Methods for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system and related apparatuses thereof

0001The present invention relates to methods for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system and related apparatuses thereof.

0002A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and a radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).

0003If an RRC connection is disconnected due to radio link failure, an RRC re-establishment procedure needs to be initiated to re-establish the RRC connection. During the RRC re-establishment procedure, a UE resumes a signal radio bearer 1 (SRB1) and configures a lower layer to re-activate security (including integrity protection and ciphering) using the previously configured algorithm immediately when receiving an RRC Connection Re-establishment message from an E-UTRAN. To resume all radio bearers other than the SRB1, the E-UTRAN shall initiate an RRC Connection reconfiguration procedure after the RRC connection is re-established, wherein the RRC Connection reconfiguration procedure is to modify the RRC connection. However, it Is not clearly specified how to resume SRBs and data radio bearers (DRBs) after the RRC Connection re-establishment procedure and the subsequent RRC connection reconfiguration in some scenarios. Hence, a mechanism for synchronizing PDCP operations after RRC connection re-establishment needs to be improved.

0004<patcit id="pcit0001" dnum="WO2005109778A1"><text>WP 2005/109778 A1</text></patcit> was used as a basis for the preamble of the independent claims and discloses a node implementing an RLC (Radio Link Control) entity and being for use in a mobile communications system transmits a sequence of RLC SDUs (SDU = Service Data Unit) towards a peer node. As a result of re-establishment of an RLC entity not all SDUs may have been received. The peer notifies the node which is the next SDU that the peer expects to receive (by transmitting the next SDU number to the node). The node resumes transmission from that SDU onwards. This may or may not lead to re-transmission of SDUs transmitted before the RLC re-establishment. As an alternative, the peer does not notify the node of the next SDU that it expects to receive, but instead the node retransmits any unacknowledged SDUs, together with the SDU number of the first retransmitted SDU. The peer then discards any duplicate SDUs.

0005This in mind, the present invention aims at providing methods for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system and related apparatuses to solve the abovementioned problems.

0006This is achieved by a communication device according to claim 1, and a method according to claim 3. The dependent claim pertains to corresponding further developments and improvements.

0007As will be seen more clearly from the detailed description following below, a method used in an E-UTRAN for synchronizing PDCP operations after a RRC connection re-establishment procedure with a user equipment (UE) includes: initiating an RRC reconfiguration procedure to resume all radio bearers other than a signaling radio bearer 1 (SRB1) when an RRC connection is re-established, and re-transmitting a designated group of PDCP Service Data Units (SDUs) to the UE when a data radio bearer (DRB) mapped on Radio Link Control (RLC) Acknowledged Mode (AM) is resumed; a method used in an UE for synchronizing PDCP operations after an RRC connection re-establishment procedure with an E-UTRAN includes: resuming all radio bearers other than an SRB1 when an RRC connection is re-established, and receiving a designated group of PDCP Service Data Units (SDUs) transmitted from the E-UTRAN when a data radio bearer (DRB) mapped on Radio Link Control (RLC) Acknowledged Mode (AM) is resumed; an E-UTRAN for synchronizing PDCP operations after an RRC connection re-establishment procedure with a UE includes: means for initiating an RRC Reconfiguration procedure to resume all radio bearers other than a SRB1 when an RRC connection is re-established, and means for re--transmitting a designated group of PDCP SDUs to the UE when a DRB mapped on RLC AM is resumed; and a UE for synchronizing PDCP operations after an RRC connection re-establishment procedure with an E-UTRAN includes: means for resuming all radio bearers other than an SRB1 when an RRC connection is re-established, and means for receiving a designated group of PDCP SDUs transmitted from the E-UTRAN when a DRB mapped on RLC AM is resumed.

