WO2018172605A1

WO2018172605A1 - Radio link management

Info

Publication number: WO2018172605A1
Application number: PCT/FI2018/050170
Authority: WO
Inventors: Ahmad AWADA; Ingo Viering
Original assignee: Nokia Technologies Oy
Priority date: 2017-03-20
Filing date: 2018-03-08
Publication date: 2018-09-27
Also published as: EP3603197A4; EP3603197A1

Abstract

According to an example aspect of the present invention, there is provided an apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus a least to cause the apparatus to participate in a communication session wherein a control plane is received from at least two access nodes (510), monitor radio link quality of at least two radio links comprised in the communication session (520), and select a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links (530).

Description

RADIO LINK MANAGEMENT

FIELD

The present invention relates to the field of wireless communication, such as, for example, wireless cellular communication with multi-connectivity.

BACKGROUND

Wireless communication may comprise establishment of radio links between mobiles, which may be referred to as user equipments, and base stations. In a traditional case, a mobile may roam in a coverage area of a cellular network, maintaining an attachment, and radio link, with the base station that is closest to the mobile. As the mobile moves, such that it leaves a coverage area of a cell controlled by a base station with which it has a radio link and enters another cell, of another base station, the mobile may undergo a handover procedure, to switch to the new cell.

In more advanced networks, a mobile may communicate simultaneously with more than one base station, which may be referred to a soft handover, for example. New radio links may be added to the communication session as suitable cells become available, and radio links may be removed from the communication session when the links become undesirable or untenable in a radio sense, for example, if distance between the mobile and the respective base station increases. A soft handover provides diversity gain, as all the links are unlikely to be in a fade at the same time. A mobile may also operate under multi-connectivity where the mobile is connected with and, receives and/or transmits data to at least two access nodes, such as base stations, for example.

When a radio link is active, its quality may be monitored, to provide inputs to handover decisions and power control algorithms, for example. Generally if a radio link declines in quality, the network may seek to replace it with a new radio link, to which traffic may be handed over. Thus, overall the user may experience smooth service without interruptions, as handovers may be handled in the background.

A radio link failure, RLF, is a situation where a radio link is determined to have become inoperative, for example due to continuity of the radio link failing at the physical layer. A RLF may occur, for example, in case interference near the mobile unexpectedly peaks. In practical terms, a determination RLF has occurred may be based on a signal level threshold, a signal- over-noise level threshold, a signal-to-interference-plus-noise ratio threshold, a block error rate threshold or a loss of synchronization, for example. SUMMARY OF THE INVENTION

According to some aspects, there is provided the subject matter of the independent claims. Some embodiments are defined in the dependent claims. According to a first aspect of the present invention, there is provided an apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to cause the apparatus to participate in a communication session wherein a control plane is received from at least two access nodes, monitor radio link quality of at least two radio links comprised in the communication session, and select a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links. Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select the time duration based on at least one of a first delay value and a second delay value

· the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a longer one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links

· the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a shorter one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being high, wherein the at least two radio links comprise exactly two radio links

· the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select both the first delay value and the second delay value, to be applied consecutively, responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links

· the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to wait for the time duration for the first radio link to recover before determining the radio link failure of the first radio link • the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to report a radio link failure of the second radio link responsive to the first radio link recovering while the radio link quality of the second radio link is low

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to a physical layer quality threshold not being met by the second radio link

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to a radio link failure timer running concerning the second radio link

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to the second radio link being in a radio link failure state

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a longer one of the first delay value and the second delay value responsive to radio link qualities of all other radio links other than the first radio link being low

• the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a shorter one of the first delay value and the second delay value responsive to a radio link quality of at least one radio link other than the first radio link being high

• receiving the control plane comprises receiving a signalling radio bearer.

According to a second aspect of the present invention, there is provided an apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to store information comprising a first delay value and a second delay value, and cause transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

In some embodiments of the second aspect, the apparatus is configured to, in transmitting the information to the user equipment, instruct the user equipment concerning selecting the delay when the user equipment is engaged in a communication session wherein a control plane is received in the user equipment from at least two access nodes.

