WO2019192708A1

WO2019192708A1 - Client device and network access node for efficient random access procedure

Info

Publication number: WO2019192708A1
Application number: PCT/EP2018/058758
Authority: WO
Inventors: Bengt Lindoff; Gustaf Claeson; Rama Kumar Mopidevi; Chaitanya TUMULA; Wenquan HU
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2018-04-05
Filing date: 2018-04-05
Publication date: 2019-10-10

Abstract

The invention relates to an efficient random access procedure for a client device (100) in a wireless communication system (500). The client device (100) determines an uplink time synchronization status for a radio link configured for a certain service type. The service type may have strict latency or reliability constraints. When the uplink time synchronization status is determined to be unreliable, the client device (100) initiate a random access procedure to restore uplink time synchronization. The random access procedure may be initiated even if a data buffer of the client device (100) is empty. Thereby, uplink time synchronization can be restored before the client device (100) has data to transmit or receive, resulting in reduced delays.

Description

CLIENT DEVICE AND NETWORK ACCESS NODE FOR EFFICIENT RANDOM ACCESS PROCEDURE

Technical Field

The invention relates to a client device and a network access node for efficient random access procedure. Furthermore, the invention also relates to corresponding methods and a computer program.

Background

The growth of wireless data traffic over the past three decades has been relentless. The upcoming fifth-generation (5G) wireless cellular communication system, new radio (NR), is expected to carry 1000 times more traffic while maintaining high reliability. Another critical requirement of 5G is support for ultra-low latency services, where latency corresponds to the time required for transmitting a message through the network.

The current fourth-generation (4G) wireless cellular communication system, LTE, has a nominal latency of about 50ms. However, this is currently unpredictable and can go up to several seconds. Moreover, it is mainly optimized for mobile broadband traffic with target block error rate (BLER) of 10e-1 before re-transmission.

There is a consensus that the future of many services, e.g. industrial control, traffic safety, medical, and internet services, depends on wireless connectivity with guaranteed consistent latencies of 0.5ms or less, as well as exceedingly stringent reliability of residual BLERs below 10e-5. The projected enormous capacity growth is achievable through conventional methods of moving to higher parts of the radio spectrum and network densifications. However, significant reductions in latency, while guaranteeing ultra reliability and low latency communication (URLLC) services, will put several challenges on the design of the 5G wireless communication system.

URLLC services will require the UE to be in-sync with the serving cell and in an active state, while the URLLC service is ongoing. Furthermore, a variety of URLLC services can be expected from chatty application with frequent transmission of small data packets with tough latency and reliability requirements, to alarm systems where sensors may in-frequently report alarms, but once alarming, the latency need to be short and reliability need to be high. A wide range of QoS levels for different URLLC services are therefore expected. This puts for instance requirements on uplink synchronization procedures defined in the NR standard for such services.

Summary

An objective of embodiments of the invention is to provide a solution which mitigates or solves the drawbacks and problems of conventional solutions.

The above and further objectives are solved by the subject matter of the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

According to a first aspect of the invention, the above mentioned and other objectives are achieved with a client device for a wireless communication system, the client device being configured to

determine an uplink time synchronization status for a radio link between the client device and a network access node when the client device is in connected mode with the network access node, wherein the radio link is configured for a first service type;

initiate a random access procedure upon a determination that the uplink time synchronization status is unreliable.

In this disclosure the term service type can be interpreted to mean a service having certain characteristics or quality of service requirements, e.g. a service having certain latency and/or reliability/error constraints, etc.

The random access procedure can be initiated directly upon a determination that the uplink time synchronization status is unreliable. In other words, as soon as it is determined that the uplink time synchronization status is unreliable the random access procedure can be triggered.

An advantage of the client device according to the first aspect is that the risk for violating delay requirements for data packets with low latency requirements is reduced, and hence the overall link performance is improved.

In an implementation form of a client device according to the first aspect, the client device is further configured to

determine the uplink time synchronization status to be unreliable if a first timer has expired. An advantage with this implementation form is that the client device has a consistent determination of unreliable uplink synchronization status and thereby re-synchronization is triggered directly once needed. This improves the overall link performance.

In an implementation form of a client device according to the first aspect, the first timer is a time alignment timer associated with the radio link.

