Modeling and dynamic cancellation of TX-RX leakage in FDD transceivers

Frequency division duplex (FDD) transceivers employing the direct-conversion radio architecture are known to suffer from transmitter-receiver signal leakage problems. The presence of such leakage signal can impose stringent linearity requirements in receiver components that are difficult to fulfill in practice. Good example is second-order intermodulation distortion (IM2) due to transmitter leakage signal, stemming from finite IIP2 of receiver mixers, falling directly on top of the weak received signal. This paper carries out detailed modeling and proposes a dynamic cancellation technique for such TX-RX leakage, enabling sufficient TX-RX isolation in FDD mode without any extra analog/RF filtering. The technique is based on creating a replica of the undesired transmitter leakage signal and subtracting it from the down-converted signal in the receiver path, taking also the essential transmitter nonidealities into account. Simulation results show that the proposed method is able to effectively push transmitter leakage induced IM2 below the receiver noise floor.