Specifications

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DSSS and Bluetooth
(Minimum Shift Keying (MSK))
(DSSS Receiver)
The IEEE 802.11b specification uses Binary Phase Shift Keying
(BPSK) and Quadrature Phase Shift Keying (QPSK) modulation.
PSK works by detecting the phase of the incoming signal and
determining its value. The phase relationships are 180° out of
phase in the BPSK scheme. This means that the bit was either a
one or a zero, and the receiver simply looks for phase inversions.
In QPSK, signals are 90° out of phase, and provide twice the
throughput of the BPSK system.
(DSSS Spectrum (BPSK modulation))
0° 90°
BPSK
0n/a
QPSK
00 01
MSK and GMSK
Minimum Shift Keying (MSK) is a special form of FSK that uses
minimum spacing and no phase inversions. It uses a continuous
phase modulation scheme where frequency changes occur at
the carrier zero crossings. With MSK the difference between the
frequency of a logical zero and a logical one is always equal to
half the data rate. Thus, the modulation index is 0.5 for MSK, as
shown in the diagram below:
Bit Value MSK Output
odd even Freq sense
11Hi +
-1 1 Lo -
1-1Lo +
-1 -1 Hi -
A large problem with using MSK for high speed applications is
that the MSK is not compact enough to fully utilize the available
RF bandwidth. To more efficiently use available bandwidth, it
is necessary to reduce the energy of the MSK upper sidelobes
via lowpass filtering. To do this, a Gaussian filter, characterized
by a Gaussian distribution (bell shaped curve), is used to provide
a cutoff frequency with very little overshoot in its impulse
response. Gaussian Minimum Shift Keying (GMSK) is used by
Quatech's QTM-8424 DSSS modem modules. GMSK makes
much more efficient use of bandwidth and power than does
MSK due to its low base-band and harmonic content. The
diagram below provides a comparison of the two waveforms.
MSK
Waveform
GMSK
Waveform
+90°
-90°
(MSK vs. GMSK modulation)
Direct Sequence Spread Spectrum
Direct Sequence radios use the PN code to divide or slice up the
data to be transmitted from “bits” into “chips.” The chipping
rate is generally an order of magnitude faster than the data signal.
These chips are then modulated and transmitted. It is the job of
the PN-code to make the transmitted signal wide. Properly spread
signals have a sinc
2
x type PSD.