E-UTRAN from all the other RATs provided by system information which UE supports. Otherwise, cell reselection to another RAT shall be performed based on Srxlev criteria

Prior Art

The prior art methods of cell selection detailed above suffer from disadvantages in some scenarios.

For dense cellular deployments with many cells and many different PLMNs the configuration of EHPLMN(s) by the network to the UE does not provide a sufficient amount of balance between the minimisation of the UE power required to make and report RSRP and RSRQ to the NAS layer from different cells, and the network having sufficient control over which cells the UEs are allowed to select and camp on.

When a UE "camps on" a cell it monitors system information and is typically ready to receive paging messages from that cell.

For heterogeneous networks it is also clear that the RSRP and RSRQ method of cell selection does not give the UE sufficient information in order to make intelligent cell selection decisions which may be driven by the UE specific service requirements (such as low mobility, desired QoS , etc). For example small cells are difficult to connect to for faster-moving UEs, but mobility is not taken into account in current cell measurements made and reported to the base station for cell selection/reselection

In the current procedures the strongest cell is the cell on a particular carrier that is considered strongest according to the layer 1 cell search procedure as defined above and in 3GPP TS 36.213: "E-UTRA; Physical layer procedures", and 3GPP TS 36.214: "E-UTRA; Physical layer; Measurements".

Much of the existing prior art relating improvement in performance involves MDT (Minimisation of Drive Tests) types of operation in which the UE makes more detailed measurements and reports these to the network on a less frequent basis then the current measurements.

The design principles of MDT in REL- 0 are:

Support for both real-time and non-real-time measurement reporting

User consent for measurements and methods to limit the impact on UE power consumption and storage memory requirements

Measurement reporting is done at same time as capture of time and location allowing the network to know exactly where and when the UE performed the reported measurements

The motivation for the collection of the measurements results is that network performance can be optimised without the need for expensive and time consuming dedicated drive tests to try and find possible coverage problems in a given area or location. According to one embodiment of a first aspect of the invention there is provided a method in a terminal of a wireless communications system, the wireless communications system comprising the terminal and base stations providing cells which can be selected to serve the terminal. The method comprises: the terminal accessing cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and the terminal applying one or more criteria to choose whether to monitor broadcast messages from the potential serving, the criteria including at least one criterion based on the cell combination information.

Hence the terminal accesses information relating not just to the potential serving cell but also to at least one other cell in a criterion to choose whether to monitor broadcast information from the potential serving cell. This information may be stored in the terminal and/or received from the cell(s).

This invention has different design objectives from the MDT prior art, for example to provide extra information to the UE to enable better informed decisions to be taken internally in the UE as to what cell selection procedures need to be performed and/or what suitable cells should be monitored to check which might be the most appropriate for use by the UE. The text refers in general to cell selection, but this should be taken to encompass cell selection and cell re-selection.

The operation of the potential serving cell in combination with the at least one other cell to communicate with the terminal can use any suitable technique, for example carrier aggregation and/or dual connectivity.

The at least one criterion can be any suitable criterion, and may for example give a threshold for anticipated downlink and/or uplink communication properties, For example the criterion can be with respect to an estimate of the quality of service, QoS, which would be available if the potential serving cell and the other cell were both selected as serving cells. The QoS can include the data rate which would be available if the potential serving cell became a primary serving cell and the at least one other cell became a secondary serving cell. Alternatively or additionally, any other measure of QoS may be used. In some embodiments, the cell combination information is stored in the terminal, and used in conjunction with a desired operational mode of the terminal to select cells from which to monitor broadcast information.

The method may comprise the terminal accessing cell combination information about the operation of more than one potential serving cell; and the terminal applying one or more criteria to choose whether to monitor broadcast messages from each potential serving cell. Although in one scenario a single potential serving cell may be considered, (for example to see if a single suitable cell in one network is suitable before changing to a different frequency and network), in other scenarios there may be several potential serving cells, and the method chooses one or more of these cells from which to monitor broadcasts.

In one implementation, the broadcast messages include at least one reference signal from the potential serving cell(s), and the terminal measures the received signal power of the at least one reference signal if it has chosen to monitor the broadcast messages.

