DESCRIPTION
Title of the invention:
METHODS AND APPARATUS FOR MANAGING A HANDOVER BETWEEN BASE STATIONS
Technical Field:
The present invention relates to mobile telecommunication networks, particularly but not
exclusively networks operating according to the 3rd Generation Partnership Project (3GPP)
standards or equivalents or derivatives thereof. The invention has particular although not
exclusive relevance to the management of the handover of a mobile device from one base station
to another.
Background Art:
Under the 3GPP standards, a NodeB (or an eNB in LTE) is the base station via which
mobile devices connect to the core network. Recently the 3GPP standards body has adopted an
official architecture and started work on a new standard for home base stations (HNB). Where
the home base station is operating in accordance with the (Long Term Evolution) LTE standards,
the HNB is sometimes referred to as a HeNB. A similar architecture will also be applied in the
WiMAX network. In this case, the home base station is commonly referred to as a femto cell.
For simplicity, the present application will use the term HeNB to refer to any such home base
station and will use the term NodeB generically to refer to other base stations (such as the base
station for the macro cell in which a HeNB operates). The HeNB will provide radio coverage
(for example, 3G/4G/WiMAX) within the home, small and medium enterprise, shopping Malls
etc and will connect to the core network via a suitable public network (for example via an ADSL
link to the Internet) or operator network and in the case of the 3GPP standards, via an optional
HeNB gateway (HeNB-GW) which typically will aggregate traffic from several HeNBs.
For a HeNB connected behind a HeNB gateway, two pairs of mobile telephone
associated connections are established, one between the HeNB and the HeNB gateway, and
another between the HeNB gateway and the mobile management entity (MME) within the core
network. Mapping between the two contexts associated with a specific mobile telephone is
performed by the HeNB gateway, and therefore for a single mobile telephone connected to a
HeNB behind a HeNB gateway, context and resources are allocated at both the HeNB and the
HeNB gateway.
In common with other NodeBs, HeNBs are able to communicate with each other (and
other NodeBs) using the X2 interface. In particular, HeNBs are able to coordinate a handover
process between source and target base stations using the X2 interface without overall control by
the core network. During an X2 based handover between two HeNBs, the target HeNB
explicitly releases the UE context at the source HeNB using the X2AP UE Context Release
procedure. However, as the X2 interface is used to coordinate the handover, for a source HeNB
behind a HeNB gateway, no message relating to release of the UE context traverses through the
HeNB gateway, and therefore the HeNB gateway does not have any means to understand the
mobile telephone has been handed over via an X2 based handover to the target HeNB.
Summary of the Invention:
Thus, according to the present handover processes, whenever an X2 based handover
takes place where a source HeNB is behind a HeNB gateway, and the target HeNB is not behind
the same HeNB gateway, the UE context and related resources assigned by the HeNB gateway to
the mobile telephone will not be released. Thus, there exists a need to adapt the current processes
to overcome this problem.
Although for efficiency of understanding for those of skill in the art, the invention will be
described in detail in the context of a 3G system (UMTS, LTE), the principles of the invention
can be applied to other systems (such as WiMAX) in which mobile telephones or User
Equipment (UE) communicate with one of several base stations with the corresponding elements
of the system changed as required.
Accordingly, preferred embodiments of the present invention aim to provide methods and
apparatus which overcome or at least alleviate the above issues.
According to a first aspect of the invention, there is provided a base station for
communicating with a plurality of mobile devices and coupled to a core network via a home base
station gateway, the base station comprising a connection management module configured to
perform a handover process and comprising means for transferring a communication link
associated with a mobile device from the base station to another base station, the other base
station coupled to the core network other than via the home base station gateway; means for
receiving a message from the other base station indicating that handover has been completed,
and means operable in response to receiving the message indicating that handover has been
completed to send an indication to the home base station gateway that the mobile station is no
longer coupled to the base station.
The base station may comprise a home base station, for example a HeNB, and the home
base station gateway may comprise a HeNB gateway for connecting the HeNB to the core
network.
The means for transferring a communication link to another base station may comprise
means for transferring the communication link to a further home base station, or further HeNB.
The further HeNB may be coupled to the core network via a different home base station gateway,
or HeNB gate way.
The means for transferring a communication link to another base station may further
comprise means for transferring the communication link to a macro cell associated with the other
base station, for example an eNB.
The handover process may comprise a X2 based handover procedure, and the message
indicating that handover has been completed may comprise a X2 UE Context Release message.
According to a further aspect of the invention, there is provided a method of facilitating
handover in a communications network, the method comprising performing a handover of a
mobile device from a source base station coupled to a core network via a base station gateway to
a target base station coupled to the core network other than via the base station gateway,
receiving a message from the target base station indicating that handover has been completed,
and in response to receiving the message indicating that handover has been completed, sending
an indication to the base station gateway that the mobile station is no longer coupled to the base
station.
According to a further aspect of the invention, there is provided a home base station
gateway for communicating with a plurality of base stations and coupled to a core network, the
base station gateway comprising a mobile telephone registration module configured to receive an
indication from a base station coupled to the base station gateway that a communication link
associated with a mobile device has been transferred from the base station to a further base
station and further configured to release a context associated with the mobile device in response
to receiving the indication.
