# Key Technologies for THz Wireless Link by Silicon CMOS Integrated Circuits
## 2 300 GHz Band Transceiver Using Silicon Integrated Circuits
### 300 GHz Band Transmitter Using Silicon Integrated Circuit  
Even for the same silicon integrated circuit, the architecture of 300 GHz band transmitter differs according to \(f_{\text{max}}\). In the SiGe integrated circuit, as in the usual transceiver, as shown in Figure 2a, a power amplifier can be used for a 300 GHz band transmitter [11,12]. On the other hand, in CMOS integrated circuits, power amplifiers normally included in transceivers are not available. Therefore, to realize a 300 GHz band CMOS transmitter, a method shown in Figure 2b was proposed in which a modulated signal with an intermediate frequency of 80 GHz is multiplied by 3 in the final stage to generate a 240 GHz signal [13]. However, as shown in Figure 3a, not only the center frequency but also the frequency band expands three times in the multiplier, so the wide frequency band is not fully utilized even in the 300 GHz band. To fully utilize the wide frequency band, it is necessary to use a mixer for up-converting the center frequency without changing the frequency band (as shown in Figure 3b) at the final stage of the transmitter. However, since the \(f_{\max}\) of the metal-oxide-semiconductor field-effect transistor (MOSFET) of the CMOS integrated circuit is low, the conversion gain of the mixer is below 0 dB and the output power is not large. It is noted that even if a power amplifier is not used, the output power can theoretically be increased when multiple mixers are connected in parallel for power combination. However, parallel connection of the mixers is not easy. This is because the mixer, which multiplies the frequency-converted signal by the local oscillation signal (LO), has two inputs and one output. These three signals must be divided and combined in parallel as shown in Figure 4 to connect mixers in parallel. When binary trees are used for each of the three input/output signal lines, the intersection of the signal lines necessarily occurs. This not only complicates the layout but also degrades the signal due to the intersection, so that performance improvement cannot be expected. Therefore, in order not to degrade the signal, a mixer with two superimposed signals as the input at the final stage of the transmitter was used [14; 15]. A frequency doubler circuit, called a square mixer since input signals are squared, is used for this mixer. Figure 5 shows the transmitter architecture  
Figure 3: Comparison of the cases using a multiplier and a mixer in the final stage. (**a**) When the N multiplier is used, both the center frequency and the bandwidth is multiplied by N. (**b**) When using a mixer, the frequency shifts upward (up-converted) by the local oscillation frequency (LO). IF: intermediate frequency, RF: radio frequency, BW: bandwidth. Figure 2: Comparison of 300 GHz band transmitters. (**a**) Since the maximum oscillation frequency (\(f_{\max}\)) of transistors used in state-of-the-art compound semiconductor integrated circuits and SiGe integrated circuits exceeds 300 GHz, the signals in the 300 GHz band can be amplified. (**b**) The power amplifier cannot be fabricated because the \(f_{\max}\) of transistors used in CMOS integrated circuits are 300 GHz or less. A 300 GHz band signal is generated by a multiplier. CMOS: complementary metal-oxide-semiconductor. using a square mixer [16]. If the frequencies of the two superimposed signals are close to each other, the two signals can be simultaneously amplified by the identical amplifier. It enables division and combination with a binary tree without complicating the layout. In the square mixer, the sum of the superimposed intermediate-frequency (IF) signal and the LO signal is squared. The generated output includes a desired signal which is the product of the two superimposed signals. On the other hand, besides the desired signal, an undesired signal consisting of the square of the IF signal and the square of the LO signal is simultaneously included.