0008In the following, the invention is further illustrated by way of example, taking reference to the accompanying drawings. Thereof <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG.1</figref> is a diagram showing the architecture of the radio interface protocol of an LTE system according to the prior art;</li><li><figref idref="f0002">FIG.2</figref> is a diagram showing an RRC re-establishment procedure of the LTE system according to the prior art;</li><li><figref idref="f0003">FIG.3</figref> is a block diagram showing a wireless communication system according to an embodiment of the present invention;</li><li><figref idref="f0004">FIG.4</figref> is a diagram illustrating PDCP SDUs received by a receiving PDCP entity;</li><li><figref idref="f0005">FIG.5</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to an exemplary embodiment of the present invention;</li><li><figref idref="f0006">FIG.6</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC Connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention;</li><li><figref idref="f0007">FIG.7</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention;</li><li><figref idref="f0008">FIG.8</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention;</li><li><figref idref="f0009">FIG.9</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention;</li><li><figref idref="f0010">FIG.10</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention; and</li><li><figref idref="f0011">FIG. 11</figref> is a flowchart illustrating a method for synchronizing PDCP operations after RRC connection re-establishment in a wireless communication system according to another exemplary embodiment of the present invention.</li></ul>

0009Please refer to <figref idref="f0001">FIG.1. FIG.1</figref> is a diagram showing the architecture of the radio interface protocol of a LTE system according to the prior art. As shown in <figref idref="f0001">FIG.1</figref>, the radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer.

0010<figref idref="f0002">FIG.2</figref> is a diagram showing an RRC re-establishment procedure of the LTE system according to the prior art. As can be seen from <figref idref="f0002">FIG.2</figref>, if an RRC connection is disconnected due to radio link failure, an RRC re-establishment procedure needs to be initiated to re-establish the RRC connection. In the beginning, the UE 110 sends an RRC Connection Re-establishment request message to the E-UTRAN 120. Upon reception of the RRC Connection re-establishment request message, the E-UTRAN 120 responds by sending an RRC Connection Re-establishment message to the UE 110. When receiving the RRC Connection Re-establishment message, the UE resumes a signal radio bearer 1 (SRB1) and configures a lower layer to re-activate security (including integrity protection and ciphering) using the previously configured algorithm immediately. In other words, integrity protection and ciphering shall be applied to all subsequent messages received and sent by the UE 110. After that, the UE 110 sends an RRC Connection re-establishment complete message to notify the E-UTRAN 120 that the RRC connection is connected again. To resume all radio bearers other than the SRB1, the E-UTRAN 120 shall initiate an RRC Connection reconfiguration procedure after the RRC connection is re-established, wherein the RRC Connection reconfiguration procedure is to modify the RRC connection.

0011However, it is not clearly specified how to resume SRBs and data radio bearers (DRBs) after the RRC Connection re-establishment procedure and the subsequent RRC connection reconfiguration in some scenarios. For example, if the UE 110 is handed over from a first eNodeB to a second eNodeB, the compressor's context in a transmitting PDCP entity has been updated while the decompressor's context in a receiving PDCP entity has not been updated. Therefore, header decompressions in the receiving PDCP entity cannot decompress the PDCP SDUs correctly after resumption. Hence, a mechanism for synchronizing PDCP operations after RRC connection re-establishment needs to be improved.

0012Please refer to <figref idref="f0003">FIG.3. FIG.3</figref> is a block diagram showing a wireless communication system 300 according to an embodiment of the present invention. The wireless communication system 300 can be an LTE system, but this should not be a limitation of the present invention, and can be wireless communication systems of other types. The wireless communication system 300 includes, but is not limited to, a UE 310 and an E-UTRAN 350. The UTRAN E-350 includes a transmitting PDCP entity 360, and the UE 310 includes a receiving PDCP entity 320. Operations of the transmitting PDCP entity 360 and the receiving PDCP entity 320 will be detailed in the embodiments below.

0013Please note that In the following embodiments, <figref idref="f0004 f0005 f0006 f0007 f0008">FIG.4-FIG.8</figref> are examples for resumption of DRBs mapped on Radio Link Control Acknowledged Mode (RLC AM), <figref idref="f0009 f0010 f0011">FIG.9-FIG.10</figref> are examples for resumption of DRBs mapped on RLC UM, and <figref idref="f0011">FIG.11</figref> is an example for resumption of an SRB1 and an SRB2.

