PHY & Radio GFSK

GFSK

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Gaussian Frequency Shift Keying -- the modulation scheme used by BLE for robust, power-efficient communication.

    


    
    
    

      Also known as:
      
      Gaussian FSK
      
    

    
  


  




  
  

    
GFSK


GFSK (Gaussian Frequency Shift Keying) is the modulation scheme used by all BLE physical layers. It encodes digital data by shifting the carrier frequency of the radio signal -- a logical 1 shifts the frequency up, and a logical 0 shifts it down -- with a Gaussian filter applied to smooth the transitions between symbols and reduce spectral spreading.


How GFSK Works


In standard FSK, abrupt frequency transitions between symbols produce wide spectral sidebands that waste bandwidth and cause adjacent-channel interference. GFSK pre-filters the data stream with a Gaussian low-pass filter before modulating the carrier, producing smooth frequency transitions that concentrate energy within the allocated channel bandwidth.


The key parameters of BLE's GFSK implementation are:


    

  • Symbol rate: 1 Msym/s for LE 1M PHY and LE Coded PHY, 2 Msym/s for LE 2M PHY
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  • Modulation index: 0.45 to 0.55 (frequency deviation of 225--275 kHz at 1 Msym/s)
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  • BT product: 0.5 (bandwidth-time product of the Gaussian filter)
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Why GFSK for BLE


BLE chose GFSK for several practical reasons. It is a constant-envelope modulation scheme, meaning the signal amplitude does not vary -- only the frequency changes. This allows the transmitter power amplifier to operate in saturation (its most efficient mode), reducing current consumption significantly compared to linear amplification schemes used by Wi-Fi.


Compared to more spectrally efficient modulations like QAM or OFDM, GFSK has lower throughput per hertz of bandwidth. However, BLE prioritizes simplicity, low power, and low silicon cost over raw throughput. The 2 MHz channel bandwidth with GFSK modulation is sufficient for BLE's target applications.


Demodulation


BLE receivers typically use a limiter-discriminator demodulator, which is simpler and lower power than the coherent demodulators required by more complex modulations. This contributes to BLE's low RX sensitivity power requirements and enables sub-milliwatt receiver implementations on modern SoCs. The simplicity of the demodulator also reduces silicon area and manufacturing cost, which is critical for high-volume IoT devices.

  

  

  
  
  




  
  
Related Terms

  

  

    
    LE 1M PHY
    
  





  

  
  

  
  

  
  
  

    
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…ever made. It transmits at 1 Megasymbol per second using GFSK modulation with a BT product of 0.5 (moderate spectral…

        
      
      
    

  

  

  
  




  
Frequently Asked Questions


  

    
    

      
      

        
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