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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