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Chapter 13. OFFSET QPSK
Offset QPSK modulation is a variation of normal QPSK, which is offered in the CDM-625.
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 (e.g., 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. In the
CDM-625, the two consequences of this are:
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.
OPERATION
13–1
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