6.0 System Performance and Integrity

6.1 Noise in Communication Systems

Noise is any unwanted signal that interferes with and corrupts the original message signal, typically introduced in the channel or at the receiver. It is random, unpredictable, and limits a system’s operating range and sensitivity.

• Types of Noise:
  • External Noise: Originates from sources outside the system, such as atmospheric noise, solar noise, and industrial noise.
  • Internal Noise: Generated by the receiver components themselves, such as thermal agitation noise and shot noise.
• Signal-to-Noise Ratio (SNR): The ratio of signal power to noise power. A higher SNR indicates better quality of the received output.
• Figure of Merit (F): The ratio of output SNR to input SNR, describing the performance of a device in the presence of noise.

6.2 Multiplexing for Channel Efficiency

Multiplexing is the process of combining multiple signals into a single signal for transmission over a shared medium. A Multiplexer (MUX) performs this combination at the transmitter, while a Demultiplexer (DEMUX) separates the signals at the receiver.

• Frequency Division Multiplexing (FDM): An analog technique where the channel’s bandwidth is divided into separate frequency bands, with each signal assigned to a different band. Used in traditional TV and radio.
• Wavelength Division Multiplexing (WDM): An analog technique used in optical fiber communications where data streams are transmitted on different wavelengths of light.
• Time Division Multiplexing (TDM): A digital technique where the channel’s time is divided into slots, and each signal is allotted a specific time slot.
  • Synchronous TDM: Each device is assigned a fixed time slot in every frame, regardless of whether it has data to send.
  • Asynchronous TDM: Time slots are allocated dynamically to devices that have data to transmit, making it more efficient than synchronous TDM.

6.3 Information Theory and Coding

Information Theory is a mathematical field that studies the quantification, storage, and communication of information. A key concept is Entropy, defined by Claude Shannon as a measure of the average information content or uncertainty of a source.

• Source Coding: Aims to reduce redundancy in a data source to improve transmission efficiency. The average code-word length for any source encoding is bounded by its entropy.
• Channel Coding: Intentionally introduces controlled redundancy into the data sequence to improve the reliability of communication over a noisy channel by allowing for error detection and correction.

6.4 Spread Spectrum Modulation for Secure Communication

Spread Spectrum is a class of modulation techniques used to provide secure communication by spreading the signal’s energy over a very wide frequency band. This makes the signal difficult to detect, interfere with, or jam. These techniques use a Pseudo-Noise (PN) sequence, a coded sequence of 1s and 0s, to spread the signal.