TR201902199T4 - Closed Subscriber Group Cell Handover - Google Patents

Abstract

A radio terminal connection may be passed from a serving base station associated with a serving cell to the CSG cell served by the CSG base station where only radio terminals belonging to the closed subscriber group (CSG) are allowed to access and receive service from the CSG cell. Radio terminals in the serving cell are generally informed that the signals sent by the CSG base station do not report the signal quality measurements to the serving base station. However, if the radio terminal in the serving cell is authorized to access and receive services from the CSG cell, the base station serving that radio terminal is instructed to report the signal quality measurements of the signals sent by the CSG base station.

Description

TECHNICAL FIELD The technical field relates to radio communications, and in particular, radio communications involving closed subscriber group radio base stations. BACKGROUND OF THE INVENTION In a typical radio communications system, radio communications terminals, referred to as radio terminals or user equipment terminals (UEs), communicate with other networks such as the Internet via a radio access network (RAN). A radio access network (RAN) covers a geographic area that is divided into cell areas, each cell served by a base station, e.g., a radio base station (RBS), also called a "NodeB" or an enhanced Node B "eNodeB" in some networks. Each base station typically serves several cells. A common installation is a 3-cell base station installation, where one base station serves three cells. A radio terminal is primarily served by a serving base station within a serving cell in which the radio terminal is located. In some technologies, communication links are established not only between a specific radio terminal and its serving cell, but also between the radio terminal and other cells. In this case, the terminal is served by multiple base stations using macrodiversity or soft handoff. A base station exchanges signals with radio terminals. The signals can be dedicated signals to and from specific radio terminals, multicast signals to a subset of radio terminals within a cell, or broadcast signals from the base station to all radio terminals within a cell. A base station broadcasts information to all radio terminals within a cell using the broadcast channel of the serving cell. Recently, small-scale base stations, sometimes called femtocells, that provide coverage for very small areas connected to broadband internet service have been introduced. Femtocells are similar to WiFi "hotspots," but are part of a cellular network rather than a wireless local area network (WLAN). While femtobase stations are much smaller and have lower output power, designed for small spaces like apartments, homes, offices, and so on, they are in many ways just another name for larger base stations, and in this case, "femto" simply means smaller. Femtobase stations provide better signal in small indoor or enclosed spaces where signal quality between conventional macro base stations and mobile phones is poor, either due to proximity to macro base station towers or simply due to building materials or other obstructions blocking the signal. Instead of using a traditional base station for access, the mobile phone uses the femtobase station to access the IP access network. Figure 1 shows an example of a cellular communications system that includes one small-scale base station and one traditional macro base station. A first building (1) contains a radio terminal (2) that receives radio signals from a macro base station (3). The macro base station (3) is connected to a core network (5) directly or through a radio access network (4). The core network (5) provides access to the Internet (6) and other networks. A second building (7) contains another radio terminal (8) that receives radio signals from a small-scale base station (9). The small-scale base station (9) may be connected to the core network (5) directly or through a radio access network (4), typically through some type of broadband access mechanism (wired or wireless). Here, the core network (5) provides access to the Internet (6) and other networks. Because a small-scale base station is located within building 7 and is typically intended to provide coverage within and around building 7, its transmit power can be significantly lower than that of a macro base station 3, which has a much larger and variable coverage area but still provides high data rate service. Small-scale base stations typically serve small cell sites where only a limited group of users can receive service from a small-scale base station. Typically, a customer can purchase a small-scale base station from a retail store and install it by connecting it to an electrical outlet and data network in a home, office, school, etc. These small-scale base stations are sometimes called home base stations, femto base stations, pico base stations, etc. A larger-scale macro base station can also provide access and service to a limited group of users, but a small-scale base station is the more typical example. For this reference, any base station that provides access and service only to a limited group of users is referred to as a closed subscriber group (CSG) base station. A CSG base station serves one or more CSG cells, which in turn allow access only to a limited group of radio terminals. A CSG cell can also be used to create networks with limited access, for example, to support enterprise networks. A base station that is not closed or limited in this manner is referred to as an "open" base station. Current cellular radio systems include, for example, the Third Generation (3G) Universal Mobile Telecommunications System (UMTS), which operates using Wide-band Code Division Multiple Access (WCDMA), and fourth generation (4G) systems, such as Long Term Evolution (LTE), which operates using Orthogonal Frequency Division Multiple Access (OFDMA) in UMTS. A key focus of the LTE and System Architecture Evolution (SAE) standardization effort is ensuring that the new network is easy to deploy and cost-effective to operate. The outlook is that the new system will be self-optimizing and self-configuring in as many ways as possible. One such aspect is the automatic addition and management of closed subscriber group cells (CSG cells) on the same frequency band as the traditional macrocell layer, with virtually no configuration required. The deployment of a CSG base station can be motivated by various expectations and presents various challenges. For example, an end-user connected to their