TECHNICAL FIELD:

The teachings in accordance with the exemplary embodiments of this invention relate generally to user plane handling for a cell change such as for a handover, and, more specifically, relate to user plane handling to enable an almost 0ms cell change.

BACKGROUND:

This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.

Certain abbreviations that may be found in the description and/or in the Figures are herewith defined as follows:

AMF Access Management Function

BS Base Station

DC Dual Connectivity

DL Downlink

eNB Base Station

gNB next generation Node B (5G NR Base Station)

HO Handover

LTE Long Term Evolution

MME Mobility Management Entity

MS Millisecond

NB NodeB (base station)

NCE Network Control Entity

NR New Radio

PDCP Packet Data Convergence Protocol

PDU Protocol Data Unit RACH Random Access Channel

RAN Radio Access Network

RRC Radio Resource Control

Rx Receive

SDU Service Data Unit

SGW Serving Gateway

SN Sequence Number

SSC Session and Service Continuity

TA Timing Advance

Tx Transmit

UL Uplink

UPF User Plane Function

In standards development cell change for handover optimization is one key issue. In a classic handover the source BS requests the resources from the target BS, which responds with the HO Command, which is then forwarded from the source to the UE. From this moment, the UE disconnects from the source BS and attempts connecting to the target BS. One proposal to provide such cell change for handover optimization includes the use of Release 14 make- before-break operations for handovers. However, this solution still requires a long interruption time e.g., approximately ~5ms.

Example embodiments of the invention work to improve cell change operations such as for handover including at least reducing this interruption time.

SUMMARY:

In an example aspect of the invention, there is a method comprising: determining, by a network device of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and based on the determining, marking with count values a sequence number of the at least one packet data unit with the count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values communicated after the indication of the cell change. A further example embodiment is a method comprising the method of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network device comprises the user equipment for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or downlink count value and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein there is communicating information of the marked sequence number of the at least one packet data unit with the source cell; wherein the synchronization with the target cell is using information comprising the marked count values with an offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the source cell after indication of the cell change; wherein the information is communicated using a radio resource control reconfiguration complete message; wherein the information communicated using the radio resource control reconfiguration complete message comprises offsets for the marked at least one of an uplink count value or a downlink count value representing a number of the at least one packet data unit that was communicated between the network device and the source cell after the indication of the cell change is received; wherein there is receiving from the source cell information comprising a handover command and an indication of an uplink offset value or a downlink offset value, wherein the uplink offset value or the downlink offset value are used by the user equipment to determine uplink count values and downlink count values for synchronization with the target cell; wherein the information is communicated using a radio resource control reconfiguration message; and wherein there is receiving from the target cell for the synchronization a first unacknowledged downlink packet data control protocol service data unit using a first sequence number that was not applied by the source cell.

In another example aspect of the invention, there is an apparatus such as a user equipment side apparatus, comprising: means for determining, by a network device of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and means, based on the determining, for marking with count values a sequence number of the at least one packet data unit with the count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values communicated after the indication of the cell change. A further example embodiment is an apparatus comprising the apparatus of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network device comprises the user equipment for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or downlink count value and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein there is means for communicating information of the marked sequence number of the at least one packet data unit with the source cell; wherein the synchronization with the target cell is using information comprising the marked count values with an offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the source cell after indication of the cell change; wherein the information is communicated using a radio resource control reconfiguration complete message; wherein the information communicated using the radio resource control reconfiguration complete message comprises offsets for the marked at least one of an uplink count value or a downlink count value representing a number of the at least one packet data unit that was communicated between the network device and the source cell after the indication of the cell change is received; wherein there is means for receiving from the source cell information comprising a handover command and an indication of an uplink offset value or a downlink offset value, wherein the uplink offset value or the downlink offset value are used by the user equipment to determine uplink count values and downlink count values for synchronization with the target cell; wherein the information is communicated using a radio resource control reconfiguration message; and wherein there is means for receiving from the target cell for the synchronization a first unacknowledged downlink packet data control protocol service data unit using a first sequence number that was not applied by the source cell.

A non-transitory computer-readable medium storing program code, the program code executed by at least one processor to perform at least the method as described in the paragraphs above.

In an example aspect of the invention, there is an apparatus, such as a user side apparatus, comprising: at least one processor; and at least one memory including computer program code, where the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to at least: determine, by a network device of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and based on the determining, mark with count values a sequence number of the at least one packet data unit with the count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values communicated after the indication of the cell change.

A further example embodiment is an apparatus comprising the apparatus of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network device comprises the user equipment for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or downlink count value and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to communicate information of the marked sequence number of the at least one packet data unit with the source cell; wherein the synchronization with the target cell is using information comprising the marked count values with an offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the source cell after indication of the cell change; wherein the information is communicated using a radio resource control reconfiguration complete message; wherein the information communicated using the radio resource control reconfiguration complete message comprises offsets for the marked at least one of an uplink count value or a downlink count value representing a number of the at least one packet data unit that was communicated between the network device and the source cell after the indication of the cell change is received; wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to receive from the source cell information comprising a handover command and an indication of an uplink offset value or a downlink offset value, wherein the uplink offset value or the downlink offset value are used by the user equipment to determine uplink count values and downlink count values for synchronization with the target cell; wherein the information is communicated using a radio resource control reconfiguration message; and wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to receive from the target cell for the synchronization a first unacknowledged downlink packet data control protocol service data unit using a first sequence number that was not applied by the source cell. In a further example aspect of the invention, there is a method comprising: determining, by a network node of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and based on the determining, marking with count values a sequence number of the at least one packet data unit with the marked count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values that is communicated after the indication of the cell change.

A further example embodiment is a method comprising the method of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network node comprises one of a source cell network node or a target cell network node for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or a downlink count value, and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein there is communicating information of the marked sequence number of the at least one packet data unit with the user equipment; wherein the synchronization with the target cell is using information comprising the marked count values with the offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the user equipment after indication of the cell change; wherein there is communicating between the source cell and the target cell information of the marked at least one of an uplink count value or a downlink count value; wherein there is communicating between the source cell and the target cell information of the at least one of the uplink count value or the downlink count value increased by the offset value; wherein the synchronization with the target cell comprises the target cell adds the offset value to the at least one of the marked at least one of an uplink count value or a downlink count value received from the source cell; wherein the communication of information of the marked count value associated with the sequence number of the at least one packet data unit with the user equipment comprises a sequence number status transfer using an indication flag identifying a status for the cell change; and wherein the information of the sequence number of the at least one packet data unit comprises a sequence number status transfer which causes the target cell to use count values received from the source cell which add offset values to the marked count values for synchronization with the user equipment. A non-transitory computer-readable medium storing program code, the program code executed by at least one processor to perform at least the method as described in the paragraphs above.

