Direct Sequence Spread Spectrum (DSSS) and Complementary Code Keying (CCK) are fundamental modulation techniques used in wireless communication, most notably in the IEEE 802.11 (Wi-Fi) standards. --- ### 1. Direct Sequence Spread Spectrum (DSSS) DSSS is a transmission method where the signal occupies more bandwidth than the information baseband requires. This is achieved by multiplying the data with a "chipping code." #### Key Electronic Components/Processes: * **PN Code Generator:** Generates a Pseudo-random Noise (PN) sequence (e.g., Barker Code). * **Modulator/Multiplier:** Combines the low-speed data bits with the high-speed chips. * **Phase Shift Keying (PSK):** Usually, BPSK or QPSK is used to modulate the resulting spread signal onto a carrier frequency. * **Correlator (Receiver Side):** Extracts the original signal by multiplying the received spread signal with the exact same PN code. | Feature | Description | | :--- | :--- | | **Spreading Code** | 11-bit Barker Sequence (10110111000) | | **Data Rates** | 1 Mbps and 2 Mbps | | **Bandwidth** | Typically 22 MHz per channel | | **Resilience** | High resistance to interference and jamming | --- ### 2. Complementary Code Keying (CCK) CCK was introduced in 802.11b to increase data rates beyond 2 Mbps without increasing the bandwidth. It replaces the fixed Barker code used in DSSS with a set of complex "complementary codes." #### Electronic Functionality: Instead of using one static spreading sequence, CCK uses a set of 64 unique 8-chip code words. The electronic circuitry selects one of these 64 codes based on the data bits being sent. * **Mapping Unit:** Groups input data into 8-bit symbols. * **Complex Code Generator:** Mathematically derives a complex sequence using Phase Shift Keying (QPSK) variations. * **I/Q Modulator:** Handles the In-phase and Quadrature components of the complex codes for transmission. | Feature | Description | | :--- | :--- | | **Data Rates** | 5.5 Mbps and 11 Mbps | | **Code Length** | 8 chips per symbol | | **Compatibility** | Backward compatible with DSSS infrastructure | | **Efficiency** | Higher spectral efficiency than standard DSSS | --- ### Comparison Summary | Parameter | DSSS (Barker) | CCK | | :--- | :--- | :--- | | **Standard** | 802.11 | 802.11b | | **Complexity** | Low | Moderate | | **Chipping Sequence** | Fixed (11-bit) | Variable (set of 64 codes) | | **Noise Immunity** | Excellent | Good |