CSG base station expects to receive the same services as a macrobase station while achieving a higher price/performance ratio compared to a macronetwork. The user may also expect higher data rates and better quality of service compared to a macrocell. Another expectation is a simple and essentially automated CSG base station setup procedure that supports handovers to and from the CSG base station for authorized users without extensive manual configuration. One problem arises from a CSG base station whose call handling capabilities are inferior to those of a macro base station and, therefore, may be less adept at performing the acceptance and authorization functions. In this case, multiple unauthorized handover requests to the CSG base station and subsequent handover rejections will degrade the performance of the CSG base station. Another problem is the desire to easily add new users authorized to use the CSG base station. Furthermore, it would be beneficial for operators to sell a CSG base station that authorizes all users with a specific subscription with the same operator to use any or a subset of the CSG base stations. Handovers in LTE and WCDMA are mobile-aided, meaning that the radio terminal reports to its own serving base station measurements of the signal quality (prediction, signal strength) received by the serving base station and other cell alternatives associated with the "candidate" base stations for handover. Other cell alternatives are identified using cell identifiers. Non-exhaustive examples of such physical identifiers include the cell's downlink encryption code number in WCDMA and the physical cell identifier in LTE, which identifies a reference signal waveform for the cell. This allows the radio terminal to report to another cell served by the same base station (assuming the base station serves more than one cell) or to another cell served by a different base station. A cell association list is a list associated with a cell that lists suitable (e.g., neighboring) candidate cells for handover. For each candidate cell, the list information includes: cell identifiers, including both physical cell identities and generic-unique cell identities; connectivity information, e.g., how a communication link can be established between the serving base station and the base station serving the candidate cell; and the cell type, e.g., CSG cell, macrocell, microcell, etc. The neighbor cell affiliation list may be stored at the base station, but it may also be stored on other nodes, possibly with regular updates/synchronization of the lists at each node. A radio terminal's measurement report transmission is typically triggered, that is, event-triggered, periodic, or periodic. For event-triggered reporting, the radio terminal sends a report to the serving base station when a configured criterion is met. An example of such a criterion is measuring the quality of a new cell during a predetermined time within a predetermined interval between the serving cell and a predetermined difference. A predetermined time and offset are provided to the radio terminal by the serving base station. Another example of this type of criterion is when a cell within a range between the previously serving cell and a predetermined offset moves out of range for a predetermined period. Event-triggered reporting results in a measurement report upon triggering an event. For event-triggered periodic reporting, the radio terminal continues to send measurement reports periodically after an event is triggered, for a predetermined number of periods, or until another trigger condition is met. For periodic reporting, the radio terminal reports measurements regularly. This was the case for second-generation systems such as GSM. The radio terminal typically considers all cells when determining whether a triggering event has been met. On the other hand, it is preferable for the radio terminal to ignore and not report alternative cells that have been declared access-denied. One way to notify various cells of their restricted status is for the serving base station to broadcast a cell blacklist over the broadcast channel (i.e., the serving cell broadcasts a blacklist of cell identifiers). Another way is to report to the radio terminal only those cells on a "whitelist" broadcast by the serving base station over the broadcast channel of the serving cell. A whitelist can be viewed as a reverse blacklist. If there are many cells on the blacklist, it may be more efficient to send the whitelist, or vice versa. A third way is for each base station serving a restricted cell to broadcast a message over the broadcast channel of the restricted cell, indicating that access to the restricted cell is prohibited, and that all radio terminals must check this notification before reporting to a cell. Examples of such notifications are "Forbidden Cell," "Restricted Cell," or "CSG Cell." Similarly, there is the potential for significant unnecessary signal processing overhead for the CSG base station. If a CSG base station is set up as a typical "open" base station, the CSG base station will have to handle numerous handover requests from unauthorized radio terminals that are not part of the CSG and will consequently have to reject these requests. This creates a significant and unnecessary processing overhead for the CSG base station. An alternative is to assign a CSG cell a Public Land Mobile Network (PLMN) identity, a PLMN identity that differs from the PLMN identity of the macrocells. In this case, radio terminals accessing the CSG cell will have both the CSG cell and macrocell PLMN identifiers listed in the allowed PLMN group, while other radio terminals will only have the PLMN identity of the macrocell. Consequently, radio terminals accessing the CSG cell will be able to handover to that cell. Since assigning unique PLMN identifiers to each CSG cell is not feasible, one approach is to assign the same PLMN identifier to all CSG cells. This means that if a radio terminal has access to one CSG cell, it will also have access to all other CSG cells associated with the same operator. However, this approach does not support a CSG cell with a more limited user group, such as all members of a family or all employees in a company. In areas with many CSG cells, radio terminals may have to consider multiple CSG cell alternatives, with the possibility that the radio terminal of another CSG cell might be a better candidate than its "own" CSG cell, i.e., a CSG specifically established to serve the radio terminal and other radio terminals within this CSG. In this case, the radio terminal's own CSG cell may not even be listed in the signal quality measurement report at the radio terminal, which is contrary to the main purpose of the CSG installation involving the radio terminal. Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); V8.1.0 describes the mobility and general access control requirements associated with CSG cells. VODAFONE GROUP, "Handover to CSG cells", 3GPP DRAFT; R2-072827, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE ; 650, ROUTE DES LUCIOLES ; F-06921 SOPHIA-ANTIPOLIS CEDEX ; FRANCE, (20070702), volume RAN WG2, no. Orlando, USA, it describes that a UE context provided by an MME may include a list of CSG cells for the UE. VODAFONE GROUP, "Triggering of measurements in LTE_ACTIVE for CSG cells", 3GPP DRAFT; R, MOBILE COMPETENCE CENTRE ; 650, ROUTE DES LUCIOLES ; F-0692l SOPHIA-ANTIPOLIS CEDEX ; FRANCE, it describes that CSG information in the UE context can be used to trigger the generation of measurement events by the UE. NOKIA ET AL., "Access control for CSG cells", 3GPP DRAFT; R2-072404 CSG CELLS, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 650, ROUTE DES LUCIOLES ; F-06921 SOPHIA-ANTIPOLIS CEDEX; FRANCE, (20070622), volume RAN WG2, no. Orlando, USA, describes a method for managing access control for home eNodeB and other types of CSG cell installations in E-UTRAN. The Tracking Domain ID is proposed as the identifier used to grant access to CSG cells for UEs in a specific User Group. A radio terminal connection from a serving base station allows only radio terminals belonging to a closed subscriber group (CSG) to access and receive service from the CSG cell. A CSG can be handovered to the serving CSG cell by a given CSG base station. Radio terminals in the serving cell are generally instructed not to report signal quality measurements of signals sent by the CSG base station to the serving base station. If a radio terminal in the serving cell is determined to be authorized to access and receive service from the CSG cell, the serving base station also sends a message instructing the radio terminal to consider the CSG cell when evaluating whether to trigger reporting of signal quality measurement reports from candidate cells to the serving base station. When reporting is triggered on the CSG cell (the radio terminal determines that the CSG cell is a handover candidate), the serving base station receives a signal quality measurement report for the CSG cell from that radio terminal. Based on the received and possibly subsequent reports, a decision can be made that a connection with this radio terminal can be handed over to the CSG base station. Because unauthorized radio terminals do not report measurements for CSG cells, the handover mechanism at the serving base station can apply the same handover mechanism to all reported cells without the risk of a radio terminal initiating a handover to a cell to which it has no access. In a non-limiting example, information instructing radio terminals not to report signal quality measurements of signals sent by the CSG base station to the serving base station could be published in the serving cell and could include a list of cells to which the information is not to be reported or a list of cells to which only the information is required to be reported. If changes are made to the list, a modified list can be published in the serving cell. In a non-restrictive example, a radio terminal may be informed not to report signal quality measurements of signals sent by the CSG base station to the serving base station using a first message indicating that the CSG cell has limited access to a CSG. Examples of such notifications are a forbidden cell, a restricted cell, or a CSG cell. In a non-restrictive example, a radio terminal accessing a CSG cell that has an established neighbor cell relationship with the radio terminal's serving cell may be informed by the serving base station using a second message indicating that the radio terminal must also include signal quality measurements of signals sent by the CSG base station in its measurement reports to the serving base station. For example, a second message from the serving base station may provide the radio terminal with a dedicated list of CSG cells indicating cells from which information is not to be reported, excluding all CSG cells to which the radio terminal has access, or a list of cells to which information must be reported, including only all CSG cells to which the radio terminal has access. In another non-limiting example, a second message from the serving base station may provide the radio terminal with a list of GSC cells that the radio terminal should consider in event-triggered measurement reporting, even if the listed CSG cells indicate that they are prohibited from access. Alternatively, the list of CSG cells may be a list of cell identifiers or a range of cell identifiers. The sending phase may involve sending a second message to the radio terminal indicating that the radio terminal should consider signal quality measurements of signals sent by the CSG base station in event-triggered measurement reporting. The first message may be sent by the serving base station or the CSG base station. In a non-limiting example, the determination phase may include receiving an authorization list of CSG cell(s) associated with the radio terminal from a central node connected to the serving base station, indicating that the radio terminal is authorized to access and receive service from the listed CSG cell(s), and comparing the listed CSG cell(s) with the CSG cell(s) in the serving cell's neighbor cell association list. In another non-limiting example, the determination phase may include a filtering phase at the central node connected to the serving base station. The central node sends a filtered authorization list of CSG cells associated with the radio terminal. The radio terminal is authorized to access and receive service from the listed CSG cell(s). All listed CSG cells are members of the serving cell's neighbor cell list. In a non-limiting example, the filtering step might involve sending the serving cell's neighbor cell association list and this radio terminal ID to the central node, comparing the radio terminal IDs in the authorization lists of all CSG cells in the serving cell's neighbor cell association list with a radio terminal ID, and generating a list of CSG cells to which the radio terminal has access and can receive service. In another non-limiting example, the filtering step might involve a central node maintaining a copy of the neighbor cell association list of all cells served by all base stations connected to the central node and an authorization list of