The method may take place as part of cell selection. The combination information may be provided to the terminal during a network selection before cell selection, such as Public Land Mobile Network PLMN selection or during another procedure before cell selection. It may for example have been provided to the terminal during a previous cell selection process.

In one embodiment, the combination information includes one or more lists of cells. It may include one or more of the following lists of cells: a list of one or more other cells that would be available as secondary cells if the potential serving cell became a primary cell, for example to support carrier aggregation; a list of one or more other cells that could be in a secondary cell group, for example to support dual connectivity; a list of one or more cells that could act as a primary cell for the potential serving cell; a list of one or more cells that are likely to be suitable when cell reselection is required.

The one or more lists can include cells which are dormant when the method takes place (and thus for which information may not otherwise have been provided).

The one or more lists can include operating information (such as throughput and/or availability information) for the cells in the list. This operating information may be of the cell in question in combination with the potential serving cell.

The criteria may further include one or more same cell criteria relating to parameters of the potential serving cell. These same cell criteria are broadcast during a network selection, such as PLMN selection, before cell selection or during another procedure before cell selection.

In contrast to some prior art methods, the same cell criteria are not necessarily obtained from physical layer measurement. For example, the same cell criteria can include one or more of: number of antennas; information relating to 3D MIMO; whether the cell is active/on/off; transmission modes supported by the cell; current traffic loading of the cell; available communication resources in the cell; the network to which the cell belongs; and location of the cell.

Terminal-specific information, such as information on previously selected cells and/or location of the terminal, may be used in an additional criterion in choosing cells from which broadcast messages are monitored for cell selection.

In some embodiments, cells are present on more than one carrier frequency and frequency layer information of the networks available for selection by the terminal is used in a further criterion for choosing cells from which broadcast messages are monitored for cell selection.

If cells are present on more than one carrier frequency, the terminal may decide to select a carrier frequency as a result of the combination information. In an embodiment of a further aspect of the invention there is provided a terminal of a wireless communications system, the wireless communications system comprising the terminal and base stations providing cells which can be selected to serve the terminal, the terminal comprising: a transmitter and a receiver arranged to communicate with the cells; and a controller arranged to control a cell selection procedure; wherein the receiver is arranged to receives cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and the controller is arranged to apply one or more criteria to choose whether to monitor broadcast messages from the potential serving cell, the criteria including at least one criterion based on the cell combination information.

In an embodiment of a still further aspect of the invention there is provided a wireless communications system comprising a terminal and base stations providing cells which can be selected to serve the terminal; the base stations comprising a transmitter and a receiver arranged to communicate with the terminal; and the terminal comprising: a transmitter and a receiver arranged to communicate with the cells; and a controller arranged to control a cell selection procedure; wherein the receiver is arranged to receives cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and the controller is arranged to apply one or more criteria to choose whether to monitor broadcast messages from the potential serving cell, the criteria including at least one criterion based on the cell combination information. In an embodiment of a yet further aspect of the invention there is provided a method in a wireless communications system comprising a terminal and base stations providing cells which can be selected to serve the terminal; wherein the terminal receives cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and applies one or more criteria to choose whether to monitor broadcast messages from the potential serving cell, the criteria including at least one criterion based on the cell combination information. The term "cell" used above is to be interpreted broadly, and may include, for example, the communication range of a transmission point or access point. As mentioned earlier, cells are normally provided by base stations. It is envisaged that the base stations will typically take the form proposed for implementation in the 3GPP LTE and 3GPP LTE-A groups of standards, and may therefore be described as an eNB (eNodeB) (which term also embraces Home eNB or HeNB) as appropriate in different situations. However, subject to the functional requirements of the invention, some or all base stations may take any other form suitable for transmitting and receiving signals from other stations.

The "terminal" referred to above may take the form of a user equipment (UE), subscriber station (SS), or a mobile station (MS), or any other suitable fixed-position or movable form. For the purpose of visualising the invention, it may be convenient to imagine the terminal as a mobile handset (and in many instances at least some of the terminals will comprise mobile handsets), however no limitation whatsoever is to be implied from this.

