M-FSK: Advantages and Disadvantages of Multiple Frequency Shift Keying

This page explores the pros and cons of M-FSK (Multiple Frequency Shift Keying). It outlines the benefits and drawbacks of MFSK.

Introduction to M-FSK

M-FSK (M-array FSK) is a variant of binary FSK, which utilizes two distinct tones. The following equations dictate how binary data is mapped in FSK (Frequency Shift Keying):

• s(t) = A * cos(2 * π * f1 * t) for Binary 1
• s(t) = A * cos(2 * π * f2 * t) for Binary 0

Here, ‘f1’ and ‘f2’ represent frequencies offset from the center frequency employed in the system.

The image below illustrates FSK modulation waveforms for binary 1 and binary 0 data.

There exists a variety of M-ary modulation techniques, including M-ary PSK, M-ary QAM, and M-ary FSK. In these techniques, digital data is transmitted by modulating the envelope, phase, or frequency of an RF carrier. These techniques map baseband digital data onto four or more RF carriers, earning them the designation “M-ary modulation.”

What is M-FSK?

MFSK uses multiple frequency tones to represent digital data before transmission. It employs narrow tone spacing to achieve higher data rates within a given bandwidth. The generalized equation for MFSK can be expressed as:

Xi(t) = A * cos (2 * π * fi * t)

Where:

• fi = fc + (2 * i - 1 - M) * fd
• fi is the frequency of the i-th signal
• fc is the center frequency
• fd is the frequency deviation
• M is the Modulation order (number of possible signals) (e.g., M = 2 for BPSK, M = 4 for QPSK)

The number of bits/symbol, K, is calculated as:

K = log2(M)

For instance, in 4FSK with a center frequency (fc) of 250 KHz and a frequency deviation (fd) of 25 KHz, we obtain 2 bits per symbol because M equals 4. The four frequencies used to map binary data (00, 01, 10, 11) are 175 KHz, 225 KHz, 275 KHz, and 325 KHz.

Typical applications of M-FSK include VHF/UHF communication, telephone signaling (DTMF), Digital Mobile Radio (DMR), and Bluetooth (FHSS).

Benefits or Advantages of MFSK

MFSK addresses some of the limitations of BFSK (Binary FSK or 2FSK) modulation.

Here are the advantages of MFSK:

• Reduced Error Rate: The increased number of frequency tones leads to a lower error rate.
• Noise Rejection: MFSK effectively rejects broadband noise and pulses at the receiver.
• Tolerance to Channel Effects: It is less susceptible to ionospheric channel effects like Doppler shift, multipath propagation, and fading.
• Robustness: MFSK is generally more robust than PSK modulation schemes.

Drawbacks or Disadvantages of MFSK

Here are the disadvantages of MFSK:

• Complex Tuning: Tuning at the receiver is more complex due to the narrow bandwidth and spacing of individual tone detectors.
• Stability Requirements: Lower Tx/Rx offset and good stability of the transceiver module are essential.
• Bandwidth Usage: MFSK utilizes more bandwidth compared to PSK modulation for the same data rate.