阅读说明：本技术 用于远程通信系统中的连接重建立的方法和装置 (Method and apparatus for connection re-establishment in a telecommunications system ) 是由 S.瓦格尔 M.萨福斯 于 2012-03-15 设计创作，主要内容包括：本发明涉及一种用于控制用户设备与网络之间的连接重建立的在所述用户设备中的方法和所述用户设备(700)。该方法包括步骤：-从该网络接收(501)定义至少一个条件的配置消息；-一经检测到通向第一小区的连接性问题(503),就评估所述至少一个条件(504)；-如果对于相邻小区(B)所述至少一个条件被满足,则应用用于对所述相邻小区发起连接重建立的至少一个特殊准则(505),所述至少一个特殊准则不同于如果所述至少一个条件没有被满足则被应用的至少一个准则。本发明此外涉及一种用于控制网络与用户设备(700)之间的连接重建立的方法和网络节点(800)中的装置。(The present invention relates to a method in a user equipment and the user equipment (700) for controlling a connection re-establishment between the user equipment and a network. The method comprises the following steps: -receiving (501) a configuration message from the network defining at least one condition; -evaluating the at least one condition (504) upon detection of a connectivity problem (503) to the first cell; -if the at least one condition is fulfilled for a neighboring cell (B), applying at least one special criterion (505) for initiating a connection re-establishment for the neighboring cell, the at least one special criterion being different from the at least one criterion applied if the at least one condition is not fulfilled. The invention further relates to a method and an arrangement in a network node (800) for controlling a connection re-establishment between a network and a user equipment (700).)

1. A method in a user equipment, UE, (700) for controlling a connection re-establishment between the user equipment and a network, the user equipment being served in a first cell (a) comprised in the network, characterized by the steps of:

-receiving (501) a configuration message from the network defining at least one condition, the at least one condition relating to determining whether the user equipment is within coverage of a neighbouring cell (B), wherein a condition is fulfilled if a measurement event is triggered for the neighbouring cell (B);

-evaluating the at least one condition (504) upon detection of a connectivity problem (503) to the first cell;

-if said at least one condition is fulfilled for said neighbouring cell (B), applying at least one special criterion (505) comprising an expiration of an additional timer (705) for initiating a connection re-establishment for said neighbouring cell, said at least one special criterion being different from at least one criterion comprising an expiration of a first timer (704) applied if said at least one condition is not fulfilled.

2. The method according to claim 1, wherein a condition is fulfilled for a neighbouring cell (B) if the received signal from said neighbouring cell is stronger than a threshold value.

3. a method according to claim 1 or 2, wherein a condition is fulfilled for a neighbouring cell (B) if a measurement report is transmitted to the network indicating that a measurement event is triggered for the neighbouring cell.

4. A method according to any of claims 1-3, wherein a condition is fulfilled for a neighbouring cell (B) if an acknowledgement is received from the network for a successfully transmitted measurement report indicating that a measurement event is triggered for the neighbouring cell.

5. a method according to any of claims 1-4, comprising a step (507) of receiving a message from the network, the message comprising an indication of at least one neighbouring cell (B) for which the at least one special criterion for initiating connection re-establishment applies if the at least one condition is fulfilled.

6. the method of any of claims 1-5, wherein when the at least one condition is satisfied, reestablishment is initiated immediately.

7. The method according to any of claims 1-5, wherein the at least one special criterion comprises an expiration of a timer (704) at an expiration value that is different from an expiration value of the timer (704) that is applied if the at least one condition is not met.

8. The method of any of claims 1-5 and 7, wherein the at least one special criterion comprises reaching a threshold value of a counter (706), the threshold being different from a threshold value for the counter (706) that is applied if the at least one condition is not met.

9. A method in a network node (800) to be included in a network for controlling connection re-establishment between the network and a user equipment (700), the network serving the user equipment in a first cell (a) included in the network, characterized by the steps of:

-sending (602) a configuration message to one or more user equipments defining at least one condition, the at least one condition relating to determining whether the user equipment is within coverage of a neighbouring cell (B), wherein a condition is fulfilled if a measurement event is triggered for the neighbouring cell (B);

-configuring (605) the one or more user equipments to evaluate the at least one condition upon detecting a connectivity problem to the first cell;

-configuring (606) the one or more user equipments to apply, if the at least one condition is fulfilled for a neighboring cell, at least one special criterion for initiating a connection re-establishment for the neighboring cell comprising an expiration of an additional timer (705), the special criterion being different from the at least one criterion applied comprising an expiration of the first timer (704) if the at least one condition is not fulfilled.

10. a method according to claim 9, wherein a condition is fulfilled for a neighbouring cell (B) if the received signal from said neighbouring cell is stronger than a threshold value.

Technical Field

The invention relates to controlling initiation of connection re-establishment upon detection of a connectivity problem.