An apparatus according to preferred embodiments of the present invention can comprise any combination of the previous method aspects. Methods according to invention embodiments can be described as computer-implemented in that they require processing and memory capability.

The apparatus according to preferred embodiments is described as configured or arranged to carry out certain functions. This configuration or arrangement could be by use of hardware or middleware or any other suitable system. In preferred embodiments, the configuration or arrangement is by software.

According to a further aspect there is provided a program which when loaded onto a terminal configures the terminal to carry out the method steps according to any of the preceding method definitions or any combination thereof.

In general the hardware mentioned may comprise the elements listed as being configured or arranged to provide the functions defined. For example this hardware may include a receiver, a transmitter (or a combined transceiver), a processor, memory/storage medium, a user interface and other hardware components generally found in a terminal. The invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The invention can be implemented as a computer program or computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, one or more hardware modules. A computer program can be in the form of a standalone program, a computer program portion or more than one computer program and can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a data processing environment. A computer program can be deployed to be executed on one module or on multiple modules at one site or distributed across multiple sites on the vehicle or in the back-end system and interconnected by a communication network.

Method steps of the invention can be performed by one or more programmable processors executing a computer program to perform functions of the invention by operating on input data and generating output data.

The invention is described in terms of particular embodiments. Other embodiments are within the scope of the following claims. For example, the steps of the invention can be performed in a different order and still achieve desirable results.

Brief Description of the Drawings

Reference is made, by way of example only, to the accompanying drawings in which: Figure 1 illustrates a basic system architecture in LTE;

Figure 2 illustrates a Small Cell Network, SCN;

Figure 3 shows the various channels in LTE;

Figure 4 is a flow diagram illustrating idle mode state transitions in cell selection; Figure 5 is a flow diagram of a general embodiment;

Figure 6 is a schematic diagram of a terminal or UE to which the present invention may be applied;

Figure 7 is an overview diagram of part of a network, illustrating a UE with a PCell and an SCell;

Figure 8 is an overview diagram of part of a network, illustrating a UE with a PCell and a PSCell; and

Figure 9 is a flowchart of steps in a method embodying the present invention. Figure 5 is a general flow chart with an overview of the method. In step S12, the terminal accesses cell combination information. In step S14 the terminal applies criteria to choose whether to monitor broadcasts from the cells. The criteria include at least one criterion based on the cell combination information. In step S16, the terminal monitors broadcasts from the chosen cells. In step S18, the terminal can select a cell from the chosen (pre-selected) cells from which broadcasts have been monitored. As an alternative, another frequency layer may be chosen at this stage.

Figure 6 shows the hardware structure of a terminal 10 suitable for use with invention embodiments, including an antenna 802, transmission and reception unit(s) 804, a controller 806 and a storage medium or memory 808.

Conceptually (and using the LTE example and terminology), invention embodiments can allow control of the amount of cells for the UE has to perform cell selection procedures such as synchronisation and reading of SIB1 and calculation of RSRP for cell selection purposes. This is achieved by allowing the UE to use extra information to make more informed decisions on the cells on which it needs to perform measurements for cell selection. The overall aim of this extra information may be viewed primarily as allowing the UE to (pre)select cell(s) which will have the highest probability of being able to provide a UE with sufficient data rate and QoS suitable for the traffic which that specific UE can support. The cell need not be selected directly; for example the UE may select a frequency layer/band form those available, and monitor broadcasts of cells operating in that frequency layer. Such extra information can allow the UE to prioritise already stored information about the cells for which the UE could perform cell access, in a similar way to the current UE procedures for PLMN selection. An aspect of invention embodiments is that signalling relating to PLMNs can be used by the network to allow the UE to make a more informed choice of cell to select.

