Bidirectional Communication Circuits for a 120-GHz PMF Data Link in 40-nm CMOS

Three different bidirectional polymer microwave fiber (PMF) links are presented and discussed. The proposed solutions exploit the following orthogonal signaling dimensions: wave direction, wave polarity, and signal polarity. First, a 120-GHz substrate integrated waveguide directional coupler is implemented and exploits the wave direction orthogonality. The 4-port on-board hybrid coupler achieves measured data rates of, respectively, 6 and 5.5 Gbps in the up- and down-link over a distance of 8 m. Second, a 120-GHz ortho-mode transducer (OMT) is micro-machined and exploits the wave polarity orthogonality. The micro-machined polarization duplexer has a measured isolation of 45 dB over the entire $F$ -band. For the horizontal and vertical paths, the insertion loss is measured with a 40-cm-long Teflon fiber and varies between -5 and -9 dB. Finally, a 120-GHz in-band full-duplex (IBFD) PMF transceiver with tunable electrical-balance duplexer (EBD) and on-chip bow-tie antenna are implemented in a 40-nm bulk CMOS technology. In this case, the duplexing principle is relying on signal polarity. The self-interference (SI) cancellation is achieved with a fully differential transformer-based EBD, resulting in a simulated SI suppression of 30 dB over a bandwidth of more than 14 GHz. The insertion loss of the duplexer is less than 11 dB from TX to fiber and less than 12 dB from fiber to RX at a frequency of 120 GHz. The balancing network provides an impedance locking range from 45 to 93 $\Omega $ and 38 to 266 fF at 120 GHz. For IBFD operation, the maximum measured data rate is 6.2 Gbps. The transceiver with integrated antenna occupies an area of 1.8 mm $\times $ 1.53 mm and has a dc power consumption of 73 mW in total.