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Understanding the 802.11a Wireless Communication Protocol: Architecture and Functions

This document provides a comprehensive overview of the IEEE 802.11a standard for wireless communication, detailing its architecture, data transmission techniques, and functional tasks of the transmitter. Key topics include packet format, preamble synchronization, the scrambler, convolutional encoder, puncturer, interleaver, mapper, and the Inverse Fast Fourier Transform (IFFT). It explores modulation schemes, data rates ranging from 6 to 54 Mbps, and the use of Orthogonal Frequency Division Multiplexing (OFDM). The significance of the cyclic prefix for synchronization and equalization is also discussed, providing insights into the operation of this wireless standard.

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Understanding the 802.11a Wireless Communication Protocol: Architecture and Functions

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  1. 802.11a By YasirAteeq

  2. Table of Contents • INTRODUCTION • TASKS OF TRANSMITTER • PACKET FORMAT • PREAMBLE • SCRAMBLER • CONVOLUTIONAL ENCODER • PUNCTURER • INTERLEAVER • MAPPER • IFFT • CYCLIC EXTEND • Q & A…..?

  3. Introduction • IEEE Standard for wireless communication • Data rate from 6 to 54Mbps • Frequency of Operation: 5Ghz band • Modulation: BPSK, QPSK and 16-QAM • Multiplexing :Orthogonal Frequency Division Multiplexing • Unit of data transmission-OFDM symbol

  4. Big Picture of Transmitter of 802.11a

  5. Tasks of Transmitter Tasks: • Encodes data for forward error correction • Maps data into complex pairs & distributes them among the different frequency indices • Transform frequency data into time domain

  6. Packet Format

  7. Preamble • synchronize the demodulator on receiver side for the incoming OFDM signal • The preamble consists of 12 symbols • The receiver uses the long symbols for fine-tuning • It takes 16 microseconds to train the receiver after first receiving the frame.

  8. OFDM • IEEE 802.11a uses OFDM with 64 sub-carriers: • 48 data sub-carriers • 4 pilot sub-carriers • 12 guard sub-carriers • (at spectrum edges) • Symbol rate: 250 ksps • Channel bandwidth: 20 MHz • Sub-carrier spacing: 312.5 kHz • (= 20 MHz/64) • Symbol duration: 4 μs

  9. Rate Dependent parameters of 802.11a Transmit at 3 data rates: (mandatory) •6Mb/s –1 24-bit input data frame per OFDM symbol •12Mb/s –2 24-bit input data frames per OFDM symbol •24Mb/s –4 24-bit input data frames per OFDM symbol

  10. Scrambler • Processes 1 input bit per cycle Simultaneously generates 1 scramble sequence bit and computes 1 output bit • Repeatedly generates a 127-bit scramble sequence • Scrambling w/ LFSR and its G(x) = x7+ x4+ 1 • seed 1011101

  11. Convolutional Encoder Design The convolutional encoder must use the industry-standard generator polynomials, g0 = 133(8) and g1 = 171(8), of rate R = 1/2 Convolutional encoder(K=7)

  12. Puncturer • Puncturing is a very useful technique to generate additional rates from a single convolutional code • The basic idea behind puncturing is to not transmit some of the output bits from the convolutional encoder, thus increasing the rate of the code • Puncturer Vector

  13. Interleaver The interleaver is defined by a two-step permutation First Step: The first permutation ensures that adjacent coded bits are mapped onto nonadjacent subcarriers. Second Step: The second ensures that adjacent coded bits are mapped alternately onto less and more significant bits of the constellation

  14. Mapper The Mapper has two stages Stage-1: First converts each sub-block into a complex number representing BPSK, QPSK, or 16-QAM constellation points Stage-2: The resulting 48 complex pairs are then normalized by Kmod Kmod Table

  15. IFFT • Inverse Fast Fourier transform (IFFT) combines the subcarriers before transmission • A 64-point IFFT is used; the coefficients 1 to 26 are mapped to the same numbered IFFT inputs, while the coefficients –26 to –1 are copied into IFFT inputs 38 to 63 • The rest of the inputs, 27 to 37 and the 0 (dc) input, are set to zero. This mapping is illustrated in Figure

  16. Cyclic Prefix • Guard interval is a cyclic prefix, where a certain number of samples at the end of the time-domain OFDM symbol are copied to the beginning. • The cyclic prefix is used in a variety of ways in different OFDM system implementations, including timing synchronization, frequency synchronization, and carrier equalization

  17. References http://www.its.ohiou.edu/kruse/its437/slides-80211phy.pdf http://www.sti-innsbruck.at/fileadmin/documents/vn-ws0809/10-VN-WLAN.pdf http://csg.csail.mit.edu/6.884/projects/group1-report.pdf http://csg.csail.mit.edu/6.884/projects/group1-presentation-4up.pdf http://standards.ieee.org/findstds/interps/802.11a-1999.html http://www.uotechnology.edu.iq/dep-eee/lectures/4th/Communication/Information%20theory/4.pdf

  18. Q & As?

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