Technical Field

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

Background

The 5G cellular system, also called new radio (NR), is currently being standardized. NR is targeting radio spectrum from below 1 GHz up to and above 60 GHz. To allow for such diverse radio environments not only different system bandwidths will be supported, but also different numerologies, such as different sub-carrier-spacings (SCS). Furthermore, for carriers over 10 GHz multiple antennas and beamforming will be needed to combat the higher path loss at such high radio frequencies. When beamforming is used, a next generation nodeB (gNB) transmits data in several directions in different transmit beams. The user equipment (UE) also designated as client device therefore has to tunes its own receive antennas in different receive beam directions to communicate with the gNB. In order for the UE to be able to detect and track the transmit beams of the gNB, the UE need to perform beam monitoring. Hence, the gNB transmits known pilot signals in adjacent beams, which the UE receives and uses to detect possible transmit beams to switch to in case of changes in the radio environment. The principles behind beam monitoring can be compared to the cell search in legacy long term evolution (LTE), wideband code division multiple access (WCDMA) and high speed packet access (HSPA) systems. In such systems, the UE on a regular basis need to scan neighbouring cells for possible handover candidates.

Each possible connection between the UE and the gNB is called a beam pair link (BPL), where a BPL consists of the best match between a transmit beam and a receive beam. The gNB will configure a set of BPLs for the UE to monitor. The configured set of monitored BPLs may be based on which BPL the UE has detected. This set can for example comprise all the BPLs associated with control channels and data channels between the gNB and the UE. The gNB will also configure a set of serving BPLs which will be used to transmit associated control information to the UE. The set of serving BPLs is a subset or equal to the set of monitored BPLs. The UE monitors the quality of the set of monitored BPLs and reports the quality in beam measurement report to the gNB. When a monitored BPL beam becomes stronger than the current serving BPL a beam switch could be initiated. The exact procedure for the beam switching is not yet defined in the NR standard. One approach could be that the UE triggers a beam measurement report comprising the event that a target BPL is stronger than the current serving BPL. Another scenario would be that the gNB determines, e.g. using uplink management procedures, that a target BPL has become a suitable serving BPL. The gNB could then order a beam switch to the target BPL.

The beam switch procedure needs to be fast, especially at frequencies above 10 GHz where the radio channel may change quickly due to blocking. Applying LTE methods would result in a slow beam switch procedure, which may result in poor throughput and hence poor user experience.

Summary

An objective of implementation forms 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 implementation forms of the present 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

switch a transmit beam from a first uplink Beam Pair Link, BPL, to a second uplink BPL, wherein the first uplink BPL is a serving uplink BPL and the second uplink BPL is a target uplink BPL for uplink data transmissions to a network access node;

determine if the client device is uplink time aligned with the network access node on the second uplink BPL;

derive an uplink timing advance for the second uplink BPL if the client device is determined to be uplink time aligned with the network access node on the second uplink BPL. An uplink BPL in this disclosure can be understood to be a beam pair comprising a client device transmit beam and a network access node receive beam. A downlink BPL in this disclosure can be understood to be a beam pair comprising a network access node transmit beam and a client device receive beam. Furthermore, to switch a transmit beam in this disclosure can be understood to mean to change from a currently serving uplink BPL to a target uplink BPL, such that the target uplink BPL becomes the new serving uplink BPL. The client device in this disclosure typically determines if the client device is uplink time aligned, i.e. synchronized in the uplink, with the network access node on the second uplink BPL prior to switching the transmit beam.

Moreover, when the client device is uplink time aligned the client device derives the uplink timing advance without performing a random access procedure in this disclosure. The client device may instead use information currently available in the client device, thereby the network access node does not have to be inquired.

A client device according to the first aspect provides a number of advantages over conventional solutions. An advantage of the client device is that by checking whether the client device is uplink time aligned or not with the network access node on the second uplink BPL, uplink interference can be avoided when switching to the second uplink BPL. Furthermore, no random access procedure is needed when the client device is uplink time aligned. Thereby, the transmit beam switch can be performed faster compared to conventional solutions.

In an implementation form of a client device according to the first aspect, both the first uplink BPL and the second uplink BPL are associated with the same network access node. In other words the client device may switch the uplink BPLs but will still be connected to the same network access node after the transmit beam switching.

