# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 2 300 GHz Band Transceiver Using Silicon Integrated Circuits
### 300 GHz Band CMOS Transceiver Module  
In Sections 2.2 and 2.3, the technology of the CMOS integrated circuit that is the core of the 300 GHz band transceiver was introduced. In the measurement of the CMOS chip introduced in these sections, a waveguide probe attached to the wafer proper was used. Although wireless communication is possible by attaching an antenna to the waveguide probe, transceivers are not practical for wireless communication using a wafer proper. It is important to connect an antenna that converts space radio waves and electric signals in the CMOS chip without using a wafer proper and waveguide probes. Here, on-chip antennas are sometimes used for millimeter-wave CMOS transceivers [19]. However, the gain of the on-chip antenna is small, and its application is limited to short-distance wireless communication of several centimeters. On the other hand, it is necessary to use an antenna with high gain to extend the communication distance. Since the interface of the high gain antenna is generally a waveguide, an interface connecting the CMOS chip and the waveguide is required. In consideration of practical application, a CMOS-to-waveguide interface is proposed using a conventional multi-layer glass epoxy printed board as shown in Figure 12[20]. The CMOS chip is flip-chip mounted on a multilayer printed circuit board. The 300 GHz band signal generated by the CMOS chip is transmitted using a microstrip line as short as 200 \(\upmu\)m on the printed circuit board. A waveguide conversion probe similar in shape to the patch antenna was formed at the tip of the microstrip line. A quarter wavelength back-short structure required for waveguide conversion was formed in the multilayer substrate and a metal waveguide flange was mounted on the surface of the printed circuit board. This realizes CMOS waveguide conversion. Figure 11: (a) Chip micrograph of 300 GHz band CMOS receiver and (b) measurement results of constellations of received signal. The input signal of the receiver was generated by a 300 GHz band CMOS transmitter. EVM: error vector magnitude, BER: bit error rate. Figure 10: 300 GHz band CMOS receiver using a fundamental mixer as the first stage. To use a fundamental mixer with small conversion loss, 300 GHz is required for the local oscillation frequency. Since the 300 GHz signal cannot be amplified by the CMOS circuit, a 300 GHz signal with the output power required for the fundamental mixer is generated by connecting the outputs of the doubler in parallel. Figure 13 shows the reflection characteristics at the opening of the waveguide flange and the photograph of the module. CMOS waveguide conversion using a printed circuit board was realized. However, the minimum value of the reflection characteristic shifts downward from 300 GHz to 250 GHz due to the estimation errors in the material constant of the substrate and the output reflection characteristic of the CMOS chip. Experiments of wireless communication were carried out using the fabricated CMOS transmission module. To compare and evaluate the performance of this module, it is necessary to compare with the result of other terahertz communication experiment. On the other hand, in each piece of communication experiment, it is not valid to simply compare the data rate and the communication distance because the modulation scheme, antenna gain, frequency (wavelength), and allowable bit error rate are different. The communication distance \(d_{\mathrm{n}}\) in which these parameters are normalized [20; 21] is:  
\[d_{\mathrm{n}}\equiv\frac{d}{\lambda}\sqrt{\frac{\mathrm{SNR}}{G_{\mathrm{t}} G_{\mathrm{r}}}} \tag{1}\]  
where \(d\) is the communication distance, \(\lambda\) is the wavelength, \(\mathrm{SNR}\) is the signal-to-noise ratio of the receiver, and \(G_{\mathrm{t}}\) and \(G_{\mathrm{r}}\) are the antenna gain of the transmitter and the receiver, respectively.