An advantage with this implementation form is that random access is triggered directly once the time alignment timer has expired. This improves overall link performance and reduces the risk for violating delay requirements.

In an implementation form of a client device according to the first aspect, the first timer is configured for the first service type.

An advantage with this implementation form is that random access is only triggered for the first service type having low latency requirements and not for other services and hence the random access load in the wireless communication system is maintained on an acceptable level.

start the first timer when a second timer has expired, wherein the second timer is a time alignment timer associated with the radio link.

An advantage with this implementation form is that the network access node can make a more flexible configuration about when the client device should trigger the random access procedure thereby reducing the overall random access load in the wireless communication system.

start the first timer when downlink control information addressed for the client device has been decoded.

An advantage with this implementation form is that the uplink synchronization timer is started from a time instance when the client device previously had uplink synchronization and hence a reliable measure of uplink synchronization is achieved. In an implementation form of a client device according to the first aspect, the client device is further configured to

receive a control message from the network access node, wherein the control message comprises a timer value for the first timer.

An advantage with this implementation form is that the client device knows for how long it can be expected to be in uplink synchronization with this particular network access node, wherein the uplink synchronization can be dependent on the cell deployment and therefore optimized performance per cell is achieved.

determine the uplink time synchronization status to be unreliable if the uplink timing for the client device has drifted outside a pre-defined window.

An advantage with this implementation form is that the client device can explicitly determine whether uplink synchronization has been drifted too much and hence random access is triggered only when needed and thereby random access load in the wireless communication system is kept on a minimum level.

initiate the random access procedure independently of a current data buffer status associated with the radio link.

An advantage with this implementation form is that the random access is triggered directly when unreliable synchronization is determined by the client device and thereby the likelihood of violating the latency requirements for low latency packets is decreased. Therefore, improved link performance is achieved.

initiate the random access procedure even if the current data buffer status is empty buffer.

An advantage with this implementation form is that the random access procedure is triggered directly when unreliable synchronization is determined even where there is currently no data to transmit. Thereby the likelihood of violating the latency requirements for low latency packets is decreased, when these packets arrive for transmission in a transmit buffer the (relatively slow) random access procedure does not have to be triggered anymore. Therefore, improved link performance is achieved.

In an implementation form of a client device according to the first aspect, the random access procedure is a non-contention based random access procedure.

An advantage with this implementation form is that pre-defined random access procedures used for other purposes also can be used for maintaining uplink synchronization and thereby the client device can reuse software code and/or hardware reducing implementation cost in the client device.

In an implementation form of a client device according to the first aspect, the first service type has at least one of a latency constraint and a reliability constraint.

An advantage with this implementation form is that the random access procedure is triggered for services requiring latency constraints and reliability constraints. Thereby the random access load in the wireless communication system is minimized.

In an implementation form of a client device according to the first aspect, at least one of the latency constraint and the reliability constraint is associated with at least one of: a quality of service flow identity, a network slice selection assistance information configuration, a radio resource control parameter, and a medium access control parameter.

determine the latency constraint based on a delay threshold.

An advantage with this implementation form is that the client device can determine based on the delay threshold whether random access procedures according to embodiments of the invention should be used or whether conventional solutions should be used. Thereby, reduced random access load is achieved in the wireless communication system.

determine the reliability constraint based on an error rate threshold. An advantage with this implementation form is that the client device can determine based on the rate threshold whether random access procedures according to embodiments of the invention should be used or whether conventional solutions should be used. Thereby, reduced random access load is achieved in the wireless communication system.

In an implementation form of a client device according to the first aspect, initiate the random access procedure comprises

transmit a random access preamble in a random access resource to the network access node, wherein at least one of the random access preamble and the random access resource is associated with at least one of the latency constraint and the reliability constraint.

An advantage with this implementation form is that the network access node can determine the purpose of the random access procedure quickly and therefore take appropriate actions for fast re-synchronization of the uplink. Hence, improved link performance is achieved.

In an implementation form of a client device according to the first aspect, at least one of the random access preamble and the random access resource is configured by the network access node or pre-defined in the client device.