According to a third aspect of the present invention, there is provided a method comprising causing an apparatus to participate in a communication session wherein a control plane is received from at least two access nodes, monitoring radio link quality of at least two radio links comprised in the communication session, and selecting a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

Various embodiments of the third aspect may comprise at least one feature from the following bulleted list:

• selecting the time duration based on at least one of a first delay value and a second delay value

• selecting a longer one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links

• selecting a shorter one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being high, wherein the at least two radio links comprise exactly two radio links

• selecting both the first delay value and the second delay value, to be applied consecutively, responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links

• waiting for the time duration for the first radio link to recover before determining the radio link failure of the first radio link

• reporting a radio link failure of the second radio link responsive to the first radio link recovering while the radio link quality of the second radio link is low

• considering the radio link quality of the second radio link to be low responsive to a physical layer quality threshold not being met by the second radio link

• considering the radio link quality of the second radio link to be low responsive to a radio link failure timer running concerning the second radio link

• considering the radio link quality of the second radio link to be low responsive to the second radio link being in a radio link failure state

• selecting a longer one of the first delay value and the second delay value responsive to radio link qualities of all other radio links other than the first radio link being low

• selecting a shorter one of the first delay value and the second delay value responsive to a radio link quality of at least one radio link other than the first radio link being high • receiving the control plane comprises receiving a signalling radio bearer.

According to a fourth aspect of the present invention, there is provided a method comprising storing, in an access node, information comprising a first delay value and a second delay value, and causing transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

In some embodiments of the fourth aspect, in transmitting the information to the user equipment, the user equipment is instructed concerning selecting the delay when the user equipment is engaged in a communication session wherein a control plane is received in the user equipment from at least two access nodes.

According to a fifth aspect of the present invention, there is provided an apparatus comprising means for participating in a communication session wherein a control plane is received from at least two access nodes, means for monitoring radio link quality of at least two radio links comprised in the communication session, and means for selecting a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

According to a sixth aspect of the present invention, there is provided an apparatus comprising means for storing information comprising a first delay value and a second delay value, and means for causing transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

According to a seventh aspect of the present invention, there is provided a non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least cause the apparatus to participate in a communication session wherein a control plane is received from at least two access nodes, monitor radio link quality of at least two radio links comprised in the communication session, and select a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

According to an eighth aspect of the present invention, there is provided a non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least store information comprising a first delay value and a second delay value, and cause transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

According to a ninth aspect of the present invention, there is provided a computer program configured to cause a method in accordance with at least one of the third and fourth aspects to be performed.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates an example system in accordance with at least some embodiments of the present invention;

FIGURE 2A illustrates RLF detection;

FIGURE 2B illustrates SRB delivery in multi-connectivity;

FIGURE 3 illustrates an example apparatus capable of supporting at least some embodiments of the present invention;

FIGURE 4 illustrates signalling in accordance with at least some embodiments of the present invention;

FIGURE 5 is a flow graph of a method in accordance with at least some embodiments of the present invention, and

FIGURE 6 is a flow graph of a method in accordance with at least some embodiments of the present invention.

EMBODIMENTS

In multi-connectivity communication sessions, a mobile may select a radio link failure timer solution based at least partly on an instantaneous radio link quality of at least one further radio link in the communication session. In detail, when the communication session comprises at least one further radio link that has an acceptable or good radio link quality, the mobile may determine a radio link failure sooner than in the case where the communication session has no further radio link that has an acceptable or good radio link quality. Thus when other radio links are strong, a failing radio link may be removed soon, and when the other link or links are weak, more time is given for the failing radio link to recover, to potentially avoid a re-establishment procedure.

In some prior art implementations, a single timer is configured per radio link and the value of the timer is static and can be configured either to short or conservative, longer, value. The prior- art does not teach setting the timer/time duration for radio link failure (RLF) based on the radio link quality of the other link or links. In other words, prior art does not teach to dynamically change/determine the timer/time duration based on the quality of the other link or links. FIGURE 1 illustrates an example system in accordance with at least some embodiments of the present invention. Mobile 110, which may comprise, for example, a user equipment, mobile phone, smartphone, tablet device, laptop computer, desktop computer or another device configured to act as a user equipment of a cellular or non-cellular communication system, is in wireless communication with base stations 120 and 130. The base station may be an example of an access node/point through which the mobile may access the network, for example.

To achieve interoperability, mobile 110 and base stations 120 and 130 may be configured to operate in accordance with a same communication technology, such as, for example, a cellular or non-cellular technology. Examples of cellular communication technologies include wideband code division multiple access, WCDMA, long term evolution, LTE, and fifth generation, 5G, also known as new radio, NR. Examples of non-cellular communication technologies include wireless local area network, WLAN, and worldwide interoperability for microwave access, WiMAX, technologies. However, in some embodiments, the base stations may also operate under different radio access technologies (RATs), such as LTE and 5G.

