WO2017081360A1

WO2017081360A1 - Multi-connectivity of terminal device in cellular system

Info

Publication number: WO2017081360A1
Application number: PCT/FI2016/050742
Authority: WO
Inventors: Yang Liu; Amaanat ALI
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2015-11-10
Filing date: 2016-10-21
Publication date: 2017-05-18

Abstract

There is provided a solution for operating signalling radio bearers in a network node of a cellular communication system. According to an aspect, a method comprises: establishing, in a network node of a cellular communication system, a first signalling radio bearer for a terminal device; establishing, in the network node, a second signalling radio bearer for the terminal device as a back-up signalling radio bearer for the first signalling radio bearer, wherein the second signalling radio bearer is routed between the network node and the terminal device via a different route than the first signalling radio bearer and through at least one mediating network node; and in response to detecting, by the network node, a determined event in characteristics of the first signalling radio bearer, causing transmission of a downlink control message to the terminal device over the second signalling radio bearer without reconfiguration of the second signalling radio bearer between said detecting and said transmission.

Description

DESCRIPTION

TITLE

MULTI-CONNECTIVITY OF TERMINAL DEVICE IN CELLULAR SYSTEM

TECHNICAL FIELD

The invention relates to wireless communications in a cellular communication system and, in particular, to carrying out control signaling for a terminal device connected to multiple access nodes.

BACKGROUND

Modern fourth generation cellular systems provide terminal devices with multi-connectivity where a terminal device may be connected to a radio access network via a plurality of access nodes concurrently. Such methods may be employ carrier aggregation principles, for example. With the emergence of the next generation of the cellular systems, new requirements may be set for such multi-connectivity.

BRIEF DESCRIPTION

According to an aspect, there is provided the subject matter of the independent claims. Some embodiments are defined in the dependent claims.

One or more examples of implementations are set forth in more detail in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

In the following embodiments will be described in greater detail with reference to the attached drawings, in which

Figure 1 illustrates a wireless communication system to which embodiments of the invention may be applied;

Figure 2 illustrates a process for configuring signaling radio bearers according to an embodiment of the invention;

Figure 3 illustrates a signaling diagram of a process for configuring multiple signaling radio bearers for a terminal device according to an embodiment of the invention;

Figures 4 and 5 illustrate embodiments of Figure 3; and

Figures 6 and 7 illustrate block diagrams of structures of apparatuses according to some embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The following embodiments are exemplifying. Although the specification may refer to "an", "one", or "some" embodiment(s) in several locations of the text, this does not necessarily mean that each reference is made to the same embodiment(s), or that a particular feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.

Embodiments described may be implemented in a radio system, such as in at least one of the following: Universal Mobile Telecommunication System (UMTS, 3G) based on basic wideband-code division multiple access (W-CDMA), high-speed packet access (HSPA), Long Term Evolution (LTE), LTE-Advanced, and/or 5G system. The present embodiments are not, however, limited to these systems.

The embodiments are not, however, restricted to the system given as an example but a person skilled in the art may apply the solution to other communication systems provided with necessary properties. One example of a suitable communications system is the 5G system, as listed above. 5G is likely to use multiple-input-multiple-output (MIMO) multi- antenna transmission techniques, many more base stations or nodes than the current network deployments of LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller local area access nodes and perhaps also employing a variety of radio technologies for better coverage and enhanced data rates. 5G will likely be comprised of more than one radio access technology (RAT), each optimized for certain use cases and/or spectrum. 5G mobile communications will have a wider range of use cases and related applications including video streaming, augmented reality, different ways of data sharing and various forms of machine type applications, including vehicular safety, different sensors and real-time control. 5G is expected to have multiple radio interfaces, namely below 6GHz, cmWave and mmWave, and also being integradable with existing legacy radio access technologies, such as the LTE. Integration with the LTE may be implemented, at least in the early phase, as a system, where macro coverage is provided by the LTE and 5G radio interface access comes from small cells by aggregation to the LTE. In other words, 5G is planned to support both inter-RAT operability (such as LTE-5G) and inter-RI operability (inter-radio interface operability, such as below 6GHz - cmWave, below 6GHz - cmWave - mmWave). One of the concepts considered to be used in 5G networks is network slicing in which multiple independent and dedicated virtual subnetworks (network instances) may be created within the same infrastructure to run services that have different requirements on latency, reliability, throughput and mobility.

It should be appreciated that future networks will most probably utilize network functions virtualization (NFV) which is a network architecture concept that proposes virtualizing network node functions into "building blocks" or entities that may be operationally connected or linked together to provide services. A virtualized network function (VNF) may comprise one or more virtual machines running computer program codes using standard or general type servers instead of customized hardware. Cloud computing or cloud data storage may also be utilized. In radio communications this may mean node operations to be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. It should also be understood that the distribution of labor between core network operations and base station operations may differ from that of the LTE or even be nonexistent. Some other technology advancements probably to be used are Software-Defined Networking (SDN), Big Data, and all-IP, which may change the way networks are being constructed and managed.