Background

Communication devices such as User Equipment (UE) are also referred to as, for example, mobile terminals, wireless terminals, and/or mobile stations. The user equipment is capable of communicating wirelessly in a cellular communication network or a wireless communication system (sometimes also referred to as a cellular radio system or a cellular network). The communication may be performed between, for example, two user equipments, between a user equipment and a regular telephone, and/or between a user equipment and a server via a Radio Access Network (RAN) comprised within the cellular communication network and possibly one or more core networks.

the user device may further be referred to as a mobile phone, a cellular phone, a laptop computer, or a web surfboard with wireless capability, just to mention some further examples. A user device in this context may be, for example, a portable, pocket-storable, hand-held, computer-included, or vehicle-mounted mobile device capable of communicating voice and/or data with another entity, such as another user device or a server, via the RAN. The concept of user equipment also includes devices with communication capabilities of machine type nature, such as sensors, measuring devices, etc. that are not necessarily in any interaction with the user.

The cellular communication network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which, depending on the technology and terminology used, is sometimes referred to as e.g. "base station", "eNodeB", "NodeB", "B node", or BTS (base transceiver station). Based on the transmission power and thus also on the cell size, these base stations may have different categories, such as e.g. macro eNodeB, home eNodeB, or pico base station. A cell is a geographical area where radio coverage is provided by a base station at a base station site.

LTE mobility

mobility management is a challenging task in cellular communication systems and well-functioning mobility performance is crucial for the quality experienced by the end user. The radio resource control protocol RRC (see 3GPP TS 36.331) is the main signaling protocol for configuration, reconfiguration, and general connection handling in LTE radio access networks (E-UTRAN). RRC controls many functions such as connection setup, mobility, measurements, radio link failure, and connection recovery.

User equipment UE in LTE can be in two RRC states: RRC _ CONNECTED and RRC _ IDLE. In the RRC _ CONNECTED state, mobility is controlled by the network based on, for example, measurements provided by the user equipment. That is, the network decides when and to which cell the user equipment should be handed over based on, for example, measurement reports provided by the user equipment. The network, i.e. the LTE radio base station, referred to as eNB in E-UTRAN, configures various measurement events, thresholds etc. based on which the user equipment then sends reports to the network, enabling the network to make a decision to handover the user equipment to a stronger cell when the user equipment moves away from the current cell.

Fig. 1 illustrates a simplified signaling scheme for an LTE handover HO procedure. It should be noted that the HO command is in fact prepared in the target eNB (i.e. the eNodeB to which the user equipment is to be handed over), but the message is transmitted via the source eNB. That is, from the perspective of the user equipment, the message is from the source eNB.

In RRC IDLE, mobility is handled by UE-based cell selection, where a flooded (nomadic) user equipment selects the "best" cell to camp on based on various specified criteria and parameters, e.g., broadcasted in the cell. For example, various cell or frequency layers may be prioritized over other cell or frequency layers such that a user equipment attempts to camp on a particular cell as long as the measured quality of the beacon or pilot in that cell is better than some other beacon or pilot received from other cells by a threshold.

The present disclosure focuses primarily on the problems associated with network controlled (i.e. for LTE user equipment in RRC _ CONNECTED state) mobility as described above. The problems associated with failed handoffs are therefore described in further detail below.

In a normal situation, and when an RRC _ CONNECTED user equipment is moving out of coverage of a first cell (also referred to as a source cell), it should be handed over to a neighboring cell, also referred to as a target cell or a second cell, before dropping a connection to the first cell. That is, it is desirable that the connection be maintained throughout the handover with no or minimal disruption so that the end user is unaware of the handover in progress. To be successful in this regard, it is necessary that

Transmitting by the user equipment and receiving by the source eNB a measurement report indicating a need for mobility, an

The source eNB has sufficient time to prepare for handover to the target cell (by requesting handover from the target eNB controlling the target cell, among other things), and

The user equipment receives a handover command message from the network, the handover command message being prepared by a target eNB controlling a target cell and sent to the user equipment via a source cell, see fig. 1.

In addition, and in order for the handover to be successful, the user equipment must eventually successfully establish a connection to the target cell, which in LTE requires a successful random access request in the target cell, and subsequent transmission of a HO complete message from the user equipment to the target eNB. It should be noted that in naming a message, the specifications may be slightly different.

It is therefore clear that for a successful handover it is necessary to initiate a sequence of events leading to a successful handover sufficiently early that the radio link to the first cell over which this signalling occurs does not deteriorate too much before completion of the signalling. If this degradation occurs before the handover signaling is completed in the source cell (i.e., the first cell), the handover is likely to fail. Such a handover failure is clearly undesirable. The current RRC specification thus provides various triggers, timers, and thresholds in order to configure the measurements sufficiently that the need for handover can be detected reliably and early enough.

In fig. 1, the illustrated measurement report is triggered by a measurement event called the A3 event, which in short means that a neighbor cell is found to be better than the current serving cell by an offset. This means that the measurement report is sent to the network when the criterion or criteria associated with the event is/are fulfilled. There are many different measurement event types, and it should be noted that there are multiple events that can be configured to trigger reporting.

A network node controlling a cell, such as an eNodeB in LTE terminology, maintains a neighbor cell relation list. Whenever reference is made to a neighbor cell in the present disclosure, it should be understood as a reference to a cell that is typically included in a neighbor cell relation list of a network node. The neighbor cell is thus a cell that is often a candidate for handover. In some cases, the network node maintains a neighbor cell list relating to each cell it is controlling. From the user equipment's perspective, a neighbor cell is a cell that is nearby or overlaps with the cell to which the user equipment is currently connected.