In another example aspect of the invention, there is an apparatus, such as a network side apparatus, comprising: means for determining, by a network node of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and means, based on the determining, for marking with count values a sequence number of the at least one packet data unit with the marked count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values that is communicated after the indication of the cell change.

A further example embodiment is an apparatus comprising the apparatus of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network node comprises one of a source cell network node or a target cell network node for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or a downlink count value, and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein there is means for communicating information of the marked sequence number of the at least one packet data unit with the user equipment; wherein the synchronization with the target cell is using information comprising the marked count values with the offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the user equipment after indication of the cell change; wherein there is means for communicating between the source cell and the target cell information of the marked at least one of an uplink count value or a downlink count value; wherein there is means for communicating between the source cell and the target cell information of the at least one of the uplink count value or the downlink count value increased by the offset value; wherein the synchronization with the target cell comprises the target cell adds the offset value to the at least one of the marked at least one of an uplink count value or a downlink count value received from the source cell; wherein the communication of information of the marked count value associated with the sequence number of the at least one packet data unit with the user equipment comprises a sequence number status transfer using an indication flag identifying a status for the cell change; and wherein the information of the sequence number of the at least one packet data unit comprises a sequence number status transfer which causes the target cell to use count values received from the source cell which add offset values to the marked count values for synchronization with the user equipment.

In an example aspect of the invention, there is an apparatus, such as a network side apparatus, comprising: at least one processor; and at least one memory including computer program code, where the at least one memory and the computer program code are configured, with the at least one processor, to cause the apparatus to at least: determine, by a network node of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and based on the determining, mark with count values a sequence number of the at least one packet data unit with the marked count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values that is communicated after the indication of the cell change.

A further example embodiment is an apparatus comprising the apparatus of the previous paragraph, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit; wherein the network node comprises one of a source cell network node or a target cell network node for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or a downlink count value, and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit; wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to communicate information of the marked sequence number of the at least one packet data unit with the user equipment; wherein the synchronization with the target cell is using information comprising the marked count values with the offset value added; wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the user equipment after indication of the cell change; wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to communicate between the source cell and the target cell information of the marked at least one of an uplink count value or a downlink count value; wherein the at least one memory including the computer program code is configured with the at least one processor to cause the apparatus to communicate between the source cell and the target cell information of the at least one of the uplink count value or the downlink count value increased by the offset value; wherein the synchronization with the target cell comprises the target cell adds the offset value to the at least one of the marked at least one of an uplink count value or a downlink count value received from the source cell; wherein the communication of information of the marked count value associated with the sequence number of the at least one packet data unit with the user equipment comprises a sequence number status transfer using an indication flag identifying a status for the cell change; and wherein the information of the sequence number of the at least one packet data unit comprises a sequence number status transfer which causes the target cell to use count values received from the source cell which add offset values to the marked count values for synchronization with the user equipment.

A communication system comprising the network side apparatus and the user equipment side apparatus performing operations as described above.

BRIEF DESCRIPTION OF THE DRAWINGS:

The foregoing and other aspects of embodiments of this invention are made more evident in the following Detailed Description, when read in conjunction with the attached Drawing Figures, wherein:

FIG. 1 shows Make-before-break handover solution where the UE maintains the radio communication with source BS for some time after receiving a handover command;

FIG. 2 shows a high level block diagram of various devices used in carrying out various aspects of the invention;

FIG. 3A shows an alternative Method 1 used in place of an alternative Method 2 in accordance with example embodiments of the invention for marking UF and DF PDUs exchanged before offset related counting is started;

FIG. 3B shows an alternative Method 2 used in place of an alternative Method 1 in accordance with example embodiments of the invention for marking UF and DF PDUs which are a last one exchanged before offset related counting is started:

FIG. 4 shows Alternative 1 in accordance with example embodiments of the invention with dynamic offsets X and Y signaled by a UE; and

FIG. 5 shows Alternative 2 in accordance with example embodiments of the invention with fixed offsets for DF and UF COUNT values; FIG. 6 shows a method in accordance with example embodiments of the invention which may be performed by an apparatus; and

FIG. 7A and FIG. 7B each show a method in accordance with example embodiments of the invention which may be performed by an apparatus.

DETAILED DESCRIPTION:

In this invention, there is proposed novel operations related to at least user plane handling to enable almost a 0ms cell change such as for handover.

In 3GPP Rel.14, the first Work Item on minimizing the HO interruption time in LTE, concluded that a solution specified is to keep the communication with the source as long as the radio allows. This specified solution is called make-before-break. This did not eliminate the interruption time completely, as the connection to the source cell was kept only until the UE was expected to execute an initial transmission to the target cell (more details can be found in 3GPP TS 36.331 and 3GPP TS 36.300, by seeking for“make-before-break” and/or“RACH- less”). These Rel-l4 conclusions are the starting point for new standards related work.

In the sequel, the term base station (BS) is used to refer to an enhanced Node B (eNB) in LTE or next generation Node B (gNB) in 5G system. For illustration, FIG.1 depicts the make-before- break solution specified in LTE Rel- 14 where the UE maintains the connection to the source BS after receiving the HO command.

As shown in FIG. 1 there is a UE 110, a Source BS 170, and a Target BS 180. At step 1101 of FIG. 1 the Source BS 170 sends a measurement control to the UE 110. Then packet data 1102 is exchanged. At step 1103 of FIG. 1 the UE provided measurement reports to the Source BS 170. As shown in step 1104 of FIG. 1 there is, based on the measurement reports, the Source BS 170 making a Handover decision. As shown in step 1105 of FIG. 1 a handover request is sent from the Source BS 170 to the Target BS 180. At step 1106 of FIG. 1 Admission Control is performed by the Target BS 180. A handover Request Acknowledgment is sent from the Target BS 180 to the Source BS 170 as shown in step 1107 of FIG. 1. Then a Handover Command is sent from the Source BS 170 to the UE 110 as shown in step 1108 of FIG. 1. As shown in step 1109 of FIG. 1 packet data is exchanged between the UE 110 and the Source BS 170. Then at step 1110 of FIG. 1 the Source BS 170 stops communication with the UE 110. At step 1111 of FIG. 1 the Source BS 170 sends SN Status Transfer message to the Target BS 180. At step 11 12 of FIG. 1 there is synchronization by the UE 110 with the Target BS 180. Then at step 1113 as in FIG. 1 there is a UL allocation and TA provided by the Target BS 180 to the UE 110. At step 1114 of FIG. 1 the Target BS 180 is receiving from the UE 110 an RRC Reconfiguration Complete indication. At step 1115 of FIG. 1 the UE 110 and the Target BS 180 are exchanging packet data. Then at step 1116 of FIG. 1 the Target BS 180 sends a UE Context Release to the Source BS.

