WO2011020481A1

WO2011020481A1 - Method for supporting handover decisions of a mobile terminal in a mobile cellular communication network

Info

Publication number: WO2011020481A1
Application number: PCT/EP2009/006012
Authority: WO
Inventors: Marcus Schoeller; Frank Zdarsky; Stefan Schmid
Original assignee: NEC Europe Ltd
Current assignee: NEC Europe Ltd
Priority date: 2009-08-19
Filing date: 2009-08-19
Publication date: 2011-02-24
Anticipated expiration: 2012-02-19
Also published as: JP2013502783A

Abstract

A method for supporting handover decisions of a mobile terminal in a cellular mobile communication network, said network comprising mobile femtocells, is characterized in the steps of detecting the presence of a mobile femtocell close to said mobile terminal, evaluating a possible joint movement of said mobile terminal with said mobile femtocell, and handing over of said mobile terminal to said mobile femtocell in case said evaluation yields that a joint movement with said mobile femtocell is given.

Description

METHOD FOR SUPPORTING HANDOVER DECISIONS OF A MOBILE TERMINAL IN A MOBILE CELLULAR COMMUNICATION

NETWORK

The present invention relates to a method for supporting handover decisions of a mobile terminal in a cellular mobile communication network, said network comprising mobile femtocells.

In mobile cellular networks, a mobile terminal - in 3GPP commonly denoted as user equipment (UE) - is typically located within the coverage range of multiple macro cell base stations (BSs) at the same time. The decision, which BS shall serve the UE, depends on various factors, the most important factor being the quality of the radio channels between the UE and the BSs in question. When the radio channel between UE and serving BS deteriorates below a given threshold, a decision to hand over the UE to another BS with better radio channel quality has to be made and the handover process initiated. To prepare this handover the BS generally provides a list of other BSs in the vicinity to the UE to minimize the frequencies to be scanned by the UE for better signals.

In a traditional mobile cellular network, handovers are mostly due to UE movement relative to static BSs. Handovers are typically initiated as late as possible, to reduce signaling overhead and oscillations (so called "ping pong effect"), yet early enough to minimize call dropping.

A current development in mobile cellular networks is that of mobile BSs with extremely small coverage ranges ("mobile femtocells", MFCs), which are, for example, deployed in public transportation means like busses or trains. From the viewpoint of a mobile network operator, such femtocells have the advantage of improving network capacity due to their smaller cell sizes, which allow a higher spatial reuse. The small and constant distance between a mobile femtocell and a passenger's terminal allows constantly high channel qualities at a lower transmit power during the whole duration of the trip. For the passenger, lower transmit power additionally means a longer battery-lifetime of his mobile terminal. Furthermore, the possibility to aggregate communication of several mobile terminals on the link between the mobile femtocell and the macrocell allows for a more efficient use of network resources by lowering the signaling and data forwarding overhead. Mobile femtocells are also an enabler for new kinds of mobility-related and location-based services.

However, the deployment of MFCs significantly increases the complexity of deciding when to perform a handover and which new BS or MFC to hand over to. Due to the mobile femtocells' small coverage area and their mobility it is very difficult to make any predictions whether the radio channel quality experienced by a UE entering the coverage area of an MFC will be sufficiently high over a sufficiently long time period so that a handover makes sense from the network operator's and/or the UE's viewpoint.

It is therefore an object of the present invention to improve and further develop a method for supporting handover decisions of the initially describe type in an efficient way such that signaling overhead is reduced, unnecessary oscillations are widely avoided, and network capacity is enhanced.

In accordance with the invention, the aforementioned object is accomplished by a method comprising the features of claim 1. According to this claim such a method is characterized in the steps of detecting the presence of a mobile femtocell close to said mobile terminal, evaluating a possible joint movement of said mobile terminal with said mobile femtocell, and handing over of said mobile terminal to said mobile femtocell in case said evaluation yields that a joint movement with said mobile femtocell is given.

