Comtech EF Data CDM-625A - Manual (Page 721)

Q–1
Appendix Q. OFFSET QPSK
OPERATION
OQPSK is the acronym for Offset Quadrature Phase Shift Keying modulation. OQPSK is a
variation of normal QPSK that is offered in the CDM-625A 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 in-phase (I) and quadrature (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-625A, 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.