New circulator structure with high isolation for time division duplexing radio systems

In this paper, we have proposed the new loop circulator structure with high isolation for time division duplexing (TDD) mobile radio system such as the high speed portable internet service (WiBro) in Korea. It is the important factor to isolate between the transmit signal and the receive signal for TDD system, because the white noise generated by the transmitter affect the noise figure of receiver system during the receiving period. Therefore, new technique to increase the isolation of a circulator is presented as adding the two additional circulators to the antenna port and the receiver input port, respectively. Because a proposed structure has twice isolation value than that of the single circulator, it has advantage for a high power TDD system for which a RF switch can not be used. about 20dB, the thermal noise amplified by the high power RF amplifier can be injected to the receiver during the receiving period. Besides, high RF power signal is leaked by a circulator with low isolation during the transmitting period and the receiver can be damaged. Accordingly, an additional device for protecting the receiver is required. As above mentioned, the isolation of the time division duplexing radio system is very important to reduce the noise figure of receiver. In this paper, new technique to increase the isolation of a circulator is presented as adding the two additional circulators to the antenna port and the receiver input port. The strength of transmit signal leaked to a receiver is isolated by the isolation of two circulators, and the transmit signal reduced by the transmission loss of two circulators is radiated through the antenna. Therefore, the function of new loop circulator structure is identical to that of the conventional single circulator, but its isolation is twice than that of a single circulator.