GMSK:

 
Abbreviation for Gaussian Minimum Shift Keying.
 

Figure 1: GMSK in the I/Q-Plane

 

 

Use of GMSK:

  • GMSK is the modulation scheme that is used in plain GSM . The function of GMSK is illustrated in figure 1: Each input bit causes a +/- 90° phase shift of the RF-vector in the I/Q-plane.
  • As the name suggests, GMSK is a special form of MSK or Minimum Shift Keying which is a digital modulation scheme .

Introduction to Minimum Shift Keying (MSK):

  • Dependent on the input bit value, MSK will switch between two RF-frequencies (FT + ft) and (FT-ft).
  • The specific difference between MSK on one hand and FSK (Frequency Shift Keying) on the other hand is that MSK deploys a logical XOR-operation on two subsequent input bits prior to the actual modulation (see table 1). This XOR-operation results in less bit shifts (in average) which obviously results in less frequency changes of the modulation frequency (see figure 2).
  • Still, even MSK applies comparibly hard frequency shifts between the two frequencies (FT + ft) and (FT-ft) which results in a large spectrum requirement. The prevention of these hard frequency shifts is the difference between MSK and GMSK.
Bit N Bit (N+1) XOR Output Frequency
0 0 0 FT + ft
0 1 1 FT - ft
1 0 1 FT - ft
1 1 0 FT + ft

Table 1: The XOR-Operation in Minimum Shift Keying (MSK)

 

Figure 2: Principles of Minimum Shift Keying (MSK)

Principles of GMSK:

  • GMSK deploys all principles of MSK but in addition filters the rectangular shaped output bit stream of the XOR-operation with a Gaussian shaped filter.
  • The result is a smoother transition from one bit value to the next (figure 3) which also provides for a smoother transition of the modulator output frequency.

Figure 3: Filtering of the XOR-Output with a Gaussian Shaped Filter

Characteristics of GMSK:

Bandwidth Requirements: One of the most important advantages of GMSK is the comparibly small spectrum requirement. In GSM, only 200 kHz of spectrum are required for one carrier.
Amplitude Modulation: GMSK contains no amplitude modulation which provides for cheaper power amplifiers and less spectrum requirement.
Throughput Rate: The throughput rate of GMSK is moderate compared to modulation schemes like 16-QAM or 8-PSK.
Simplicity and Implementation: The implementation of GMSK is rather simple and cheap which is an important asset when it comes to the production of mobile stations.

[2GTS 05.04]