JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year MOBILE COMMUNICATIONS AND NETWORKS Previous Question Paper (August/September - 2021 - R18)

Cell splitting and cell sectoring are fundamental techniques used in mobile communication systems to increase the capacity of a network by accommodating more users without requiring additional spectrum allocation.

Cell Splitting is the process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power. When a cell reaches its maximum capacity, it is split into several smaller cells. Because the transmit power is reduced, the interference is localized, and the frequency reuse distance remains constant. This allows the same frequencies to be reused more frequently in a given geographic area, effectively increasing the system capacity by a factor equal to the number of splits.

Cell Sectoring involves replacing a single omnidirectional antenna at the base station with several directional antennas, each radiating within a specific sector (e.g., 120 or 60 degrees). By dividing the cell into sectors, the co-channel interference is reduced because a mobile unit only receives signals from a subset of the surrounding cells. This reduction in interference allows for a tighter frequency reuse pattern, which increases the overall capacity of the system. While sectoring improves capacity, it also increases the complexity of the base station hardware and requires more handovers as mobile units move between sectors.

In summary, while cell splitting increases capacity by increasing the number of base stations and reducing cell size, cell sectoring increases capacity by reducing co-channel interference through the use of directional antennas within the existing cell structure.

Fading-time dispersion parameters describe the time-dispersive nature of a mobile radio channel, which causes the signal to spread in time, leading to inter-symbol interference (ISI). These parameters are derived from the power delay profile of the channel and are critical for characterizing wideband channels.

The key parameters are:

1. Mean Excess Delay: This is the first moment of the power delay profile, defined as the weighted average of the delays of all multipath components. It is calculated as the sum of the product of each delay and its corresponding power, divided by the total received power.
2. RMS Delay Spread: This is the square root of the second central moment of the power delay profile. It provides a measure of the time dispersion of the channel. A larger RMS delay spread indicates a more significant time dispersion, which increases the likelihood of frequency-selective fading and ISI.
3. Excess Delay Spread: This is defined as the time interval during which the multipath power spectral density falls below a specific threshold (e.g., 10 dB or 20 dB) relative to the maximum peak of the power delay profile. It defines the window within which most of the signal energy is received.

These parameters are essential for determining the Coherence Bandwidth of the channel, which is inversely proportional to the RMS delay spread. If the signal bandwidth is greater than the coherence bandwidth, the channel is considered frequency-selective, leading to significant distortion that requires equalization or multicarrier modulation techniques like OFDM to mitigate.