Figure 1 illustrates an example of a cellular communication system to which embodiments of the invention may be applied. Cellular radio communication networks, such as the Long Term Evolution (LTE), the LTE-Advanced (LTE-A) of the 3^rd Generation Partnership Project (3GPP), or the predicted future 5G solutions, are typically composed of at least one network node, such as a network node 1 10, providing a cell 100. Each cell may be, e.g., a macro cell, a micro cell, femto, or a pico cell, for example. The network node 1 10 may be an evolved Node B (eNB) as in the LTE and LTE-A, or any other apparatus capable of controlling radio communication and managing radio resources within a cell. For 5G solutions, the implementation may be similar to LTE-A, as described above. The network node 1 10 may be called a base station or an access node. The cellular communication system may be composed of a radio access network of network nodes 1 10, 1 12, e.g. eNBs, each controlling a respective cell or cells 100, 102. The network node 1 10 may control a macro cell 100 providing wide area coverage for terminal devices 120. The network nodes 1 10 to 1 14 may also be called access nodes because they provide the terminal devices 120 with wireless access to other networks such as the Internet. Additionally, one or more local area access nodes 1 12 may be arranged within a control area of a network node 1 10 controlling a macro cell 100. The local area access node 1 12 may provide wireless access within a sub-cell 102 that may be comprised within a macro cell 100. Examples of the sub- cell may include a micro, pico and/or femto cell. Typically, the sub-cell provides a hot spot within a macro cell. The operation of the local area access node 1 12 may be controlled by a network node 1 10 under whose control area the sub-cell is provided.

The network nodes 1 10, 1 12 may support Dual Connectivity (DC) or multi-connectivity in which the terminal device 120 has established multiple connections with the radio access network comprising the network nodes 1 10,1 12 and, in some embodiments, other network nodes. In 5G multi-connectivity, the terminal device 120 may connect to two or three 5G access nodes simultaneously for improving bit rate performance through multiple downlink streams, as well as signal strength and resilience. In the multi-connectivity scenario, the terminal device 120 may establish one radio resource control (RRC) connection with the network node 1 10 and another RRC connection with the local area access node 1 12 for improved performance of communications. Some embodiments of the invention described below relate to a multi-connectivity scenario in which the terminal device 120 is connected to multiple network nodes, e.g. network nodes 1 10, 1 12, such that one of the network nodes, e.g. the macro-cell network node 1 10, operates as an anchor point for an RRC connection. In other words, the terminal device may logically have only one RRC connection with the radio access network even though it exchanges messages and data concurrently with multiple network nodes. Control signaling related to the local area access node 1 12 may be encapsulated into the single RRC connection. The anchor point may also serve as an anchor point with respect to tracking mobility of the terminal device 120 in the cellular communication system.

Some functions of the RRC protocol include connection establishment and release functions, broadcast of system information, radio bearer establishment, reconfiguration and release, RRC connection mobility management procedures, paging notification and release and outer loop power control. By means of the signalling functions, the RRC connection is used to configure user and control planes of the connection of the terminal device and to enable implementation of radio resource management strategies.

The operation of the RRC may be guided by a state machine which defines certain specific states for the terminal device. The different states in this state machine may be associated with different amounts of radio resources available to the terminal device and these are the resources that the UE may use when it is present in a given specific state. Since different amounts of resources are available at different states the quality of the service that the user experiences and the energy consumption of the UE are influenced by this state machine.

Further with respect to the multi-connectivity, the network node 1 10 alone or together with the other network node 1 12 may employ carrier aggregation in which the terminal device 120 is allocated with resources from a plurality of component carriers that may be on contiguous frequency bands or on non-contiguous frequency bands. One network node 1 10 may provide one component carrier, e.g. a primary component carrier, while another network node 1 12 may provide another component carrier, e.g. a secondary component carrier. The network node 1 10 operating the primary component carrier may carry out scheduling of resources on all component carriers, or each network node 1 10, 1 12 may control scheduling of the component carrier it operates. Alternatively, the network node 1 10 may provide one component carrier, e.g. a primary component carrier, as well as another component carrier, e.g. a secondary component carrier. In the case of multiple network nodes in the communication network, the network nodes may be connected to each other with an interface. LTE specifications call such an interface as X2 or S1 interface. Other communication methods between the network nodes may also be possible. The network nodes 1 10 to 1 12 may be further connected via another interface to a core network 130. The LTE specifications specify the core network as an evolved packet core (EPC), and the core network may comprise a mobility management entity (MME) 132 and a gateway node 134. The MME may handle mobility of terminal devices in a tracking area encompassing a plurality of cells and also handle signaling connections between the terminal devices and the core network 130. The gateway node 134 may handle data routing in the core network 130 and to/from the terminal devices.

In a multi-connectivity scenario where the terminal device is connected to a plurality of network nodes, one of the network nodes may provide both control plane and user plane connections with the core network, e.g. S1 and S1 -U connections of the LTE, while the other network node(s) provide only the user plane connection, e.g. S1 -U.

