Patent Application
20140050197
The present invention relates to a method for providing information in a cellular wireless communication system, or more particularly to a method according to the preamble of claim 1. Furthermore, the invention also relates to a method in a base station, a computer program, a computer program product, and a base station device.
Normally a mobile station (MS) in active mode in a cellular wireless communication system is handed over from one cell to the next as it moves through the system, and data can be transmitted and received without significant interruptions due to these handovers.
A handover (HO) procedure can consist of many steps. In many cellular wireless communication systems a HO is: 1) network controlled, i.e. the MS is commanded by the network when to connect to another cell; 2) prepared, i.e. the target cell to which the MS is moving to is prepared; and 3) MS assisted, i.e. the MS provides measurement reports before HO to the serving cell to assist the decision to do HO preparation of target cell(s), and when to leave the serving cell/connect to the target cell.
In the context of HO, the serving cell before HO is often referred to as the source cell. After successful HO the target cell becomes the new serving cell. In Long Term Evolution (LTE) the HO is a “hard handover”, which means that the UE radio link is switched from one (source) cell to another (target) cell. In Universal Mobile Telecommunications System (UMTS) hard handovers are used exclusively for Time Division Duplex (TDD) mode, but may also be used for Frequency Division Duplex (FDD) mode.
In the following discourse, the focus is on the intra frequency LTE HO procedure, but the procedures are similar for the LTE inter Radio Access Technology (RAT) and LTE inter frequency HO procedures. The intra E-UTRAN in RRC_CONNECTED state is a User Equipment (UE) assisted network controlled HO, with HO preparation signalling in E-UTRAN.
A HO is initially triggered by a measurement report sent from a UE to a serving eNB (E-UTRAN NodeB). The serving eNB configures how the UE shall take measurements, and under what conditions a measurement report shall be triggered and sent to the eNB.
To assist mobility control decisions, the UE can measure several different cells and report the results to the network. Different networks and network deployments can have different detailed behaviour, but in most systems it is natural to trigger HO when signal reception from a target cell is better than from a source cell.
For the case of intra-frequency HO in a reuse-one system (i.e. in a system where the source cell and the target cell uses exactly the same frequency resources) there are strong interference management benefits in (always) keeping the UE connected to the cell with the best signal strength. In the measurement report, the UE includes the reason for the trigger of a HO, e.g. target cell signal stronger than serving cell signal, and measurements of a Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ) of the serving cell and several neighbour cells including the target cell. To reduce ping-pong effects where a UE is handed over repeatedly between two cells a HO offset is often added to the HO trigger condition: target cell signal should be better than the serving cell signal by an offset, wherein the offset value >0 dB.
When a serving eNB receives a measurement report from a UE and if the eNB wishes to HO the UE to another cell, the eNB performs a HO preparation to that cell. HO preparation involves a signalling exchange between one (serving) eNB and another (target) eNB. The source cell requests the HO (Handover Request) and passes over UE context information; and the target cell decides if it can admit the UE (Call Admission Control) and either accepts or rejects the HO. In an acceptance message (Handover Request Ack), the target cell includes parameters required by the UE to allow it to communicate to the target cell—these parameters being grouped into a transparent container. The source cell may prepare multiple cells for HO.
Following a successful preparation, the HO execution takes place. The source cell issues a HO Command to the UE—this is the RRCConnectionReconfiguration message and carries the transparent container. If, and when, the UE receives this message correctly the UE synchronises to the new target cell and sends a synchronisation message on the Random Access Channel (RACH). The target cell then issues an allocation to the UE so that the UE can send a HO confirmation message to the target cell (RRCConnectionReconfiguration-Complete message).
In the final steps (Handover Completion), which do not involve the UE, the source eNB (serving the source cell) is able to forward data (un-acknowledged downlink packets) to the target eNB (serving the source cell), and the S1-U interface from the Serving Gateway (S-GW) must be switched from the source to the target cell (“path switch”). Finally, if the HO is successful, the target eNB issues a UE Context Release message to the source eNB.
However, it is possible for a HO to fail at different points because of a Radio Link Failure (RLF) or a failure by the RACH. A RACH failure during a HO is called “Handover Failure” in 3GPP TS36.331, but for the remainder of this disclosure the term HO failure is used to include both RLF and RACH failures.
After a HO failure, the UE attempts a RRC re-establishment which is described in specifications 3GPP TS36.300 and 3GPP TS36.331. The UE firstly tries to find the strongest cell that it can detect (“cell selection”), and then the UE sends a RRCConnection-ReestablishmentRequest to the cell that it has selected. If this selected cell has prior knowledge of the UE and details regarding the UE connection (e.g. security parameters, this is called the “UE Context”) then the cell can send a RRCConnectionReestablishment and the re-establishment will succeed which means that the UE remains in Radio Resource Control (RRC) connected state.
