6.9 Radio Interface Modulation

The modulation technique used with standard GSM is known as Gaussian Minimum Shift
Keying (GMSK). GMSK had its roots in Frequency Shift Keying (FSK) where each binary 1
and 0 is transmitted in the form of one of two different frequencies:

Modulation - Frequency Shifftt Keying

• Two frequencies are used to represent the two binary levels

Modulation - Miiniimum Shiifftt Keyyiinngg ((MSK))

• MSK utilises procedures to minimise the phase change when alternating
frequencies

Minimum Shift Keying (MSK) is a special form of FSK that uses minimum spacing and no
phase inversions. It uses a continuous phase modulation scheme where frequency changes
occur at the carrier zero crossings. With MSK the difference between the frequency of a
logical zero and a logical one is always equal to half the data rate.
A significant problem with using MSK for high speed applications is that it is not compact
enough to fully utilize the available RF bandwidth. To more efficiently use available
bandwidth, it is necessary to reduce the energy of the MSK upper side lobes by feeding the
signal through a low pass filter. To do this, a Gaussian filter, characterized by a Gaussian
distribution (bell shaped curve), is used to provide a cut-off frequency with very little
overshoot in its impulse response and therefore reduces adjacent channel interference.
Therefore GMSK is able to make much more efficient use of bandwidth and power than does
MSK due to its low base-band and harmonic content.

 

Modulation - Gaussian Minimum Shift Keying

• Data pulses are shaped using a Gaussian
filter:
• Smoothes phase transitions
• Gives a constant envelope
• Reduces adjacent channel interference