GFSK

GFSK, or Gaussian frequency-shift keying, is a digital modulation technique in which information is conveyed by shifting the instantaneous frequency of a carrier between two or more predefined frequencies. Before the frequency modulation occurs, the binary data stream is passed through a Gaussian filter, which smooths the transitions and reduces the bandwidth of the transmitted signal compared with plain FSK. This Gaussian pre-filtering makes GFSK more spectrally efficient and easier to manage in crowded radio environments while maintaining a continuous phase.

How it works in practice: The data stream is typically encoded as binary symbols, with each symbol selecting one of two (or more) frequencies separated by a fixed deviation. The Gaussian filter shapes the bit transitions, controlling the bandwidth-time trade-off via the BT parameter, where smaller BT limits side lobes but increases intersymbol interference, and larger BT widens the spectrum but reduces ISI. The resulting signal is a continuous-phase modulation with a constant envelope, which is advantageous for power-efficient amplification.

Relation to related schemes: Gaussian minimum shift keying (GMSK) is a special case of GFSK that uses a Gaussian filter together with a specific modulation index to achieve near-constant envelope and very good spectral efficiency; while GFSK describes the general approach of Gaussian-filtered FSK, GMSK is a particular implementation often associated with GSM.

Applications: GFSK is widely used in short-range wireless systems, most notably as the modulation for Bluetooth Basic Rate (BR/EDR). It is also employed in various sub-GHz and 2.4 GHz transceivers where bandwidth control and amplifier efficiency are important.

Advantages and considerations: The main benefits are improved spectral efficiency and compatibility with non-linear, power-efficient amplifiers due to the near-constant envelope. Trade-offs include the need to select an appropriate BT value to balance bandwidth and intersymbol interference, as well as potentially more complex transmitter and receiver design to realize the Gaussian shaping and coherent demodulation.