# Patent

## Title
Operation of base station in a cellular communications network

## Abstract

In one embodiment, a method of forming a neighbor cell list in a base station of a cellular communications network comprises, at periodic intervals, and for specified durations: requesting the or each wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect; receiving reports from the or each wireless device that is in connected mode with the base station; and operating with a neighbor cell list compiled on the basis of the received reports.

## Background
TECHNICAL FIELD
The present disclosure relates generally to the operation of a base station in a cellular communications network.
BACKGROUND
In certain cellular communications networks, the network operator allocates a pool of cell identifiers, and each base station uses one of these identifiers in its transmissions. The number of available identifiers is less than the number of base stations, and so the identifier used by a base station will probably also be used by other base stations. Steps can therefore be taken to ensure that the identifier used by a base station will not also be used by other nearby base stations.
Each base station also compiles a list of its neighboring cells, to be used when a mobile device is preparing a handover from one cell to another. The presence of multiple nearby base stations using the same cell identifier means that it is not always possible for the base station to compile a neighbor cell list that accurately reflects the state of the network. A neighbor cell list may be complied based on measurement reports received from a wireless device, with the measurement reports containing information about particular base stations. The presence of two base stations using the same cell identifier may then mean that the received information is associated with the wrong base station.



BRIEF DESCRIPTION OF THE DRAWINGS
To provide a more complete understanding of the present disclosure and features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying figures, wherein like reference numerals represent like parts, in which:
FIG. 1 is a schematic illustration of a cellular communications network in accordance with an embodiment.
FIG. 2 is a schematic illustration of a base station in the cellular communications network of FIG. 1.
FIG. 3 is a flow chart, illustrating a method in accordance with one embodiment.
FIG. 4 is a flow chart, illustrating a method in accordance with a second embodiment.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
Overview
There is described in one example embodiment a method of forming a neighbor cell list in a base station of a cellular communications network. The method comprises, at periodic intervals, and for specified durations, requesting at least one wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect. Reports are received reports from the or each wireless device that is in connected mode with the base station, and the base station operates with a neighbor cell list compiled on the basis of the received reports.
In more particular embodiments, the method comprises creating the neighbor cell list on the basis of the received reports. In other embodiments, the method comprises forwarding information from the received reports to a core network node of the cellular communications network; and receiving the neighbor cell list complied on the basis of the received reports from the core network node. The periodic intervals and the specified durations may each be separately configurable.
The method may further comprise reconsidering an identifier used by the base station itself in response to the received reports, such that the identifier used by the base station is not in use by another base station meeting a proximity condition. The method may further comprise, in certain embodiments, reporting to a core network node of the cellular communications network instances where one of the wireless devices in connected mode reports that it can detect signals from another base station that is using a same physical cell identifier as said base station. The method may also comprise, in certain embodiment, reporting to a core network node of the cellular communications network instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.
In another example embodiment, there is provided a base station for a cellular communications network, comprising: a processor; and a memory, said memory containing instructions executable by the processor. The base station is operative to:
at periodic intervals, and for specified durations, request at least one wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect; receive reports from the or each wireless device that is in connected mode with the base station; and operate with a neighbor cell list compiled on the basis of the received reports.
EXAMPLE EMBODIMENTS
FIG. 1 illustrates a part of a cellular communications network, comprising multiple base stations 10, 12, 14, 16, 18, 20, 22. Each of these base stations has a connection to a core network 24 of the cellular communications network. The connections between the base stations 10, 12, 14, 16, 18, 20, 22 and the core network 24 may be wired, or wireless, or a mixture of wired and wireless connections. It will be appreciated that a typical cellular communications network will include many more base stations than are shown here, but FIG. 1 shows enough of the network for an understanding of the present method.
The cellular communications network may use any communications technology. In one embodiment, the cellular communications network operates using the Long Term Evolution (LTE) technology, and this description will relate to that specific network technology, but the methods described herein can be used in any suitable network.
