Significantly improved peak rates of 1 Gbps in the downlink and 500 Mbps in the uplink are required for a Long Term Evolution-Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. Carrier Aggregation (CA) is introduced to the LTE-A system to accommodate the required improved peak rates, compatibility with the LTE system and full use of spectrum resources.
Carrier aggregation refers to presence of a plurality of component carriers (CCs) in both the uplink and the downlink in a cell instead of an approach in which there is only one pair of carriers in the LTE system and earlier radio communication systems. In the system with carrier aggregation, the respective component carriers may be consecutive or inconsecutive, the maximum bandwidth of each component carrier is 20 MHz for compatibility with the LTE system, and the bandwidths of the component carriers may be the same or different.
A random access of a User Equipment (UE) in the LTE system arises generally from the following several reasons.
In a first case, a User Equipment (UE) in a Radio Resource Control Idle (RRC_IDLE) status accesses the system, which is also referred to as an initial access.
In a second case, a UE initiates a random access after a radio link fails, which is also deemed as an initial access.
In a third case, a random access is required for a UE during a cell handover.
In a fourth case, a UE in a Radio Resource Control Connected (RRC_CONNECTED) status has downlink data incoming.
In a fifth case, a UE in an RRC_CONNECTED status has uplink data incoming.
In the third and fourth cases, if there is a dedicated preamble, a non-contention random access can be available, and FIG. 1 illustrates a non-contention random access procedure which generally includes the following three steps.
For a message 0: An eNodeB (eNB) assigns a UE with an index of a random access preamble (ra-PreambleIndex) for a non-contention random access and a mask index of a physical random access channel for the random access (ra-PRACH-MaskIndex). For a non-contention random access due to incoming downlink data, the ra-PreambleIndex and the ra-PRACH-MaskIndex are transmitted to the UE over a Physical Downlink Control Channel (PDCCH), and for a non-contention random access due to a handover, the ra-PreambleIndex and the ra-PRACH-MaskIndex are carried in a handover command transmitted to the UE.
For a message 1: The UE transmits the dedicated preamble specified in the message 0 to the eNodeB over the PRACH resource specified in the ra-PRACH-MaskIndex according to the received ra-PreambleIndex and ra-PRACH-MaskIndex. The eNodeB calculates an uplink Timing Advance (TA) from the message 1 upon reception of the message 1.
For a message 2: The eNodeB transmits a random access response including the uplink TA to the UE to notify the UE of the timing advance for subsequent uplink transmission.
A contention random access can be available to a random access due to any of the other random access reasons, and FIG. 2 illustrates a contention random access procedure which generally includes the following four steps.
For a message 1: A UE selects a random access preamble and a PRACH resource and transmits the selected random access preamble to an eNodeB over the PRACH resource.
For a message 2: The eNodeB calculates an uplink TA upon reception of the preamble and transmits to the UE a random access response including at least the uplink TA and a UL grant for a message 3.
For the message 3: The UE performs uplink transmission over the UL grant specified in the message 2, and the contents of the uplink transmission of the message 3 vary from one random access reason to another, for example, a Radio Resource Control (RRC) connection establishment request is transmitted in the message 3 for a random access reason of an initial access.
For a message 4: The eNodeB transmits a contention resolution message to the UE, and the UE can judge from the message 4 whether the random access succeeds.
The UE performs establishment of uplink synchronization according to the uplink TA acquired in the foregoing random access flow. The uplink synchronization is for the purpose of keeping the UE and the eNodeB in uplink synchronization so that the UE transmits uplink data and also transmits feedback information of a Hybrid Automatic Repeat Request (HARQ) for downlink data.
Uplink synchronization has to be maintained after the uplink synchronization is established. Uplink synchronization is maintained by the eNodeB as stipulated in the LTE system. FIG. 3 illustrates an uplink synchronization maintenance procedure where the eNodeB and the UE maintain an uplink TA timer (TAT) respectively; the eNodeB recalculates an uplink TA according to uplink transmission of the UE and then transmits a Time Advance command (TA command) including the uplink TA to the UE and starts the TAT; if the UE can not receive the TA command correctly, the UE transmits a Negative Acknowledgement (NACK) message to the eNodeB, and the eNodeB retransmits a TA command and restarts the TAT after a period of time, T1; or if the UE receives the TA command correctly, the UE starts the TAT of the UE and transmits an Acknowledgement (ACK) message to the eNodeB, and the UE performs uplink transmission according to the TA in the TA command; and the eNodeB restarts the TAT of the eNodeB upon reception of the ACK message transmitted from the UE. The eNodeB performs the foregoing procedure at an interval of time, T2, which is shorter than the length of timing of the TAT. The eNodeB considers a specific UE as being synchronized if its TAT for the UE is not timeout. The UE considers itself as being synchronized if the TAT maintained by the UE itself is not timeout.
The inventors have identified during making of the invention the following technical problem in the prior art.
An uplink synchronization establishment and maintenance method for an LTE-A multi-carrier system has been absent so far, thus making it impossible to perform uplink synchronization between an eNodeB and a UE in the multi-carrier system.