Why This Topic Is Important for ECET 2026
Multiplexing is one of the most repeated topics in ECET ECE Communication Systems.
It is used in TV broadcasting, telephone networks, fiber systems, 5G, satellite links, OFDM, Wi-Fi, and IoT communication.
Questions come from:
- Types of multiplexing
- Bandwidth usage
- Guard bands
- Time slot calculations
- Numerical problems on sampling & channel capacity
Mastering multiplexing helps you score full marks from this chapter.
📘 Concept Notes (Deep Explanation)
1. What is Multiplexing?
Multiplexing is a technique where multiple signals share the same communication channel without interfering with each other.
Goal:
- Increase efficiency
- Reduce cost of communication
- Utilize bandwidth properly
Two broad categories:
- Analog Multiplexing
- Digital Multiplexing
2. Frequency Division Multiplexing (FDM)
- Multiple signals are transmitted simultaneously in a channel
- Each signal is assigned a unique frequency band
- Used in: Radio, TV broadcasting, Cable TV, Satellite
Each channel has a guard band to avoid overlap.
Guard band example formula:
Example:
Three FDM channels each with 4 kHz, guard band of 1 kHz
Total bandwidth:
3. Time Division Multiplexing (TDM)
- Signals occupy the same channel at different time slots
- Used in: Digital telephony, PCM, 4G/5G, Optical systems
Frame time formula:
Example:
4 users, each requires 1 ms sampling time
Frame time =
4. Synchronous TDM vs. Statistical TDM
Synchronous TDM
- Each user gets fixed time slot, even if no data
- Wastage occurs
Statistical TDM
- Slots assigned only when needed
- Higher efficiency
5. WDM – Wavelength Division Multiplexing
Used in fiber optic communication.
- Each signal is a different wavelength (λ)
- Similar to FDM but in optical domain
Channel spacing:
6. Code Division Multiplexing (CDMA)
- All users transmit simultaneously
- Differentiated by unique codes
- Used in 3G, GPS
Orthogonality condition:
⚙️ Formulas List (QuickLaTeX Only)
🔟 10 MCQs (Conceptual + Numerical)
Q1. Multiplexing is used to:
a) Increase power
b) Reduce bandwidth
c) Share a channel among users
d) Improve noise immunity
Q2. FDM requires ______ for separating channels.
a) Time slots
b) Guard bands
c) Chips
d) Codes
Q3. 4 FDM channels each require 5 kHz and guard band = 1 kHz. Total BW?
Q4. TDM frame consists of 8 users, each needs a sampling period of 0.5 ms. Find frame time.
Q5. In TDM, if slot time = 200 μs and 6 slots exist, what is frame time?
Q6. WDM is used in:
a) Coaxial cable
b) Fiber optics
c) Wireless
d) Coax + Wireless
Q7. CDMA uses ______ to separate users.
a) Different frequencies
b) Time slots
c) Unique codes
d) Guard bands
Q8. If sampling frequency is 8 kHz and 8 channels exist in TDM PCM, what is bit rate?
(bits/sample = 8)
Q9. Orthogonal codes satisfy:
a) Sum = 1
b) Sum = -1
c) Sum = 0
d) Sum = 10
Q10. In FDM, if guard band is reduced to zero, what occurs?
a) No distortion
b) Overlapping of channels
c) Higher SNR
d) Fixed time slots
✅ Answer Key (WordPress-friendly table)
| Q No | Answer |
|---|---|
| 1 | c |
| 2 | b |
| 3 | 23 kHz |
| 4 | 4 ms |
| 5 | 1.2 ms |
| 6 | b |
| 7 | c |
| 8 | 512 kbps |
| 9 | c |
| 10 | b |
🧠 Explanations (Step-by-Step)
Q1 → c
Multiplexing shares one channel among multiple users. Others are unrelated.
Q2 → b
FDM assigns different frequency bands and uses guard bands to prevent overlap.
Q3
4 channels × 5 kHz = 20 kHz
Guard bands = 3 × 1 kHz = 3 kHz
Total = 23 kHz
Q4
Q5
6 slots × 200 μs
= 1200 μs = 1.2 ms
Q6 → b
WDM works ONLY in fiber optics.
Q7 → c
CDMA separates users by unique spreading codes.
Q8
= 8 × 8000 × 8
= 512,000 bps = 512 kbps
Q9 → c
Orthogonal codes produce zero cross-correlation.
Q10 → b
Zero guard band → channels overlap → severe interference.
🎯 Motivation / Why Practice Matters
Multiplexing questions in ECET are fast-solve, formula-based, and perfect for boosting your score. Practicing daily helps you:
- Quickly calculate bandwidth, frame time, bit rate
- Understand differences between FDM, TDM, WDM, CDMA
- Gain accuracy in numerical MCQs
- Build strong communication fundamentals for GATE-level questions
If you master multiplexing, you secure easy marks in the exam.
📲 CTA
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