An advantage with this implementation form is that the network access node in this case can use narrow beams in communication with the client device, and hence increase the signal quality and thereby increase the data throughput over the radio interface.

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

determine if the client device is uplink time aligned with the network access node on the second uplink BPL based on a non-expired transmission timer for the second uplink BPL.

An advantage with this implementation form is that it provides a simple way of using available information to determine whether the client device is uplink time aligned or not. This also implies faster determination whether the client device is uplink time aligned with the network access node on the second uplink BPL. In an implementation form of a client device according to the first aspect, the client device is further configured to

derive the uplink timing advance for the second uplink BPL based on an uplink timing advance value for the second uplink BPL.

Generally, a timing advance corresponds to how long before a downlink reception the client device need to start an uplink transmission, such that the uplink transmission reaches the network access node at a correct time instance. Hence, the timing advance is a measure of the time period it takes for a radio signal to propagate from a client device to the network access node.

An advantage with this implementation form is that by using an uplink timing advance value, the uplink timing advance for the second uplink BPL can be more accurately derived. In an implementation form of a client device according to the first aspect, the uplink timing advance value for the second uplink BPL is differentially encoded in relation to an uplink timing advance for the first uplink BPL, and the client device is further configured to

derive the uplink timing advance for the second uplink BPL based on the differentially encoded uplink timing advance value for the second uplink BPL and the uplink timing advance for the first uplink BPL.

An advantage with this implementation form is that the uplink timing advance for the second uplink BPL can be accurately derived, even in situations where the uplink timing advance value for the second uplink BPL is differentially encoded in relation to the uplink timing advance for the first uplink BPL. Furthermore, by using differentially encoded uplink timing advance values the signalling overhead over the radio interface is reduced.

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

receive the uplink timing advance value for the second uplink BPL from the network access node.

The uplink timing advance value for the second uplink BPL is in this disclosure typically received prior to the switching of the transmit beam.

An advantage with this implementation form is that the network access node can ensure that the client device receives a valid uplink timing advance value. In an implementation form of a client device according to the first aspect, the uplink timing advance for the second uplink BPL is valid for a plurality of second uplink BPLs. An advantage with this implementation form is that the derived uplink timing advance for the second uplink BPL can be shared and used when switching the transmit beam to other second uplink BPLs for which the same uplink timing advance is valid.

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

determine if the client device is uplink time aligned with the network access node on the second uplink BPL based on a downlink reception timing for a second downlink BPL associated with the second uplink BPL. An advantage with this implementation form is that the check whether the client device is uplink time aligned or not can be performed even when no explicit information about uplink time alignment is available to the client device.

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

derive the uplink timing advance for the second uplink BPL based on the downlink reception timing for the second downlink BPL.

An advantage with this implementation form is that the uplink timing advance for the second uplink BPL can be derived even when no explicit information about the uplink timing advance value is available to the client device.

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

derive the uplink timing advance for the second uplink BPL based on a timing difference value and an uplink timing advance for the first uplink BPL, wherein the timing difference value represents a difference between a downlink reception timing for a first downlink BPL associated with the first uplink BPL and the downlink reception timing for the second downlink BPL.

An advantage with this implementation form is that by using the timing difference value and the uplink timing advance for the first uplink BPL, the uplink timing advance for the second uplink BPL can be more accurately derived even when no explicit information about the uplink timing advance value is available to the client device.

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

transmit the timing difference value to the network access node.

An advantage with this implementation form is that the network access node is able to validate the timing difference value and use it to derive an uplink timing advance value which may be used upon a transmit beam switch.

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

switch the transmit beam from the first uplink BPL to the second uplink BPL in response to a beam failure for at least one of the first uplink BPL and the first downlink BPL.

An advantage with this implementation form is that a beam recovery can be performed in the case a beam failure occurs. In an implementation form of a client device according to the first aspect, the client device is further configured to

perform a random access procedure on the second uplink BPL if the client device is determined not to be uplink time aligned with the network access node on the second uplink BPL.

An advantage with this implementation form is that it provides a fallback to a conventional method of acquiring an uplink timing advance if the client device is not uplink time aligned.

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

receive a BPL switch indication from the network access node at a first time instance; switch the transmit beam from the first uplink BPL to the second uplink BPL according to the BPL switch indication. An advantage with this implementation form is that it provides a network controlled transmit beam switch, which is initiated by the network access node. In an implementation form of a client device according to the first aspect, the client device is further configured to

determine a second time instance based on the first time instance and a predefined rule; perform an uplink data transmission on the second uplink BPL at the second time instance according to the timing advance for the second uplink BPL.