Radio link failure and RRC connection re-establishment

It may happen that the user equipment looses the coverage of the cell to which the user equipment is currently connected. This may occur in the case when the user equipment enters a fading dip (fading dip), or in the case where a handover is required but fails for one or another reason as described above. This is particularly true if the "handover region" is very short, as will be described further below.

As described in 3GPP TS 36.300, TS 36.331 and TS 36.133 and as outlined below, the quality of the radio link is typically monitored in the user equipment, e.g. on the physical layer.

Upon detecting that the physical layer experiences a problem according to the criteria defined in TS 36.133, the physical layer sends an indication of the detected problem, referred to as an out-of-sync indication, to the RRC protocol. After a configurable number N310 of such successive indications, a timer T310 is started. If the link quality does not improve (recover) when T310 is running, i.e. there are no N311 consecutive "in sync" indications from the physical layer, a radio link failure RLF is declared in the user equipment, see fig. 2.

the currently relevant timers and counters described above are listed in fig. 4 for reference. The user equipment may read these timer values and counter constants from the system information broadcast in the cell. Alternatively, it is possible to configure the user equipment with the value of the UE-specific timer and the counter constant using dedicated signaling, i.e. wherein specific values and constants are given to specific user equipments with messages directed only to one or more specific user equipments.

The functions of the timers and counters used for monitoring radio link failures in LTE are presented in the table of figure 3.

If T310 expires, the user equipment initiates a connection re-establishment to resume the RRC connection in progress. This procedure includes cell selection by the user equipment. That is, the RRC _ CONNECTED user equipment will try to autonomously find a better cell to connect with, since the connection to the previous cell failed according to the described measurements. It may happen that the user equipment returns to the first cell in any way, but the same procedure is then also performed. As further described, for example, in 3GPP TS 36.304, once a suitable cell is selected, the user equipment requests a connection to be reestablished in the selected cell. It is important to note the difference in mobility behaviour as a result of RLF in user equipment based cell selection compared to commonly applied network controlled mobility.

If the re-establishment is successful, this depends among other things on whether the selected cell and the eNB controlling the cell are ready to maintain the connection to the user equipment, which implies that the re-establishment request is ready to be accepted, the connection between the user equipment and the eNB can be restarted. In LTE, the re-establishment procedure includes a random access request in the selected cell, followed by higher layer signaling, where the user equipment sends a message with content based on which the user equipment can be identified and authenticated. This is needed so that the network can trust that it knows exactly which user equipment is trying to perform the re-establishment.

Failure of re-establishment means that the user equipment goes to RRC _ IDLE and the connection is released. To continue the communication, a new RRC connection has then to be requested and established. For example, if the eNB receiving the re-establishment request cannot identify the user equipment requesting the re-establishment, a failure may occur. Such a situation may occur if the receiving eNB has not been informed or is ready for a possible re-establishment from this user equipment.

The reason for introducing the timer T31x and the counter N31x described above is to add some freedom and hysteresis for configuring the criteria for when the radio link should be considered as failed and needs to be re-established. This is desirable because if the result is that the loss of link quality is temporary and the user equipment successfully recovers the connection without any further action or procedure, for example, before T310 expires or the counter reaches the value N310, this will negatively impact the end user's performance if the connection is prematurely dropped.

The re-establishment procedure will be described below:

A network node (such as an eNB) controlling the target cell receives a recovery request message, such as an RRC connection reestablishment request, from the user equipment. In response to this message, the target eNB may respond with an RRC connection reestablishment message sent to the user equipment, by which the target eNB accepts the reestablishment request. The message may include various configuration parameters so that the connection can be adapted and continued in the new cell. Other message names may of course be applicable, such as any reference to cell reselection or handover.

upon receipt of the re-establishment message, the user equipment may now process the content of the message and resume the RRC connection according to the content and the commands provided therein. Typically, the user equipment will further send an acknowledgement message to the eNB of the target cell, wherein the acknowledgement message indicates that the communication between the user equipment and the target eNB can now be resumed. For example, the RRC connection re-establishment request, the re-establishment message, and the subsequent acknowledgement message will typically include a field for supporting secure identification of the user equipment, and a field for supporting contention resolution, i.e. so that the user equipment and its connection can be unambiguously and securely identified.

The recent and rapid uptake of mobile broadband has resulted in a need to increase the capacity of cellular networks. One solution to achieve this capacity increase is to use a denser network comprising several layers of cells with different sizes: macro cells ensure large coverage with cells encompassing large areas, while micro-cells, pico-cells, and even femto-cells are deployed in hot spot areas where there is a large demand for capacity. Those cells typically provide connectivity in a much smaller area, but by adding additional cells and the radio base station controlling those cells, the capacity increases as these new cells offload the macro cells. Such networks are known as heterogeneous networks or hetnets. Figure 9 shows a user equipment moving from the coverage of pico-cell a into the coverage of macro-cell B.

Different "layers" of cells can be deployed on the same carrier, i.e., in a reuse-1 fashion, these small cells can be deployed on different carriers, and even on various layers using different technologies, e.g., 3H/HSPA is deployed on macro-and micro-layers and LTE is deployed on pico-layer, as one non-exclusive example.

