---
title: "FDMA vs TDMA; operation of FHMA"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:fdma-tdma-fhma"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/fdma-tdma-fhma
site: BCT Engineering
---

# FDMA vs TDMA; operation of FHMA

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2078 Chaitra, 4+4 marks
- Video: [FDMA, TDMA, CDMA and SDMA in one video 🔥 | Wireless Communication](https://www.youtube.com/watch?v=sArkTtUMDgg) — Er Sahil ka Gyan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/fdma-tdma-fhma
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Compare between FDMA and TDMA. Explain the operation of FHMA.

## Short answer

FDMA assigns each user a separate frequency band with continuous transmission; TDMA shares one carrier in time slots with burst transmission and requires tight synchronization. FHMA pseudo-randomly hops each user's carrier over a wide band, and the synchronized receiver de-hops to recover the signal, with coding handling occasional collisions.

## Answer

#### Comparison of FDMA and TDMA

| Point | FDMA | TDMA |
|---|---|---|
| Basic idea | Each user gets a separate frequency band | Users share one carrier in different time slots |
| Transmission | Continuous | Burst (discontinuous) |
| Bandwidth per channel | Narrow (e.g. 30 kHz AMPS) | Wider (e.g. 200 kHz GSM) |
| Synchronisation | Not needed | Tight time sync and guard times needed |
| Equalizer | Usually not needed (low ISI) | Needed (high bit rate, ISI) |
| Duplexer | Needed (FDD, Tx and Rx together) | Not needed (Tx and Rx in different slots) |
| Handoff | Hard, mobile cannot measure easily | Mobile-assisted handoff in idle slots |
| Overhead | Guard bands | Guard time, sync and training bits |
| Base station cost | One transceiver per user | One transceiver serves many users |
| Example | AMPS, NMT | GSM, IS-136 |

#### Operation of FHMA
Frequency Hopping Multiple Access is a digital multiple access method in which the **carrier frequency of each user changes (hops) in a pseudo-random way** inside a wide band. The instantaneous signal is narrowband, but over time it covers the whole wideband channel.

```text
 Transmitter
 data -> [Modulator] -> (x) -> [BPF] -> antenna
         (FSK)          ^
                  [Freq. synthesizer]
                        ^
                  [PN code generator]

 Receiver
 antenna -> (x) -> [IF filter] -> [Demod] -> data
             ^
       [Freq. synthesizer] <- [PN gen] (synchronised)
```

Steps:
1. Data modulates a carrier (usually BFSK or M-FSK).
2. A PN generator selects a frequency from the synthesizer; the mixer moves the signal to that hop frequency.
3. The hopping pattern is unique to each user, so two users rarely occupy the same frequency at the same time.
4. The receiver uses the same PN sequence, synchronised, to "de-hop" the signal back to a fixed IF, then demodulates it.
5. If two users hit the same frequency (a **collision**), errors occur; error-correcting codes and interleaving recover them.

- **Slow FH**: several symbols per hop (GSM, 217 hops/s). **Fast FH**: several hops per symbol.
- Advantages: immunity to narrowband interference and fading, and a much less severe near-far problem than in DS-CDMA. Example: Bluetooth (1600 hops/s over 79 channels).

## Check yourself

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

1. Which statement about duplexers is correct?
   A. FDMA (FDD) needs a duplexer; TDMA does not since Tx and Rx use different slots
   B. TDMA needs it, FDMA does not
   C. Both never need it
   D. Both always need it
   Answer: A. In FDMA Tx and Rx work together; in TDMA they work in different slots. (Key point: Duplexer)

2. How does handoff differ?
   A. TDMA has mobile-assisted handoff using idle slots
   B. FDMA has MAHO
   C. Both are soft
   D. TDMA cannot hand off
   Answer: A. The mobile can measure neighbours during idle slots in TDMA. (Key point: Handoff)

3. How does the FHMA receiver recover data?
   A. Synchronised PN generator de-hops to fixed IF, then demodulates
   B. Uses a fixed oscillator
   C. Uses a codebook
   D. Uses averaging
   Answer: A. Synchronised hopping lets the receiver return to a fixed IF. (Key point: FHMA de-hopping)

4. What happens on a collision in FHMA?
   A. Errors in that hop, corrected by FEC and interleaving
   B. Link breaks permanently
   C. Nothing
   D. Users swap codes
   Answer: A. Errors are confined to the hop and recovered by coding. (Key point: Collision)

## Related in Wireless Communication

- [TDMA versus FDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atdma-vs-fdma) — Wireless Communication, chapter 7
- [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
- [Advantages of TDMA over FDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atdma-advantages-over-fdma) — Wireless Communication, chapter 7
- [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7
- [Multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-fdma-vs-cdma) — Wireless Communication, chapter 7
- [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7

## Related in other subjects

- [FDMA vs TDMA; T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-tdma-t1-e1) — Communication Systems, chapter 5