According to the invention it has been recognized that the increasing deployment of mobile femtocells opens up promising opportunities for both end users as well as network operators, but at the same time requires new mechanisms regarding the performance of economically reasonable handover decisions. To solve the problems outlined above the present invention proposes first that the mobile terminal detects the presence of a mobile femtocell close to the mobile terminal. Detecting the presence of a mobile femtocell close to a mobile terminal means that mechanisms are provided that make the mobile terminal getting aware that it is located within the coverage area of a mobile femtocell.

However, it has been further recognized that just noticing the presence of a mobile femtocell is not sufficient for performing effective handovers due to the MFC's mobility. Consequently, in a further step the method according to the present invention proposes to evaluate a possible joint movement of a mobile terminal with a mobile femtocell. Dependent on the outcome of this joint movement evaluation process it is decided whether a handover of the mobile terminal to the mobile femtocell is performed or not. Having identified a jointly moving mobile femtocell enables improved pro-active accelerated hand-over decision algorithms relative to those for static BSs, resulting in an improved frequency reuse within the macro cell and a reduced signal overhead. The proposed solution allows a UE to find/choose the best available cell while on the move, and it prevents (or at least minimizes) erroneous handovers to mobile femtocells which do not move jointly with the UE. Further, the method according to the present invention is energy-efficient due to low power communication of mobile terminals with femtocells.

When referred to "mobile terminal" hereafter, this terminology includes but is not limited to a user equipment (UE), a mobile phone, a handset, a PDA (personal digital assistant), a computer, or any other user device capable of operating in a cellular mobile telecommunication network.

To enable mobile terminals to easily notice the presence of a mobile femtocell, it may be provided that mobile femtocells indicate the fact that they are mobile as part of the system information they regularly broadcast. In particular in a 3GPP architecture, the information whether a femtocell is mobile or not may be included in System Information Block Type 1 (e.g. according to 3GPP TS 25.331 and TS 36.331 ) of the system information, which are broadcasted by a femtocell. Either a Boolean indicating its mobility or an undefined GPS coordinate (-1 ,-1 ) or both can be used. Alternatively or additionally, the System Information Block Type 4 (e.g. according to 3GPP TS 25.331 and TS 36.331 ) of the system information may be employed. The SystemlnformationBlockType4 contains neighboring cell related information relevant only for intra-frequency cell re-selection. The IE includes cells - A - with specific re-selection parameters as well as blacklisted cells. Again, either a Boolean indicating a mobile femtocell's mobility or an undefined GPS coordinate (- 1 ,-1 ) or both can be used. Similar extensions to other architectures, i.e., WiMAX, can be applied.

According to a further preferred embodiment, other femtocells and/or macro cells of the network may broadcast in their neighbor cell list the fact that a specific mobile femtocell is mobile. In this context, the neighbor cell list announcement may be extended to host mobile femtocells and a special marker may be introduced to mark mobile femtocell entries in this neighbor cell list. Further it may be provided that mobile terminals actively perform radio-based measurements for such announcements of mobile femtocells.

According to another embodiment that may be employed independently or in combination with one or more of the aforementioned embodiments, presence beacons are used to announce mobile femtocells. In this regard it may be provided that either the mobile femtocells itself or another radio-, light- or sound-based technology sends out a periodic signal with a sufficiently low power so that it is only receivable by mobile terminals located within close vicinity, e.g. within the interior of a public transport like a bus or a tram. The radio-, light- or sound-based technology may include for example, but not limited to, RFID tags (Radio Frequency IDentification), Bluetooth, ZigBee, NFC (Near Field Communication), light-modulation transmitters, ultrasonic transducers, etc. The signals emitted by these devices may encode an identifier for the mobile femtocell the presence of which they announce.

Independent of which of the aforementioned specific methods for making a mobile femtocell visible for its environment is implemented, it may be provided that the fact that the mobile terminal notices the mobile femtocell (e.g. by receiving a respective presence beacon) serves as an implicit trigger for the mobile terminal to start the mobile femtocell detection process.

Concerning the movements of a mobile terminal it proves to be advantageous that a mobile terminal, while being in idle mode, detects that it is moving based on system information received from different macro cells and femtocells of the network. For instance, frequent changes of macro cells and/or location areas can be used as indication for movements of the mobile terminal and as an explicit trigger for the mobile terminal to actively start detecting the presence of a mobile femtocell, e.g. by starting to perform active scans for the presence of mobile femtocells.