In an LTE network, the radio access network is referred to as the Evolved UMTS Terrestrial Radio Access Network (EUTRAN), and the base stations 10, 12, 14, 16, 18, 20, 22 are commonly referred to as eNodeB's. The core network 24 includes multiple modes, performing respective standardized functions. For an understanding of the present method, it is sufficient to note that, in the case of the LTE cellular communications network, the core network 24 takes the form of the Evolved Packet Core (EPC), and includes a Mobility Management Entity (MME) node 25, which has multiple functions.
The MME node 25 is responsible for idle mode User Equipment (UE) paging, and is involved in the bearer activation/deactivation process and is also responsible for choosing the Serving Gateway (SGW) 26 for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. The MME node 25 is also responsible (together with the Home Subscriber Server 27) for authenticating the user. Thus, the MME node 25 checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME node 25 is the termination point in the network for ciphering/integrity protection and handles the security key management.
The Serving Gateway (SGW) 26 routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies. For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE.
The Home Subscriber Server 27 is a central database that contains user-related and subscription-related information. The HSS 27 is thus involved in mobility management, call and session establishment support, user authentication and access authorization.
A Packet Data Network (PDN) Gateway (PGW) 28 provides connectivity from the UE to external packet data networks. A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs. The PGW performs functions such as policy enforcement, packet filtering for each user, and charging support.
Each eNodeB 10, 12, 14, 16, 18, 20, 22 connects over a S1-AP protocol interface with the MME node 25, and connects over the GTP-U interface with the respective SGW node 26.
In addition, each eNodeB 10, 12, 14, 16, 18, 20, 22 can connect to other eNodeB's over a respective X2 interface.
Each base station provides coverage for devices in its coverage area, or cell. In a typical cellular communications network, there are macrocells that have a relatively large coverage area, with the macrocells together providing coverage across the whole of the area served by the network. In addition, there are smaller cells, which provide additional coverage in parts of the area served by the network, where the demand requires this additional capacity. For example, the smaller cells may be provided in urban areas, within different parts of a building, etc. In this illustrated embodiment, the base stations 10, 12, 14, 16, 18, 20, 22 are small cell base stations, although the methods described herein may be used in any base stations where required. In addition, although this description relates to an embodiment in which the method relates to a licensed radio small cell, the methods described can be applied at any radio access point that has a cell identifier in a configurable network as described here.
In addition, although the method is described herein with reference to an example in which each base station provides coverage for a single cell, the method is also applicable to examples in which one or more of the base stations in the radio access network serves more than one cell.
FIG. 1 shows four wireless devices 30, 32, 34, 36, which are in the cell served by the base station 10. It will be appreciated that a typical cellular communications network will include many more wireless devices than are shown here, but FIG. 1 shows enough of the network for an understanding of the present method.
The wireless devices 30, 32, 34, 36 may be of any type suitable for bidirectional communication with the cellular communications network. For example, the devices may be mobile phones, smartphones, portable computers or the like, that are intended to be operated by a user while moving, or may be devices such as wireless routers that are intended to operate from a generally fixed location, or may be devices such as tracking devices, or devices using Machine-Type Communications (MTC) that are intended to operate without direct human intervention while moving or stationary. In a typical network, most of the wireless devices are user equipment devices (UEs), and that term is also used in the following description to describe a traditional user equipment device, or any other device, component, element, endpoint, or object capable of initiating or receiving voice, audio, video, media, or data exchanges within the communication system.
FIG. 2 illustrates the general form of a base station in the cellular communications network, in this case the base station 10, although the other base stations may have generally the same form. Specifically, the base station 10 includes a transceiver/communications module 40 and a data processing and control unit 42.
The transceiver/communications module 40 is able to provide the required communications over the radio interface in the cellular communications network through a connection to an antenna 44. Thus, in this illustrated case of an LTE eNodeB, the communications over the radio interface use the Evolved UMTS Terrestrial Radio Access (E-UTRA) protocols. In this illustrated embodiment, the transceiver/communications module 40 is able to communicate over system downlink channels with user equipment devices that are present in the network, and is able to receive signals that are transmitted by the user equipment devices over system uplink channels.
It is known that some base stations are additionally able to detect signals transmitted by other base stations, and are thus able to receive and decode signals that are transmitted on system downlink channels. However, this capability typically requires additional hardware, for example in the form of radio receiver circuitry, in the base station. In this example embodiment, the base station 10 does not have the ability to detect signals on system downlink channels, and therefore relies on reports received from the connected user equipment devices in order to obtain information about the network configuration.