Radio link 112 connects mobile 110 with base station 120, and radio link 113 connects mobile 110 with base station 130. Radio links 112 and 113 may be arranged in accordance with a same communication technology as mobile 110 and base stations 120 and 130, to achieve interoperability. As said, base stations 120 and 130, as well as mobile 110, may be capable of communicating also with other communication technologies. The expression "base station" is a terminological choice by which it is not intended to limit the disclosure to any specific technology. Depending on a communication technology used, the expression "access point" may be used alternatively to "base station". The expression "access node" may comprise either a base station or an access point.

Inter-base station link 123 enables the base stations to exchange information directly. For example, handover-related information may be communicated over inter-base station link 123. This link may be referred to as an X2 interface, for example, depending on the technology in use.

Interface 124 connects base station 120 to network node 140. Interface 134 connects base station 130 with network node 140. Network node 140 may comprise, for example, a base station controller or a core network node, such as, for example, a mobility management entity, gateway or switch. Network node 140 may be interfaced with further nodes, which are not illustrated in FIGURE 1, via interface 141. Inter-base station link 123, interface 124, interface 134 and/or interface 141 may comprise wire-line connections, for example. While base station 120 and base station 130 are in the example of FIGURE 1 connected to the same network node 140, in general not all base stations need be connected to a same node. In the absence of inter- base station link 123, base station 120 and base station 130 may communicate via network node 140, or more generally via a core network, for example. Radio links 112 and 113 are comprised in an active communication session of mobile 110. Mobile 110 may receive information via both radio links on downlink parts of the links, and/or mobile 110 may transmit information on uplink parts of the links. To maintain the radio links, mobile 110 may participate in power control procedures for the radio links, and mobile 110 may be configured to monitor radio link quality of both radio links.

In some implementations of a dual connectivity communication sessions, mobile 110 receives a control plane over only one of the two radio links, and mobile 110 receives a user plane over both radio links. Base station 120, for example, may assume a role of master base station and provide the control plane, while base station 130 may assume a role of secondary base station in a dual connectivity communication session.

In a dual connectivity communication session, mobile 1 10 may be configured to detect a RLF if physical layer problems occur with the radio link that provides connectivity to the master base station, MeNB, which in this example is base station 120. Such a RLF may trigger connection re-establishment, which may be, in terms of delay and signalling, a costly procedure. Physical layer problems with a radio link to a secondary base station, SeNB, in this example radio link 113, may trigger a so-called secondary RLF, S-RLF, which does not lead to connection re-establishment. A S-RLF may be reported to the network, which may, for example, seek to replace the failing radio link with a new one without interrupting the radio link to the master base station.

RLF detection for the master radio link in dual connectivity may use similar parameters as in a single connection communication session, because the control plane is received, as in the single connection case, via only the one radio link and the communication session as a whole depends on this single radio link. RLF detection for S-RLF in dual connectivity may use a shorter RLF timer, for example. By receiving the control plane it may be meant that a signalling radio bearer, SRB, is received. The control plane may comprise a radio resource control, RRC, layer, for example comprised in a SRB. The control plane may handle radio-specific functionality which depends on the state of mobile 110. Mobile may be in an idle state or a connected state, for example. A data radio bearer, DRB, may be used in the user plane, distinct from the control plane. In at least some embodiments, a SRB or control plane is not used to communicate user data, such as, for example, data of a user application the user of the mobile 110 uses.

In some implementations of a multi-connectivity session, mobile 110 may receive both user plane and control plane information via both radio links 112 and 113. The situation is thus different from the dual-connectivity case where the control plane is only received via one radio link. Since the communication session is now not dependent on any single radio link, the RLF detection procedure can be different than in the single connectivity or single RRC dual connectivity cases. Multi-connectivity sessions may have a MeNB and at least one SeNB. In general, mobile 110 may determine RLF in case a radio link quality of the affected radio link is low for a time period that exceeds a threshold time duration, which may be defined in terms of one or more timer value, for example. For example, where the radio link quality drops below a first threshold, an out-of-sync indication may be generated in mobile 110 every 200 milliseconds, for example. Responsive to a predefined number, referred to herein as N310, of the out-of-sync indications, a RLF timer, referred to here as timer T310, may be started. In case timer T310 expires, the RLF is determined by mobile 110. Timer T310 may be interrupted in case the radio link recovers while the timer is running. Recovery of the radio link may be initiated by the radio link quality increasing over a second threshold. When the radio link quality is over the second threshold, in-sync indications may be generated, for example every 100 milliseconds. Responsive to a predefined number of the in-sync indications, referred to herein as N311, the RLF timer T310 may be interrupted. This procedure will be described in more detail in connection with FIGURE 2A.