According to another embodiment it is the network that detects that a mobile terminal is moving. Such movement detection, which may be based on frequent location area updates, may be performed by the network's MSC (Mobile Switching Center) or the SGSN/MME (Serving GPRS Support Node/Mobility Management Entity) that handles mobility management for the mobile terminal. In case the network detects movements of the mobile terminal, the network may trigger the mobile terminal to actively perform radio-based measurements in order to identify the most appropriate mobile femtocell for the mobile terminal. The trigger may be realized by way of paging the mobile terminal, e.g. with a specific page cause.

Advantageously, the trigger from the network to the mobile terminal to actively perform radio-based measurements is performed depended on the network's explicit knowledge of the potential presence of mobile femtocells. For instance, the network could easily detect that a mobile terminal is located on a certain railroad on which mobile femtocells are available, and as a result trigger the mobile terminal to perform the active scan. In such case, the network could also indicate the specific mobile femtocell information as part of the neighbor cell list.

With respect to an effective detection of a joint movement between a mobile terminal and a mobile femtocell it may be provided that an announcement of context information related to the presence of a mobile femtocell is correlated with context information of the mobile terminal. The term "context" is to be understood in a broad sense according to a definition given by Dey et al. ('Towards a better understanding of context and context-awareness", proceedings of the Workshop on the What, Who, Where, When and How of context-awareness, affiliated with a CHI 2000 conference on human factures in computer systems, New York, NY 2000). Accordingly, context may include "any information that can be used characterized of the situation of entities (i.e. whether a person, place or object) that are considered relevant to the interaction between a user and an application, including the user and the application themselves. Context is particularly the location, identity and state of people, groups and computations in physical objects." For instance, according to an embodiment of the present invention the announcement of specific context information that a mobile femtocell is onboard a bus of line 4 can be correlated with the context information of a mobile terminal that the user is about to board or get off that bus. Such context information may be inferred from the user's calendar, from a movement history or from the public transport's time schedule. Alternate context information may include, but is not limited to, geographical position together with direction and speed of movement, e.g. inferred form GPS information, vehicular sensors, etc.

With respect to an enhanced flexibility the context information of a mobile terminal may be communicated either to a present mobile femtocell or to the mobile terminal's operator network or to both of them. The correlation of information to detect joint movement between a mobile terminal and a mobile femtocell can then be done either on the mobile femtocell or in the operator network, respectively, depending on the specific circumstances.

According to a preferred embodiment the evaluation of a possible joint movement of a mobile terminal with a mobile femtocell includes the step of analyzing over a predefined measurement time period the signal strength the mobile terminal receives from the mobile femtocell. Further it may be provided that a mobile terminal assumes to move jointly with a mobile femtocell in case the signal strength is stable within a predefined range and above a predefined threshold over the predefined measurement time period. The measurement time period as well as the signal strength threshold may be automatically adjusted based on past performance and/or context information (e.g. based on the mobile femtocell's name/ID, location, history, etc.).

According to another preferred embodiment that uses network resources to full capacity, joint movement is detected by a mobile femtocell performing measurements of the signal strength of non-attached mobile terminals located in its vicinity. Such joint movement detection at the femtocell can be done if an appropriate medium access technology - like e.g. OFDMA (Orthogonal Frequency Division Multiple Access) - or secondary radio for measurements exists that allow signal strength measurements of mobile terminals that are located in the mobile femtoceN's coverage area, but are not attached to it. A joint movement can be assumed to have been detected if the signal strength received from a non- attached mobile terminal changes (semi-)synchronously with the signal output power of the mobile femotcell over time and the femtocell is moving. Again, the measurements time period and the signal strength threshold may be automatically adjusted.

To take further advantage of the available network resources, joint movement detection may be carried out by a macrocell. If an appropriate medium access technology, e.g. OFDMA, or a secondary radio measurement exist, which allow signal strength measurement of mobile terminals and femtocells, a macrocell can correlate changes of signal strength of jointly moving mobile terminals and femtocells. In any case, having detected a mobile femtocell and joint movement of a mobile terminal with it, an expedited handover of the mobile terminal to the mobile femtocell can be executed.