The transceiver/communications module 40 is also connected via a connection 46 to the core network 24 of the cellular communications network. The S1-AP protocol interface used for communications with the MME node 25, and the GTP-U interface used for communications with the respective SGW node 26 are together known as the S1 interface. As mentioned above, the communications between the base station 10 and the core network 24 can take place over a wired or wireless link, using any suitable technique.
More generally, the transceiver/communications module 40 includes network elements for receiving, transmitting, and/or otherwise communicating data or information in a network environment.
The data processing and control unit 42 includes at least one processor 48, and at least one memory 50, and the memory 50 stores a program, containing instructions for causing the processor to perform various processes, including those described herein.
The processor 48 may be a single device or may include multiple devices operating together, in order to execute any type of instructions associated with the data to achieve the operations detailed herein. The processor 48 may operate with fixed logic or programmable logic (e.g., software/computer instructions executed by the processor) and the elements identified herein could be a programmable processor, programmable digital logic (e.g., a field programmable gate array [FPGA], an erasable programmable read only memory (EPROM), an electrically erasable programmable ROM (EEPROM)) or an ASIC that includes digital logic, software, code, electronic instructions, or any suitable combination thereof.
The memory 50 may similarly be provided as a single device, or as multiple devices operating together for storing information to be used in performing the activities described herein. The memory 50 may take the form of, or may include, Random Access Memory (RAM), Read Only Memory (ROM), Electrically Erasable Programmable Read Only Memory (EEPROM), Flash memory, etc., in a fixed or removable component, device, element, or object.
FIG. 3 illustrates a first process performed in the base station 10, in accordance with an embodiment of the disclosure.
It is important to note that the steps shown in FIG. 3 illustrate one example of a process. Some of these steps may be deleted or removed where appropriate, or these steps may be modified or changed considerably without departing from the scope of the present disclosure. In addition, while the operations are described as being sequential, they may be executed concurrently with, or in parallel to, one or more additional operations.
The process shown in FIG. 3 starts at step 70, at a time when the base station 10 is in operation in the cellular communications network. At this time, the base station 10 is operating with a physical layer identifier that it broadcasts with its transmissions. In the illustrative embodiment described here, in which the cellular communications network is an LTE network, the identifier is a Physical Cell Identity (PCI). In the case in which the cellular communications network is a Universal Mobile Telecommunications System (UMTS) network, the identifier may be a Primary Scrambling Code (PSC). In either case, the number of available identifiers is considerably smaller than the number of base stations expected to be present in the network. Therefore, the process of installation of the base station 10 will typically include a process for selecting an identifier that minimizes the likelihood that any wireless device will receive signals from more than one cell using the same identifier.
For example, on installation, the base station may perform a process in which it receives a pool of identifiers available for use by base stations within the network. As mentioned above, one application of the method is the situation where the base stations 10, 12, 14, 16, 18, 20, 22 are small cell base stations. It is typical for a network operator to reserve a relatively small pool of identifiers for use by small cells, and so, on installation, the small cell is informed of this pool of available identifiers. The pool of identifiers may for example be received from an external management system. Having received information identifying the available identifiers, the base station may then select a temporary identifier allowing it to enter service. Selecting the temporary identifier may comprise checking for identifiers in use by other base stations within a threshold proximity of the base station 10, and allocating as the temporary identifier an identifier which is not in use by any base station within the threshold proximity. The threshold proximity may for example be the detection range of the base station 10 or may be a set radius below which it is possible that base station coverage may overlap.
The base station 10 may then instruct its connected wireless devices, that is User Equipment devices (UEs), to attempt to detect signals transmitted by other base stations, and to report the identifiers that are in use in the signals that they detect. The base station then analyses the received reports of identifiers from among the received pool that are in use by other base stations. The base station 10 may then assign for its subsequent use a final identifier that is selected from the received pool of identifiers, but that is different from the temporary identifier. For example, the final identifier may be selected on the basis that it is not among the selected identifiers being used by neighboring base stations, as reported by the connected wireless devices.
At the start of the process in step 70, the base station 10 has also established a neighbor cell list, which it uses in order to prepare for handovers, as wireless devices move around in the coverage area of the base station. Wireless devices make measurements of the strengths of signals that they receive from other base stations, and report these measurements to the base station 10, together with the respective physical layer identifiers broadcast by those other base stations. Based on these reports, the base station 10 is able to compile a list of neighbor cells that are potential handover targets, because they transmit signals that can be received with an acceptable signal strength by at least some wireless devices that are in the coverage area of the base station 10.
