---
title: "Short note: Frequency division multiplexing"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:short-note-fdm"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdm
site: BCT Engineering
---

# Short note: Frequency division multiplexing

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2073 Bhadra (CS I), 5 marks; 2070 Magh (CS I), 5 marks (asked 2 times)
- Video: [Frequency Division Multiplexing (FDM) Explained](https://www.youtube.com/watch?v=UwWmDwbsDBs) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdm
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Write a short note on Frequency Division Multiplexing (FDM).

## Short answer

FDM transmits multiple signals simultaneously by assigning each a separate frequency band via modulation. Each message band-limited, modulates subcarrier (often SSB-SC), BPFs limit slots with guard bands, summed and transmitted. Receiver uses BPFs and demodulators. Used in analog telephony, broadcasting, cable TV, satellite FDMA, OFDM. Advantages: no sync needed, simple for analog. Disadvantages: crosstalk, many sharp filters, wasted guard bands.

## Answer

**Frequency Division Multiplexing (FDM)** sends several signals at the same time over one channel by giving each signal a **separate frequency band**. Each message modulates a different carrier; the modulated signals are added and transmitted together.

```text
Tx: m1->[Mod fc1]-+          Rx: +->[BPF1]->[Demod]->m1
    m2->[Mod fc2]-+->(sum)->ch->-+->[BPF2]->[Demod]->m2
    m3->[Mod fc3]-+              +->[BPF3]->[Demod]->m3
```

**Working**
1. Each message is band-limited by a LPF.
2. It modulates its own subcarrier (often SSB-SC to save bandwidth).
3. Band-pass filters limit each modulated signal to its slot; **guard bands** separate adjacent slots.
4. All are summed and sent (possibly modulating a main carrier).
5. At the receiver, band-pass filters separate the channels and demodulators recover the messages.

Total bandwidth $\approx N \times (\text{channel bandwidth}) + \text{guard bands}$; e.g. 12 voice channels in 4 kHz slots need 48 kHz (telephone group, 60–108 kHz).

**Applications:** analog telephone trunks (group, supergroup), AM/FM broadcasting, cable TV, FM stereo, satellite FDMA, and OFDM in Wi-Fi/4G as a digital form.

**Advantages:** no synchronization between channels, simple for analog signals, continuous transmission.
**Disadvantages:** crosstalk from nonlinearity (intermodulation) and imperfect filters, needs many sharp filters and stable oscillators, wasted guard bands, and fading affects a whole channel.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In FDM each message:
   A. Is time-sliced
   B. Modulates a different carrier so signals occupy separate bands
   C. Shares the same carrier
   D. Is coded with PN sequence
   Answer: B. Modulated signals are summed and sent together. (Key point: Working)

2. Guard bands in FDM are used to:
   A. Increase data rate
   B. Prevent adjacent channel interference
   C. Provide synchronization
   D. Boost power
   Answer: B. They separate adjacent slots given imperfect filters. (Key point: Guard bands)

3. At the FDM receiver channels are separated by:
   A. Band-pass filters, then demodulators
   B. Time gates
   C. Correlators
   D. Decoders only
   Answer: A. BPFs isolate each slot. (Key point: Receiver)

4. 12 voice channels in 4 kHz slots need:
   A. 12 kHz
   B. 48 kHz
   C. 4 kHz
   D. 96 kHz
   Answer: B. 12 x 4 = 48 kHz (60-108 kHz group). (Key point: Bandwidth)

## Related in Communication Systems

- [FDM and its method in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-method) — Communication Systems, chapter 5
- [FDM principle; SCPC and DAMA FDMA](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-scpc-dama) — Communication Systems, chapter 5

## Related in other subjects

- [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3×
- [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×
- [Need for multiplexing and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-need-wdm) — Telecommunications, chapter 3
- [Comparison of FDM, TDM and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-wdm) — Telecommunications, chapter 3, asked 2×