adaptive modulation and coding for ofdm systems

8/3/2019 Adaptive Modulation and Coding for OFDM Systems

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Adaptive Modulation and Coding

Suk Chan Kim

Pusan National University


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Introduction

Adaptive code-rate for OFDM system

Adaptive modulation for BICM-OFDM system

Conclusion

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Adaptive transmission techniques improve the performance of wireless

communication system by adjusting transmission parameter like

-, ,

adaptively.

When adaptive transmission is applied to OFDM system, transmission

parameters of each subcarrier are adapted to each subchannel. Adaptivemo u at on or unco e system ave een stu e w e y.

e s u y a ap ve co e-ra e an a ap ve co e mo u a on or

system.

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-

Code-rate of each subcarrier is adapted to each subchannel optimally.

o o ta n erent co e-rate or eac su carr er, rate-compat e punctureconvolutional (RCPC) code is used

Received signal-to-noise ratio (SNR) is used for measure of subchannel

state to select a ro riate code-rate for each subcarrier.

Transmission power and modulation for each subcarrier is fixed for

simplication of the adaptation algorithm

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-

Code-rate for each subcarrier are selected according to switching levels.

Received SNR and code-rate of each subcarrier is shown in figure.

e t su carr er se ects t e co e-rate r2 s nce t e rece ve sbetween s2 and s3.

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Our optimization problem is

, , ,

: set of switching level,

c Ts

s

, : average co e-ra e,

( , ) : average BER,

c

BER

s

s

: target BER.T

BER

,

optimazation.

( , ) ( , ) ( , )c T

J R BER BER = + s s s

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Rayleigh fading

BPSK modulation.

arent convo ut ona co e: = 4, g = (15,17,13)8. RCPC code-rates: 8/9, 8/10, 8/14, 8/18, 8/22, 8/24.

arge = - or - .

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-

Average code-rate of adaptive code-rate OFDM system

at BER = 10-4 and 10-6.

0.8

0.9

0.7

-rate

0.5

.

veragecod

0.3

0.4

Average BER = 10-4

0 5 10 15 200.2

Average SNR (dB)

Average BER = 10-6

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Average BER of adaptive and fixed code-rate OFDM system.

10-2

1/3 adaptive

10-3

1/2 adaptive

2/3 adaptive

1/3 fixed

1/2 fixed

-5

10-4

rProbability

10-6

10

AverageBitErr

10-7

0 2 4 6 8 10 12 14 16 18 20

10-8

Es/No [dB]

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Code-rate of each subcarrier was adapted to subchannel state to maximize

data rate and to satisfy average BER.

To get different code-rate and to reduce complexity, RCPC code was used

for each subcarrier.

We showed that lar e SNR ain was achieved b ada tive code-rateOFDM system.

Since we use BPSK modulation, data rate is saturated at high SNR. If highlevel modulation is used, we can expect more increase of data rate at high

.

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-

OFDM system overcome ISI caused from multipath fading using large

number of subcarrier and guard interval.

Channel coding is essential for OFDM system to cope with faded

subcarriers.

Bit-interleaved coded modulation BICM is widel used in broadbandwireless communication since it has high capacity in fading channel and it

use bit-interleaver and simple encoder/decoder.

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-

Adaptive modulator choose modulation level for each subcarrier according

to subchannel state and maps coded bits to QAM symbol using Gray

.

Demodulator produces the bit metric using transmission mode of each

subcarrier.

`

TransmitBinar data

OFDMBICM

Encoder Interleaver Modulator, ,

Add CP

channelEstimation

AWGN

Transmission mode

from Transmitter

Receive

Binar dataDecoder DeInterleaver Demodulator

, ,

Remove CP

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Our adaptation policy is to minimize BER under the constant throughput.

-1N

0 1c

,..., 0

: the number of coded bits for th subcarrier

Nk

m m k

km k

=

c: the number of coded bits for a OFDM symbolN

OFDM-BICM system by assuming ideal bit interleaving.

21

22

0

| |1( , , ) exp | | ()4

dN

kk

kc

h f d m E x zN

=

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.

Using Lagrange method, our adaptation problem is simplified by

minH m

22| |where exp | | .

km

h H m E x z

=

k , , , , , ,

discrete resource allocation problem.

We find Lagrange multiplier that satisfy the given constraintNc usingiterative method.

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.

Example of finding iteratively givenNc = 256

iteration

0 1.000000 336 70.939687

1

0

Nkk

m=

1

0

Nkk

H=

1 0.211130 214 4.676390

. .

3 0.366540 242 12.005676

4 0.441766 261 19.545568

5 0.396836 256 17.418943

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Coded bits per subcarrier mk= {0,1 2,4,6,8}.

64 subcarrier, 4.0s OFDM symbol duration.

xponent a y ecay ng mu t pat ntens ve pro e. Delay spread ofrms= 0.2s, max= 0.8s

onvo u ona co e w = , g= 8, 8 , c= , ,

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Bit allocation result for a realization of the multipath channel

when SNR = 18 dB,Nc = 256.

1

1.5

f|h|

0

0.5

Square

Subcarrier

4

6

bits/

rrier

0 10 20 30 40 50 600

1

2

Co

de

Su

bc

su carr er

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BER whenRc=1/2,Nc = 128, 256.

100

=

10-1

c

Nonadaptive (Nc

= 128)

Adaptive (Nc

= 256)

Nondaptive (N = 256)

-

10-2

10-4

10BE

10-5

4 6 8 10 12 14 16 1810

-6

SNR (dB)

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BER whenRc=2/3,Nc = 128, 256.

100

10-1

c

Nonadaptive (Nc

= 128)

Adaptive (Nc

= 256)

Nonada tive N = 256

10-2

10-4

10-

BE

10-5

6 8 10 12 14 16 18 20 2210

-6

SNR (dB)

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BER whenRc=3/4,Nc = 128, 256.

100

=

10-1

c

Nonadaptive (Nc = 128)

Adaptive (Nc

= 256)

Nonadaptive (N = 256)

-

10-2

10-4

10BE

10-5

8 10 12 14 16 18 20 22 2410

-6

SNR dB

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Adaptive modulation for BICM-OFDM system was considered.

oun s m n m ze un er t e constant t roug put.

o e s were a oca e o eac su carr er us ng scre e agrange

optimization.

Simulation results show that adaptive BICM-OFDM system got large SNR

gain over nonadaptive one.

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adaptive modulation and coding for ofdm systems

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