Patent Description:
A cellular telecommunications network includes a plurality of base stations which each provide telephony and data services to a plurality of User Equipment (UE) within a coverage area. Traditional base stations typically had coverage areas of several square kilometres, but the capacity was shared amongst all UEs. To increase this capacity, "home" base stations were introduced (which are sometimes called femto base stations, pico base stations, micro base stations or metro base stations depending on their coverage area), and the traditional base stations are now often referred to as "macro" base stations. These home base stations had much smaller coverage areas than their macro base station counterparts but increased the overall capacity of the network.

Cellular networks have also introduced inter-base station messages (such as the X2 message in the Long Term Evolution (LTE) protocol) to allow inter base station communication. These messages are typically used to communicate their operating parameters (e.g. their Physical Cell Identifier (PCI)), load management information, and to coordinate a handover of a UE to a target base station. A "handover" is the transfer of a UE from its serving base station to the target base station with minimal disruption when it is determined that the target base station should thereafter provide telephony and data services to that UE.

In order to support inter-base station messaging, each base station must store information regarding other base stations in the network. This information typically includes an identifier for the other base station, such as the enhanced Cell Global Identifier (eCGI), and an IP address of the other base station. Each base station must also set up and maintain a connection with every other base station. These requirements were acceptable in cellular networks with a relatively small number of macro base stations. However, as the number of base stations has increased with the introduction of home base stations, these requirements have become a burden and it is increasing difficult for base stations (especially macro base stations with many neighbouring home base stations) to update their stored information and maintain the inter base station connections.

In the LTE protocol, this problem was addressed by the introduction of a network element that inter-connects neighbouring base stations such that inter base station messages may be communicated indirectly via the network element. This is known as an X2 Gateway (X2 GW), although other network elements such as the X2 Broker may also serve the same purpose. The X2 GW is used to transfer X2 Application Protocol (X2 AP) messages between a source and target base station by encapsulating it within an X2 AP Message Transfer message. The X2 AP Message Transfer message includes a Radio Network Layer (RNL) header which identifies both the source and target base station, and a payload including the encapsulated X2 AP message. On receipt of an X2 AP Message Transfer message from the source base station, the X2 GW decodes the RNL header to identify the target base station, and forwards the X2 AP Message Transfer message to the identified target base station. This is a more scalable solution as the source base station only needs to establish and maintain a single connection with the X2 GW for a plurality of target base stations (rather than a connection with each of the plurality of target base stations).

Document "<NPL>, discloses scenarios where X2 communications could imply high resource utilisation.

<CIT> discloses an access point that may perform a method for X2 communication set up in a wireless communication network. The method may include discovering a neighbor node at an access point, optionally in response to detecting a new neighbor node, or an address parameter change at a neighbor node.

<CIT> discloses technology for initiating a communication interface in a wireless communication system. In an embodiment, a neighbor node is discovered at an access point. The subject technology receives, via a network message in response to discovering the neighbor node, an address indication associated with the neighbor node for configuration of the communication interface. It is then determined whether to initiate one of a direct communication interface or indirect communication interface for communication with the neighbor node based on the address indication in the received network message.

<CIT> provides measures for optimized signaling in relay- enhanced access networks, said measures exemplarily comprising receipt of at least one signaling message concerning at least one relay node of a relay-enhanced access network over at least one predetermined signaling interface, concentration of signaling concerning a respective relay node in terms of irrelevancy and/or redundancy from the at least one signaling message, and forwarding of the concentrated signaling in a signaling message over the at least one predetermined signaling interface towards the respective relay node. Said measures may exemplarily be applied for optimizing X2 messaging in relay-enhanced LTE access networks.

According to a first aspect of the invention, there is provided a method as claimed in Claim <NUM>.

Present invention provides the advantage that the number of inter-base station messages having the same content that must be sent from the source base station to a relay component for onward transmission to N target base stations is reduced from N to <NUM>. This is particularly advantageous in scenarios where the source base station must update all of its neighbouring base stations upon a change in of its transmission frame pattern or scheduling pattern.

According to a second aspect of the invention, there is provided an X2 Gateway, X2GW, as claimed in Claim <NUM>.