An advantage with this implementation form is that the uplink data transmission on the second uplink BPL is performed at a pre-determined time instance, allowing the client device to be uplink time aligned with the network access node already at the start of the uplink data transmission on the second uplink BPL.

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

receive a timing difference value from a client device, wherein the timing difference value represents a timing difference between a downlink reception timing for a first downlink BPL associated with a first uplink BPL and a downlink reception timing for a second downlink BPL associated with a second uplink BPL, wherein the first uplink BPL is a serving uplink BPL and the second uplink BPL is a target uplink BPL for uplink data transmissions from the client device to the network access node;

derive an uplink timing advance value for the second uplink BPL based on the timing difference value;

transmit the uplink timing advance value for the second uplink BPL to the client device. A network access node according to the second aspect provides a number of advantages over conventional solutions. An advantage of the network access node is that the timing difference value from the client device can be validated by the network access node and used to derive an uplink timing advance value for the second uplink BPL. The uplink timing advance value for the second uplink BPL may be used upon a transmit beam switch to the second uplink BPL, ensuring time aligned uplink data transmissions on the second uplink BPL. Hence, uplink interference due to miss-timing of uplink data transmissions is avoided.

In an implementation form of a network access node according to the second aspect, the timing difference value is received in a beam measurement report from the client device. An advantage with this implementation form is that the timing difference value can be provided to the network access node using an already existing type of report. Hence, no new report type has to be defined. In an implementation form of a network access node according to the second aspect, the network access node is further configured to

determine a transmission timer configuration parameter (for a client device side transmission timer) for the second uplink BPL;

transmit the transmission timer configuration parameter for the second uplink BPL to the client device.

An advantage with this implementation form is that the client device is provided with the transmission time configuration parameter and can accordingly adjust its transmission timer for the second uplink BPL. Hence the client device knows for how long time the second uplink BPL can be considered uplink time aligned.

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

switching a transmit beam from a first uplink Beam Pair Link, BPL, to a second uplink BPL, wherein the first uplink BPL is a serving uplink BPL and the second uplink BPL is a target uplink BPL for uplink data transmissions to a network access node;

determining if the client device is uplink time aligned with the network access node on the second uplink BPL;

deriving an uplink timing advance for the second uplink BPL if the client device is determined to be uplink time aligned with the network access node on the second uplink BPL.

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 receiving a timing difference value from a client device, wherein the timing difference value represents a timing difference between a downlink reception timing for a first downlink BPL associated with a first uplink BPL and a downlink reception timing for a second downlink BPL associated with a second uplink BPL, wherein the first uplink BPL is a serving uplink BPL and the second uplink BPL is a target uplink BPL for uplink data transmissions from the client device to the network access node;

deriving an uplink timing advance value for the second uplink BPL based on the timing difference value;

transmitting the uplink timing advance value for the second uplink BPL 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 code means, which when run by processing means causes said processing means to execute any method according to the present 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 present 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 present 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 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 according to an embodiment of the invention;

- Fig. 7 shows downlink reception timings and uplink transmission timings at a client device according to an embodiment of the invention;

- 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. 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 1 10 coupled to the transceiver 104. 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 should 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.

The client device 100 is configured to switch a transmit beam from a first uplink beam pair link (BPL) 510 (shown in Fig. 5) to a second uplink BPL 512 (shown in Fig. 5). The first uplink BPL 510 is a serving uplink BPL and the second uplink BPL 512 is a target uplink BPL for uplink data transmissions to a network access node 300 (shown in detail also in Fig. 3). The client device 100 is further configured to determine, prior to switching the transmit beam, if the client device 100 is uplink time aligned with the network access node 300 on the second uplink BPL 512. The determination whether the client device 100 is uplink time aligned with the network access node 300 on the second uplink BPL 512 may be based on a non-expired transmission timer for the second uplink BPL 512 or based on a downlink reception timing for a second downlink BPL 512 ^' (shown in Fig. 5) associated with the second uplink BPL 512. Furthermore, the client device 100 is configured to derive an uplink timing advance for the second uplink BPL 512 if the client device 100 is determined to be uplink time aligned with the network access node 300 on the second uplink BPL 512.

