## 3 Network Background
### Ultra-Dense 5G/6G Cellular Networks
#### 3.3.1 Type of Cell Coverage/Base Stations  
There are different types of cell sizes with different characterizations. For example, small cells are characterized by low-power, short-range wireless access points that enhance the network capacity and coverage in dense environments. They include femtocells, picocells, microcells, relay nodes, and RRHs, each designed for specific applications, as presented in Figure 3. These cells play a crucial role in 6G ultra-dense networks, enabling high-speed data, low latency, and seamless connectivity in smart cities and IoT ecosystems, as shown in Table 3. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Cell Types** & **Placement** & **Coverage** & **Served Users** & **Power Consumption** & **Backhaul** & **Access** \\ \hline Macrocells & Outdoor & 1–50 km & & 60 w & Fiber/microwave & Open access \\ Microcell & Outdoor & 200–2000 m & Up to & 10 w & Non-ideal & Open/closed/ \\ Femtocell & Indoor & 10–30 m & & \(\leq\)100 mW & & (broadband/DSL) & hybrid access \\  & (unplanned) & & & & Outdoor (0.25–2.00 W) & Ideal & \\ Picocells & Outdoor/indoor & 4–200 m & Up to 64 users & Indoor (\(\leq\)100 mW); & (fiber/microwave) & Open access \\  & (planned) & & & Outdoor (0.25–2.00 W) & Ideal & \\  & (planned) & & & & Indoor (\(\leq\)100 mW); & (fiber/microwave) & \\ Relay & Outdoor/indoor & Up to 100 m & & Outdoor (0.25–2.00 W) & (out-of-band/in-band) & \\ \hline \end{tabular}
\end{table} 
Table 3: Characteristics of various types of small cells. Macrocells are large BSs designed to provide broad coverage in both urban and rural areas. Their antennas are typically mounted on towers, rooftops, or other elevated structures to ensure optimal signal propagation. Macrocell BSs operate at high transmission power, typically in the range of tens of watts, enabling them to cover long distances. In rural areas and highways, macrocells play a crucial role in providing widespread connectivity, although operators often limit coverage to essential voice services due to the high cost of deployment and the lower number of users. Network planning tools, combined with real-world measurements, are used to optimize macrocell deployment and ensure efficient coverage [36]. * Microcell  
Microcells are small cellular base stations with a typical coverage range of less than 2 km, deployed to enhance the network capacity in densely populated areas such as urban buildings and highways. They offer a cost-effective alternative to picocells for creating high-density mobile networks while using power control mechanisms to limit their coverage area. Although microcells improve channel capacity, they introduce challenges in radio resource management and are more susceptible to short-term fluctuations in traffic and environmental changes, such as new buildings, compared to macrocells [37]. * Picocells  
Picocells are small BSs installed by network operators to provide coverage within a 100 m range, primarily in hotspots (indoor and outdoor), to offload traffic from macrocells and enhance network capacity [4; 5]. They typically support tens of active users and operate with a transmission power of up to 33 dBm. Picocells use fiber or microwave links for backhauling, similar to macrocells, ensuring low latency and high-bandwidth connectivity [4]. * Femtocell  
Femtocells are user-installed indoor BSs designed to provide coverage in homes, offices, and meeting rooms, serving a limited number of users. They operate at a transmission power of less than 20 dBm and cover a range of tens of meters, significantly improving the indoor signal strength [4]. Femtocells connect to the network via consumer broadband  
Figure 3: Comparison of small-cell and macrocell coverage and output power. services, such as DSL, cable, or fiber, making them an effective solution for enhancing indoor connectivity where most data traffic is generated. * RRH RRHs are RF units that extend macrocell coverage to remote areas by connecting to a central BS through fiber optics or microwave links [28].