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Summary SDR4all OFDM

Channel Estimation using SDR4All: a Case Study

Leonardo S. Cardoso Merouane Debbah

SUPELEC, Gif-sur-Yvette, France

Alcatel-Lucent - Supelec Chair in Flexible Radio
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Summary

Objectives of the course:

Present a real-life channel estimation system and the underlying

mechanisms required to make it work

In this course youll see:

SDR4All (a tool for implementing actual transmissions);

Hardware related channel impairments;

Channel estimation based on OFDM;
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SDR4all
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SDR4allWhats SDR4all anyways?

SDR: Software Defined Radio Hardware: radio frequency (RF) part (medium and high

frequency); Software: Base-band radio.

SDR4all: an easy to use platform for creating and testingcommunication schemes

Hardware: SDR4all cards (USRPs will be used in this course); Software: Matlab toolbox + windows drivers + glue software.


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SDR4allSDR frontend

Includes simple blocks, such as: MODEM - Bit/symbol manipulation - Error detection - Test;

and also elaborate blocks, such as:

Full transmission/reception chains - Channel sounding;

http://www.sdr4all.eu/

S S O
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SDR4allHardware Backend

Controller (mother) board (USB Controller + FPGA)

RF control: Selection of frequency, rates, clock speeds... USB 2.0 link: Selection of rates, buffer handling... Decimation/Interpolation: Sampling rates, filtering...

RF board (up to four) RF circuitry: ISM band (2.4GHz), filtering, amplifiers... Sampling: analog to digital / digital to analog converters. Glue logic: Configuration buses, board selection...

S SDR4 ll OFDM
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SDR4allOverview

USRPUSRP

DRIVER DRIVER

MATLAB MATLAB

Baseband TX Baseband RX

USB USB

computer computer

S SDR4 ll OFDM
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SDR4allInside the card

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SDR4allSISO vs. MIMO



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y


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OFDM

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OFDMPrinciples

OFDM: Orthogonal Frequency Division Multiple Access

a particular form of multi-carrier transmission;

suitable to frequency selective channels with high bandwidth;

transforms a frequency-selective wide-band channel into a setof flat narrowband channels;

robust against large delay spreads by preserving orthogonality

in the frequency domain;

cyclic redundancy at the transmitter reduces the complexity toonly FFT processing.

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OFDMFrequency selective fadign vs. Flat fading

Frequency selective fading: coherence bandwith is smallerthan that of the signal;

Flat fading: coherence bandwith is smaller than that of thesignal;

Coherence band (Wc):

Wc 1

2Td

Tdis the delay spread (time between first and lastelectromagnetic path)

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OFDMFrequency selective fadign vs. Flat fading

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OFDMPrinciples

bn

sampling

rate T

DAC

P S

MODULATOR

modulation

DEMODULATOR

parallel

conversionto serial

guardinterval

insertion

digital to

converter

demodulation

sampling

rate T

P S

rn

ADC

r t

serial toparallel

conversion

analog todigital

converter

guardinterval

suppressionanalog

s1 ks2 k

sN k

s k x k

x1 k

x2 k

x3 k

xN k

xN D 1 k

xN D 1 k

xN k

x1 k

xN k

C t

xn x t

rcp1 k

rcp

D kr1 k

r2 k

r3 k

rN k

rcp k y k

y1 k

yN k

h1

hN

r k

rcp

N D k

FF 1

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OFDMConsiderations

FFT: transforms a convolution with the channel in time domain into

a multiplication in the frequency domain; helps reduce the equalization process into a 1-tap one;

Cyclic Prefix: allows for the use of an FFT; helps guarantee the orthogonality of the system by taking care

of block-interference;
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