In a single or dual connectivity case with single control plane link, the RLF timer may be set to a longer time, to give the radio link a longer time duration to, possibly, recover, before triggering the costly re-establishment procedure. A radio link may recover, for example, in case the radio channel comes out of a deep fade, as deep fades may be transient as the mobile moves, for example. As another example, the radio link may recover in case a short burst of spurious interference ceases. On the other hand, in multi-connectivity, when the control plane is available to mobile 110 over more than one radio link, a shorter RLF timer may be employed if there is at least one other radio link comprised in the session that is not experiencing a low signal quality. In this way, a failing radio link may be reported faster, and data queued in the respective base station may be re-routed quicker over the radio link that is not experiencing low quality. The quicker reporting of a RLF does not endanger the communication session itself, as the session may be maintained using the remaining radio link or radio links, providing the control plane, which is not, or are not, experiencing low signal quality. On the other hand, in a multi-connectivity communication session, a longer RLF timer may be more beneficial in case the other radio link or radio links comprised in the session are experiencing low signal quality. For example, in a two-radio link case as illustrated in FIGURE 1, radio link 112 may trigger a burst of out-of-sync indications, causing the RLF timer to start. Now, in case radio link 113 simultaneously experiences a low radio link quality, the communication session as a whole may indeed be endangered by a potential RLF in radio link 112, since there may be no further copy of the control plane mobile 110 might fall back on, in case radio link 112 is determined to have a radio link failure, RLF.

Mobile 110 may be configured to select a length of the RLF timer dynamically, in dependence of radio link quality of at least one further radio link of the communication session. In detail, responsive to another link in the communication session having low radio link quality, mobile 110 may select a longer RLF timer value, to give the failing radio link a longer time duration to, possibly, recover. In an example with three radio links in a multi-connectivity communication session, mobile 110 may be configured to use a shorter RLF timer responsive to there being among the two other radio links at least one radio link that does not have low radio link quality, since the communication session may continue at least based on that link and the copy of the control plane conveyed over it. On the other hand, if both other radio links have low radio link quality, the longer RLF timer value may be used.

Where all radio links in a multi-connectivity communication session have low radio link quality, mobile 110 may attempt to notify the network of this, for example by transmitting a message with an indication of this.

In a first implementation option, mobile 110 has two timer values reflecting a shorter RLF timer length and a longer RLF timer length, respectively. To use the longer RLF timer length, mobile 110 selects the timer value reflecting the longer RLF timer length and to use the shorter RLF timer length, mobile 110 selects the timer value reflecting the shorter RLF timer length.

In a second implementation option, mobile 110 has two timer values reflecting a shorter RLF timer length and a supplementary timer length, respectively. To use the longer RLF timer length, mobile 110 selects the timer value reflecting the shorter RLF timer length and the supplementary timer value, and to use the shorter RLF timer length, mobile 110 selects the timer value reflecting the shorter RLF timer length. In the first case, the supplementary timer is run after the shorter timer expires, and the RLF is only determined if also the supplementary timer expires without the radio link recovering before this.

In general, in case a RLF timer is running for one radio link in a two-radio link multi- connectivity communication session and, during this time, the other radio link is determined to provide sufficiently good quality, above a predetermined threshold, for example, mobile 110 may at once report a Secondary RLF, S-RLF, to the network, since the communication session is secure over the other link, and the failing link may be quickly replaced with a new one.