The further base station is not coupled to the core network via the base station gateway,
for example the further base station may be directly coupled to the core network or may be
coupled via a different home base station gateway.
The transfer of the communication link may comprise an X2 based handover procedure.
The base station may be a home base station, such as a HeNB.
According to a further aspect of the invention, there is provided a method of facilitating
handover in a communications network, the method comprising receiving at a base station
gateway an indication from a base station coupled to the base station gateway that a
communication link associated with a mobile device has been transferred from the base station to
a further base station, and in response to receiving the indication, releasing a context associated
with the mobile station.
According to a further aspect of the invention, there is provided a home base station
gateway for communicating with a plurality of base stations and coupled to a core network, the
base station gateway comprising a mobile telephone registration module configured to identify a
path switch request from a base station, the path switch request associated with a communication
link transferred to the base station, the mobile telephone registration module further configured
to assign a context to a mobile device associated with the communication link in response to
identifying the path switch request.
The base station may be a home base station such as an HeNB, and may be coupled to
the core network via the base station gateway. The transfer of the communication link to the
base station may comprise an X2 based handover procedure.
According to a further aspect of the invention, there is provided a base station for
communicating with a plurality of mobile devices and coupled to a core network via a home base
station gateway, the base station comprising a connection management module configured to
perform a handover process comprising: transferring a communication link associated with a
mobile device to the base station from another base station, the other base station coupled to the
core network other than via the home base station gateway, the handover process further
comprising receiving a message from the other base station providing information identifying the
mobile device, and storing the information identifying the mobile device at the base station.
The base station may comprise a home base station, for example a HeNB. The transfer
of the communication link may comprise an X2 based handover procedure.
The information identifying the mobile device comprises an MME UE S1AP ID value.
According to a further aspect of the invention, there is provided a user equipment
adapted to operate with a base station as described above.
According to a further aspect of the invention, there is provide a computer program
product comprising computer program code configured to perform any of the above described
methods.
According to a further aspect of the invention, there is provided a base station for
communicating with a plurality of mobile devices and coupled to a core network via a home base
station gateway, the base station comprising a connection management module configured to
perform a handover process comprising transferring a communication link associated with a
mobile device from the base station to another base station, the other base station coupled to the
core network other than via the home base station gateway, the handover process further
comprising receiving a message from the other base station indicating that handover has been
completed, and in response to receiving the message indicating that handover has been
completed sending an indication to the home base station gateway that the mobile station is no
longer coupled to the base station.
According to a further aspect of the invention, there is provided a base station for
communicating with a plurality of mobile devices and coupled to a core network via a home base
station gateway, the base station comprising a connection management module configured to
perform a handover process and comprising means for transferring a communication link
associated with a mobile device to the base station from another base station, the other base
station coupled to the core network other than via the home base station gateway; means for
receiving a message from the other base station providing information identifying the mobile
device, and means for storing the information identifying the mobile device at the base station.
Brief Description of the Drawings:
Embodiments of the invention will now be described, by way of example, with reference
to the accompanying drawings in which:
Figure 1 schematically illustrates a mobile telecommunication system;
Figures 2a and 2b schematically illustrate hand over scenarios in the mobile
telecommunication system of Figure 1;
Figure 3 is a block diagram of a home eNB forming part of the system shown in Figure 1;
Figure 4 is a block diagram of a home base station gateway forming part of the system
shown in Figure 1;
Figure 5 is a timing diagram indicating a handover procedure by which communication
with a mobile telephone is transferred between a source and target home base station;
Figure 6 is a timing diagram indicating a handover procedure by which communication
with a mobile telephone is transferred between a source and target home base station served by
different serving gateways in the core network;
Figure 7 schematically illustrates a further hand over scenario in the mobile
telecommunication system of Figure 1; and
Figure 8 is a timing diagram indicating a portion of the handover procedure of Figure 5.
Detailed Description of Exemplary Embodiments:
Overview
Figure 1 schematically illustrates a mobile (cellular) telecommunication system 1 in
which a user of a mobile telephone 3 can communicate with other users (not shown) via a
'home' base station (HeNB) 11-1, which he may be permitted to access. The HeNB 11-1 is
coupled via a home base station gateway (HeNB-GW) 15, to the core telephone network 8
including mobile management entity/serving gateway (MME/S-GW) 6-1 . In the
telecommunication system 1 the user may also communicate with the other users (not shown) via
an HeNB 1-2 which, in this example, is also coupled via the same home base station gateway
(HeNB-GW) 15, to the mobile management entity/serving gateway 6-1.
In the example of Figure 1, the HeNBs 11-1 and 11-2 connect to a common HeNB-GW
15 via a suitable network connection such as an ADSL or cable connection providing an Internet
connection and are programmed with the IP address of the HeNB-GW 15 so that all uplink
communications are transmitted via HeNB-GW 15. HeNB 11-3 provides a further cell area in
which a user may connect via HeNB 11-3 and a suitable internet connection such as an ADSL or
cable connection to a mobile management entity/serving gateway 6-2 without connecting to
HeNB-GW 15. It is noted that while HeNB 11-3 is shown as connecting to a different MME/SGW
6-2 than HeNBs 11-1 and 11-2, HeNB 11-3 could connect to the same MME/S-GW 6-1
without connecting via HeNB GW 15.
