Communication System Fundamentals Notes

Introduction to Communication System Fundamentals

This document provides an overview of communication system fundamentals, covering modulation techniques, digital communication, M-ary encoding, pulse modulation, and digital modulation.

Course Learning Outcomes

Upon completing this course, students will be able to:

1. Apply the concept of electronic communication systems using appropriate diagrams and standard formulas (C3, PLO1).
2. Systematically assemble related communication equipment to measure appropriate signal parameters (P4, PLO5).
3. Demonstrate teamwork abilities to complete assigned tasks during practical work sessions (A3, PLO9).

Chapter 2: Modulation Techniques

This chapter covers various modulation techniques, focusing on both analog and digital modulation methods.

Learning Outcomes

Students will be able to:

1. Remember and understand modulation and demodulation.
2. Remember and understand Analog Modulation.
3. Understand digital information in a communication system.
4. Understand M-ary encoding.
5. Remember Pulse Modulation.
6. Remember and understand Pulse Code Modulation (PCM).
7. Understand and solve problems related to Sampling, Quantization, and Encoding processes in PCM.
8. Remember and apply digital modulation techniques.

Modulation and Demodulation

Modulation

Modulation is essential for transmitting low-frequency information signals over standard transmission media. It involves transforming a low-frequency baseband signal into a higher-frequency passband signal. For instance, modulating a low-frequency audio signal onto a high radio-frequency (RF) signal.

• Definition: Modulation is the process of changing one or more properties of a high-frequency analog carrier signal in proportion to the values of the information signal.
• The information (modulating) signal alters the carrier signal by changing its amplitude, frequency, or phase to produce a modulated signal.
• The modulated signal is the carrier signal that has been modified by the information signal.

Modulation Process

Modulation is performed in a transmitter by a circuit called a Modulator. The information can be in analog or digital form, and the modulator can perform either analog or digital modulation. The information signal combines with the carrier in the modulator to produce a high-frequency modulated signal.

Demodulation

Demodulation is the reverse process of modulation. It extracts the information signal from the modulated carrier signal.

• Demodulation is performed in a receiver by a circuit called a Demodulator.
• The demodulated signal is the original information signal.

Types of Modulation

Modulation techniques can be broadly classified into analog, digital, and pulse modulation.

• Analog Modulation: Both the information signal and the carrier signal are in analog waveform (Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM)).
• Digital Communication: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK), Quadrature Amplitude Modulation (QAM)
• Digital Modulation: The information signal is in digital waveform, while the carrier signal is in analog waveform.
• Pulse Modulation: The information signal is in analog waveform, while the sampling signal is in digital waveform (Pulse Code Modulation (PCM), Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM), Pulse Position Modulation (PPM), Delta Modulation, Delta Sigma Modulation).
• Pulse modulation is necessary to convert the analog signal to a digital signal and vice versa for digital transmission.
• Digital Modulation is necessary to convert the digital signal to an analog signal and vice versa for digital radio.

vc(t) = Vp \sin(2\pi ft + \Theta)

where:

• v_c(t) = time-varying sine wave of Carrier signal voltage
• V_p = peak amplitude (volts)
• f = frequency (hertz, Hz)
• \Theta = phase shift (radians)

Necessity of Modulation

Modulation is necessary for several reasons:

1. Efficient Radiation: It is challenging to radiate low-frequency signals from an antenna in the form of electromagnetic energy. Modulation increases the frequency of the information signal.
2. Signal Conversion: To convert analog signal to digital signal and vice versa for matching with communication medium and communication needs
3. Multiplexing: Information signals often occupy the same frequency band; modulation converts each station's information to a different frequency band or channel to avoid interference and to increase more number of signal.
4. Bandwidth Increase: To increase the bandwidth of the signal.
5. Antenna Size Reduction: To reduce the antenna height and size.
6. Equipment Complexity Reduction: To reduce equipment complexity.

MODEM stands for Modulator-Demodulator.

Analog Modulation

In Analog Modulation, both the Information Signal and Carrier signal are in analog waveform.

Types of Analog Modulation

1. Amplitude Modulation
2. Frequency Modulation
3. Phase Modulation

Amplitude Modulation (AM)

• Definition: Amplitude Modulation (AM) is the process of changing the amplitude (V_p) of an analog carrier signal in proportion to the amplitude of the analog information signal.
• In AM, the amplitude (V_p) of the carrier signal is varied proportionally to the information signal.
• The frequency (f) and phase (\Theta) of the carrier signal remain unchanged.
• When the information signal's amplitude increases, the carrier signal's amplitude also increases, and vice versa.

vc(t)=Vc \sin(2 \pi fc t) vm(t) = Vm \sin(2 \pi fm t)
v{AM}(t) = (Vc \sin(2 \pi fc t))(1 + m \sin(2 \pi fm t))

• The information/modulating signal modulates the amplitude of the carrier signal to produce a high-frequency AM modulated signal using an AM Modulator circuit.
• The shape of the AM modulated signal is called the AM envelope. This “envelope” contains the information signal.