In a classic handover (i.e. without Rel-l4 LTE enhancements: make-before-break and RACH- less), the source BS requests the resources from the target BS, which responds with the HO Command, which is then forwarded from the source to the UE. From this moment, the UE disconnects from the source BS and attempts connecting to the target BS. The source BS, right after forwarding the HO Command, stops any communication with the UE over the User Plane, and sends the COUNT Status to the target (c.f. SN Status Transfer in Fig. 1), which is used it to enable encryption of the user plane. Since the UE also knows the COUNT Status at the moment it received HO Command, it can encrypt/decrypt communication with the target BS, while the COUNT Status itself is not exchanged over radio and as such it cannot be intercepted.

The Rel. l4 Make-before-break solution enabled the UE to communicate with the source slightly longer, i.e. beyond the moment the HO Command was received as shown in Fig.l . However, the source BS had to wait with sending the SN Status Transfer to the target until the communications stopped, i.e., it was left for network implementation when the source BS stops the communication. And the target BS, as explained above, could start communication with the UE only once the COUNT status was received. Therefore, some interruption time of ~5ms has remained [TS 36.133]

Therefore, at least one problem that can be addressed in accordance with example embodiments of the invention includes:

How to provide the COUNT Status before the communication with the source BS stops, so that at the moment the UE switches, the target BS or gNB is ready to start communication (or that the communication with the target BS or gNB starts even before the communication with the source BS or gNB stops, if the UE is equipped with two transmitters) (?) It is noted that a solution to this problem should consider that due to security reasons, the UE shall not provide the“last” COUNT to the target BS: this has to be known at both ends independently.

As mentioned above, in Rel. l4, solutions to minimize interruption time during HO were considered. The selected Make-before-break solution allows the UE to communicate with the source BS longer, beyond the moment the HO Command is received, but still, the COUNT Status is sent only once the communication stops. Release 14 LTE work was predominantly focused on control plane enhancements and did not specify any new approaches to SN Status reporting.

Also, in R2- 1802473 and R2- 1708028 the problem is addressed. The proposed solution, however, relies on quasi-DC approach, where the source BS sends ready PDCP PDUs to the target, where they are only ciphered. However, the papers do not discuss the issue how to start the ciphering at the target BS side, i.e. when to provide the COUNT status and how to synchronise it with the UE.

One key aspect to solve the aforementioned problem in accordance with example embodiments of the invention is to enable the UE to use PDCP Sequence Numbers (SNs) after the COUNT Status is sent from the source BS to the target BS. Effectively, this means certain offset of the numbering must be created and synchronised among the UE, source and target BSs. In accordance with the example embodiments of the invention this may be achieved in any one of two alternative ways or solutions:

1) Dynamic offset: The source BS provides the target BS with the COUNT status at the moment the HO Command was sent. The UE stores the COUNT values at that moment, too, and when it is ready to switch user plane to the target BS, it provides the target BS with the offset (delta) over the stored COUNT status. The target BS adds this offset to the received COUNT values and thus is ready to start communication; or

2) Fixed offset: The source BS anticipates the amount of data that it will be able to exchange with the UE before it switches to the target BS. The source BS sends the HO Command together with this anticipated value - the UE knows then up to what SN number it may keep communication with the source. The source BS sends the COUNT status to the target BS already increased by that anticipated offset.

Fixed offset solution 2 has some disadvantages, namely it relies on the“intelligence” of the source BS to anticipate correctly the offset. If the offset is too small, the UE has to break the communication before it is ready to switch to the target BS and the interruption time may still occur; if the offset is too big, there will be a gap in the SN numbering that will slow down data processing at the PDCP layer.

The enablers for the offset solution, in either of the two variants, include:

• The ability to mark the UL and DL PDUs which are the last one exchanged before the offset related counting is started; and

• Forwarding the DL data to the target BS simultaneously with sending it to the UE. The forwarded PDCP SDUs shall be numbered in the same way as the PDUs sent to the UE. Also, the information on the delivered PDUs shall be forwarded to the target BS, so that it can clear delivered data from its DL buffer to avoid duplicating DL data. If duplicating application data is acceptable, the UE may continue exchanging data with the source BS even after sending the RRC Reconfiguration Complete, either using remaining pool of SN numbers that are higher than those increased by the pre-allocated offset in the Fixed offset solution 2, or in a separate PDCP entity in Dynamic offset solution 1 as stated above.

In a classic (or enhanced in Rel.l4) HO, the source BS and the UE know when the radio communication between them stops and therefore they both exactly know the corresponding COUNT. When the COUNT status is to be stored at the same time at the source BS and the UE, while the communication continues in uninterrupted manner, the first step is to mark the last PDU, separately in UL and DL, after which the offset related counting starts. For instance, this may be done by the UE for the UL COUNT and by the source BS for the DL COUNT: once the UE receives the HO command it waits for the delivery confirmation of the UL PDCP PDU and stores the SN number of the first one received. The source BS does the same for DL PDCP PDUs. The UE and the source BS exchange the“marked” SN numbers. This method is referred to as Method 1. Alternatively, in Method 2 the UE and the source BS start marking all the PDUs sent after the HO is sent/received: a new flag, e.g.“post-HO PDU”, is then added to the PDUs exchanged with the source BS after the HO Command is received. The source BS and the UE consider as the starting point for counting the first marked DL/UL PDU that has been delivered.

Example embodiments of the invention at least enable, in response to an indication of a handover, exchanging data with a source cell even after COUNT is sent to the target cell. To achieve these example embodiments of the invention either a dynamic or fixed offset is used with respect to the COUNT value, stored at the moment HO command was sent. Thus, the essence should be to protect those relative values/offsets and their use with respect to legacy COUNT (with the aim to enable longer packet exchange, longer than just until HO command is received).

