FHSS PHY

Overview

Frequency-Hopping Spread Spectrum (FHSS) was one of the three original physical layers defined in the IEEE 802.11-1997 standard, alongside DSSS and Infrared (IR). It operated in the 2.4 GHz ISM band and transmitted data by rapidly hopping across a sequence of narrow 1 MHz subchannels in a pseudo-random pattern.

The FHSS PHY supported 1 Mbps and 2 Mbps data rates using DBPSK and DQPSK modulation, respectively. Although it offered robustness to interference, it was eventually replaced by DSSS (and later OFDM) PHYs offering higher throughput.

Key Characteristics

Parameter | Description |

|------------|————-| | Frequency Band | 2.4 GHz ISM (2.400–2.4835 GHz) | | Hop Channel Width | 1 MHz | | Number of Hop Channels | Up to 79 | | Minimum Hop Channels (per FCC) | 75 | | Modulation | DBPSK (1 Mbps), DQPSK (2 Mbps) | | MAC Access | CSMA/CA (DCF) | | Typical Dwell Time | 100–400 ms | | PLCP Format | Preamble + Header + PSDU | | Status | Obsolete (replaced by DSSS in 802.11b) |

Operating Principle

FHSS transmits data by hopping between narrow subchannels according to a predefined pseudo-random sequence known to both transmitter and receiver.

Each data packet (PPDU) is transmitted on one hop frequency or across a few consecutive hops. After the dwell time expires, both transmitter and receiver move to the next frequency in the hop sequence.

This method spreads transmissions across the band, reducing interference from narrowband sources and enabling multiple co-located FHSS networks.

Modulation and Data Rates

Data Rate | Modulation | Bits per Symbol | Description |

|------------|————-|-----------------|————-| | 1 Mbps | DBPSK | 1 | Differential BPSK | | 2 Mbps | DQPSK | 2 | Differential QPSK |

  • Symbol duration: 1 µs (for both rates)

  • Clocking: 1 Msymbol/s

  • Spreading method: frequency-hopping (not code spreading)

Hopsets and Frequency Sequences

The 802.11 FHSS PHY defined multiple hopsets (groups of sequences) designed to minimize collisions between networks.

  • Hopset: a list of 79 (or ≥75) 1 MHz channels.

  • Hop sequence: pseudo-random permutation of hopset.

  • Hop spacing: minimum channel index separation to reduce adjacent-channel interference.

  • Dwell time: time spent on each hop before moving to the next frequency.

Typical example: .. code-block:: none

Hopset (simplified) = { f1, f4, f11, f32, f54, … f79 } Dwell time (T_dwell) = 100 ms Sequence repeats after 79 × T_dwell = 7.9 seconds.

Regulatory Constraints

Parameter | FCC Requirement |

|------------|—————–| | Min. number of hop channels | 75 | | Max. dwell time per channel | 400 ms | | Max. 20 dB bandwidth per hop | ≤ 1 MHz | | Hop pattern repetition interval | ≥ 30 s (typical for 75 hops × 400 ms) | | Max. transmit power (2.4 GHz FHSS) | 1 W EIRP (under spread-spectrum rule) |

PLCP Frame Structure

FHSS PHY used a Physical Layer Convergence Procedure (PLCP) to standardize the MAC interface.

Field | Description |

|--------|————-| | Preamble | Synchronization and rate detection | | PLCP Header | Length, rate, and service fields | | PSDU | MAC frame payload |

All MAC-layer features (DCF, ACK, RTS/CTS, NAV) remained identical across PHYs.

MAC and PHY Interaction

  • The MAC (DCF) operates independently of the hopping process.

  • Physical carrier sense monitors only the current hop frequency.

  • Virtual carrier sense (NAV) functions as usual.

  • DCF backoff, DIFS, SIFS, and ACK timing remain unchanged.

  • Beacons include FH Parameter Set elements describing hopping behavior.

FH Parameter Set

APs advertise the FH Parameter Set IE in their Beacon or Probe Response frames:

Parameter | Description |

|------------|————-| | Hop Set | Index of predefined channel set | | Hop Pattern | Identifier for pseudo-random sequence | | Hop Index | Current position in sequence | | Dwell Time | Duration of each hop | | Hop Modulus | Number of frequencies used | | Hop Offset | Start offset for sequence |

Stations use these fields to synchronize hopping with the AP.

Synchronization Procedure

  1. STA performs active or passive scanning on 2.4 GHz channels.

  2. Receives Beacon containing FH Parameter Set.

  3. Extracts hop sequence, dwell time, and hop index.

  4. Aligns its local hopping clock to match AP.

  5. Begins hopping in sync with AP to maintain association.

Once synchronized, all management and data frames are exchanged on the current hop frequency.

Carrier Sense and DCF Operation

  • Physical carrier sense: Detects RF energy on current hop channel.

  • Virtual carrier sense (NAV): Same Duration-based rule as DSSS PHY.

  • Collision avoidance: Random backoff across slots (t_slot = 50 µs typical).

  • SIFS/DIFS: Same absolute durations as defined in the 802.11 MAC layer.

DCF thus works identically over FHSS and DSSS PHYs.

Advantages

  • Robustness: Resistant to narrowband interference.