0014Please refer to <figref idref="f0004">FIG.4. FIG.4</figref> is a diagram illustrating PDCP SDUs received by the receiving PDCP entity 320 shown in <figref idref="f0003">FIG.3</figref>. As shown in <figref idref="f0004">FIG.4</figref>, five PDCP SDUs 410-450 are in-sequence transmitted by the transmitting PDCP entity 360. In this embodiment, the PDCP SDUs 410-450 are transmitted in RLC AM, thus the receiving PDCP entity 320 needs to send an RLC acknowledgement to confirm the reception of the PDCP SDUs. After a DRB mapped on RLC AM is resumed, the PDCP SDUs 420 and 450 are successfully received while the PDCP SDUs 410, 430, and 440 are not successfully received by the receiving PDCP entity 320 due to radio link failure. Actually, the PDCP SDU 410 is successfully received but has not been confirmed yet by a lower layer of the receiving PDCP entity 320. To recover the missing PDCP SDUs, three solutions are proposed below.

0015[Solution-1]: After the DRB mapped on RLC AM is resumed, the transmitting PDCP entity 360 retransmits all the PDCP SDUs 410-450 from the first PDCP SDU 410 of which a successful delivery of corresponding PDCP PDUs have not been confirmed by the lower layer. This solution can solve the PDCP SDU missing issue above, but wastes radio resource due to re-transmission.

0016[Solution-2]: After the DRB mapped on RLC AM is resumed, the transmitting PDCP entity 360 retransmits the PDCP SDUs of which the successful delivery of the corresponding PDCP PDUs has not been confirmed by the lower layer. In other words, the PDCP SDUs 410, 430, and 440 are re-transmitted. This solution can improve the method mentioned in the first solution, but still wastes radio resource due to re-transmission.

0017[Solution-3]: After the DRB mapped on RLC AM is resumed, the receiving PDCP entity 320 transmits a PDCP status report configured by RRC to the transmitting PDCP entity 360. After that, the transmitting PDCP entity 360 re-transmits the PDCP SDUs negatively acknowledged in the PDCP status report. In other words, the PDCP SDUs 430 and 440 are re-transmitted according to the PDCP status report. If an in-sequence delivery of upper layer PDUs is needed in the second solution and the third solution, the receiving PDCP entity 320 needs to decipher the received out-of sequence PDCP SDUs and reorder them. A timer is started for reordering the received PDCP SDUs when the DRB mapped on RLC AM is resumed. When the timer expires, the reordering is done and the received PDCP SDUs are delivered to an upper layer.

0018The abovementioned solutions 1-3 are shown in <figref idref="f0005">FIG.5</figref>, which is a flowchart illustrating a method for synchronizing PDCP operations after RRC Connection Re-establishment in a wireless communication system according to an exemplary embodiment of the present invention. The method includes the following steps: <dl id="dl0001" compact="compact"><dt>Step 502:</dt><dd>A DRB mapped on RLC AM is resumed.</dd><dt>Step 504:</dt><dd>A designated group of PDCP SDUs are not successfully received due to radio link failure.</dd><dt>Step 510:</dt><dd>Re-transmitting all the PDCP SDUs from the first PDCP SDU of which the successful delivery of the corresponding PDCP PDUs have not been confirmed by the lower layer.</dd><dt>Step 520.</dt><dd>Re-transmitting the PDCP SDUs of which the successful delivery of the corresponding PDCP PDUs have not been confirmed by the lower layer.</dd><dt>Step 530:</dt><dd>Transmitting a PDCP status report configured by RRC to the transmitting PDCP entity.</dd><dt>Step 532:</dt><dd>Re-transmitting the PDCP SDUs negatively acknowledged in the PDCP status report.</dd></dl>

0019For a DRB mapped on RLC AM, header decompression in the receiving PDCP entity 320 may not work after resumption. For example, five compressed PDCP SDUs are not transmitted successfully by the transmitting PDCP entity 360 due to radio link failure. The compressor's context in the transmitting PDCP entity 360 has been updated but the decompressor's context in the receiving PDCP entity 320 has not been updated. To solve this problem, two solutions are proposed as below.

0020[Solution-4]: After a DRB mapped on RLC AM is resumed, the header compression protocol is reset by the transmitting PDCP entity 360 and the decompression protocol is reset by the receiving PDCP entity 320.

0021[Solution-5]: After a DRB mapped on RLC AM is resumed, header compression and decompression protocols are not reset. The receiving PDCP entity 320 does not perform a header decompression on the received out-of-sequence PDCP SDUs in a reordering buffer if the out-of-sequence PDCP SDUs are received due to RLC re-establishment. The UE 310 further includes a timer Discard_Timer associated with the received PDCP SDUs, wherein the timer Discard_Timer is started to reorder the received out-of-sequence PDCP SDUs in the reordering buffer to generate received in-sequence PDCP SDUs when a DRB mapped on RLC AM is resumed. When the timer Discard_Timer expires, the receiving PDCP entity 320 performs a header decompression on the received in-sequence PDCP SDUs in the reordering buffer and then delivers the PDCP SDUs to the upper layer after decompression.

