Download - Constant Envelope Modulation Techniques
Constant Envelope Modulation Schemes
Constant Envelope Modulation• Amplitude of the carrier is constant,
regardless of the variation in the modulating signal– Better immunity to fluctuations due to fading. – Better random noise immunity– Power efficient
• They occupy larger bandwidth
EE 552/452 Spring 2007
Frequency Shift Keying (FSK)
• The frequency of the carrier is changed according to the message state (high (1) or low (0)).
Continues FSK
Integral of m(x) is continues.
))(22cos()(
))(2cos()(
t
fc
c
dxxmktfAts
tfAts
0)(bit Tt0 )22cos()(
1)(bit Tt0 )22cos()(
b2
b1
tffAts
tffAts
c
c
One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation).
FSK Bandwidth• Limiting factor: Physical capabilities of the
carrier• Not susceptible to noise as much as ASK
• Applications– On voice-grade lines, used up to 1200bps– Used for high-frequency (3 to 30 MHz) radio transmission– used at higher frequencies on LANs that use coaxial cable
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Binary Frequency Shift Keying (BFSK)
• The most common form of FSK is Binary FSK (BFSK)• Two binary values represented by two different frequencies
( f1 and f2 )
• less susceptible to noise than ASK• used for
1. up to 1200bps on voice grade lines2. high frequency radio (3 to 30MHz)3. even higher frequency on LANs using coaxial cable
0),2cos(
1),2cos()(
2
1
binarytfA
binarytfAts
0 0 1 1 0 1 0 0 0 1 0
f2 f2 f1 f1 f2 f1 f2 f2 f2 f1 f2
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Multiple FSK (MFSK)
• More than two frequencies (M frequencies) are used• More bandwidth efficient compared to BFSK• More susceptible to noise compared to BFSK• MFSK signal:
elementsignalperbitsofnumberL
elementssignaldifferentofnumberM
frequencydifferencethef
frequencycarrierthef
fMiff
where
MitfAts
L
d
c
dci
ii
2
)12(
1),2cos()(
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Multiple FSK (MFSK)
• MFSK signal:
• Period of signal element
• Minimum frequency separation
• MFSK signal bandwidth:
elementsignalperbitsofnumberL
elementssignaldifferentofnumberM
fMiff
where
MitfAts
L
dci
ii
2
)12(
1),2cos()(
ddd MffMW 2)2(
periodbitTperiodelementsignalTLTT bsbs ::,
)(2/12)/(12/1 ratebitLfTfLTfT dbdbds
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Example
• With fc=250KHz, fd=25KHz, and M=8 (L=3 bits), we have the following frequency assignment for each of the 8 possible 3-bit data combinations:
• This scheme can support a data rate of:
KHzMfWbandwidth
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
KHzf
ds 4002
425111
375110
325101
275100
225011
175010
125001
75000
8
7
6
5
4
3
2
1
KbpsHzbitsLfT db 150)25)(3(22/1
dci fMiff )12(
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Example
• The following figure shows an example of MFSK with M=4. An input bit stream of 20 bits is encoded 2bits at a time, with each of the possible 2-bit combinations transmitted as a different frequency.
dc
dc
dc
dc
dci
fffi
fffi
fffi
fffi
fMiff
3411
310
201
3100
)12(
4
3
2
1
FSK Coherent Detection
Noncoherent FSK
Relationship between baud rate and bandwidth in FSK
Minimum frequency-shift keying
Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent FSK. In MSK the difference between the higher and lower frequency is identical to half the bit rate.
As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal.
A variant of MSK called GMSK is used in the GSM mobile phone standard.
MSK modulation
MSK reception
Minimum Shift Keying spectra
Relationship between baud rate and bandwidth in FSK
Gaussian Minimum frequency-shift keying
•Gaussian minimum shift keying or GMSK is a continuous-phase frequency-shift keying modulation scheme.
•It is similar to standard minimum-shift keying (MSK); however the digital data stream is first shaped with a Gaussian filter before being applied to a frequency modulator.
•This has the advantage of reducing sideband power, which in turn reduces out-of-band interference between signal carriers in adjacent frequency channels.
GMSK spectral shaping
GMSK spectra shaping
EE 552/452 Spring 2007
Simple GMSK modulation and demodulation
Digital GMSK demodulator