Gaussian-Filtered MSK (GMSK). In this subsection, we will address the GMSK modulation
scheme, specifically the interaction of the Gaussian LPF with the MSK waveform [34]. As mentioned
earlier, lowering the BTb product will reduce the out-of-band emissions; this is clearly shown in Fig. 2.68
when we view their respective PSD plots [35??“40].
90 CHAPTER TWO
MSK and OQPSK Power Spectral Density (PSD)
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0
0 0.5 1 1.5 2 2.5 3 3.5
Frequency Offset from Carrier (Hz)
PSD (dB)
MSK PSD
OQPSK PSD
FIGURE 2.67 PSD comparison of MSK with OQPSK modulation schemes.
Transmit Power Spectral Density (PSD)
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0
0 0.5 1 1.5 2 2.5 3 3.5 4
Normalized Frequency ( f Ts)
Power Spectral Density (dB)
GMSK (BT = 0.5)
GMSK (BT = 0.3)
GMSK (BT = 0.16)
MSK
FIGURE 2.68 GMSK PSD plot comparison for various BTb product values.
This spectral compactness is achieved by slowing down the phase transitions. Without the filtering,
the phase transitions produced a
90-degree phase shift in a single bit time duration. After the
insertion of the Gaussian LPF, the achievable phase shifts now become
(90 ) degrees, where
increases as BTb decreases.
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