If however the UE context is lacking the re-establishment request is rejected and the UE drops to RRC idle state, which results in further delay before the UE can transit to RRC connected state and recommence any data communication. The “UE Context” may be passed to a cell during the HO procedure or at some other point in time. This transfer is called HO Preparation. Note also that the RRCConnectionReestablishmentRequest carries three fields, the Cell Radio Network Temporary Identifier (C-RNTI) of the UE in the serving cell where failure occurred, the Physical Cell Identity (PCI) of this cell, and the shortMAC-I calculated using the Identity (ID) of the re-establishment cell.
The procedure for a successful RRC connection re-establishment is shown in
The hard HO in the Universal Mobile Telecommunications System (UMTS) is very similar in many respects to the above description—i.e. also being UE assisted but network controlled, which means that the UE is configured to send triggered measurement reports but the network decides when to execute the HO; exploits preparation (using Radio Link Setup procedure); is “backward” HO, which means that the source cell sends the HO command to the UE and the UE replies to the target cell; and is completed by inter-node signalling.
Furthermore, RLF is described in specifications 3GPP TS36.300 and 3GPP TS36.331. One form of RLF is driven by out-of-sync detection by Layer 1. A radio problem detection procedure is started when a UE receives a certain number of consecutive “out-of-sync” indications from lower layers. The number of consecutive indications is specified by the threshold N310. When this happens, the UE starts a timer T310. In case the UE receives a certain (N311) consecutive “in-sync” indications from lower layers while T310 is running, the UE shall stop the timer and return to normal operation. If T310 expires then a RLF is recognised to have occurred (“declared”) by the UE.
Following the declaration of a RLF, the UE attempts cell selection. If the UE manages to find a cell to connect to within the cell selection phase, the UE will attempt to re-establish RRC to this cell. If, on the other hand, the UE does not find a cell within the cell selection phase (T311), the UE goes back to idle mode and may start looking for cells on other RATs, examples of which are LTE, UMTS, WiMaX and GSM EDGE Radio Access Network (GERAN).
A RLF can also be declared by the Radio Link Control (RLC) layer of the UE when a maximum number of transmissions have been reached for transmission of an uplink RRC signalling packet, but the packet has still not been delivered successfully. Additionally, if the random access during the HO fails (T304 timeout) the UE behaves as if a RLF had occurred. In the present discourse the term RLF relates to any of the above mentioned events.
Moreover, a RLF report was introduced in 3GPP Rel-9 to enable an eNB receiving a RLF indication message to distinguish between Mobility Robustness Optimisation (MRO) related problems and coverage problems. This was done by including a set of neighbour cell measurements indicating the signal strength at the time of failure. With the help of this, the eNB is able to see if there is an alternative neighbour cell that might have been used, or if there is no neighbour detected in the case of a coverage hole.
The RLF report carries information about:
In Rel-9, if a RLF during a HO is followed by a successful RRC Re-establishment, it is possible to include a RLF Report in a RLF INDICATION message that is sent from a eNB where re-establishment takes place to a eNB that was serving the UE at the point of RLF. The capability of the UE to provide the RLF Report is indicated by a flag in the RCConnectionReestablishmentComplete message. The RLF Report is then provided to the eNB where re-establishment took place using the UE Information procedure.
Furthermore, in 3GPP there has been considerable study into Self-Organising Networks (SON) for LTE. One part of this is the Handover Parameter Optimisation also known as the above mentioned MRO which is aiming at optimising mobility parameters. It has not been specified which HO parameters shall be optimised, but examples include the HO hysteresis (also called offset) and the Time to Trigger (TTT) parameters. The aims of the optimisation are to reduce HO failures whilst at the same time not having more HOs than are necessary. The MRO functionality is distributed in the Evolved-UTRAN (E-UTRAN), i.e. every eNB has its own MRO optimisation function. To assist optimisation, signalling has also been defined between eNBs to help identify HO failure events.
The following is the text describing the use-case of Handover Parameter Optimisation also known as MRO in section 22.5 of specification 3GPP TS36.300, 9.2.0:
Further, according to specification 3GPP TS32.522, the MRO is expected to: a) meet a specified HO failure rate target, and b) minimize the number of HO events whilst meeting this target value. Studies have though shown that HO failure rate reduction and HO count reduction are contradictory requirements; if the failure rate is reduced the HO count increases, and vice versa.
