Comtech EF Data DMD-2050E - Manual (Page 74)

DMD2050E Universal Satellite Modem Theory of Operation
MN-DMD2050E Revision 2 3–32
Traditional Full Duplex Link Duplex Link with DoubleTalk Carrier-in-Carrier
Figure 3-16. Duplex Link Optimization
Because acquiring the delay and frequency offset of the interfering carrier is fundamentally a
correlation operation, anything deterministic in the interfering carrier (within the correlation
window of the algorithm) will potentially produce false correlation peaks and result in incorrect
delays and/or frequency. Normally, this is not a problem, since energy dispersal techniques are
utilized in the vast majority of commercial and military modems. However, it is something that
must be kept in mind when troubleshooting a system that utilizes the DoubleTalk Carrier-in-
Carrier technique for signal suppression.
One possible way to mitigate false peaks is to narrow the correlation window. For example, if the
delay is known to be around 240ms, set the minimum search delay to 230ms and the maximum
search delay to 250ms.
As all advances in modem technologies – including advanced modulation and FEC techniques –
approach their theoretical limits of power and bandwidth efficiencies, DoubleTalk Carrier-in-
Carrier allows satellite users to achieve spectral efficiencies (bps/Hz) that cannot be achieved
with modulation and FEC alone. Table 3-1 illustrates how DoubleTalk Carrier-in-Carrier, when
used with 16-QAM, approaches the bandwidth efficiency of 256-QAM (8bps/Hz).
Table 3-1. Spectral Efficiency using DoubleTalk Carrier-in-Carrier
Modulation
and Code Rate
Spectral Efficiency (bps/Hz)
Traditional
SCPC
Carrier-in-Carrier
BPSK 1/2 0.50 1.00
QPSK 1/2 1.00 2.00
QPSK 2/3 1.33 2.67