Comtech EF Data CDM-600/600L - Manual (Page 171)

11–1
Chapter 11. OFFSET QPSK
OPERATION
Offset QPSK modulation is a variation of normal QPSK, which is offered in the
CDM-600/600L Open Network Satellite Modem. Normal, bandlimited 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 bandlimited 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.