Method and apparatus for handling out-of-order packets during handover in a wireless communication system

Abstract

Techniques for sending packets and maintaining synchronization during handover is described. A user equipment (UE) may be handed over from a source base station to a target base station. The source base station may forward packets for the UE to the target base station, which may receive the packets out of order. In one design, the target base station may determine whether each packet can be sent in order to the UE, send the packet if it can be sent in order, and discard the packet otherwise. In another design, the target base station may re-order packets received within a re-ordering window and may send the re-ordered packets to the UE. In yet another design, the target base station may process each packet received out of order as if the packet is in order, e.g., by incrementing a hyper-frame number (HFN) or re-assigning the packet with a later sequence number.

1 . A method for wireless communication, comprising:
 receiving a sequence of packets from a first entity at a second entity;   determining whether each packet in the sequence can be sent in order to a third entity;   sending to the third entity each packet that can be sent in order; and   discarding each packet that cannot be sent in order.   
   
   
       2 . The method of  claim 1 , wherein the first entity is a source base station, the second entity is a target base station, and the third entity is a user equipment (UE), and wherein the receiving, the determining, the sending, and the discarding are performed by the target base station during handover of the UE from the source base station to the target base station. 
   
   
       3 . The method of  claim 1 , wherein the determining whether each packet in the sequence can be sent in order comprises
 maintaining a pointer for a sequence number of a latest packet sent to the third entity, and   determining whether each packet in the sequence can be sent in order to the third entity based on a sequence number of the packet and the pointer.   
   
   
       4 . The method of  claim 1 , wherein the determining whether each packet in the sequence can be sent in order comprises
 determining that a packet cannot be sent in order to the third entity if the packet has an earlier sequence number than a sequence number of a packet already sent to the third entity.   
   
   
       5 . The method of  claim 1 , wherein the determining, the sending, and the discarding are performed as each packet in the sequence is received from the first entity, without buffering the packet at the second entity. 
   
   
       6 . The method of  claim 1 , wherein the sequence of packets comprises a sequence of Packet Data Convergence Protocol (PDCP) service data units (SDUs). 
   
   
       7 . The method of  claim 1 , wherein the receiving the sequence of packets comprises receiving the sequence of packets from the first entity via a packet-switched interface. 
   
   
       8 . An apparatus for wireless communication, comprising:
 at least one processor configured to receive a sequence of packets from a first entity at a second entity, to determine whether each packet in the sequence can be sent in order to a third entity, to send to the third entity each packet that can be sent in order, and to discard each packet that cannot be sent in order.   
   
   
       9 . The apparatus of  claim 8 , wherein the at least one processor is configured to maintain a pointer for a sequence number of a latest packet sent to the third entity, and to determine whether each packet in the sequence can be sent in order to the third entity based on a sequence number of the packet and the pointer. 
   
   
       10 . The apparatus of  claim 8 , wherein the at least one processor is configured to determine that a packet cannot be sent in order to the third entity if the packet has an earlier sequence number than a sequence number of a packet already sent to the third entity. 
   
   
       11 . An apparatus for wireless communication, comprising:
 means for receiving a sequence of packets from a first entity at a second entity;   means for determining whether each packet in the sequence can be sent in order to a third entity;   means for sending to the third entity each packet that can be sent in order; and   means for discarding each packet that cannot be sent in order.   
   
   
       12 . The apparatus of  claim 11 , wherein the means for determining whether each packet in the sequence can be sent in order comprises
 means for maintaining a pointer for a sequence number of a latest packet sent to the third entity, and   means for determining whether each packet in the sequence can be sent in order to the third entity based on a sequence number of the packet and the pointer.   
   
   
       13 . The apparatus of  claim 11 , wherein the means for determining whether each packet in the sequence can be sent in order comprises
 means for determining that a packet cannot be sent in order to the third entity if the packet has an earlier sequence number than a sequence number of a packet already sent to the third entity.   
   
   
       14 . A computer program product, comprising:
 a computer-readable medium comprising:
 code for causing at least one computer to receive a sequence of packets from a first entity at a second entity, 
 code for causing the at least one computer to determine whether each packet in the sequence can be sent in order to a third entity, 
 code for causing the at least one computer to send to the third entity each packet that can be sent in order, and 
 code for causing the at least one computer to discard each packet that cannot be sent in order. 
   
   
   
       15 . A method for wireless communication, comprising:
 receiving a sequence of packets from a first entity at a second entity;   re-ordering the packets in the sequence; and   sending the re-ordered packets from the second entity to a third entity.   
   
   
       16 . The method of  claim 15 , wherein the first entity is a source base station, the second entity is a target base station, and the third entity is a user equipment (UE), and wherein the receiving, the re-ordering, and the sending are performed by the target base station during handover of the UE from the source base station to the target base station. 
   
   
       17 . The method of  claim 15 , wherein the sequence of packets is received by the second entity during a time period determined by a re-ordering window. 
   
   
       18 . The method of  claim 15 , wherein the re-ordering the packets comprises
 starting a timer in response to receiving a first packet in the sequence from the first entity,   buffering the first packet if not received in order from the first entity,   buffering subsequent packets not received in order from the first entity prior to expiration of the timer, and   re-ordering the buffered packets after expiration of the timer.   
   
   
       19 . The method of  claim 18 , further comprising:
 processing each packet received from the first entity after expiration of the timer without buffering the packet.   
   
   
       20 . The method of  claim 18 , further comprising:
 receiving a packet from the first entity after expiration of the timer;   determining whether the packet can be sent in order to the third entity;   sending the packet to the third entity if the packet can be sent in order; and   discarding the packet if cannot be sent in order.   
   