In general, in a multi-connectivity communication session, in at least some embodiments, a radio link failure is detected by mobile 110 once RLF timers expire for all radio links comprised in the communication session. Since the control plane may be received over any single radio link, re-establishment is needed only in case all radio links of the communication session are down. Reporting the RLF detected in case all radio links are down may be difficult for the mobile, since the radio links are not operative. A radio link failure of a single radio link from among the radio links in a multi-connectivity session may be reported as an S-RLF. In case the S-RLF involves the radio link to the MeNB, the master role may be transferred to a SeNB. In general, the timer values employed may be the same for MeNB and SeNB RLF determination, or, alternatively, MeNB and SeNBs may have differing timer values. The RLF timer values may be configured to the mobile by the network, for example. Alternatively, the mobile may be furnished with a mathematical process the mobile may use, to select dynamically the RLF timer length in dependence of the quality of at least one other radio link comprised in the session with a finer granularity than selecting from among a shorter and a longer wait time. For example, a mathematical function may take as input a SINR quality value of another radio link, or links, and produce as output a RLF timer length in milliseconds. There are various options for avoiding unnecessary RRC connection re-establishment. In some embodiments, a RRC connection re-establishment request is not sent, if the other radio link is available/sufficient. Instead mobile 110 may use a short timer/time duration for radio link monitoring (RLM) before sending an S-RLF report, when the radio link of the other cell is sufficient. Now, if a S-RLF occurs e.g. on the MeNB and the radio quality of the other link in SeNB is sufficient, the UE may in some embodiments send an S-RLF report to the network which will transfer the role from MeNB to SeNB and, therefore, connection re-establishment may be avoided. On the other hand, if the S-RLF is reported for the SeNB, the MeNB may in some embodiments try to find a replacement for the SeNB, again avoiding connection re- establishment. The mobile may be configured to send a RRC connection re-establishment request only if both links are down.

In some embodiments, mobile 100 may dynamically switch to different time durations for RLF detection. For example, if the mobile detects that at a time point A the link quality from one link is low, for example, below a predetermined threshold, which may be derived empirically or mathematically, for example, and that the link quality from other existing link(s) is also low, the mobile may set a longer timer duration. On the other hand, the same mobile may at a time point B, different than time point A, choose to use a shorter time duration for the RLF detection, if the link quality of at least one of the other link(s) is high, for example above the predetermined threshold.

Let us assume in an embodiment that a second link is of high quality, causing the RLF timer of a first link to be set to a shorter value at a time point when the RLF timer is started at the first link. Then let us assume that the link quality from the second link changes from high to low, during the RLF timer is running on the first link. In such case, the mobile may immediately switch into using the longer RLF timer value, assuming the short one has not yet elapsed. Analogously, a dynamic switch from using a long RLF timer into using a short RLF timer may take place as well, in case the link quality from the second link switches from low to high, during the long RLF timer is running for the first link. Therefore, it is clear that mobile 100 may in some embodiments dynamically select the time duration it is using, even when the RLF timer is running. Advantages obtained by the presently described solution may comprise that, firstly, secondary RLF may be reported quickly, using a shorter RLF timer, when another radio link in the communication session works well. This enables quick re-transmission of data queued behind the failed radio link. Secondly, where the other radio link has low radio link quality, the connection is given some more time to recover, in many cases avoiding a re- establishment procedure and its associated signalling. This is because the radio links may recover after some time they drop below Qout (see Figure 2A). If we configure short values for both/all the RLF timers, then RLF will declared rapidly and may lead to a high number of false detections.

FIGURE 2A illustrates RLF detection. Time advances from the left towards the right. Radio link quality increases from the bottom towards the top. Two radio link quality thresholds are provided on the vertical axis, Qin and Qout, which correspond to the second threshold and first threshold, respectively, as discussed above. These thresholds may be defined in terms of 2% and 10% block-error-rate, BLER, for example, on a physical downlink control channel, PDCCH. When measuring the radio link quality, a SINR metric may be used, for example. A metric may, in general, comprise radio link quality information.

Responsive to the radio link quality declining below the first threshold, Qout, mobile 110 begins generating out-of-sync indications, as illustrated. In this example, an out-of-sync indication is generated every 200 milliseconds as long as the radio link quality remains below threshold Qout. Once N310 out-of-sync indications have been generated, the RLF timer, denoted as T310 in FIGURE 2 A, is started. In the illustrated example, T310 has a length of 500 milliseconds. In the example of FIGURE 2A, the radio link quality increases over the second threshold, Qin, before the RLF timer expires, and in-sync indications are generated, as illustrated, every 100ms as long as the radio link quality remains over Qin. However, to interrupt the RLF timer, N311 in-sync indications would be needed, which are not generated before the timer expires, therefore a RLF is determined at T310 timer expiry in the example of FIGURE 2A. The frequencies at which the out-of-sync and in-sync indications are generated may be configured by the network, for example. FIGURE 2B illustrates SRB delivery in multi-connectivity. A signalling radio bearer, SRB, is delivered to user equipment UE, corresponding to mobile 110 of FIGURE 1, via two base stations, MeNB and SeNB. The SRB is conveyed from the main base station, MeNB, to secondary base station, SeNB, via an interface which may correspond, for example, to inter- base station link 123 of FIGURE 1. In the absence of such a link, the SRB may be conveyed to SeNB from the core network. As can be seen from FIGURE 2B, in case physical layer PHY of MeNB develops a problem, the SRB may still be delivered to the mobile via the SeNB.

