Introduction to Analog and Digital Communications
(A concise, structured overview for quick reference)
1. Fundamental Concepts
| Concept |
Definition |
Typical Use |
| Signal |
A physical quantity that conveys information (e.g., voltage, current, electromagnetic wave). |
Both analog and digital systems. |
| Medium (Channel) |
The physical path through which a signal travels (wired, wireless, optical fiber). |
Determines bandwidth, attenuation, noise. |
| Modulation |
Process of varying a carrier to embed information. |
Essential for transmission over limited‑bandwidth channels. |
| Demodulation |
Reverse of modulation; extracts the original information from the received carrier. |
Performed at the receiver. |
| Noise |
Unwanted random disturbances that corrupt the signal (thermal, shot, interference). |
Impacts error rates and required SNR. |
2. Analog Communication
2.1 What Is Analog?
- Continuous‑time, continuous‑amplitude representation.
- Information is encoded directly onto the amplitude, frequency, or phase of a carrier wave.
2.2 Key Analog Modulation Schemes
| Scheme |
Carrier Parameter Varied |
Typical Bandwidth (relative to message) |
Advantages |
Disadvantages |
| Amplitude Modulation (AM) |
Amplitude |
≈ 2 × message bandwidth |
Simple transmitter/receiver |
Poor noise immunity |
| Frequency Modulation (FM) |
Frequency |
≈ 2 × (Δf + fm) |
Good noise resistance, capture effect |
Larger bandwidth |
| Phase Modulation (PM) |
Phase |
Similar to FM |
Efficient for digital data (see PSK) |
Requires coherent detection |
2.3 Analog System Block Diagram
Message (baseband) → Encoder (optional) → Modulator → Channel → Demodulator → Decoder → Recovered Message
2.4 Performance Metrics
- Signal‑to‑Noise Ratio (SNR) – ratio of signal power to noise power.
- Carrier-to-Noise Ratio (C/N) – used for carrier‑based systems.
- Bandwidth Efficiency (η) – bits/s per Hz (low for pure analog).
- Error Probability – expressed as Bit Error Rate (BER) for analog‑digital hybrid systems.
3. Digital Communication
3.1 What Is Digital?
- Discrete‑time, discrete‑amplitude representation (bits: 0/1).
- Information is first quantized and encoded before transmission.
3.2 Digital Modulation (Key Schemes)
| Scheme |
Symbol Set |
Bits per Symbol |
Typical Bandwidth |
Notes |
| ASK (On‑Off Keying) |
{0, A} |
1 |
Same as baseband |
Simple, vulnerable to amplitude noise |
| FSK (Frequency Shift Keying) |
{f₀, f₁} |
1 |
Slightly larger than baseband |
Better noise immunity |
| PSK (Phase Shift Keying) |
{0°, 180° …} |
1–2 (BPSK, QPSK) |
Efficient |
Requires coherent detection |
| QAM (Quadrature Amplitude Modulation) |
Constellation of amplitude/phase points |
2–6+ (16‑QAM, 64‑QAM…) |
High spectral efficiency |
Sensitive to both amplitude & phase noise |
| OFDM (Orthogonal Frequency Division Multiplexing) |
Multiple orthogonal sub |
|
|
|