ntal component) was incident upon a single polarized receive antenna, the electrical and mechanical imperfections would introduce a small amount of ellipticity to the polarization of the signal. The signal can be thought of as having both vertical and horizontal compo- nents. The ratio of the resulting horizontal to vertical components is the defined as XPD. Active self-interference cancellation techniques in the analog domain generate a replica of the transmit signal and then adjust it to match the self-interference channel, making the replica similar to the self-interference signal in order to subtract it from the total received signal. This replica can be generated either in the analog domain or in the digi- tal domain before the DAC. The self-interference cancellation signal is performed in the same domain from which it was created; thus no additional ADC/DAC is required. Replication of the transmission signal in the analog domain can be achieved by tapping the transmitter chain, using a power splitter, or using a balanced-to-unbalanced circuit in the case of two separate antennas. Experiments show the practical benefits of the latter approach relative to phase shifter, notably the flatter response within a wide frequency band. After creating an exact negative replication of the RF reference signal, the replica is adjusted by delay and attenuation elements to match the self-interference [40,41] (Fig. 3.40). It can be observed in the experiments that the nonlinear components of self-interference can be 80 dB higher than the receiver noise floor. A large fraction of this component may be eliminated along with the linear self-interference by self-interference cancellation techni- ques in the analog and propagation domains, but the residual nonlinear self-interference in the digital domain, which amounts to 10-20 dB, needs to be canceled. In general, nonlinear self-interference cancellation methods are added to the linear methods to achieve optimal performance. A generic model for approximation of the nonlinear f