Shift Keying by Erol Seke For the course Communications OSMANGAZI - - PowerPoint PPT Presentation
Shift Keying by Erol Seke For the course Communications OSMANGAZI UNIVERSITY Basic PCM Binary 1 is represented by voltage A Binary 0 is represented by voltage B Amplitude Shift Keying (ASK) Binary - ASK case carrier If A and B has
by Erol Seke For the course “Communications”
OSMANGAZI UNIVERSITY
Basic PCM
Binary 1 is represented by voltage A Binary 0 is represented by voltage B
Amplitude Shift Keying (ASK)
carrier If A and B has opposite signs then there will be a phase discontinuity at bit-value changes Binary - ASK case
Frequency Shift Keying (FSK)
Use different frequency values (finite number of) instead of different amplitudes
f2 f1
Example : Binary FSK Binary 1 is represented by a sinusoid with frequency f1 Binary 0 is represented by a sinusoid with frequency f2 Note: Amplitude does not change, phase is not an issue
Phase Shift Keying (PSK)
Use different phase values (finite number of), and we get PSK Note: Amplitude and frequency do not change Example : Binary PSK (BPSK) Binary 1 is represented by a sinusoid with 0 phase Binary 0 is represented by a sinusoid with phase π Carrier with phase A Carrier with phase A+π
)) ( 2 cos( ) ( t s ft A t BPSK 1 ) ( binary for binary for t s
Cosine and Sine are Orthonormal t t
) ( ) (
2 1
dt t t ) (
2 t
) (
1 t
f1 f1
same frequencies A sinusoidal signal with any phase (at frequency f1) can be obtained by a weighted sum of these basis waveforms and
) (
1 t
) (
2 t
cos sin
I Q I Q
) 2 cos( ) 2 sin( ) ( ft ft t s
In phase component Quadrature component
BPSK S1 Symbol S2 Binary 1
) 2 cos( t fc ) 2 cos( t fc
Signal I Q 1
1 A binary stream 1 1 1 1 1 1 1 Phase changes
Example shows 1 period per bit. This is not necessary. Infact in real systems there are many periods per bit duration.
1/z Binary BPSK mod. DBPSK mod. DBPSK DPCM
Differential BPSK
Advantage : Non-Coherent Detection is possible 1 0-1 change 1-0 change Changes can be easily detected even when there is no reference carrier Disadvantage : A bit error affects detection of all remaining bits
Carriers with different phases
) 2 cos( t fc ) 2 cos(
1
t fc ) 2 cos(
2
t fc ) 2 cos(
2
t fc
Binary Input Memory PSK
General PSK
... Demux Combine multiple bits (usually b bits for M=2b in M-ary PSK)
00 11 01 10
Quadrature PSK S1 Symbol S2 Binary 00 11
) 2 cos( t fc ) 2 cos( t fc
Signal I Q 1
S3 S4 01 10
) 2 / 2 cos( t fc ) 2 / 2 cos( t fc
1
00 11 01 10
QPSK S1 Symbol S2 Binary 00 11
) 4 / 2 cos( t fc ) 4 / 5 2 cos( t fc
Signal I Q 0.707 0.707 S3 S4 01 10
) 4 / 3 2 cos( t fc ) 4 / 3 2 cos( t fc
0.707 0.707
Binary value
I channel Q channel I Q I+Q channel QPSK
Modulated In-Phase carrier Modulated Quadrature-Phase carrier
) 2 cos( t fc ) 2 sin( t fc
I Q
0111011...01 binary stream I-Q mod. QPSK
2 2
Q I M ) ( tan 1 Q I
Im Qm
QPSK
Ir = [ 0.7071 -0.7071 -0.7071 0.7071 ] Qr = [ 0.7071 0.7071 -0.7071 -0.7071 ]
Ir = [1 0 -1 0] Qr = [0 1 0 -1]
QPSK
Binary 000 001
) 2 cos( t fc ) 4 / 2 cos( t fc
Signal I Q 1 011 010
) 2 / 2 cos( t fc ) 4 / 3 2 cos( t fc
0.707 0.707 1
0.707 110 111
) 8 / 5 2 cos( t fc ) 8 / 7 2 cos( t fc
101 100
) 8 / 9 2 cos( t fc ) 8 / 11 2 cos( t fc
0.707 -0.707 8-PSK
8-PSK I Q
000 001 010 011 100 101 110 111
I Q
000 001 010 011 100 101 110 111 4 different I and Q values 000 001 010 011 100 101 110 111 Gray-coded
Ir = [1 0.7071 0 -0.7071 -1 -0.7071 0 0.7071 ] Qr = [0 0.7071 1 0.7071 0 -0.7071 -1 -0.7071 ]
8-PSK
(bit assignments are different than shown in previous slide)
Ir = [0.9239 0.3827 -0.3827 -0.9238 0.9238 0.3827 -0.3827 -0.9238] Qr = [0.3827 0.9238 0.9238 0.3827 -0.3827 -0.9238 -0.9238 -0.3827]
8-PSK