Communication system, base station, user equipment and timing synchronization method for base station thereof

A user equipment (UE), located in a communication coverage of a first base station and a second base station, comprising:

• a transceiver, exchanging information with the first base station and the second base station;
• and
• a processor, coupled to the transceiver,
• wherein the processor exchanges information with the first base station through the transceiver, the transceiver receives a first instruction message from the first base station, the processor transmits a reference message to the second base station through the transceiver according to the first instruction message, the transceiver receives a synchronization message from the second base station and receives a second instruction message from the first base station, the processor records a round-trip time (RTT) of transmitting and receiving the reference message and the synchronization message with the second base station according to the second instruction message and transmits the RTT to the first base station through the transceiver, wherein the reference message and the synchronization message are periodic signals.