---
title: "Frequency hopped multiple access (FHMA)"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:fhma-operation"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-operation
site: BCT Engineering
---

# Frequency hopped multiple access (FHMA)

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2080 Bhadra, 6 marks; 2080 Baisakh, 7 marks; 2079 Bhadra, 4 marks; 2076 Bhadra, 5 marks; 2075 Bhadra, 3 marks (asked 5 times)
- Video: [Spread Spectrum Multiple Access - Mobile Radio Propagation - Mobile Communication System](https://www.youtube.com/watch?v=6r7pOoogNBo) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-operation
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Explain the working of the Frequency Hopped Multiple Access (FHMA) technique using its transmitter and receiver blocks, with an example.

## Short answer

In FHMA, each user’s narrowband modulated signal (usually FSK) hops pseudo-randomly across many channels following a unique PN code. The transmitter uses a frequency synthesizer driven by the PN code; the receiver employs a synchronized synthesizer to de-hop to a fixed IF and demodulate. An example: Bluetooth with 79 channels hopping 1600 times per second.

## Answer

**Frequency Hopped Multiple Access (FHMA)** is a digital multiple access technique in which the carrier frequency of each user **changes (hops) pseudo-randomly** over a wide band, following a PN code unique to that user. At any instant a user occupies only one narrow channel, but over time the signal is spread over the whole band. Different users hop with different codes, so they rarely collide.

```text
 TRANSMITTER
 data->[FSK/BFSK mod]->(×)->[BPF]-> wideband channel
                        ^
            [freq. synthesizer]<-[PN code gen.]

 RECEIVER
 channel->(×)->[BPF/IF]->[FSK demod]-> data
           ^
  [freq. synthesizer]<-[PN code gen.] (synchronised)
```

**Transmitter**
1. Data modulates a narrowband carrier, usually with **FSK**.
2. A **PN code generator** gives a new code word every hop period; the **frequency synthesizer** converts it into a hop frequency.
3. The mixer moves the modulated signal to that frequency. The **hop rate** is the number of frequency changes per second.

**Receiver**
1. An identical PN generator, **synchronised** with the transmitter, drives a local synthesizer.
2. Mixing with this local signal **de-hops** the signal back to a fixed IF.
3. A conventional FSK demodulator recovers the data. Signals of other users, hopping on different patterns, fall outside the IF filter most of the time.

**Example:** a system has 8 channels f₀ … f₇ (3-bit PN code). User A's code gives the sequence f₃, f₇, f₁, f₄ …; user B's code gives f₅, f₂, f₆, f₀ …. In each hop interval they are on different frequencies, so both share the band. If both land on the same frequency in one hop (a collision), only that hop's bits are damaged, and coding/interleaving corrects them. Bluetooth is a real example: 79 channels of 1 MHz, 1600 hops/s. GSM uses slow frequency hopping (217 hops/s) for diversity.

**Types:** slow hopping (several symbols per hop) and fast hopping (several hops per symbol).

**Advantages:** resistance to narrowband interference and fading, security, and a less severe near–far problem than DS-CDMA. **Disadvantages:** fast frequency synthesizers are needed, and collisions cause errors.

## Check yourself

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

1. In FHMA, what makes different users share the band with few collisions?
   A. Each user has a different data rate
   B. Each user's carrier hops pseudo-randomly following its own PN code
   C. All users hop on the same code
   D. Each user has a fixed channel
   Answer: B. Different PN codes give different hop patterns, so users rarely occupy the same channel at once. (Key point: FHMA principle)

2. What is the role of the frequency synthesizer in the FHMA transmitter?
   A. It converts PN code words into hop frequencies for the mixer
   B. It modulates data with FSK
   C. It filters the output
   D. It corrects errors
   Answer: A. The PN code generator drives the synthesizer, which produces the hop carrier. (Key point: Transmitter blocks)

3. How does the receiver recover the data?
   A. A free-running oscillator is used
   B. A synchronised identical PN generator and synthesizer de-hop the signal to a fixed IF, then FSK demodulation
   C. It averages over all channels
   D. It needs no synchronisation
   Answer: B. Synchronised de-hopping brings the signal to a fixed IF for a normal FSK demodulator. (Key point: Receiver de-hopping)

4. If two users land on the same frequency in a hop, what happens?
   A. The whole call is lost
   B. Only that hop's bits are damaged and coding/interleaving corrects them
   C. Both users hop together forever
   D. The system shuts down
   Answer: B. Collisions are occasional and limited to one hop, so FEC with interleaving repairs them. (Key point: Collisions)

## Related in Wireless Communication

- [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
- [OFDM transmitter and receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-transmitter-receiver) — Wireless Communication, chapter 4, asked 5×
- [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
- [Direct sequence and frequency hopped SS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-fhss-techniques) — Wireless Communication, chapter 4
- [Multiple access, TCDMA and TDFH](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Atcdma-tdfh) — Wireless Communication, chapter 7
- [Spread spectrum and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4

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- [Stereo FM transmitter and receiver; output of mono receiver](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Astereo-fm-mono-receiver) — Communication Systems, chapter 3
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