Comtech EF Data CDM-550T - Manual (Page 91)

Chapter 8. OFFSET QPSK
OPERATION
Offset QPSK modulation is a variation of normal QPSK, which is offered in the CDM-550T.
Normal, bandlimited QPSK produces an RF signal envelope which 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 sinx/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. The two
consequences of this are:
1) Acquisition may be longer, especially at low data rates.
2) The acquisition threshold is higher than for normal QPSK, although the demodulator
will maintain lock down to its normal levels. The acquisition thresholds are as
follows:
7.0 dB Eb/No for Rate 1/2
5.2 dB Eb/No for Rate 3/4
4.8 dB Eb/No for Rate 7/8
4.0 dB Eb/No for Uncoded operation (No FEC)
8–1