DSSS PHY =============== IEEE 802.11 Direct Sequence Spread Spectrum (DSSS) PHY ======================================================= .. contents:: :local: :depth: 3 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 = :math:`b(t)` - Spreading code (chips) = :math:`c(t)` - Transmitted signal = :math:`s(t) = b(t) \times c(t)` Each data bit is represented by **11 chips**, where each chip has a duration: .. math:: T_c = \frac{T_b}{11} At 1 Mbps data rate: .. math:: 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**: .. code-block:: none +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): .. code-block:: none +----------------+----------------+----------------+ | 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): .. code-block:: none 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: .. math:: R = \sum_{i=1}^{11} r_i \cdot c_i where: - :math:`r_i` = received chip - :math:`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 ------- .. figure:: _static/dsss_spreading.svg :align: center :alt: DSSS spreading process Illustration of DSSS modulation and 11-chip Barker spreading sequence used in IEEE 802.11. .. tab-set:: .. tab-item:: physical_rates in 802.11b DSSS PHY layer ============ ================= ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== 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 ============ ================= ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== .. tab-set:: .. tab-item:: List of channels (802.11b) ===================== ======================= ====================== ============== 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 ===================== ======================= ====================== ============== .. tab-set:: .. tab-item:: List of Bands (802.11b) ======================= ====================== ====================== ============================================================ 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) ======================= ====================== ====================== ============================================================