Requirements a) and b) above reflect the fact that meeting the failure rate is of higher priority. A typical failure rate target would be 1%, and the MRO should not engineer a failure rate below the target because this will cause more HO than if the failure rate was equal to the target value. Looking more closely at requirement a), 3GPP defined 3 possible targets in specification 3GPP TS32.522 which may be used together or in part. These targets are presented in table 1 below.
There is a need for a source eNB to judge whether a RLF related to a HO failure for a UE was followed by a successful RRC Re-establishment. In other words, whether the UE was able to stay in RRC Connected state or a transition to RRC Idle state took place after the RLF. This knowledge is required for mainly two reasons:
An object of the present invention is to provide a method for providing information which may be used for tuning/improving mobility parameters and/or making performance measurements. Another object of the invention is to provide a method which makes a small impact on existing system regarding signalling, protocols, etc. A yet another object of the invention is to provide an alternative method for providing information in a cellular wireless communication system.
According to one aspect of the invention, the objects are achieved with a method for providing information in a cellular wireless communication system, wherein each cell in said cellular wireless communication system is served by a base station, and said cellular wireless communication system employs a procedure in which a mobile station suffering from a radio link failure (RLF), when being connected to a cell, may attempt to re-establish a connection in another cell, comprising the steps of:
Embodiments of the method in a communication system above are disclosed in the dependent claims 2-16.
According to another aspect of the invention, the objects are also achieved with a method in a base station for providing information in a cellular wireless communication system, wherein each cell in said cellular wireless communication system is served by a base station, and said cellular wireless communication system employs a procedure in which a mobile station suffering from a radio link failure (RLF), when being connected to a cell, may attempt to re-establish a connection in another cell, comprising the steps of:
The method in a base station above involves according to an embodiment: transmitting said information to a first base station serving said first cell or to a third base station serving said third cell directly or via one or more X2 and/or S1 interfaces over one or more other base stations in said cellular wireless communication system.
The invention also relates to a computer program and a computer program product when run in a computer causes the computer to execute the method in a base station described above.
According to yet another aspect of the invention, the objects are also achieved with a base station device for providing information in a cellular wireless communication system, wherein each cell in said cellular wireless communication system is arranged to be served by a base station, and said cellular wireless communication system employs a procedure in which a mobile station suffering from a radio link failure (RLF), when being connected to a cell, may attempt to re-establish a connection in another cell, being configured to:
The base station device according to the invention may also be arranged according to the different embodiment of the methods above.
The present invention enables the collection of data independent on the capability of the mobile station. The only thing that is required is that the mobile station is capable of performing RRC re-establishment. The information regarding the outcome of RRC re-establishment request for a mobile station is reliably provided to a base station, according to the invention, to be used e.g. for tuning mobility parameters and/or providing one or more performance measurements. Another advantage with the present invention is that it captures the actual success rate by feeding back the actual outcome of the re-establishment attempt.
Other advantages and applications of the present invention will be apparent from the following disclosure.
The appended drawings are intended to clarify and explain different embodiments of the present invention in which:
The invention enables a source eNB to determine whether a RRC re-establishment attempt following a radio link failure (RLF) was successful or not. Using this knowledge the source eNB can among other things: take measurements, such as PM to be passed over Itf-N, in support of the MRO targets Rate of failures related to HO without RRC state transition and Rate of failures related to HO with RRC state transition; and optimize against these failure targets.
The present invention achieves these and other goals by a method for providing information in a cellular wireless communication system. Preferably, each cell in the cellular wireless communication system is only served by one Base Station (BS), but the BS may serve more than one cell in the system.
The cellular wireless communication system further employs a HO procedure in which a MS suffering from a radio link failure (RLF), when being connected to a cell, may attempt to re-establish a connection in another cell. The method comprises the steps of: detecting a RLF for a MS while connected to a first cell; requesting a RRC re-establishment for the MS in a second cell after the RLF; and providing information, whether the RRC re-establishment for the MS was successful or not, to the first cell or to a third cell, wherein the third cell is the cell to which the MS was connected before the first cell.
In a preferred embodiment of the invention the step of providing information involves: transmitting the information in a RLF indication message to a first BS serving the first cell, or transmitting the information in a HO report message to a third BS serving the third cell.
According to another embodiment, the information is transmitted in a RLF indication message from the second cell to the first cell and in a HO report message from the first cell to the third cell.
According to yet another embodiment of the invention, the information whether the RRC re-establishment for the MS was successful or not is represented as a binary flag in a RLF indication message or in a HO report message.