According to a second aspect of the invention, the above mentioned and other objectives are achieved with a network access node for a wireless communication system, the network access node being configured to

configure a radio link between a client device and the network access node, wherein the radio link is configured for a first service type;

determine a timer value for a first timer to be used by the client device to determine an uplink time synchronization status for the radio link;

generate a control message comprising the timer value for the first timer; and transmit the control message to the client device.

An advantage of a network access node according to the second aspect is that network access node can configure timers for the client device to be used for keeping the uplink in synchronization. Hence, fast re-synchronization is possible so that time periods when the client device is in unreliable uplink synchronization state is minimized. Hence, improved link performance is achieved.

According to a third aspect of the invention, the above mentioned and other objectives are achieved with a method for a client device, the method comprises

determining an uplink time synchronization status for a radio link between the client device and a network access node when the client device is in connected mode with the network access node, wherein the radio link is configured for a first service type;

initiating a random access procedure upon a determination that the uplink time synchronization status is unreliable.

The method according to the third aspect can be extended into implementation forms corresponding to the implementation forms of the client device according to the first aspect. Hence, an implementation form of the method comprises the feature(s) of the corresponding implementation form of the client device.

The advantages of the methods according to the third aspect are the same as those for the corresponding implementation forms of the client device according to the first aspect.

According to a fourth aspect of the invention, the above mentioned and other objectives are achieved with a method for a network access node, the method comprises

configuring a radio link between a client device and the network access node, wherein the radio link is configured for a first service type;

determining a timer value for a first timer to be used by the client device to determine an uplink time synchronization status for the radio link;

generating a control message comprising the timer value; and

transmitting the control message to the client device.

The method according to the fourth aspect can be extended into implementation forms corresponding to the implementation forms of the network access node according to the second aspect. Hence, an implementation form of the method comprises the feature(s) of the corresponding implementation form of the network access node.

The advantages of the methods according to the fourth aspect are the same as those for the corresponding implementation forms of the network access node according to the second aspect. The invention also relates to a computer program, characterized in program code, which when run by at least one processor causes said at least one processor to execute any method according to embodiments of the invention. Further, the invention also relates to a computer program product comprising a computer readable medium and said mentioned computer program, wherein said computer program is included in the computer readable medium, and comprises of one or more from the group: ROM (Read-Only Memory), PROM (Programmable ROM), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically EPROM) and hard disk drive.

Further applications and advantages of the embodiments of the invention will be apparent from the following detailed description.

Brief Description of the Drawings

The appended drawings are intended to clarify and explain different embodiments of the invention, in which:

- Fig. 1 shows a client device according to an embodiment of the invention;

- Fig. 2 shows a method according to an embodiment of the invention;

- Fig. 3 shows a network access node according to an embodiment of the invention;

- Fig. 4 shows a method according to an embodiment of the invention;

- Fig. 5 shows a wireless communication system according to an embodiment of the invention;

- Fig. 6 shows a flow chart of a method according to an embodiment of the invention;

- Fig. 7a-b shows timelines for the initiation of a random access procedure according to an embodiment of the invention; and

- Fig. 8 shows signalling between a network access node and a client device according to an embodiment of the invention.

Detailed Description

Fig. 1 shows a client device 100 according to an embodiment of the invention. In the embodiment shown in Fig. 1 , the client device 100 comprises a processor 102, a transceiver 104 and a memory 106. The processor 102 is coupled to the transceiver 104 and the memory 106 by communication means 108 known in the art. The client device 100 further comprises an antenna or antenna array 1 10 coupled to the transceiver 104, which means that the client device 100 is configured for wireless communications in a wireless communication system. That the client device 100 is configured to perform certain actions can in this disclosure be understood to mean that the client device 100 comprises suitable means, such as e.g. the processor 102 and the transceiver 104, configured to perform said actions.

According to embodiments of the invention the client device 100 is configured to determine an uplink time synchronization status for a radio link between the client device 100 and a network access node 300 (shown in Fig. 3), when the client device 100 is in connected mode with the network access node 300. The radio link is configured for a first service type. The client device 100 is further configured to initiate a random access procedure directly upon a determination that the uplink time synchronization status is unreliable. In other words, as soon as it is determined that the uplink time synchronization status is unreliable the random access procedure is triggered.

