SF

Module03_-_deck_1_dec2024

Module Overview

Title: WLANs and Spread SpectrumPublisher: Wiley Sybex, A Wiley BrandModule Number: 6

Key Topics Covered

  • Spread Spectrum Techniques

    • Frequency Hopping Spread Spectrum (FHSS)

      • Basics: FHSS modulates the radio signal by rapidly switching frequencies to minimize the impact of potential interference and provide a secure, stable connection.

      • Key Parameters:

        • Hop Time: The duration between frequency changes, typically measured in milliseconds, which affects the system's resilience to interference.

        • Dwell Time: The time the system transmits on a particular frequency before changing to the next, balancing reliability and data throughput.

        • Hopping Pattern: The sequence of frequencies used during transmission; a complex pattern enhances security and reduces the chance of detection by unauthorized interceptors.

      • Examples: Commonly used in Bluetooth technologies, enabling short-range communication through dynamic frequency changes.

  • Direct Sequence Spread Spectrum (DSSS)

    • Description: This technique spreads radio frequency signals across a larger bandwidth than the data size requires, enhancing resilience against interference and eavesdropping.

    • Advantages: It exhibits superior resistance to narrowband interference, improving the reliability and quality of wireless connections, particularly in environments with high potential for signal disruption.

    • Types of DSSS:

      • DSSS: Standard implementation used for spreading bandwidth.

      • HR-DSSS (High Rate DSSS): Enhanced version supporting higher data rates while maintaining the robustness of DSSS.

      • OFDM (Orthogonal Frequency-Division Multiplexing): A more advanced technique providing greater data throughput by splitting signals across numerous subcarriers.

  • PHY Modulation and Coding (Data Rates)

    • Modulation Techniques Used in IEEE 802.11 Standards:

      • 802.11 (Original): Offered 1 & 2 Mbps using BPSK (Binary Phase Shift Keying) and QPSK (Quadrature Phase Shift Keying).

      • 802.11b: Increased capacities to 5.5 & 11 Mbps leveraging QPSK and CCK (Complementary Code Keying).

      • 802.11a: Featured data rates ranging from 6 to 54 Mbps employing diverse modulation schemes such as QPSK and 16-QAM.

      • 802.11g: Provided backward compatibility with 802.11b while offering variable data rates.

      • 802.11n: Introduced new Modulation and Coding Scheme (MCS) allowing support for up to 64-QAM, facilitating higher data rates.

      • 802.11ac: Offered significant advancements in data rates, enabling up to 256-QAM.

      • 802.11ax (Wi-Fi 6): Featured 1024-QAM, further enhancing throughput, efficiency in crowded environments, and reducing latency.

      • 802.11be (Wi-Fi 7): The upcoming standard designed to use 4096-QAM for even greater performance and capacity.

    • Modulation Methods:

      • ASK: Amplitude Shift Keying, varies the amplitude of the carrier wave.

      • FSK: Frequency Shift Keying, changes the frequency of the carrier wave.

      • PSK: Phase Shift Keying, alters the phase of the carrier, enabling distinct data representation based on phases.

    • Quadrature Amplitude Modulation (QAM): Combines both amplitude and phase shift, allowing for efficient data transmission with multiple bits per symbol, e.g., 16-QAM uses 16 unique phase and amplitude combinations.

  • Error Vector Magnitude (EVM):

    • Definition: A measure to assess the performance of a receiver or transmitter, quantifying the deviation from expected signal points.

    • Significance: Especially critical in high-order modulations like 1024-QAM, where precision in signal transmission significantly affects quality and reliability.

  • Barker Code and Coding Techniques:

    • Barker Code: A method that converts data bits into a wider chip code to evade interference through complex sequences.

    • X-OR Operation: Combines bits using an Exclusive-OR operation to produce pseudorandom sequences spread across a broader frequency band, thereby enhancing security.

    • Coding Objective: Aims to minimize transmission errors and ensure accurate data recovery, with coding rates such as 1/2, 2/3, and 3/4 indicating different levels of redundancy.

  • OFDM (Orthogonal Frequency-Division Multiplexing):

    • Mechanism: Divides data into multiple subcarriers, each transmitting unique pieces of information simultaneously, enhancing integrity and reducing latency.

    • OFDM Subcarriers: Consists of data subcarriers that carry the main signal, pilot subcarriers for synchronization, and null subcarriers that diminish interference risk.

    • Subcarrier Spacing and Symbol Time: Critical factors in ensuring effective transmission, allowing efficient usage of frequencies without interference between channels.

Channel Specifications

  • 2.4 GHz Channels:

    • Channel Details: Includes 14 channels ranging from 2.400 to 2.500 GHz, typically subject to higher levels of interference due to congestion and overlapping channels.

  • 5 GHz Channels:

    • Details: Offers multiple UNII bands with a wide frequency range of 5.15 to 5.85 GHz, providing lower congestion and reduced interference vulnerabilities than 2.4 GHz Bands.

  • 6 GHz Wi-Fi Channels:

    • New Allocations: Recently designated U-NII bands operate from 5925 to 7125 MHz, accommodating wider channels and increased use cases, supporting multiple channel widths for expanded user demand.

Future Channel Considerations

  • Channel Availability: Future expansion to include 20, 40, 80, and 160 MHz channels in the 6 GHz spectrum, anticipated to support the growing requirement for higher data throughput and more simultaneous user connections.