Mocomtech CIM-550 - Manual (Page 108)

CiM-550 IP Enabled Satellite Modem Rev. 2
Theory of Operation CD/CIM550.IOM
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After decoding, the recovered clock and data pass to the de-framer (if EDMAC is
enabled) where the overhead information is removed. Following this, the data passes to
the Plesiochronous/Doppler buffer, which has a programmable size, or may be bypassed.
From here, the receive clock and data signals are routed to the terrestrial interface, and
are passed to the externally connected DTE equipment.
6.2 O FFSET QPSK O PERATION
Offset QPSK modulation is a variation of normal QPSK, which is offered in the CiM-
550. 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:
Acquisition may be longer, especially at low data rates.
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)