continuos phase frequency shift keying(cpfsk)
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Transcript of continuos phase frequency shift keying(cpfsk)
BY:
MOKA DINESH
M.TECH 1ST YEAR
PONDICHERRY UNIVERSITYDEPARTMENT OF ELECTRONICS ENGINEERING
CONTINUOUS-PHASE FREQUENCY-SHIFTKEYING
MODULATION
ANALOG DIGITAL
MEMORY LESS MEMORY BASED
CPFSK CPM
WHY CPFSK?In Fsk Abrupt Switching From one oscillator output to another in successive signaling intervals results in relatively large spectral side lobes outside of the main spectral band of the signal.
Oooscillator1
Oscillator2
digital modulated signal
Discontinues phase generate unwanted
spectrum
CONTINUOUS-PHASE FREQUENCY-SHIFTKEYING
CPFSK is a memory base modulation scheme in which phase is constrained to be continuous.
Voltage controlled oscillator
Phase continuous fskCPFSK
Let us consider PAM signal
Where,• {In} = sequence of amplitudes and • g(t) =rectangular pulse of amplitude 1/2T and duration T second
The equivalent low pass waveform v(t) is expressed as
Where,• fd = the peak frequency deviation• ɸo = the initial phase of the carrier
The carrier-modulated signal expressed as
where φ(t; I) represents the time-varying phase of the carrier
Although d(t) contains discontinuities, the integral of d(t) is continuous.
The phase of the carrier is determined by integrating the above equation(1) in the interval nT ≤ t ≤ (n+1)T
1
Then
• Here, θn represents the memory (accumulation of all symbols up to time (n-1)T
• h is called the modulation index.
Phase diagramsThe phase diagrams are also called phase tress.
Phase tress for cpfsk is piecewise linear because the pulse g(t) is rectangular.
Smoother phase trajectories and phase tress are obtained by using pulses that do not contain discontinuities such as the class of raised cosine pulses.
State trellis for binary cpfsk with h=1/2
The phase trajectories ɸ(t;I) for binary cpfsk
State diagramAn alternative representation to the state trellis is the state diagram which also illustrates the state transtions at the time instants t=NT.
Only the possible phase states and their transitions are displayed in the state diagram ,here time does not appear explicitly as a variable.
The state diagram for the cpfsk signal with h=1/2 is shown below
Relationship between cpm , cpfsk and mskCpfsk becomes a special case of a general class of continuous phase modulated(cpm) signal in which the carrier phase is
Msk is a special form of binary cpfsk in which the modulation index h=1/2 and g(t) is a rectangular pulse of duration T.
Relationship between cpm ,cpfsk and msk
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