Before describing the example embodiments of the invention in further detail, reference is made to FIG. 2 for illustrating a simplified block diagram of various electronic devices that are suitable for use in practicing the example embodiments of this invention. FIG. 2 shows a block diagram of one possible and non-limiting exemplary system in which the example embodiments of the invention may be practiced. In FIG. 2, a user equipment or mobile station (UE) 110 is in wireless communication with a wireless network 100. A UE is a wireless, typically mobile device that can access a wireless network. The UE 110 includes one or more processors 120, one or more memories 125, and one or more transceivers 130 interconnected through one or more buses 127. Each of the one or more transceivers 130 includes a receiver Rx, 132 and a transmitter Tx 133. The one or more buses 127 may be address, data, or control buses, and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit, fiber optics or other optical communication equipment, and the like. The one or more transceivers 130 are connected to one or more antennas 128. The one or more transceivers 130 have multi-connectivity configurations and communicate over the wireless network 100 or any other network. The one or more memories 125 include computer program code 123 executed by the one or more processors 120. The one or more processors 120 may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. For instance, the one or more memories 125 and the computer program code 123 may be configured, with the one or more processors 120, to cause the user equipment 110 to perform one or more of the operations as described herein. The UE 110 communicates with gNB 170 and the gNB 180 via a wireless link 111. The gNB 170 (NR/5G Node B or possibly an evolved NB) is a base station (e.g., for NR or LTE long term evolution) that communicates with devices such as gNB 180 and UE 110 of FIG. 2. The gNB 170 provides access to wireless devices such as the UE 110 to the wireless network 100. The gNB 170 includes one or more processors 152, one or more memories 155, one or more network interfaces (N/W I/F(s)) 161, and one or more transceivers 160 interconnected through one or more buses 157. Each of the one or more transceivers 160 includes a receiver Rx 162 and a transmitter Tx 163. The one or more transceivers 160 are connected to one or more antennas 158. The one or more memories 155 include computer program code 153 executed by the one or more processors 152. The one or more processors 152 may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. The one or more memories 155 and the computer program code 153 are configured to cause, with the one or more processors 152, the gNB 170 to perform one or more of the operations as described herein. The one or more network interfaces 161 and 191 and the one or more transceivers 160 that have multi-connectivity configurations and communicate over the wireless network 100 or any other network. Such communication can be between the gNB 170, the gNB 180, and the UE 110 via the links 176 and 111. In addition, two or more gNB 170 may communicate with another gNB or eNB using, e.g., links 176. The links 176 may be wired or wireless or both and may implement, e.g., an Xn interface. Further the links 176 may be through other network devices such as, but not limited to an NCE/AMF/UPF device such as the NCE/AMF/UPF 198 of FIG. 2.

The gNB 180 (NR/5G Node B or possibly an evolved NB) is a base station (e.g., for NR or LTE long term evolution) that communicates with devices such as the gNB 170 and/or UE 110 and/or the wireless network 100. The gNB 180 includes one or more processors 182, one or more memories 195, one or more network interfaces (N/W I/F(s)) 191, and one or more transceivers 190 interconnected through one or more buses 187. Each of the one or more transceivers 190 includes a receiver Rx 192 and a transmitter Tx 183. The one or more transceivers 190 are connected to one or more antennas 185. The one or more transceivers 190 have multi- connectivity configurations and communicate over the wireless network 100 or any other network. The one or more memories 195 include computer program code 193 executed by the one or more processors 182. The one or more processors 182 may be implemented also as an integrated circuit or through other hardware such as a programmable gate array. The one or more memories 155 and the computer program code 153 are configured to cause, with the one or more processors 182, the gNB 180 to perform one or more of the operations as described herein. The one or more network interfaces 181 communicate over a network such as via the links 176. Two or more gNB 170 or gNB 180 may communicate with another gNB and/or eNB or any other device using, e.g., links 176. The links 176 maybe wired or wireless or both and may implement, e.g., anXn interface. Further, as stated above the links 176 may be through other network devices such as, but not limited to an NCE/AMF/UPF device such as the NCE/AMF/UPF 198 of FIG. 2.

The one or more buses 157 and 187 may be address, data, or control buses, and may include any interconnection mechanism, such as a series of lines on a motherboard or integrated circuit, fiber optics or other optical communication equipment, wireless channels, and the like. For example, the one or more transceivers 160 and/or 190 may be implemented as a remote radio head (RRH) 203 and/or 205, with the other elements of the gNB 170 being physically in a different location from the RRH, and the one or more buses 157 could be implemented in part as fiber optic cable to connect the other elements of the gNB 170 to a RRH.

It is noted that description herein indicates that“cells” perform functions, but it should be clear that the gNB that forms the cell will perform the functions. The cell makes up part of a gNB. That is, there can be multiple cells per gNB.

The wireless network 100 may include a network control element (NCE), an access management function (AMF)/ user plane function (UPF) such as the NCE/AMF/UPF 198 that may include AMF (Access Management Function)/UPF (User Plane Function) functionality, and which provides connectivity with a further network, such as a telephone network and/or a data communications network (e.g., the Internet). The gNB 170 is coupled via a link 131 to the NCE/AMF/UPF 198. The gNB 180 is coupled via a link 200 to the NCE/AMF/UPF 198. Further, the gNB 180 is coupled via links 176 to the gNB 170. The links 131, 176, and/or 200 may be implemented as, e.g., an NG2 or NG3 interface.

The NCE/AMF/UPF 198 includes one or more processors 175, one or more memories 171, and one or more network interfaces (N/W I/F(s)) 197, interconnected through one or more buses coupled with the link 185. The one or more memories 171 include computer program code 173. The one or more memories 171 and the computer program code 173 are configured to, with the one or more processors 175, cause the NCE/AMF/UPF 198 to perform one or more operations which may be needed to support the operations in accordance with the example embodiments of the invention. The wireless network 100 may implement network virtualization, which is the process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization involves platform virtualization, often combined with resource virtualization. Network virtualization is categorized as either external, combining many networks, or parts of networks, into a virtual unit, or internal, providing network- like functionality to software containers on a single system. Note that the virtualized entities that result from the network virtualization are still implemented, at some level, using hardware such as processors 152, 182, or 175 and memories 155, 195, and 171, and also such virtualized entities create technical effects.