FIGURE 3 illustrates an example apparatus capable of supporting at least some embodiments of the present invention. Illustrated is device 300, which may, comprise, or be comprised in, for example, a mobile communication device such as mobile 110 of FIGURE 1. In applicable parts, device 300 may also, comprise, or be comprised in a base station (in general an access node). The base station may be e.g. a node B, or evolved node B, or a gNB (as referred to in 5G). Comprised in device 300 is processor 310, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor 310 may comprise more than one processor. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings or a Steamroller processing core produced by Advanced Micro Devices Corporation. Processor 310 may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor 310 may comprise at least one application- specific integrated circuit, ASIC. Processor 310 may comprise at least one field-programmable gate array, FPGA. Processor 310 may be means for performing method steps in device 300. Processor 310 may be configured, at least in part by computer instructions, to perform actions.

Device 300 may comprise memory 320. Memory 320 may comprise random-access memory and/or permanent memory. Memory 320 may comprise at least one RAM chip. Memory 320 may comprise solid-state, magnetic, optical and/or holographic memory, for example. Memory 320 may be at least in part accessible to processor 310. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be means for storing information. Memory 320 may comprise computer instructions that processor 310 is configured to execute. When computer instructions configured to cause processor 310 to perform certain actions are stored in memory 320, and device 300 overall is configured to run under the direction of processor 310 using computer instructions from memory 320, processor 310 and/or its at least one processing core may be considered to be configured to perform said certain actions. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be at least in part external to device 300 but accessible to device 300.

Device 300 may comprise a transmitter 330. Device 300 may comprise a receiver 340. Transmitter 330 and receiver 340 may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter 330 may comprise more than one transmitter. Receiver 340 may comprise more than one receiver. Transmitter 330 and/or receiver 340 may be configured to operate in accordance with global system for mobile communication, GSM, wideband code division multiple access, WCDMA, long term evolution (LTE), New Radio, , IS-95, wireless local area network, WLAN, Ethernet and/or worldwide interoperability for microwave access, WiMAX, standards, for example.

Device 300 may comprise a near-field communication, NFC, transceiver 350. NFC transceiver 350 may support at least one NFC technology, such as NFC, Bluetooth, Wibree or similar technologies.

Device 300 may comprise user interface, UI, 360. UI 360 may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device 300 to vibrate, a speaker and a microphone. A user may be able to operate device 300 via UI 360, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory 320 or on a cloud accessible via transmitter 330 and receiver 340, or via NFC transceiver 350, and/or to play games. Device 300 may comprise or be arranged to accept a user identity module 370. User identity module 370 may comprise, for example, a subscriber identity module, SIM, card installable in device 300. A user identity module 370 may comprise information identifying a subscription of a user of device 300. A user identity module 370 may comprise cryptographic information usable to verify the identity of a user of device 300 and/or to facilitate encryption of communicated information and billing of the user of device 300 for communication effected via device 300.

Processor 310 may be furnished with a transmitter arranged to output information from processor 310, via electrical leads internal to device 300, to other devices comprised in device 300. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to memory 320 for storage therein. Alternatively to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise processor 310 may comprise a receiver arranged to receive information in processor 310, via electrical leads internal to device 300, from other devices comprised in device 300. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from receiver 340 for processing in processor 310. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver. Device 300 may comprise further devices not illustrated in FIGURE 3. For example, where device 300 comprises a smartphone, it may comprise at least one digital camera. Some devices 300 may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. Device 300 may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device 300. In some embodiments, device 300 lacks at least one device described above. For example, some devices 300 may lack a NFC transceiver 350 and/or user identity module 370.

