---
title: "FHMA principle and hybrid techniques for near-far"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:fhma-hybrid-near-far"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-hybrid-near-far
site: BCT Engineering
---

# FHMA principle and hybrid techniques for near-far

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2074 Bhadra, 4+6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-hybrid-near-far
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Explain the principle of Frequency Hopping Multiple Access. Briefly describe two hybrid spread spectrum multiple access techniques which can mitigate near-far problem.

## Short answer

FHMA pseudo-randomly hops each user's carrier over a wide band, so at any instant each user occupies one narrow channel. Hybrid DS/FHMA adds DS spreading before hopping, making near and far users rarely collide on the same hop. TCDMA uses different cell codes and time slots per user, so near and far users of a cell are never received together.

## Answer

#### Principle of Frequency Hopping Multiple Access (FHMA)
In FHMA the **carrier frequency of each user is changed pseudo-randomly** over a wide band, following a PN hopping pattern unique to that user. At any instant each user occupies only one narrow channel, but over time it uses the whole band. Different users have different hopping patterns, so they rarely collide.

```text
 freq ^
  f5  |      A           B
  f4  |  B         A
  f3  |            B          A
  f2  |      B           A
  f1  |  A                    B
      +-------------------------> time (hops)
```

- Transmitter: data → FSK modulator → mixer driven by a frequency synthesizer controlled by a PN generator → wideband channel.
- Receiver: a locally generated, synchronised hopping pattern "de-hops" the signal to a fixed IF before demodulation.
- **Slow hopping**: one or more symbols per hop (GSM). **Fast hopping**: several hops per symbol.
- A collision (two users on the same frequency at the same time) causes errors that are corrected by coding and interleaving.
- Advantages: resistance to narrowband jamming and frequency-selective fading, security. Example: Bluetooth hops 1600 times/s over 79 channels of 1 MHz.

#### Hybrid techniques that mitigate the near-far problem
**1. Hybrid DS/FHMA**
- The signal is first DS-spread by a PN code, then its centre frequency hops pseudo-randomly.
- A strong near user and a weak far user are rarely on the same hop frequency at the same time, so the near user cannot swamp the far user's despreading. Thus the near-far effect is avoided.
- Drawback: complex; soft handoff is hard since the base must synchronise to many hopping signals.

```text
 d(t)->(x)c(t)->(x)->[BPF]->channel
                  ^
        [synth] <- [hop PN]
```

**2. Time Division CDMA (TCDMA)**
- Different cells use different spreading codes.
- Inside a cell, users are given time slots, so only one CDMA user per cell transmits at any time.
- Since near and far users of one cell are never received together, the near-far effect disappears. During handoff the user's code is switched to the code of the new cell.
- Drawback: needs time synchronisation; capacity per cell limited by slots.

```text
 Cell A, code C_A: |u1|u2|u3|u1|u2|u3|
 Cell B, code C_B: |u4|u5|u6|u4|u5|u6|
```

## Check yourself

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

1. In FHMA, what changes pseudo-randomly with time?
   A. The data rate
   B. The carrier frequency following a PN hopping pattern
   C. The PN code length
   D. The modulation type
   Answer: B. Each user's carrier hops over a wide band according to its own PN pattern. (Key point: FHMA principle)

2. How does the FHMA receiver recover the signal?
   A. It uses a locally generated, synchronised hopping pattern to de-hop to a fixed IF
   B. It uses a bigger antenna
   C. It averages over many hops without synchronisation
   D. It applies a Walsh code
   Answer: A. The receiver's synchronised hop pattern brings the signal to fixed IF before demodulation. (Key point: FHMA receiver)

3. What distinguishes fast hopping from slow hopping?
   A. Fast hopping uses fewer frequencies
   B. Slow hopping has several hops per symbol
   C. Slow hopping has one or more symbols per hop; fast hopping has several hops per symbol
   D. They differ only in modulation
   Answer: C. Slow FH sends symbols within a hop (GSM), fast FH hops several times per symbol. (Key point: Slow vs fast hopping)

4. Why does TCDMA avoid the near-far problem?
   A. Only one CDMA user per cell transmits at any time, so near and far users are never received together
   B. Hopping makes users appear orthogonal
   C. Power is always the same
   D. It uses no spreading
   Answer: A. Time slots within a cell prevent a near and far user of the same cell being received simultaneously. (Key point: TCDMA vs near-far)

## Related in Wireless Communication

- [SDMA and hybrid SS techniques against near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Asdma-hybrid-near-far) — Wireless Communication, chapter 7
- [Hybrid spread spectrum and FHMA types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-ss-fhma-types) — Wireless Communication, chapter 7
- [Near-far effect and a hybrid SS technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Anear-far-hybrid-technique) — Wireless Communication, chapter 7
- [Multiple access and near-far effect mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-near-far) — Wireless Communication, chapter 7
- [Self jamming, FHMA and hybrid SS techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aself-jamming-fhma-hybrid) — Wireless Communication, chapter 7
- [FHMA principle and near-far effect in CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-near-far-cdma) — Wireless Communication, chapter 7

## Related in other subjects

- [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2×
- [Near-far problem in CDMA](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acdma-near-far) — Data Communication, chapter 6
- [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4×
- [RF safety practices and reactive/near/far fields](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Arf-safety-practices-near-far-field) — RF and Microwave Engineering, chapter 7