---
title: "Short note: FDMA in satellite communications"
subject: "Communication Systems"
chapter: 5
key: "semester_6/communication_systems:short-note-fdma-satellite"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-fdma-satellite
site: BCT Engineering
---

# Short note: FDMA in satellite communications

Communication Systems · Chapter 5: Multiplexing Techniques

- Asked in: 2074 Bhadra (CS I), 4 marks; 2073 Bhadra (CS I), 5 marks (asked 2 times)
- Video: [FDMA in satellite communication || FDMA || Frequency division multiple access](https://www.youtube.com/watch?v=OPSBZ986FI8) — Ravi Teja Creative Catchers !!
- 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-fdma-satellite
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5

## Question

Write a short note on FDMA in satellite communications.

## Short answer

FDMA in satellite divides transponder bandwidth among earth stations, each gets own frequency band. Types: MCPC (multiple channels per carrier, FDM/FM) and SCPC (single channel per carrier, can be demand-assigned DAMA). Issues: intermodulation from nonlinear TWT requires back-off, guard bands waste spectrum, power control needed. Simpler than TDMA but less efficient.

## Answer

**FDMA (Frequency Division Multiple Access)** in satellite communication lets many earth stations share one satellite transponder by giving **each station its own frequency band** within the transponder bandwidth. All stations transmit continuously and at the same time.

```text
   Transponder (e.g. 36 MHz)
 |  ES-A  |g|  ES-B  |g|  ES-C  |g| ...
 <------------ frequency ------------>
```

**Types**
- **MCPC (Multiple Channels Per Carrier):** each earth station FDM-multiplexes many voice channels onto one carrier (FDM/FM/FDMA). Pre-assigned, suited to heavy fixed routes.
- **SCPC (Single Channel Per Carrier):** each voice/data channel has its own carrier. Can be **demand assigned (DAMA)**, e.g. INTELSAT **SPADE** system: about 800 channels at 45 kHz spacing in a 36 MHz transponder, assigned on request.

**Features and problems**
- Simple: no network-wide timing or synchronization needed.
- **Intermodulation:** the transponder's TWT amplifier is nonlinear; many carriers produce intermodulation products, so the amplifier is operated with **input/output back-off** (a few dB), which reduces capacity and power.
- **Guard bands** between carriers waste spectrum.
- Power control needed so that a strong carrier does not suppress weak ones (capture effect).
- Capacity falls sharply as the number of carriers rises.

**Comparison:** FDMA is the oldest and simplest satellite access method; TDMA uses the transponder more efficiently (single carrier at saturation) but needs precise timing; CDMA allows overlapping use with spread spectrum.

## Check yourself

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

1. In satellite FDMA, each earth station is given:
   A. A time slot
   B. Its own frequency band in the transponder
   C. A spreading code
   D. A polarisation only
   Answer: B. All stations transmit simultaneously in different bands. (Key point: FDMA principle)

2. Why is back-off required in FDMA?
   A. The TWT is nonlinear and many carriers cause intermodulation products
   B. To increase power
   C. To get timing
   D. To save spectrum
   Answer: A. Back-off reduces intermodulation but also reduces power efficiency and capacity. (Key point: Intermodulation)

3. SCPC means:
   A. Single channel per carrier, each channel with its own carrier
   B. Many channels on one carrier
   C. Single code per channel
   D. Satellite carrier power control
   Answer: A. SCPC (e.g. SPADE) can be demand-assigned; MCPC puts many FDM channels on one carrier. (Key point: SCPC)

4. A benefit of FDMA compared with TDMA is:
   A. Higher efficiency
   B. No network-wide timing/synchronization needed
   C. No guard bands
   D. No intermodulation
   Answer: B. FDMA is simple, but TDMA uses the transponder more efficiently. (Key point: Advantage over TDMA)