# Experimental Demonstration of Terahertz-Wave Signal Generation for 6G Communication Systems
## 1 Introduction  
Recently, the data traffic in different communication systems is increasing rapidly, so it is imperative to develop new spectrum resources for next-generation networks (e.g., 6G). To meet the requirements of 6G and its applications, such as the Internet of Things (IoT), space communication networks, wireless data centers, precise telemedicine control, and holographic communication [1, 2], THz band communications have attracted considerable attention and are considered as a key technology in pillar for 6G networks [3]. THz frequency range closes the gap between the millimeter wave (mmWave) and the optical frequency band. In the comparison between THz communication and mmWave communication, THz communications own more spectrum resources that offer higher data rates in the range of hundreds of gigabits per second (Gbps) to some terabit per second (Tbps) [4].  
The THz frequency band has the potential to enhance communication performance in challenging propagation environments by leveraging reflected paths. As a result, THz technology is seen as the most promising option for 6G networks [5]. Research has explored the electromagnetic spectrum from 0.1 THz to 10 THz, corresponding to wavelengths ranging from 3 mm to 30 \(\upmu\)m, known as the low-THz band [6; 7; 8; 9]. The equipment requirements for the low-THz band are less demanding than those for higher THz frequencies, yet it retains many characteristics of THz signals, making low-THz a prominent area of research [9].  
High-frequency electromagnetic radiation can be understood as either particle processed through photonic devices (optical medium) or as waves managed by electronic devices (mmWave medium) [10]. Consequently, the design of modern photonic and electronic THz transceivers has been enhanced to enable efficient signal generation, modulation, and radiation [11; 12; 13; 14]. In this context, metamaterials are a key solution for enhancing performance and enabling applications in 5G and 6G communication systems, while dielectric materials play a vital role in designing electronic devices such as antennas and sensors for future IoT applications, as indicated in [15] and [16], respectively.  
The photonic solutions can be utilized to achieve higher carrier frequencies, making them essential for applications that require high data rates, although they tend to have lower output power and integration levels [10]. These photonic approaches employ various technologies, such as quantum cascade lasers [17], uni-travelling carrier photodiodes [18], photoconductive antennas [19], and optical down-conversion systems [20].  
Additionally, advancements in the understanding of light's topological phase are being harnessed to enable robust transmission of THz topological valleys through multi-sharp curves in integrated THz waveguides [21].  
Figure 1 categorizes the different THz generation techniques. Given our focus on photonic solutions for THz generation, we will delve deeper into the key technologies utilized in this area including uni-travelling carrier photodiodes (UTC-PDs) and OFCG. UTC-PDs are known for their high-speed performance, which is essential in our THz signal generation experiments where, in our research, we utilize UTC-PDs to achieve efficient signal modulation and enhance the overall performance of our THz systems. Next, the UTC-PD is introduced as it directly influences the quality and feasibility of our THz signal generation methods.