Characteristics of AM Signals

• Carrier Signal (v_c): High frequency, fixed frequency, and amplitude.
• Modulating Signal (v_m): Low frequency, may contain a single frequency or multiple frequencies.
• Amplitude Modulated Signal (v_{AM}): The amplitude of the carrier signal is varied by the modulating signal. Frequency and phase remain constant.

AM Wave (Frequency Domain)

• Consists of a carrier frequency (fc) and two sidebands: upper sideband (fc + fm) and lower sideband (fc - f_m).

B_{AM} = 2B

Angle Modulation (FM & PM)

Frequency Modulation (FM) and Phase Modulation (PM) are both forms of Angle Modulation. Whenever the frequency of a carrier is varied, the phase is also varied and vice versa. FM and PM must both occur whenever either form of angle modulation is performed. The difference between FM and PM lies in which property of the carrier (the frequency or phase) is directly varied by modulating signal and which property is indirectly varied.

• Frequency Modulation (FM): Frequency is varied directly in accordance with the modulating signal
• Phase Modulation (PM): Phase is varied directly in accordance with the modulating signal

Frequency Modulation (FM)

• Definition: FM is the process of changing the frequency of an analog carrier signal in proportion to the amplitude of the analog information signal.
• In FM, the carrier amplitude and phase remain constant, while the carrier frequency is varied by the modulating signal.
• The amount of carrier frequency change is proportional to the amplitude of the information signal.

vc(t) = Vc \sin(2 \pi fc t) vm(t) = Vm \sin(2 \pi fm t)
v{FM}(t) = Vc \cos[2 \pi fc t + mf \sin(2 \pi f_m t)]

• This modulation is performed by a circuit called an FM Modulator.

Phase Modulation (PM)

• Definition: PM is the process of changing the phase of an analog carrier signal in proportion to the amplitude of the information signal.
• In PM, the carrier amplitude and frequency remain constant, while the carrier phase is varied by the modulating signal.
• As the modulating signal amplitude increases, the carrier phase increases, and vice versa.

vc(t) = Vc \sin(2 \pi fc t) vm(t) = Vm \sin(2 \pi fm t)
v{PM}(t) = Vc \cos[2 \pi fc t + mp \sin(2 \pi f_m t)]

Comparison of AM, FM, and PM

Digital Communication

Digital Communication covers digital transmission and digital radio.

• Digital Transmission: Digital signals are transferred between two or more points in a communication system. There is NO analog carrier, and the original source information may be in digital or analog form.
• Digital Radio: Transmission of digitally-modulated analog carrier signals between two or more points. The information signal and demodulated signal are in digital form, while the carrier signal and modulated signal are in analog form.

Types of Digital Communication

• Digital Transmission
• Digital Radio
• Pulse Code Modulation (PCM)
• Pulse Modulation (PAM, PWM, PPM)
• Delta Modulation
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
• Quadrature Amplitude Modulation (QAM)
• Delta-Sigma Modulation

Basic Elements of a Digital Communication System

1. Information Source: Analog (audio, voice) or Digital (teletype signal).
2. Source Encoder: Converts the information signal into digital signals by formatting and compressing the signals (e.g., PCM, ASCII).
3. Channel Encoder: Error correction coding to reduce the probability of error.
4. Digital Modulator: Converts serial bits into electric signals to transmit them on the channel (e.g., ASK, FSK, PSK).
5. Channel: Physical medium for transmitting signals from transmitter to receiver (atmosphere, twisted pair cable, optical fiber).
6. Digital Demodulator: Converts the electric signals back to serial bits.
7. Channel Decoder: Reconstructs the original serial bits from the code symbols.
8. Signal Decoder: Converts back the serial bits into the original source information signal.

Advantages of Digital Communication

1. Noise Immunity: Digital signals are less susceptible to interference caused by noise.
2. Reduction of Errors: Errors caused by noise can be detected and corrected systematically.
3. High Security: Digital systems are more secure because of data encryption
4. Interfacing: Easier to interface digital circuits compared to analog circuits.
5. Processing and Multiplexing: Ease of processing and multiplexing.
6. Cost-Effective: Inexpensive digital circuitry can be used extensively.

