Comtech EF Data CDM-625 - Manual (Page 369)

13–1
Chapter 13. OFFSET QPSK
OPERATION
OQPSK is an acronym for Offset Quadrature Phase Shift Keying modulation. OQPSK is a variation
of normal QPSK that is offered in the CDM-625 Advanced Satellite Modem. 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 (e.g., a saturated power
amplifier), there is a tendency for the carefully filtered spectrum to degrade. This phenomenon
is termed spectral regrowth, 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 realign 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. In the
CDM-625, the two consequences of this are as follows:
1. Demodulator acquisition may be longer than standard QPSK, especially at low symbol rates.
2. The acquisition threshold is slightly higher than for normal QPSK. This effect is only an issue
for LDPC Rate 1/2 and TPC Rate 21/44 code rates, where the Eb/No values are typically less
than 2 dB. In this case, the acquisition and tracking threshold is approximately 1dB higher
than for QPSK.