There are several ways how to design and further develop the teaching of the present invention in an advantageous way. To this end, it is to be referred to the patent claims subordinate to patent claim 1 on the one hand, and to the following explanation of a preferred example of an embodiment of the invention illustrated by the drawing on the other hand. In connection with the explanation of the preferred example of an embodiment of the invention by the aid of the drawing, generally preferred embodiments and further developments of the teaching will be explained. In the drawings

Fig.1 schematically illustrates an example scenario in which the method according to the present invention can be applied, and

Fig.2 is a diagram showing the results of different signal strength measurements upon which handover decisions can be based. The scenario depicted in Fig. 1 shows a person P waiting at a bus stop for his line to arrive. Multiple busses and trams pass by which person P does not enter either because the vehicle is not stopping at this bus stop or because it is the wrong line. Upon entering a bus the person's P mobile terminal UE should detect the onboard femtocell. The onboard femtocell detection may be performed by the UE receiving respective presence beacon announcements emitted by the onboard femtocell on a regular basis.

As concerns the mobile femtocell announcements, a femtocell can indicate whether it is mobile or not as part of the System Information Block Type 1 , which are broadcasted by a femtocell. Either a Boolean indicating the femtoceH's mobility or a undefined GPS coordinate (-1 ,-1 ) or both can be used as follows:

SystemlnformationBlockTypel SEQUENCE {

cellAccessRelatedlnfo SEQUENCE {

plmn-IdentityList PLMN-IdentityList,

trackingAreaCode TrackingAreaCode,

cellldentity Cellldentity,

cellBarred ENUMERATED {barred, notBarred}, intraFreqReselection ENUMERATED {allowed, notAllowed}, csg-Indication BOOLEAN,

csg-Identity BIT STRING (SIZE (27))

},

cellSelectionlnfo SEQUENCE {

q-RxLevMin Q-RxLevMin,

q-RxLevMinOffset INTEGER (1..8)

},

p-Max P-Max

freqBandlndicator INTEGER (1..64),

schedulinglnfoList SchedulinglnfoList,

tdd-Config TDD-Config

ceLL-mobiLe BOOLEANj

cell-Location GPS Coordinates / -1,-1 for mobiLe ceLL

Alternatively, the announcements may be based on Systemlnformation BlockType4 that contains neighboring cell related information relevant for intra- frequency cell re-selection. The IE includes cells with specific re-selection parameters as well as blacklisted cells. Either a Boolean indicating a femtocell's mobility or an undefined GPS coordinate (-1 ,-1 ) or both can be used as follows:

SystemInformationBlockType4 : : = SEQUENCE {

intraFreqNeighCellList IntraFreqNeighCellList

intraFreqBlackCellList IntraFreqBlackCellList

csg-PhysCellldRange PhysCellldRange

•••

}

IntraFreqNeighCelllnfo ::= SEQUENCE {

physCellld PhysCellld,

q-OffsetCell Q-OffsetRange,

ceLL-mobiLe BOOLEAN,

cβLL-Location GPS Coordinates / -1_}-1 for mobiLe ceLL

}

Having detected the mobile femtocell, the relative movement of the UE compared to this mobile femtocell has to be derived.

To this end the UE analyzes the signal strength it receives from the mobile femtocell over a predefined measurement time period. In case the signal strength is stable within a predefined range and above a predefined threshold over the predefined measurement time period, the mobile terminal assumes to move jointly with the mobile femtocell and hands over to it.

Basically, in the public transport scenario as illustrated in Fig. 1 , the following different situations can be regarded as common:

• The UE's of all passengers boarding a bus should be handed over to that bus' MFC, which is static relative to the passengers' UEs. Ideally, this should be done as early as possible and not only when the UE leaves its serving BS' coverage range.

• People waiting at a bus stop for a different line should not be handed over to the MFC of a passing bus, but should rest attached to their serving BS.