Applications of Digital Communication

• ADC – Analog to Digital Converter
• DAC – Digital to Analog Converter
• MODEM – Modulator-Demodulator
• Digital Camera and Video
• Broadband digital subscriber lines (DSL)
• Telemetry and Teleconferencing
• Compact Disk (CD) and Hard Disk Drive
• Personal Communication System (PCS)
• Satellite Communication System

M-ary Encoding

• M-ary is a term derived from the word binary. M = represents a digit that corresponds to the number of conditions or levels or combinations possible for a given number of binary variables, n.
• For example, a digital signal with 4 possible conditions (either voltages, levels, frequencies, phases and so on) is an M-ary system where M = 4.

M = 2^n
where:

• n = number of bits
• M = number of conditions, or levels, or combinations possible with n bits
  n = \log_2 M

Pulse Modulation

In Pulse Modulation, the Information Signal is in analog waveform, while the Sampling signal is in digital waveform. There is NO carrier signal in pulse modulation.
This modulation is necessary to convert the analog signal to digital signal for digital transmission.

Pulse Modulation Technique

• Definition: Pulse Modulation (PM) is a process of sampling the analog information signals into sampled signal before converting those into digital signals.
• Properties of sampling pulses signal such as width, position and amplitude will be varied in proportion with amplitude of information signal.

Types of Pulse Modulation

1. Pulse Width/Duration Modulation (PWM @ PDM)
2. Pulse Position Modulation (PPM)
3. Pulse Amplitude Modulation (PAM)
4. Pulse Code Modulation (PCM)

• PWM: Width of the pulses is varied proportional to the analog amplitude information signal. Amplitude and Position of pulses are constant.
• PPM: Position of the pulses is varied proportional to the analog amplitude information signal. Amplitude and Width of pulses are constant.
• PAM: Amplitude of the pulses is varied proportional to the analog amplitude information signal. Width and Position of pulses are constant.
• PCM: The analog information signal is sampled into a PAM signal and then converted to a serial n-bit binary code for transmission.

Pulse Code Modulation (PCM)

• PCM is a digital pulse modulation technique used to convert analog signals to digital signals.
• It is not a type of modulation but a form of digitally coding analog signals.
• PWM, PPM, and PAM signals are discrete-time signals, but they are not represented in a single binary digit (bits).
• PCM consists of three steps to digitize an analog signal:
  • Sampling
  • Quantizing
  • Encoding
  • APPLICATION:
• In electronic communication circuit, the PCM technique is applied at
• Analog to Digital Converter (ADC) device - in Coder
• Digital to Analog Converter (DAC) device - in Decoder
• In digital telephony Multiplexing (TDM-PCM)
• Digital PABX
• Digital Audio recording
• CD laser disks, etc.

PCM Process

1. Sampling: Converts an analog signal to a discrete-time signal (PAM signal).

   • The sampling rate (fs) must be at least twice the maximum frequency (fm) of the information signal (Nyquist Sampling Theorem).

   fs \geq 2fm
   Ts = \frac{1}{fs}

2. Quantization: Rounds off the amplitudes of the sampled signal to a countable number of quantization levels (L).

   L = 2^n

   where:

   • n = number of bits per level
   • L = number of finite quantization levels

3. Encoding: Translates the quantized signal into a decimal code number and then converts this number to a representative binary sequence. The Bit Rate for PCM is:
   Bit Rate = f_s \times n

PCM Decoder

PCM Decoder follows these steps:

• We use a decoder and hold circuit that holds the amplitude value of a pulse till the next pulse arrives. This will produce a staircase PAM signal.
• We pass this PAM signal through a low pass filter which has the same cutoff frequency as the original information signal at sender. The filter will smooth the staircase amplitude of PAM signals into an analog signal.

Digital Modulation

Digital Modulation is the process of changing one of the characteristics of an analog carrier signal based on the information in digital data.

Types of Digital Modulation

1. Amplitude Shift Keying (ASK)
2. Frequency Shift Keying (FSK)
3. Phase Shift Keying (PSK)
4. Quadrature Amplitude Modulation (QAM)

• ASK: Amplitude (V_p) of the analog carrier signal is varied proportional to the digital information signal.
• FSK: Frequency (f) of the analog carrier signal is varied proportional to the digital information signal.
• PSK: Phase (\Theta) of the analog carrier signal is varied proportional to the digital information signal.
• QAM: Both amplitude (V_p) and phase (\Theta) of the analog carrier signal are varied proportional to the digital information signal.