Comtech EF Data SNM-1010L - Manual (Page 178)

SNM-1010L Data/Control Modem Revision 0
Theory of Operation MN/SNM1010L.OM
Table 6-1 provides a listing of all the available TPC modes and rates:
Table 6-1. Available TPC Modes
Data Rate (bps) Symbol Rate (sym/s)
Modulation Type Encoding Type Minimum Maximum Minimum Maximum
BPSK 5/16 Turbo 2,400 781,250 4,800 2,500,000
BPSK 21/44 Turbo 2,400 1,193,000 4,800 2,500,000
QPSK/OQPSK 3/4 Turbo 7,200 3,750,000 4,800 2,500,000
8PSK 3/4 Turbo 384,000 5,000,000 4,800 2,500,000
6.5.2 End-to-End Processing Delay
In many cases, FEC methods that provide increased coding gain do so at the expense of
increased processing delay. However, with TPC, this increase in delay is very modest.
Table 6-2 shows, for the modem, the processing delays for the major FEC types,
including the three TPC modes:
Table 6-2. Turbo Product Coding Processing Delay Comparison
FEC Mode (64 kbps data rate) End-to-end delay, ms
Viterbi, Rate 1/2 12
Sequential, Rate 1/2 74
Viterbi Rate 1/2 + Reed Solomon 266
Sequential Rate 1/2 + Reed Solomon 522
Turbo Product Coding, Rate 3/4, QPSK/OQPSK 79
Turbo Product Coding, Rate 21/44, BPSK 64
Turbo Product Coding, Rate 5/16, BPSK 48
Note that in all cases, the delay is inversely proportional to data rate, so for 128 kbps, the
delay values would be half of those shown above.
It can be clearly seen that the concatenated Reed-Solomon cases increase the delay very
significantly (due mainly to interleaving/de-interleaving), while the TPC cases yield
delays which are less than or equal to Sequential decoding.
It can be seen that the 8PSK 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.
6.5.3 Comparison of all TPC Modes
6–16