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): n f f 1 2 2 T The minimum frequency separation is occurred for n=1 .
This mean that the separation frequency for MSK is equal : 1 f f 1 2 T 2 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.
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 f 3 dB BT Bitrate BT = 0.3 for GSM.
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
Recommend
More recommend