The radio system of Figure 1 may support Machine Type Communication (MTC). MTC may enable providing service for a large amount of MTC capable devices, such as the at least one terminal device 120. The at least one terminal device 120 may comprise mobile phones, smart phones, tablet computers, laptops and other devices used for user communication with the radio communication network, such as a MTC network. These devices may provide further functionality compared to the MTC scheme, such as communication link for voice, video and/or data transfer. However, in MTC perspective the at least one terminal device 120 may be understood as a MTC device. It needs to be understood that the at least one terminal device 120 may also comprise another MTC capable device, such as a sensor device providing position, acceleration and/or temperature information to name a few examples.

In MTC, the radio communication network may need to handle a massive amount of uncoordinated accesses by the MTC devices. As the amount of MTC devices may be quite high, network access may be a limiting factor, compared to the conventional network limitations, where interference and/or limited coverage may pose a problem. Most of the MTC devices may have a small amount of data to be transmitted in sporadic fashion. This may enable the MTC devices to spend majority of time in sleep mode, disconnected from the network node 1 10 to 1 16 and/or the cellular communication network. Thus, the MTC devices may have a requirement of very small energy small energy consumption.

Some embodiments described below are related to managing a control plane connection between the terminal device and a network node, e.g. the network node 1 10. A signaling radio bearer (SRB) may be defined as a radio bearer (RB) used only for the transmission of control messages or signaling messages of the control plane connection. Such messages may include radio resource control (RRC) and/or non-access stratum (NAS) messages. The NAS messages may relate to a communication protocol between the core network 130 and the terminal device 120 while the RRC messages may relate to a communication protocol between a network node of the radio access network and the terminal device.

Figure 2 illustrates an embodiment for managing signaling radio bearers in a multi- connectivity scenario, for example. The process of Figure 2 may be carried out by the network node 1 10, for example. Referring to Figure 2, the process comprises: establishing, in a network node of a cellular communication system, a first signalling radio bearer for a terminal device (block 200); establishing, in the network node, a second signalling radio bearer for the terminal device as a back-up signalling radio bearer for the first signalling radio bearer (block 202), wherein the second signalling radio bearer is configured between the network node and the terminal device via a different route than the first signalling radio bearer and through at least one mediating network node; and in response to detecting a determined event in characteristics of the first signalling radio bearer, causing transmission of a downlink control message to the terminal device over the second signalling radio bearer without reconfiguration of the second signalling radio bearer between said detecting and said transmission (block 204).

In the embodiment of Figure 2, the second signalling radio bearer may be established proactively for the event that the first signalling radio bearer becomes unusable for transferring the control messages, e.g. RRC and/or NAS messages, between the network node and the terminal device. As the second signalling radio bearer has been established before the event, the second signalling radio bearer may be taken into use fast and even without any reconfiguration or setting up after the detection of the event and before the first control message after the event is transferred over the second signalling radio bearer. 2. In an embodiment, no control message is transferred over the second signalling radio bearer between the establishment of the second signalling radio bearer and the detection of the determined event in the characteristics of the first signalling radio bearer.

In an embodiment, the network node executing the process of Figure 2 is the network node 1 10, e.g. a macro cell network node, the terminal device is the terminal device 120, and the mediating network node is the local area access node 1 12.

In an embodiment, the terminal device 120 has multiple connections with the cellular communication system, e.g. one connection through the network node 1 10 and another connection through the local area access node 1 12. The terminal device may have even further concurrent, parallel connections with the cellular communication system. The terminal device 120 may have only one control plane connection with the network node 1 10 and a plurality of user plane connections with the different access nodes, including or excluding the access node 1 10.

Let us now describe an embodiment of the procedure of Figure 2 with reference to a signalling diagram of Figure 3. Referring to Figure 3, the terminal device 120 and the network node 1 10 may establish the first signalling radio bearer (SRB) in step 300. The first SRB may be established in response to a connection request from the terminal device 120 to the network node 1 10. Step 300 may comprise establishment of an RRC connection between the network node 1 10 and the terminal device 120 and an establishment of a core network connection between the core network 130 and the terminal device 120. The first SRB may be configured to transfer control messages of both the RRC connection and the core network connection. In the LTE terminology, the core network connection may correspond to the NAS connection. In an embodiment, the first SRB is established over a physical radio link having the network node 1 10 and the terminal device 120 as end points of the radio link. As a consequence, only one radio link is associated with the first SRB in this embodiment.

In step 301 , a plurality of data connections are established for the terminal device 120. The data connection may refer to the user plane connection configured to transfer user data between the terminal device and the cellular communication system. One data connection may be established between the network node 1 10 and the terminal device 120, and another data connection may be established between the network node 1 12 and the terminal device 120. The first SRB may be used to carry control messages of the RRC connection and the core network connection of all data connections. Each network node 1 10, 1 12 providing a data connection may independently allocate radio resources to a data connection it manages, or the network node 1 10 managing the first SRB may allocate the radio resource for all data connections, depending on the embodiment. When steps 300 and 301 have been completed, the network node 1 10 may determine to add the second SRB as the back-up SRB. The second SRB may be established as soon as the network node has determined that there are at least two routes for transferring information with the terminal device, e.g. at least two network nodes provide the terminal device 120 with a connection. In step 302, the network node 1 10 may send a SRB addition request to the network node 1 12 via an interface established between the network nodes 1 10, 1 12, e.g. according to an X2 application protocol (X2AP) in the X2 interface of the LTE. The SRB addition request may request the network node 1 12 to establish the second SRB for the terminal device 120 specified in the SRB addition request and to reserve radio resources for the second SRB. In this case, the network node 1 12 is the above-mentioned mediating network node.