The computer readable memories 125, 155, 171, and 195 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The computer readable memories 125, 155, 171, and 195 may be means for performing storage functions. The processors 125, 155, 171, and 195 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on a multi-core processor architecture, as non-limiting examples. The processors 120, 152, 175, and 182 may be means for performing functions, such as controlling the UE 110, gNB 170, gNB 180, and other functions as described herein.

Example embodiments of the invention can work to improve at least handover operation with novel operations which can use Packet Data Convergence Protocol (PDCP) sequence numbers. The PDCP supports the following functions: transfer of data (user plane or control plane);

maintenance of PDCP SNs;

header compression and decompression using the ROHC protocol;

ciphering and deciphering;

integrity protection and integrity verification;

timer based SDU discard;

routing for split bearers;

duplication;

reordering and in-order delivery; out-of-order delivery; and

duplicate discarding.

Two methods in accordance with example embodiments of the invention are shown in FIG. 3A and FIG. 3B. FIG. 3A shows a Method 1 and an alternative Method 2 in accordance with example embodiments of the invention for marking UL and DL PDUs which are a last one exchanged before offset related counting is started. As shown in FIG. 3A the alternative Method 2 is not used, and as such is crossed out.

As shown in FIG. 3 A there is a UE 110, a Source BS 170, and a Target BS 180. At step 3001 of FIG. 3 A the Source BS 170 sends a measurement control to the UE 110. Then packet data 3002 is exchanged. At step 3004 of FIG. 3A the UE provided measurement reports to the Source BS 170. As shown in step 3005 of FIG. 3A there is, based on the measurement reports, the Source BS 170 making a Handover decision. As shown in step 3006 of FIG. 3A a handover request is sent from the Source BS 170 to the Target BS 180. At step 3007 of FIG. 3A Admission Control is performed by the Target BS 180. A handover Request Acknowledgment is sent from the Target BS 180 to the Source BS 170 as shown in step 3008 of FIG. 3A. Then a Handover Command is sent from the Source BS 170 to the UE 110 as shown in step 3009 of FIG. 3 A. As shown in step 3010 of FIG. 3 A packet data is exchanged between the UE 110 and the Source BS 170. At this stage there is a Method 1 option implemented in alternative to a Method 2 option in accordance with example embodiments of the invention.

In Method 1 3011 of FIG. 3 A in accordance with example embodiments of the invention there is at step A of Method 1 the UE 110 waits for the ACK of UE PDCP PDUs and store the SN of the first one received. At this stage the Source BS 170 also waits for the ACK of DF PDCP PDUs and store the SN of the first one received as shown in step B of Method 1. Then as shown in step C of Method 1 the UE 110 sends to the Source BS 170 the marked SN for the UE and at step D of Method 1 the Source BS 170 send the UE 110 the marked SN for DL.

Following the Method 1 3011 of FIG. 3 A there is at step 3013 synchronization of the UE 110 with the Target BS 180. At step 3014 of FIG. 3A there is UL Allocation and TA for the UE 110. At step 3015 of FIG. 3A there is an RRC Reconfiguration Complete message communicated between the UE 110 and the Target BS 180. After this packet data is exchanged between the UE 110 and the Target BS 180 at step 3016 of FIG. 3A. Then at step 3017 of FIG. 3 A there is a UE Context Release sent from the target BS 180 to the Source BS 170. FIG. 3B shows a Method 2 and an alternative Method 1 in accordance with example embodiments of the invention for marking UL and DL PDUs which are a last one exchanged before offset related counting is started. As shown in FIG. 3B the alternative Method 1 is not used, and as such is crossed out.

As shown in FIG. 3B there is a UE 110, a Source BS 170, and a Target BS 180. At step 3001 of FIG. 3B the Source BS 170 sends a measurement control to the UE 110. Then packet data 3002 is exchanged. At step 3004 of FIG. 3B the UE provided measurement reports to the Source BS 170. As shown in step 3005 of FIG. 3B there is, based on the measurement reports, the Source BS 170 making a Handover decision. As shown in step 3006 of FIG. 3A a handover request is sent from the Source BS 170 to the Target BS 180. At step 3007 of FIG. 3B Admission Control is performed by the Target BS 180. A handover Request Acknowledgment is sent from the Target BS 180 to the Source BS 170 as shown in step 3008 of FIG. 3B. Then a Handover Command is sent from the Source BS 170 to the UE 110 as shown in step 3009 of FIG. 3 A. As shown in step 3010 of FIG. 3 A packet data is exchanged between the UE 110 and the Source BS 170. At this stage there is a Method 2 option implemented in alternative to the Method 1 option in accordance with example embodiments of the invention.

In Method 2 3012 of FIG. 3B in accordance with example embodiments of the invention there is at step A of Method 2 the UE 110 marks UE PDUs with“post-HO PDU.” At step B of Method 2 of FIG. 3B there is storing the SN of the first marked DF PDCP PDU. As shown in step C of Method 2 of FIG. 3B there is the Source BS 170 marking DF PDCP PDUs with“post- HO PDU.” Further, in step D of Method 2 as in FIG. 3B there is storing the SN of the first marked UF PDCP PDU.

Then following the Method 2 3012 of FIG. 3B there is at step 3013 synchronization of the UE 110 with the Target BS 180. At step 3014 of FIG. 3B there is UF Allocation and TA for the UE 110. At step 3015 of FIG. 3B there is an RRC Reconfiguration Complete message communicated between the UE 110 and the Target BS 180. After this packet data is exchanged between the UE 110 and the Target BS 180 at step 3016 of FIG. 3B. Then at step 3017 of FIG. 3B there is a UE Context Release sent from the target BS 180 to the Source BS 170.

Other steps following these operations as in FIG. 3A and FIG. 3B can depend on the which of the following two alternatives is applied: Alternative 1 : Dynamic Offset

• The source BS sends the SN STATUS TRANSFER to the target BS with the stored DL/UL COUNT values, i.e., COUNT = Hyper Frame Number (HFN) + SN. The message may be enhanced to contain a flag indicating the status is for an enhanced handover;

• The UE starts counting the UL and DL PDCP PDUs and when it is ready to send the RRC Reconfiguration Complete message to the target BS, it may stop communication with the source. The RRC message must be enhanced to include the SN offsets X & Y for DL and UL in the RRC message, respectively, i.e., X and Y represent the number of DL/UL PDCP PDUs that were received/transmitted by the UE from source BS after receiving the HO command until the transmission of RRC Reconfiguration Complete message; and

• The target BS adds the offsets to the UL and DL COUNT values received before and initiates communication, starting from the first forwarded DL PDCP SDU that has not been acknowledged yet.