radio terminals for each CSG cell. The central node receives a radio terminal ID from the serving base station, compares the radio terminal IDs in the authorization lists of all CSG cells in the serving cell's association list with this radio terminal ID, and generates a list of CSG cells to which the radio terminal has access and service. The CSG cell may be part of a CSG network consisting of CSG cells to which the radio terminal is authorized to access and request service. Each CSG network cell has an associated cell identifier. In such a case, the determination phase involves receiving a range of CSG cell identifiers from a central node connected to the serving base station. In a non-limiting example, the determination phase may involve receiving an authorization list of CSG cell(s) associated with the radio terminal from the previous serving base station, indicating that the radio terminal is authorized to access and receive service from the listed CSG cell(s). The radio terminal can then compare the listed CSG cell(s) with the CSG cell(s) in the serving cell's neighbor cell association list to determine whether any of the authorized CSG cell(s) are potential handover candidates. If the radio terminal's connection is handover from a first cell served by a first base station to a second cell served by a second base station, the first base station can send the authorization list of the CSG cell(s) to the second target base station. In a non-limiting example, the central node could be a database containing subscriber and subscription information, such as a Home Subscriber Server (HSS), and the authorization list of CSG cells could be retrieved from the HSS upon network contact. Other examples of subscription information databases include a home location record (HLR), a visitor location record (VLR), and a gateway location record (GLR). The term HSS will be used as a representation of a subscriber and subscription information database. During subsequent handovers to other base stations, the authorization list of CSG cells is transmitted to the destination base station. A radio terminal is provided to communicate with a serving base station serving a cell, where the radio terminal belongs to a CSG cell served by a closed subscriber group (CSG) base station. In this case, only radio terminals belonging to the CSG are allowed to access and receive service from the CSG cell. The radio terminal contains radio transceiver circuitry and a data processing system electronic circuitry connected to the radio transceiver circuitry. The circuitry obtains a measure of the quality of the signals sent by neighboring base stations and reports the signal quality measurements to the macro base station for possible handover. The terminal first receives the first piece of information, which instructs the radio terminal not to report signal quality measurements of signals sent by CSG base stations to the macro base station. It then receives the second piece of information, which overrides the first piece of information, which instructs the radio terminal to report signal quality measurements of signals sent by the CSG base station to the macro base station. The circuit determines the signal quality of a signal sent by the CSG base station and reports the CSG base station signal quality measurement to the macro base station. In a non-limiting example, when the radio terminal receives instructions to switch to the CSG base station, the electronic circuit instructs the radio transceiver circuit to establish communication with the CSG base station. In a non-limiting example, the second piece of information includes a range of CSG cell identifiers. The electronic circuit measures the signal quality of a signal received by any CSG base station with a cell identifier within the range and considers these CSG base station signal quality measurements in triggering the measurement report. In a non-limiting example, if a radio terminal cannot receive or maintain service from a serving base station, the electronic circuitry can scan the signal quality of a signal received by any nearby CSG base station and select one of these CSG base stations to provide service. An authorized CSG base station is preferentially assigned a higher selection priority than an unauthorized CSG base station. The central node contains an interface for communicating with base stations in general, including closed subscriber group (CSG) base stations, and a memory for storing a CSG authorization list that identifies radio terminals authorized to access and receive service from CSG cells. A controller in the node sends the CSG authorization lists to a serving station serving a radio terminal in the cell served by a base station when the CSG authorization list contains a CSG cell that is a neighboring cell to the serving cell and to which the radio terminal is authorized to access. Otherwise, the controller does not send the CSG authorization list to the serving base station. The controller can allow changes to the CSG authorization list. The memory can also store a neighbor cell relationship (NCR) list for the serving cell, and the controller can use the NCR list to determine whether the CSG cell is a neighbor cell. The controller can obtain the radio terminal ID from the serving base station. The controller can also obtain the NCR list from the serving base station. In a non-limiting example application, the CSG cell is part of a CSG network consisting of CSG cells to which the radio terminal is authorized to access and request service. Each CSG network cell has an associated cell identifier. Instead of determining a list of CSG identifiers, the controller determines a range of CSG cell identifiers for the CSG network and sends the CSG cell identifier range to the serving base station. In another non-restrictive example embodiment, the controller determines a CSG monitoring area associated with one or more CSG cells that the radio terminal is authorized to access. When the central node is notified that the radio terminal has accessed the serving cell, the controller checks whether the CSG monitoring area corresponds to a monitoring area of the serving cell and, if so, notifies the serving base station of one or more CSG cells. In another non-restrictive example embodiment, the controller determines a CSG PLMN ID associated with one or more CSG cells that the radio terminal is authorized to access. When the central node is notified that the radio terminal has accessed the macrocell, the controller checks whether the CSG PLMN ID corresponds to a corresponding PLMN ID associated with the macrocell and, if so, notifies the macro base station of one or more CSG cells. Although embodiments have been summarized above, the claimed embodiments are defined by appended claims 1 to 14. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a macro base station and a small-scale base station in a cellular communications system; Figure 2 shows a cellular communications system including macro and CSG cells and base stations; Figure 3 is a functional block diagram of an exemplary LTE mobile radio communications system including macro and CSG base stations; Figure 4 is a non-limiting exemplary functional block diagram of a base station; Figure 5 is a non-limiting exemplary functional block diagram of a radio terminal; Figure 6 is a non-limiting exemplary signaling diagram of a central node that stores cell lists; Figure 7 is a signaling diagram showing non-limiting exemplary signaling procedures for CSG cell management to support handover to CSG base stations; and Figure 8 shows the example in Figure 7 in a different way. DETAILED DESCRIPTION In the following description, specific details such as specific nodes, functional elements, techniques, protocols, standards, etc. are provided for illustrative, but not exhaustive, purposes to facilitate understanding of the technology described. In some cases, to avoid cluttering the description with unnecessary detail, detailed descriptions of well-known procedures, devices, techniques, etc. are not provided. The figures show individual function blocks. As will be appreciated by those skilled in the art, the functions of these blocks can be implemented using discrete hardware circuits, using software programs and data in conjunction with a suitably programmed microprocessor or general-purpose computer, using an application-specific integrated circuit (ASIC), using programmable logic arrays, and/or using one or more digital signal processors (DSPs). Figure 2 conceptually illustrates a cellular communications system containing macro and CSG cells and base stations. This simplified example shows two adjacent macro cells (MI and M2) and corresponding macro base stations (MAC BS1 and MAC BSZ). Within macro cell (M1), there is a second CSG cell (2) with an associated CSG base station (CSG BSZ) adjacent to it. The broadcast radio signals, designated R1-R4, arriving at a UE/radio terminal from each of these base stations are shown. It is assumed that Macro BS1 is the base station currently serving the radio terminal and that the radio terminal is capable of sending received signal quality measurement reports to the base station (Macro BS1) serving Macro BS2, CSG BS1 and CSG BSZ. If the radio terminal approaches CSG cell (1) or CSG cell (2), an opportunity arises for handover to one of these CSG cells, depending on whether the radio terminal is a member of the CSG for this cell. The technology in this application is well suited for an LTE system and therefore It is described in this context to provide an example and non-limiting context for illustration. However, this technology can be used in any modem cellular communication system and is not limited to LTE, e.g., WCDMA, etc. Figure 3 shows an example of an LTE type mobile communication system 10. An E-UTRAN 12 includes E-UTRAN NodeBs (eNodeBs) 18 (only one shown) that provide E-UTRA user plane and control plane protocol terminations towards a user equipment (UE) terminal 20 via a radio interface. Also shown are two CSG base stations 19. The base stations 18 and 19 include an Evolved Packet Core (EPC) 14 and a System Architecture Evolution (SAE) module that includes a Mobility Management Element (MME) via an S1 interface. It communicates with the gateway. The MME/SAE gateway is shown as a single node (22) in this example and is similar in many ways to an SGSN/GGSN gateway in UMTS and GSM/EDGE. However, the MME and SAE can be separate nodes. MME/SAE gateways can communicate via an 810 interface. The MME/SAE may include a memory (23) to store cell lists for base stations, including, for example, neighbor cell association (NCR) lists, black and/or white cell lists, etc. The E-UTRAN (12) and the EPC (14) together form a Public Area Mobile Network (PLMN). The MMEs/SAE Gateways 22 are connected directly or indirectly to the Internet 16 and other networks. Base stations 18 and 19 can communicate with the Home Subscriber Server (HSS) 24, possibly via the MME/SAE 22. Radio networks generally use globally unique cell identifiers, as well as non-unique physical identifiers of base stations/cells, to support data measurement procedures. For the LTE example arrangement described, there is a unique identifier for each cell, referred to as a Cell Global Identifier (CGI), and a physical layer 1 identifier for the same cell, referred to as a Physical Cell Identity (PCI), which is not long enough to be unique for each cell in the PLMN. In LTE, the current example assumption is that there are 168 cell group identifiers and 504 unique PCI values, each consisting of three identifiers within each group. For received signal quality measurements, radio terminals measure the base station's reference symbol received power (RSRP) by monitoring the received base station's broadcast reference symbols. These metrics are used during initial cell selection and when performing handoffs. Therefore, from each radio terminal's perspective, it is important that there is a local one-to-one correspondence between the PCIs and CGIs the radio terminal can detect. This means that when a radio terminal reports a PCI to a serving cell base station, if a cell association is established, the serving base station can associate the reported PCI with an entry in the serving cell's neighbor cell association list. In a WCDMA system, a PCI may correspond to an "encryption code," where in one example, there may be 512 different encryption codes. The systems shown in Figures 2 and 3, and other similar types of systems, describe technology that reduces the resources associated with radio terminals searching their own CSG cells and limits unnecessary CSG cell measurements and reporting unless the radio terminal is authorized. This improves overall network performance. Radio terminals are supported during handover from a BS to a CSG cell, while also greatly reducing the negative impact of all radio terminals in the network performing measurements and reporting on unauthorized CSG cells. In a non-restrictive example embodiment, all CSG base stations broadcast a notification over the CSG cell's broadcast channel indicating that a CSG cell has restricted access to a CSG. Examples of such notifications are a prohibited cell, a restricted cell, or a CSG cell. The