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 switching 202 a transmit beam from a first uplink BPL 510 to a second uplink BPL 512. The first uplink BPL 510 is a serving uplink BPL and the second uplink BPL 512 is a target uplink BPL for uplink data transmissions to a network access node 300. The method 200 further comprises determining 204 prior to switching the transmit beam if the client device 100 is uplink time aligned with the network access node 300 on the second uplink BPL 512. Furthermore, the method 200 comprises deriving 206 an uplink timing advance for the second uplink BPL 512 if the client device 100 is determined to be uplink time aligned with the network access node 300 on the second uplink BPL 512. Fig. 3 shows a network access node 300, such as a base station, according to an embodiment of the invention. In the implementation 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 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 should 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 receive a timing difference value Δ (shown in Fig. 7) from a client device 100. The timing difference value Δ represents a timing difference between a downlink reception timing for a first downlink BPL 510 ^' (shown in Fig. 5) associated with a first uplink BPL 510 and a downlink reception timing for a second downlink BPL 512 ^' associated with a second uplink BPL 512, where the first uplink BPL 510 is a serving uplink BPL and the second uplink BPL 512 is a target uplink BPL for uplink data transmissions from the client device 100 to the network access node 300. The network access node 300 is further configured to derive an uplink timing advance value 540 (shown in Fig. 8) for the second uplink BPL 512 based on the timing difference value Δ. Furthermore, the network access node 300 is configured to transmit the uplink timing advance value 540 for the second uplink BPL 512 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 receiving 402 a timing difference value Δ from a client device 100. The timing difference value Δ represents a timing difference between a downlink reception timing for a first downlink BPL 510 ^' associated with a first uplink BPL 510 and a downlink reception timing for a second downlink BPL 512 ^' associated with a second uplink BPL 512, where the first uplink BPL 510 is a serving uplink BPL and the second uplink BPL 512 is a target uplink BPL for uplink data transmissions from the client device 100 to the network access node 300. The method 400 comprises deriving 404 an uplink timing advance value 540 for the second uplink BPL 512 based on the timing difference value Δ. Furthermore, the method 400 comprises transmitting 406 the uplink timing advance value 540 for the second uplink BPL 512 to the client device 100.

Fig. 5 shows a wireless communication system 500 according to an embodiment of the present invention. The wireless communication system 500 comprises a client device 100 and network access node 300 configured to operate in the wireless communication system 500. In the wireless communication system 500, beamforming is used such that data is transmitted in several directions in different BPLs between the client device 100 and the network access node 300. In Fig. 5, a first downlink BPL 510 ^' associated with a first uplink BPL 510, as well as a second downlink BPL 512 ^' associated with a second uplink BPL 512, is shown. The uplink and downlink can in some cases be considered reciprocal to each other which implies that there is an uplink to downlink correspondence hence the association between a downlink BPL and an uplink BPL herein. This means that the transmit beam for an uplink BPL and the reception beam for the associated downlink BPL at the client device 100 generally have the same direction. Furthermore the transmit beam for the downlink BPL and the reception beam for the associated uplink BPL at the network access node 300 generally have the same direction. However, any number of uplink and/or downlink BPLs may exist between the client device 100 and the network access node 300 without deviation from the scope of the invention. The first uplink BPL 510 is a serving uplink BPL and the second uplink BPL 512 is a target uplink BPL for uplink data transmissions from the client device 100 to the network access node 300. Hence, the first uplink BPL 510 is currently used for uplink data transmissions from the client device 100 to the network access node 300, while the second uplink BPL 512 is a candidate for a transmit beam switch.

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 any number of network access nodes 300 without deviating from the scope of the invention. Fig. 6 shows a flow chart illustrating an embodiment of the invention. In step 602, the client device 100 obtains an indication to perform a transmit beam switch from a first uplink BPL 510 to a second uplink BPL 512, herein called a BPL switch indication. The BPL switch indication may be received from the network access node 300, in e.g. a downlink physically layer 1 or layer 2 control message. In this case, the client device 100 receives a BPL switch indication from the network access node 300 at a first time instance. In some cases, the BPL switch indication may be based on a detected beam failure for at least one of the first uplink BPL 510 and the first downlink BPL 510 ^' , which may be detected either by the client device 100 or by the network access node 300. Beam failure in this disclosure means that the quality of one of the serving beams, the first uplink BPL 510 or the first downlink BPL 510 ^' , has become so low that communication over the serving beam is not reliable. A beam failure is typically detected based on estimates of the decoding performance on a control channel being below a predefined quality threshold.