As is well known, a mobile telephone 3 may be transferred from a cell associated with a
first HeNB 11-2 to a second cell associated with a second HeNB 11-3. The process of
performing the handover HO is illustrated schematically in Figure 1 and described in more detail
below with reference to Figure 6. The MME 6-1, and or MME 6-2, keeps track of mobile
devices and their connections to the base stations as they move through the network in a database
of Mobility Management Entity (MME) and Evolved Packet System (EPS) bearer contexts.
Figures 2a and 2b show further handover scenarios in which context and resources
allocated to a mobile station at the HeNB gateway 15 may not be released after an X2 based
handover of the mobile station 3.
In the scenario illustrated in Figure 2a, mobile telephone 3 is transferred from a cell
associated with source HeNB 11-2 to target HeNB 11-3. In this case, source HeNB 11-2 is
coupled to the core network 8 via a first HeNB GW 15-1, while target HeNB 11-3 is connected
to the core network 8 via HeNB GW 15-2.
The scenario illustrated in Figure 2b is similar to the handover scenario of Figure 1,
except that the target base station comprises eNB 13, rather than a further HeNB. Some
implementations of the telecommunication network 1 may allow such handovers to take place,
and in such implementations the problem addressed by embodiments of the invention will
manifest.
In each of the handover scenarios illustrated in Figures 1, 2a and 2b, embodiments of the
invention provide a method of facilitating release of context and resources assigned to a mobile
telephone 3 at the HeNB gateway 15-1 when an X2 based handover procedure is used to
handover mobile telephone from a source HeNB coupled to the core network 8 via the HeNB
gateway 15-1 to a target base station that is not coupled via the same HeNB gateway 15-1.
As will be recognized by the skilled person, core network 8 may include many other
entities and modules required to provide the communication network 1, which have not been
shown in the figures to aid clarity. Such entities include the packet data network (PDN) gateway
for routing packet data to a mobile device.
Home Base Station (HeNB)
Figure 3 is a block diagram illustrating the main components of one of the home base
stations (HeNB) 11 shown in Figure . As shown, the HeNB 1 includes transceiver circuitry 51
which is operable to transmit signals to, and to receive signals from, the mobile telephone 3 via
one or more antennae 53 and which is operable to transmit signals to and to receive signals from
the HeNB-GW 15 via an HeNB-GW interface 55. The operation of the transceiver circuitry 5 1
is controlled by a controller 57 in accordance with software stored in memory 59. The software
includes, among other things, an operating system 61, a base station registration module 63, a
mobile telephone registration module 65, and a connection management module 67.
The base station registration module 63 is operable to register the HeNB with the HeNBGW
15 and the telephone registration module 65 is operable to register the user equipment 3
with the HeNB 11 and with the HeNB-GW 15. The connection management module 67 is
operable to manage the connections of mobile telephones camped on the cell of the HNB 11 and
to broadcast system information relating to the cell to mobile telephones 3 in the vicinity.
On conclusion of a successful handover of a mobile telephone, the connection
management module 67 of a target HeNB 11 is configured to transmit a message to inform the
source HeNB to release context and resources associated with the mobile telephone. Upon
receipt of this message, the mobile telephone registration module 65 is operable to release the
context and resources associated with the handed over mobile telephone.
HeNB Gateway (GW)
Figure 4 is a block diagram illustrating the main components of the HeNB gateway
(HeNB-GW) 15 shown in Figure 1. As shown, the HeNB-GW 15 includes a transceiver circuit
101 which is operable to transmit signals to, and to receive signals from, the HeNB 11-1, 11-2
via the HeNB interface 103 and which is operable to transmit signals to and to receive signals
from the MME/S-GW 6 in the core network 8 via a core network interface 105. The operation of
the transceiver circuit 101 is controlled by a controller 107 in accordance with software stored in
memory 109. The software includes, among other things, an operating system 111, a HeNB
registration module 113, and a mobile telephone registration module 115. The HeNB
registration module 113 is operable to control the registration of HeNBs 11-1, 11-2 with the
gateway and the mobile telephone registration module 115 is operable to control the registration
of the user equipment 3 and to perform access control as required.
The mobile telephone registration module is further operable to receive an indication
from a home base station connected to the core network 8 via the home base station gateway,
that a user equipment previously connected to the base station, and therefore having a context
registered at the home base station gateway, has been handed over to a target base station not
coupled to the home base station gateway. In response to receiving the indication that the user
equipment has been handed over, the mobile telephone registration module is operable to release
the context and resources associated with the user equipment 3.
X2 Based Handover
An X2 based handover of a mobile telephone 3 between home base stations 11-2 and 11-
3 illustrated in Figures 1, 2a, and 2b will now be described, by way of example only, with
reference to the timing diagram of Figure 5.
