Low Noise Amplifier (LNA) systems are provided as part of base stations in wireless access networks and are typically positioned between the Base Transceiver System (BTS) of a base station and an antenna. Typically, the LNA system is positioned close to the antenna, at the top of an antenna mast or other supporting structure, and is called a Tower Mount Amplifier (TMA) or a Tower Top LNA (TTLNA). The main purpose of the LNA system is to amplify a received signal before forwarding the signal to a receiver in the BTS where the signal is demodulated and decoded.
Conventional LNA systems designed for Time Division Duplex (TDD) wireless systems or devices require either separate Transmit & Receive cables or require an additional control line/signal to switch between transmit and receive bursts as well as synchronization with the base station to perform the switching operation. In a Time Division Duplex (TDD) transmission scheme transmission and reception are performed in separate time periods. Examples of TDD transmission schemes are Worldwide Interoperability for Microwave Access (WIMAX) as defined in IEEE 802.16 and UMTS TDD.
FIG. 1 shows a first type of LNA system (TTLNA 20) with only one feeder cable 26 connecting the BTS 10 and TTLNA 20 which is used for both transmit and receive. Because a TDD transmission scheme will only transmit or receive a signal at any time (and will never simultaneously transmit and receive) the single feeder 26 can be used on a time-shared basis. A Tx/Rx switch 21 alternately connects the TX path 22 of the TTLNA 20 to the antenna 30, to convey a high-power signal for transmission, or connects the antenna 30 to the receive path 23 of the TTLNA 20 to convey a relatively low power received signal. Transmit/receive control signaling 27 is required between the base station BTS 10 and the Tx/Rx switch 21 of the TTLNA in order to implement switching between the receive and transmit signal paths at required times. This requires a standardised Tx/Rx signaling interface which can be difficult to realize where the LNA system 20 and base station 10 are supplied by different manufacturers. Furthermore, it is difficult to realize a Tx/Rx switch 21 which can switch at high power and with low loss. The transmit path 22 and receive path 23 are shown in FIG. 1 as being combined at the lowermost end by a circulator 25 although this could be replaced by another Tx/Rx switch, similar to switch 21, which is also under the control of signaling 27.
FIG. 2 shows another LNA system 40 which offers lower loss in transmit and receive paths as compared to the system of FIG. 1, at the expense of having two RF feeder cables 41, 42. One feeder cable 41 is dedicated to the transmit path and a second feeder cable 42 is dedicated to the receive path. This implies a high cost as well as some additional leasing and maintenance burden, and decreases the reliability of such systems.
It is an object of the present invention to provide an improved LNA system which mitigates the drawbacks of the above described LNA systems.