According to a third aspect of the invention, there is provided a method as claimed in Claim <NUM>.

According to a fourth aspect of the invention, there is provided a first base station as claimed in Claim <NUM>.

According to a fifth aspect of the invention, there is provided a computer program as claimed in Claim <NUM>.

According to a sixth aspect of the invention, there is provided a computer readable carrier medium comprising the computer program as claimed in Claim <NUM>.

A first embodiment of a cellular telecommunications network <NUM> will now be described with reference to <FIG>. <FIG> illustrates a macro base station ("MeNB") <NUM> and a first, second and third home base station ("HeNB") <NUM>, <NUM>, <NUM>. These base stations each serve a plurality of User Equipment (UE), not shown for simplicity, about their respective coverage areas. In this embodiment, the base stations <NUM>, <NUM>, <NUM>, <NUM> communicate using the Long Term Evolution (LTE) protocol.

<FIG> also shows an X2 Gateway ("X2 GW") <NUM>, a Mobility Management Entity (MME) <NUM> and a HeNB Gateway ("HeNB GW") <NUM>. The form of each respective connection between each entity is also illustrated in <FIG>.

The MeNB <NUM> is shown in more detail in the schematic diagram of <FIG>. The MeNB <NUM> includes a first transceiver <NUM>, a processor <NUM>, memory <NUM> and a second transceiver <NUM>, all connected via bus <NUM>. In this embodiment, the first transceiver <NUM> is an antenna configured for transmissions according to the LTE protocol, and the second transceiver <NUM> is a wired optical fibre connection (typically known as a backhaul) with one or more cellular core networking nodes, including the X2 GW <NUM> and the MME <NUM>. Memory <NUM> is configured to store an X2 connection table which includes information regarding each established X2 connection with other base stations. In this embodiment, the X2 connection table includes an identifier for the neighbouring base station of each established X2 connection, and an IP address to use when sending an X2 message to that neighbouring base station. This X2 connection table will be discussed in more detail in the description of the embodiment of the method of the present invention.

The first, second and third HeNBs <NUM>, <NUM>, <NUM> are of similar form to the MeNB <NUM>, but its components differ slightly such that they are more suited for transmissions about smaller coverage areas (typically covering a user's premises). Furthermore, in this embodiment, the second transceivers of both the second and third HeNBs <NUM>, <NUM> are also connected to the X2 GW <NUM> and the HeNB GW <NUM> (with an onward connection from the HeNB GW <NUM> to the MME <NUM>), but the second transceiver of the first HeNB <NUM> is not connected to either the X2 GW <NUM> or HeNB GW <NUM> (and has a direct connection to the MME <NUM>).

The MeNB <NUM>, and second and third HeNBs <NUM>, <NUM> are also configured to store, in memory, an IP address for the X2 GW <NUM>. In this embodiment, the MeNB <NUM> and second and third HeNB <NUM>, <NUM> are pre-configured with this information (but they may also be updated with new data upon receipt of a new configuration file via their second transceivers).

The X2 GW <NUM> is shown in more detail in the schematic diagram of <FIG>. The X2 GW <NUM> includes a first transceiver <NUM>, a processor <NUM> and memory <NUM>, all connected via bus <NUM>. In this embodiment, the first transceiver <NUM> is a wired optical fibre connection with the MeNB <NUM> and the second and third HeNBs <NUM>, <NUM>. Memory <NUM> stores an eNB association table which stores connection information for all base stations that have registered with that X2 GW <NUM>, including their enhanced Cell Global Identifier (eCGI) and Transport Network Layer (TNL) address.

An embodiment of a method will now be described with reference to <FIG> and <FIG>. <FIG> illustrates an embodiment of a method of establishing an indirect X2 connection between the MeNB <NUM> and the second HeNB <NUM> via the X2 GW <NUM>.