As those skilled in the art will appreciate, the X2 interface is the interface via which base
stations communicate with each other. In an X2 based handover the handover process is
coordinated between the source and target base stations without overall control by the core
network 8. X2 based handover is therefore advantageous as it reduces load on the core network
8.
As shown in Figure 5, once a mobile telephone 3 establishes communication with a
source HeNB 11-2 the source HeNB 11-2 relays packet data between the mobile telephone 3 and
the MME/S-GW 6-1 of the core network 8 (via the HeNB-GW 15). During communication
between the mobile telephone 3 and source HeNB 11-2 properties of the communication
between the mobile telephone 3 and the HeNB 11-2 and other base stations are measured to
assist in control of the mobile telephone's connection mobility. The HeNB 11-2 is operable to
trigger the mobile telephone 3 to send a measurement report to the HeNB 11-2.
Based on the measurement report information and/or other radio resource management
(RRM) information, the source HeNB 11-2 can determine whether or not to hand the mobile
telephone 3 off to a target HeNB 11-3. In the event that it is determined that the mobile
telephone 3 is to be handed off to a target HeNB 11-3, the source HeNB 11-2 transmits a
handover request 42 to the target HeNB 11-3 which includes information necessary for the target
HeNB 11-3 to prepare the handover, such as information relating to quality of service and other
parameters.
The target HeNB 11-3 may perform admission control functions dependent on the
received information and, if the necessary resources can be granted by the target HeNB 11-3,
then the target configures the required resources to provide the required quality of service etc.
Once the physical and link layer (LI & L2) resources have been configured, the target HeNB 11-
3 sends a handover request acknowledgement message 44 to the source HeNB 11-2. The
handover request acknowledgement message 44 includes a transparent container to be sent to the
mobile telephone as a radio resource control, RRC, message to perform the handover.
Once the source HeNB 11-2 has received the handover request acknowledgement 44, or
as soon as the transmission of the handover command is initiated in the downlink, data
forwarding can be performed so that data received at the source HeNB 11-2 is forwarded to the
target HeNB 11-3 to be relayed to the mobile telephone 3.
The target base station 11-3 informs the MME6 in the core network 8 of the handover of
the user equipment using a path switch request 50. The purpose of the path switch request
procedure is to request that the downlink GTP (GPRS tunnelling protocol) tunnel is directed to a
GTP tunnel endpoint at the target HeNB 11-3.
An example of a path switch request message 50 is set out in Table 1. In Table 1 the
following abbreviations are used: E-RAB is used to indicate the evolved UMTS Terrestrial
Radio Access Network - Radio Access Bearer; IE is used to indicate an information element;
GTP-TEID is used to indicate a GPRS tunnelling protocol Tunnel Endpoint Identifier; EUTRAN
CGI is used to indicate the evolved UMTS Terrestrial Radio Access Network Cell
Global Identity.
Table 1.
In response to the path switch request the MME transmits an update bearer request 54 or
a create session request to the serving gateway which forms part of the MME/S-GW 6 within the
core network 8 as shown in Figure 1, which may in turn relay this request 58 to a packet data
network gateway (not shown).
In the example described above with reference to Figure 4 the source and target HeNB
are served by the same serving gateway in the core network 8. In this case, as shown in Figure 5
a modify bearer request message 54 is sent from the MME to the serving gateway and from the
serving gateway to the PDN gateway. As shown in Figure 5 these messages are acknowledged
by the serving gateway and by the PDN gateway. In the example shown in Figure 6 the source
and target base stations (e.g. HeNB 11-2, 11-3) are served by different serving gateways. In this
example, to perform serving gateway relocation the MME sends a create session request to the
target serving gateway. The target serving gateway then sends a modify bearer request message
to the PDN gateway. Once this has been done and the target base station has sent a release
resource message to the source base station the MME sends a delete session request to the source
serving gateway.
After the down link path is switched in the core network 8, packets destined for the
mobile telephone 3 are sent to the target HeNB 11-3. The target HeNB 11-3 then sends a X2AP
UE Context Release message 46 to the source HeNB 11-2, to inform the source HeNB 11-2 that
the resources previously reserved at the source HeNB 11-2 for the connection to the mobile
telephone can be released, although any ongoing data forwarding may continue.
As noted above, the X2AP UE Context Release message 46 is not provided to the HeNB
gateway 15. Furthermore, as the target HeNB 11-3 is not connected to the core network 8 via
the HeNB gateway 15, the path switch request/response message does not traverse the HeNB
gateway 15. Thus, according to embodiments of the invention, the source HeNB 11-2 upon
receipt of the X2AP Context Release message requests release of the UE context towards the
HeNB gateway 15. Thus, a message 48 from the source HeNB 11-2 traverses the HeNB
gateway 15, and provides an indication to the HeNB gateway 15 that the resources and context
associated with the mobile telephone 3 can be released.
Thus according to embodiments of the invention, for any hand over scenario in which a
mobile station 3 is transferred from a cell associated with a source HeNB 11 coupled to the core
network 8 via a HeNB gateway 15 to a cell associated with a target base station that is not
coupled to the HeNB gateway 15, such as the examples shown in Figures 1, 2a and 2b, the
source HeNB 11 provides a Context Release indication message 48 that traverses the HeNB
gateway 15 and informs the gateway 15 that the context and resources associated with the mobile
telephone 3 can be released.