There is currently a great interest in investigating the potential of such hetnets. However, as briefly discussed above, it has also been found that hetnets may lead to an increased rate of handover failures. One reason is that the handover region in hetnets can be very short, meaning that the handover may fail because the user equipment loses coverage of the source cell before the handover to the target cell can be completed. For example, when the user equipment leaves the pico-cell, it may happen that the coverage boundary of the pico-cell is so sharp that the user equipment cannot receive any handover command towards the macro-cell before releasing the coverage of the pico-cell, see fig. 4.

Similar problems may occur when a user equipment connected to a macro cell suddenly enters a pico cell on the same carrier. It may now happen that the control channel of the pico cell interferes with the signals that the user equipment needs to receive from the macro cell in order to complete the handover, and the handover thus fails.

In a failed handover of the type exemplified above, the user equipment will eventually attempt to re-establish the RRC connection. But this can only occur after the process before the recovery process has been completed, as described above: thus, the user equipment will observe "out of sync" on layer 1 (L1) for the source cell, these events will be counted on the layer 3 (L3) level (i.e. RRC) as described above until N310 such consecutive events have occurred, and then T310 will be started. Only when T310 has expired, the user equipment can initiate a re-establishment procedure by searching for a better cell to connect in order to resume RRC connection.

It is clear that this count (up to N310) and the waiting for the expiration of T310 will result in an undesirable interruption of the connectivity, which is likely observable to the end user.

One can therefore argue that the network should configure the relevant counters and timers with small values (N310 and T310 as non-exclusive examples) in order to speed up the recovery. However, if these radio problems are not due to an imminent handover, but only to a sudden fading dip, this may lead to a premature loss of connection.

Disclosure of Invention

As a consequence of the problems described hereinbefore, there is difficulty in finding suitable criteria to trigger connection re-establishment, such as the values of correlation counters and timers, which will be suitable for both the handover problem described herein, and the more general case of fading channels experiencing time-varying fluctuations in path loss and interference.

It is therefore an object of embodiments of the present invention to provide a solution for reducing the number of handover failures resulting in an end-user perceived connection release.

More specifically, a first aspect of one embodiment of the present invention relates to a method in a user equipment, UE, for controlling a connection re-establishment between the user equipment and a network, the user equipment being served in a first cell comprised in the network. The method comprises the following steps:

-receiving a configuration message from the network defining at least one condition;

-evaluating said at least one condition upon detection of a connectivity problem towards the first cell;

-if said at least one condition is fulfilled for a neighbouring cell, applying at least one special criterion for initiating a connection re-establishment for said neighbouring cell, said at least one special criterion being different from the at least one criterion applied if said at least one condition is not fulfilled.

a second aspect of an embodiment of the invention relates to a method in a network node to be comprised in a network for controlling connection re-establishment between the network and a user equipment, the network serving the user equipment in a first cell comprised in the network. The method comprises the following steps:

-sending a configuration message defining at least one condition to one or more user equipments;

-configuring the one or more user equipments to evaluate the at least one condition upon detecting a connectivity problem to the first cell;

-configuring the one or more user equipments to apply, if the at least one condition is fulfilled for a neighboring cell, at least one special criterion for initiating a connection re-establishment for the neighboring cell, the special criterion being different from the at least one criterion applied if the at least one condition is not fulfilled.

A third aspect of one embodiment of the present invention relates to a user equipment adapted for controlling connection re-establishment between the user equipment and a network, the user equipment being served in a first cell comprised in the network. The user equipment includes:

-a receiver configured to receive a configuration message from the network defining at least one condition;

-a processor comprising circuitry configured to evaluate whether the at least one condition is fulfilled upon detection of a connectivity problem towards the first cell, and circuitry configured to apply at least one special criterion for initiating a connection re-establishment for a neighboring cell if the at least one condition is fulfilled for the neighboring cell, the at least one special criterion being different from the at least one criterion applied if the at least one condition is not fulfilled.

a fourth aspect of one embodiment of the invention relates to an arrangement in a network node to be comprised in a network, the arrangement being adapted to control connection re-establishment between the network and a user equipment, the network serving the user equipment in a first cell comprised in the network. The device includes:

-a transmitter configured to transmit a configuration message defining at least one condition to one or more user equipments;

-processing circuitry arranged to configure the one or more user equipments to: evaluating the at least one condition upon detecting a connectivity issue to the first cell; and if the at least one condition is met for a neighboring cell, applying at least one special criterion for initiating a connection re-establishment for the neighboring cell, the at least one special criterion being different from the at least one criterion applied if the at least one condition is not met.

Accordingly, embodiments are directed to reducing outage times in challenging radio environments where handover failures are likely to occur. A fast connection re-establishment will reduce any interruption time experienced by the end user. Particular embodiments will also ensure a higher likelihood of successful connection recovery.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings and claims.

Drawings

The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of preferred embodiments as illustrated in the accompanying drawings.

Figure 1 shows a simplified signaling diagram illustrating an LTE handover procedure;

Fig. 2 shows how radio link failure, RLF, is declared in LTE;

Figure 3 shows how RLF is monitored in LTE;

fig. 4 illustrates the concept of a handover region for a pico/macro cell change relative to a macro/macro cell change;

Fig. 5 shows a flow diagram of a method according to embodiments performed in a user equipment;

Fig. 6 shows a flow diagram of a method according to embodiments performed in a network node;

FIG. 7 schematically illustrates a user equipment according to embodiments;

Fig. 8 schematically illustrates an arrangement in a network node according to various embodiments.

