Early handover preparation and early handover command
A handover procedure for a terminal device in a system comprising the terminal device, a source network access node to which the terminal device is initially connected, and one or more candidate target network access nodes. The method includes determining that a first measurement for radio channel conditions for a first candidate target access node satisfies a first predefined criterion; receiving at the terminal device connection information for use by the terminal device for connecting to the first candidate target network access node; establishing a second measurement for radio channel conditions between the terminal device and one of the one or more candidate target network access nodes; determining that the second measurement of radio channel conditions satisfies a second predefined criterion; and transmitting from the terminal device an indication that the second measurement of radio channel conditions for the first candidate target network access node satisfies the second predefined criterion.
1. A terminal device for use in a wireless telecommunications system comprising the terminal device, a source network access node and one or more candidate target network access nodes, wherein the terminal device comprises controller circuitry and transceiver circuitry configured to operate together such that the terminal device is operable to:
establish a first measurement of radio channel conditions between the terminal device and a first candidate target access node;
determine if the first measurement of radio channel conditions for the first candidate target access node satisfies a first predefined criterion, and if so, transmit to the source network access node an indication that the first measurement of radio channel conditions for the first candidate target network access node satisfies the first predefined criterion, wherein the first predefined criterion is satisfied if the first measurement of radio channel conditions indicates a quality characteristic for the radio channel conditions at the time of the first measurement exceeds a first predefined threshold;
receive and store an indication of first connection information for use by the terminal device for connecting to the first candidate target network access node;
establish a second measurement of radio channel conditions between the terminal device and the first candidate target network access node;
determine if the second measurement of radio channel conditions satisfies a second predefined criterion, and if so, transmit to the source network access node an indication that the second measurement of radio channel conditions for the first candidate target network access node satisfies the second predefined criterion, wherein the second predefined criterion is satisfied if the second measurement of radio channel conditions indicates a quality characteristic for the radio channel conditions at the time of the second measurement exceeds a second predefined threshold, and the second predefined criterion is different from the first predefined criterion;
receive an indication the terminal device should connect to the first candidate target network access node, and, in response thereto,
transmit signalling to the first candidate target network access node using the stored first connection information to initiate a connection to the first candidate target network access node.
2. The terminal device of claim 1 , wherein the first and second predefined criteria are associated with different thresholds for the same quality characteristic.
3. The terminal device of claim 1 , wherein the first and second predefined criteria are associated with different quality characteristics.
4. The terminal device of claim 1 , wherein the terminal device is further configured to:
establish a first measurement of radio channel conditions between the terminal device and a second candidate target access node;
determine if the first measurement of radio channel conditions for the second candidate target access node satisfies the first predefined criterion, and if so, transmit to the source network access node an indication that the first measurement of radio channel conditions for the second candidate target network access node satisfies the first predefined criterion;
receive and store an indication of second connection information for use by the terminal device for connecting to the second candidate target network access node.
5. The terminal device of claim 4 , wherein the terminal device is configured to simultaneously store the first connection information associated with the first candidate target access node and the second connection information associated with the second candidate target access node.
6. The method of claim 1 , wherein the terminal device is configured to discard the stored first connection information so that it does not go on to initiate a connection to the first candidate target network access node using the stored first connection information if one or more of the following conditions is met:
the terminal device does not determine the second measurement of radio channel conditions for the first candidate target network access node satisfies the second predefined criterion within a predefined validity period associated with the stored first connection information;
the terminal device connects to a different candidate target network access node after having stored the first connection information;
the terminal device determines a first measurement of radio channel conditions between the terminal device and a second candidate target access node satisfies the first predefined criterion after having stored the first connection information;
the terminal device receives signalling from the source network access node to indicate the terminal device should discard the stored first connection information;
the terminal device determines a third measurement of radio channel conditions between the terminal device and the first candidate target access node satisfies a third predefined criterion before the terminal device determines the second measurement of radio channel conditions for the first candidate target network access node satisfies the second predefined criterion.
7. The terminal device of claim 1 , wherein the indication that the first measurement of radio channel conditions for the first candidate target network access node satisfies the first predefined criterion and/or the indication that the second measurement of radio channel conditions for the first candidate target network access node satisfies the second predefined criterion comprises at least one of:
a radio resource control message;
layer two, L2, signalling, such as a medium access control, MAC, header;
layer one, L1, signalling, such as in association with a scheduling request or other control information.
8. The terminal device of claim 1 , wherein the indication the terminal device should connect to the first candidate target network access node comprises as a least one of:
an indication of an identifier for the first candidate target network access node;
a radio resource control message;
layer two, L2, signalling, such as a medium access control, MAC, control element;
layer one, L1, signalling, such as a physical downlink control channel, PDCCH, order.
9. The terminal device of claim 1 , wherein the indication of first connection information for use by the terminal device for connecting to the first candidate target network access node comprises an early handover command for the first candidate target network access node.
10. A terminal device for use in a wireless telecommunications system comprising the terminal device, a source network access node and one or more candidate target network access nodes, wherein the terminal device comprises controller circuitry and transceiver circuitry configured to operate together such that the terminal device is operable to:
establish a first measurement of radio channel conditions between the terminal device and a first candidate target access node;
determine if the first measurement of radio channel conditions for the first candidate target access node satisfies a first predefined criterion, and if so, transmit to the source network access node an indication that the first measurement of radio channel conditions for the first candidate target network access node satisfies the first predefined criterion;
receive and store an indication of first connection information for use by the terminal device for connecting to the first candidate target network access node;
establish a further measurement of radio channel conditions between the terminal device and the first candidate target network access node;
determine if the further measurement of radio channel conditions satisfies a further predefined criterion, and if so, discard the first connection information, wherein the further predefined criterion is satisfied if the further measurement of radio channel conditions indicates a quality characteristic for the radio channel conditions at the time of the further measurement fails to exceed a predefined threshold.
11. A method performed by a source network access function in a wireless telecommunications system, the wireless telecommunications system comprising a terminal device, a source network access node, and one or more candidate target network access nodes, the method comprising:
receiving from the terminal device a first indication that a first measurement of radio channel conditions for a first candidate target network access node for the terminal device satisfies a first predefined criterion, wherein the first predefined criterion is satisfied if the first measurement of radio channel conditions indicates a quality characteristic for the radio channel conditions at the time of the first measurement exceeds a first predefined threshold;
in response to receiving the first indication, transmitting, to the terminal device, an indication of first connection information for use by the terminal device for connecting to the first candidate target network access node;
receiving from the terminal device a second indication that a second measurement of radio channel conditions for the first candidate target network access node for the terminal device satisfies a second predefined criterion, wherein the second predefined criterion is different from the first predefined criterion, and wherein the second predefined criterion is satisfied if the second measurement of radio channel conditions indicates a quality characteristic for the radio channel conditions at the time the second measurement exceeds a second predefined threshold; and
in response to receiving the second indication, transmitting to the terminal device an indication the terminal device should proceed to initiate a connection to the first candidate target network access node using the first connection information.