Modifications and Alternatives
A number of detailed embodiments have been described above. As those skilled in the
art will appreciate, a number of modifications and alternatives can be made to the above
embodiments whilst still benefiting from the inventions embodied therein.
While the above discussion relates to the hand over of a user equipment from a source
HeNB 11-2 connected to the core network 8 via the HeNB gateway, to a target HeNB 11-3 not
connected via the same HeNB gateway 15, it is noted that a hand over in the reverse direction
(i.e. from a base station not coupled to a HeNB gateway 15 to a target HeNB 11-2 coupled to the
core network 8 via HeNB GW 15) may also lead to issues not currently addressed in the
standards. An example of such a handover is shown in Figure 7 in which mobile telephone 3 is
transferred from source HeNB 11-3 to target HeNB 11-2. It will be recognized that other
example transfers include situations analogous to those shown in Figures 2a and 2b, in which the
handover occurs in the opposite direction to that shown.
Figure 8 illustrates the exchange of messages between a target HeNB 11-2 connected to
the MME 6 of the core network 8 via a HeNB gateway 15 in order to instruct the core network 8
to switch the termination point of the connection to a mobile telephone 3 transferred to the target
HeNB 11-2. As coordination of the handover to the target HeNB uses the X2 interface, the first
message relating to the mobile telephone 3 that traverses the HeNB gateway 15 is the Sl-AP
Path Switch Request Message 50. Thus, this is the first occasion for which a UE associated
signalling message for the handed over mobile telephone 3 is received at the HeNB gateway. In
order to support this scenario, new functionalities may be provided in the HeNB gateway 15
according to some embodiments of the invention. In particular:
• The Mobile Telephone Registration Module 115 of the HeNB gateway 15 may be
further configured to allocate the UE context upon reception of the SI Path Switch
Request Message.
• The HeNB gateway 15 may provide a proxy function for subsequent messages
associated with the handed over UE 3 between the HeNB and the core network 8,
including allocating a eNB UE SIAP IDGW towards the MME 6, and a MME UE
SIAP IDQW towards the HeNB 11-2. The HeNB gateway will exchange the MME
S1AP UE ID and eNB S1AP UE ID values between the global and as necessary
according to the proxy function.
• The HeNB gateway may be required to include the actual MME UE S1AP ID value
in the S1AP Path Switch Acknowledgement message to the target HeNB for use in
further handovers.
Furthermore, in order to support further X2 based handover procedures, the target HeNB
11-2 should store eNB UE SIAP ID and MME UE S1AP ID values associated with the mobile
telephone 3 and received from the source HeNB or eNB as part of the X2 based handover
procedure.
The target HeNB can then store the MME UE S1AP ID value received from the source
eNB during handover for use in future handovers, while using the MME UE S AP IDGW value
provided by the HeNB GW 15 for all other signalling towards the HeNB GW 15 and MME 6.
Alternatively, the HeNB GW 15 may provide separate MME UE SIAP IDGW and MME
UE SIAP ID2 values to the HeNB 11-2 associated with the mobile telephone 3. In this case, the
HeNB 11-2 will store the MME UE SIAP ID2 value for use in future X2 based handover
procedures.
In the above embodiments, a mobile telephone based telecommunications system was
described. As those skilled in the art will appreciate, the signalling techniques described in the
present application can be employed in other communications system. Other communications
nodes or devices may include user equipment devices such as, for example, personal digital
assistants, laptop computers, web browsers, etc.
In the embodiments described above, the mobile telephone and the HNB each include
transceiver circuitry. Typically this circuitry will be formed by dedicated hardware circuits.
However, in some embodiments, part of the transceiver circuitry may be implemented as
software run by the corresponding controller.
In the above embodiments, a number of software modules were described. As those
skilled in the art will appreciate, the software modules may be provided in compiled or uncompiled
form and may be supplied to the HNB or to the mobile telephone as a signal over a
computer network, or on a recording medium. Further, the functionality performed by part or all
of this software may be performed using one or more dedicated hardware circuits. However, the
use of software modules is preferred as it facilitates the updating of the base stations, gateways,
and the mobile telephones in order to update their functionalities.
The description of the handover process provided above has been given with specific
reference to home NodeBs however embodiments of the invention may be applied to other base
stations such as eNodeBs or RAN base stations.
While the above discussion relates to a hand over between a HeNB 11-2 coupled to a
core network via a HeNB gateway 15, and a further HeNB 11-3 coupled directly to the core
network, it is noted that in other embodiments of the invention, the hand over may be between
two HeNBs 11 coupled to the core network via different HeNB gateways.
Various other modifications will be apparent to those skilled in the art and will not be
described in further detail here.
The following is a detailed description of the way in which the present inventions may be
implemented in the currently proposed 3GPP standard. Whilst various features are described as
being essential or necessary, this may only be the case for the proposed 3GPP standard, for
example due to other requirements imposed by the standard. These statements should not,
therefore, be construed as limiting the present invention in any way.
