[PDF] OFDM Basics Introduction to OFDM. • Discussion of

View - Download OFDM Basics

Previous PDF

Next PDF

Multi-Carrier Transmission

over Mobile Radio Channels

Jean-Paul M.G. Linnartz

Philips Research and TU/e

Outline

• Introduction to OFDM • Discussion of receivers for OFDM and MC-CDMA • Intercarrier Interference, FFT Leakage • New receiver designs • Simulation of Performance • Conclusions

OFDMOFDM: a form of MultiCarrier Modulation.

• Different symbols are transmitted over different subcarriers • Spectra overlap, but signals are orthogonal. • Example: Rectangular waveform -> Sinc spectrum

Applications

Fixed / Wireline:

• ADSL Asymmetric Digital Subscriber Line

Mobile / Radio:

• Digital Audio Broadcasting (DAB) • Digital Video Broadcasting - Terrestrial (DVB-T) • Hiperlan II • Wireless 1394 •4G (?) I-FFT: OFDM TransmissionTransmission of QAM symbols on parallel subcarriers

Overlapping, yet orthogonal subcarriers

cos( c t+ s t) cos( c t) cos( c t+i s t) cos( c t+ (N-1) s t) User symbols

Serial-to-

parallel

Serial-to-

Parallel

I-FFT

Parallel-to-

Serial

I-FFT: OFDM TransmissionTransmission of QAM symbols on parallel subcarriers

Overlapping, yet orthogonal subcarriers

otherwise 0,0 1 stj s k TteTt k

1,...,1,0 ; with

0 csk Nkk Although the subchannels overlap, they do not interfere with each other at f =f k ;(k = 0, 1,...,N c -1). Indeed, they are orthogonal: s T lk lkdttt 0*

OFDM Subcarrier Spectra

Symbol duration : inverse of

subcarrier spacing plus cyclic prefix

Sampling rate : inverse of

transmit bandwidth

Frequency

Pulse shape in time domain: rectangle (t /NT

s

TsNNfTsNNfTsNNfaTNNtats

cpcp cpN n nF scpN n n sinsinc S(f))( 1 01 0

OFDM Subcarrier Spectra

OFDM signal strength versus

frequency.

Rectangle <- FFT -> Sinc

before channel after channel

Frequency

Cyclic Prefic / Cyclix postfixThe length of the cyclic prefix should be made longer than the experienced impulse response to avoid ISI and ICI. However, the transmitted energy increases with the length of the cyclic prefix. The SNRloss due to the insertion of the CP is given by where T cp denotes the length of the cyclic prefix and T=T cp +T s is the length of the transmitted symbol.

CP OFDM SYMBOL

T cp T s T TTSNR cp loss

1log10

10 Coded OFDMReceived signal at a subcarrier is not affected by transmitted symbols in any other subcarrier Symbol data can be recovered using simple single tap data estimation Viterbi decoder helps recover bits from subcarriers in deep fade 10 10 10 10 00 NNNN nn aa HH yy 10 11 1 0 10 00 NN yy HH aa N

Static Environment

Ave BER curves:

slope ~ degree of diversity on fading channel lmfj lm ls ehH )(2 Single Frequency NetworksOFDM is robust against delay spread

We can "mis"use this by transmitting

a synchronous signal from two transmit sites

The Wireless Multipath Channel

OFDM and MC-CDMA in a

rapidly time-varying channel Doppler spread is the Fourier-dual of a delay spread

Mobile Multipath Channel

Collection of reflected waves, each

with • random angle of arrival • random delay

Angle of arrival is uniform

Doppler shift is cos(angle)

U-shaped power density spectrum

Doppler Spectrum

Crosstalkcaused by Doppler

For uniform angles of arrivals of waves, ICI power spilled from transmit subcarrierninto received subcarrierm= n+ equals where f is the maximum Doppler shift, and P T the local mean received power, per subcarrier 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10 -4 10 -3 10 -2 10 -1 10 0

Normalized Doppler [fm/fsub]

Power, Variance of ICI

P0

P1 P2 P3

Power or variance of ICI

Doppler spread / Subcarrier Spacing

Neighboring subcarrier

2nd tier subcarrier

3rd tier subcarrier

)}(Ȧ)ȦȦȦ(exp{ȦȦsinc2 211
0 mnjTjTnjmnD siisicI i siinm w 1 1 22
1sinc 8 E xdxxff PP sTnnnnch Y= [y 0 , y 1 , ..,y N1 T ,withy m n a n m,n T s m,n isthe 'transfer' for a signal transmitted at subcarriern and received at subcarrierm,

ICI caused by Doppler

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 10quotesdbs_dbs17.pdfusesText_23

[PDF] fft code example

[PDF] fft code for arduino

[PDF] fft code in c

[PDF] fft code in verilog

[PDF] fft code python

[PDF] fft codechef

[PDF] fft complex number

[PDF] fft complex number frequency

[PDF] fft complex number input

[PDF] fft complex number meaning

[PDF] fft complex number result

[PDF] fft convolution complexity

[PDF] fft eigenvalues

[PDF] fft example arduino

[PDF] fft example by hand