An illustration of the first alternative or Alternative 1 is shown in FIG. 4.

As shown in FIG. 4 there is a UE 110, a Source BS 170, and a Target BS 180. At step 4001 of FIG. 4 the Source BS 170 sends a measurement control to the UE 110. Then packet data 4002 is exchanged. At step 4003 of FIG. 4 the UE provided measurement reports to the Source BS 170. As shown in step 4005 of FIG. 4 there is, based on the measurement reports, the Source BS 170 making a Handover decision. As shown in step 4006 of FIG. 4 a handover request is sent from the Source BS 170 to the Target BS 180. At step 4007 of FIG. 4 Admission Control is performed by the Target BS 180. A handover Request Acknowledgment is sent from the Target BS 180 to the Source BS 170 as shown in step 4008 of FIG. 4. Then a Handover Command is sent from the Source BS 170 to the UE 1 10 as shown in step 4009 of FIG. 4. As shown in step 4010 of FIG. 4 packet data is exchanged between the UE 110 and the Source BS 170. At this stage there is implementation of the Method 1 or the Method 2 as shown in FIG. 3 A and 3B as described above. Then there performed is an Alternative 1 operation 4014 in accordance with the example embodiments of the invention. This Alternative 1 operation 4014 is one of two alternative operations which may be performed in accordance with the Examiner embodiments of the invention. Another alternative operation in accordance with example embodiments of the invention is labeled Alternative 2 and is described below with regards to FIG. 5.

As shown in FIG. 4 the Alternative 1 operation 4014 includes a step A where SN Status transfer operation from the Source BS 170 to the Target BS 180 includes a flag =“enhanced HO” and stored DF/UF COUNT values. Then shown in step B of Alternative 1 operation 4014 of FIG. 4 there is synchronization of the UE 110 with the Target BS 180. In step C of Alternative 1 operation 4014 of FIG. 4 there is from the UE 110 to the Target BS 180 an RRC Reconfiguration Complete message including offsets X & Y for DF and UF COUNT values. Then as shown in step E of Alternative 1 operation 4014 of FIG. 4 there is deriving by the Target BS 180 COUNT values by adding offsets to already received COUNT values from the source BS 170.

Following the Alternative 1 operation 4014 of FIG. 4 there is at step 4016 packet data communicated between the UE 110 and the Source BS 180. Then as shown in step 4017 of FIG. 4 there is a UE context release sent by the Target BS 180 to the Source BS 170.

Alternative 2: Fixed offset

• The source estimates the offset, e.g. based on the data rate, separately for UF and DF, and provides these values to the UE together with the HO command. The RRC Reconfiguration (containing handover command) must be enhanced to carry the offset information;

• The source BS sends the SN STATUS TRANSFER to the target BS including the COUNT values increased by the offset sizes. There is no need to add any new flag, since the target handles this information as in the classic HO scenario; and

• Once the marking of the last UF and DF PDUs is executed, the UE exchanges the data with the source as long as the max value of SN indicated from the source BS is not reached. Further, in accordance with example embodiments of the invention this Fixed offset solution can include that the UE sends the last transmitted /received PDCP PDU in terms of its offset from a Fixed offset in an RRC-Reconfiguration-eomplete. This would enable the Target BS to determine where to start the new numbers without Gap. In addition, the BS can use this to determine how to set the fixed offset values better in future cases.

When the UE switches to the target and sends there the RRC Reconfiguration Complete, the target sends the first unacknowledged DL PDCP SDU using the first SN number indicated before in the SN STATUS TRANSFER. Similarly, the UE uses the DL/UL COUNT values stored after HO command for the communication with the target BS. For clarity, alternative with fixed offsets for DL/UL COUNT values is shown in FIG. 5.

As shown in FIG. 5 there is a UE 110, a Source BS 170, and a Target BS 180. At step 5001 of FIG. 5 the Source BS 170 sends a measurement control to the UE 110. Then packet data 5002 is exchanged. At step 5003 of FIG. 5 the UE provided measurement reports to the Source BS 170. As shown in step 5005 of FIG. 5 there is, based on the measurement reports, the Source BS 170 making a Handover decision. As shown in step 5006 of FIG. 5 a handover request is sent from the Source BS 170 to the Target BS 180. At step 5007 of FIG. 5 Admission Control is performed by the Target BS 180. A handover Request Acknowledgment is sent from the Target BS 180 to the Source BS 170 as shown in step 5008 of FIG. 5. Then as shown in FIG. 5 Alternative 2 5014 is performed.

As shown in Alternative 2 5014 of FIG. 5 there is a step A where the Source BS 170 estimates the SN offsets X and Y for DL and UL. At step B of Alternative 2 5014 of FIG. 5 a Handover Command including SN offsets X and Y is sent from the Source BS 170 to the UE 110. In step C of Alternative 2 5014 of FIG. 5 packet data is exchanged between the Source BS 170 and the UE 110. In step D of Alternative 2 5014 of FIG. 5 the UE 110 stores the DL/UL COUNT values increased by offsets X and Y. Then as shown in Alternative 2 5014 of FIG. 5 there is performing either the Method 1 5011 or the Method 2 5012 in accordance with example embodiments of the invention as described herein. The Method 1 5011 or the Method 2 5012 may be executed before step D of Alternative 2 5014 of FIG. 5.

Then in Alternative 2 5014 of FIG. 5 at Step E there is Exchanging data by the UE 110 with the source BS 170 as long as COUNT values are not reached. At step F of Alternative 2 5014 of FIG. 5 there is an SN Status Transfer between the Source BS 170 and the Target BS 180. At step G of Alternative 2 5014 there is Synchronization performed by the UE 110 with the Target BS 180. At step H of Alternative 2 5014 of FIG. 5 there is UL Allocation and TA communicated with UE 110. At step I of Alternative 2 5014 of FIG. 5 an RRC Reconfiguration Complete message is communicated between the UE 110 and the Target BS 180. In step J of Alternative 2 5014 there is using by the UE 110 DL/UL COUNT values stored after HO command for communication with Target BS 180. Then at step K of Alternative 2 5014 of FIG. 5 there is using by the Target BS 180 the DL/UL COUNT values received from the source BS 170 for communication with the UE 110.

Following the Alternative 2 operation 5014 of FIG. 5 there is at step 5016 packet data communicated between the UE 110 and the Target BS 180. Then as shown in step 5017 of FIG. 5 there is a UE context release sent by the Target BS 180 to the Source BS 170.