Introduction
During the last RAN3 meeting (RAN3#70bis) meeting, discussion paper [1] highlighted
the open issue related to X2 based mobility for HeNB. As discussed in the paper, for the certain
deployment scenarios with HeNB GW, after the X2 based HO the HeNB GW will be left with
hanging UE contexts and related resources. RAN3 acknowledged the issue and it was decided to
discuss further and find a solution for this problem. This discussion paper analyzes the issue and
proposes potential solutions.
2 Description
It was acknowledged that the problem will happen during the X2 HO in two cases:
(a) The source HeNB is connected via the HeNB GW and target HeNB connected
directly to the MME.
(b) The source HeNB and target HeNB are connected to different HeNB GWs.
Since the source HeNB is connected behind the HeNB GW, therefore two pair of UE
associated connection is established, one between HeNB and HeNB GW and another between
HeNB GW and MME. The mapping of these two UE associated signalling connected for the
same UE is performed by the HeNB GW. Therefore, UE context and related resources are
allocated both at the HeNB and the HeNB GW.
During the X2 based HO, the target HeNB explicitly releases the UE Context at the
source HeNB using the X2AP UE Context Release procedure. On the other hand, since S1AP C
Plane message i.e. S1AP Path Switch Request/ Response message does not transverse through
the HeNB GW therefore, the HeNB GW does not have any means to understand that the UE has
been handed over (via X2) to the other HeNB. As a result, the UE context information and
related resources assigned by the HeNB GW will keep hanging. Three possible solutions have
been discussed during the last RAN3 meeting.
1. The source HeNB GW releases the UE context by itself
2. The source HeNB GW releases the UE context as the request from the source
HeNB
3. The source HeNB GW releases the UE context as the command from the MME
Solution 1 considered as an inadequate because this solution can possibly address (further
investigation needed) the deployment scenario where HeNB GW Sl-U plane transverse through
the HeNB GW, noting that Sl-U concentration in the HeNB GW is optional. Solution 3 can
work but seen as significant changes for the MME behaviour to address only this particular use
case. Therefore, it is reasonable to investigate more on the solution 2 which does not have any
impact on the MME.
The basic essence of solution 2 is that source HeNB requests the release of UE context
towards the HeNB GW. There are two possible options to handle this.
Option 1: Re-use the existing S1AP UE Context Release Request - eNB initiated procedure
The benefit of option 1 is that there is no need to define any new procedure. On the other
hand, there are some drawbacks for this solution. Firstly, this procedure is only meant to be used
for abnormal cases (as per TS 36.413) and not for the normal X2 HO. secondly, additional
signalling during X2HO since HeNB has to trigger the release of UE context towards the HeNB
GW. Thirdly, the HeNB GW has to look every S1AP UE CONTEXT RELEASE REQUEST
from the HeNB and then decide whether this message to be sent to MME (for the existing use
case of this procedure) or terminate at the HeNB GW in case it is triggered due to X2 based HO.
And, finally HeNB GW need to trigger UE Context Release (MME initiated) procedure (class 1)
after receiving S1AP UE CONTEXT RELEASE REQUEST from the HeNB, means more
signalling.
Option 2 : Define a new class 2 procedure along the line of X2AP UE Context Release
Indication procedure
The second easy mechanism is to define a new class 2 procedure along the line of X2AP
UE Context Release procedure. The source HeNB will trigger the procedure upon reception of
X2AP UE Context Release from the target HeNB. The HeNB GW behaviour in this case would
be simple because the the HeNB does not need to check the cause of every UE CONTEXT
RELEASE REQUEST message. Of course there is disadvantage that HeNB will need to
terminate UE associated signalling message which is an exception. As an alternative non UE
associated signalling could be also used.Based on the above evaluation, our preference is option
2 because it requires less signalling and less HeNB/ HeNB GW implementation complexity.
Proposal 1: NEC proposes to agree on option 2 in order to resolve the UE Context Release
issue at the HeNB GW.
The necessary changes in TS 36.413 and TS 36.300 is as described below (Annex). In
case of agreement, NEC is happy to provide the necessary CRs.
3 Conclusion and Proposals
Proposal 1: NEC proposes to agree on option 2 in order to resolve the UE Context Release
at the HeNB GW issue.
4 References
[1] TS 36.300 E- UTRAN stage-2
[2] R3-1 10108 UE Context Release in the source HeNB GW
Annex A.1
This section describes the necessary stage-2/3 changes needed in the specification.
8. 3.x UE Context Release Indication
8. 3.X.1 General
The UE Context Release Indication procedure is initiated by the eNB to signal to indicate
the source MME that UE context shall be released.
The procedure uses UE-associated signalling.
8. 3.X.2 Successful Operation
The UE Context Release Indication procedure is initiated by the eNB. By sending the UE
CONTEXT RELEASE INDICATION message the target eNB informs the MME of X2
Handover success and triggers the release of resources. This procedure is only triggered by the
eNB if different values for MME UE SIAP ID and MME UE SIAP ID2 are assigned by the
MME.
Upon reception of the UE CONTEXT RELEASE message, the MME may release UE
Context and related resources associated to the UE context.