However, the inventor has also realised that the information may be represented by a presence or absence of a field in a RLF report or in RLF indication message; of presence or absence of a field in a RLF report or in a HO report message. Preferably, the field is an establishment cell identity (TD) field or a time from re-establishment to establishment field. Using existing fields is a compact and efficient way of implicitly transmitting the information
The information may also according to another embodiment of the invention be provided to the first cell and/or the third cell when a time period after a successful HO to the first cell is less than a threshold value TMax.
In the case that the RRC re-establishment is requested in the second cell, the information is preferably transmitted by a second BS serving the second cell.
According to another embodiment of the invention the second and third cell is the same cell, which includes the case where there is a too early HO failure in which a RLF occurs shortly after a successful handover HO.
In the exemplary cases below different HO failure modes in a LTE system are captured and discussed in relation to the method according to the present invention, i.e. the impact of the present invention on a LTE system as specified by 3GPP for different cases. However, it should be understood that the present invention is not limited to the embodiments described above or to the following cases, but also relates to and incorporates all embodiments within the scope of the appended independent claims.
It should further be noted that the exemplary cases below only are only concerned with the RRC state transitions of HO failures that can be detected—if the RRCConnection-ReestablishmentRequest message of the re-establishment attempt following the failure is not received then the failure is not detectable.
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Case 9
Signalling impact on LTE system: the RLF INDICATION (when required) and the HANDOVER REPORT (when required) should include a 1-bit flag denoting a “Re-establishment Success”.
According to another embodiment of the invention, instead of using explicit signalling in RLF indication messages, knowledge of the UE capability and an examination of the contents of the RLF indication message can be used. If a RLF report is present in the RLF indication message, and the UE is only capable of sending a RLF report when re-establishment succeeds (this is the Rel-9 3GPP behaviour), this means that the UE succeeded in RRC re-establishment. If the RLF report is not included in the RLF indication, and the UE is capable of sending the RLF report, it may be concluded that it is likely that the RRC re-establishment failed. This embodiment therefore uses implicit transfer of information.
According to another embodiment of the invention, it is possible to generate a RLF indication message with a RLF report if the re-establishment succeeds or if it fails. The recipient of a RLF indication with a RLF report according to this embodiment may identify some difference between the message in these two cases and thus identify whether the re-establishment succeeded or not. Indeed, a difference in the message contents could be used instead of an explicit flag. For example, in the failed re-establishment case the RLF report (or possibly in the main part of the RLF indication) could carry the ID of the cell where establishment took place, but this ID would be missing in the other case (since there has been no establishment). Another example could be the time period from re-establishment attempt to successful establishment (again this is not applicable to the successful case and would not be included).
According to yet another embodiment of the invention, an alternative to including explicit signalling of the outcome of a RRC re-establishment in the HO report, another possibility would be to collect statistics on a cell pair in the cell receiving the RLF indication message, and merge this statistics in OAM of the system. For example, assuming case 8 described above, eNB serving/handling cell B can store one counter for all “wrong cell” failure events between the cell pair (A, C) and one counter for all successful re-establishments associated for each of these failure events. However, in order to obtain full statistics for cell A, the OAM must collect statistics for all failure events involving cell A as source cell in all neighbouring cells. A person skilled in art can also see other possible combinations to retrieve the desired statistics, such as storing the number of rejected re-establishment together with the total number of failure events.
It should be understood that the cellular wireless communication system may be any relevant cellular system, such as LTE, UMTS, CDMA2000, WiMaX and GERAN. Accordingly, the BS may be any of a eNB, NodeB and BTS; and the MS any of UE, MS and SS; or any relevant MS or BS having the corresponding functions.
The invention also relates to a method in a BS comprising the steps of: receiving a RRC re-establishment request from a MS; registering a RLF for the MS while connected to a first cell;
and providing information, whether the RRC re-establishment for the MS was successful or not, to the first cell or to a third cell, wherein the third cell is the cell to which the MS was connected before the first cell. The information may be transmitted to a first base station serving the first cell and/or to a third base station serving the third cell directly or via one or more X2 and/or S1 interfaces over one or more other base stations.
Furthermore, as understood by the person skilled in the art, a method in a BS according to the present invention may be implemented in a computer program, having code means, which when run in a computer causes the computer to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may consist of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.
Moreover, the invention also relates a BS device being configured to: receive a RRC re-establishment request from a MS; register a RLF for the MS while connected to a first cell; and provide information, whether the RRC re-establishment for the MS was successful or not, to the first cell and/or to a third cell, wherein the third cell is the cell to which the MS was connected before the first cell.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CN2010/075869 | 8/11/2010 | WO | 00 | 11/5/2013 |
