In GMSK, what is the purpose of the Gaussian low-pass filter placed before the frequency modulator?

It smooths the phase path so side lobes and out-of-band power fall, while the envelope stays constant

It removes the carrier so the signal can be detected non-coherently

It makes the envelope vary so the signal can use linear amplifiers

It increases the modulation index above 0.5 to widen the spectrum

What happens when the BT product of a GMSK filter is made smaller (for example from infinity to 0.3)?

The spectrum becomes narrower but ISI increases

The spectrum becomes wider but ISI decreases

Both the spectrum width and ISI decrease

The envelope stops being constant

2070 Bhadra · 3 marks

Short note: GMSK modulation technique

Short answer

GMSK is constant-envelope MSK whose data passes through a Gaussian low-pass filter, smoothing phase and reducing side lobes. The BT product trades spectrum compactness against ISI; GSM uses BT=0.3, allowing efficient nonlinear amplifiers and coherent or non-coherent detection.

Write a short note on GMSK modulation technique.

Gaussian Minimum Shift Keying (GMSK) is a constant-envelope digital modulation obtained by passing NRZ data through a Gaussian low-pass filter before MSK (FM with modulation index h = 0.5).

data->[Gaussian LPF (BT)]->[FM/VCO, h = 0.5]-> GMSK

The filter smooths the phase path, so side lobes and adjacent-channel interference are much lower than MSK.
BT product (3 dB filter bandwidth × bit period) sets the trade-off: smaller BT gives a narrower spectrum but more ISI. GSM uses BT = 0.3.
Constant envelope allows efficient non-linear (class C) amplifiers.
Can be detected coherently (like MSK) or non-coherently with an FM discriminator; GSM uses an MLSE equalizer to handle the ISI.
Used in GSM (270.833 kbps in 200 kHz), DECT and, in GFSK form, Bluetooth.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Short note: GMSK modulation technique

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