Comtech EF Data SDM-300A - Manual (Page 245)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–9
Notes:
1. The occupied bandwidth is defined at the width of the transmitted spectrum taken at the
–10 dB points on the plot of power spectral density. This equates (approximately) to
1.19 x symbol rate for the SDM-300A TX filtering.
2. Included for comparison purposes.
Observe that the 8-PSK Rate 3/4 Turbo performance closely approaches that of the Rate
2/3 TCM/Reed-Solomon case – the BER performance is within approximately 0.4 dB.
However, it should be noted that the Rate 3/4 Turbo mode is 20% more bandwidth
efficient than the TCM case. The additional advantages of Turbo (lower delay,
performance during fades etc) should also be considered.
9.7 Uncoded Operation (No FEC)
There are occasions where a user may desire to operate a satellite link with No FEC of
any kind. For this reason, the SDM-300A offers this uncoded mode for three modulation
types - BPSK, QPSK and Offset QPSK. However, the user should be aware of some of
the implications of using this approach.
PSK demodulators have two inherent undesirable features. The first of these is known as
‘phase ambiguity’, and is due to the fact the demodulator does not have any absolute
phase reference, and in the process of carrier recovery, the demodulator can lock up in
any of K phase states, where K = 2 for BPSK, K = 4 for Offset QPSK. Without the ability
to resolve these ambiguous states there would be a 1-in-2 chance that the data at the
output of the demodulator would be wrong, in the case of BPSK. For Offset QPSK, the
probability would be 3 in 4.
The problem is solved in the case of BPSK by differentially encoding the data prior to
transmission, and then performing the inverse decoding process. This is a very simple
process, but has the disadvantage that it doubles the receive BER. For every bit error the
demodulator produces, the differential decoder produces two.
The problem for QPSK is more complex, as there are 4 possible lock states, leading to 4
ambiguities. When FEC is employed, the lock state of the FEC decoder can be used to
resolve two of the four ambiguities, and the remaining two can be resolved using serial
differential encoding/decoding. However, when No FEC is being used, an entirely
different scheme must be used. Therefore, in QPSK, a parallel differential
encoding/decoding technique is used, but has the disadvantage that it again doubles the
receive BER.
Offset QPSK is a different situation again, where the ambiguities result not only from not
having an absolute phase reference, but also not knowing which of the two parallel paths
in the demod, I or Q, contains the half-symbol delay. Another type of differential
encoding is used, but yet again the error rate is doubled, compared to ideal.
NOTE: Whenever uncoded operation is selected, the modem automatically uses the
differential encoder/decoder appropriate for the modulation type. It cannot be disabled.