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802.11 a

802.11 a. By Yasir Ateeq. Table of Contents. INTRODUCTION TASKS OF TRANSMITTER PACKET FORMAT PREAMBLE SCRAMBLER CONVOLUTIONAL ENCODER PUNCTURER INTERLEAVER MAPPER IFFT CYCLIC EXTEND Q & A…..?. Introduction. IEEE Standard for wireless communication Data rate from 6 to 54Mbps

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802.11 a

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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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