   
       21 . An apparatus for wireless communication, comprising:
 at least one processor configured to receive a sequence of packets from a first entity at a second entity, to re-order the packets in the sequence, and to send the re-ordered packets from the second entity to a third entity.   
   
   
       22 . The apparatus of  claim 21 , wherein the at least one processor is configured to start a timer in response to receiving a first packet in the sequence from the first entity, to buffer the first packet if not received in order from the first entity, to buffer subsequent packets not received in order from the first entity prior to expiration of the timer, and to re-order the buffered packets after expiration of the timer. 
   
   
       23 . The apparatus of  claim 22 , wherein the at least one processor is configured to process each packet received from the first entity after expiration of the timer without buffering the packet. 
   
   
       24 . The apparatus of  claim 22 , wherein the at least one processor is configured to receive a packet from the first entity after expiration of the timer, to determine whether the packet can be sent in order to the third entity, to send the packet to the third entity if the packet can be sent in order, and to discard the packet if cannot be sent in order. 
   
   
       25 . A method for wireless communication, comprising:
 receiving a first packet with a first sequence number;   processing the first packet for transmission to a receiving entity;   receiving a second packet with a second sequence number earlier than the first sequence number, the second packet being received out of order with respect to the first packet; and   processing the second packet for transmission to the receiving entity, the second packet being processed as if the second packet is later than the first packet.   
   
   
       26 . The method of  claim 25 , wherein the receiving entity is a user equipment (UE), and wherein the first and second packets are forwarded by a source base station to a target base station during handover of the UE from the source base station to the target base station. 
   
   
       27 . The method of  claim 25 , further comprising:
 incrementing a hyper-frame number (HFN) in response to receiving the second packet out of order, the HFN being used to process the second packet.   
   
   
       28 . The method of  claim 25 , wherein the processing the first packet comprises ciphering the first packet with a first count comprising a hyper-frame number (HFN) and the first sequence number, and wherein the processing the second packet comprises
 incrementing the HFN in response to receiving the second packet out of order, and   ciphering the second packet with a second count comprising the incremented HFN and the second sequence number.   
   
   
       29 . The method of  claim 25 , wherein the processing the second packet comprises
 re-assigning the second packet with a third sequence number later than the first sequence number, and   processing the second packet with the third sequence number for transmission to the receiving entity.   
   
   
       30 . The method of  claim 29 , further comprising:
 receiving a third packet with the third sequence number;   re-assigning the third packet with a fourth sequence number later than the third sequence number; and   processing the third packet with the fourth sequence number for transmission to the receiving entity.   
   
   
       31 . The method of  claim 25 , wherein the first and second packets are processed as each packet is received, without buffering the first and second packets. 
   
   
       32 . An apparatus for wireless communication, comprising:
 at least one processor configured to receive a first packet with a first sequence number, to process the first packet for transmission to a receiving entity, to receive a second packet with a second sequence number earlier than the first sequence number, the second packet being received out of order with respect to the first packet, and to process the second packet for transmission to the receiving entity, the second packet being processed as if the second packet is later than the first packet.   
   
   
       33 . The apparatus of  claim 32 , wherein the at least one processor is configured to cipher the first packet with a first count comprising a hyper-frame number (HFN) and the first sequence number, to increment the HFN in response to receiving the second packet out of order, and to cipher the second packet with a second count comprising the incremented HFN and the second sequence number. 
   
   
       34 . The apparatus of  claim 32 , wherein the at least one processor is configured to re-assign the second packet with a third sequence number later than the first sequence number, and to process the second packet with the third sequence number for transmission to the receiving entity. 
   
   
       35 . A method for wireless communication, comprising:
 receiving a sequence of packets from a target base station at a user equipment (UE), wherein the sequence of packets is forwarded by a source base station to the target base station during handover of the UE from the source base station to the target base station, and wherein the target base station discards at least one forwarded packet that cannot be sent in order to the UE, or receives the forwarded packets out of order and re-orders the packets prior to transmission to the UE, or receives a forwarded packet out of order and processes the packet as if received in order; and   processing each packet in the sequence to recover the packet.   
   
   
       36 . The method of  claim 35 , wherein the processing each packet in the sequence comprises
 incrementing a hyper-frame number (HFN) if the packet has a smaller sequence number than a sequence number of a preceding packet in the sequence, and   deciphering the packet with a count comprising the HFN and the sequence number of the packet.   
   
   
       37 . The method of  claim 35 , further comprising:
 delivering recovered packets in the sequence to upper layers, wherein at least one recovered packet is delivered out of order to the upper layers; and   re-ordering data in the recovered packets in the upper layers.   
   
   
       38 . An apparatus for wireless communication, comprising:
 at least one processor configured to receive a sequence of packets from a target base station at a user equipment (UE), and to process each packet in the sequence to recover the packet, wherein the sequence of packets is forwarded by a source base station to the target base station during handover of the UE from the source base station to the target base station, and wherein the target base station discards at least one forwarded packet that cannot be sent in order to the UE, or receives the forwarded packets out of order and re-orders the packets prior to transmission to the UE, or receives a forwarded packet out of order and processes the packet as if received in order.   
   
   
       39 . The apparatus of  claim 38 , wherein for each packet in the sequence the at least one processor is configured to increment a hyper-frame number (HFN) if the packet has a smaller sequence number than a sequence number of a preceding packet in the sequence, and to decipher the packet with a count comprising the HFN and a sequence number of the packet. 
   
   
       40 . The apparatus of  claim 38 , wherein the at least one processor is configured to deliver recovered packets in the sequence to upper layers, to deliver at least one recovered packet out of order to the upper layers, and to re-order data in the recovered packets in the upper layers.