Upon receiving the SRB addition request in step 302, the network node 1 12 may configure the second SRB for the terminal device 120. The configuration may comprise making a radio resource allocation for the second SRB and, optionally, configuring other parameters such as physical layer, MAC layer, radio link control (RLC) layer, and/or packet data convergence protocol (PDCP) layer parameters) of the second SRB. The radio resource reservation may be a periodic or a static reservation of a frequency radio resource. Upon reserving the radio resource for the second SRB and, optionally, configuring other parameters of the second SRB, the network node 1 12 may send an acknowledgment message to the network node 1 10. The acknowledgment message may respond to the SRB addition request of step 302 and acknowledge configuration of the second SRB. The acknowledgment message may also comprise an information element indicating the radio resources reserved for the second SRB and, optionally, at least one other information element indicating the other parameters of the second SRB.

Upon receiving the acknowledgment message in step 304, the network node may transmit a SRB configuration message to the terminal device 120 in step 306. The SRB configuration message may configure the terminal device 120 to add the second SRB through the network node 1 12. The SRB configuration message may indicate the radio resources reserved by the network node 1 12 for the second SRB. The SRB configuration message may be transferred over a channel of the first SRB, e.g. on a dedicated control channel, or on a channel not associated with the first SRB, e.g. on a broadcast control channel. Upon receiving the SRB configuration message in step 306, the terminal device 120 may add the second SRB for the network node 1 10 and associated the radio resources indicated in the received SRB configuration message with the second SRB. In step 308, the terminal device may transmit a SRB configuration completed message to the network node 1 12 in the radio resources associated with the second SRB. The SRB configuration completed message may indicate that the terminal device 120 has established the second SRB and is ready to transfer control messages over the second SRB. Upon receiving the SRB configuration completed message from the terminal device 120 in step 308, the network node may forward the SRB configuration completed message to the network node 1 10 in step 310. Upon receiving the SRB configuration completed message in step 310, the network node 1 10 may complete the establishment of the second SRB. After completion of the setup of the second SRB, the network node 1 12 may maintain the resource reservation of the second SRB (block 312), e.g. maintain periodic or static reservation of the radio communication resources for the second SRB. Meanwhile, the control messages may be transferred between the network node 1 10 and the terminal device only through the first SRB in step 314. Data may be transferred over the plurality of data connections established in step 301 (step 316).

In an embodiment, the setup of the first SRB and the second SRB may comprise a RRC connection setup procedure specified in the LTE specifications or a similar control connection setup. In an embodiment, the SRB configuration in step 300 and steps 302 to 310 comprises exchange of security keys to be used in encryption of the control messages of the SRB. The exchange of security keys may also relate to PDCP setup with respect to the SRB. In an embodiment, the SRB configuration in step 300 and steps 302 to 310 comprises negotiation and establishment of one or more logical channels for the SRB. The establishment of the logical channels may be a part of medium access control (MAC) configuration of the SRB.

While the control messages are exchanged exclusively via the first SRB and while the reserved radio resources of the second SRB are maintained but not necessarily used to transfer any control messages, an event may be detected in the characteristics of the first SRB (block 318). In an embodiment, the event is a disconnection of the first SRB. In another embodiment, the event is degradation of a link quality of the first SRB below a determined threshold level. The threshold level may be set to such a level where the first SRB remains connected but a link quality is still considered to be too low for reliable exchange of control messages. The event may be detected in the network node, e.g. on the basis of measurements of link quality between the network node 1 10 and the terminal device 120 or upon receiving no uplink control messages from the terminal device 120 within a determined time interval. Upon detecting the event in block 318, the network node 1 10 may determine to employ the second SRB and transmit a subsequent control message to the terminal device via the network node 1 12 over the second SRB (steps 320, 322). As described above, steps 320 and 322 may be carried out without any reconfiguration of the second SRB between 318 and 320. For example, the control message may be transferred in the radio resources the network node 1 12 has reserved for the second SRB in block 312.

As seen from the description of Figure 3, the second SRB may be established without a need to transfer any control messages over the second SRB at the time of establishing the second signalling radio bearer in steps 302 to 310. The need for the actual use of the second SRB may occur only after the establishment of the second SRB, e.g. upon execution of block 318. As a result of proactive establishment of the second SRB, a delay in the communication caused by the event in the first SRB is reduced because the back-up SRB has already been prepared and is ready for use. The reduction of the delay compared with the situation where the second SRB is established only after block 318 may be in the order of the duration of the establishment of the second SRB, e.g. dozens of milliseconds. Such a delay could cause full disconnection of the terminal device in certain implementations. Therefore, the back-up SRB that has been readily established may reduce the number of for reestablishment procedures in the cellular communication system. This is advantageous especially in ultra-reliable and very low latency communications, such as wireless vehicular communications, e.g. V2X wireless coordination . V2X refers to implementations where a communication device of a vehicle is capable of communicating with other vehicles and/or other communication infrastructures. V2X may refer to remotely-controlled or even self- driven vehicles. Generally, V2X implementations may require ultra-reliable and very fast communication between different self-driving cars and between cars and infrastructure.

