Chapter 5 Analog Transmission

Chapter 5 of Data Communications and Networking by Behrouz A. Forouzan, titled "Analog Transmission", focuses on converting both digital and analog data into analog signals for transmission over a bandpass channel. Below are the detailed notes:

1. Digital-to-Analog Conversion

• Definition: Digital-to-analog conversion involves changing the characteristics of an analog signal (carrier) based on the information in digital data.

• Types of Digital-to-Analog Modulation:

  1. Amplitude Shift Keying (ASK): The amplitude of the carrier signal is changed to represent digital data. Only two levels are commonly used, representing binary data.

  2. Frequency Shift Keying (FSK): The frequency of the carrier signal is varied to represent data. The phase and amplitude remain constant.

  3. Phase Shift Keying (PSK): The phase of the carrier is altered to represent data. Both amplitude and frequency remain unchanged.

  4. Quadrature Amplitude Modulation (QAM): Combines ASK and PSK by modulating both the amplitude and phase of the carrier. QAM is more efficient and widely used.

• Carrier Signal: This is the high-frequency signal altered by modulation to carry data. Modulation changes the amplitude, frequency, or phase of the carrier signal.

• Bit Rate and Baud Rate:

  • Bit Rate: The number of bits transmitted per second.

  • Baud Rate: The number of signal elements transmitted per second. For analog transmission, baud rate can be less than or equal to the bit rate, depending on the modulation technique.

2. Analog-to-Analog Conversion

• Definition: Analog-to-analog conversion is the process of modulating an analog signal to represent another analog signal.

• Types of Analog Modulation:

  1. Amplitude Modulation (AM): The amplitude of the carrier is varied in accordance with the modulating signal. The frequency and phase remain constant.

  2. Frequency Modulation (FM): The frequency of the carrier signal is varied by the amplitude of the modulating signal. Amplitude and phase stay constant.

  3. Phase Modulation (PM): The phase of the carrier is varied based on the amplitude of the modulating signal, while the frequency and amplitude remain unchanged.

3. QAM (Quadrature Amplitude Modulation)

• Explanation: QAM is the most efficient form of modulation, combining ASK and PSK. It uses two carriers (in-phase and quadrature) to create signal elements, improving data transmission efficiency by transmitting more bits per signal element.

4. Bandwidth Requirements

• The required bandwidth depends on the signal rate. For instance:

  • ASK and PSK need a bandwidth proportional to the signal rate.

  • FSK requires additional bandwidth due to frequency differences between carrier signals.

5. Summary

• Digital-to-analog conversion methods (ASK, FSK, PSK, and QAM) modulate the carrier signal to represent digital data. The most efficient method is QAM.

• Analog-to-analog conversion methods (AM, FM, PM) are used to modulate analog signals over bandpass channels.

This chapter provides an in-depth understanding of how digital and analog data can be transmitted using modulation techniques over analog channels .