Processor 310, memory 320, transmitter 330, receiver 340, NFC transceiver 350, UI 360 and/or user identity module 370 may be interconnected by electrical leads internal to device 300 in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device 300, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention. FIGURE 4 illustrates signalling in accordance with at least some embodiments of the present invention. On the vertical axes are disposed, from the left, base station 120, mobile 110 and base station 130 of FIGURE 1, and on the right, a further base station, SeNB. Time advances from the top toward the bottom.

Initially, in phase 410, a multi-connectivity communication session is in progress between mobile 110 and base stations 120 and 130. In phase 420, mobile 110 determines a RLF concerning the radio link it has with base station 130. In determining the RLF, mobile 110 uses, as described above, a shorter or longer RLF timer in dependence of a radio link quality of the radio link mobile 110 has with base station 120. The RLF concerning the radio link to base station 130 is reported to the network via base station 120, and the network, in this example, directs mobile 110 to establish a radio link with base station SeNB. The radio link establishment process is schematically illustrated in FIGURE 4 as phase 430. In phase 440, mobile 110 is engaged in a multi-connectivity communication session with base stations 120 and SeNB.

FIGURE 5 is a flow graph of a method in accordance with at least some embodiments of the present invention. The phases of the illustrated method may be performed in mobile 110, or in a control device configured to control the functioning thereof, when installed therein for example.

Phase 510 comprises causing an apparatus to participate in a communication session wherein a control plane is received from at least two access nodes. Phase 520 comprises monitoring radio link quality of at least two radio links comprised in the communication session. Finally, phase 530 comprises selecting a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links. FIGURE 6 is a flow graph of a method in accordance with at least some embodiments of the present invention. The phases of the illustrated method may be performed in base station 120, or in a control device configured to control the functioning thereof, when installed therein, for example. Phase 610 comprises storing, in an access node, information comprising a first delay value and a second delay value. Phase 620 comprises causing transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links. The at least two active radio links may be comprised in a communication session wherein a control plane is received from at least two base stations.

In an embodiment at least some of the functionalities of the apparatus of FIGURE 3, such as the base station, or in general an access node/point, 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. The apparatus utilizing such shared architecture, may comprise a remote control unit (RCU), such as a host computer or a server computer, operatively coupled, for example via a wireless or wired network, to a remote radio head (RRH) located in the base station. In an embodiment, at least some of the described processes 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 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 net-works, into the server computer or the host computer, for example, 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.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the 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. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality. INDUSTRIAL APPLICABILITY

At least some embodiments of the present invention find industrial application in avoiding unnecessary radio link re-establishment or re-connection.

ACRONYMS LIST

5G fifth generation

BLER block-error-rate

LTE long term evolution

MeNB master base station

NR new radio

PDCCH physical downlink control channel

RCU remote control unit

RLF radio link failure

RLM radio link monitoring

RRC radio resource control

RRH remote radio head

SeNB secondary base station

S-RLF secondary RLF

SINR signal to interference plus noise ratio

SRB signalling radio bearer

WCDMA wideband code division multiple access

WiMAX worldwide interoperability for microwave access

WLAN wireless local area network

REFERENCE SIGNS LIST

110 mobile

120, 130 base station

140 network node

112, 113 radio link

123 inter-base station link

124, 134, interface

141

300 - 370 structure of the device of FIGURE 3

410 - 440 signaling phases of the method of FIGURE 4

510 - 530 phases of the method of FIGURE 5

610 - 620 phases of the method of FIGURE 6

Claims

CLAIMS:

1. An apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to:

- cause the apparatus to participate in a communication session wherein a control plane is received from at least two access nodes;

- monitor radio link quality of at least two radio links comprised in the communication session, and

- select a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

2. The apparatus according to claim 1, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select the time duration based on at least one of a first delay value and a second delay value.

3. The apparatus according to claim 2, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a longer one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links.

4. The apparatus according to claim 2 or 3, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a shorter one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being high, wherein the at least two radio links comprise exactly two radio links.

5. The apparatus according to claim 2, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select both the first delay value and the second delay value, to be applied consecutively, responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links.

6. The apparatus according to any of claims 1 - 5, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to wait for the time duration for the first radio link to recover before determining the radio link failure of the first radio link.

7. The apparatus according to any of claims 1 - 6, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to report a radio link failure of the second radio link responsive to the first radio link recovering while the radio link quality of the second radio link is low.

8. The apparatus according to any of claims 1 - 7, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to a physical layer quality threshold not being met by the second radio link.