Some cell information (e.g. that could be broadcast) that a UE might use to (pre-) select a cell (e.g. for initial access, camping-on), or to choose other frequency bands (and potentially PLMNs) to search as well could be any of:

• A list of other cells that would be available as secondary cells (SCells) if the selected cell became a primary cell (PCell) for the UE (including carriers in unlicensed spectrum)

o This could take into account the UE capability to support carrier aggregation with the relevant frequency band combinations

• A list of other cells that could be in a secondary cell group (SCG) in dual connectivity

o This could take into account the UE capability to support dual connectivity with the relevant frequency band combinations Note the above lists could contain possibly dormant cells (in the RAN3 - overall

UTRAN architecture - sense, e.g. in some low power energy saving state), which the UE could not otherwise detect

• A list of other info e.g. related to throughput/availability for cells in either or both of the above lists (which may or may not be in their SIBs)

· A list of cell(s) which could be a PCell for this cell (if the cell is not available as an SCell for some reason)

• A list of cell(s) which are likely to be suitable when a reselection is needed.

On this basis the UE can select a cell that is most likely to be able to provide high enough throughput and/or QoS (for example when serving as a PCell and configured together with possible SCells or a PSCell). In addition or alternatively to the UE receiving this as broadcast information it may also be possible that this information could be UE specific, such as information on previously selected cells. Additionally, as the UE knows its location, this could be used as an additional input into the UE based decision making process for cell selection.

Some possibilities for additional information relate directly to the cell itself: These are any of:

• Number of antennas (or other info relating to 3D MIMO)

· Availability as a PCell

• Whether cell on/off is active

• Supported transmission modes

• Current traffic loading Detailed Embodiments

In general, unless otherwise indicated, the embodiments described below are based on technology such as LTE, in which the network comprises multiple base stations or eNodeBs each supporting one or more cells and multiple devices are allowed to attach to the network. The invention is expected to be most useful when cells are present on more than one carrier frequency.

In general a cell can broadcast information which allows the UE to make a selection of a defined frequency layer (or some other criteria) of PLMNs already stored in the UE - this saves UE processing time and allows the cell to be selected to be most appropriate to the UE traffic requirements.

A first specific embodiment is shown in Figure 7. The figure shows use of an RRH to form an SCell within a PCell. The RRH is linked to the eNB main transmitter, using optical fibre. The connection from the eNB to the S-GW is over the S1 interface. The UE can receive signals from both the SCell and the PCell. Other cells in the network are not depicted. Here, a UE uses information about the combination of the RRH cell and the eNB cell to make a selection of (a) cell(s) from candidate cells from which to monitor broadcast information. This then allows the UE to make a selection of a particular PCell which can be combined together with another SCell to allow the UE to use carrier aggregation to meet a specific UE based service requirement such as high data rate transmission. One benefit of this is that the UE can immediately be configured to set up either Carrier Aggregation (CA) or Dual Connectivity (DC) with reduced control signalling, which will reduce the delay in moving the UE from idle to connected mode and therefore reduce total system latency for a given application.

A second embodiment shown in figure 8 has a similar arrangement, but a master eNB and a second EnB (SeNB) are each connected over the S1 interface to the S-GW. The MeNB and SeNB are linked over the X2 interface. The oval above the UE and eNB in this figure shows that the SeNB can also use CA - i.e. it has an PCell and and SCell that the UE can connect to. This embodiment envisages the UE using information about the combination of the SeNB cell and MeNB cell to make a selection of which candidate cell that the UE monitors in order to allow the network to configure dual connectivity (multiple radio bearers through 2 independent eNBs) to the UE. The dual connectivity operation allows the transmission of data bearers to the UE through both a first PCell and a PSCell (and optionally SCells on the secondary eNB using Carrier aggregation).

A third embodiment includes additional consideration of any of various selection criteria that the UE would use to help it select a cell with the highest probability of being able to provide the UE with the required radio resources. This is in addition to a cell- combination-information-based criterion. Such selection criteria can include: information on previously selected cells, information broadcasted by an eNB, the available communication resources available in the cell or combination of cells, the PLMN of the cell, and the frequency layer information of the PLMNs available for section by the UE.

A fourth embodiment uses location information additionally alternatively to the selection criteria in the third embodiment. The local information can be obtained by and in the UE, and could be used to restrict cells considered in the selection procedure. This can save the UE battery power in assessing other cells. The general LTE procedure for all embodiments is shown in figure 9.

