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Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Minimum Shift Key Modulation is another type of digital modulation technique used to convert a digital signal into analog signals. It is also called Minimum-shift keying (MSK) or Advance Frequency Shift Keying because it is a type of continuous-phase frequency-shift keying.

Key features of Minimum Shift Key Modulation or Minimum-shift keying (MSK)

  • Minimum-shift keying or MSK was first developed by the Collins Radio employees Melvin L. Doelz and Earl T. Heald in the late 1950s.
  • It is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period.
  • Minimum Shift Keying is the most effective digital modulation technique. It can be implemented for almost every stream of bits much easier than the Phase Shift Key, Frequency Shift Key and Amplitude Shift Key of digital modulation technique.
  • The Minimum Shift Keying's concept is based on the positioning of bits such as even bits and odd bits for the given bitstream and the bit positioning frequency generating table.
  • MSK is the most widely used digital modulation technology because of its ability and flexibility to handle "One(1)" and "Zero(0)" transition of binary bits.

Working of Minimum-shift keying (MSK)

  • In Minimum-shift keying, bits are separated in even and odd bits and each bit's duration is doubled.
  • After that, frequency is separated into two types of frequencies f1 and f2. Here, f1 determines/denotes the low frequency, and f2 denotes the high frequency.
  • Original or inverted signals are chosen from the frequency generating table according to the bit values if they are even or odd.
  • The curve for higher frequency takes a complete wave from 0 to π, and the curve for low frequency takes a wave 0 to π/2 within the same interval of time.

Let's take an example to demonstrate the working of Minimum-shift keying and draw a curve for a given bit stream. Let's consider a bit stream 1011010. Here, we have to find the MSK curve for this bit stream.

Rules for drawing MSK Curve

Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Step 1: First, draw the curve according to the bit value of amplitude. If the bit is zero, it would have amplitude while is the bit is zero, it does not have amplitude.

Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Step 2: Now, start with the odd bit. If the bit's value is one, draw the curve above the x-axis twice as long as the original one. If the bit's value is zero, draw the curve below the x-axis twice as long as the original size.

Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Step 3: Now, draw the curve for high and low frequency, as shown in the following graph. It would remain the same for any problem.

Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Step 4: This is the final step. Now, draw the final curve according to the frequency generating table. In the following diagram, the blue colored curve represents the final obtained MSK curve.

Minimum Shift Key Modulation/ Minimum-shift keying (MSK)

Gaussian Minimum Shift Keying (GMSK)

Gaussian Minimum Shift Keying or GMSK is very much similar to standard minimum-shift keying (MSK), but the digital data stream is first shaped with a Gaussian filter before it is applied to a frequency modulator.

The GMSK form of modulation is based on frequency shift keying that has no phase discontinuities. It provides efficient use of the spectrum as well as enabling high-efficiency radio power amplifiers.

It has much narrower phase shift angles than most MSK modulation systems.

Usage of Gaussian Minimum Shift Keying (GMSK)

GMSK is mainly used in the following technologies:

  • Global System for Mobile Communications (GSM)
  • Bluetooth
  • Satellite Communications
  • Automatic Identification System (AIS) for maritime navigation.

Advantage of Gaussian Minimum Shift Keying (GMSK)

  • The biggest advantage of using GMSK is that it reduces the sideband power, reducing out-of-band interference between signal carriers in adjacent frequency channels.
  • GMSK provides high spectral efficiency.

The disadvantage of Gaussian Minimum Shift Keying (GMSK)

  • It increases the modulation memory in the system that causes interference within a symbol, making it more challenging to differentiate between different transmitted data values.
  • It requires more complex channel equalization algorithms, such as an adaptive equalizer at the receiver.
  • It has high power consumption.

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