JNTU B.Tech III MOBILE COMMUNICATIONS AND NETWORKS Previous Question Paper (August - 2022 - R18)

Conventional mobile telephone systems, such as early Mobile Telephone Service (MTS) and Improved Mobile Telephone Service (IMTS), were limited by several factors:

• Limited Capacity: These systems relied on a single, high-power transmitter covering a large area, meaning only a few channels were available for a large number of users, leading to frequent call blocking.
• High Transmit Power: Because the base station had to cover a massive radius, mobile units required high-power transmitters, resulting in bulky, power-hungry equipment.
• Lack of Scalability: Adding more users was impossible without allocating more spectrum, which was not feasible due to the scarcity of frequency bands.
• Poor Spectral Efficiency: The inability to reuse frequencies in adjacent areas meant that the entire available spectrum was consumed by a single coverage area, preventing efficient utilization of the radio resource.
• Interference: Large coverage areas were highly susceptible to co-channel interference from distant systems operating on the same frequency.

Frequency reuse is the core concept of cellular networks that allows the same radio frequencies to be used simultaneously in different geographic areas, known as cells, separated by a sufficient distance to minimize interference.

Key aspects of frequency reuse include:

• Cellular Structure: The service area is divided into small, hexagonal-shaped regions called cells, each served by a low-power base station.
• Cluster Concept: A group of cells that collectively use the entire available spectrum is called a cluster. If the total number of available duplex channels is S and each cell uses a subset of k channels, the cluster size N is defined as N = S/k.
• Reuse Distance: To maintain signal quality, cells using the same frequency set must be separated by a minimum distance, known as the reuse distance (D). This is determined by the formula:

where R is the cell radius and N is the number of cells per cluster. By increasing the number of clusters within a geographic area, the network capacity increases proportionally, as the same frequencies are reused across the entire network coverage.