Handover of unacknowledged mode bearer in a wireless communication system

A method performed by a source radio network node, the method comprising:

• establishing, with a wireless device, an unacknowledged mode bearer with a source link;
• transmitting, to the wireless device, a handover command that commands the wireless device to perform a make-before-break handover from the source link provided by the source radio network node to a target link provided by a target radio network node, the make-before-break handover comprises resetting a downlink data packet sequence number transmitter status for the unacknowledged mode bearer after or as part of the make-before-break handover;
• preserving the downlink data packet sequence number transmitter/receiver status at the source radio network node for the unacknowledged mode bearer by using the same downlink data packet sequence number transmitter/receiver status for the unacknowledged mode bearer before and after the make-before-break handover;
• and
• transmitting, from the source radio network node to the target radio network node, control signaling indicating the preserved downlink data packet sequence number transmitter/receiver status at the source radio network node for the unacknowledged mode bearer.

A method performed by a target radio network node, the method comprising:

• transmitting, from the target radio network node, a handover command to a source radio network node that commands a wireless device to perform a make-before-break handover from a source link provided by the source radio network node to a target link provided by the target radio network node, the make-before-break handover comprises resetting a downlink data packet sequence number transmitter/receiver status for an unacknowledged mode bearer after or as part of the make-before-break handover;
• and
• receiving, from the source radio network node, control signaling indicating a preserved downlink data packet sequence number transmitter/receiver status at the source radio network node for the unacknowledged mode bearer,
• wherein the downlink data packet sequence number transmitter/receiver status is preserved at the source radio network node for the unacknowledged mode bearer by using the same downlink data packet sequence number transmitter/receiver status for the unacknowledged mode bearer before and after the make-before-break handover.

A wireless device comprising:

• communication circuitry;
• and
• processing circuitry configured to:
• establish to a source radio network node, an unacknowledged mode bearer with a source link;
• receive a handover command that commands the wireless device to perform a make-before-break handover from the source link to a target link;
• perform the make-before-break handover from the source link to the target link, the make-before-break handover comprising establishing a connection to a target radio network node with the target link and resetting a downlink data packet sequence number receiver status for the unacknowledged mode bearer after or as part of the make-before-break handover;
• after receiving the handover command, continue to receive from the source link downlink data packets for the unacknowledged mode bearer, until after the wireless device establishes the connection with the target link;
• after the wireless device establishes the connection with the target link, receive from the target link downlink data packets for the unacknowledged mode bearer;
• preserve a downlink data packet sequence number receiver status at the wireless device for the unacknowledged mode bearer, by using the same downlink data packet sequence number receiver status for the unacknowledged mode bearer before and after the make-before-break handover;
• and
• based on the downlink data packet sequence number receiver status as preserved, monitor for data packets received in duplicate from the source link and the target link for the unacknowledged mode bearer.

A source radio network node comprising:

• communication circuitry;
• and
• processing circuitry configured to:
• establish, with a wireless device, an unacknowledged mode bearer with a source link;
• transmit, to the wireless device, a handover command that commands the wireless device to perform a make-before-break handover from the source link provided by the source radio network node to a target link provided by a target radio network node, the make-before-break handover comprises resetting a downlink data packet sequence number transmitter status for the unacknowledged mode bearer;
• preserve the downlink data packet sequence number transmitter status at the source radio network node for the unacknowledged mode bearer by using the same downlink data packet sequence number transmitter status for the unacknowledged mode bearer before and after the make-before-break handover;
• and
• transmit, from the source radio network node to the target radio network node, control signaling indicating the preserved downlink data packet sequence number transmitter status at the source radio network node for the unacknowledged mode bearer.

A target radio network node comprising:

• communication circuitry;
• and
• processing circuitry configured to:
• transmit, from the target radio network node, a handover command to a source radio network node that commands a wireless device to perform a make-before-break handover from a source link provided by the source radio network node to a target link provided by the target radio network node, the make-before-break handover comprises resetting a downlink data packet sequence number transmitter status for an unacknowledged mode bearer after or as part of the make-before-break handover;
• and
• receive, from the source radio network node, control signaling indicating a preserved downlink data packet sequence number transmitter status at the source radio network node for the unacknowledged mode bearer,
• wherein the downlink data packet sequence number transmitter/receiver status is preserved at the source radio network node for the unacknowledged mode bearer by using the same downlink data packet sequence number transmitter/receiver status for the unacknowledged mode bearer before and after the make-before-break handover.