Fig. 2 shows a flow chart of a corresponding method 200 which may be executed in a client device 100, such as the one shown in Fig. 1 . The method 200 comprises determining 202 an uplink time synchronization status for a radio link between the client device 100 and a network access node 300 when the client device 100 is in connected mode with the network access node 300. The radio link is configured for a first service type. The method 200 further comprises initiating 204 a random access procedure directly upon a determination that the uplink time synchronization status is unreliable.

Fig. 3 shows a network access node 300 according to an embodiment of the invention. In the embodiment shown in Fig. 3, the network access node 300 comprises a processor 302, a transceiver 304 and a memory 306. The processor 302 is coupled to the transceiver 304 and the memory 306 by communication means 308 known in the art. The network access node 300 may be configured for both wireless and wired communications in wireless and wired communication systems, respectively. The wireless communication capability is provided with an antenna or antenna array 310 coupled to the transceiver 304, while the wired communication capability is provided with a wired communication interface 312 coupled to the transceiver 304.

That the network access node 300 is configured to perform certain actions can in this disclosure be understood to mean that the network access node 300 comprises suitable means, such as e.g. the processor 302 and the transceiver 304, configured to perform said actions. The network access node 300 is configured to configure a radio link between a client device 100 and the network access node 300. The radio link is configured for a first service type. The network access node 300 is further configured to configure a first timer based on the first service type. The first timer is used by the client device 100 to determine an uplink time synchronization status for the radio link. Furthermore, the network access node 300 is configured to generate a control message 502 comprising a timer value for the first timer and transmit the control message 502 to the client device 100.

Fig. 4 shows a flow chart of a corresponding method 400 which may be executed in a network access node 300, such as the one shown in Fig. 3. The method 400 comprises configuring 402 a radio link between a client device 100 and the network access node 300, where the radio link is configured for a first service type. The method 400 further comprises configuring 404 a first timer based on the first service type, where the first timer is used by the client device 100 to determine an uplink time synchronization status for the radio link. Furthermore, the method 400 comprises generating 406 a control message 502 comprising a timer value for the first timer and transmitting 408 the control message 502 to the client device 100.

Fig. 5 shows a wireless communication system 500 according to an embodiment of the invention. The wireless communication system 500 comprises a client device 100 and a network access node 300 configured to operate in the wireless communication system 500. For simplicity, the wireless communication system 500 shown in Fig. 5 only comprises one client device 100 and one network access node 300. However, the wireless communication system 500 may comprise any number of client devices 100 and network access nodes 300 without deviating from the scope of the invention.

In the embodiment shown in Fig. 5, the client device 100 is in connected mode with the network access node 300 and a radio link RL is configured between the client device 100 and the network access node 300. The radio link RL is configured for a first service type which can be understood to mean that the radio link RL between the client device and network access node 300 is used for communicating data of a specific service type that may for instance be services requiring low latency and/or high reliability. When the first service type has strict latency and/or reliability constraints, it becomes important for the client device 100 to maintain the uplink time synchronization for the radio link RL to the network access node 300. In conventional systems, when a client device loses its uplink time synchronization, a random access procedure is initiated to restore uplink time synchronization. However, the random access procedure is initiated only upon the client device having data to transmit or receive. For services with strict latency and/or reliability constraints, such as e.g. ultra reliability and low latency communication (URLLC) services, the latency of the random access procedure becomes a problem. Consequently, there is a need to improve the initiation of random access procedure for certain service types. According to embodiments of the invention a client device 100 is therefore configured to determine the uplink time synchronization status for the radio link RL and initiate a random access procedure directly if the uplink time synchronization status for the radio link RL is determined to be unreliable. The determination that the uplink time synchronization status is unreliable may be based on a first timer or based on uplink timing, as will now be described with reference to Fig. 6.

Fig. 6 shows a flow chart of a method 600 for initiating a random access procedure for a radio link configured for a first service type according to an embodiment of the invention. The method 600 may be performed in a client device, such as e.g. the client device 100 shown in Fig. 1 . In step 602, the client device 100 is configured with a radio link. The radio link is a radio link between the client device 100 and a network access node 300, where the client device 100 is in connected mode with the network access node 300. Furthermore, the radio link is configured for a first service type. In embodiments, the first service type for which the radio link is configured may have at least one of a latency constraint and a reliability constraint. At least one of the latency constraint and the reliability constraint may be associated with at least one of: a quality of service flow identity, a network slice selection assistance information configuration, a radio resource control parameter, and a medium access control parameter. The first service type may be a service with strict latency constraint and/or reliability constraint, such as e.g. an URLLC service requiring residual block error rate (BLER) of 10e-5 and latency of 1 ms. The client device 100 may determine the latency constraint of the first service type e.g. based on a delay threshold and may determine the reliability constraint e.g. based on an error rate threshold.

