# User Association Performance Trade-Offs in Integrated RF/mmWave/THz Communications
## 6 Results
The number of base stations is an extremely large number, 50-100, which would likely be needed within a 2000 m \(\times\) 2000 m area to provide coverage. We consider a network with 200 RF UEs and 6 RF BSs, 80 mmW UEs and 21 mmW BSs, and for THz 10 UEs and 30 BSs. The network has 80 high-data users, 70 low-latency users, and 50 high-reliability users for RF; 30 high-data users, 20 low-latency users, and 30 high-reliability users for mmW; and 4 high-data users, 3 low-latency users, and 3 high-reliability users for THz. The results demonstrate that our algorithm associates users more effectively than the performance of max-SINR (this is evaluated by comparing standard deviation) balances the load across BSs, including those with lower transmit power that are underutilized by traditional approaches. Our proposed algorithm maintained an average standard deviation around 0.5 for the RF and mmW bands and 0.3 for THz band, unlike max-SINR, which has a standard deviation of 6.3456 for RF, 2.1543 for mmW, and 0.6972 for THz for all user categories as shown in Figure 2a,b. It is worth mentioning that max-SINR may show an acceptable performance in the THz band, but this is only because the number of users is much fewer than the number of BSs, so most of the distribution will be zeros, except for the associated ones, and this keeps the standard deviation low. However, that is not the only issue, as the figures are only considering independent frequency bands assuming three independent networks, and each one is operating in a separate band. When we evaluated an integrated network of RF, mmW, and THz BSs and UEs working together (a coexisting network of RF/mmW/THz), we concluded that any UE association algorithm, not just max-SINR that relies on SINR values or even the blockage score like many proposed methods in the literature, will fail in 6G coexisting networks. SINR for RF is the maximum value, which skews the results towards SINR RF after a certain point. No users are associated with the mmW or THz BSs. On the other hand, our algorithm still works well and provides a fair load distribution despite the wide spectrum of SINR values. Figure 3 shows the SINR distribution plot and visualizes how SINR values are distributed across all users in each frequency band. The SINR distribution plot provides valuable insights into the quality of the wireless communication links between the users and the base stations in each frequency band. The y-axis values indicate the count of occurrences of a particular SINR range in the histogram plot. This count is not directly related to the total number of users in the system. The count represents how many data points (in this case, SINR values) fall within each specified bin (SINR range) on the x-axis. The histogram is constructed using the SINR values of users across the entire system, and it is possible for multiple users to have similar or the same SINR values, leading to multiple counts in the same SINR range. Figure 2: User association comparison. (**a**) max-SINR for three bands. (**b**) Proposed algorithm for three bands. max-SINR has a skewed distribution as in Figure 3a, which suggests that some users experience much better SINR than others, indicating an uneven user experience. As shown in Figure 3b, our proposed algorithm has a bell-shaped distribution, which indicates that most users experience SINR values around a certain central value, indicating good overall performance. Figure 4 shows a comparison between the SINR coverage probability of three users (RF, millimeter, and THz users) in our network as a function of the SINR threshold for our proposed user association algorithm. THz and mmW bands can offer higher data rates and capacity due to their wider bandwidths, but their coverage distance may be limited due to higher path loss and attenuation. MW bands may provide more extended coverage distances and better penetration through obstacles but with comparatively lower data rates and capacity.