• Passengers getting off a bus should be handed over to the local BS as soon as possible, because the signal from the departing bus will quickly be lost. • Passengers walking through a train with multiple MFCs may require very timely handovers between MFCs, but should not be handed over to a BS, neither while the train is moving nor while it is stopped at a train station.

Fig. 2 shows one example embodiment of the disclosed invention based on measurement results of received signals from various mobile femtocells. A bus passing by is only in communication range for a very short time (rhombic measurement points). A bus stopping in front of a person will be in communication range of the person's UE until it moves on (quadratic measurement points). As the person's selected bus arrives and the person enters this bus the signal strength of the mobile femtocell will stay high over a longer time (triangular measurement points). In comparison, the signal strength received from a macro cell is constantly high (x-shaped measurement points).

These different measurement base lines can be utilized to detect which bus a passenger entered implying with which femtocell the passenger's UE should be attached. If the signal strength of a mobile femtocell stays high over a predefined time period, as is the case for the triangular measurement points, the UE should be switched to this femtocell. This requires that mobile femtocells announce themselves as such type of femtocells. Therefore, a descriptor is added to the cell announcement. Secondly, a time interval has to be defined after which the likelihood of joint movement is high. This interval is scenario dependent.

Many modifications and other embodiments of the invention set forth herein will come to mind the one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing description and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

C l a i m s

1. Method for supporting handover decisions of a mobile terminal in a cellular mobile communication network, said network comprising mobile femtocells, c h a r a c t e r i z e d i n the steps of

detecting the presence of a mobile femtocell close to said mobile terminal, evaluating a possible joint movement of said mobile terminal with said mobile femtocell, and

handing over of said mobile terminal to said mobile femtocell in case said evaluation yields that a joint movement with said mobile femtocell is given.

2. Method according to claim 1 , wherein a mobile femtocell indicates the fact that it is mobile as part of the System Information that it broadcasts.

3. Method according to claim 1 or 2, wherein other femtocells and/or macro cells of said network broadcast in their neighbor cell list the fact that a mobile femtocell is mobile.

4. Method according to any of claims 1 to 3, wherein presence beacons are used to announce mobile femtocells.

5. Method according to claim 4, wherein said presence beacons are sent out by a mobile femtocell itself or by another radio-, light-, and/or sound-based technology.

6. Method according to any of claims 1 to 5, wherein said mobile terminal, while being in idle mode, detects that it is moving based on system information received from different macro cells of said network.

7. Method according to claim 6, wherein said detection of mobile terminal movement serves as a trigger for said mobile terminal to actively start detecting the presence of a mobile femtocell.

8. Method according to any of claims 1 to 7, wherein movements of said mobile terminal are detected by said network, in particular by said network's MSC (Mobile Switching Center).

9. Method according to any of claims 1 to 8, wherein said network, in case it detects that said mobile terminal is moving, triggers said mobile terminal, in particular by way of paging, to actively perform radio-based measurements.

10. Method according to claim 9, wherein said trigger is performed by said network dependent on said network's explicit knowledge of potential presence of mobile femtocells.

11. Method according to any of claims 1 to 10, wherein an announcement of context information related to the presence of a mobile femtocell is correlated with context information of said mobile terminal.

12. Method according to claim 11 , wherein context information of said mobile terminal is communicated to said mobile femtocell and/or to the operator network.

13. Method according to any of claims 1 to 12, wherein the evaluation of a possible joint movement of said mobile terminal with a mobile femtocell includes the step of analyzing over a predefined measurement time period the signal strength said mobile terminal receives from said mobile femtocell.

14. Method according to claim 13, wherein said mobile terminal assumes to move jointly with said mobile femtocell in case said signal strength is stable within a predefined range and above a predefined threshold for said predefined measurement time period.

15. Method according to claim 14, wherein said measurement time period and/or said threshold are automatically adjusted based on past performance and/or context information.

16. Method according to any of claims 1 to 15, wherein a mobile femtocell performs measurements of the signal strength of non-attached mobile terminals located in its vicinity in order to detect joint movement.

17. Method according to any of claims 1 to 16, wherein joint movement is detected by said network by performing signal strength measurements of femtocells and of mobile terminals and by correlating the measurement results.