In step 604, the client device 100 monitors an uplink time synchronization status for the radio link configured for the first service type. Based on the monitoring in step 604, the client device 100 determines in step 606 if the uplink time synchronization status is unreliable. If the uplink time synchronization status is determined to be unreliable, i.e. the outcome of the determination in step 606 is YES, the client device 100 initiates a random access procedure in step 608. On the other hand, if the uplink time synchronization status is determined to not be unreliable in 606, i.e. the outcome of the determination in step 606 is NO, the client device 100 continuous to monitor the uplink time synchronization status for the radio link in step 604.

According to an embodiment of the invention the monitoring in step 604 and the determination in step 606 may be based on a first timer. In this case, step 604 may be initiated when the first timer is started and may comprise the client device 100 monitoring the first timer. The client device 100 may further in step 606 determine the uplink time synchronization status to be unreliable if the first timer has expired.

The first timer may in embodiments be a time alignment timer associated with the radio link. The time alignment timer (in NR one might exist per Timing Advance Group (TAG)) controls how long the MAC entity of the client device 100 considers the Serving Cells belonging to the associated TAG to be uplink time aligned. Hence, the client device 100 may in step 604 monitor the time alignment timer and in step 606 determine the uplink time synchronization status to be unreliable if the time alignment timer has expired. The client device 100 may be configured with the time alignment timer by the network access node 300. The time alignment timer (TA timer) is the timer defined in conventional systems for the client device 100 (or UE) to know when uplink synchronization is reliable. In such conventional systems once expired the client device 100 performs random access once data arrives in the uplink buffer. However, in embodiments in which the first timer is a TA timer directly upon the TA timer expiring the client device 100 would initiate a random access procedure.

However, in further embodiments the first timer may instead be configured for the first service type, i.e. be a timer configured to maintain uplink time synchronization for a radio link configured for the first service type. The first timer may be started when a second timer such as e.g. a time alignment timer has expired. Hence, the client device 100 may in embodiments start the first timer when a second timer has expired, where the second timer is a time alignment timer associated with the radio link. The client device 100 may be configured with the time alignment timer by the network access node 300. The client device 100 monitors the time alignment timer and when the time alignment timer expires, the client device 100 starts the first timer. The start of the first timer initiates step 604 and the client device 100 thereby starts to monitor the first timer. If the client device 100 determines in step 606 that the first timer has expired, the client device 100 initiates a random access procedure in step 608, to recover the uplink synchronization.

In embodiments, the client device 100 may be configured with more than one time alignment timer, e.g. when multiple serving cells are active for the client device 100. The client device 100 may e.g. be configured with a time alignment timer for each timing advance group (TAG) of serving cells. In this case, if a subset of the time alignment timers expires, the client device 100 may start the first timer. The subset of the time alignment timers may comprise one or more of the configured time alignment timers. Hence, depending on the subset of the time alignment timers, one or more time alignment timers need to be expired before the first timer is started. Furthermore, the first timer may in embodiments be started if the time alignment timer associated to the TAG of serving cells delivering the first service type has expired. The subset of the time alignment timers triggering the start of the first timer may be configured by the network access node 300.

Furthermore, when the first timer is configured for the first service type, the client device 100 may start the first timer when downlink control information (DCI) addressed for the client device 100 has been decoded since the first timer in this case can be considered as an inactivity timer triggered by reception of control information that may indicate reception or transmission of data.

The first timer may be started with a timer value defining the time before a random access procedure is initiated. The timer value of the first timer may be pre-defined in the client device 100 or received from the network access node 300. In the latter case, the client device 100 may receive the timer value from the network access node 300, e.g. in a control message 502 as will be described below with reference to Fig. 8. In embodiments, the timer value may be zero such that immediate recovery of uplink time synchronization is requested.

