HRDSSS PHY

IEEE 802.11b High-Rate Direct Sequence Spread Spectrum (HR/DSSS) PHY

Overview

The High-Rate Direct Sequence Spread Spectrum (HR/DSSS) PHY, defined in IEEE 802.11b-1999, is an extension of the original DSSS PHY from IEEE 802.11-1997. It increased data rates from 1–2 Mbps to 5.5 Mbps and 11 Mbps while maintaining full backward compatibility with legacy DSSS systems.

HR/DSSS operates in the 2.4 GHz ISM band, using Complementary Code Keying (CCK) as its primary modulation technique, and optionally supports Packet Binary Convolutional Coding (PBCC) for enhanced robustness.

Key Characteristics

Parameter | Description |

|------------|————-| | Frequency Band | 2.4 GHz ISM (2.400–2.4835 GHz) | | Channel Bandwidth | 22 MHz | | Channel Spacing | 5 MHz | | Data Rates | 1, 2, 5.5, 11 Mbps | | Modulation | DBPSK, DQPSK, CCK, PBCC (optional) | | Spreading Code | 11-chip Barker (1/2 Mbps), 8-chip CCK (5.5/11 Mbps) | | Chip Rate | 11 Mchips/s | | Access Mechanism | CSMA/CA (DCF) | | Compatibility | Backward compatible with DSSS PHY |

Evolution from DSSS

The HR/DSSS PHY extends the DSSS system as follows:

  • Maintains 11 MHz chip rate and 22 MHz channel bandwidth.

  • Introduces CCK (Complementary Code Keying) to encode multiple bits per symbol.

  • Adds PBCC as an optional higher-robustness alternative.

  • Introduces short preamble mode for faster synchronization.

  • Retains backward compatibility with legacy DSSS (1 & 2 Mbps) systems.

Backward Compatibility

HR/DSSS stations can interoperate with legacy DSSS stations through:

  1. Dual Modulation Support — HR/DSSS PHYs must support 1 Mbps and 2 Mbps DBPSK/DQPSK with 11-chip Barker spreading.

  2. Preamble Signaling — Long preamble (DSSS-compatible) indicates 1/2 Mbps rates; short preamble signals HR/DSSS-only operation.

  3. Rate Field in PLCP Header — Encodes current PHY data rate (1–11 Mbps).

Operating Principle

Like DSSS, HR/DSSS uses spreading to reduce interference, but employs multi-bit symbol encoding (via CCK) for higher throughput.

Each data bit sequence is first encoded, then spread using orthogonal chip sequences derived from complex-valued complementary codes.

The general process:

Data bits → Symbol mapping → CCK encoder → 8-chip code sequence
          → BPSK/QPSK modulation → RF upconversion → Transmission

Complementary Code Keying (CCK)

CCK is a form of code-based modulation using complex-valued spreading sequences of length 8 chips per symbol.

### Codeword Generation

Each 8-chip codeword is generated from 4 or 8 input bits and 4 phase parameters (φ₁–φ₄):

\[c_k = e^{j(\phi_1 + b_1 k_1 + b_2 k_2 + b_3 k_3 + b_4 k_4)}\]

The four phase terms define complementary codes that minimize autocorrelation and cross-correlation between sequences.

### Symbol Mapping

Data Rate | Bits per Symbol | Chips per Symbol | Symbol Rate |

|------------|—————-|------------------|————–| | 5.5 Mbps | 4 | 8 | 1.375 Msymbol/s | | 11 Mbps | 8 | 8 | 1.375 Msymbol/s |

Each CCK symbol spans 8 chips, transmitted at 11 Mchips/s.

The spreading gain is 10.4 dB (same as DSSS), but each symbol carries more data bits.

Modulation Summary

Data Rate | Modulation | Spreading | Notes |

|------------|————-|------------|——-| | 1 Mbps | DBPSK | 11-chip Barker | Legacy DSSS | | 2 Mbps | DQPSK | 11-chip Barker | Legacy DSSS | | 5.5 Mbps | CCK (4 bits/symbol) | 8-chip CCK | HR/DSSS | | 11 Mbps | CCK (8 bits/symbol) | 8-chip CCK | HR/DSSS | | Optional | PBCC | 8-chip convolutional | Optional mode |

Packet Binary Convolutional Coding (PBCC)

PBCC is an optional modulation technique offering similar throughput to CCK but with enhanced noise immunity and forward error correction (FEC).

  • Encodes data using a rate-½ convolutional code.

  • Modulates symbols with DQPSK.

  • Provides better BER performance under multipath fading.

  • Not mandatory; few commercial chipsets implemented PBCC.

PLCP Frame Structure

The HR/DSSS PLCP layer is backward compatible with DSSS.

Field | Description |

|--------|————-| | Preamble | Long (144 bits) or Short (72 bits) | | PLCP Header | Rate, Length, and Service fields | | PSDU | MAC payload |

### Preamble Variants

Type | Duration | Compatibility | Description |

|-------|———–|---------------|————-| | Long | 144 bits | Legacy DSSS | Used for mixed networks | | Short | 72 bits | HR/DSSS only | Faster synchronization |

  • Long preamble uses Barker-modulated SYNC field.

  • Short preamble uses shorter SYNC for improved efficiency.

Carrier Sense and DCF Operation

  • Uses CSMA/CA (DCF) identical to other PHYs.

