HRDSSS PHY =============== IEEE 802.11b High-Rate Direct Sequence Spread Spectrum (HR/DSSS) PHY ==================================================================== .. contents:: :local: :depth: 3 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: .. code-block:: none 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): .. math:: 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: .. code-block:: none 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: .. math:: 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 ------------------------------ .. code-block:: none |<-- 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 .. 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 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 ============ ================= ========== === ================== ========== ============ ======= ====== ============ ========= ====== ========= =============================================================================== .. 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) ======================= ====================== ====================== ============================================================