30-03-2010, 12:34 PM
GMSK:
Gaussian Filtered Minimum Shift Keying
Gina Colangelo
EE194 - SDR
Topics
Background
Minimum Shift Keying (MSK)
Gaussian Filtering
Spectral Efficiency GMSK vs. MSK
Common Applications
Introduction
Digital modulation schemes are becoming increasingly important in wireless communication systems.
Using an unfiltered binary data stream to modulate an RF carrier will produce an RF spectrum of considerable bandwidth.
Techniques have been developed to minimize this bandwidth, improve spectral performance, and ease detection.
Background
MSK or Minimum Shift Keying is a digital modulation scheme in which the phase of the remains continuous while the frequency changes.
GMSK or Gaussian-filtered Minimum Shift Keying differs from MSK in that a Gaussian Filter of an appropriate bandwidth is used before the modulation stage.
We will first discuss MSK to understand the need for GMSK.
MSK “ Minimum Shift Keying
MSK is a continuous phase FSK (CPFSK) where the frequency changes occur at the carrier zero crossings.
MSK is unique due to the relationship between the frequency of a logic 0 and 1.
The difference between the frequencies is always ½ the data rate.
This is the minimum frequency spacing that allows 2 FSK signals to be coherently orthogonal.
MSK “ How It Works
The baseband modulation starts with a bitstream of 0â„¢s and 1â„¢s and a bit-clock.
The baseband signal is generated by first transforming the 0/1 encoded bits into -1/1 using an NRZ filter.
This signal is then frequency modulated to produce the complete MSK signal.
The amount of overlap that occurs between bits will contribute to the inter-symbol interference (ISI).
Example of MSK
1200 bits/sec baseband MSK data signal
Frequency spacing = 600Hz
Pros of MSK
Since the MSK signals are orthogonal and minimal distance, the spectrum can be more compact.
The detection scheme can take advantage of the orthogonal characteristics.
Low ISI (compared to GMSK)
Cons of MSK
The fundamental problem with MSK is that the spectrum has side-lobes extending well above the data rate (See figure on next slide).
For wireless systems which require more efficient use of RF channel BW, it is necessary to reduce the energy of the upper side-lobes.
Solution “ use a pre-modulation filter:
Straight-forward Approach: Low-Pass Filter
More Efficient/Realistic Approach: Gaussian Filter
The Need for GMSK
Gaussian Filter
Impulse response defined by a Gaussian Distribution “ no overshoot or ringing (see lower figure)
BT “ refers to the filter™s -3dB BW and data rate by:
Notice that a bit is spread over more than 1 bit period. This gives rise to ISI.
For BT=0.3, adjacent symbols will interfere with each other more than for BT=0.5
GMSK with BT=8 is equivalent to MSK.
Trade-off between ISI and side-lobe suppression (top and bottom figures)
The higher the ISI, the more difficult the detection will become.
GMSK “ Applications
An important application of GMSK is GSM, which is a time-division multiple-access system.
For this application, the BT is standardized at 0.3, which provides the best compromise between increased bandwidth occupancy and resistance to ISI.
Ninety-nine percent of the RF power of GMSK signals is specified to confine to 250kHz (+/- 25kHz margin from the signal), which means that the sidelobes need to be virtually zero outside this frequency band and the ISI should be negligible.
References
Haykin, S. 2001: Communication Systems. 4th ed. New York, NY. John Wiley & Sons.
GMSK: Practical GMSK Data Transmission
http://eetchinaARTICLES/2003AUG/PDF/2003AUG29_NTEK_AN.PDF
Minimum Shift Keying: A Spectrally Effiecient Modulation
http://elet.polimi.it/upload/levantin/Si...y_1979.pdf