notification implements a general rule that radio terminals should not report the signal quality of broadcast signals received from CSG cells that include such notification on the broadcast channel unless specific commands are sent from the serving base station instructing the radio terminal to consider signal quality measurements associated with a particular CSG cell in triggering a measurement report for the radio terminal. In another non-limiting example embodiment, the serving base station broadcasts a "blacklist" on the broadcast channel of the serving cell containing the physical cell identities (e.g., PCIs in LTE) of all CSG cells registered as neighboring cells to the serving cell. This prevents the reporting of measurements from CSG cells by unauthorized radio terminals. Other techniques for transmitting a similar type of message to the radio terminal are also possible. The serving base station may, if desired, send a message to one or more radio terminals informing them that they must consider measurements related to a specific CSG cell when triggering a measurement report, even if the CSG base station serving a CSG cell broadcasts a notification over the CSG cell's broadcast channel that the CSG cell has restricted access to the CSG or that the CSG cell is on a "blacklist" broadcast by the serving base station over the serving cell's broadcast channel. Examples of such notifications include a forbidden cell, a restricted cell, or a CSG cell. A "blacklist" can also be sent to individual radio terminals on a dedicated basis. This forbidden cell information used in blacklists can be obtained by the base station when a radio terminal connects to a cell served by the base station, e.g., after a successful transition to the cell or after the radio terminal registers to the cell. At this point, the serving base station checks with a central node (e.g., a database of subscriber and subscription information, such as an MME in LTE or an SGSN or HSS in WCDMA) whether the radio terminal is authorized to access any CSG cells that are neighbors of the macro base station. The central node sends a list of these neighboring CSG cells to the serving base station. The serving base station then sends an instruction to the radio terminal that, if access to one or more CSG cells on the serving base station's neighbor cell association (NCR) list is authorized, the serving base station requests that the radio terminal consider these listed and authorized CSG cells when triggering a measurement report, even if the CSG base stations serving these CSG cells broadcast a notification over the broadcast channel indicating that these CSG cells have restricted access to a CSG or are on a "blacklist." The serving base station can update the blacklist by sending a new "blacklist" to the radio terminal, replacing any previous blacklists. This way, only authorized radio terminals will consider CSG cells as "candidate cells" when triggering a measurement report. Unauthorized radio terminals will ignore CSG cells when triggering a measurement report. The forbidden cell information used in the blacklists can also be obtained by the base station from a central node such as the HSS during radio terminal registration. This forbidden cell information can then be passed to the next target base station during handover, e.g., using a message "bearer" that transfers information associated with the radio terminal from one serving base station to the next target base station during handover. Non-limiting example methods for compiling neighbor cell association lists are described in the US application "Self Configuring and Optimization of Cell Neighbors in Wireless Telecommunications Networks" filed on 28 February 2007 and PCT filed on 28 February 2008 in the name of the same applicant. The association information between a CSG cell and authorized radio terminals stored in the central node can be accessed and modified by the operator, the CSG cell owner, etc., through a user interface. The serving base station preferably processes the radio terminal measurement reports and performs handover procedures regardless of whether they belong to a CSG cell or to another macrocell. However, handover procedures adapted to the target cell type, such as a CSG cell, may be used. When a radio terminal performs cell selection without macrocell coverage, it may consider cell connection candidates as restricted, prohibited, or blacklisted cells, unlike the general rule when macrocell coverage is available. The radio terminal may also monitor its own CSG cell identifier value (preliminary PCI) to first attempt to connect to its own CSG cell. If this fails, the radio terminal attempts to connect to other CSG cells it can detect, as the radio terminal may be authorized for other CSG cells of which it is unaware. In the LTE context, the existing Non-Access Layer (NAS) protocol between a core network node and the radio terminal can be extended to inform the radio terminal of the current PCI values of the CSG cells to which it is authorized to connect. Another example alternative is to send a short message service (SMS) or a multimedia message service (MMS) message or the like to the radio terminal containing this information. Figure 4 is a non-limiting example functional block diagram of a base station, which may be an open base station 18 or a CSG base station 19. The base station 18, 19 includes one or more radio transceivers 32 connected to one or more antennas 30 for communicating with radio terminals via the radio interface. The radio transceiver(s) 32 are connected to a data processing system 34, which includes a baseband processor 36 that performs baseband operations on the information to be sent via the radio transceiver(s) 32 and on the signals received from the radio transceiver(s) 32. The data processing system 34 also includes a cell reporting processor 38 which receives radio signal quality measurements from the transceiver(s) 32 and probable scanned broadcast information from the radio terminals receiving service, as well as from the radio terminals initiating access in a cell selection procedure for use in handover and cell selection operations, as well as in generating communication cell association lists. A neighboring cell association list is stored in a memory 40. The base station communicates with other network elements through one or more interfaces 42. Radio terminals are authorized based on signals sent through the interface 42. Furthermore, the access of a new neighboring cell is followed by communication establishment through the interface 42 to determine connectivity information to the