Table 3-1. Spectral Efficiency Using Doubletalk Carrier-In-Carrier; Figure 3-16. Duplex Link Optimization - Comtech EF Data DMD-2050E Installation And Operation Manual

DMD2050E Universal Satellite Modem
Traditional Full Duplex Link
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

MN-DMD2050E Revision 2
Duplex Link with DoubleTalk Carrier-in-Carrier

Figure 3-16. Duplex Link Optimization

Modulation
and Code Rate
BPSK 1/2
QPSK 1/2
QPSK 2/3
Spectral Efficiency (bps/Hz)
Traditional
Carrier-in-Carrier
SCPC
0.50
1.00
1.33
Theory of Operation
1.00
2.00
2.67
3–32