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8-1Modulation and DemodulationPE/TRD/GR/0109 12.02/EN February, 2000

"Confidentialinformation--maynotbecopiedordisclosedwithoutpermission".

Modulation and Demodulation

Section 8

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Information physical transmission through the air

After completing this lesson, you will be able to:

- Briefly describe GMSK: digital modulation used in GSM

- Present how demodulation works through equalization

- Describe the channel decoding method

Objectives

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Source

decoding

Channel

decoding

De-interleaving

Burst

deformatting

Deciphering

Demodulation

equalization

Digitizing and

source coding

Channel

coding

Modulation

Ciphering

Burst formatting

Interleaving

Transmission

Diversity

section 6

section 7

section 8

section 10

Location In Transmission and Reception Chains

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"Confidentialinformation--maynotbecopied

ordisclosedwithoutpermission".

Demodulation

(Equalization)

AIR INTERFACE

SPEECH

DATA

SIGNALING

SPEECH

DATA

SIGNALINGDeciphering

De-interleaving

Decoding

Modulation

MODULATED

SIGNAL

RECEIVED

SIGNAL

0 1

SOFT BITSBITS

BITS

Modulation and Demodulation Overview

BITS

Ciphering

Interleaving

Coding

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Q

+ 90 '0''1' - 90

I

Bit 0Bit 1

phase shift + 90 phase shift - 90

Bit b=0

MSK

GMSK

-Tb Tb0

90

t

( t)

relativepower(dB)

frequency(MHz)

3 dB

Fc

271 kHz

GMSK spectrum

GMSK Modulation

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carrier

frequencyFi

sampling

at 4xFi&

quantization

FREQUENCY

TRANSPOSER

4xFi

analog

transposition

at Fi

maximizesC

Il + Ic + Ir+N0

2xFi

200 kHz

0

- 10

- 20

- 30- 40

- 50

- 60

F

C

IrIl Ic

GMSK

modulation

spectrum

F+0.2F-0.2

baseband

optimized

filtering

relativepower(dB)

frequency(MHz)

Fi

ANTI-ALIASING

FILTER

A/D

CONVERTER

Fi

Digitization and Preprocessing

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2 x 57

SOFT BITS

PER BURST

BUFFER

MAIN

DIVERSITY

M

D

CHANNEL

SOUNDING

COMBINATION

(MRC)

EQUALIZER

(VITERBI)

DE-INTERLEAVING

DECODER

(VITERBI)

456BITS

260BITS

DEMODULATION

DECODING

TA C/I RXLEV RXQUALDECIPHERING

Demodulation and Decoding

8 bursts

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PROPAGATION

Known at thereception

Training

sequenceData

Tail

bitData

Tail

bit

Sentburst

&

ReceivedTraining

Sequence

Received

burst

-

ESTIMATED FILTER

Channel Sounding

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possible sentsequence of 57

bits over 257

COMPARISON OF 57 BITS:

the estimated sequence is the most

likelyone, minimizing the least mean

square error

NOTUSED

needs 257

comparisons

of 57 bits!!

possible receivedsequence over 257

received sequence

ESTIMATED FILTER

Demodulation: a Complex Method

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forea

choft

he16

partialsequ

ences

two possibly received sequences

ESTIMATED FILTER

Adding 0 or 1

Channel filtering

Choice of the best sequenceaccording to the maximum likelihood criterion

COMPARISON WITH

THE RECEIVED SEQUENCE

finally Choice of the best sequence out of 16

according to the maximum likelihood criterion

Sent Sequence Estimation: Viterbi Algorithm

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POSSIBLE

SEQUENCES

SIGNAL

GENERATOR

DEMUX

training sequence

information

receivedsamples

INCREMENT

CALCULATOR

(metrics)

soft

bits

0XXX

1XXX

CHANNEL

SOUNDING

VITERBI

ALGORITHM

Demodulation with Equalization: Viterbi Algorithm

Viterbi Equalizer

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received

sequence

estimated sent sequence which

verifies the maximum likelihood

criterion (soft bits levels)

difference betweenestimated and effectively

received sequence = NOISE

each estimated value has

its own likelihood

Bit Error Rate (RXQUAL)

involves all the bitslikelihoods

S = LogP(r / 0)P(r / 1)

1 0

1XXX 0XXX

p(r/1) p(r/0)

r

P

RXQUAL (burst) = =1

1141

11

114

+=

ek

Sk

1

114 1

114

P Serrork

k=

( )

Bit Error Rate Estimation

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260 bits = 20 ms of FR speech

Class I: 378 bits

456 bits from de-interleaving

189 bits

Convolutional Decoding (r = 1/2)

Class II78 bits

50 132 43

Class Ia

50 bits

Class Ib

132 bits

Class II

78 bits

Parity Code Tail Bits

Channel Decoding: TCH Full Rate

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00001000

0100

1100

0010

1010

01101110

0001

1001

0101

1101

0011

1011

0111

1111

0 1 2 3 4 5 6 7 time (bit period)

state of the encoder

0 0 0 1 1 1 1 0 1 0 0 1 0 1 0 1

0/00 0/00

1/110/10

1/11

1/01

1/01

0/00

0 1

1

1

2

2

2

3

errors in the demodulated sequence

path with the minimum

cumulated metric

cumulative metrics with

hard decisions

Viterbi Decoding with Hard Decisions

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0 1 2 3 4 5 6 7 time (bit period)

+ 8

+ 1

- 1

- 8

0000

1000

0100

1100

0010

0/00 0/00

1/11

0/10

1/11

12

12

13

1

28

2

21

8

26

5

X /XX Maximum Cumulative Path

Cumulative Path Metric

Branch Metric (Partial Metric)

+7 +5

+12

0

1

Viterbi Decoding with Soft Decisions

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Exercise

Exercises: Use the Viterbi Trellis

Exercise 1: Hard BitsYou received the following sequence of 16 hard bits (the left bit is the firstreceived): 1001000010111101.

The coding was done with the convolutional codes used for full ratespeech.

Question: find the 8 bits of the original information.

Exercise 2: Soft Bits

You received the following sequence of 6 hard bits with their associatedsoft bits, so in total 24 bits:

1/011 0/001 0/110 1/011 0/011 0/101

The coding was done with the convolutional codes used for full ratespeech.

Question: find the 3 bits of the original information.

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"Confidentialinformation--maynotbecopie

dordisclosedwithoutpermission".

1- What are the GSM modulation characteristics?

2- What is the purpose of channel sounding?

3- How is the signal demodulated?

4- What is the interest of soft decisions?

5- Give the principle of Viterbi Algorithm. When is it used?

Check Your Learning

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dordisclosedwithoutpermission".

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