In full-duplex radio communication systems like eUTRAN, CDMA-2000, the radio transmitter (Tx) is active at the same time as the radio receiver (Rx). The Tx and the Rx will be using separate dedicated frequency bands and the Tx-Rx isolation is ensured by duplex ﬁlters. However, agile duplexers required for multiband operation are almost non-existent while dedicating a bank of narrowband ﬁlters is bulky and incurs considerable switching losses. In this article we propose an approach that dramatically reduces the complexity of the RF frontend, ﬁrst by replacing the duplex ﬁlter with a spatial ﬁlter and second, by co-designing the ﬁltering antennas and the RF frontend. The spatial ﬁlter is synthesized by equipping the Tx with redundant antennas. By properly weighting the Tx antennas, the Tx signal is selectively attenuated in the Rx direction. The spatial ﬁlter can be tuned to different frequency bands as long as the antennas are made tunable. Moreover, the spatial ﬁlter may directly beneﬁt from the balanced architecture of the power ampliﬁers (PAs) thus reducing the total system complexity and insertion loss. Finally, simulation and initial measurement results are provided in a challenging low-frequency band, serving as a proof-of-concept.
I E E E Transactions on Antennas and Propagation, 2013, Vol 61, Issue 4, p. 2241-2251