---
title: "FDM principle; SCPC and DAMA FDMA"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:fdm-scpc-dama"
source: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-scpc-dama
site: BCT Engineering
---

# FDM principle; SCPC and DAMA FDMA

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2071 Bhadra (CS I), 2+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/fdm-scpc-dama
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Describe the principle of frequency division multiplexing (FDM). Briefly explain SCPC and DAMA types of FDMA.

## Short answer

FDM shares a channel by assigning each signal a separate frequency band. SCPC allocates a dedicated carrier per channel, whereas DAMA dynamically assigns carriers from a common pool on demand, improving efficiency for sporadic traffic.

## Answer

#### Principle of FDM
**Frequency division multiplexing** shares the bandwidth of one channel among many signals by giving each signal its own frequency band at the same time. Each message is band-limited and modulates a different carrier $f_{c1}, f_{c2}, \dots$. The modulated signals are added and sent together. Guard bands between bands prevent overlap. At the receiver, band-pass filters tuned to each band separate the signals, and demodulators recover the messages.

```text
 Spectrum of composite FDM signal
   | ch1 |g| ch2 |g| ch3 |g| ch4 |
   fc1      fc2     fc3     fc4      f
   (g = guard band)
```

When the same idea is used to let many earth stations share one satellite transponder, it is called **FDMA**.

#### SCPC (Single Channel Per Carrier)
- Each voice or data channel gets its **own carrier** inside the transponder band, instead of first being grouped into a multi-channel FDM baseband.
- Example: a 36 MHz transponder divided into narrow channel slots (e.g. 45 kHz each).
- Often used with **voice activation**: the carrier is switched off during silence, which saves transponder power.
- Suits thin-route traffic (small stations with only a few circuits).
- In **pre-assigned SCPC**, channels are fixed to station pairs, so they sit idle when not in use.

#### DAMA (Demand Assigned Multiple Access)
- Channels are not fixed. All stations share a **common pool of carrier frequencies**, and a channel is given to a station only when a call is requested. It is returned to the pool when the call ends.
- A common signalling channel (central or distributed control) handles the requests.
- Example: the INTELSAT **SPADE** system (SCPC, PCM, multiple access, demand assignment) uses about 800 SCPC channels at 45 kHz spacing in a 36 MHz transponder.
- Advantage: far better use of the transponder when traffic is random and many stations have light traffic.

| Point | Pre-assigned SCPC | DAMA |
|---|---|---|
| Channel allotment | Fixed | On demand |
| Efficiency for light traffic | Low | High |
| Control | Simple | Needs signalling channel |

## Check yourself

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

1. How does the FDM receiver separate the signals?
   A. Band-pass filters tuned to each band, then demodulators
   B. Time-slot gating
   C. Correlators with codes
   D. A single low-pass filter
   Answer: A. Each band is picked out by a BPF and demodulated to recover its message. (Key point: FDM principle)

2. What is SCPC?
   A. Each voice/data channel gets its own carrier within the transponder band
   B. Many channels grouped into one FDM baseband on one carrier
   C. Time slots in a burst
   D. Spread-spectrum coding
   Answer: A. Single Channel Per Carrier gives each channel its own narrow carrier, often voice-activated to save power. (Key point: SCPC)

3. What is the key feature of DAMA?
   A. Channels are fixed to station pairs
   B. A pool of frequencies is assigned to a station only when a call is requested and returned afterwards
   C. Only one station uses the transponder
   D. All channels are always on
   Answer: B. DAMA assigns channels on demand via a signalling channel, giving better use for random, light traffic. (Key point: DAMA)

4. Which is correct for pre-assigned SCPC compared with DAMA?
   A. Pre-assigned has low efficiency for light traffic and simple control
   B. Pre-assigned has high efficiency and needs a signalling channel
   C. Both allot on demand
   D. Pre-assigned needs complex control
   Answer: A. Fixed channels sit idle when unused (low efficiency) but need simple control; DAMA is efficient but needs signalling. (Key point: SCPC vs DAMA)

## Related in Communication Systems

- [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2×
- [FDMA; FDM in telephone hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-fdm-telephony-hierarchy) — Communication Systems, chapter 5, asked 2×
- [FDM and its method in telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdm-telephony-method) — 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×
- [Working principle of FDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-principle) — Telecommunications, chapter 3
- [Advantages and disadvantages of FDMA, TDMA, CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afdma-tdma-cdma-pros-cons) — Wireless Communication, chapter 7