  • CCA detects energy or valid DSSS/CCK correlation.

  • NAV ensures virtual carrier sense.

  • DIFS, SIFS, and backoff timing identical to DSSS.

DCF operation sequence:

Medium idle → DIFS → Backoff → Transmit (CCK/Barker)
Medium busy → Freeze backoff → Wait for DIFS → Resume countdown

Spectral Characteristics

Parameter | Value |

|------------|——–| | Channel Bandwidth | 22 MHz | | Chip Rate | 11 Mchips/s | | Symbol Rate | 1.375 Msymbol/s | | Processing Gain | 10.4 dB | | Occupied Bandwidth | ~17 MHz | | Spreading Codes | 11-chip Barker, 8-chip CCK |

Backward Compatibility Example

Mixed DSSS + HR/DSSS BSS:

  1. AP transmits beacons at 1 Mbps (Barker modulation).

  2. Legacy STA receives at 1/2 Mbps.

  3. HR/DSSS STA switches to 5.5 or 11 Mbps (CCK) for data after association.

  4. Control frames (RTS/CTS/ACK) usually use 1 Mbps or 2 Mbps.

This ensures all stations can detect and decode management/control frames.

Performance and Processing Gain

The processing gain (due to spreading) remains approximately:

\[G_p = 10 \log_{10}(11) \approx 10.4 \, \text{dB}\]

However, CCK’s code diversity improves performance in multipath environments.

Advantages

  • Higher throughput (5.5 & 11 Mbps).

  • Backward compatible with DSSS.

  • Robust against multipath fading.

  • Supports short preamble for efficiency.

  • Same DCF and MAC behavior.

Limitations

  • Confined to 2.4 GHz band (susceptible to interference).

  • Limited spectral efficiency compared to OFDM.

  • Channel overlap (only 3 non-overlapping channels).

  • PBCC optional — not universally supported.

Channelization (2.4 GHz)

Channel | Center Frequency (MHz) | Overlap |

|----------|————————-|----------| | 1 | 2412 | overlaps 2–4 | | 6 | 2437 | overlaps 4–8 | | 11 | 2462 | overlaps 8–13 |

Same as DSSS; only 1, 6, and 11 are non-overlapping in North America.

Receiver Operation

  1. Detects RF energy.

  2. Synchronizes using preamble correlation.

  3. Determines data rate from PLCP header.

  4. Performs CCK or Barker de-spreading.

  5. Demodulates and decodes data.

Example Transmission Sequence

|<-- DIFS -->|<- Backoff ->|--- PLCP Preamble ---|--- PSDU (CCK) ---| SIFS | ACK |
Idle Medium     Contention       Synchronization        Data Frame     Wait  Control

Typical Parameter Values

Parameter | Typical Value |

|------------|—————-| | Chip Rate | 11 MHz | | Symbol Rate | 1.375 Msymbol/s | | Modulation | DBPSK/DQPSK/CCK | | Preamble Length | 72 or 144 bits | | Processing Gain | 10.4 dB | | Operating Range | 100–150 m (indoor) |

Historical Context

Year | Event |

|------|——–| | 1997 | DSSS PHY introduced (1–2 Mbps) | | 1999 | HR/DSSS (802.11b) ratified (5.5/11 Mbps) | | 2000 | 802.11b becomes dominant Wi-Fi technology | | 2003 | Superseded by OFDM PHYs (802.11a/g) |

Summary

Concept | Description |

|----------|————-| | Medium | 2.4 GHz ISM band | | Modulation | DBPSK, DQPSK, CCK, PBCC | | Data Rates | 1, 2, 5.5, 11 Mbps | | Spreading | 11-chip Barker or 8-chip CCK | | Access | CSMA/CA (DCF) | | Compatibility | Backward with DSSS | | Successor | 802.11g (OFDM, 54 Mbps) |

References

  • IEEE Std 802.11b-1999, Clause 18 — HR/DSSS PHY Specification

  • IEEE Std 802.11-2016, Clause 16 (Legacy DSSS/HR-DSSS)

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

  • Stallings, W. Wireless Communications and Networks

  • IEEE 802.11 Working Group — PHY Evolution Records

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))

DSSS

11 chip barker

DBPSK

20

64

312.5 KHz

3.2 us

0.8 us

4 us

1

1/11

44

1 Mbps

(1 / 4 µs) × (1 / (1/11)) = 250 kbps × 11 = 1 Mbps

DSSS

11 chip barker

DQPSK

20

64

312.5 KHz

3.2 us

0.8 us

4 us

2

1/11

44

2 Mbps

(2 / 4 µs) × (1 / (1/11)) = 500 kbps × 11 = 2 Mbps

HR/DSSS

CCK

DQPSK

20

64

312.5 KHz

3.2 us

0.8 us

4 us

4

1/8

44

5.5 Mbps

(4 / 4 µs) × (1 / (1/8)) = 1 Mbps × 8 = 8 Mbps → maps to 5.5 Mbps in standard

HR/DSSS

CCK1

DQPSK

20

64

312.5 KHz

3.2 us

0.8 us

4 us

8

1/8

44

11 Mbps

(8 / 4 µs) × (1 / (1/8)) = 2 Mbps × 8 = 16 Mbps → maps to 11 Mbps in standard

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)