# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 2 Simulation Model for THz Generation
This combined optical signal then passes through a variable optical attenuator (VOA), which allows for precise control of the optical power level, ensuring optimal conditions for detection and modulation. Figure 4: Optical spectrum before UTC-PD. Following this, the attenuated optical signal is directed onto the UTC-PD, a high-speed photodetector that converts the optical signal into a corresponding electrical signal. The UTC-PD's role is critical in generating a high-frequency electrical signal that embodies the modulated information, in this case, QPSK modulation, which is essential for high-capacity THz communication. The figure also highlights the key components involved in this conversion process, including the optical couplers, the VOA, and the photodiode, along with the signal pathways connecting them. Once the electrical THz signal is generated, it is transmitted to the subsequent detection and demodulation stages, which are also represented in the diagram. The visual layout clarifies the sequence of operations on the receiver side, illustrating how the optical input is transformed into a modulated THz carrier and subsequently processed for data retrieval. Overall, Figure 5 provides an intuitive and detailed overview of the entire receiver architecture, emphasizing the flow of the optical and electrical signals, the role of each component, and the operational steps necessary to realize the THz communication system with QPSK modulation. Following this, the generated photonic THz signal is amplified using an electrical amplifier equipped with a Bessel bandpass filter that has a center frequency of 0.1 THz and a bandwidth of 1.5 GHz, achieving a transmission gain of 35 dB. A 0.1 THz local oscillator (LO) signal is then employed for coherent demodulation. Ultimately, the transmitted data is extracted from the 0.1 THz RF signal utilizing digital signal processing (DSP) algorithms, which include the cascaded multi-modulus algorithm (CMMA), frequency down conversion, equalization, dispersion compensation, phase offset estimation, frequency offset estimation (FOE), and bit error rate (BER) calculation. The key parameters of the proposed system are summarized in Table 2.  
In communication systems, modulation plays a vital role in the effective transmission of information. The choice of modulation scheme involves balancing various factors, including cost, complexity, radio frequency (RF) channel characteristics, component technologies, and noise levels. Given that THz frequency bands are not yet widely utilized, the design process faces technical challenges, such as significant channel losses, power management, and oscillator phase stability. Therefore, selecting an appropriate modulation scheme for THz communication systems is essential. This work explores several modulation formats,  
Figure 5: Electrical RF spectrum after PD of generated THz signal at 100 GHz. including QPSK, 16-QAM, 64-QAM, and 256-QAM, within the THz generation context, and examines how different modulation orders influence the resulting THz signal. Figure 6 illustrates the relationship between bit error rate (BER) and received optical power. The results show that for QAM signals, the performance was very poor. Hence, using QPSK is a recommended choice for these applications. As observed, our current implementation reliably supports only QPSK, with BER exceeding acceptable thresholds (>10\({}^{-3}\)) for 16/64/256-QAM under tested channel conditions--primarily due to phase noise sensitivity from low-cost oscillators at higher symbol rates, nonlinear distortion in the power amplifier degrading EVM, and channel es