The process shown in FIG. 3 involves entering an observation period at periodic intervals and so, in step 70, it is determined whether a specified time interval has passed since the base station entered service, or since the previous observation period. The specified time interval may for example be within the range from 10 minutes to 24 hours, for example 1 hour. The time interval may be configurable by an operator. The specified time interval is chosen to balance the advantage of obtaining up-to-date information against the disadvantage of performing additional signalling and measurements. The time interval may be adjusted automatically. For example observations may be carried out at shorter intervals in response to received measurements reports that indicate significant changes in the network. Thus, if the measurements reports received from connected UEs indicate that a nearby base station has gone out of service, or that a nearby base station has changed its transmission power by a significant amount, or that a new base station has entered service, new observations may be carried out after a reduced time interval, so that the information that is received reflects the changed network configuration.
If it is determined in step 70 that the specified time interval has not passed, then the step is repeated until the specified time interval has passed, at which point the process passes to step 72, and a new observation period is entered.
In step 72, the base station 10 sends a message to all wireless devices that are in connected mode, that is to say that they are in active radio operation. For example, such a message may be sent to all UEs that are in a call or data session. In this illustrated example of the LTE cellular communications network, the base station is able to request wireless devices that are in connected mode to perform certain operations.
Specifically, in step 72, the base station 10 sends a Radio Resource Control (RRC) Connection Reconfiguration message to wireless devices that are in connected mode, having an active radio connection through that base station. The RRC Connection Reconfiguration message instructs the wireless devices to report all cells that they can detect, and to read and report the EUTRAN Cell Global ID (ECGI) of each such cell. Unlike the physical cell identifier that is broadcast by each base station in its regular transmissions, the ECGI uniquely identifies the cell.
The sending of the RRC Connection Reconfiguration messages, instructing the wireless devices to report the cells that they can detect, and to read and report the EUTRAN Cell Global ID (ECGI) of each such cell, may be achieved by sending a general request to all wireless devices that are in connected mode at the time that the observation period is entered. Alternatively, a message may be sent to each wireless device separately, at a time when it would ordinarily be making measurements in any event. For example, a wireless device may start making measurements relating to the neighboring cells when the strength of signals that it receives from the base station falls below a certain threshold. The base station may send the RRC Connection Reconfiguration message, instructing the wireless device to report the cells that it can detect, and to read and report the EUTRAN Cell Global ID (ECGI) of each such cell, when it receives such measurement reports from the wireless device.
In step 74, the base station receives the reports from the wireless devices. Based on these received reports, the base station is able to identify neighboring cells, transmissions from which can be received by the wireless devices.
In step 76, the base station sends a request to a core network node, for example the Mobility Management Entity (MME) node 26, requesting the IP addresses of each of the neighboring cells identified in the reports from the wireless devices.
In step 78, the base station determines whether the observation period is complete. The duration of the observation period may for example be specified to be within the range from 1 minute to 30 minutes, for example 5 minutes. The specified duration of the observation period may be configurable by an operator. The length of the observation period is chosen to balance the advantage of obtaining the maximum amount of information against the disadvantage of performing additional signalling and measurements. The length of the observation period may be adjusted automatically.
If it is determined in step 78 that the observation period is not yet complete, then the process returns to step 72, and the RRC Connection Reconfiguration message is sent to any wireless devices that have entered connected mode since step 72 was performed previously, or that have started to transmit measurement reports to the base station. In that case, steps 74 and 76 are also repeated.
If it is determined in step 78 that the observation period is complete, then the process passes to step 80. In step 80, the base station determines whether it is necessary for it to reconsider the identifier that it is using.
For example, if one of the connected wireless devices reports that it can detect signals from another base station that is using the same physical cell identifier as the base station to which that wireless device is connected, then it may be necessary for that base station to change its identifier. In that case, the base station preferably waits for a time when it has no active connections to wireless devices, and repeats the process that it used on installation, as described above, for selecting a physical cell identifier.
In step 82, the base station uses the information obtained from the reports received from the wireless devices, in order to modify, if necessary, its neighbor cell list.
