# User Association Performance Trade-Offs in Integrated RF/mmWave/THz Communications
## 1 Introduction
Although THz and mmW bands offer much larger bandwidths compared to RF, the impact of interference and noise becomes more critical with the increased bandwidth, and practical implementation challenges arise [9]. Therefore, the greatly expanded spectrum at the mmW and THz frequencies will be key enablers for 6G applications that will require terabit data rates and low latency. However, overcoming challenges like blockage, absorption losses, and limited range will require innovative user association techniques [12]. The seamless integration of high- and low-frequency bands' cells imposes a number of challenges since different BSs will have different transmission powers, coverage areas and data rate capabilities and need to cater to different types of UEs. Therefore, developing a proper UE-BS association algorithm for diverse 6G networks is a tough task [13]. Such an appropriate algorithm shall not only improve the QoS performance for each UE, but shall also ensure fairness for both UEs and BSs [14]. UE association methods have been widely used in cellular networks to optimize network performance by selecting the best serving cell or frequency band for each UE. Traditional user association strategies that simply maximize the SINR ratio [15] cannot be applied here. This will lead to imbalanced load distributions across frequency bands and cells since the UEs tend to associate with the lowest-frequency (RF) band BSs, which would have better better SINR due to the better propagation characteristics and higher transmit powers. This imbalanced load distribution will result in the inefficient utilization of the network's resources, where RF band cells would be overloaded, while the THz/mmW band would be underutilized. To achieve an equitable load distribution and efficient resource utilization, more sophisticated user association strategies that go beyond simply maximizing the received signal strength are needed. Individual user association, just considering only one band at a time, will not yield globally optimized results. In this paper, we propose a unified multi-band user association algorithm to optimize the user throughput and satisfaction in next-generation 6G wireless networks considering all three bands. We consider three types of user cases: enhanced broadband, low latency and high-reliability connections, each with different QOS requirements. Our proposed algorithm aims to associate each user with the best available BS using the best possible band considering real-time channel conditions across bands, while accounting for user preferences. It considers key factors, such as signal strength and link blockage probability that vary significantly by frequency. The algorithm recomputes user-BS scores periodically and reassociates users in batches to dynamically balance loads. Through comprehensive system-level simulations under realistic scenarios, we demonstrate the effectiveness of our approach in efficiently utilizing network resources across diverse operating conditions. The multi-band association technique paves the way for optimized performance in emerging multi-band wireless networks.