Upon reception of a BPL switch indication, the client device 100 determines, in step 604, if the client device 100 is uplink time aligned with the network access node 300 on the second uplink BPL 512. The determination whether the client device 100 is uplink time aligned or not may for example either be based on a non-expired transmission timer for the second uplink BPL 512 or based on a downlink reception timing for a second downlink BPL 512 ^' associated with the second uplink BPL 512. Further details related to step 604 and the two determining methods will be described below.

If the outcome of the determining in step 604 is No, i.e. it is determined that the client device 100 is not uplink time aligned with the network access node 300 on the second uplink BPL 512, the client device 100 performs a random access procedure on the second uplink BPL 512 in step 606. The random access procedure may be performed on a random access channel (RACH) in a frequency-time resource that may be associated to the second uplink BPL 512 at a time instant occurring after the first time instance. The random access procedure may be performed according to procedures defined in standards. During the random access procedure the client device receives a timing advance value from the network access node 300.

On the other hand, if the outcome of the determining in step 604 is Yes, the client device 100 derives an uplink timing advance for the second uplink BPL 512, in step 608 without inquiring the network access device 300. Hence, the client device 100 can derive the uplink timing advance based on information already available inside the client device 100. The uplink timing advance may be derived in different ways and is dependent on the determining step 604 as will be described further below.

In step 610, the client device 100 switches the transmit beam from the first uplink BPL 510 to the second uplink BPL 512 according to the BPL switch indication.

In step 612, the client device 100 performs an uplink data transmission on the second uplink BPL 512 at a second time instance according to the uplink timing advance for the second uplink BPL 512. The uplink timing advance for the second uplink BPL 512 used by the client device 100 in step 612 is the uplink timing advance derived in step 608. The second time instance is determined by the client device 100 based on the first time instance and a predefined rule.

As described above in relation to step 604 in Fig. 6, the client device 100 may determine if the second uplink BPL 512 is time aligned in different ways. For example based on a transmission timer or based on downlink reception timing.

Firstly, an embodiment using a transmission timer will be described. In such an embodiment, the client device 100 determines whether it is uplink time aligned with the network access node 300 on the second uplink BPL 512 based on a non-expired transmission timer for the second uplink BPL 512. Hence, step 604 will comprise determining whether the transmission timer has expired or not. The transmission timer may e.g. be a timing advance timer and may have been configured due to at least one of the following reasons:

a) An uplink beam management procedure has taken place. In such an uplink beam management procedure, the client device 100 has been configured to transmit sounding-reference-symbols (SRS) in a set of BPLs (comprising at least the second uplink BPL 512) and the network access node 300 has measured the uplink timing for respective BPLs, and hence transmitted timing advance commands and configured timing advance timers, including the transmission timer, for the respective BPLs to the client device 100.

b) The client device 100 has used the second uplink BPL 512 as a serving uplink BPL at an earlier time instance and the corresponding transmission timer has not yet expired. c) The client device 100 has performed a random access procedure on the second uplink BPL 512 earlier and the corresponding, by the network access node 300 configured, transmission timer has yet not expired. If no transmission timer is configured for the second uplink BPL 512 or the transmission timer is determined to have expired, the client device 100 is considered not uplink time aligned with the network access node 300 on the second uplink BPL 512 and step 606 will be performed. On the other hand, if the transmission timer for the second uplink BPL 512 is determined to be non-expired, i.e. is still running, the client device 100 is considered uplink time aligned with the network access node 300 on the second uplink BPL 512 and step 608 will be performed. In step 608 in this embodiment, the client device 100 derives the uplink timing advance for the second uplink BPL 512 based on an uplink timing advance value 540 for the second uplink BPL 512 already available the client device 100. The uplink timing advance value 540 for the second uplink BPL 512 may have been received from the network access node 300, as shown in Fig. 8, prior to switching the transmit beam, e.g. when the transmission timer has been (re)started.