Fig. 9 illustrates movement of user equipment between different cell coverage, in accordance with various embodiments.

Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

The present invention is described within the context of E-UTRAN (also known as LTE). It should be understood that the problems and solutions described herein are equally applicable to radio access networks and user equipment implementing other access technologies and standards. LTE is used as an example technology to which the present invention is suitable, and is therefore particularly useful for understanding the problem and solving the problem in the present description.

in the following, various alternative embodiments for implementing the invention will be described using non-exclusive examples.

Consider a user equipment in connected mode that implements network controlled mobility and is connected to a first cell (e.g., pico-cell). For the notation "connected to a cell", it should be noted that this implies that the user equipment is configured to transmit and/or receive data in at least this cell. Sometimes, this cell is also referred to as "serving cell", or "primary cell", PCell. The cell is typically controlled by one network node, such as a radio base station, which means that "connected to a cell" is in this respect equivalent to the user equipment "connected to the first radio base station", i.e. the radio base station controlling the aforementioned cell. Sometimes this radio base station is called serving eNB or serving radio base station.

In this example, the pico-cell is "surrounded" by macro-cells that provide coverage in a much larger area, see fig. 9. When the user equipment is moving out of the coverage of a small pico cell, it quickly releases the coverage of the pico cell to which it is currently connected, see fig. 4. In this case, there will be a rather high likelihood that the user equipment will drop the connection to the pico-cell before the handover is completed, which means that the user equipment will experience a radio link failure resulting in an undesirable interruption in connectivity, as described earlier.

the same problem may also occur if the user equipment moves from a macro cell to another cell (e.g. pico-cell) where there is a very short handover area. This problem is further magnified if the user equipment is moving at high speed.

embodiments of the invention provide the following possibilities: the evaluation of normal or "normal" radio link failures is bypassed for performing a connection re-establishment in a particular neighbouring cell in case at least some condition is fulfilled for said cell.

Fig. 5 schematically illustrates method steps performed by a User Equipment (UE) that is served in a first cell, which in this example is a pico cell, according to embodiments of the invention. In step 501, the user equipment receives a configuration message from the network defining at least one condition that will be evaluated upon detection of a connectivity problem to the first cell. The configuration message may be received on a broadcast channel or via dedicated signaling.

The condition may relate to determining whether the user equipment is within coverage of any neighbouring cells. This evaluation of whether the user equipment is within the coverage of the neighboring cell may be based on whether the signal received from the neighboring cell is stronger than a certain value (e.g., a threshold value). Various quality parameters for defining the "strength" of such a received signal can be implemented, such as received signal power measurements and measurement criteria on path loss and on interference. This includes, for example, Reference Symbol Received Power (RSRP) and Reference Symbol Received Quality (RSRQ).

the network may configure the user equipment with a threshold or thresholds that define whether the user equipment should consider itself within the coverage area of the indicated cell. The network may also configure the user equipment with a quality parameter that the user equipment will use when evaluating whether the user equipment is within the coverage area of the indicated cell. If the RSRQ of a cell, e.g., as indicated in the example outlined above, is greater than x dB, where x represents a configured threshold value, the user equipment considers itself to be within the coverage area of that cell.

If the user equipment identifies that said at least one condition is fulfilled for a plurality of neighbouring cells, e.g. identifies that it is within coverage of a plurality of cells, there may be additional criteria for deciding a priority of selection among the cells. For example, the user equipment may select the best cell, wherein "best" is defined in accordance with the above measured criteria, e.g. such that the cell with the strongest signal or the best signal quality is selected. Alternatively, there may be additional priority criteria defined, such as relative priority between cells included in the neighbor cell list, or offsets defining: for example, if a cell a or a cell on frequency layer F _ a is better than another cell B or a cell on another frequency layer F _ B in measured criteria by x dB, then the cell a or the cell on frequency layer F _ a should be selected.

When the user equipment moves out of the coverage area of the first cell, which in this example is a pico-cell, it will detect a physical layer problem or other problem related to connectivity to the first cell (i.e. pico-cell), see step 503. The detection of the physical layer problem may include: receiving an out-of-sync indication from the physical layer, or reading a reference signal received power, RSRP, or a reference signal received quality, RSRQ, towards the first cell below a certain absolute level. The network may configure the user equipment with a threshold or thresholds for RSRP or RSRQ that define whether the user equipment should consider that a connection to the source cell is problematic. According to another alternative, the detection of the connectivity problem to the first cell comprises: a step of starting a timer upon triggering a measurement event related to handover, and detecting a connectivity problem if no handover has been performed before the timer expires.

In step 504, the user equipment evaluates whether the at least one condition is satisfied. The condition may, as already mentioned, involve determining whether the user equipment is within the coverage of a neighbouring cell. This evaluation is done by checking whether certain one or more conditions related to the neighbouring cells are fulfilled, as will be described further below. In this case, the user equipment will implement a fast connection re-establishment to such a cell by applying a specific criterion or criteria for initiating a connection re-establishment in said cell, see step 505.

In a particular embodiment, a condition is satisfied when the received signal from the neighboring cell is stronger than a threshold value.