base station serving the new cell, as well as other information about the cell, such as the cell type. Once a candidate cell is reported, the cell reporting processor 38 can continue monitoring the candidate cell by receiving measurements from the transceiver 32. The measurements associated with the candidate cell are sent to a handoff processor 43, which determines if and when a handoff is considered beneficial. Once the handoff processor 43 determines to proceed with a handoff, the handoff procedure is performed by using the transceiver 32 to notify the radio terminal through one of the interfaces 42 to the next serving base station and through the interfaces 42 to a core network node. Figure 5 is a non-limiting exemplary functional block diagram of a radio terminal. The radio terminal 20 includes a radio transceiver 60 connected to one or more antennas 58 for communicating with base stations over the radio interface. The radio transceiver 60 is connected to a data processing system 62, which includes a baseband processor 64 that performs baseband processing on the information to be sent via the radio transceiver 60 and on the signals received from the radio transceiver 60. The data processing system 62 also includes a radio signal measurement processor 66, which controls the transceiver 60 to measure and possibly filter the radio signal quality and possibly scan broadcast signals from neighboring base stations, including CSG base stations, and the information contained in these signals, and which triggers a measurement report by evaluating criteria configured by the serving base station. When a trigger condition is met, the measurement processor sends measurement reports to a serving base station. As explained above, in triggering the measurement report, the measurement processor 66 considers all cells except CSG cells, which are limited and which the serving base station has not specifically instructed the radio terminal to consider, despite being limited. The radio terminal 20 also includes a user interface to allow a user to program the radio terminal and communicate with the radio terminal. Figure 6 is a non-limiting exemplary functional block diagram of a central node 70. The central node 70 includes a controller 78 to perform monitoring, data processing, and control functions and communicates with other network nodes through a suitable interface 76. A memory, such as a CSG cell radio terminal authorization list 74, and optionally a database for storing neighbor cell association lists 72 and other lists, is provided for various cells in the networks. The CSG cell radio terminal authorization list 74 is updated via interface 76 by synchronizing the included list with information at another central node. A radio terminal's identifier (e.g., IMSI) can be added/modified/removed from the CSG cell-radio terminal authorization list 74 by the network operator or by the owner of the CSG base station. This can be done, for example, through the radio terminal operator web interface. A cell's neighbor cell association list can be synchronized with the base station serving a cell. Figure 7 is a signal diagram showing an example non-restrictive signaling procedure for CSG cell management to support handover to a CSG base station, assuming an example non-restrictive LTE architecture. Figure 9 helps illustrate the example in Figure 7. The central node (for LTE, this could be the MME) sends an Authorized_CSGcell_list message to the serving base station, informing the serving base station of authorized CSG cells that a particular radio terminal is served by that base station. This signal is designated as 'A' in the example in Figure 8. The serving base station sends a measurement control signal to the dedicated radio terminal, for example, a Forbidden/Restricted/CSG Cell Report containing a list or range of cell identifiers of authorized CSG cells, instructing the radio terminal to consider the group of CSG cells when triggering a measurement report, even if they are prohibited, restricted, or included on a blacklist broadcast globally by the serving base station over the serving cell's broadcast channel. This signal is designated as 'B' in the example in Figure 9. The radio terminal complies with this instruction and, while receiving service from the same base station and if the condition for triggering a measurement report containing any of the listed CSG cells is met, the radio terminal sends a measurement report (Meas_report CSG cell) to the serving base station containing the cell ID of the triggering cell. The measurement report triggering mechanism itself is the same for all cells, but only authorized cells are considered in the measurement report triggering mechanism. All handover candidate cells considered when triggering a measurement report are those to which the radio terminal is authorized to access and receive service. If the serving base station, possibly after receiving subsequent measurement reports related to the triggering cell, decides that the radio terminal's connection should be handed over to the reported CSG cell, the serving base station sends a Handover_Request message to the CSG base station locally via 'C' in Figure 8, possibly via multiple interfaces and nodes (in the LTE architecture, via the central node/MME) shown in Figure 8. The MME/CN checks its CSG cell radio terminal authorization list (74) and determines whether the radio terminal is an authorized member of the CSG for this cell. In the example in Figure 8, a handoverAuthorization_confirmation message is sent to the CSG base station via 'D' and possibly multiple interfaces and nodes, as shown in Figure 8, and the handover to the CSG cell is performed. Additional problems may exist in large closed subscriber group networks. Consider a radio terminal authorized to access a large CSG network consisting of many CSG cells. Each time a radio terminal performs a handover to a new serving base station, the serving base station receives from the central node/MME/HSS a list containing the cell identifiers (e.g., CGIs) of all CSG cells to which the radio terminal is authorized. This list can be quite long, making communication between the serving base station and the MME costly in terms of communication resources. To address this problem, a CSG network can use a defined range of cell identifiers (e.g., CGI or PCI). All cells with a cell ID (e.g., CGI or PCI) within the range are assumed to belong to the CSG network. For the MME to inform the serving macro base station that a radio terminal is authorized in all CSGs belonging to a particular network, it only needs to report the range of cell