Communication Systems Notes — B.Tech ECE Sem 4

Communication Systems — Introduction

Communication System Block Diagram:

Info Source → Transmitter → Channel → Receiver → Destination
               (Modulate)   (Noise)   (Demodulate)

Channel Types:

• Wired: coaxial cable, fiber optic, twisted pair
• Wireless: radio waves, microwave, satellite, infrared

Frequency Bands: | Band | Frequency | Use | |------|-----------|-----| | LF | 30-300 kHz | AM radio | | MF | 300kHz-3MHz | AM broadcast | | HF | 3-30 MHz | Shortwave radio | | VHF | 30-300 MHz | FM, TV | | UHF | 300MHz-3GHz | TV, 4G | | SHF | 3-30 GHz | Satellite, 5G |

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1. Amplitude Modulation (AM)

Message: m(t) = Am·cos(2πfm·t)
Carrier: c(t) = Ac·cos(2πfc·t)

AM Signal: s(t) = Ac[1 + ka·m(t)]·cos(2πfc·t)
         = Ac[1 + μ·cos(2πfm·t)]·cos(2πfc·t)

Modulation Index: μ = Am/Ac (0 ≤ μ ≤ 1)
μ > 1 → over-modulation → distortion!

Spectrum: 3 components
  fc (carrier), fc+fm (USB), fc-fm (LSB)
  Bandwidth = 2fm

Power:
  Pt = Pc(1 + μ²/2)
  Efficiency = (μ²/2)/(1+μ²/2)
  At μ=1: efficiency = 33.3% (wasteful!)

Types:

• DSB-SC: Double Sideband Suppressed Carrier (efficient)
• SSB: Single Sideband (halved bandwidth)
• VSB: Vestigial Sideband (TV broadcasting)

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2. Frequency Modulation (FM)

Instantaneous frequency: fi = fc + kf·m(t)
FM Signal: s(t) = Ac·cos(2πfc·t + 2πkf·∫m(t)dt)

Frequency Deviation: Δf = kf·Am
Modulation Index: β = Δf/fm

FM Bandwidth (Carson's Rule):
  BW = 2(Δf + fm) = 2fm(β + 1)

For β >> 1 (wideband FM): BW ≈ 2Δf
For β << 1 (narrowband FM): BW ≈ 2fm (like AM)

Commercial FM: fc=88-108MHz, Δf=75kHz, fm max=15kHz
  BW = 2(75+15) = 180kHz per station

FM Advantages:

• Better noise immunity (SNR improvement)
• Constant amplitude → no power variation
• Used: FM radio, VHF voice communication

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3. Noise in Communication

Thermal Noise: N = kTB
  k = Boltzmann's constant (1.38×10⁻²³ J/K)
  T = Temperature (Kelvin)
  B = Bandwidth (Hz)

SNR = Signal Power / Noise Power = S/N
SNR_dB = 10 log₁₀(S/N)

Figure of Merit (comparison):
  AM DSB-LC: SNR_o = μ²Ac²/(2N₀B) / (1+μ²/2)
  FM:        SNR_o = 3β²(β+1)·(S/N)  ← much better!
  FM improvement over AM increases with β

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4. Digital Modulation

ASK (Amplitude Shift Keying)

0 → carrier off (or low amplitude)
1 → carrier on (full amplitude)
OOK (On-Off Keying) = binary ASK
Bandwidth = bit rate
Susceptible to amplitude noise

FSK (Frequency Shift Keying)

0 → f1 (e.g., 1070 Hz)
1 → f2 (e.g., 1270 Hz)
Bandwidth = 2Δf + Rb
Noise immune (constant amplitude)
Old modems used FSK

PSK (Phase Shift Keying)

BPSK (Binary PSK):
  0 → 0° phase
  1 → 180° phase
  1 bit/symbol, highest noise immunity

QPSK (Quadrature PSK):
  00 → 45°
  01 → 135°
  11 → 225°
  10 → 315°
  2 bits/symbol, same BW as BPSK
  Used in satellite, CDMA

8-PSK: 3 bits/symbol, 8 phases

QAM (Quadrature Amplitude Modulation)

16-QAM: 4×4 constellation → 4 bits/symbol
64-QAM: 8×8 → 6 bits/symbol
256-QAM: 16×16 → 8 bits/symbol (cable TV, 4G LTE)
1024-QAM: 10 bits/symbol (5G, WiFi 6)

Higher order = more bits/Hz but needs better SNR
Spectral Efficiency = bits/symbol × 1/BW

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5. OFDM — Orthogonal Frequency Division Multiplexing

Wideband channel → N narrow subcarriers
Each subcarrier: low data rate, long symbol time
All subcarriers transmitted simultaneously

Key benefits:
1. ISI immunity: symbol duration >> delay spread
2. Cyclic prefix: absorbs multipath echoes
3. Simple equalization: 1-tap per subcarrier
4. Flexible spectrum: turn off subcarriers

OFDM Implementation:
  IFFT (Tx) / FFT (Rx) — efficient computation
  N subcarriers, spacing = 1/T (orthogonal)

LTE: 15kHz subcarrier spacing, 2048-point FFT
5G NR: 15/30/60/120/240 kHz (flexible numerology)
WiFi 802.11a: 52 subcarriers, 20MHz channel

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6. Mobile Networks

4G LTE

Multiple Access: OFDMA (Downlink), SC-FDMA (Uplink)
Peak: 150 Mbps DL, 50 Mbps UL (Category 4)
Latency: ~30ms
Modulation: up to 256-QAM
MIMO: Multiple antennas → spatial multiplexing

5G NR (New Radio)

Frequency bands:
  Sub-6GHz (FR1): wide coverage, penetration
  mmWave (FR2, 24-100GHz): ultra high speed, short range

Key technologies:
  Massive MIMO: 64-256 antennas
  Beamforming: directional transmission
  Network Slicing: virtual networks for different services
  MEC (Multi-access Edge Computing): low latency

Peak speed: 20 Gbps DL
Latency: <1ms (URLLC use case)
Use cases: eMBB, URLLC, mMTC

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Exam Important Questions

1. AM signal ka mathematical expression aur bandwidth calculate karo
2. FM vs AM — noise immunity aur bandwidth comparison
3. BPSK, QPSK, 8-PSK ka constellation diagram draw karo
4. Shannon's capacity theorem explain karo with example
5. OFDM kyun use karte hain? Cyclic prefix ka kaam kya hai?
6. 16-QAM mein kitne bits per symbol? Advantage kya hai?
7. 4G LTE aur 5G NR mein kya technical differences hain?
8. Modulation index kya hai? Over-modulation problem kya hai?

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Viva Questions

Q: Superheterodyne receiver kya hai? A: RF signal → mixer + local oscillator → fixed IF (Intermediate Frequency) signal. IF pe filtering aur amplification zyada stable aur cheap hai. AM radios mein IF = 455kHz, FM = 10.7MHz. Almost all radios use this.

Q: Nyquist sampling theorem kya kehta hai? A: Sampling frequency ≥ 2 × maximum signal frequency. CD audio: 20kHz audio → 44.1kHz sampling rate. Undersampling → aliasing (wrong frequencies appear).

Q: Spread Spectrum kyun use karte hain? A: LPI/LPD (Low Probability of Intercept/Detection) — military. Jamming resistance. Multiple users same spectrum — CDMA (3G). GPS bhi spread spectrum use karta hai.