In step S30 the UE is in idle mode, possibly following PLMN selection. Hence the UE can make use of any stored information on the USIM regarding, for example, the priority of the PLMN and associated RAT. In step S32 the UE starts cell selection. In step S34 the UE employs a combination of stored information and a desired operational mode to determine cells and/or a carrier frequency to monitor. For example the stored information could have been provided previously or during PLMN selection and it relates to cell combination information. Thus, for the first embodiment referring to carrier aggregation, the cell combination information might be that the RRH shown is available to provide a secondary cell if the EnB is used as a primary cell. Further information may be other potential SCells. Additional information may be throughput availability for the SCells listed. This can be used to limit the number of cells for which broadcast information is monitored or even to narrow to a single cell for which broadcasting information is monitored. In step S36 the previously mentioned monitoring of broadcast information takes place. In step S38 the UE stores information that has been monitored. This allows its use in cell selection in a subsequent iteration. Finally in step S40 the UE uses the monitoring information to select a cell for initial access or to camp on.

Summary

Invention embodiments can provide a method in which a UE (secondary station) in a mobile communications system selects a cell from which to monitor broadcast messages, at least one criterion for the selection being based on the operation of the selected cell in combination with at least one other cell to communicate with the UE.

Information may be provided to the UE concerning operation of the selected cell in combination with at least one other cell.

The criterion can be linked to an estimate of the QoS which would be available if the selected cell and the at least one other cell became serving cells for the UE. The operation of the selected cell and the at least one other cell may be carrier aggregation or dual connectivity. The QoS could be the data rate which would be available if the selected cell became a primary serving cell and the at least one other cell became a secondary serving cell. For example, if the UE is connected to an eNB and is using or would like to use radio bearers to provide large data rates then the best cell to camp on would be the cell which is able to support the potential required data rates required by the UE. Other factors that may affect the available data rates include the current traffic loading of the cell and the available communication resources in the cell.

Also, the selection criterion may be pre-determined information. The selection criterion may be on broadcast information. The selection criterion may be on communication resources available in the cells. The selection criterion can include the PLMN. Hence, a UE can receive broadcast information which allows the UE to make a selection of a defined frequency layer (or some other criteria) of PLMNs already stored in the UE. Location information can be used to restrict cells considered in the cell selection procedure. Finally, a selection criterion can include a measurement on a cell exceeding a threshold.

Various modifications are possible within the scope of the present invention.

For convenience, the invention has been described with respect to specific cells. However, the invention can be applied without the necessity for cells, and may be described in terms of the communications between different stations (including base stations supporting cells, mobile stations (e.g. D2D), and other types of station such as relays, and to communication via Remote Radio Heads of base stations).

The invention is equally applicable to LTE FDD and TDD, and to mixed TDD/FDD implementations (i.e. not restricted to cells of the same FDD/TDD type). The principle can be applied to other communications systems such as UMTS. Accordingly, references in the claims to a "terminal" are intended to cover any kind of user device, subscriber station, mobile terminal and the like and are not restricted to the UE of LTE. In any of the aspects or embodiments of the invention described above, the various features may be implemented in hardware, or as software modules running on one or more processors. Features of one aspect may be applied to any of the other aspects.

The invention also provides a computer program or a computer program product for carrying out any of the methods described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein. A computer program embodying the invention may be stored on a computer-readable medium, or it may, for example, be in the form of a signal such as a downloadable data signal provided from an Internet website, or it may be in any other form.

It is to be clearly understood that various changes and/or modifications may be made to the particular embodiments just described without departing from the scope of the claims.

Claims

A method in a terminal of a wireless communications system, the wireless communications system comprising the terminal and base stations providing cells which can be selected to serve the terminal, the method comprising:

the terminal accessing cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and

the terminal applying one or more criteria to choose whether to monitor broadcast messages from the potential serving, the criteria including at least one criterion based on the cell combination information.

A method according to claim 1 , wherein the operation of the potential serving cell in combination with the at least one other cell to communicate with the terminal uses carrier aggregation and/or dual connectivity.