The neighbor cell list that is complied by a base station is used in preparing handovers. In general terms, wireless devices that are connected to the base station can be requested to make measurements of the signals received from the cells that are included on the neighbor cell list. If any wireless device finds that it is receiving a stronger signal from one of the neighbor cells than from the serving cell, then a handover from the serving cell to that neighbor cell may be advantageous.
In this case, therefore, the base station seeks to include in its neighbor cell list all of the neighbor cells whose transmissions can be detected by connected wireless devices with a certain signal strength.
Referring to FIG. 1, by way of an example, it can be seen that, if the wireless devices 30, 32, 34, 36 are all connected to the base station 10, then the wireless device 30 may report that it is able to detect signals transmitted by the base stations 14, 16, 18, amongst others, while the wireless device 36 may report that it is able to transmit signals detected by the base stations 18, 20, 22, amongst others. Therefore, the neighbor cell list compiled by the base station 10 should preferably include at least the base stations 14, 16, 18, 20, and 22.
If the base station simply relies on the measurement reports from the wireless devices, which contain the physical cell identifiers broadcast by the other base stations, then there is the possibility that reports will be received from two base stations which share the same identifier, and the base station will be unable to recognize that the received reports related to different base stations. For example, referring back to the example illustrated in FIG. 1, it is quite possible that the base stations 14 and 20 could share a physical cell identity. If the base station 10 simply relies on the measurement reports from the wireless devices 30 and 36, it would be unable to recognize that the respective reports from the wireless devices 30 and 36 related to different base stations. In that case, the neighbor cell list would typically include only one neighbor having that identity, and further reports relating to the same cell identity would be assumed to relate to the same cell. For example, reports from the wireless device 30 relating to the base station 14 might be assumed to relate to the base station 20. An attempted handover could then fail, because the wireless device 30 might be instructed to attempt a handover to the base station 20, when it is in fact unable to detect signals from that base station.
Therefore, the method described herein allows the compilation of a neighbor cell list that includes all of the relevant cells. In step 72, the wireless device 30 would report the ECGIs of the base stations 14, 16, and 18, while the wireless device 36 would report the ECGIs of 18, 20, and 22. In compiling its neighbor cell list, therefore, the base station 10 would be able to separately identify the base stations 14 and 20, even though they share a physical cell identity. Thus, even though there is collision between the physical cell identities used by the base stations 14 and 20, the base station 10 is able to include them both in its neighbor cell list. The result is that, when a user equipment device needs to consider a handover, it is able to make measurements that reflect this situation, and is able to handover to the cell that is able to provide improved coverage.
The base station 10 is therefore able to compile a neighbor cell list that more accurately reflects its wireless environment.
FIG. 4 illustrates a second process performed in the base station 10, in accordance with an embodiment of the disclosure.
It is important to note that the steps shown in FIG. 4 illustrate one example of a process. Some of these steps may be deleted or removed where appropriate, or these steps may be modified or changed considerably without departing from the scope of the present disclosure. In addition, while the operations are described as being sequential, they may be executed concurrently with, or in parallel to, one or more additional operations.
The process shown in FIG. 4 starts at step 100, at a time when the base station 10 is in operation in the cellular communications network. As described with reference to FIG. 3, the base station 10 is operating with a physical layer identifier that it broadcasts with its transmissions. In the illustrative embodiment described here, in which the cellular communications network is an LTE network, the identifier is a Physical Cell Identity (PCI). In the case in which the cellular communications network is a Universal Mobile Telecommunications System (UMTS) network, the identifier may be a Primary Scrambling Code (PSC). In either case, the number of available identifiers is considerably smaller than the number of base stations expected to be present in the network. Therefore, the process of installation of the base station 10 will typically include a process for selecting an identifier that minimizes the likelihood that any wireless device will receive signals from more than one cell using the same identifier.
At the start of the process in step 100, the base station 10 has also established a neighbor cell list, which it uses in order to prepare for handovers, as wireless devices move around in the coverage area of the base station.
The process shown in FIG. 4 involves entering an observation period at periodic intervals and so, in step 100, it is determined whether a specified time interval has passed since the base station entered service, or since the previous observation period. The specified time interval may for example be within the range from 10 minutes to 24 hours, for example 1 hour. The time interval may be configurable by an operator. The specified time interval is chosen to balance the advantage of obtaining up-to-date information against the disadvantage of performing additional signalling and measurements. The time interval may be adjusted automatically, for example observations may be carried out at shorter intervals in response to received measurements reports that indicate significant changes in the network.
