1

Digital Modulation

Lecture 17

2

s t E tT

t

i Mt T

io1

2

1 20

( ) ( ) cos( )

, ,...,

= +

=

≤ ≤

ω Φ

Amplitude Shift Keying (ASK)

3

Baseband filtered ASK

4

FSK Waveforms

5

s t ET

t

i Mt T

i i( ) cos( )

, ,...,

= +

=

≤ ≤

2

1 20

ω Φ

Frequency Shift Keying (FSK)

6

s t ET

t i M

i Mt T

i o( ) cos( / )

, ,...,

= +

=

≤ ≤

2 2

1 20

ω π

Phase Shift Keying (PSK)

7

Baseband filtered PSK

8

s t E tT

t t

i Mt T

ii

o i( ) ( ) cos[ ( )]

, ,...,

= +

=

≤ ≤

2

1 20

ω Φ

Amplitude & Phase Shift Keying (APK)

9

2,4 and 8-PSK constellations

10

SER of coherent M-PSK

11

Why M-PSK ? (M>4)

•  The last figure clearly demonstrates that as M becomes larger than 4, there is a power efficiency penalty. The question is why do we pay this penalty. The answer is in the next figure.

12

Bandwidth of M-PSK

13

Is FSK used in cellular systems?

•  We know that FSK is a basic digital modulation format.

•  Is it frequently used in cellular systems? •  If not, why not?

14

Signal separation in FSK

15

SER performance of M-FSK, M=2k

16

What about QAM?

•  We now know that PSK is the most popular modulation for many wireless systems. But M-PSK for M>8 is not used in practice. Clearly as M becomes large, putting the points on a single circuit reduces the distance for a given average power (or energy) as shown next.

17

a

b

c d

4 different 8-QAM constellations

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Average Peak Peak to average

a 6 10 1.66

b 6.8 11.8 1.7

c 6 8 1.33

d 4.7 7.46 1.58

Comparison of the 8-QAM constellations

19

AWGN

Loss of Synch

Inter_ ference

Phase Noise

16QAM with different impairments

20

Bandwidth requirements

For PSK or QAM for FSK

B r rM

B Mr M rM

Brr

M

sb

sb

s

b

= + =+

= =

− < ≤

( ) ( )log log

::: ( ):

1 1

0 1

2 2

αα

α α

: bandwidth in Hzsymbol rate in spsbit rate in bpsroll off factor number of points in the constellation

21

Detection Types

•  Coherent detection •  Differential detection (also called

differentially coherent detection) •  Noncoherent detection (e.g. envelope

detection)

22

Non-coherent detection of ASK

23

Coherent ASK detection

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Carrier recovery for ASK

25

Noncoherent FSK Detection

26

PLL based detection of FSK

27

Coherent FSK Detection

28

Envelope detection of M-FSK

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Coherent detection of BPSK

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Coherent detection of PSK

31

State diagram of QPSK

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Phase changes in QPSK

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Envelope variations in QPSK