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802.11 a By Yasir Ateeq

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

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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

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Big Picture of Transmitter of 802.11a

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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

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Packet Format

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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.

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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

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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

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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

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

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Puncturer 1.Puncturing is a very useful technique to generate additional rates from a single convolutional code 2.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 3.Puncturer Vector

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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

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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

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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

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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

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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

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Q & A s?

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