IDs (e.g., CGI or PCI) in the CSG network. To determine whether a corresponding radio terminal is authorized in a CSG cell, the serving base station can then only check whether the neighboring CSG cell has a cell ID (e.g., CGI or PCI) that is within range. This limits the required signaling amount between the central node/MME and the serving base station. If desired, the operator can charge the CSG network owner for the service of providing the cell ID range. Private users or small companies can use a small range of cell IDs, which is cost-effective, while larger companies or organizations can use a larger range of cell IDs, for which the operator can charge a higher fee. This service is valuable because the CSG owner does not need to explicitly authorize every radio terminal in every CSG cell. When a radio terminal is authorized to access a CSG network, it automatically becomes authorized in every cell belonging to the CSG network. Furthermore, instead of separate measurement control signals directed between the serving base station and individual radio terminals, the cell ID range can be used if desired. Radio terminals can simply be requested to consider all cells belonging to the cell ID range when triggering a measurement report, even if the cells contain one or more restricted/prohibited/CSG cells or are on the "blacklist" of the serving cell. This approach is particularly useful when the number of CSG cells is large and they belong to the same operator. Additionally, the cell ID range can be assigned to a dedicated radio terminal, replacing the blacklist broadcast by the serving base station over the broadcast channel of the serving cell. In the event of a radio terminal migration, information can be exchanged between the new serving cell and a central node/MME/HSS. This information contains the list of CSG cells to which the radio terminal has access. For most cells in the network, the CSG cell-to-radio terminal authorization information is not important, as these cells are not adjacent to any cells in the CSG cell authorization list. In this case, CSG cell authorization information is not required at the serving base station. Consider the example where the radio terminal owner has a CSG cell installed at their home and the user is visiting another city. In this case, the cell to which the radio terminal is switching is not a neighbor of the radio terminal's home CSG cell. Therefore, it is preferable to filter the CSG authorization list sent from the central node/MME to the connected serving base station to include only CSG cells that also belong to the neighboring cell of the serving cell. One example and preferred way to achieve this is to store the NCR lists of all cells served by base stations connected to a given central node/MME. In this case, the central node can compare the neighbor cell association list of the serving cell with the CSG cell-to-radio terminal authorization list for the radio terminal in question. This synchronization of the neighbor list association lists between the central node/MME and the base stations generates relatively little signaling during handovers. Another way to achieve this is to store a base station's NCR list only at the base station and not at the central node/MME, and to send the neighbor cell association list to the central node/MME after each handover. Filtering is also performed at the central node. This approach requires significant signaling from the new serving cell and the central node/MME. Several other solutions exist for filtering the CSG authorization list sent between the central node/MME and the macro base station. The CSG authorization list can be filtered based on monitoring fields. The central node/MME knows the monitoring area of the CSG cells to which the radio terminal is authorized to access. When a new radio terminal is requested to join a base station's cell, the MME checks whether any authorized CSG cells belong to the same monitoring area as the serving base station. Only authorized CSG cells that also belong to the same monitoring area as the serving cell are included in the CSG authorization list sent by the central node/MME to the serving base station. The CSG authorization list can also be filtered by PLMN ID. The MME knows the PLMN ID of the CSG cell(s) to which a radio terminal is authorized to access. When a new radio terminal is requested to join a base station's cell, the MME checks whether any authorized CSG cells belong to a PLMN whose PLMN ID matches the serving base station's PLMN ID. The term "compatible" is used to cover situations where an operator has several different PLMN identities, or where several operators with different PLMN identities coexist in a network. Only authorized CSG cells that also belong to a compatible PLMN with the serving base station are included in the CSG authorization list sent by the central node/MME to the serving base station. During a radio terminal handover, information may be exchanged between the old serving cell and the new serving cell. This information may include the authorization list of the CSG cells to which the radio terminal has access. With this type of signaling support, only the base station with which the radio terminal originally registered needs to contact the central node, e.g., the HSS or the visitor location register (VLR), and obtain the authorization list of the CSG cell(s) to which the radio terminal has access. This authorization list of the CSG cell(s) to which the radio terminal has access is then transmitted to the next serving base station as part of the radio terminal handover process. The advantages of the technology described above include supporting flexible CSG management, where radio terminals can easily be added to and removed from each CSG cell's authorization list, and supporting low-cost handover from a general cell to a CSG cell. Furthermore, the use of cell ID ranges allows the solution to scale across CSG cell networks, and when using blacklists, the effort involved in determining whether a cell is denied access is reduced. While various arrangements are illustrated and explained in detail, the requirements are not limited to any specific arrangement or example. No part of the foregoing description is intended to imply that any particular element, stage, interval, or function is so essential that it is required to be included within the scope of the claims. The scope of patent subject matter is defined solely by the claims. The scope of legal protection is defined by the words used in the permitted claims and their equivalents.

Claims (1)

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