If it is determined in step 100 that the specified time interval has not passed, then the step is repeated until the specified time interval has passed, at which point the process passes to step 102, and a new observation period is entered.
In step 102, the base station 10 sends a message to all wireless devices that are in connected mode, that is to say that they are in active radio operation, for example in a call or data session. In this illustrated example of the LTE cellular communications network, the base station is able to request wireless devices that are in connected mode to perform certain operations.
Specifically, in step 102, the base station 10 sends a Radio Resource Control (RRC) Connection Reconfiguration message to all wireless devices that are in connected mode, having an active radio connection through that base station. The RRC Connection Reconfiguration message instructs the wireless devices to report all cells that they can detect, and to read and report the EUTRAN Cell Global ID (ECGI) of each such cell. Unlike the physical cell identifier that is broadcast by each base station in its regular transmissions, the ECGI uniquely identifies the cell.
In step 104, the base station receives the reports from the wireless devices.
In step 106, the base station sends a request to a core network node, for example the Mobility Management Entity (MME) node 26, requesting the IP addresses of each of the neighboring cells identified in the reports from the wireless devices.
In step 108, the base station determines whether the observation period is complete. The duration of the observation period may for example be specified to be within the range from 1 minute to 30 minutes, for example 5 minutes. The specified duration of the observation period may be configurable by an operator. The length of the observation period is chosen to balance the advantage of obtaining the maximum amount of information against the disadvantage of performing additional signalling and measurements. The length of the observation period may be adjusted automatically.
If it is determined in step 108 that the observation period is not yet complete, then the process returns to step 102, and the RRC Connection Reconfiguration message is sent to any wireless devices that have entered connected mode since step 102 was performed previously. In that case, steps 104 and 106 are also repeated.
If it is determined in step 108 that the observation period is complete, then the process passes to step 110. In step 110, the base station determines whether it is necessary to send any information to the core network.
For example, if one of the connected wireless devices reports that it can detect signals from another base station that is using the same physical cell identifier as the base station 10, then it may be necessary for one of the base stations to change its identifier. In that case, the base station 10 may report this occurrence to the core network, for example to the MME node 26 in the core network shown in FIG. 1, so that the core network can take action to resolve this collision, such as by requiring one base station to change its physical layer identifier.
Similarly, if two of the connected wireless devices report that they can detect signals from base stations that are using the same physical cell identifier, but the base station 10 is able to determine that these are two different base stations having different ECGIs, then it may be advantageous for one of the base stations to change its physical layer identifier. Thus, the base station 10 may report this occurrence to the core network, for example to the MME node 26 in the core network shown in FIG. 1, so that the core network can determine whether it is preferable to take action to resolve this collision, such as by requiring one base station to change its physical layer identifier.
In step 112, the base station uses the information obtained from the reports received from the wireless devices, in order to modify, if necessary, its neighbor cell list.
If the base station simply relies on the measurement reports from the wireless devices, which contain the physical cell identifiers broadcast by the other base stations, then there is the possibility that reports will be received from two base stations which share the same identifier, and the base station will be unable to recognize that the received reports related to different base stations.
Therefore, the method described herein allows the compilation of a neighbor cell list that includes all of the relevant cells. In step 102, the connected wireless devices would report the ECGIs of the base stations that they can detect, and so the base station 10 would be able to separately identify the base stations mentioned in the reports, even if they share a physical cell identity. Thus, even though there is collision between the physical cell identities used by the base stations 14 and 20, the base station 10 is able to include them both in its neighbor cell list. The result is that, when a user equipment device needs to consider a handover, it is able to make measurements that reflect this situation, and is able to handover to the cell that is able to provide improved coverage.
The base station 10 is therefore able to compile a neighbor cell list that more accurately reflects its wireless environment.
Numerous other changes, substitutions, variations, alterations, and modifications may be ascertained by one skilled in the art and it is intended that the present disclosure encompass all such changes, substitutions, variations, alterations, and modifications as falling within the scope of the appended claims. Applicant wishes to note that it does not intend, by any statement in the specification, to limit this disclosure in any way that is not otherwise reflected in the appended claims.

