GFSK (Gaussian Frequency Shift Keying) differences & advantages over FSK mudulation
Research 2006/05/11 23:25The Bluetooth radio chip uses GFSK Modulation. It is natural to assume that GFSK is quite like FSK Modulation, however the differences between the 2 are substantial in implementation and results (output)
Question 1
What are the physical Differences between an FSK & GFSK Modulator, and how do their results vary?
Answer 1
An FSK Modulator is much the same as a GFSK Modulator ,but GFSK uses a Gaussian filter as well. In a GFSK modulator everything is the same as a FSK modulator except that before the baseband pulses (-1, 1) go into the FSK modulator, it is passed through a gaussian filter to make the pulse smoother so to limit its spectral width.
Question 2
What is Gaussian Filtering?
Answer 2
Gaussian filtering is one of the very standard ways for reducing the spectral width, it is called Pulse Shaping. If we use -1 for fc-fd and 1 for fc+fd, once when we jump from -1 to 1 or 1 to -1, the modulated waveform changes rapidly, which introduces large out-of-band spectrum. If we change the pulse going from -1 to 1 as -1, -.98, -.93 ..... .96, .99, 1, and we use this smoother pulse to modulate the carrier, the out-of-band spectrum will be reduced.
Question 3
So, why is GFSK implemented in Bluetooth Radios , and not FSK , is it cheaper?
Answer 3
The spectral width for FSK is unlimited, comparatively, there is a limitation on GFSK. fc "climbs slowly" to fd in GFSK, However, in the case of FSK, fc "jumps sharply" to fd, which greatly decreases spectral efficiency.
GFSK is not implemented in the Bluetooth radio unit for cheaper chips (since you need to put an extra pulse-shaping filter in front of the modulator), but for spectral efficiency.
ps.
1. Gaussian filtering에 대해 자세히 알고 싶으면 아래 사이트를 참고할 것!
http://homepages.inf.ed.ac.uk/rbf/HIPR2/gsmooth.htm
http://www.fho-emden.de/~hoffmann/gauss25092001.pdf
2. FSK는 무선 랜에서 사용되는 변조 방식임.
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