# A Full-Duplex 60 GHz Transceiver with Digital Self-Interference Cancellation
## 3 Design and Implementation
### RF Isolation from the Transmitter Chain to the Receiver  
The SI can occur at any stage in the transmitter chain. It may come from the 60 GHz band within the transceiver chipset, PCB, antennas, and reflected signal. The 12 GHz IF signals and the direct baseband leakage from the transmitter chain may also be available at the receiver system as the SI. Some efforts are made at the level of chip design to reduce the on-chip leakages for the 60 GHz signals. The floor plan is optimized to maximize the distance between the PA and LNA layouts with additional guard rings and isolation wells around them. In addition, isolated power, ground, and I/O pads are used for transmitter and receiver chains to avoid signal coupling. At the circuit level, all the sub-blocks except the PA and LNA are implemented using a fully differential architecture to minimize the effects of the coupling signal in the common mode or supply networks. The 0.18 \(\upmu\)m BiCMOS can achieve a signal isolation of around 60 dB for a distance of 50 \(\upmu\)m [27], and it is good enough for the proposed low-power, short-distance design without extra active cancellation.  
Two adjacent antipodal Fermi antennas are embedded on the FR4 RF PCB separately for the transmitter and receiver. The antenna has a half-power beamwidth of 17\({}^{\circ}\) and 24\({}^{\circ}\) in the E plane and H plane, respectively [25]. Figure 4 shows the top view of the fabricated PCB with a pair of antennas and corresponding interconnection transmission lines. The quadrature baseband signals are placed on the sides of the PCB as shown in Figure 4. The double-row 40-pin connector is placed on the right side for power supply and control signals from the motherboard. The left cross-sectional view of the PCB with antenna beams is shown in Figure 4 and marks the thickness of the different materials used in the PCB design. Two separate antennas avoid the difficulties of signal isolation within the antenna, unlike the single-antenna FD in [11]. For the antenna design, a pair of conducting arms are placed symmetrically on both sides of the RO4003 substrate layer. Moreover, the whole antenna is placed on a slot of the FR4 board that is much thicker than the antenna.  
The narrow-beam antenna is designed to deliver maximum energy in the direction of the antenna tip, pointing with a small-angle coverage. The TX and RX antennas will send power in the marked direction, and the energy propagated between them is minimized. The measurement shows around a 30 dB drop for the energy propagation in the orthogonal direction compared with the beam direction [25]. The thick FRantennas also blocks the direct propagation path between them to further reduce power transfer. The board-level measurement is performed to extract the energy leakage through the near-field of the antenna and the mutual coupling. A Keysight PNA-N5247 network analyzer is used to analyze the board-level leakage signal across the interested band. The measurement is performed with a 5 dBm input signal fed to one of the antennas, and the received signal power in the other antenna is measured. The measured S21 shows more than 80 dB of attenuation in the interested frequency ranges from 57 GHz to 67 GHz. The SI leakage in the board is lower than the noise floor, and it can be ignored.