8. 3.X.3 Unsuccessful Operation
Not applicable.
8. 3..4 Abnormal Conditions
Not applicable.
Next Change
9.1.1.5 UE CONTEXT RELEASE Indication
This message is sent by the eNB to the MME to indicate that resources can be released.
Direction: eNB MME.
IE/Group Name Presence Range IE type and Semantics Criticality Assigned
reference description Criticality
Message Type M 9.2.13 YES ignore
eNB UE SIAP ID M eNB UE X2AP ID Allocated at YES reject
9.2.24 the source
eNB
MME UE SIAP ID M eNB UE X2AP ID Allocated at YES reject
9.2.24 the target
eNB
Changes for TS 36.300
4.6.2 Functional Split
The HeNB hosts the same functions as an eNB as described in section 4.1, with the
following additional specifications in case of connection to the HeNB GW:
Discovery of a suitable Serving HeNB GW;
A HeNB shall only connect to a single HeNB GW at one time, namely no S Flex
function shall be used at the HeNB:
The HeNB will not simultaneously connect to another HeNB GW, or
another MME.
The TAC and PLMN ID used by the HeNB shall also be supported by the HeNB
GW;
Selection of an MME at UE attachment is hosted by the HeNB GW instead of the
HeNB;
HeNBs may be deployed without network planning. A HeNB may be moved from
one geographical area to another and therefore it may need to connect to different
HeNB GWs depending on its location.
- On completion of X2 HO, release of UE Context and triggering the UE Context
Release procedure.
Regardless of HeNB GW connection:
The HeNB may support the LIPA function. See section 4.6.5 for details.
The HeNB GW hosts the following functions:
Relaying UE-associated S1 application part messages between the MME serving
the UE and the HeNB serving the UE;
Terminating non-UE associated S1 application part procedures towards the HeNB
and towards the MME. Note that when a HeNB GW is deployed, non-UE
associated procedures shall be run between HeNBs and the HeNB GW and
between the HeNB GW and the MME.
Optionally terminating Sl-U interface with the HeNB and with the S-GW.
Supporting TAC and PLMN ID used by the HeNB.
X2 interfaces shall not be established between the HeNB GW and other nodes.
- Termination of the UE Context Release procedure and releasing the UE Context
based on the indication from the HeNB.
A list of CSG IDs may be included in the PAGING message. If included, the HeNB GW
may use the list of CSG IDs for paging optimization.
In addition to functions specified in section 4.1, the MME hosts the following functions:
Access control for UEs that are members of Closed Subscriber Groups (CSG):
- In case of handovers to CSG cells, access control is based on the target CSG ID
provided to the MME by the serving E-UTRAN.
Membership Verification for UEs handing over to hybrid cells:
- In case of handovers to hybrid cells Membership Verification is triggered by the
presence of the Cell Access Mode and it is based on the target CSG ID provided
to the MME by the serving E-UTRAN.
CSG membership status signalling to the target E-UTRAN in case of
attachment/handover to hybrid cells and in case of the change of membership
status when a UE is served by a CSG cell or a hybrid cell.
Supervising the eNB action after the change in the membership status of a UE.
- Routing of handover messages and MME configuration transfer messages
towards HeNB GWs based on the TAI contained in these messages.
NOTE:The MME or HeNB GW should not include the list of CSG IDs for paging
when sending the paging message directly to an un-trusted HeNB or eNB.
- The MME may support the LIPA function with HeNB. See details of this support
in section 4.6.5.
Annex A.2
9.1.5.9 PATH SWITCH REQUEST ACKNOWLEDGE
This message is sent by the MME to inform the eNB that the path switch has been
successfully completed in the EPC.
Direction: MME eNB.
IE/Group Name Presence Range IE type and Semantics Criticality Assigned
reference description Criticality
Message Type M 9.2.1.1 YES reject
MME UE S1AP ID M 9.2.3.3 YES ignore
eNB UE SlAP ID M 9.2.3.4 YES ignore
UE Aggregate O 9.2.1.20 YES ignore
Maximum Bit Rate
E-RAB To Be 0..1 YES ignore
Switched in
Uplink List
>E-RABs 1 to EACH ignore
Switched in
»E-RAB ID M 9.2.1.2 -
»Transport M 9.2.2.1 -
Layer Address
»GTP-TEID M 9.2.2.2 -
E-RAB To Be 0 E-RAB List a value for E- YES ignore
Released List 9.2.1.36 RAB ID shall
only be
present once
in E-RAB To
Be Switched
in Uplink List
IE + E-RAB
to Be
Released List
IE
Security Context M 9.2.1.26 One pair of YES reject
{NCC, NH}
is provided
Criticality 0 9.2.1.21 YES ignore
Diagnostics
MME UE S1AP ID 0 9.2.3.3 This IE YES reject
2 indicates the
MME UE
S1AP ID
assigned by
the MME
This application is based upon and claims the benefit of priority from United Kingdom
patent application No. 1102883.4, filed on February 18, 20 11, the disclosure of which is
incorporated herein in its entirety by reference.
