Complementary Code Keying

802.11 direct-sequence systems use a rate of 11 million chips per second. The original DS PHYs divided the chip stream up into a series of 11-bit Barker words and transmitted 1 million Barker words per second. Each word encoded either one bit or two bits for a corresponding data rate of 1.0 Mbps or 2.0 Mbps, respectively. Achieving higher data rates and commercial utility requires that each code symbol carry more information than a bit or two.

Straight phase shift encoding cannot hope to carry more than a few bits per code word. DQPSK requires that receivers distinguish quarter-cycle phase differences. Further increasing the number of bits per symbol would require processing even finer phase shifts, such as an eighth-cycle or sixteenth-cycle shift. Detecting smaller phase shifts is more difficult in the presence of multipath interference and requires more sophisticated (and thus expensive) electronics.

Instead of continuing with straight phase-shift keying, the IEEE 802.11 working group turned to an alternate encoding method. Complementary code keying (CCK) divides the chip stream into a series of 8-bit code symbols, so the underlying transmission is based on a series of 1.375 million code symbols per second. CCK is based on sophisticated mathematical transforms that allow the use of a few 8-bit sequences to encode 4 or even 8 bits per code word, for a data throughput of 5.5 Mbps or 11 Mbps. In addition, the mathematics underlying CCK transforms allow receivers to distinguish between different codes easily, even in the presence of interference and multipath fading. Figure 12-16 illustrates the use of code symbols in CCK. It is quite similar to the chipping process used by the slower direct-sequence layers; the difference is that the code words are derived partially from the data. A static repeating code word such as the Barker word is not used.

Figure 12-16. Code symbols in CCK

Barker spreading, as used in the lower-rate, direct-sequence layers, uses a static code to spread the signal over the available frequency band. CCK uses the code word to carry information, as well as simply to spread the signal. Several phase angles are used to prepare a complex code word of eight bits.

Introduction to Wireless Networking

Overview of 802.11 Networks

11 MAC Fundamentals

11 Framing in Detail

Wired Equivalent Privacy (WEP)

User Authentication with 802.1X

11i: Robust Security Networks, TKIP, and CCMP

Management Operations

Contention-Free Service with the PCF

Physical Layer Overview

The Frequency-Hopping (FH) PHY

The Direct Sequence PHYs: DSSS and HR/DSSS (802.11b)

11a and 802.11j: 5-GHz OFDM PHY

11g: The Extended-Rate PHY (ERP)

A Peek Ahead at 802.11n: MIMO-OFDM

11 Hardware

Using 802.11 on Windows

11 on the Macintosh

Using 802.11 on Linux

Using 802.11 Access Points

Logical Wireless Network Architecture

Security Architecture

Site Planning and Project Management

11 Network Analysis

11 Performance Tuning

Conclusions and Predictions

802.11 Wireless Networks The Definitive Guide
802.11 Wireless Networks: The Definitive Guide, Second Edition
ISBN: 0596100523
EAN: 2147483647
Year: 2003
Pages: 179
Authors: Matthew Gast

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