The User Plane protocol for Xn may remain as it is specified now, but the forwarding must be done using the numbered frames. This protocol may also be used to provide the information about delivered PDUs (as specified for PDCP duplication in dual connectivity).

FIG. 6 shows a method in accordance with example embodiments of the invention which may be performed by an apparatus. FIG.6 illustrates operations which may be performed by a device such as, but not limited to, a device such as the gNB 170 and/or gNB 180 and/or UE 110 as in FIG. 2. As shown in step 610 of FIG. 6 there is receiving, by a network device of a communication network, an indication of a cell change of a user equipment from a source cell to a target cell of the communication network. Then as shown in step 620 of FIG. 6 there is, based on the indication of the cell change, determining a count value associated with a start of communication with the user equipment for the cell change, wherein the count value is determined using an offset value based on at least one packet data unit communicated by the source cell with the user equipment after the indication of the cell change is received.

In accordance with the example embodiments as described in the paragraph above, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit, and wherein the offset value is determined using a sequence number of a last packet data unit at a stop of the at least one packet data convergence protocol packet data unit communicated from the source cell to the user equipment. In accordance with the example embodiments as described in the paragraphs above, wherein the network device comprises a target network device for the cell change, and wherein the count value is a downlink count value, the method comprising applying the offset value to the last packet data unit to determine the count value for the start of the communication with the user equipment for the cell change.

In accordance with the example embodiments as described in the paragraphs above, wherein the at least one packet data convergence protocol packet data unit is communicated from the source cell to the target cell after the indication of the cell change, and wherein each of the at least one packet data convergence protocol packet data unit communicated from the source cell to the target cell after the cell change comprises a post handover indication flag for use by the target cell to determine the offset value to apply to the count value.

In accordance with the example embodiments as described in the paragraphs above, wherein the offset value is communicated between the source cell and the target cell in a radio resource control reconfiguration message.

In accordance with the example embodiments as described in the paragraphs above, wherein the radio resource control reconfiguration message comprises sequence number offset values for use with each of an uplink and a downlink communication with the user equipment.

In accordance with the example embodiments as described in the paragraphs above, there is determining a sequence number associated with a last acknowledged delivery confirmation of the at least one of an uplink and downlink packet data convergence protocol packet data unit communicated after the indication of the cell change is received; and communicating with the target cell a message comprising a sequence number status transfer indicating the sequence number and a Hyper Frame number for use by the target cell to determine the offset value.

In accordance with the example embodiments as described in the paragraphs above, wherein the target cell uses a first unacknowledged packet data unit sequence number indicated before the sequence number status transfer for the start of the communication with the user equipment for the cell change.

In accordance with the example embodiments as described in the paragraphs above, wherein the message comprises a status flag for indicating the cell change is an enhanced handover. In accordance with the example embodiments as described in the paragraphs above, wherein the count value is for use to enable ciphering of exchanged data after the cell change.

A non-transitory computer-readable medium (Memory(ies) 155 and/or Memory(ies) 195 and/or Memory(ies) 125 as in FIG. 2) storing program code (Computer Program Code 153 and/or Computer Program Code 193 and/or Computer Program Code 123 as in FIG. 2), the program code executed by at least one processor (Processor(s) 152 and/or Processors 182 and/or Processor(s) 120 as in FIG. 2) to perform the operations as at least described in the paragraphs above.

In accordance with an example embodiment of the invention as described above there is an apparatus comprising: means for receiving (remote radio head (RRH) 203 and/or 205 and/or transceivers 130; Memory(ies) 155 and/or Memory(ies) 195 and/or Memory(ies) 125; Computer Program Code 133 and/or Computer Program Code 193 and/or Computer Program Code 123; and Processor(s) 152 and/or Processors 182 and/or Processor(s) 120 as in FIG. 2), by a network device )gNB 180, gNB 170, and/or UE 110 as in FIG. 2) of a communication network (network 100 as in FUG. 2), an indication of a cell change of a user equipment (UE 110 as in FIG. 2) from a source cell to a target cell of the communication network. Then means based on the indication of the cell change, for determining (Memory(ies) 155 and/or Memory(ies) 195 and/or Memory(ies) 125 as in FIG. 2; Computer Program Code 133 and/or Computer Program Code 193 and/or Computer Program Code 123; and Processor(s) 152 and/or Processors 182 and/or Processor(s) 120 as in FIG. 2) a count value associated with a start of communication with the user equipment for the cell change, wherein the count value is determined using an offset value based on at least one packet data unit communicated by the source cell with the user equipment after the indication of the cell change is received.

In the example aspect of the invention according to the paragraphs above, wherein at least the means for communicating, selecting, associating, and indicating comprises a non-transitory computer readable medium [Memory(ies) 155 and/or Memory(ies) 195 and/or Memory(ies) 125 as in FIG. 2] encoded with a computer program [Computer Program Code 153 and/or Computer Program Code 193 and/or Computer Program 123 as in FIG. 2] executable by at least one processor [Processor(s) 152 and/or Processors 182 and/or Processor(s) 120 as in FIG. 2] · FIG. 7A and FIG. 7B each show a method in accordance with example embodiments of the invention which may be performed by an apparatus.

FIG. 7A illustrates operations which may be performed by a device such as, but not limited to, a device (e.g., the UE 110 as in Fig. 2). As shown in step 710 of FIG. 7A there is determining, by a network device of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change. Then as shown in step 720 of FIG. 7A there is, based on the determining, marking with count values a sequence number of the at least one packet data unit with the count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values communicated after the indication of the cell change.

In accordance with the example embodiments as described in the paragraph above, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit.

In accordance with the example embodiments as described in the paragraphs above, wherein the network device comprises the user equipment for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or downlink count value and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit.

In accordance with the example embodiments as described in the paragraphs above, there is communicating information of the marked sequence number of the at least one packet data unit with the source cell.

In accordance with the example embodiments as described in the paragraphs above, wherein the synchronization with the target cell is using information comprising the marked count values with an offset value added. In accordance with the example embodiments as described in the paragraphs above, wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the source cell after indication of the cell change.

In accordance with the example embodiments as described in the paragraphs above, wherein the information is communicated using a radio resource control reconfiguration complete message.

In accordance with the example embodiments as described in the paragraphs above, wherein the information communicated using the radio resource control reconfiguration complete message comprises offsets for the marked at least one of an uplink count value or a downlink count value representing a number of the at least one packet data unit that was communicated between the network device and the source cell after the indication of the cell change is received.