0022The abovementioned solutions 4-5 are shown in <figref idref="f0006">FIG.6</figref>, which is a flowchart illustrating a method for synchronizing PDCP operations after RRC Connection Re-establishment in a wireless communication system according to another exemplary embodiment of the present invention. The method includes the following steps: <dl id="dl0002" compact="compact"><dt>Step 602:</dt><dd>A DRB mapped on RLC AM is resumed.</dd><dt>Step 604:</dt><dd>Header decompression in the receiving PDCP entity cannot work after resumption.</dd><dt>Step 610:</dt><dd>Reset the header compression and de-compression protocols.</dd><dt>Step 620:</dt><dd>Start a timer Discard_Timer to reorder received PDCP SDUs to generate received in-sequence PDCP SDUs after a DRB mapped on RLC AM is resumed.</dd></dl><dl id="dl0003" compact="compact"><dt>Step 622:</dt><dd>When the timer expires, perform a header decompression on the received in- sequence PDCP SDUs.</dd></dl>

0023A new base-key (KeNB) is always derived corresponding to the RRC Connection Re-establishment procedure. However, the lifetime of the new key is decreased due to waste of usable HFN and Sequence Number space if state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN, and RX_HFN are not initiated to zero. The state variable Next_PDCP_TX_SN indicates the PDCP sequence number of the next PDCP SDU. The state variable TX_HFN indicates the HFN value for the generation of the COUNT value used for the PDCP PDUs. The state variable Next_PDCP_RX_SN indicates the next expected PDCP sequence number by the receiving PDCP entity. The state variable RX_HFN indicates the HFN value for the generation of the COUNT value used for the received PDCP PDUs. To avoid the inefficient key usage, one solution is proposed as below.

0024[Solution-6]: After a DRB mapped on RLC AM is resumed, the state variables of Next_PDCP_TX_SN and TX_HFN are reset to initial values by the transmitting PDCP entity 360. The state variables of Next_PDCP_RX SN and RX_HFN are reset to initial values by the receiving PDCP entity 320.

0025The abovementioned solution 6 is shown in <figref idref="f0007">FIG.7</figref>, which includes the following steps: <dl id="dl0004" compact="compact"><dt>Step 702:</dt><dd>A DRB mapped on RLC AM is resumed.</dd><dt>Step 704:</dt><dd>The lifetime of a new key KeNB is decreased due to waste of usable HFN and Sequence Number space.</dd><dt>Step 710:</dt><dd>Reset the state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN, and RX_HFN to initial values, respectively.</dd></dl>

0026As mentioned above, a new base-key (KeNB) is always derived corresponding to the RRC Connection Re-establishment procedure. A new ciphering key is also generated from the new KeNB. However, it is not clear which ciphering key (old or new) is used to cipher and decipher the retransmitted PDCP PDUs proposed in the solutions 1-3. To solve this problem, one solution is proposed as below.

0027[Solution-7]: The retransmitted PDCP PDUs are ciphered with the new ciphering key derived by a new KeNB generated by the RRC Connection Re-establishment procedure.

0028The abovementioned solution 7 is shown in <figref idref="f0008">FIG.8</figref>, which includes the following steps: <dl id="dl0005" compact="compact"><dt>Step 802:</dt><dd>Perform an RRC Connection Re-establishment procedure.</dd><dt>Step 804:</dt><dd>After a DRB mapped on RLC AM is resumed, re-transmit a designated group of PDCP SDUs being not successfully received.</dd><dt>Step 806:</dt><dd>Generate a new KeNB corresponding to the RRC Connection Re- establishment procedure, and generate a new ciphering key from the new KeNB.</dd><dt>Step 810:</dt><dd>Utilize the new ciphering key to cipher the re-transmitted PDCP PDUs.</dd></dl>

0029For a DRB mapped on RLC UM, header decompression in the receiving PDCP entity 320 may not work after resumption. For example, five compressed PDCP SDUs are not transmitted successfully by the transmitting PDCP entity 360 due to radio link failure. The compressor's context in the transmitting PDCP entity 360 has been updated but the decompressor's context in the receiving PDCP entity 320 has not been updated. To solve this problem, one solution is proposed as below.