Ultra-reliable communications may be defined as an operation mode not present in today's cellular wireless systems and it may refer to provision of certain level of communication service almost 100 % of the time. Examples of possible applications are reliable cloud connectivity, V2X wireless coordination (coordination based on short messages) and sensor-based alarms in critical systems. Ultra-reliable communication may be sensitive to channel impairments such as fading, shadowing and interference. The embodiments of the invention may enable ultra-reliable communications by providing the ready-to-use back-up SRB. In an embodiment of Figure 3, a procedure executed in the network node 1 12 may comprise: acquiring, in the network node 1 12 from the network node 1 10, a request for setting up a second signalling radio bearer for the terminal device 120 as a back-up signalling radio bearer for the first signalling radio bearer, wherein the second signalling radio bearer is routed between the network node and the terminal device via a different route than the above-described first signalling radio bearer; in response to the request, setting up the second signalling radio bearer for the terminal device; receiving a downlink control message destined to the terminal device over the second signalling radio bearer and forwarding the downlink control message to the terminal device over the second signalling radio bearer. In an embodiment of Figure 3, a procedure executed in the terminal device 1 12 may comprise: acquiring, in the network node 1 12 from the network node 1 10, a request for setting up a second signalling radio bearer for the terminal device 120 as a back-up signalling radio bearer for the first signalling radio bearer, wherein the second signalling radio bearer is routed between the network node and the terminal device via a different route than the above-described first signalling radio bearer; in response to the request, setting up the second signalling radio bearer through the different route; and receiving a downlink control message over the second signalling radio bearer.

In the embodiment of Figure 3, the first SRB is established over the direct radio link between the network node 1 10 and the terminal device 120 while the second SRB is established through the mediating network node 1 12. In general, the first and second SRB may be established through other routes as long as the routes employ at least partially different radio links and, thus, different routes between the network node 1 10 and the terminal device 120.

In the embodiment of Figure 3, the second SRB may be established as soon as the multi- connectivity of the terminal device 120 has been established, e.g. at any point between 301 and 318. Accordingly, the trigger for the establishment of the second SRB may be establishment of the first SRB (step 300) or establishment of the multi-connectivity (Step 301 ). In some embodiments, the second SRB is established before or after any control messages are exchanged over the first SRB. In other embodiments, the second SRB may be established even in the case where the terminal device has a single data connection with the cellular communication system. For example, the terminal device may be determined to be in a coverage area of the network node 1 12 and the network node 1 10 managing the first SRB may determine to create the back-up SRB through the network node 1 12 even in the case where no data connection has been or will be established between the terminal device and the network node 1 12.

Figure 4 and 5 illustrate embodiments where the network node 1 10 may initially employ only the first SRB in the exchange of the control messages without the back-up SRB. In other words, the network node 1 10 and the terminal device 120 may exchange control messages over the first SRB in a situation where the second SRB has not yet been established (see upper 314 in Figure 4). Upon detecting an event in link quality of the first SRB in block 400, the network node may trigger the establishment of the second SRB. Thereafter, the process may proceed as described above in connection with Figure 3. The event detected in block 400 may be an event in a link quality of the first SRB, e.g. degradation of the link quality below a determined threshold level. The network node 1 10 may monitor the link quality of the SRB by carrying out measurements and/or monitoring traffic statistics such as bit error rate (BER), block error rate (BLER), or packet loss. The measurements may include measuring a received signal strength indicator (RSSI) of a radio signal received from the terminal device. The measurements may be focused on an uplink reference signal transmitted by the terminal device in which case the RSSI may be a reference signal received power (RSRP). LTE specifications define also reference signal received quality which takes into account the RSSI over the number of resource blocks (bandwidth) of the measurements. Any metric representing the link quality of a radio link between the network node 1 10 and the terminal device may be used as the basis for the detection in block 400. In an embodiment, the threshold level is set to such a level where the first SRB is still operational. As a consequence, the second SRB is created in a situation where the first SRB is still operational for transferring the control messages. Assuming that the first SRB does not disconnect between 400 and 310 in Figure 4, radio resources of the second SRB are saved for other use between steps 300 and 310 without compromising the disconnection of the terminal device 12.

