802.11a 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

Frequency of Operation: 5Ghz band

Modulation: BPSK, QPSK and 16-QAM

Multiplexing :Orthogonal Frequency Division Multiplexing

Unit of data transmission-OFDM symbol

Big Picture of Transmitter of 802.11a

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

Packet Format

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.

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

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

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

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

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

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

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

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

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

References

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

Q & As?