JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year PRINCIPLES OF ELECTRONIC COMMUNICATIONS Previous Question Paper (September - 2021 - R16)

Gain is the ratio of the output power or voltage of a system to its input power or voltage. It represents the amplification of a signal. Mathematically, it is expressed as:

Attenuation is the reduction in the strength of a signal as it travels through a medium, such as a cable or free space. It is the opposite of gain and is caused by factors like resistance, absorption, and scattering. It is expressed as:

Decibels (dB) are a logarithmic unit used to express the ratio of two power levels. Because communication systems often deal with signals spanning many orders of magnitude, the decibel scale simplifies calculations by converting multiplication and division into addition and subtraction. The formula is:

These terms are critical in communications for the following reasons:

• System Budgeting: Engineers use dB to calculate the total link budget of a communication system. By adding gains (amplifiers) and subtracting attenuations (cables, connectors, free-space path loss), they determine if the signal arriving at the receiver is strong enough to be detected above the noise floor.
• Signal Integrity: Attenuation dictates the maximum distance a signal can travel before it requires regeneration or amplification. For example, in fiber optic communication, attenuation is measured in dB/km, which determines the spacing required between repeaters.
• Amplification Requirements: Gain specifications help in selecting the appropriate hardware, such as Low Noise Amplifiers (LNAs) at the receiver front-end to boost weak signals without adding excessive noise.

Example: If a signal enters a cable with 100 mW of power and exits with 10 mW, the attenuation is 10. In decibels, this is 10 log₁₀(10/100) = -10 dB, indicating a 10 dB loss. Conversely, if an amplifier increases a 1 mW signal to 100 mW, the gain is 100, which is 10 log₁₀(100) = +20 dB.