According to an alternative embodiment, a condition is met if a measurement event (e.g., a 3) is triggered for the neighbor cell. Alternatively, if a measurement report is transmitted to the network indicating that a measurement event is triggered for a neighboring cell, a condition is satisfied for the neighboring cell. According to yet another alternative, a condition is fulfilled for a neighbouring cell if an acknowledgement is received from the network for a successfully transmitted measurement report indicating that a measurement event is triggered for the neighbouring cell.

a benefit of the embodiments, where the condition depends on measurement events already defined for the user equipment, is that no new measurements for the target cell need to be defined.

a benefit of these two latter embodiments (i.e. where one condition is met when a report is transmitted and an acknowledgement of the report is received, respectively) is that they allow the network to know the intention of the user equipment to move towards the neighbouring cell (which in this case is the target cell) and to be able to initiate the required preparation, such as target cell preparation. It should be noted that the a3 event is merely an example, and other measurement events may be applied in the context of different embodiments.

If the at least one condition is met for the neighboring cell, then at least one special criterion for initiating connection re-establishment for the neighboring cell is applied, per step 505. The special criterion is different from at least one criterion to be applied for initiating a connection re-establishment if the at least one condition is not met.

If the at least one condition is not met for the neighboring cell, the user equipment should apply the normal criterion or criteria for initiating the connection re-establishment, see step 506, and complete the normal re-establishment procedure, possibly based on e.g. a conventional cell selection procedure.

In optional step 507, the user equipment receives an indication of at least one cell from the network for which said at least one special criterion should apply, provided that the at least one condition is fulfilled. The indication may in some embodiments be included in the configuration message received in step 501.

The indication of at least one cell provided to the user equipment in steps 501 or 507 may take different forms. According to a particular embodiment, the indication comprises an indication of all neighbouring cells of the first cell. In this case, there is no specific indication or need to specify a particular cell, but only an indication sent from the network to one or more user equipments stating that the user equipment should apply a special criterion or criteria that allow for a fast re-establishment solution whenever the at least one specific condition for the fast re-establishment procedure is fulfilled.

According to another specific embodiment, the indication comprises an indication of a specific frequency layer, whereby all neighbouring cells on the frequency layer are indicated. This means that, in case the at least one condition is fulfilled, the user equipment should apply a fast connection re-establishment solution towards cells on a certain frequency layer, instead of on another frequency layer, in case said at least one condition is fulfilled. Also in such a case, there may be no need to indicate a particular cell on that layer, since all neighboring cells on that particular frequency layer are indicated with a single indication.

according to yet another specific embodiment, the indication comprises a list of indicated cells for which the user equipment should perform a fast re-establishment of the connection provided that the at least one condition is fulfilled. If the indication is provided in the form of a list of cells, the cells in the list will be identified by cell identities (typically by their physical cell identities, PCIs), but other indexing or identification methods for identifying the cells in the list may be applicable. Such a list may also include an indication of: in which frequency layer the cell indicated by the identity code will be found.

The indication provided in step 507 may be provided to the user equipment on a broadcast channel (BCCH in LTE terminology), or it may be provided by dedicated signalling, where one message is sent to a specific user equipment or group of user equipments, but not to all user equipments in the cell. Broadcast signaling typically implies that the message or messages are sent to some or all of the user equipments within the coverage of the cell. These messages are then repeated, often periodically, so that the user equipment entering the cell can receive the message or messages at the next occasion. The dedicated signalling from the network usually implies that the message is directed to a specific receiver or group of receivers by specifically determining the address of this user equipment or group of user equipments in the transmission of the message. Thus, the message may be sent only once, followed by an acknowledgement from the receiving user equipment.

The application of the criterion or criteria to be met in order to initiate a connection re-establishment may in some embodiments comprise using a counter and/or a timer to assess when the user equipment should initiate a connection re-establishment in the indicated cell. These counters and/or timers may be the same as those used during the "normal" recovery process. For example, the network configures the user equipment with a counter value N310 equal to 10, and a timer expiration value T310 equal to 500 milliseconds. A "normal" connection re-establishment is then initiated after 10 consecutive "out of sync" indications, followed by the start and expiry of the timer T310, i.e. followed by a latency of 0.5 seconds. Only then does the user equipment select a cell for which the user equipment performs a re-establishment request.

According to a particular embodiment, the re-establishment is initiated immediately for a neighbouring cell when said at least one condition is fulfilled for that cell. This means that the re-establishment is initiated immediately as soon as the at least one condition is fulfilled, i.e. no additional criteria for initiating the connection re-establishment are applied. The following non-limiting examples will be used to explain such embodiments.

The user equipment is configured by the network to initiate a connection re-establishment to a neighbouring cell upon detecting a connectivity problem to its serving cell, provided that the a3 event has been triggered for that neighbouring cell. Let us assume that the connectivity problem is detected by receiving an out-of-sync indication from the physical layer. According to this particular embodiment, the user equipment will then stop the counter N310 counting out of synch indications from the physical layer, or the timer T310 controlling the radio link failure, and immediately initiate a connection re-establishment towards the neighboring cell for which the a3 event has been triggered, i.e. towards the target cell.

