Digital Communication and Switching Overview

Digital Communication and Switching

• Course: Digital Communication and Switching
• Semester: I AY 2024-25
• Lecturer: Lenin Joseph (LJ)
• Date: Dec 24, 2024

Outcome #1

• Describe various elements of Digital Communication systems.

Electronic Communication Systems

• A collection of electronic circuits enabling transmission and/or reception of information electrically.

Block Diagram of a Communication System

• Information Source → Transmitter → Channel (Communication Medium) → Receiver → User of Information
• Includes a message signal, transmitted signal, received signal, estimate of message signal, and noise

Analog and Digital Transmission

• Analog transmission: Transfer of continuously varying analog signal.
• Digital transmission: Transfer of discrete messages.
• Digital communication: Messages represented by pulses (baseband) or wave forms (passband) using digital modulation.

Block Diagram of Digital Communication System

• Shows the components involved in a digital communication system and their interconnections (Analog Information Source, A/D Converter, Source Encoder, Channel Encoder, Modulator, Channel, Demodulator, Source Decoder, D/A Converter, Digital Information Sink)

Elements of Digital Communication System - Information Sources

• Message can be analog (e.g., voice) or digital (e.g., computer data).

Analog to Digital Converter

• Converts analog signal to discrete digital form (0s and 1s) through Sampling, Quantization, and Coding.

Sampling

• Process of converting continuous-time signal to discrete-time signal.
• "Samples" taken at regularly spaced intervals.

Analog to Digital Conversion - Nyquist Sampling Theorem

• Original signal can be reconstructed at the receiver with minimal distortion if the sampling rate is equal to or greater than twice the maximum frequency of information signal.
• fs ≥ 2fm(max), where fs = minimum Nyquist sampling rate, and fm(max) = maximum information signal frequency.
• For bandwidth limited signals, Nyquist rate is greater than or equal to twice the bandwidth (2 * (f2 – f1)), or where f2 and f1 are upper and lower limits of the bandwidth.

Analog to Digital Conversion - Aliasing

• Distortion called "aliasing" or "fold-over distortion" occurs if the sampling rate is less than twice the signal frequency.
• A low-pass filter, called an antialiasing filter, is usually placed before the A/D converter to eliminate aliasing.

Analog to Digital Conversion - Sampling Methods

• Ideal Sampling: Impulse at each sampling instant

• Natural Sampling: Sample amplitude follows original signal during sample duration

• Flat-top Sampling: Analog signal sampled using a sample-and-hold circuit, resulting in a sample with a constant amplitude over the sampling interval.

Analog to Digital Conversion - Example

• Calculating signal frequency, fourth harmonic, and minimum sampling frequency (Nyquist rate) from a rectangular wave's period.

Analog to Digital Conversion - Quantization

• Process of assigning analog signal samples to predetermined discrete levels.
• Divides the voltage range into a number of levels (L).
• Number of quantization levels (L) depends on the number of bits per sample (n): L = 2ⁿ.

Analog to Digital Conversion - Quantization Step/Zone Width

• Δ = (S_max - S_min) / L, where Δ is quantization step/zone width, S_max is the maximum signal amplitude, S_min is the minimum signal amplitude, and L is the number of quantization levels

Analog to Digital Conversion - How Quantization is Done

• Divide the signal voltage range into zones, calculate the midpoint of each zone, and approximate the values of the original signal within a zone to the midpoint value.

Analog to Digital Conversion - Quantization Error

• The difference between the original value and its quantized value.
• Smaller error with more quantization levels, thus increasing the data rate (D = fs * n).

Analog to Digital Conversion - Coding

• Assigning unique binary code to each quantization level.
• Number of bits per sample (n) determined by the number of quantization levels (L): n = log₂ L.

Source Encoder

• Accepts digitized source and encodes it into a shorter digital signal.
• Source coding (or Entropy Encoding) compresses data by replacing frequently occurring codes with shorter ones and less frequent ones with longer ones.
• Aims to remove redundancy, reducing bandwidth requirements.

Source Encoding Types

• Lossless compression - retains all original information, using techniques like Huffman coding, run-length encoding and Lempel-Ziv.
• Lossy compression - discards some information to reduce data size, as in MPEG and JPEG compression.

Channel Encoder

• Accepts the output of the source encoder and systematically adds extra bits for error control.
• Error control: detect and/or correct errors in the information-bearing bits.

Channel Coding Methods

• Block coding: Encodes blocks of information bits with additional error control bits. Ex. Hamming codes.
• Convolution coding: Continuously interleaves information and error control bits, encoding in a continuous fashion. Example: Viterbi algorithm.

Modulator

• Converts coded bit stream to electrical waveform suitable for transmission over the channel.
• Digital modulation impresses digital symbols onto a signal suitable for transmission.
• Baseband modulation used for short distances.
• Line codes create square pulses to represent different symbols, and avoid ambiguity for recovery upon reception. (ex. NRZ, RZ, Manchester, Miller Codes)
• Passband modulation (carrier modulation): used for long distances and wireless transmission; adjusts signal parameters (amplitude, frequency, phase) of a carrier signal. (ex. ASK, FSK, PSK, QAM)

Communication Channel

• Transmission path, connecting source & destination.
  • Physical medium, with limited bandwidth.
  • Subject to amplitude and phase distortion, attenuation, and noise. -Types: Telephone channels, coaxial cables, optical fibers, microwave radio, satellite channels
  • Important parameters: Signal-to-Noise Ratio (SNR), Bandwidth

Communication Channel Models

• AWGN (Additive White Gaussian Noise) Channel: Simplest model, adding noise with flat and linear characteristics.

  • No bandwidth limitations, unless signal bandwidth is similar to channel bandwidth.
  • Practical channels include line-of-sight radio, wideband coaxial cables under good weather circumstances.

• Bandlimited Channel: Signal bandwidth smaller than the channel bandwidth; channel has a limited bandwidth.

  • Intersymbol Interference (ISI): Signal pulses are distorted, extending beyond their intended time intervals, impairing correct data recovery.
  • Techniques exist to combat this interference. (channel equalization)

• Fading Channel: Signal amplitude and phase change rapidly over short periods or distances due to multipath interference.

  • Multipath waves combine, leading to resultant signal variations (amplitude and phase) - experienced by wireless communications.
  • Doppler shift: Time-varying effect on multipath components due to movement of objects, producing phase and amplitude changes.

Demodulator

• Recovers the message from the received modulated signal, removing the carrier wave and reversing the modulating process.
• Outcome is a coded bit stream.

Channel Decoder

• Recovers information bits from the coded binary stream.
• Error detection and correction may be performed during recovery.

Source Decoder

• Decompresses the binary stream back to its original format.

Digital to Analog Converter

• Converts the digital data to an analog signal needed by the information sink.

Information Sink

• Destination for the received information (e.g., human, computer).

Advantages of Digital Communication Systems

• Noise Immunity
• Flexible Hardware Implementation
• Ease of Regeneration
• Ease of Multiplexing
• Reliable, cheaper digital circuits
• Strong signal processing capability
• Capability of error detection and correction

Disadvantages of Digital Communication Systems

• Large System Bandwidth
• System Synchronization

This quiz covers the fundamental concepts of Digital Communication systems, including the elements, transmission methods, and the block diagram of communication systems. Students will explore both analog and digital transmission techniques used in electronic communication.