# User Association Performance Trade-Offs in Integrated RF/mmWave/THz Communications
## 1 Introduction  
Sixth-generation wireless networks are expected to support a wide range of emerging applications, including augmented reality, tactile Internet, wireless brain interfaces, connected robotic systems, and holographic telepresence. To enable these applications, 6G will require extremely high data rates with low latency and high reliability [1]. As the current sub-6 GHz spectrum is becoming crowded and cannot meet the growing demands, mmW and THz frequency bands have been proposed to provide the needed additional spectrum and higher data rates [2, 3]. Broadly, RF includes all frequencies from around 9 kHz to 300 GHz, encompassing both microwaves and lower frequencies. This is the broadest definition. More specifically, RF is often defined as 3 kHz to 30 GHz, with microwaves considered separately from 30 GHz to 300 GHz. For cellular communications (Telecom), \"RF bands\" commonly refers to frequencies below 6 GHz (aka sub-6 GHz), which includes licensed LTE and 5G bands. mmW and THz technologies are poised to enable key capabilities that will drive many emerging applications. Specifically, the use of higher frequencies allows for the highly accurate estimation of wireless channel characteristics and device-positioning parameters. This level of precision will underpin the delivery of location-aware services that have become increasingly prevalent. In addition, the massive bandwidths available at mmWand THz frequencies facilitate multi-gigabit data rates and wireless connections with ultra-low latency. Together, the abilities to accurately localize, deliver high data throughput, and minimize latency will be crucial for applications such as augmented/virtual reality, autonomous systems, telemedicine, and industrial automation that require real-time, context-aware wireless connectivity. mmW and THz communications are widely seen as essential technologies for fully realizing these next-generation applications [4; 5]. The International Telecommunication Union (ITU) defines mmW frequencies as ranging from 30 GHz to 300 GHz. This is a broad definition encompassing the whole millimeter wavelength range, which offers multi gigabit data rates due to the large available bandwidth. For 5G cellular applications (also defined by the US Federal Communications Commission), mmW is often referred to more narrowly as frequencies between 24 GHz and 100 GHz. This captures the mid-band mmW frequencies being used for 5G. Currently, in Canada, 5G mmW describes radio signals in three frequencies: 26, 28, and 38 GHz. However, mmW signals have high attenuation and cannot penetrate solid materials and walls. This leads to blockage issues and requires LOS or highly directional transmissions [6]. THz bands (0.3 to 10 THz) or wavelengths between 1 mm and 100 \(\upmu\)m provide even higher data rates and capacity, enabling terabit wireless applications. For communication applications, some sources define THz as frequencies above 100 GHz, i.e., bordering the upper end of mmWave bands. This definition includes lower THz frequencies. The term \"THz gap\" is sometimes used to refer specifically to frequencies between 0.3 and 3 THz [7], reflecting the challenges of this mid-band range. Some sources define THz communication beginning at wavelengths shorter than 1 mm, or frequencies above 1 THz. This excludes lower THz frequencies. However, it faces challenges due to high atmospheric and molecular absorption losses, strong signal attenuation, and limited propagation range [8; 9]. To compensate for the high propagation losses at THz/mmW frequencies, 6G networks will utilize \"ultra-massive MIMO\" techniques with large antenna arrays embedded in surfaces, dense arrays of plasma nano-antennas that can be integrated into walls and objects to provide highly directional beamforming gains. The integration of satellite, optical, and molecular communications with intelligent reflecting surfaces will help provide a truly ubiquitous connectivity envisioned for 6G networks [10; 11].