In an alternative example embodiment, the at least one special criterion includes: expiration of a timer at an expiration value that is different from an expiration value of the timer that was applied for a "normal" connection re-establishment (i.e., if the at least one condition is not satisfied); and/or reaching a threshold value for a counter that is different than the threshold value for the counter that is applied if the at least one condition is not met (i.e., for normal connection re-establishment). Thus, the user equipment may implement and use different values of the timer and/or counter to evaluate when the user equipment should initiate a connection re-establishment when the at least one condition has been met.

Thus, according to embodiments of the present invention, the user equipment may perform RRC connection re-establishment to a neighboring cell according to a "normal" connection re-establishment procedure before ending the counting and expiration of the timer (e.g., T310), provided that the at least one condition is met.

for example, the counter threshold value may be set to 3 as a special criterion, whereby the special criterion implies that the user equipment may initiate re-establishment to the neighboring cell for which the at least one condition is fulfilled after 3 "out of synch" indications. Alternatively, or in addition, the timer expiration value may be set to 100 ms as another special criterion, so that the user equipment may initiate re-establishment to the neighboring cell for which the at least one condition is satisfied 100 ms after T310 was started.

In another alternative embodiment, the at least one special criterion involves: in addition to the at least one counter and/or timer used to initiate the connection re-establishment "normally", at least one different counter and/or timer is used which governs when the user equipment is allowed to initiate the connection re-establishment to the cell for which the condition is fulfilled. For example, the initiation of a re-establishment in the cell for which this condition is satisfied may be associated with a counter value of "3" for counter N410 and a timer value of "100 ms" for timer "T410". The naming of these timers and counters should be seen as an example only and does not imply any limitation to the applicability of the embodiments of the invention. The same counter "N410" and timer "T410" may be used to govern the recovery for all neighboring cells. There may also be separate values for the counter and timer for each neighbor cell or group of neighbor cells.

Numerous variations and combinations are possible. The values of the counter and the timer for evaluating when the user equipment should attempt to re-establish a connection to a neighboring cell, both for the case when the at least one condition is met or not, may in some embodiments be configurable by the network, see optional step 502 in fig. 5 and 603 in fig. 6.

Embodiments of the present invention also include a network aspect for configuring the user equipment with at least one condition for evaluation to determine whether the user equipment should implement a fast connection re-establishment procedure to a particular neighboring cell. On the network side, the rationale for providing such at least one condition to a user equipment or group of user equipments to be evaluated upon detection of a connectivity problem to the first (i.e. serving) cell is that handover is particularly challenging in some deployments. Thus, it often happens that people must not resort to connection restoration as initiated by the user device in a particular zone. Thus, there is an incentive to accelerate such recovery where such problems are likely.

Fig. 6 schematically illustrates method steps performed by a network node according to embodiments of the invention. In step 602, the network node sends a configuration message to one or more user equipments served in a first cell controlled by the network node, the message defining at least one condition. In the foregoing in connection with the description of the method performed by the user equipment, examples of such conditions have been described. In step 605, one or more user equipments are configured by the network node to evaluate the at least one condition upon detecting a connectivity problem towards the first cell. In step 606, the one or more user equipments are configured by the network node to apply, if the at least one condition is fulfilled for a neighboring cell, at least one special criterion for initiating a connection re-establishment for the neighboring cell, which is different from at least one criterion to be applied if the at least one condition is not fulfilled.

In an optional step 607, the network node sends an indication of at least one cell for which the at least one special criterion for initiating connection re-establishment applies, if the at least one condition is fulfilled. The indication of at least one cell can take a form different to that described hereinbefore in relation to the description of the method performed by the user equipment.

In an optional step 600, the network node receives information from one or more neighboring network nodes regarding a previous re-establishment request in order to establish those cells that should be indicated to the one or more user equipments in step 607. Thus, the network uses statistics from, for example, previous handover failures, in order to establish those neighbor cells that should be indicated. For example, if there is information in the eNB that handover from its cell a to cell B often fails, the network may add cell B as the indicated cell for which the user equipment should conduct fast connection re-establishment by applying the at least one special criterion. This information, e.g. failed handovers, may be received, e.g. from neighboring network nodes over X2, S1 or O & M interfaces.

The network node may thus be arranged to collect information relating to connectivity issues, such as handovers that have failed, in order to provide such information to other enbs controlling neighbouring cells. For example, the network node B controlling cell B discovers that the user equipment often arrives in cell a after a failed handover from this cell, see fig. 9. The network node B controlling cell B may receive this information in an RLF-report message, see 3GPP TS 36.331, and the information element may befailedPCellId. The network node B collects this information and aggregates it directly or via some aggregation nodes (e.g., O)&m node) to the network node a controlling cell a, whereby the network node can add cell B as the indicated cell for which the user equipment should perform a fast recovery.

In an optional step 601, the network node establishes those cells that should be indicated to the user equipment transmitted in step 607. This can be done by utilizing information or statistics of previous connectivity issues, which are received from neighboring network nodes as described in the foregoing or which are available themselves.

in step 604, which is an optional step in relation to certain embodiments, the network prepares neighbouring cells which have been indicated to the one or more user equipments in step 607 by sending a message to at least one second network node (e.g. a radio base station or an eNodeB in LTE terminology) controlling such indicated cells so that the second network node is ready to receive a connection re-establishment request from a certain user equipment currently connected to the first cell. The preparing may include sending a handover request from the first network node to the second network node. If both cells are controlled by the same network node, no message needs to be sent, but the network node itself may be ready for connection re-establishment from the UE1 in the indicated cell.