In step S1, the MeNB <NUM> establishes an X2 connection with the X2 GW (hereinafter, an "X2 GW connection"). This is typically performed when a base station is initialised for the first time or following a reboot. To establish this X2 GW connection, the MeNB <NUM> sends a registration X2 AP Message Transfer message to the X2 GW <NUM> (explained in more detail below). Before this message can be sent, the MeNB <NUM> establishes a Stream Control Transmission Protocol (SCTP) association with the X2 GW <NUM>. The MeNB <NUM> initiates the SCTP association using the X2 GW's IP address (known from pre-configuration), and the MeNB <NUM> and X2 GW <NUM> exchange TNL addresses (in this embodiment, their IP addresses). Once the SCTP association has been established, the MeNB <NUM> sends the registration X2 AP Message Transfer message to the X2 GW <NUM>. The registration X2 AP Message Transfer includes an RNL header having the MeNB's eCGI as the source eNB Information Element (IE), but the target eNB IE and payload portions of the message are void. The X2 GW <NUM> is configured to interpret the registration X2 AP Message Transfer message having void target eNB IE and payload portions as a registration message, and responds by registering the MeNB <NUM> in memory <NUM>. Accordingly, the X2 GW <NUM> updates its eNB association table in memory to indicate that the MeNB <NUM> is a registered base station, and to identify both its eCGI and TNL (i.e. IP) address. Following step S1, the MeNB <NUM> has a <NUM>st X2 GW connection with the X2 GW <NUM>.

The second HeNB <NUM> also establishes an X2 GW connection with the X2 GW <NUM> (the <NUM>nd X2 GW connection in <FIG>) in the same manner. The skilled person will understand that it is unlikely for these registrations to be contemporaneous, but they are performed by both the MeNB <NUM> and second HeNB <NUM> at some point in time before the remaining steps of this embodiment of the invention. Once registered with the X2 GW <NUM>, both the MeNB <NUM> and second HeNB <NUM> begin normal base station operation providing telephony and data services to one or more UEs.

In step S2, the MeNB <NUM> receives a measurement report from a UE which identifies the second HeNB <NUM> by its eCGI. The MeNB <NUM> has no stored data regarding the second HeNB <NUM> and receipt of this measurement report therefore constitutes a detection of a new neighbour base station. In response, in step S3, the MeNB <NUM> sends an X2 AP Message Transfer message to the X2 GW <NUM>, in which the source eNB IE is the MeNB <NUM> eCGI, the target eNB IE is the second HeNB's eCGI, and the payload portion is an X2 Setup message.

On receipt of this message, in step S4, the X2 GW <NUM> determines that the message is destined for the second HeNB <NUM> (from the target eNB IE), looks up the second HeNB's IP address from memory <NUM>, and forwards the X2 AP Message Transfer message to the second HeNB <NUM>.

In step S5, the second HeNB <NUM> responds to the X2 Setup message with an X2 Setup Response message. Again, this is encapsulated into an X2 AP Message Transfer message (with the source eNB IE and target eNB IE being the second HeNB's eCGI and MeNB's eCGI respectively), which is sent via the X2 GW to the MeNB <NUM>. Following this message exchange, the MeNB <NUM> and second HeNB <NUM> have an established (indirect) X2 connection, which is via both the <NUM>st X2 GW connection (between the MeNB <NUM> and the X2 GW <NUM>) and the <NUM>nd X2 GW connection (between the X2 GW <NUM> and the second HeNB <NUM>).

In step S6, the MeNB <NUM> populates its X2 connection table with a new entry identifying the second HeNB and an IP address for the X2 GW <NUM>.

The above method is initiated each time the MeNB <NUM> detects a new neighbouring base station (e.g. the first and third HeNBs <NUM>, <NUM>). As the first HeNB <NUM> is not associated with the X2 GW <NUM>, a direct X2 connection is established and the X2 connection table identifies the first HeNB <NUM> and the first HeNB's IP address. As the third HeNB <NUM> is associated with the X2 GW <NUM>, an indirect X2 connection is established (which in part uses the same X2 GW connection between the MeNB <NUM> and X2 GW <NUM>) and the X2 connection table identifies the third HeNB <NUM> and the X2 GW's IP address. This is illustrated in the following table:.

