A Series N-Path Code Selective Filter for Transmitter Rejection in Full-Duplex Communication

A series code selective filter (CSF) for full-duplex communication is presented. The CSF passes a desired signal in a frequency band only when the signal is tagged with a pseudonoise (PN) code that matches the code at the filter. This approach is shown to reject a transmit signal in the same band as a desired receive signal. The coded transmitter signal is orthogonal through appropriate code selection from a codebook and the PN sequence spreads the signal energy outside the filter bandwidth (BW). The CSF achieves a self-interference rejection of 23.2 dB over a 10-MHz BW using on-chip generated Walsh codes. The CSF operates from 300 to 675 MHz and consumes 28.2 mA from a 1-V supply at 500 MHz. The CSF achieves an IIP3 of 15.7 dBm. The CSF was implemented in a 45-nm CMOS silicon on insulator process and occupies an area of 0.9 mm².