Breaking the Transmitter-Receiver Isolation Barrier in Mobile Handsets with Spatial Duplexing

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.