Once the indirect X2 connections have been established, then X2 messages may be transmitted to the second and third HeNBs <NUM>, <NUM> via the X2 GW <NUM>. This will now be described with reference to <FIG>, which illustrates an overview of the MeNB <NUM> sending an X2 Application Protocol ("X2 AP") message to all neighbouring base stations with an established X2 connection. In this example, the X2 AP message indicates the Time Division Duplex (TDD) pattern that is being used by the MeNB <NUM>.

In step S20, the MeNB <NUM> determines whether it has an established X2 connection with a plurality of neighbouring base stations via the same X2 GW. In this example, the MeNB <NUM> has indirect X2 connections with both the second and third HeNBs <NUM>, <NUM> via the X2 GW <NUM> (which involves the same X2 GW connection between the MeNB <NUM> and the X2 GW <NUM>). The following embodiment describes the transmission of the X2 AP message to both the second and third HenBs <NUM>, <NUM>.

In step S21, the MeNB <NUM> prepares a first X2 AP Message Transfer message for both the second and third HeNBs <NUM>, <NUM>. This first message includes an RNL header, which includes the MeNB <NUM> in the source eNB Information Element (IE) and both the second and third HeNBs <NUM>, <NUM> in the target eNB IE, and a payload portion including the X2 AP message. The MeNB <NUM> sends the first X2 AP Message Transfer message to the X2 GW <NUM>.

In step S22, the X2 GW <NUM> receives the first X2 AP Message Transfer message and decodes the RNL header. The X2 GW <NUM> therefore determines that the message is addressed to both the second and third HeNBs <NUM>, <NUM>. In step S23, the X2 GW <NUM> prepares a second X2 AP Message Transfer message, which includes an RNL header identifying the second HeNB <NUM> only and a payload portion including the X2 AP message, and prepares a third X2 AP Message Transfer message, which includes an RNL header identifying the third HeNB <NUM> only and a payload portion including the X2 AP message.

In step S24, the X2 GW <NUM> sends the second X2 AP Message Transfer message to the second HeNB <NUM> and sends the third X2 AP Message Transfer message to the third HeNB <NUM>.

The present invention is therefore advantageous in that the number of X2 AP Message Transfer messages that must be sent to an X2 Gateway for onward transmission to N base stations is reduced from N to <NUM>. In the above example in which the MeNB <NUM> supports dynamic TDD and is sending its TDD pattern to all of its neighbours, the processing involved in the MeNB <NUM> to prepare all of these messages and the processing involved in the X2 GW <NUM> for forwarding all of these messages is significantly reduced. This method is also advantageous in any other situation in which the content of the X2 AP message is the same for many neighbouring base stations, such as when a source base station sends an X2 AP message identifying its Almost Blank Subframes (ABS) to its neighbours for enhanced Inter-Cell Interference (elCIC) mitigation.

The above embodiments describe the invention in terms of the LTE protocol. However, the skilled person will understand that the present invention may be applied to any cellular network of any protocol which includes an intermediate node between base stations, and the X2 Gateway is just one example.

In the above embodiment, the target eNB IE of the X2 AP Message Transfer message is expanded to include a plurality of target eNBs, rather than just a single target eNB as in the prior art. This expansion may be performed in the same manner as for the "Served Cell" IE in the X2 Setup Request message (of 3GPP TS36. <NUM>, section <NUM>.

Claim 1:
A method of sending an X2 message between a first base station (<NUM>) and a plurality of recipient base stations in a cellular telecommunications network, wherein the X2 message is transmitted via an X2 Gateway, X2GW, (<NUM>) the method comprising the steps of:
the X2GW (<NUM>) receiving a first X2 message from the first base station (<NUM>) via an optical fibre connection, wherein the first X2 message includes:
a first address portion identifying a second and third recipient base station (<NUM>, <NUM>), and
a first content portion;
the X2GW (<NUM>) transmitting a second X2 message to the second recipient base station (<NUM>), the second X2 message including:
a second address portion identifying the second recipient base station (<NUM>), and
the first content portion; and
the X2GW (<NUM>) transmitting a third X2 message to the third recipient base station (<NUM>),
the third X2 message including:
a third address portion identifying the third recipient base station (<NUM>), and
the first content portion,
wherein the first content portion includes a transmission frame pattern of the first base station (<NUM>) or a scheduling pattern of the first base station (<NUM>).