  • Coexistence: Multiple FHSS networks can operate with minimal overlap.

  • Security: Eavesdroppers must follow hopping sequence.

  • Regulatory benefit: Spread-spectrum classification allowed higher power limits.

Limitations

  • Low data rate (1–2 Mbps max).

  • Complex synchronization (requires hopping alignment).

  • High management overhead (FH Parameter Set negotiation).

  • Inefficient spectral usage vs. DSSS or OFDM.

  • Limited vendor adoption — replaced by DSSS in 802.11b (11 Mbps).

Processing Gain (Example)

Approximate processing gain:

\[G_p = 10 \log_{10}(N)\]

For a hopset of 79 channels:

\[G_p = 10 \log_{10}(79) \approx 19 \text{ dB}\]

This quantifies robustness to narrowband interference across the hopping band.

Timing Example

If 75 hop channels are used and dwell time is 400 ms:

\[T_{\text{sequence}} = 75 \times 0.4 \text{ s} = 30 \text{ s}\]

Thus, the full hop pattern repeats every 30 seconds.

Coexistence with Bluetooth

Bluetooth (also FHSS-based) uses 79 1 MHz channels with 1600 hops/s. Because 802.11 FHSS uses slow hopping (100–400 ms dwell), the two technologies may collide occasionally, but the probability of repeated interference is low due to independent sequences.

Historical Context

Year | Event |

|------|——–| | 1997 | FHSS defined in IEEE 802.11-1997 | | 1999 | 802.11b DSSS (11 Mbps) replaces FHSS | | 2000+ | FHSS WLAN products discontinued | | 2002 | OFDM (802.11a/g) dominates industry |

No major Wi-Fi vendors shipped FHSS-based 802.11b devices; by 1999, DSSS had completely replaced FHSS for all practical WLAN use.

Advantages vs DSSS Summary

Feature | FHSS | DSSS |

|----------|——|------| | Spread Mechanism | Frequency hopping | Code spreading | | Data Rate | 1–2 Mbps | Up to 11 Mbps | | Range | Short–medium | Medium–long | | Interference Robustness | Strong against narrowband | Strong against wideband | | Implementation Complexity | High (sync & timing) | Moderate | | Market Adoption | Low | High |

Practical Implementation Notes

  • Ensure dwell-time and hop-count comply with regional regulations.

  • Include FH Parameter Set in Beacons and Probe Responses.

  • Select dwell time large enough to transmit full MAC frames.

  • Resynchronize after beacon loss or long power-save intervals.

  • Avoid mixed PHY BSS (FHSS cannot interoperate with DSSS or IR PHY).

Summary

Concept | Description |

|----------|————-| | Medium | 2.4 GHz ISM, frequency-hopping | | Hop Width | 1 MHz | | Channels | 75–79 | | Data Rates | 1 Mbps (DBPSK), 2 Mbps (DQPSK) | | Dwell Time | 100–400 ms typical | | Access | CSMA/CA (DCF) | | Advantages | Interference resilience, coexistence | | Drawbacks | Low throughput, sync complexity | | Successor | DSSS (802.11b), OFDM (802.11a/g/n) |

References

  • IEEE Std 802.11-1997, Clause 15 — FHSS PHY Specification

  • IEEE Std 802.11b-1999, Annex (PHY evolution overview)

  • Gast, M. 802.11 Wireless Networks: The Definitive Guide, O’Reilly

  • Kurose & Ross, Computer Networking, 8th Edition

  • IEEE 802.11 FHSS Tutorial (IEEE ComSoc archives)

Figures

Frequency-Hopping Sequence

Example FHSS hop timeline showing 1 MHz hop channels and pseudo-random hopping pattern.

802.11 MCS

spreading/coding

Modulation

BW

Total-Sub-Carriers

FSP

Tdata=1/FSP

GI

symbol

Bits/symbol

Code rate

Usable

Rate

Formula (Usable Rate = (Bits/Symbol ÷ Symbol Duration) × (1 / Code Rate))

FHSS

GFSK (whitening)

2-FSK

1

N/A

1 MHz

1 µs

N/A

1 µs

1

1

1

1 Mbps

(1 / 1 µs) × 1 = 1 Mbps

FHSS

GFSK (whitening)

4-FSK

1

N/A

1 MHz

1 µs

N/A

1 µs

2

1

2

2 Mbps

(2 / 1 µs) × 1 = 2 Mbps

Channel Number

Center Frequency (MHz)

Frequency Range

DFS Required

1

2412

2401 – 2423

No

2

2417

2406 – 2428

No

3

2422

2411 – 2433

No

4

2427

2416 – 2438

No

5

2432

2421 – 2443

No

6

2437

2426 – 2448

No

7

2442

2431 – 2453

No

8

2447

2436 – 2458

No

9

2452

2441 – 2463

No

10

2457

2446 – 2468

No

11

2462

2451 – 2473

No

12

2467

2456 – 2478

No

13

2472

2461 – 2483

No

14

2484

2473 – 2495

No

Band Name

Frequency Range (GHz)

Frequency Range (MHz)

Channels

ISM Band (Global)

2.400 – 2.4835

2400 – 2483.5

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 (12, 13, 14 vary by region)