0030[Solution-8]: After a DRB mapped on RLC UM is resumed, the header compression protocol is reset by the transmitting PDCP entity 360 and the decompression protocol is reset by the receiving PDCP entity 320.

0031The abovementioned solution 8 is shown in <figref idref="f0009">FIG.9</figref>, which includes the following steps: <dl id="dl0006" compact="compact"><dt>Step 902:</dt><dd>A DRB mapped on RLC UM is resumed.</dd><dt>Step 904:</dt><dd>Header decompression in the receiving PDCP entity cannot work after resumption.</dd><dt>Step 910:</dt><dd>Reset the header compression and de-compression protocols.</dd></dl>

0032In this embodiment, a DRB mapped on RLC UM is resumed. As mentioned above, a new KeNS is always derived corresponding to the RRC Connection Re-establishment procedure However, the lifetime of the new key is decreased due to waste of usable HFN and Sequence Number space if state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN, and RX_HFN are not initiated to zero. To avoid the inefficient key usage, one solution is proposed as below.

0033[Solution-9]: After a DRB mapped on RLC UM is resumed, the state variables of Next_PDCP_TX_SN and TX_HFN are reset to initial values by the transmitting PDCP entity 360. The state variables of Next_PDCP_RX_SN and RX_HFN are reset to initial values by the receiving PDCP entity 320.

0034The abovementioned solution 9 is shown in <figref idref="f0010">FIG.10</figref>, which includes the following steps: <dl id="dl0007" compact="compact"><dt>Step 1002:</dt><dd>A DRB mapped on RLC UM is resumed.</dd><dt>Step 1004:</dt><dd>The lifetime of a new key KeNB is decreased due to waste of usable HFN and Sequence Number space.</dd><dt>Step 1010:</dt><dd>Reset the state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN, and RX_HFN to initial values, respectively.</dd></dl>

0035In this embodiment, SRB1 and SRB2 are resumed. As mentioned above, a new KeNS is always derived corresponding to the RRC Connection Re-establishment procedure. However, the lifetime of the new key is decreased due to waste of usable HFN and Sequence Number space if state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN, and RX_HFN are not initiated to zero. To avoid the-inefficient key usage, one solution is proposed as beiow.

0036[Solution-10]: After an SRB1 and an SRB2 are resumed, the state variables of Next_PDCP_TX_SN and TX_HFN are reset to initial values by the transmitting PDCP entity 360. The state variables of Next_POCP_RX_SN and RX_HFN are reset to initial values by the receiving PDCP entity 320.

0037The abovementioned solution 10 is shown in <figref idref="f0011">FIG.11</figref>, which includes the following steps: <dl id="dl0008" compact="compact"><dt>Step 1102:</dt><dd>SRB1 and SRB2 are resumed.</dd><dt>Step 1104:</dt><dd>The lifetime of a new key KeNS is decreased due to waste of usable HFN and Sequence Number space.</dd><dt>Step 1110:</dt><dd>Reset the state variables of Next_PDCP_TX_SN, TX_HFN, Next_PDCP_RX_SN. and RX_HFN to initial values.</dd></dl>

0038The abovementioned embodiments are presented merely for describing features of the present invention, and in no way should be considered to be limitations of the scope of the present invention. In summary, the present invention provides a method for synchronizing PDCP operations after RRC Connection Re-establishment in a wireless communication system and a related device. Through adopting the mechanism disclosed in the present invention, the abovementioned issues raised due to the RRC Connection Re-establishment can be solved. For example, the missing PDCP SDU issue in RLC AM can be solved by adopting the solutions 1-3; the header decompression fail issue can be solved by adopting the solutions 4-5 and 8; the inefficient key usage issue can be solved by adopting the solutions 6, 9, and 10; and the ciphering key issue can be solved by adopting the solution 7. Therefore, the PDCP operations between the UE and the E-UTRAN after RRC Connection Re-establishment can be synchronized to avoid the issues mentioned above.

0039All combinations and sub-combinations of the above-described features also belong to the invention.