Comtech EF Data CDM-570 - Manual (Page 205)

12–1
Chapter 12. OFFSET QPSK
OPERATION
Offset QPSK modulation is a variation of normal QPSK, which is offered in the
CDM-570/570L. Normal, band limited QPSK produces an RF signal envelope that necessarily
goes through a point of zero amplitude when the modulator transitions through non-adjacent
phase states. This is not considered to be a problem in most communication systems, as long as
the entire signal processing chain is linear.
However, when band limited QPSK is passed through a non-linearity (for instance, a saturated
power amplifier), there is a tendency for the carefully-filtered spectrum to degrade. This
phenomenon is termed ‘spectral re-growth’, and at the extreme (hard limiting) the original,
unfiltered sin(x)/x spectrum would result. In most systems, this would cause an unacceptable
level of interference to adjacent carriers, and would cause degradation of the BER performance of
the corresponding demodulator.
To overcome the problem of the envelope collapsing to a point of zero amplitude, Offset QPSK
places a delay between I and Q channels of exactly 1/2 symbol. Now the modulator cannot
transition through zero when faced with non-adjacent phase states. The result is that there is far
less variation in the envelope of the signal, and non-linearities do not cause the same level of
degradation.
The demodulator must re-align the I and Q symbol streams before the process of carrier recovery
can take place. For various reasons this makes the process of acquisition more difficult, and
acquisition times may be longer, especially at low data rates.