When the client device 100 derives the uplink timing advance for the second uplink BPL 512 based on an uplink timing advance value 540 for the second uplink BPL 512, different methods are needed depending on whether an absolute timing advance value or a differentially encoded timing advance value is used. If the uplink timing advance value 540 for the second uplink BPL 512 is not differentially encoded, the uplink timing advance can be directly derived by the client device 100. In this case, the following equation may be used to derive the uplink timing advance for the second uplink BPL 512:

where TA512 is the uplink timing advance for the second uplink BPL 512 and TA 540 is the timing advance derived from the uplink timing advance value 540. On the other hand, if the uplink timing advance value 540 for the second uplink BPL 512 is differentially encoded in relation to an uplink timing advance for the first uplink BPL 510, the client device 100 derives the uplink timing advance for the second uplink BPL 512 based on the differentially encoded uplink timing advance value 540 for the second uplink BPL 512 and the uplink timing advance for the first uplink BPL 510. In this case, the following equation may be used to derive the uplink timing advance for the second uplink BPL 512:

where TA512 is the uplink timing advance for the second uplink BPL 512, TA510 is the uplink timing advance for the first uplink BPL 510 and TA 540 is the uplink timing advance derived from the uplink timing advance value 540.

The uplink timing advance for the second uplink BPL 512 derived by the client device 100 in step 608 may in some cases be valid for a plurality of second uplink BPLs. For example, where a plurality of second uplink BPLs, including the second uplink BPL 512, has substantially the same uplink timing advance. In such a case, the network access node 300 may group the plurality of second uplink BPLs into a timing advance group with a corresponding uplink timing advance value. Hence, determination of the uplink timing advance for any of the plurality of second uplink BPLs may comprise determining which timing advance group the second uplink BPL belongs to and use the corresponding uplink timing advance value to derive of the uplink timing advance. E.g. when the client device 100 determines that first uplink BPL 510 and the second uplink BPL 512 are in the same timing advance group, the uplink timing advance stays the same when switching from the first uplink BPL 510 to the second uplink BPL 512.

The management of the transmission timer upon a transmit beam switch may differ. In some cases, the client device 100 may keep the transmission timer for the serving uplink BPL running upon a transmit beam switch. In other cases, the transmission timer for the serving uplink BPL is reset upon a transmit beam switch and the client device 100 instead enables the transmission timer for the target uplink BPL. Furthermore, the configured transmission timer may be different for different BPLs. For example, it may be set to a first (longer) value for the serving uplink BPL and set to a second (shorter) value for the target uplink BPLs.

An embodiment where step 604 and step 608 instead are based on downlink reception timing will now be described, i.e. when the client device 100 determines whether it is uplink time aligned with the network access node 300 on the second uplink BPL 512 based on a downlink reception timing for a second downlink BPL 512 ^' associated with the second uplink BPL 512. In this case, the client device 100 has no explicit information about the uplink timing for the second uplink BPL 512, instead the client device 100 implicitly determines the uplink timing for the second uplink BPL 512 from timing information related to the first uplink BPL 510, the first downlink BPL 510 ^' and the second downlink BPL 512 ^' .

Hence, in step 604 it is determined whether the client device 100 is uplink time aligned with the network access node 300 on the second uplink BPL 512 based on whether the client device 100 has a valid downlink reception timing of the second downlink BPL 512 ^' relative to the first downlink BPL 510 ^' . A valid downlink reception timing in this context implies that the client device 100 has been able to obtain a downlink reception timing. For example, if the second downlink BPL 512 is configured with pilot signals, the client device 100 can use the pilot signals for estimating the downlink reception timing. However, if no such pilot exists for the second downlink BPL 512, the client device 100 does not have any knowledge of the downlink reception timing and a valid downlink reception timing does not exist. It is however to be noted that the downlink reception timing is also non-valid if a downlink reception timing estimate based on pilot signals is not reliable, e.g. due to poor radio conditions, such that signal-to- interference and noise ratio (SINR) or any corresponding measure is below a threshold value. If the client device 100 does not have a valid downlink reception timing of the second downlink BPL 512 ^' relative to the first downlink BPL 510 ^' , the client device 100 is considered not uplink time aligned with the network access node 300 on the second uplink BPL 512 and step 606 will be performed. On the other hand, if the client device 100 has a valid downlink reception timing of the second downlink BPL 512 ^' relative to the first downlink BPL 510 ^' , the client device 100 is considered uplink time aligned with the network access node 300 on the second uplink BPL 512 and step 608 will be performed. In step 608 in this embodiment, the client device 100 derives the uplink timing advance for the second uplink BPL 512 based on the downlink reception timing for the second downlink BPL 512 ^' , as will now be described with reference to Fig. 7.

