Comtech EF Data CDM-625A - Manual (Page 588)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix D Revision 3
D–2
Traditionally, the method for identifying an interfering carrier involves using a geo-location
system that, in turn, uses the phase offset from an adjacent satellite to triangulate the
approximate location on the surface of the earth where the interference is being generated.
While such “tried-and-true” geo-locating methods have proven beneficial to satellite operators
and service providers, they are nevertheless imprecise. For example, to find the exact location
of the transmission source in a densely populated area, you must dispatch a helicopter
equipped with a feed horn and spectrum analyzer; the time and cost associated with such
methods are significant.
By contrast, Comtech EF Data’s DVB-CID provides you with the interference source’s
identification information within seconds. Once you identify the offending carrier, you may then
contact the uplinking station and request that the station shut down or otherwise remove the
identified transmission from service.
D.3 Functional Description
In a typical network, there can be many CDM-625As with DVB-CID, and one (or more)
MCDD-100 MetaCarrier Detection Devices to verify the presence of the DVB-CID on each carrier.
In an interference situation, the MCDD-100 may be used to decode the DVB-CID of an
interfering carrier that may not be part of one’s own transmission network, as long as the
interfering carrier has an embedded DVB-CID.
The CDM-625A creates a composite carrier by first sizing the appropriate MetaCarrier, and then
by adding the spread spectrum CID (at a highly reduced power spectral density compared with
that of the host carrier).
The size of the MetaCarrier is determined based purely on symbol rate and is totally
independent of modulation and coding, resulting in two (2) discrete sizes of MetaCarrier being
combined with the host carrier. The MetaCarrier parameters are shown below:
Host Carrier Embedded DVB-CID MetaCarrier DVB-CID psd relative to host carrier psd
128 ksps to < 512 ksps 112 kchips per sec * -27.5 dB
512 ksps to < 2048 ksps 224 kchips per sec -27.5 dB
2048 ksps to < 4096 ksps 224 kchips per sec -24.5 dB
4096 ksps to < 8192 ksps 224 kchips per sec -21.5 dB
8192 ksps to 12500 ksps 224 kchips per sec -18.5 dB
*kchips per sec refers to the direct sequence spread spectrum chipping rate
As shown here, the bandwidth of the host carrier is always wider than the MetaCarrier, the
worst case being a 112 kcps MetaCarrier underneath a 128 ksps host carrier. In all
configurations of the combined carrier, the MetaCarrier raises the transmission power less than
0.1 dB above the original carrier.
Note that in accordance with the DVB specification, Carrier ID is only available when
the Transmit symbol rate is greater than, or equal to 128 ksps.