According to further embodiments of the invention the monitoring in step 604 and the determination in step 606 may be based on the uplink timing for the client device 100. In this case, step 604 may comprise the client device 100 monitoring an uplink or downlink timing for the client device 100 and step 606 may comprise the client device 100 determining the uplink time synchronization status to be unreliable if the uplink timing for the client device 100 has drifted outside a pre-defined window. In embodiments, a drift in the uplink timing may be detected in the client device 100 by monitoring a downlink timing based on known reference signals, such as e.g. synchronization signal block (SSB), channel state information reference signal (CSI-RS), or demodulation reference signal (DRMS). In this case, a drift in the downlink timing is assumed to indicate a similar or larger drift in the uplink timing. Thus, if the downlink timing monitored in step 604 is drifting outside the pre-defined window, the uplink time synchronization status is determined to be unreliable in step 606. The monitoring of the uplink or downlink timing may be performed in an entity of the client device, such as e.g. in a physical layer (PHY) or a radio resource control (RRC) layer of the client device 100.

When the client device 100 determines the uplink time synchronization status to be unreliable in step 606, the client device 100 initiates a random access procedure in step 608. The random access procedure may be performed to restore a reliable uplink time synchronization status. The client device 100 may initiate the random access procedure independently of a current data buffer status associated with the radio link. Hence, the client device 100 may initiate the random access procedure even if the current data buffer status is empty buffer. In this way, a reliable uplink time synchronization status can be maintained even when the client device 100 does not have any data to transmit or receive. By maintaining the uplink time synchronization for the radio link, the client device 100 can transmit or receive data without delay, i.e. without having to wait for performing a random access procedure until data arrives in the data buffer so as to restore uplink synchronization.

Step 608 may comprise the client device 100 transmitting a random access preamble in a random access resource to the network access node 300. At least one of the random access preamble and the random access resource may be associated with at least one of the latency constraint and the reliability constraint associated with the first service type. The random access preamble and/or the random access resource may be configured by the network access node 300 or pre-defined in the client device 100.

The random access procedure may be a contention based or a non-contention based random access procedure. Furthermore, whether a contention based or a non-contention based random access procedure is used may depend on at least one of the latency constraint and the reliability constraint associated with the first service type. When the random access procedure is a non-contention based random access procedure, the client device 100 may use a random access preamble and random access resources which are configured by the network access node 300 or indirectly identified based on the TAG where the time alignment timer has expired. Furthermore, the client device 100 may for a non-contention based random access procedure use a random access preamble and random access resources reserved for non contention based beam failure recovery (BFR) associated with a serving beam. The serving beam may here e.g. correspond to a CSI-RS resource index (CRI) or SSB index. The network access node 300 can distinguish the random access procedure due to expiry of the first timer from the random access procedure due to a BFR based on whether the non-contention based random access procedure is associated with a serving beam or a non-serving beam. If the received random access preamble and/or random access resources are associated with a serving beam then the random access procedure is initiated based on the expiry of the first timer. On the other hand, if the received random access preamble and/or random access resources are associated with a non-serving beam then the random access procedure is initiated based a BFR.

Figs. 7a-b show timelines for the initiation of a random access procedure according to embodiments of the invention. In Figs. 7a-b, the determination of the uplink time synchronization status for a radio link and thereby the initiation of a random access procedure is based on a first timer T1 configured for the first service type. In Fig. 7a, the first timer T1 is started at time instance t1 . As described with reference to Fig. 6, the first timer T1 may be started based on the expiry of a time alignment timer (shown in Fig. 7b) or based on decoded downlink control information (not shown in Figs.). The first timer T 1 runs with a timer value and expires at time instance t2. When the first timer T1 expires at time instance t2 the uplink time synchronization status is determined to be unreliable and a random access procedure is initiated, as indicated by the random access RA transmission in Fig. 7a. In reply to the random access RA transmission, the client device 100 receives a random access response RAR at time instance t3. The random access response RAR may comprise information such as e.g. a timing advance command, that the client device 100 can use to restore uplink time synchronization for the radio link. Hence, after the time instance t3 the uplink time synchronization status for the radio link is again determined to be reliable.

