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Comtech EF Data CDM-760 User Manual

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Appendix J

Revision 2

CDM-760 Advanced High-Speed Trunking Modem

MN-CDM760

J–7

J.4.1

DoubleTalk Carrier-in-Carrier Cancellation Process

The state-of-the-art signal processing technology employed via CnC continually estimates and

tracks all parametric differences between the local uplink signal and its image within the

downlink. Through advanced adaptive filtering and phase locked loop implementations, it

dynamically compensates for these differences by appropriately adjusting the delay, frequency,

phase and amplitude of the sampled uplink signal, resulting in excellent cancellation

performance.

When a conventional full duplex satellite connection is established between two sites, separate

satellite channels are allocated for each direction. If both directions transmitted on the same

channel, each side would normally find it impossible to extract the desired signal from the

aggregate due to interference originating from its local modulator. However since this

interference is produced locally, it is possible to estimate and remove its influence prior to

demodulation of the data transmitted from the remote location.

For the CnC cancellation, it is necessary to provide each demodulator with a copy of its local

modulator’s output.

Figure J-5. DoubleTalk Carrier-in-Carrier Signals

Referring to Figure J-5: Modem 1 and Modem 2 transmit signals S1 and S2 respectively. The

satellite receives, translates, and retransmits the composite signal. The composite downlink

signal S1* + S2* is received at Modem 1 and Modem 2. The modem’s returned image of the

uplink signal (not including the desired carrier) differs from the originally-transmitted signal

primarily in terms of phase, frequency, and delay offsets.

Referring to Figure J-6: For round trip delay estimation, a search algorithm is utilized that

correlates the received satellite signal to a stored copy of the local modulator’s transmitted

signal. The interference cancellation algorithm uses the composite signal and the local copy of

S1 to estimate the necessary parameters of scaling (complex gain/phase), delay offset and

frequency offset. The algorithm continuously tracks changes in these parameters as they are

generally time-varying in a satellite link.