Fig. 7 shows downlink reception timings and uplink transmission timings at the client device 100 at two different times, a first time TO and a second time T1 . The first time TO represents a time instance before the transmit beam switch, i.e. when the first uplink BPL 510 is a serving uplink BPL, while the second time T1 represents a time instance after the transmit beam switch, i.e. when the second uplink BPL 512 is a serving uplink BPL. In Fig. 7 the downlink reception timing of the first downlink BPL 510 ^' relative to the uplink transmission timing of the first uplink BPL 510 a first time TO is shown. Furthermore, Fig. 7 shows the downlink reception timing of the second downlink BPL 512 ^' relative to the uplink transmission timing of the second uplink BPL 512 at the second time T1 . The client device 100 may derive the uplink timing advance for the second uplink BPL 512, denoted TA512 in Fig. 7, based on a timing difference value Δ and an uplink timing advance for the first uplink BPL 510, denoted TA510 in fig. 7. The timing difference value Δ as previously described represents a difference between a downlink reception timing for a first downlink BPL 510 ^' , denoted DRTs-io- in Fig. 7, and the downlink reception timing for the second downlink BPL 512 ^' , denoted DRTsi2- in Fig. 7. These downlink reception timings may in some cases be obtained from channel estimation of reference signal transmissions from the network access node 300 in the first downlink BPL 510 ^' and the second downlink BPL 512 ^' , respectively. Thus, the timing difference value Δ is typically an estimated value and may be derived based on the following equation:

where Δ is the timing difference value, DRT ₅ 12- is the downlink reception timing for the second downlink BPL 512 ^' , and DRTs-io- is the downlink reception timing for the first downlink BPL 510 ^' . In an example, the following equation may be used to derive the uplink timing advance for the second uplink BPL 512 from the timing information shown in Fig. 7 and described above:

TA ₅ i2=max(TA ₅ io-2A, 0) (4) where TA512 is the uplink timing advance for the second uplink BPL 512, TA510 is the uplink timing advance for the first uplink BPL 510, and Δ is the timing difference value from equation (3). The max function is used to avoid negative timing advance values. As the timing difference value Δ typically is an estimation and due to noise in estimations the value 2Δ may be larger than TA510. Hence, the max function in equation (4) ensures that the timing advance is 0 or larger.

Furthermore, the client device 100 may transmit the timing difference value Δ to the network access node 300. This allows the network access node 300 to derive an uplink timing advance value 540, as will now be described with reference to Fig. 8. In step I in Fig. 8, the client device 100 transmits the timing difference value Δ for the second uplink BPL 512 to the network access node 300. The timing difference value Δ may e.g. be transmitted in a beam measurement report, i.e. the network access node 300 may receive the timing difference value Δ in a beam measurement report from the client device 100. Upon a transmit beam switch to the second uplink BPL 512, the network access node 300 validates the timing difference value Δ. The validation checks if the timing difference value Δ is a realistic value with regards to e.g. cell size and possible maximum reception delay differences between different downlink BPLs taking deployment factors like likelihood for line-of-sight into account. If the timing difference value Δ is deemed valid, the network access node 300 derives the uplink timing advance value 540 for the second uplink BPL 512 based on the timing difference value Δ, as shown in step II. In step III, the derived uplink timing advance value 540 is transmitted by the network access node 300 to the client device 100.

In addition, the network access node 300 may configure the client device 100 with a transmission timer for the second uplink BPL 512. In this case, the network access node 300 determines a transmission timer configuration parameter 530 for the second uplink BPL 512, as shown in step IV. The network access node 300 further transmits the transmission timer configuration parameter 530 for the second uplink BPL 512 to the client device 100, in step V. Upon reception of the transmission timer configuration parameter 530, the client device 100 starts the transmission timer based on the transmission timer configuration parameter 530. 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 the present 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 node 300 herein may also be denoted as a radio network access node, an access network access node, an access point, or a base station, e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter, "eNB", "eNodeB", "NodeB" or "B node", depending on the technology and terminology used. The radio network access nodes 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 network access node 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 network access node 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 present 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 present 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 implementations within the scope of the appended independent claims.