A method according to claim 1 or 2, wherein the at least one criterion is with respect to an estimate of the quality of service, QoS, which would be available if the potential serving cell and the other cell were both selected as serving cells.

A method according to claim 3, wherein the QoS includes the data rate which would be available if the potential serving cell became a primary serving cell and the at least one other cell became a secondary serving cell.

A method according to any of the preceding claims, wherein the cell combination information is stored in the terminal, and used in conjunction with a desired operational mode of the terminal to select cells from which to monitor broadcast information.

A method according to any of the preceding claims, wherein the method comprises the terminal accessing cell combination information about the operation of more than one potential serving cell; and the terminal applying one or more criteria to choose whether to monitor broadcast messages from each potential serving cell.

7. A method according to any preceding claim, wherein the broadcast messages include at least one reference signal from the potential serving cell(s), and the terminal measures the received signal power of the reference signal if it has chosen to monitor the broadcast messages.

8. A method according to any of the preceding claims, wherein the method takes place as part of cell selection and wherein the combination information is provided to the terminal during a network selection before cell selection, such as Public Land Mobile Network PLMN selection or during another procedure before cell selection.

9. A method according to any of the preceding claims, wherein the combination information includes one or more of the following lists of cells:

a list of one or more other cells that would be available as secondary cells if the potential serving cell became a primary cell, for example to support carrier aggregation;

a list of one or more other cells that could be in a secondary cell group, for example to support dual connectivity;

a list of one or more cells that could act as a primary cell for the potential serving cell;

a list of one or more cells that are likely to be suitable when cell reselection is required.

10. A method according to claim 9, wherein the one or more lists include cells which are dormant when the method takes place.

11. A method according to claim 9 or 10, wherein the one or more lists include operating information such as throughput and/or availability information for the cells in the list, preferably in combination with the potential serving cell.

12. A method according to any of the preceding claims, wherein the criteria further include one or more same cell criteria relating to parameters of the potential serving cell. 13. A method according to any of the preceding claims, wherein the same cell criteria are broadcast during a network selection, such as PLMN selection, before cell selection or during another procedure before cell selection.

14. A method according to claim 12 or 13, wherein the same cell criteria are not obtained from physical layer measurement.

15. A method according to claim 12, 13 or 14, wherein the same cell criteria include one or more of:

number of antennas

information relating to 3D MIMO

whether the cell is active/on/off

transmission modes supported by the cell

current traffic loading of the cell

available communication resources in the cell

the network to which the cell belongs

location of the cell.

16. A method according to any of the preceding claims, wherein terminal-specific information, such as information on previously selected cells and/or location of the terminal, is used in an additional criterion in choosing cells from which broadcast messages are monitored for cell selection.

17. A method according to any of the preceding claims, wherein cells are present on more than one carrier frequency and frequency layer information of the networks available for selection by the terminal is used in a further criterion for choosing cells from which broadcast messages are monitored for cell selection.

18. A method according to any of the preceding claims, wherein cells are present on more than one carrier frequency and the terminal decides to select a carrier frequency as a result of the combination information. 19. A terminal of a wireless communications system, the wireless communications system comprising the terminal and base stations providing cells which can be selected to serve the terminal, the terminal comprising:

a transmitter and a receiver arranged to communicate with the cells; and a controller arranged to control a cell selection procedure; wherein

the receiver is arranged to receives cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and

the controller is arranged to apply one or more criteria to choose whether to monitor broadcast messages from the potential serving cell, the criteria including at least one criterion based on the cell combination information.

20. A wireless communications system comprising a terminal and base stations providing cells which can be selected to serve the terminal;

the base stations comprising a transmitter and a receiver arranged to communicate with the terminal; and

the terminal comprising:

21. A method in a wireless communications system comprising a terminal and base stations providing cells which can be selected to serve the terminal; wherein the terminal receives cell combination information about the operation of a potential serving cell in combination with at least one other cell to communicate with the terminal; and applies one or more criteria to choose whether to monitor broadcast messages from the potential serving cell, the criteria including at least one criterion based on the cell combination information.