A method for carrier aggregation, being applied for a time division duplexing, TDD, system or a collaborative system composed of a TDD system and a frequency division duplexing, FDD, system, and comprising: in a first time interval, sending, by a radio access point, a downlink signal to a user terminal by using a first guard band between the TDD system and the FDD system and a bidirectional communication band of the TDD system, or/and, in a second time interval, receiving, by the radio access point, an uplink signal sent by the user terminal using a second guard band between the TDD system and the FDD system and the bidirectional communication band; wherein the first guard band is located between a downlink band of the FDD system and the bidirectional communication band; and the second guard band is located between a uplink band of the FDD system and the bidirectional communication band; wherein the radio access point is a micro-cell radio access point; the first guard band comprises a first sub-band used for one-way communication and a second sub-band used for bidirectional communication; and the step of sending, by the radio access point, the downlink signal to the user terminal comprises: using, by the radio access point, the first sub-band or/and the second sub-band of the first guard band, and the bidirectional communication band to send the downlink signal to the user terminal; wherein the radio access point is a micro-cell radio access point; the second guard band comprises a third sub-band used for one-way communication and a fourth sub-band used for bidirectional communication; and the step of receiving, by the radio access point, the uplink signal sent by the user terminal comprises: receiving, by the radio access point, the uplink signal which is sent by the user terminal to the micro-cell radio access point using the third sub-band or/and the fourth sub-band of the second guard band and the bidirectional communication band. in a first time interval, sending, by a radio access point, a downlink signal to a user terminal by using a first guard band between the TDD system and the FDD system and a bidirectional communication band of the TDD system, or/and, in a second time interval, receiving, by the radio access point, an uplink signal sent by the user terminal using a second guard band between the TDD system and the FDD system and the bidirectional communication band; wherein the first guard band is located between a downlink band of the FDD system and the bidirectional communication band; and the second guard band is located between a uplink band of the FDD system and the bidirectional communication band; wherein the radio access point is a micro-cell radio access point; the first guard band comprises a first sub-band used for one-way communication and a second sub-band used for bidirectional communication; and the step of sending, by the radio access point, the downlink signal to the user terminal comprises: using, by the radio access point, the first sub-band or/and the second sub-band of the first guard band, and the bidirectional communication band to send the downlink signal to the user terminal; wherein the radio access point is a micro-cell radio access point; the second guard band comprises a third sub-band used for one-way communication and a fourth sub-band used for bidirectional communication; and the step of receiving, by the radio access point, the uplink signal sent by the user terminal comprises: receiving, by the radio access point, the uplink signal which is sent by the user terminal to the micro-cell radio access point using the third sub-band or/and the fourth sub-band of the second guard band and the bidirectional communication band.