## Claims

1. A method of forming a neighbor cell list in a base station of a cellular communications network, the method comprising, at periodic intervals, and for specified durations:
requesting at least one wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect;
receiving reports from each wireless device that is in connected mode with the base station;
operating with a neighbor cell list compiled on the basis of the received reports; and
reporting instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.

2. A method as claimed in claim 1, further comprising:
creating the neighbor cell list on the basis of the received reports.

3. A method as claimed in claim 1, further comprising:
forwarding information from the received reports to a core network node of the cellular communications network; and
receiving the neighbor cell list complied on the basis of the received reports from the core network node.

4. A method as claimed in claim 1, wherein the periodic intervals are configurable.

5. A method as claimed in claim 1, wherein the specified durations are configurable.

6. A method as claimed in claim 1, further comprising:
reconsidering an identifier used by the base station itself in response to the received reports, such that the identifier used by the base station is not in use by another base station meeting a proximity condition.

7. A method as claimed in claim 1, further comprising:
reporting to a core network node of the cellular communications network instances where one of the wireless devices in connected mode reports that it can detect signals from another base station that is using a same physical cell identifier as said base station.

8. A method as claimed in claim 1, further comprising reporting to a core network node of the cellular communications network:
instances where one of the wireless devices in connected mode reports that it can detect signals from another base station that is using a same physical cell identifier as said base station; and
instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.

9. A base station for a cellular communications network, comprising:
a processor; and
a memory, said memory containing instructions executable by the processor, wherein said base station is operative to:
at periodic intervals, and for specified durations, request at least one wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect;
receive reports from each wireless device that is in connected mode with the base station;
operate with a neighbor cell list compiled on the basis of the received reports; and
report instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.

10. A base station as claimed in claim 9, wherein the base station is further operative to:
create the neighbor cell list on the basis of the received reports.

11. A base station as claimed in claim 9, wherein the base station is further operative to:
forward information from the received reports to a core network node of the cellular communications network; and
receive the neighbor cell list complied on the basis of the received reports from the core network node.

12. A base station as claimed in claim 9, wherein the periodic intervals are configurable.

13. A base station as claimed in claim 9, wherein the specified durations are configurable.

14. A base station as claimed in claim 9, wherein the base station is further operative to:
reconsider an identifier used by the base station itself in response to the received reports, such that the identifier used by the base station is not in use by another base station meeting a proximity condition.

15. A base station as claimed in claim 9, wherein the base station is further operative to:
report to a core network node of the cellular communications network instances where one of the wireless devices in connected mode reports that it can detect signals from another base station that is using a same physical cell identifier as said base station.

16. A base station as claimed in claim 9, wherein the base station is further operative to report to a core network node of the cellular communications network:
instances where one of the wireless devices in connected mode reports that it can detect signals from another base station that is using a same physical cell identifier as said base station; and
instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.

17. A computer program product, comprising a non-transitory storage medium, containing instructions executable by a processor associated with a base station for a cellular communications network, wherein said base station is operative to:
at periodic intervals, and for specified durations, request at least one wireless device that is in connected mode with the base station to report a unique identifier for each other base station that it is able to detect;
receive reports from each wireless device that is in connected mode with the base station;
operate with a neighbor cell list compiled on the basis of the received reports; and
report instances where one or more of the wireless devices in connected mode reports that it can detect signals from two other base stations that are using a same physical cell identifier as each other, but have different unique identifiers.

18. A computer program product as claimed in claim 17, wherein at least one of the periodic intervals and the specified durations is configurable.

19. A computer program product as claimed in claim 17, wherein said base station is further operative to:
create the neighbor cell list on the basis of the received reports.

20. A computer program product as claimed in claim 17, wherein said base station is further operative to:
forward information from the received reports to a core network node of the cellular communications network; and
receive the neighbor cell list complied on the basis of the received reports from the core network node.