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APPENDIX A
GFSK MODULATION AND
DEMODULATION
Since it is chosen to use software defined radio for the relay node, it is necessary to provide a software modulation and demodulation of the signals transmitted on the physical
layer of Bluetooth. This appendix describes how these signals shall be modulated and
demodulated, and some additions to these procedures in order to optimise this.
Gaussian Frequency Shift Keying is a subclass of CPFSK (Continuous-Phase Frequency
Shift Keying) and is related to MSK (Minimum Shift Keying). Stated shortly GFSK
modulation is Gaussian low-pass filtered binary pulses, resulting in smooth or rounded
pulses, which are then frequency modulated. Thus GFSK may be viewed as more similar to the analog modulation FM than to FSK, this is also evident in the demodulation
procedure, see Section A.1.
In addition to the property of a continuous phase, like CPFSK, a GFSK modulated
signal is also smooth at all times, and thus exhibits no discontinuities in neither the
phase nor frequency. This ensures a limited spectral bandwidth of the GFSK modulated
signal [21].
The minimum frequency deviations for GFSK in Bluetooth shall never be smaller than
115kHz, and the data is transmitted with a symbol rate of 1MSamples/sec [5].
The relation between the modulation index and frequency deviation is stated in (A.1)
where fd is the frequency deviation in Hertz, h is the modulation index, T is the symbol
time, and R is the symbol rate [21].
h=
74
2fd
= 2fd T
R
(A.1)