Jawaharlal Nehru Technological University Hyderabad B.Tech III Year MOBILE COMMUNICATIONS AND NETWORKS Previous Question Paper (February/March - 2022 - R18)

In a cellular system, the Carrier-to-Interference (C/I) ratio is a critical metric for determining signal quality. For an omnidirectional antenna system, the interference is caused by co-channel cells using the same frequency set. The C/I ratio is defined as the ratio of the received carrier signal power from the serving base station to the sum of the interference powers from all co-channel base stations.

Assuming a signal propagation model where the received power follows a path loss exponent n, the power received at a mobile unit at distance d from a base station is proportional to d^-n. For a standard hexagonal cellular layout with a cluster size N, the distance to the nearest co-channel interferers is approximately D, where D = R√3N, with R being the cell radius.

The C/I ratio for the first tier of six co-channel interferers is given by:

Assuming all interferers are at approximately the same distance D from the mobile unit, the expression simplifies to:

Substituting the frequency reuse factor N using the relation D/R = √3N, we obtain:

In decibels, this is expressed as:

For a typical mobile radio environment where the path loss exponent n is 4, the expression becomes:

This result shows that for omnidirectional antennas, the C/I ratio is highly dependent on the cluster size N. A larger N increases the distance between co-channel cells, thereby reducing interference and improving the C/I ratio, though at the cost of reduced spectral efficiency per cell.

Diversity techniques are used in mobile communications to mitigate the effects of multipath fading by providing the receiver with multiple versions of the same signal, which are likely to experience independent fading. The primary goal is to improve the signal-to-noise ratio (SNR) and reliability of the communication link without increasing transmit power or bandwidth.

The main diversity techniques are categorized as follows:

• Time Diversity: This technique transmits the same information at different time intervals. The separation between transmissions must be greater than the coherence time of the channel to ensure independent fading. It is effective in fast-fading channels but results in reduced data throughput.
• Frequency Diversity: This involves transmitting the signal over different carrier frequencies. The frequency separation must exceed the coherence bandwidth of the channel. While effective, it is bandwidth-inefficient as it requires multiple frequency channels for a single information stream.
• Space (Antenna) Diversity: This is the most common technique in cellular systems, utilizing multiple antennas at the transmitter or receiver. If antennas are spaced sufficiently apart (typically several wavelengths for base stations or half a wavelength for mobiles), the received signals undergo independent fading. It does not require additional bandwidth or power.
• Polarization Diversity: This utilizes antennas with different polarizations, such as vertical and horizontal. Since signals with different polarizations experience independent fading paths, this technique provides diversity gain in a compact form factor, often used in mobile handsets.
• Angle Diversity: This uses directional antennas at the receiver to capture signals arriving from different angles. It is particularly useful in line-of-sight environments or microcellular systems to reduce interference and fading.

To combine these signals, various combining techniques are employed:

• Selection Combining: The receiver selects the branch with the highest instantaneous SNR. It is simple to implement but does not utilize the information from other branches.
• Maximal Ratio Combining (MRC): The signals from all branches are weighted by their individual signal-to-noise ratios and then summed. This provides the optimal performance in terms of SNR but requires complex hardware to estimate channel gains.
• Equal Gain Combining: All branches are weighted equally and summed. It is simpler than MRC and provides performance close to the optimal, as it does not require complex gain estimation for each branch.