In the embodiment of Figure 4, the network node 1 10 carries out the measurements in order to make the detection in block 400. Figure 5 illustrates an embodiment where the terminal device 120 carries out measurement of quality of the first SRB. The process of Figure 5 may start as the embodiment of Figure 4 with the establishment of the first SRB and the multi-connectivity in steps 300 and 301 , respectively. Additionally, control messages may be transferred over the first SRB before block 500. The terminal device 120 may measure similar link quality metrics described above but from a downlink reference signal transmitted by the network node 1 10. Upon detecting on the basis of the measurements that the link quality of the first SRB is blow a determined threshold level (block 500), e.g. the same threshold level employed in block 400 or another threshold level, the terminal device may generate and transmit in step 502 an uplink control message indicating the poor link quality associated with the first SRB and triggering the establishment of the second SRB. Upon receiving the message in step 502, the network node 1 10 may initiate the establishment of the second SRB in the above-described manner. The message transferred in step 502 may be a MAC control element (MAC CE), for example.

Yet another embodiment employs the measurements of the terminal device in the network node 1 10 such that the terminal device 120 is configured to measure the downlink quality of the radio link between the network node 1 10 and the terminal device and transmit uplink measurement reports comprising results of the measurements to the network node. The network node may then base the detection on these measurement results in block 400.

In both embodiments of Figures 4 and 5, the establishment of the second SRB may be triggered upon detecting degraded link quality in the first SRB while the first SRB is still operational. If the link quality of the first SRB is detected to improve after the establishment of the second SRB, e.g. link quality is above a second threshold level associated with better link quality than the threshold level used in blocks 400 and 500, the network node 1 10 may trigger release of the second SRB. As a consequence, radio resources may be released for other use.

Figures 6 and 7 provide apparatuses according to some embodiments of the invention. Figure 6 illustrates an apparatus configured to carry out the functions described above in connection with the network node 1 10. Figure 7 illustrates an apparatus configured to carry out the functions described above in connection with the terminal device 120. Each apparatus may comprise a communication control circuitry 10, 30, such as at least one processor, and at least one memory 20, 40 including a computer program code (software) 22, 42 wherein the at least one memory and the computer program code (software) are configured, with the at least one processor, to cause the respective apparatus to carry out any one of the embodiments of each apparatus described above.

The memories 20, 40 may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The memory may comprise a configuration database 24, 44 for storing configuration data for communicating in a cell over a radio interface. For example, the configuration databases 24, 44 may store the configurations of the established SRBs, e.g. the above-described first SRB and second SRB. The apparatuses may further comprise a communication interface (TX/RX) 26, 46 comprising hardware and/or software for realizing communication connectivity according to one or more communication protocols. The communication interface may provide the apparatus with communication capabilities to communicate in the cellular communication system and enable communication the network node 1 10 and terminal device 120, for example. The communication interface 26, 46 may comprise standard well-known components such as an amplifier, filter, frequency-converter, (de)modulator, and encoder/decoder circuitries and one or more antennas. The communication interfaces 26, 46 may comprise radio interface components providing the network node 1 10 and the terminal device 120 with radio communication capability in the cell 100 and with other network nodes such as the network node 1 12.

In an embodiment of Figure 6, at least some of the functionalities of the network node 1 10 may be shared between two physically separate devices, forming one operational entity. Therefore, the apparatus may be seen to depict the operational entity comprising one or more physically separate devices for executing at least some of the described processes.

Thus, the apparatus of Figure 6, utilizing such a shared architecture, may comprise a remote control unit (RCU), such as a host computer or a server computer, operatively coupled (e.g. via a wireless or wired network) to a remote radio head (RRH) located in a base station site. In an embodiment, at least some of the described processes of the network node 1 10 may be performed by the RCU. In an embodiment, the execution of at least some of the described processes may be shared among the RRH and the RCU. In such a context, RCU may comprise the components illustrated in Figure 6, and the communication interface 26 may provide the RCU with the connection to the RRH. The RRH may then comprise radio frequency signal processing circuitries and antennas, for example. In an embodiment, the RCU may generate a virtual network through which the RCU communicates with the RRH. In general, virtual networking may involve a process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization may involve platform virtualization, often combined with resource virtualization. Network virtualization may be categorized as external virtual networking which combines many networks, or parts of networks, into the server computer or the host computer (i.e. to the RCU). External network virtualization is targeted to optimized network sharing. Another category is internal virtual networking which provides network-like functionality to the software containers on a single system. Virtual networking may also be used for testing the terminal device.

In an embodiment, the virtual network may provide flexible distribution of operations between the RRH and the RCU. In practice, any digital signal processing task may be performed in either the RRH or the RCU and the boundary where the responsibility is shifted between the RRH and the RCU may be selected according to implementation.

Referring to Figure 6, the apparatus may comprise a control circuitry 12 carrying out control plane signalling with terminal devices, other access nodes of the radio access network, and with nodes of the core network 130. The control circuitry 12 may carry out signalling in steps 300, 301 , 302, 304, 306, 310, 314, 320, and 502 in the network node 1 10.

