Communication Systems

[Ramya_Velayutham]

Communication Systems

Communication systems focus on transmitting information from one point to another efficiently and reliably. Here's a detailed explanation of the key topics:

1. Analog Communication

Analog communication involves transmitting continuous signals that vary with time.

Amplitude Modulation (AM)

• Definition: The amplitude of the carrier wave is varied in proportion to the message signal while keeping frequency and phase constant.
• Advantages:
  • Simple implementation.
• Disadvantages:
  • Poor noise immunity and low efficiency.
• Applications:
  • AM radio broadcasting.

Frequency Modulation (FM)

• Definition: The frequency of the carrier wave is varied in proportion to the amplitude of the message signal while keeping amplitude constant.
• Advantages:
  • Better noise immunity and improved sound quality compared to AM.
• Applications:
  • FM radio, audio broadcasting.

Phase Modulation (PM)

• Definition: The phase of the carrier wave is varied in proportion to the message signal.
• Applications:
  • Used as a part of digital modulation schemes like QAM.

2. Digital Communication

Digital communication transmits discrete signals (binary data) and offers higher reliability and efficiency.

Pulse Code Modulation (PCM)

• Steps:
  1. Sampling: The analog signal is sampled at discrete intervals (following the sampling theorem).
  2. Quantization: Sampled values are rounded to the nearest available level.
  3. Encoding: Quantized levels are converted into binary codes.
• Advantages:
  • High noise immunity.
  • Easy to multiplex multiple signals.
• Applications:
  • Telephony, audio CDs.

Modulation Techniques

Digital modulation maps binary data onto carrier signals.

• Amplitude Shift Keying (ASK):
  • Amplitude of the carrier varies with binary data.
  • Applications: Low-speed data transmission.
• Frequency Shift Keying (FSK):
  • Frequency of the carrier varies with binary data.
  • Applications: Modems, radio signals.
• Phase Shift Keying (PSK):
  • Phase of the carrier varies with binary data.
  • Types:
    • BPSK: Binary Phase Shift Keying (two phases for binary 0 and 1).
    • QPSK: Quadrature Phase Shift Keying (four phases to encode two bits per symbol).
  • Applications: Wireless communication, satellite systems.
• Quadrature Amplitude Modulation (QAM):
  • Combines ASK and PSK by varying both amplitude and phase.
  • Applications: Broadband communication, Wi-Fi.

3. Information Theory and Coding

Information theory provides the theoretical foundation for efficient and reliable communication.

Key Concepts:

• Entropy (HHH):
  • Measures the average information content of a message.
• Channel Capacity (CCC):
  • Maximum rate at which information can be transmitted over a channel without errors.

Error Correction:

• Forward Error Correction (FEC):
  • Redundant data is added to enable error detection and correction at the receiver.
• Common Codes:
  • Hamming Code: Corrects single-bit errors.
  • Cyclic Redundancy Check (CRC): Detects burst errors.
  • Turbo Codes and LDPC: Used in modern high-speed communication systems.

Data Compression:

• Lossless Compression:
  • No data is lost (e.g., Huffman coding, arithmetic coding).
• Lossy Compression:
  • Some data is discarded for higher compression ratios (e.g., JPEG, MP3).

4. Antennas and Wave Propagation

Antennas are devices that radiate or receive electromagnetic waves.

Key Parameters:

• Gain:
  • Measure of an antenna’s ability to focus energy in a specific direction.
• Directivity:
  • Ratio of radiation intensity in a specific direction to the average intensity.
• Bandwidth:
  • Frequency range over which the antenna operates efficiently.

Types of Antennas:

• Dipole Antenna: Basic antenna used in communication systems.
• Parabolic Reflector: High-gain antenna for satellite communication.
• Patch Antenna: Compact, used in mobile devices and IoT.
• Yagi-Uda Antenna: Directional antenna used in TV reception.

Wave Propagation:

Wave propagation describes how electromagnetic waves travel through space.

1. Ground Wave Propagation:
   • Follows the Earth’s surface.
   • Used for AM radio broadcasting.
2. Sky Wave Propagation:
   • Reflected by the ionosphere.
   • Used for shortwave and HAM radio.
3. Line-of-Sight (LOS) Propagation:
   • Requires a clear path between transmitter and receiver.
   • Used for microwave, satellite, and mobile communication.

Applications of Communication Systems

1. Telecommunication:
   • Voice, video, and data transmission.
2. Broadcasting:
   • Radio and TV services.
3. Wireless Networks:
   • Wi-Fi, cellular communication (4G/5G).
4. Satellite Communication:
   • Global positioning systems (GPS), weather monitoring.
5. IoT and Smart Devices:
   • Machine-to-machine communication.