# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 2 300 GHz Band Transceiver Using Silicon Integrated Circuits
### 300 GHz Band Receiver Using Silicon Integrated Circuit  
As shown in Figure 8a, the receiver of the SiGe integrated circuit can use a low-noise amplifier [11; 12]. Furthermore, in recent receivers of SiGe integrated circuits, by using a 300 GHz band driver amplifier as an LO signal, it is possible to improve the conversion gain of the fundamental mixer and to remove the low-noise amplifier. As a result, conversion gain of +7 dB and a noise figure of 17 dB are realized without a low-noise amplifier [16]. In a receiver of a CMOS integrated circuit, however, since a low-noise amplifier cannot be used as shown in Figure 8b, the down conversion mixer must be used for the first stage. Figure 9 shows a block diagram of a 300 GHz band CMOS receiver [17; 18]. How can the noise figure of the receiver be lowered without using a low-noise amplifier?  
Figure 6: Suppression of undesired signal generated by square mixer. In the square mixer, the square of the superimposed IF and LO signals is generated. Undesired signals are canceled by extracting the difference between the output signals generated by two types of square mixers given “IF – LO” and “IF – LO” signals.  
Figure 7: (**a**) Chip micrograph of 300 GHz band CMOS transmitter, and (**b**) measurement results of constellation and output spectrum.  
To lower the noise figure of the receiver, not only the noise figure but also the conversion loss of the first-stage mixer must be reduced. In general, harmonic mixers are often used for ultra-high frequency down-conversion mixers to lower the frequency of the LO. However, since a harmonic mixer has large conversion loss and high noise figure, performance of the receiver deteriorates. To suppress performance degradation of the receiver, it is necessary to use a fundamental mixer, in which the frequency of the local oscillator is the same as the input 300 GHz band. On the other hand, since the conversion gain of the mixer is proportional to the power of the LO, a LO with sufficiently large output is required to suppress the conversion loss. However, in CMOS integrated circuits, driver amplifiers for LO are unavailable. Therefore, the 300 GHz band CMOS receiver utilizes a LO generating the output power required for the fundamental mixer by parallel combination like the transmitter as shown in Figure 10[17; 18].  
As with the transmitter, a 300 GHz band receiver was fabricated with a 40 nm CMOS process. The chip micrograph is shown in Figure 11a. The LO block occupies a large area of the chip by the parallel combiner. The measurement result of the receiving circuit is shown in Figure 11b. The modulation signal of the 300 GHz band given to the receiver was generated by the 300 GHz band CMOS transmitter shown in Figure 7. In the fabricated 300 GHz band CMOS receiver, the maximum data rate of 32 Gb/s is obtained at 16 QAM.  
Figure 8: Comparison of 300 GHz band receivers. (**a**) In the compound semiconductor integrated circuit and the SiGe integrated circuit, since the received signal is amplified by a low-noise amplifier and then converted into the low frequency signal by a down conversion mixer, the influence on the receiver performance by the noise figure of the mixer is small. (**b**) Since the CMOS integrated circuit cannot use a low-noise amplifier, a fundamental mixer becomes the first stage of the receiver, which has a significant influence on the performance of the receiver. When a fundamental mixer with small conversion loss is used, the local oscillation signal cannot be amplified.  
Figure 9: Block diagram of 300 GHz band CMOS receiver. For CMOS integrated circuits, 300 GHz band low-noise amplifier (LNA) and LO driver cannot be used.