The apparatus may further comprise a control message processor 18 configured to carry out messaging with the terminal device 120 and/or with other network nodes of the cellular communication system on a higher layer of a protocol stack. The control message processor 18 may trigger establishment of a control connection with the terminal device 120 and carry out messaging with the terminal device over one or more SRBs of the control connection. In a similar manner, the control message processor may communicate with other terminal devices in a control area of the network node 1 10, e.g. in the cell 100. The control message processor may comprise, as a sub-circuitry, a signalling radio bearer configuration circuitry 14. The SRB configuration circuitry 14 may handle the SRB(s) of the control connections with the terminal devices, e.g. establishment, operation, and release of the SRBs. The SRB configuration circuitry may carry out operations described in connection with steps 200, 202, 204, 318, 400, and trigger establishment and release of the SRBs. The SRB configuration circuitry 14 may further control the control circuitry to carry out transmission and reception of the control messages over the established SRBs and transmission and reception of signalling messages related to the establishment and release of the SRBs, including the first and second SRB described above.

The apparatus may further comprise a data communication circuitry 16 configured to carry out transmission and reception of payload data over user plane connections established with the terminal devices in the cell 100. The data communication circuitry may be configured to carry out data transfer in step 316.

The apparatus of Figure 6 is applicable not only to the network node 1 10 but also to the network node 1 12. In such an embodiment, the SRB configuration circuitry 14 may be configured to carry out establishment of the second SRB and associated signalling in steps 302, 304, 308, 310, 312, 32, 322 in the network node 1 12.

Referring to Figure 7, the apparatus may comprise a control circuitry 32 carrying out control plane signalling with one or more network nodes of the cellular communication system, e.g. the network node 1 10 and 1 12. The control circuitry 32 may also carry out measurements of the link quality of established radio connections. The control circuitry 32 may carry out steps 300, 301 , 306, 308, 314, and 322 in the terminal device 120. The apparatus may further comprise a control message processor 38 configured to carry out messaging with the network nodes 1 10, 1 12 of the cellular communication system on a higher layer of a protocol stack. The control message processor 38 may carry out messaging with the terminal device over one or more SRBs of the control connection. The control message processor may comprise, as a sub-circuitry, a signalling radio bearer configuration circuitry 34. The SRB configuration circuitry 34 may handle the SRB(s) of the control connection, e.g. establishment, operation, and release of the SRBs. The SRB configuration circuitry may carry out operations of the terminal device 120 described in connection with steps 300, 306, 308, 314, 322, 500, 502. The SRB configuration circuitry 34 may further control the control circuitry to carry out transmission and reception of the control messages over the established SRBs and transmission and reception of signalling messages related to the establishment and release of the SRBs, including the first and second SRB described above.

The apparatus may further comprise a data communication circuitry 16 configured to carry out transmission and reception of payload data. The data communication circuitry may be configured to carry out data transfer in step 316.

As used in this application, the term 'circuitry' refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of circuits and soft-ware (and/or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of processor(s)/software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present. This definition of 'circuitry' applies to all uses of this term in this application. As a further example, as used in this application, the term 'circuitry' would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and/or firmware. The term 'circuitry' would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

In an embodiment, at least some of the processes described in connection with Figures 2 to 5 may be carried out by an apparatus comprising corresponding means for carrying out at least some of the described processes. Some example means for carrying out the processes may include at least one of the following: detector, processor (including dual- core and multiple-core processors), digital signal processor, controller, receiver, transmitter, encoder, decoder, memory, RAM, ROM, software, firmware, display, user interface, display circuitry, user interface circuitry, user interface software, display software, circuit, antenna, antenna circuitry, and circuitry. In an embodiment, the at least one processor, the memory, and the computer program code form processing means or comprises one or more computer program code portions for carrying out one or more operations according to any one of the embodiments of Figures 2 to 5 or operations thereof. The techniques and methods described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a hardware implementation, the apparatus(es) of embodiments may be implemented within one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. For firmware or software, the implementation can be carried out through modules of at least one chipset (e.g. procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by processors. The memory unit may be implemented within the processor or externally to the processor. In the latter case, it can be communicatively coupled to the processor via various means, as is known in the art. Additionally, the components of the systems described herein may be rearranged and/or complemented by additional components in order to facilitate the achievements of the various aspects, etc., described with regard thereto, and they are not limited to the precise configurations set forth in the given figures, as will be appreciated by one skilled in the art.

Embodiments as described may also be carried out in the form of a computer process defined by a computer program or portions thereof. Embodiments of the methods described in connection with Figures 2 to 9 may be carried out by executing at least one portion of a computer program comprising corresponding instructions. The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. For example, the computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, a record medium, computer memory, read-only memory, electrical carrier signal, telecommunications signal, and software distribution package, for example. The computer program medium may be a non-transitory medium. Coding of software for carrying out the embodiments as shown and described is well within the scope of a person of ordinary skill in the art.

Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, the embodiment. It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. Further, it is clear to a person skilled in the art that the described embodiments may, but are not required to, be combined with other embodiments in various ways.

Claims

1 . A method comprising:

establishing, in a network node of a cellular communication system, a first signalling radio bearer for a terminal device;

establishing, in the network node, a second signalling radio bearer for the terminal device as a back-up signalling radio bearer for the first signalling radio bearer, wherein the second signalling radio bearer is configured between the network node and the terminal device via a different route than the first signalling radio bearer and through at least one mediating network node; and

in response to detecting, by the network node, a determined event in characteristics of the first signalling radio bearer, causing transmission of a downlink control message to the terminal device over the second signalling radio bearer without reconfiguration of the second signalling radio bearer between said detecting and said transmission.