Alternatively, if no prepare message is sent, the second network node controlling the indicated cell will not receive any information about any user equipment that may request a connection re-establishment in the indicated cell. Hence, the second network node does not have any user equipment context related to such user equipment. The second network node controlling the indicated cell may then retrieve the context of such user equipment from the network node controlling the first cell. This retrieval implies that the second network node requests information about the user equipment from the first node upon detecting a re-establishment request from the user equipment. The retrieval is successful if the second network node receives the necessary information about the user equipment to accept the re-establishment request. When the retrieval is successful, the second network node may accept the re-establishment from the user equipment.

A user equipment 700, UE, according to embodiments of the invention is schematically illustrated in fig. 7. The user equipment comprises a receiver 702 and a transmitter 703. The receiver is configured to receive a configuration message from the network defining at least one condition. In the foregoing in connection with the description of the method performed by the user equipment, examples of such conditions have been described. The message can be received on a broadcast channel or via dedicated signaling.

The user equipment 700 further comprises a processor 701, the processor 701 comprising: processing circuitry configured to evaluate whether the at least one condition is met upon detection of a connectivity issue towards the first cell, and circuitry configured to apply at least one special criterion for initiating a connection re-establishment for a neighboring cell if the at least one condition is met for the neighboring cell, the at least one special criterion being different from at least one criterion to be applied if the at least one condition is not met.

In some embodiments, the user equipment 700 further comprises a timer 704 and/or a counter 706. In a specific embodiment, the at least one special criterion includes: an expiration of timer 704 at an expiration value that is different from an expiration value of the timer 704 that is applied if the at least one condition is not satisfied; and/or reaching a threshold value of the counter 706 that is different than the threshold value for the counter 706 that is applied if the at least one condition is not met. In one particular embodiment, the timer 704 may be a timer T310 and the counter 706 may be a counter N310.

In a further embodiment, the user equipment 700 comprises a further different counter 707 and/or timer 705 for initiating a connection re-establishment to a neighbouring cell in addition to the counter 706 and/or timer 704 used for initiating a re-establishment to a cell if the at least one condition is not met.

The arrangement in the network node 800 is schematically illustrated in fig. 8. The network node is adapted to be comprised in a network serving a user equipment UE in a first cell comprised in the network, and is further adapted to control connection re-establishment between the network and the user equipment UE.

The apparatus includes a receiver 802 and a transmitter 803. The transmitter 803 is configured to transmit a configuration message defining at least one condition to one or more user equipments. Examples of such conditions have been described in relation to the description of the method embodiments performed by the user equipment. The message can be transmitted on a broadcast channel or via dedicated signaling.

The apparatus in the network node 800 further comprises a processor (801), the processor (801) comprising processing circuitry arranged to configure the one or more user equipments to: the at least one condition is evaluated upon detecting a connectivity issue to the first cell. The processing circuitry is further arranged to configure the one or more user devices to: applying at least one special criterion for initiating a connection re-establishment for a neighboring cell if the at least one condition is met for the neighboring cell, the special criterion being different from at least one criterion to be applied if the at least one condition is not met.

In some embodiments, the transmitter 803 is configured to transmit an indication of at least one cell for which the at least one special criterion for initiating connection re-establishment applies if the at least one condition is met. The indication of at least one cell can take a form different to that described hereinbefore in relation to the description of the method performed by the user equipment.

In some embodiments, the receiver 802 is configured to receive information related to a previous re-establishment request from one or more neighboring network nodes in order to establish the at least one cell that should be indicated to the one or more user equipments.

In some embodiments, the processor (801) comprises circuitry configured to utilize information of previous connectivity issues to establish those cells that should be indicated to the one or more user equipment. The processing circuitry may be further configured to use information or statistics received by the receiver (802) from a neighboring network node relating to a re-establishment request to establish those cells that should be indicated to the one or more user equipments.

In some embodiments, the transmitter (803) may be configured to send a prepare message to at least one neighboring network node controlling at least one cell indicated to the one or more user equipments, for preparing such network node to receive a connection re-establishment request from the one or more user equipments.

The method steps performed by the apparatus in the user equipment (700) and the network node (800) are performed by functional elements of processing circuitry in their respective processors 701 and 801. In some embodiments, these functions are performed by a suitably programmed microprocessor or microcontroller, alone or in combination with other digital hardware, which may include a Digital Signal Processor (DSP), dedicated digital logic, and so forth. Either or both of these microprocessors and digital hardware may be configured to execute program code stored in memory. Again, because various details and engineering tradeoffs associated with the design of baseband processing circuits for mobile devices and wireless base stations are well known and are not necessary to a complete understanding of the present invention, additional details are not shown herein.

The program code stored in the memory circuits may include one or several types of memory, such as Read Only Memory (ROM), random access memory, cache memory, flash memory devices, optical storage devices, and so forth, and in several embodiments, includes program instructions for performing one or more telecommunication and/or data communication protocols, as well as instructions for performing one or more of the techniques described herein. Of course, it will be appreciated that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive.