Lecture 11: Digital Modulation

Overview

• Topic: Digital Modulation (Digital Band Pass Modulation)
• Comparison: Equivalent of lectures 2 to 6 but for digital versions.
• Agenda:
  • Digital AM, FM, PM
  • Modulation and Demodulation
  • Frequency Domain Representation
  • Multi-level modulation (M-ary systems)
  • Introduction to Multiplexing (next lecture)

Review of Previous Lecture

• Topics Covered:
  • Digitization: Sampling, Quantization
  • Nyquist Rate
  • Pulse Modulation and Line Coding
  • Pulse Code Modulation (PCM)
  • Channel Capacity and Shannon-Hartley Theorem

Introduction to Digital Modulation

• Distinction:
  • Baseband Modulation: (Discussed in Lecture 10)
  • Band Pass Modulation: (Lecture 11, involves a carrier, typically used in wireless communication)
• Will cover modulation (how we encode data into a carrier) and demodulation (how we decode the received signal).

Digital Modulation Techniques

• Amplitude Shift Keying (ASK): Amplitude of the carrier is adjusted based on the digital pulse (High = 1, Low = 0).
• Frequency Shift Keying (FSK): Frequency of the carrier is adjusted based on the digital pulse (High freq = 1, Low freq = 0).
• Phase Shift Keying (PSK): Phase of the carrier changes by 180 degrees when digit transitions (1 to 0 or vice versa).

Quadrature Amplitude Modulation (QAM)

• Advantage: Can send multiple bits in a single electromagnetic wave, enhancing data transfer speed and efficiency.
• Example: 16-QAM packs 4 bits together to transmit as a single electromagnetic wave, with different phase angles and amplitudes representing various bit combinations.

Detailed Look at Each Modulation Technique

Amplitude Shift Keying (ASK)

• Binary ASK: Uses two levels (1 and 0) for amplitude.
• Characteristic: On-Off Keying (off = 0, burst of carrier = 1).
• Demodulation:
  • Non-Coherent Detection: Uses envelope detection.
  • Coherent Detection: Uses synchronous detection with local oscillators.

Frequency Shift Keying (FSK)

• Binary FSK: Two frequencies (High freq for 1, Low freq for 0).
• Generation: Can be visualized as two ASK signals added together.
• Demodulation:
  • Non-Coherent Detection: Utilizes band-pass filters and envelope detectors.

Phase Shift Keying (PSK)

• Binary PSK (BPSK): 180-degree phase shifts based on digit pulses.
• Differential PSK (DPSK): Encodes data as phase changes rather than absolute phase values.
• Demodulation:
  • Techniques vary based on type (BPSK, DPSK).

Enhancing Data Throughput

• M-ary Systems: Utilizes more than two levels to increase data rate.
• 4-ASK: Uses four amplitude levels to transmit two bits per symbol.
• QPSK: Uses four phase levels to transmit two bits per symbol.
• QAM: Combines amplitude and phase variations to transmit multiple bits per symbol (e.g., 16-QAM for 4 bits).

Applications

• ASK: Used in IR modulation (TV remotes).
• FSK: Caller ID, Bluetooth (partially).
• PSK: Wi-Fi, Enhanced GSM.
• QAM & QPSK: GPS, Mobile communications (4G/5G), Broadband, Satellite communications.

Closing Remarks

• Next Lecture: Multiplexing
• Upcoming Test: 12th May, 15% of module, two parts (A & B) with different question formats.
• Preparation: Ensure to try out the demo provided for part B.