2. The method of claim 1 , wherein the determined event is associated with a link quality of the first signalling radio bearer.

3. The method of claim 2, wherein the determined event is a disconnection of the first signalling radio bearer or detection that the first signalling radio bearer is unusable for exchange of control messages.

4. The method of claim 2, wherein the determined event is detection of the link quality of the first signalling radio bearer below a determined threshold level.

5. The method of any preceding claim, wherein said establishment of the second signalling radio bearer is triggered on the basis of measurements of a link quality of the first signalling radio bearer

6. The method of claim 5, wherein said establishment of the second signalling radio bearer is triggered when said measurements indicate that a link quality of the first signalling radio bearer is below a determined threshold, wherein the determined threshold is set at a level at which the first signalling radio bearer is still operational.

7. The method of any preceding claim, wherein said establishment of the second signalling radio bearer is triggered by reception of an uplink control message from the terminal device.

8. The method of claim 7, wherein the uplink control message is at least one of a measurement report and a medium access control message.

9. The method of any preceding claims, wherein the second signalling radio bearer is established without a need to transfer any control messages over the second signalling radio bearer at the time of establishing the second signalling radio bearer

10. The method of any preceding claim, further comprising maintaining, by the network node, resource reservations for the second signalling radio bearer before said event while no control messages are transferred over the second signalling radio bearer.

1 1 . The method of any preceding claim, wherein the first signalling radio bearer and second signalling radio bearer are configured to transfer radio resource control layer control messages.

12. The method of any preceding claim, further comprising in the network node: providing the terminal device with a plurality of radio connections with the cellular communication system, wherein one radio connection is provided through the network node and at least one further radio connection is provided through the mediating network node, wherein the network node operates as an anchor point for radio resource connectivity of said plurality of radio connections.

13. An apparatus comprising:

at least one processor, and

at least one memory comprising a computer program code, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to:

establish a first signalling radio bearer for a terminal device of a cellular communication system;

establish a second signalling radio bearer for the terminal device as a back-up signalling radio bearer for the first signalling radio bearer, wherein the second signalling radio bearer is configured between the network node and the terminal device via a different route than the first signalling radio bearer and through at least one mediating network node; and in response to detecting, by the apparatus, a determined event in characteristics of the first signalling radio bearer, cause transmission of a downlink control message to the terminal device over the second signalling radio bearer without reconfiguration of the second signalling radio bearer between said detecting and said transmission.

14. The apparatus of claim 13, wherein the determined event is associated with a link quality of the first signalling radio bearer.

15. The apparatus of claim 14, wherein the determined event is a disconnection of the first signalling radio bearer or detection that the first signalling radio bearer is unusable for exchange of control messages.

16. The apparatus of claim 14, wherein the determined event is detection of the link quality of the first signalling radio bearer below a determined threshold level.

17. The apparatus of any preceding claim 13 to 16, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to trigger said establishment of the second signalling radio bearer on the basis of measurements of a link quality of the first signalling radio bearer

18. The apparatus of claim 17, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to trigger said establishment of the second signalling radio bearer when said measurements indicate that a link quality of the first signalling radio bearer is below a determined threshold, wherein the determined threshold is set at a level at which the first signalling radio bearer is still operational.

19. The apparatus of any preceding claim 13 to 18, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to trigger said establishment of the second signalling radio bearer by reception of an uplink control message from the terminal device.

20. The apparatus of claim 19, wherein the uplink control message is at least one of a measurement report and a medium access control message.

21. The apparatus of any preceding claim 13 to 20, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to establish the second signalling radio bearer without a need to transfer any control messages over the second signalling radio bearer at the time of establishing the second signalling radio bearer

22. The apparatus of any preceding claim 13 to 21 , wherein the processor, the memory, and the computer program code are configured to cause the apparatus to maintain resource reservations for the second signalling radio bearer before said event while no control messages are transferred over the second signalling radio bearer.

23. The apparatus of any preceding claim 13 to 22, wherein the first signalling radio bearer and second signalling radio bearer are configured to transfer radio resource control layer control messages.

24. The apparatus of any preceding claim 13 to 23, wherein the processor, the memory, and the computer program code are configured to cause the apparatus to provide the terminal device with a plurality of radio connections with the cellular communication system, wherein one radio connection is provided through the network node and at least one further radio connection is provided through the mediating network node, wherein the apparatus operates as an anchor point for radio resource connectivity of said plurality of radio connections.

25. The apparatus of any preceding claim 13 to 24, further comprising radio interface components configured to provide the apparatus with radio communication capability.

26. An apparatus, comprising means for carrying out all the steps of the method according to any preceding claim 1 to 12.

27. A computer program product embodied on a distribution medium readable by a computer and comprising program instructions which, when loaded into an apparatus, execute the method according to any preceding claim 1 to 12.