In the embodiment shown in Fig. 7b, the first timer T 1 is started upon expiry of a time alignment timer TA associated with the radio link. The expiry of the time alignment timer TA and thereby the start of the first timer T1 occurs at a time instance t1 in Fig. 7b. Furthermore, before the time instance t1 the radio link is considered to be uplink time aligned but after the time instance t1 the radio link is considered to be not uplink time aligned anymore. Note that as the first timer can be started based on other events than the expiry of the time alignment timer, the first timer may in embodiments be started while the time alignment timer is still running, i.e. while the radio link is still assumed time aligned. When the first timer T1 expires at time instance t2 the uplink time synchronization status is determined to be unreliable and a random access procedure is initiated, as indicated by the random access RA transmission in Fig. 7b. In reply to the random access RA transmission, the client device 100 receives a random access response RAR at time instance t3. The random access response RAR may comprise information such as e.g. a timing advance command, that the client device 100 can use to restore uplink time synchronization for the radio link. Hence, after the time instance t3 the uplink time synchronization status for the radio link is again determined to be reliable.

As previously mentioned, the client device 100 may be configured with the first timer by the network access node 300. Fig. 8 shows signaling between the network access node 300 and the client device 100 according to such an embodiment. In step I, the network access node 300 configures a radio link between the client device 100 and the network access node 300, where the radio link is configured for a first service type. The configuration of the radio link for the first service type can be made according to conventional methods, for instance during a RRC connection setup phase. The network access node 300 further configures a first timer based on the first service type in step II. In embodiments, the first service type for which the radio link is configured may have at least one of a latency constraint and a reliability constraint. At least one of the latency constraint and the reliability constraint may be associated with at least one of: a quality of service flow identity, a network slice selection assistance information configuration, a radio resource control parameter, and a medium access control parameter. The first service type may be a service with strict latency constraint and/or reliability constraint, such as e.g. an URLLC service requiring residual block error rate (BLER) of 10e-5 and latency of 1 ms. The client device 100 may determine the latency constraint of the first service type e.g. based on a delay threshold and may determine the reliability constraint e.g. based on an error rate threshold.

In Step II the network access node 300 determines a (timer) value for the first timer to be used by the client device 100 to determine an uplink time synchronization status for the radio link. The value of the first timer can be determined by the network access node 300 in a number of different ways and in the following non-limiting examples are given thereof. In one example, the value of the first timer can be determined based on current cell load in a cell of the network access node 300 such that the client device 100 configured for the first service type triggers random access in such a way that the random access load in the system is balanced with random access due to other causes. In another example, the value of the first timer is determined based on the cell deployment, such as e.g. micro, macro, and pico cell layout associated with the network access node 300. Moreover, in yet other examples the maximum signal transmission-reception delay for the client device configured for the first service type can be used to determine the value of the first timer.

The network access node 300 generates a control message 502 comprising the timer value for the first timer in step III and further transmit the control message 502 to the client device 100, as shown in step IV. The client device 100 receives the control message 502 from the network access node 300, where the control message 502 comprises a timer value for the first timer. Hence, the client device 100 may derive the timer value for the first timer from the received control message 502 and use the timer value when determining the uplink time synchronization status for the radio link.

The client device 100 herein, may be denoted as a user device, a User Equipment (UE), a mobile station, an internet of things (loT) device, a sensor device, a wireless terminal and/or a mobile terminal, is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The UEs may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in this context may be, for example, portable, pocket-storable, hand-held, computer- comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server. The UE can be a Station (STA), which is any device that contains an IEEE 802.1 1 -conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM). The UE may also be configured for communication in 3GPP related LTE and LTE-Advanced, in WiMAX and its evolution, and in fifth generation wireless technologies, such as New Radio.

The network access nodes 300 herein may also be denoted as a radio client device, an access client device, an access point, or a base station, e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter,“gNB”,“gNodeB”,“eNB”,“eNodeB”,“NodeB” or“B node”, depending on the technology and terminology used. The radio client devices may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. The radio client device can be a Station (STA), which is any device that contains an IEEE 802.1 1 -conformant Media Access Control (MAC) and Physical Layer (PHY) interface to the Wireless Medium (WM). The radio client device may also be a base station corresponding to the fifth generation (5G) wireless systems.

