DSSS PHY

IEEE 802.11 Direct Sequence Spread Spectrum (DSSS) PHY

Overview

The Direct Sequence Spread Spectrum (DSSS) Physical Layer (PHY) was one of the three PHY options defined in the original IEEE 802.11-1997 standard, alongside FHSS and Infrared (IR).

DSSS uses spreading sequences to distribute the transmitted signal energy across a wider bandwidth, improving interference resistance and signal robustness.

It operates in the 2.4 GHz ISM band (2.400–2.4835 GHz) and forms the foundation for the 802.11b extension, which increased data rates up to 11 Mbps.

Key Characteristics

Parameter | Description |

|------------|————-| | Frequency Band | 2.4 GHz ISM | | Channel Bandwidth | 22 MHz | | Number of Channels (US) | 11 | | Data Rates | 1 Mbps, 2 Mbps (original 802.11 DSSS) | | Modulation | DBPSK (1 Mbps), DQPSK (2 Mbps) | | Spreading Sequence | 11-chip Barker code | | Chip Rate | 11 Mchips/s | | Access | CSMA/CA (DCF) | | PLCP Type | DSSS PLCP (Barker sequence preamble) |

Operating Principle

In DSSS, each information bit is multiplied by a pseudorandom chip sequence before transmission. This process spreads the signal over a wider frequency band, reducing the impact of interference and making the signal more resistant to noise.

At the receiver, the incoming signal is correlated with the same chip sequence to reconstruct the original bit stream.

Spreading Concept

Let: - Information bit = \(b(t)\) - Spreading code (chips) = \(c(t)\) - Transmitted signal = \(s(t) = b(t) \times c(t)\)

Each data bit is represented by 11 chips, where each chip has a duration:

\[T_c = \frac{T_b}{11}\]

At 1 Mbps data rate:

\[T_b = 1 \, \mu s \quad \text{and} \quad T_c = 0.0909 \, \mu s\]

so the chip rate is 11 Mchips/s.

Barker Sequence

The DSSS PHY uses a fixed 11-bit Barker sequence:

+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1

Properties: - Autocorrelation sidelobes ≤ 1 - Excellent synchronization characteristics - Provides 10.4 dB processing gain (10 × log₁₀(11))

Modulation and Data Rates

Data Rate | Modulation | Chips per Bit | Description |

|------------|————-|----------------|————-| | 1 Mbps | DBPSK | 11 | Differential BPSK, Barker spreading | | 2 Mbps | DQPSK | 11 | Differential QPSK, Barker spreading |

  • DBPSK: phase shift of 180° represents bit ‘1’, 0° represents ‘0’.

  • DQPSK: encodes two bits per symbol using four phase states.

PLCP Frame Structure

The DSSS PHY uses the Physical Layer Convergence Procedure (PLCP) to interface with the MAC layer.

Field | Description |

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

Preamble structure (long preamble, per IEEE 802.11-1997):

+----------------+----------------+----------------+
| SYNC (128 bits)| SFD (16 bits)  | PLCP Header    |
+----------------+----------------+----------------+

The SYNC field uses a continuous 101010… pattern modulated with the Barker code.

Carrier Sense and DCF Operation

The DSSS PHY integrates seamlessly with the MAC’s DCF (Distributed Coordination Function).

  • Physical Carrier Sense (CCA): - Detects energy or correlation with Barker sequence. - Indicates medium busy/idle status to MAC.

  • Virtual Carrier Sense (NAV): - Uses Duration/ID field from MAC headers. - Prevents medium access for reserved time.

DCF rules such as DIFS, SIFS, backoff, and ACK handling are identical across PHYs.

Spectral Characteristics

Parameter | Value |

|------------|——–| | Center frequency spacing | 5 MHz | | Channel bandwidth | 22 MHz | | Occupied bandwidth | ~17 MHz (99% power) | | Processing gain | 10.4 dB | | Chip rate | 11 MHz | | Symbol rate | 1 or 2 Msymbol/s |

DSSS Spectrum Sketch (conceptual):

Power
  ^
  |             **************
  |          ***              ***
  |       ***                    ***
  +------------------------------------> Frequency
              22 MHz occupied

Receiver Operation

  1. Detect incoming energy in the 2.4 GHz band.

  2. Perform correlation with 11-chip Barker sequence.

  3. Achieve synchronization (timing and carrier recovery).

  4. Demodulate DBPSK/DQPSK symbols.

  5. De-spread the signal to recover data bits.

Synchronization and De-spreading

Receiver correlation is computed as:

\[R = \sum_{i=1}^{11} r_i \cdot c_i\]

where: - \(r_i\) = received chip - \(c_i\) = known Barker chip (+1 or –1)

When R is maximized, the receiver achieves chip-level synchronization.

Advantages

  • Resistant to narrowband interference (energy is spread across 22 MHz).

  • Strong correlation properties via Barker code.

  • Simplified MAC integration (identical to FHSS/IR DCF).

  • Good multipath resilience due to spreading.

  • Foundation for 802.11b (CCK and PBCC at higher rates).

Limitations

  • Limited to 1–2 Mbps data rate.

  • Requires 22 MHz per channel → fewer available channels.

  • More susceptible to wideband interference compared to FHSS.

  • Cannot interoperate with FHSS or IR PHYs.

Evolution to 802.11b

802.11b extended DSSS with new modulation techniques:

Data Rate | Technique | Description |

|------------|————|-------------| | 5.5 Mbps | CCK | Complementary Code Keying (8-chip) | | 11 Mbps | CCK | 8-bit symbols, 8-chip spreading | | Optional | PBCC | Packet Binary Convolutional Coding |

802.11b maintained backward compatibility with original DSSS (1 and 2 Mbps) via Barker modulation, enabling mixed-rate networks.

Channelization (2.4 GHz DSSS)

Channel | Center Frequency (MHz) | Overlap |

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

Only channels 1, 6, and 11 are non-overlapping in most regions.

Practical Implementation Notes

  • Ensure transmitter bandwidth and spectral mask compliance.

  • Maintain synchronization using PLCP preamble correlation.

  • Use energy and correlation-based CCA detection modes.

  • DSSS and FHSS cannot coexist in the same BSS.

  • All interframe timings (SIFS/DIFS/EIFS) are identical to FHSS and IR.

Summary

Concept | Description |

|----------|————-| | Medium | 2.4 GHz ISM band | | Modulation | DBPSK (1 Mbps), DQPSK (2 Mbps) | | Spreading Code | 11-chip Barker | | Channel Bandwidth | 22 MHz | | Access | CSMA/CA (DCF) | | Processing Gain | 10.4 dB | | Evolution | 802.11b (CCK-based DSSS) |

References

  • IEEE Std 802.11-1997, Clause 16 — DSSS PHY Specification

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

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

  • Stallings, W. Wireless Communications and Networks

  • IEEE 802.11 Working Group Archives and Technical Reports

Figures

DSSS spreading process

Illustration of DSSS modulation and 11-chip Barker spreading sequence used in IEEE 802.11.

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

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)