Communication Systems (Electronics)

Communication System:

Source → Encoder → Modulator → Channel → Demodulator → Decoder → Destination

Modulation:

• Why? Smaller antenna, multiplexing, less noise.

Amplitude Modulation (AM):

• Carrier amplitude varies with message.
• Bandwidth: 2f_m (twice message frequency).
• Used: AM radio (medium wave).
• Modulation index μ < 1 (avoid distortion).

Frequency Modulation (FM):

• Carrier frequency varies with message.
• Bandwidth: 2(Δf + f_m) (Carson's rule).
• Better SNR than AM.
• Used: FM radio (88-108 MHz), TV audio.

Phase Modulation (PM):

• Phase of carrier varies.
• Used in: digital communication.

Digital Modulation:

• ASK (Amplitude Shift Keying).
• FSK (Frequency Shift Keying).
• PSK (Phase Shift Keying): BPSK, QPSK.
• QAM (Quadrature Amplitude Modulation): combines amplitude + phase.

Multiplexing:

• FDM (Frequency Division): different frequencies (analog).
• TDM (Time Division): time slots (digital).
• WDM: wavelengths (fiber optics).
• CDM: code division (CDMA mobile).

Antennas:

• Dipole, Yagi-Uda, parabolic, horn.
• Gain, directivity, bandwidth.
• Polarization: linear (vertical/horizontal), circular.

Propagation:

• Ground wave: below 2 MHz.
• Sky wave: 2-30 MHz, ionospheric reflection.
• Space wave (line of sight): above 30 MHz, satellite, microwave.

Mobile Communication:

• Cellular network: cells, base stations, handoff.
• Generations: 1G (analog), 2G (GSM), 3G, 4G (LTE), 5G.

Optical Fiber:

• Light propagation via total internal reflection.
• Single-mode, multi-mode.
• Very high bandwidth, low loss.

Satellite:

• Geostationary orbit (35,786 km).
• LEO, MEO satellites.
• GPS, communication, weather.

RRB JE focus: AM vs FM, basic modulation, propagation types, mobile generations.