Rates Above 2.5 Msymbols/Sec - Comtech EF Data CDM-570 Installation And Operation Manual

CDM-570/570L Satellite Modem with Optional IP Module
Forward Error Correction Options
contains the half-symbol delay. Another type of differential encoding is used, but yet again the error
rate is doubled, compared to ideal.
The second problem inherent in PSK demodulators is that of 'data false locking'. In order to
accomplish the task of carrier recovery, the demodulator must use a non-linear process. A second-
order non-linearity is used for BPSK, and a fourth-order non-linearity is used for QPSK. When
data at a certain symbol rate is used to modulate the carrier, the demodulator can lock at incorrect
frequencies, spaced at intervals of one-quarter of the symbol rate away from the carrier.
Fortunately, when FEC decoding is used, the decoder synchronization state can be used to verify
the correct lock point has been achieved, and to reject the false locks.
However, if uncoded operation is used, there is no way to recognize a data false lock. The
demodulator will indicate that it is correctly locked, but the data out will not be correct.This
problem has been almost entirely eliminated in the CDM-570/570L with the fast acquisition
algorithm which includes Fast Fourier Transform (FFT) techniques. However, there is a very
small probability that a data false lock could still occur in uncoded mode, and in this
circumstance Comtech EF Data cannot be held responsible for incorrect operation.
8.7

Rates above 2.5 Msymbols/sec

Starting with Release 1.4.1 of the CDM-570/570L firmware, the maximum
symbol rate has been increased from 2.5 to 3.0 Msymbols/sec. This has
been done without modification to the hardware, and as a consequence,
there may be a small degradation in BER versus E
above 2.5 Msymbols/sec. The degradation is as follows:
Rates from 2.5 to 2.65 Msps: degradation < 0.1 dB
Rates from 2.65 to 2.80 Msps: degradation < 0.2 dB
Rates from 2.80 to 3.00 Msps: degradation < 0.3 dB
You should take this into account when considering the BER versus E
graphs that follow.
8–9
Revision 12
MN/CDM570L.IOM
/N performance for rates
b o
/N
b o