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Quasi-coherent differential demodulator for binary PSK

IP.com Disclosure Number: IPCOM000017313D
Original Publication Date: 2000-Jul-01
Included in the Prior Art Database: 2003-Jul-25
Document File: 3 page(s) / 36K

Publishing Venue

Siemens

Related People

Holger Gryska: AUTHOR

Abstract

Differential phase shift keying (DPSK) uses only the phase difference of two successive symbols to transfer information. Therefore a small frequency offset between receiver and transmitter can be ignored and the receiver need to know the absolute phase. These characteristics make the DPSK modulation technique attractive for low cost transmission systems. In the following binary DPSK with the symbol alphabet , )( kx (1) { }

This text was extracted from an ASCII text file.
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Information / Kommunikation

Quasi-coherent differential demodulator for binary PSK

Idee: Holger Gryska, München

Differential phase shift keying (DPSK) uses only the phase difference of two successivesymbols to transfer information. Therefore a small frequency offset between receiver andtransmitter can be ignored and the receiver need to know the absolute phase. Thesecharacteristics make the DPSK modulation technique attractive for low cost transmissionsystems. In the following binary DPSK with the symbol alphabet

(1)� � { }

x k

( ) ,

Î -� � 11

is used. Every symbol is transmitted with a positive or negative phase rotation of p/2:

(2)� s k s k j x k( ) ( ) * exp * * ( )

= - æèç öø÷1 2p

This disclosure uses the following transmission model with a white-gaussian-noise channel:

(3)

The exponential function takes into account the imperfections of the down mixing to thebaseband. ? f is the frequency offset between the transmitter and receiver down mixer and T sis the symbol period. ? 0 denotes a constant phase offset between transmitter and receiver.The noise term n(k) is complex normal distributed with zero mean and s ² as variance. Afternormalising the amplitude you get at the receiver

is used. Every symbol is transmitted with a positive or negative phase rotation of p/2:

(2)� s k s k j x k( ) ( ) * exp * * ( )

= - æèç öø÷1 2p

This disclosure uses the following transmission model with a white-gaussian-noise channel:

(3)

The exponential function takes into account the imperfections of the down mixing to thebaseband. ? f is the frequency offset between the transmitter and receiver down mixer and T sis the symbol period. ? 0 denotes a constant phase offset between transmitter and receiver.The noise term n(k) is complex normal distributed with zero mean and s ² as variance. Afternormalising the amplitude you get at the receiver

Î -� � 11

[ ]

r k s k j k f T n k

s

( )

=

+ +

2

( ) ( ) * exp * * * * * ( )

0

p y

D

(4)� r k r k j x k f T k

s n

( ) ( ) * exp * * ( ) * * * ( )

1 2 2

The phase noise ?? 2n(k) is the uniformly distributed phase of a complex normal distributednoise process with a variance of twice s ² because r(k) and r(k-1) have both a variance of s ².

The DPSK demodulation is usually done with the following differential demodulator:

= -

æèç

é

ëê

p p y

D D

+ +

öø÷

ù

ûú

lm� � � { }

v k r k

= -

æèç

x k f T k

s n

+ +

p yD D

öø÷

2

( ) ( * sin * ( ) * * * ( )

1 2 2

The disadvantage of this approach is that both symbols are disturbed by noise of variances ². Therefore v(k) has a variance of twice s ² what results in a performance loss of almost3dB. To correct this disadvantage we propose to have a fixed reference symbol and to add afilter to the reference branch which reduces the variance of the reference symbol. The resultis a very effort effective implementation of a phase offset estimator and you get a quasicoherent demodulator.

p

2

Siemens Technik Report

Jahrgang 3� Nr. 8� Juli 2000

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