In accordance with the example embodiments as described in the paragraphs above, there is receiving from the source cell information comprising a handover command and an indication of an uplink offset value or a downlink offset value, wherein the uplink offset value or the downlink offset value are used by the user equipment to determine uplink count values and downlink count values for synchronization with the target cell.

In accordance with the example embodiments as described in the paragraphs above, wherein the information is communicated using a radio resource control reconfiguration message.

In accordance with the example embodiments as described in the paragraphs above, there is receiving from the target cell for the synchronization a first unacknowledged downlink packet data control protocol service data unit using a first sequence number that was not applied by the source cell.

A non-transitory computer-readable medium (Memory(ies) 125 as in FIG. 2) storing program code (Computer Program Code 123 as in FIG. 2), the program code executed by at least one processor (Processor(s) 120 as in FIG. 2) to perform the operations as at least described in the paragraphs above. In accordance with an example embodiment of the invention as described above there is an apparatus comprising: means for determining (one or more transceivers 130, Memory(ies) 125, Computer Program Code 123, and Processor(s) 120 as in FIG. 2), by a network device (UE 110 as in FIG. 2) of a communication network (Network 100 as in FIG. 2), based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change; and means, based on the determining, for marking (one or more transceivers 130, Memory(ies) 125, Computer Program Code 123, and Processor(s) 120 as in FIG. 2) with count values a sequence number of the at least one packet data unit with the count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values communicated after the indication of the cell change.

In the example aspect of the invention according to the paragraph above, wherein at least the means for determining, and marking comprises a non-transitory computer readable medium [Memory(ies) 125 as in FIG. 2] encoded with a computer program [Computer Program Code 123 as in FIG. 2] executable by at least one processor [Processor(s) 120 as in FIG. 2]

FIG. 7B illustrates operations which may be performed by a device such as, but not limited to, a device (e.g., the gNB 170 as in Fig. 2). As shown in step 750 of FIG. 7B there is determining, by a network node of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change. Then as shown in step 760 of FIG. 7B there is, based on the determining, marking with count values a sequence number of the at least one packet data unit with the marked count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values that is communicated after the indication of the cell change.

In accordance with the example embodiments as described in the paragraph above, wherein the at least one packet data unit comprises at least one packet data convergence protocol packet data unit.

In accordance with the example embodiments as described in the paragraphs above, wherein the network node comprises one of a source cell network node or a target cell network node for the cell change, wherein the marked count values comprise a marked at least one of an uplink count value or a downlink count value, and wherein the marked at least one of an uplink count value or a downlink count value are based on a hyper frame number or a sequence number associated with each of the at least one packet data unit.

In accordance with the example embodiments as described in the paragraphs above, there is communicating information of the marked sequence number of the at least one packet data unit with the user equipment.

In accordance with the example embodiments as described in the paragraphs above, wherein the synchronization with the target cell is using information comprising the marked count values with the offset value added.

In accordance with the example embodiments as described in the paragraphs above, wherein the information comprises an indication flag identifying a status for the cell change added to at least one packet data unit exchanged with the user equipment after indication of the cell change.

In accordance with the example embodiments as described in the paragraphs above, there is communicating between the source cell and the target cell information of the marked at least one of an uplink count value or a downlink count value.

In accordance with the example embodiments as described in the paragraphs above, there is communicating between the source cell and the target cell information of the at least one of the uplink count value or the downlink count value increased by the offset value.

In accordance with the example embodiments as described in the paragraphs above, wherein the synchronization with the target cell comprises the target cell adds the offset value to the at least one of the marked at least one of an uplink count value or a downlink count value received from the source cell.

In accordance with the example embodiments as described in the paragraphs above, wherein the communication of information of the marked count value associated with the sequence number of the at least one packet data unit with the user equipment comprises a sequence number status transfer using an indication flag identifying a status for the cell change. In accordance with the example embodiments as described in the paragraphs above, wherein the information of the sequence number of the at least one packet data unit comprises a sequence number status transfer which causes the target cell to use count values received from the source cell which add offset values to the marked count values for synchronization with the user equipment.

A non-transitory computer-readable medium (Memory(ies) 155 or Memory(ies) 195 as in FIG. 2) storing program code (Computer Program Code 153 or Computer Program Code 193 as in FIG. 2), the program code executed by at least one processor (Processor(s) 152, Processor(s) 182 as in FIG. 2) to perform the operations as at least described in the paragraphs above.

In accordance with an example embodiment of the invention as described above there is an apparatus comprising: means for determining (one or more transceivers 160 or one or more transceivers 190; Memory(ies) 155 or Memory(ies) 195; Computer Program Code 153 or Computer Program Code 193, and Processor(s) 152 or Processor(s) 182 as in FIG. 2), by a network node of a communication network, based on an indication of a cell change of a user equipment to a target cell of the communication network, count values for at least one packet data unit communicated before a communication stop between the source cell and the user equipment for the cell change, and means based on the determining, for marking with count values a sequence number of the at least one packet data unit with the marked count values, wherein synchronization with the target cell for the cell change is using information of an offset value based on the marked count values that is communicated after the indication of the cell change.

In the example aspect of the invention according to the paragraph above, wherein at least the means for determining, and marking comprises a non-transitory computer readable medium [Memory(ies) 155 or Memory(ies) 195 as in FIG. 2] encoded with a computer program [Computer Program Code 153 or Computer Program Code 193 as in FIG. 2] executable by at least one processor [Processor(s) 152 or Processor(s) 182 as in FIG. 2]

In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described in this Detailed Description are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the best method and apparatus presently contemplated by the inventors for carrying out the invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

It should be noted that the terms "connected," "coupled," or any variant thereof, mean any connection or coupling, either direct or indirect, between two or more elements, and may encompass the presence of one or more intermediate elements between two elements that are "connected" or "coupled" together. The coupling or connection between the elements can be physical, logical, or a combination thereof. As employed herein two elements may be considered to be "connected" or "coupled" together by the use of one or more wires, cables and/or printed electrical connections, as well as by the use of electromagnetic energy, such as electromagnetic energy having wavelengths in the radio frequency region, the microwave region and the optical (both visible and invisible) region, as several non-limiting and non- exhaustive examples.

Furthermore, some of the features of the preferred embodiments of this invention could be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the invention, and not in limitation thereof.