WO 2012/111844 PCT/JP2012/054177
CLAIMS
. A base station for communicating with a plurality of mobile devices and coupled to a
core network via a home base station gateway, the base station comprising:
a connection management module configured to perform a handover process and
comprising:
means for transferring a communication link associated with a mobile device from the
base station to another base station, the other base station coupled to the core network other than
via the home base station gateway;
means for receiving a message from the other base station indicating that handover has
been completed, and
means operable in response to receiving the message indicating that handover has been
completed to send an indication to the home base station gateway that the mobile station is no
longer coupled to the base station.
2. The base station of claim 1, wherein the base station comprises a home base station.
3. The base station of claim 1 or claim 2, wherein the means for transferring a
communication link to another base station comprise means for transferring the communication
link to a further home base station.
4. The base station of claim 3, wherein the further home base station is coupled to the core
network via a further home base station gateway.
5. The base station of claim 1, wherein the means for transferring a communication link to
another base station comprise means for transferring the communication link to a macro cell
associated with the other base station.
6. The base station of any one of the preceding claims, wherein the handover process
comprises a X2 based handover procedure.
7. The base station of any one of the preceding claims, wherein the message indicating that
handover has been completed comprises a X2 UE Context Release message.
8. A method of facilitating handover in a communications network, the method comprising:
performing a handover of a mobile device from a source base station coupled to a core
network via a base station gateway to a target base station coupled to the core network other than
via the base station gateway;
receiving a message from the target base station indicating that handover has been
completed; and
in response to receiving the message indicating that handover has been completed,
WO 2012/111844 PCT/JP2012/054177
sending an indication to the base station gateway that the mobile station is no longer coupled to
the base station.
9. A home base station gateway for communicating with a plurality of base stations and
coupled to a core network, the base station gateway comprising:
a mobile telephone registration module configured to receive an indication from a base
station coupled to the base station gateway that a communication link associated with a mobile
device has been transferred from the base station to a further base station and further configured
to release a context associated with the mobile device in response to receiving the indication.
10. The home base station gateway of claim 9, wherein the further base station is not coupled
to the core network via the base station gateway.
11. The home base station gateway of claim 9 or claim 0, wherein the transfer of the
communication link comprises an X2 based handover procedure.
12. The home base station gateway of any one of claims 9 to 11, wherein the base station
comprises a home base station
13. A method of facilitating handover in a communications network, the method comprising:
receiving at a base station gateway an indication from a base station coupled to the base
station gateway that a communication link associated with a mobile device has been transferred
from the base station to a further base station; and
in response to receiving the indication, releasing a context associated with the mobile
station.
14. A home base station gateway for communicating with a plurality of base stations and
coupled to a core network, the base station gateway comprising:
a mobile telephone registration module configured to identify a path switch request from
a base station, the path switch request associated with a communication link transferred to the
base station;
the mobile telephone registration module further configured to assign a context to a
mobile device associated with the communication link in response to identifying the path switch
request.
15. The home base station gateway of claim 14, wherein the base station is coupled to the
core network via the base station gateway.
16. The home base station gateway of claim 14 or claim 15, wherein the transfer of the
communication link to the base station comprises an X2 based handover procedure.
17. A base station for communicating with a plurality of mobile devices and coupled to a
core network via a home base station gateway, the base station comprising:
WO 2012/111844 PCT/JP2012/054177
a connection management module configured to perform a handover process comprising:
transferring a communication link associated with a mobile device to the base station from
another base station, the other base station coupled to the core network other than via the home
base station gateway, the handover process further comprising receiving a message from the
other base station providing information identifying the mobile device, and storing the
information identifying the mobile device at the base station.
18. The base station of claim 17, wherein the base station comprises a home base station.
19. The base station of claim 17 or claim 18, wherein the transfer of the communication link
comprises an X2 based handover procedure.
20. The base station of any one of claims 17 to 19, wherein the information identifying the
mobile device comprises an MME UE S1AP ID value.
2 . User equipment adapted to operate with the base station according to any of claims 1 to 7,
or 17 to 20.
22. A computer program product comprising computer program code configured to perform
the steps of claim 8 or claim 13 when executed on a processor
23. A base station for communicating with a plurality of mobile devices and coupled to a
core network via a home base station gateway, the base station comprising:
a connection management module configured to perform a handover process comprising:
transferring a communication link associated with a mobile device from the base station to
another base station, the other base station coupled to the core network other than via the home
base station gateway, the handover process further comprising receiving a message from the
other base station indicating that handover has been completed, and in response to receiving the
message indicating that handover has been completed sending an indication to the home base
station gateway that the mobile station is no longer coupled to the base station.
24. A base station for communicating with a plurality of mobile devices and coupled to a
core network via a home base station gateway, the base station comprising:
a connection management module configured to perform a handover process and
comprising: means for transferring a communication link associated with a mobile device to the
base station from another base station, the other base station coupled to the core network other
than via the home base station gateway; means for receiving a message from the other base
station providing information identifying the mobile device, and means for storing the
information identifying the mobile device at the base station.
25. A mobile terminal configured for handover, from a base station adapted to perform a
method according to claim 13.