Essay: Gaussian Minimum Shift Keying (GMSK)

Gaussian Minimum Shift Keying (GMSK) is one of the most widely used digital modulation technique. In GMSK, the phase of the carrier signal is continuously varied by a contrary signal, which has been shaped by the Gaussian shaping filter. Since it is kind of MSK, it has modulation index of value 0.5. In GMSK, the side lobe levels of the spectrum which were introduced in MSK are further minimized by passing the modulating NRZ data waveform through a pre modulation Gaussian pulse-shaping filter. The Gaussian filter width is determined by the bandwidth-time product BT (e.g., BT = 0.3 for GSM and BT = 0.5 for CDPD). As the BT product decreases, the bandwidth of the spectrum becomes narrow but this increases the inter symbol interference.
1.3.1 Advantages of GMSK:
• Improved spectral efficiency
• Reduced mail lobe over MSK
• Self synchronizing capability
• Constant envelope over entire bandwidth
• Good BER performance
• High noise immunity
• Remain undistorted even when amplifies by a non- linear amplifier.
1.3.2 Disadvantages of GMSK:
Although GMSK reduces the side lobes of the spectrum, the use of Gaussian shaping filter may cause inter symbol, interference. Power spectral density of MSK does not fall fast hence will not reduce interference completely between adjacent channels. Hence GMSK is designed with number of BT factors. BT of 0.3 gives robustness against adjacent channels.
Table 1 shows the comparison of GMSK with other techniques
Table 1 Comparison of GMSK with Other Conventional Techniques
 
1.4 Planning of the project
For this particular thesis work following steps has been followed:
In the proposed project, input image of dimensions 600×400 is transmitted through channel and received at the receiver. In the transmitter, the image undergoes convolution coding, interleaving, conversion, GMSK modulation, IFFT, pilot insertion and cyclic extension. In the channel, Additive White Gaussian Noise is added to the signal. The image is received at the receiver after performing removing of cyclic extension, pilot exertion, FFT, GMSK de-modulation, conversion, de-interleaving, and decoding.
Then the noise added signal undergoes removal of cyclic extension, pilot exertion, FFT, de-modulation, conversion, de-interleaving, decoding and the original image can be received at the receiver. The system performance can be evaluated by using BER Vs SNR plot.
This system performance using GMSK encoding technique is then compared with the performance using QPSK encoding technique.