n f f 1 2 2 T The minimum frequency separation is occurred - - PowerPoint PPT Presentation

 Fsk modulation technique is to modulate the data

signal to different frequencies to achieve effective transmission .

 MSK modulation implies the minimum frequency

separation that allows orthogonal detection as binary FSK.

 The frequency separation for coherent FSK signaling

as follows (n =any integer number ,T= symbol period):

 The minimum frequency separation is occurred for

n=1 .

T n f f 2

2 1

 

 This mean that the separation frequency for MSK is

equal :

 The minimum spacing tone is equal to half the data

rate.

 This can be expressed in terms of the modulation

index, and it is always equal to 0.5.

 But what is Gaussian MSK ?

The fundamental problem with MSK is that the spectrum is not compact enough to realize channel BW and data rate.

T f f 2 1

2 1

 

 We need to concentrate the maximum possible

power in specific limited bandwidth.

 It is necessary to reduce the energy of the MSK

upper sidelobes.

 Then we use pre-modulation low pass filter must have

a narrow BW and very little overshoot in its impulse response.

 The pre-modulation low pass filter is Gaussian low-

pass filter .

 Bandwidth-time product for Gaussian filter is BT  BT = 0.3 for GSM.

Bitrate f BT

dB 3 



Advantages of GMSK

1- GMSK has high spectral efficiency because it has minimum sidelobes than other digital modulation. 2- GMSK can be amplified by a non-linear amplifier and remain undistorted. ( power amplifier class C) 3- It provides constant envelope.

Disadvantages of GMSK

1-Requiring more complex channel equalization algorithms such as an adaptive equalizer at the receiver.

GMSK Modulation:

The two methods of GMSK transmission are : 1- Frequency modulated VCO ( FM-VCO) . The first one is simple method that consists Gaussian LPF , FM modulator with VCO and RF amplifier .

2- Quadratrue modulation

 In the GMSK transmission I an Q modulator

method is also widely used.

GMSK demodulator