9. The apparatus according to any of claims 1 - 8, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to a radio link failure timer running concerning the second radio link.

10. The apparatus according to any of claims 1 - 9, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to consider the radio link quality of the second radio link to be low responsive to the second radio link being in a radio link failure state.

11. The apparatus according to any of claims 2 or 6 - 10, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a longer one of the first delay value and the second delay value responsive to radio link qualities of all other radio links other than the first radio link being low.

12. The apparatus according to any of claims 2 or 6 - 11, wherein the at least one memory and the computer program code are configured to, with the at least one processing core, further cause the apparatus to select a shorter one of the first delay value and the second delay value responsive to a radio link quality of at least one radio link other than the first radio link being high.

13. The apparatus according to any of claims 1 - 12, wherein receiving the control plane comprises receiving a signalling radio bearer.

14. An apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to:

- store information comprising a first delay value and a second delay value, and - cause transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

15. The apparatus according to claim 14, wherein the apparatus is configured to, in transmitting the information to the user equipment, instruct the user equipment concerning selecting the delay when the user equipment is engaged in a communication session wherein a control plane is received in the user equipment from at least two access nodes.

16. A method comprising:

- causing an apparatus to participate in a communication session wherein a control plane is received from at least two access nodes;

- monitoring radio link quality of at least two radio links comprised in the communication session, and

- selecting a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

17. The method according to claim 16, comprising selecting the time duration based on at least one of a first delay value and a second delay value.

18. The method according to claim 17, wherein the method further comprises selecting a longer one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links.

19. The method according to claim 17 or 18, wherein the method further comprises selecting a shorter one of the first delay value and the second delay value responsive to the radio link quality of the second radio link being high, wherein the at least two radio links comprise exactly two radio links.

20. The method according to claim 17, wherein the method comprises selecting both the first delay value and the second delay value, to be applied consecutively, responsive to the radio link quality of the second radio link being low, wherein the at least two radio links comprise exactly two radio links.

21. The method according to any of claims 16 - 20, comprising waiting for the time duration for the first radio link to recover before determining the radio link failure of the first radio link.

22. The method according to any of claims 16 - 21, further comprising reporting a radio link failure of the second radio link responsive to the first radio link recovering while the radio link quality of the second radio link is low.

23. The method according to any of claims 16 - 22, further comprising considering the radio link quality of the second radio link to be low responsive to a physical layer quality threshold not being met by the second radio link.

24. The method according to any of claims 16 - 23, further comprising considering the radio link quality of the second radio link to be low responsive to a radio link failure timer running concerning the second radio link.

25. The method according to any of claims 16 - 24, further comprising considering the radio link quality of the second radio link to be low responsive to the second radio link being in a radio link failure state.

26. The method according to any of claims 17 or 21 - 25, further comprising selecting a longer one of the first delay value and the second delay value responsive to radio link qualities of all other radio links other than the first radio link being low.

27. The method according to any of claims 17 or 21 - 26, further comprising selecting a shorter one of the first delay value and the second delay value responsive to a radio link quality of at least one radio link other than the first radio link being high.

28. The method according to any of claims 16 - 27, wherein receiving the control plane comprises receiving a signalling radio bearer.

29. A method comprising:

- storing, in an access node, information comprising a first delay value and a second delay value, and - causing transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

30. The method according to claim 29, wherein, in transmitting the information to the user equipment, the user equipment is instructed concerning selecting the delay when the user equipment is engaged in a communication session wherein a control plane is received in the user equipment from at least two access nodes.

31. An apparatus comprising:

- means for participating in a communication session wherein a control plane is received from at least two access nodes;

- means for monitoring radio link quality of at least two radio links comprised in the communication session, and

- means for selecting a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

32. An apparatus comprising:

- means for storing information comprising a first delay value and a second delay value, and

- means for causing transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

33. A non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least:

- monitor radio link quality of at least two radio links comprised in the communication session, and - select a time duration, for determining a radio link failure of a first radio link from among the at least two radio links, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two radio links.

34. A non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least:

- store information comprising a first delay value and a second delay value, and

- cause transmission of the information to a user equipment, to instruct the user equipment to employ the information in selecting a time duration for determining radio link failure of a first radio link from among at least two active radio links of the user equipment, wherein the selection is based at least partly on radio link quality of a second radio link from among the at least two active radio links.

35. A computer program configured to cause a method in accordance with at least one of claims 16 - 30 to be performed.