Furthermore, any method according to embodiments of the invention may be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may comprise essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

Moreover, it is realized by the skilled person that embodiments of the client device 100 and the network access node 300 comprises the necessary communication capabilities in the form of e.g., functions, means, units, elements, etc., for performing the solution. Examples of other such means, units, elements and functions are: processors, memory, buffers, control logic, encoders, decoders, rate matchers, de-rate matchers, mapping units, multipliers, decision units, selecting units, switches, interleavers, de-interleavers, modulators, demodulators, inputs, outputs, antennas, amplifiers, receiver units, transmitter units, DSPs, MSDs, TCM encoder, TCM decoder, power supply units, power feeders, communication interfaces, communication protocols, etc. which are suitably arranged together for performing the solution. Especially, the processor(s) of the client device 100 and the network access node 300 may comprise, e.g., one or more instances of a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. The expression“processor” may thus represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above. The processing circuitry may further perform data processing functions for inputting, outputting, and processing of data comprising data buffering and device control functions, such as call processing control, user interface control, or the like.

Finally, it should be understood that the invention is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.

Claims

1 . A client device (100) for a wireless communication system (500), the client device (100) being configured to

determine an uplink time synchronization status for a radio link between the client device (100) and a network access node (300) when the client device (100) is in connected mode with the network access node (300), wherein the radio link is configured for a first service type; initiate a random access procedure upon a determination that the uplink time synchronization status is unreliable.

2. The client device (100) according to claim 1 , configured to

determine the uplink time synchronization status to be unreliable if a first timer has expired.

3. The client device (100) according to claim 2, wherein the first timer is a time alignment timer associated with the radio link.

4. The client device (100) according to claim 2, wherein the first timer is configured for the first service type.

5. The client device (100) according to claim 4, configured to

6. The client device (100) according to claim 4, configured to

start the first timer when downlink control information addressed for the client device (100) has been decoded.

7. The client device (100) according to any of claims 2 to 6, configured to

receive a control message (502) from the network access node (300), wherein the control message (502) comprises a timer value for the first timer.

8. The client device (100) according to any of the preceding claims, configured to

determine the uplink time synchronization status to be unreliable if the uplink timing for the client device (100) has drifted outside a pre-defined window.

9. The client device (100) according to any of the preceding claims, configured to initiate the random access procedure independently of a current data buffer status associated with the radio link.

10. The client device (100) according to claim 9, configured to

1 1 . The client device (100) according to any of the preceding claims, wherein the random access procedure is a non-contention based random access procedure.

12. The client device (100) according to any of the preceding claims, wherein the first service type has at least one of a latency constraint and a reliability constraint.

13. The client device (100) according to claim 12, wherein at least one of the latency constraint and the reliability constraint is associated with at least one of: a quality of service flow identity, a network slice selection assistance information configuration, a radio resource control parameter, and a medium access control parameter.

14. The client device (100) according to any of claims 12 to 13, wherein initiate the random access procedure comprises

transmit a random access preamble in a random access resource to the network access node (300), wherein at least one of the random access preamble and the random access resource is associated with at least one of the latency constraint and the reliability constraint.

15. A network access node (300) for a wireless communication system (500), the network access node (300) being configured to

configure a radio link between a client device (100) and the network access node (300), wherein the radio link is configured for a first service type;

determine a timer value for a first timer to be used by the client device (100) to determine an uplink time synchronization status for the radio link;

generate a control message (502) comprising the timer value; and

transmit the control message (502) to the client device (100).

16. A method (200) for a client device (100), the method (200) comprising

determining (202) an uplink time synchronization status for a radio link between the client device (100) and a network access node (300) when the client device (100) is in connected mode with the network access node (300), wherein the radio link is configured for a first service type;

initiating (204) a random access procedure upon a determination that the uplink time synchronization status is unreliable.

17. A method (400) for a network access node (300), the method (400) comprising

configuring (402) a radio link between a client device (100) and the network access node (300), wherein the radio link is configured for a first service type;

determining (404) a timer value for a first timer to be used by the client device (100) to determine an uplink time synchronization status for the radio link;

generating (406) a control message (502) comprising the timer value; and

transmitting (408) the control message (502) to the client device (100).

18. A computer program with a program code for performing a method according to claim 16 or 17 when the computer program runs on a computer.