Abbreviation for Gaussian Minimum Shift Keying. |
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Figure 1: GMSK in the
I/Q-Plane |
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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.
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- As the name suggests, GMSK is a special form of MSK or Minimum Shift
Keying which is a digital modulation scheme
.
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Introduction to Minimum Shift Keying
(MSK):
- Dependent on the input bit value, MSK will switch between two
RF-frequencies (FT + ft) and
(FT-ft).

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- 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).
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- 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.
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- 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.
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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. |
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