GMSK (Gaussian minimum-shift keying or Gaussian filtered Minimum Shift Keying) is used for modulation in GSM. It is a kind of continues-phase frequency shift keying.
GMSK is inspired by MSK, which is a form of PSK. In PSK the sideband extends out from the carrier which overcomes in MSK.
In MSK and GMSK frequency changes occur at the carrier zero crossing points, so there are no phase discontinuities. This is because the difference between frequency of logical one and logical zero states is equal to half the data rate (modulation index = 0.5). 
In MSK signal sidebands extending beyond a bandwidth equal to the data rate. It can be reduced if modulating signal is passed through a low pass filter before applying it to the carrier. The filter should have a sharp cut-off; its impulse response should show no overshoot and narrow bandwidth which is supported by GMSK.
GMSK Modulation Generation:
- Quardrature Modulator (Widely Used)-
- Using Gaussian Filter (Practically not suitable)-
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Quadrature modulator is used to generate GMSK modulation. In it the phase of a signal is 90 degrees to another one. 
The quadrature modulator uses two signals of which one signal is said to be in-phase and another in quadrature to the first. Thus, modulation index can be maintained at exactly 0.5 without adjustments. 
First of all the modulating signal is filtered using Gaussian filter. Then it is applied to a frequency modulator. In the frequency modulator modulation index is set to 0.5. 
Instead of its simple implementation it has a drawback that the modulation index must be abjectly 0.5 which is not possible due to the component tolerances drift. 
- GMSK can be amplified by a non-linear amplifier and remain undistorted as there are no elements of the signal that are carried as amplitude variations. A non-linear amplifier consumes less DC power to generate radio signal and this result in lower levels of battery consumption. 
- As in case of GMSK none of the information is carried in the form of amplitude variation, it is more resilient to noise, as most noise is based on amplitude. 
- GMSK improves spectral efficiency. 
Requirement for Success Implementation of GSM
The hardware and software requirement is more or less the same for implementing GSM but technical support and cost varies as per the size of the network.
- Two components of GSM mobile phone is the mobile radio telephone and theSubscriberIdentityModule (SIM).International Mobile Equipment Identity (IMEI) acts as the unique number for the mobile device and is stored in mobile radio telephone. International Mobile Subscriber Identity (IMSI), which is stored on SIM card act as the customer number. 
- Similarly, Base station is divided into three groups namely, the transmitting station, its components and the control unit.
- The Base Transceiver Station (BTS) comprises of transmitting and receiving equipments. 
- The Base Station Controller (BSC) acts as administer for transmit and receive resources of the connected base stations. 
- Mobile Switching Center (MSC) controls the base station. 
- Home Location Registers (HLR) stores the data, such as the name of the subscriber, his customer number and the services required by the customer. 
- Visitor Location Registers (VLR) keeps track of the location of mobile number and it also keeps record weather the number is switched on or not. 
- Authentication Center (AUC) holds algorithms and subscriber-related keys. 
- Equipment Identity Registers (EIR) holds details of mobile transceivers permitted on the on the network. 
- Gateway Mobile Switching Center (GMSC) is as interface to the land lines.
First of all voice is given as input to the receiver of GSM phone. The speech is then encoded. This encoded data then undergoes the process of encryption, which goes for modulation. After modulating the data it is transmitted by the mobile transmitter and is received by the BTS. Now at the Base Station Subsystem, received data is decoded and is passed from GSM to PSTN (Public Switched Telephone Network).
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Now, this data travels to the receiving Base Station Subsystem. There it is transferred from PSTN to GSM and undergoes the process of speech encoder. Now, it is transmitted via BTS to the receivers mobile device where it under goes the process of data demodulation, data decryption and speed decoder respectively.