Provided are a signal transmission method an interface extension apparatus and a communication system which relate to the field of communications and can solve the problem that a multi frequency electric tilt antenna with only one AISG interface cannot simultaneously communicate with a plurality of base stations. The interface extension apparatus comprises N input interfaces and M output interfaces; the input interfaces are connected to at least one base station; the output interfaces are connected to an RET; by acquiring device information about each RET control device in a plurality of RET control devices a proxy is respectively created for each RET control device according to the device information there being a mapping relationship between each RET control device and a corresponding proxy; and then a control signal received from at least one base station is modulated via the proxy corresponding to the RET control device thus sending the modulated control signal to a corresponding RET. The embodiments of the present invention are used for managing a multi frequency electric tilt antenna by a plurality of base stations at the same time.
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