Premier Mounts CDM-600 specifications MN/CDM600.IOM

CDM-600 Satellite Modem	Revision 7
Forward Error Correction Options	MN/CDM600.IOM

CAUTION

NOTE: Whenever uncoded operation is selected, the modem offers the choice between enabling and disabling the differential encoder/decoder appropriate for the modulation type.

The second problem inherent in PSK demodulators is that of ‘data false locking’.

In order to accomplish the task of carrier recovery, the demodulator must use a non-linear process. A second-order non-linearity is used for BPSK, and a fourth-order non-linearity is used for QPSK. When data at a certain symbol rate is used to modulate the carrier, the demodulator can lock at incorrect frequencies, spaced at intervals of one-quarter of the symbol rate away from the carrier. Fortunately, when FEC decoding is used, the decoder synchronization state can be used to verify the correct lock point has been achieved, and to reject the false locks.

However, if uncoded operation is used, there is no way to recognize a data false lock. The demodulator will indicate that it is correctly locked, but the data out will not be correct. In Firmware Version 1.3.1 or higher, a new signal processing algorithm avoids this problem.

When using Firmware Versions prior to 1.3.1, Comtech EF Data strongly cautions users when using uncoded operation. If the acquisition sweep width exceeds one quarter of the symbol rate, there is a very high probability that the demodulator will false lock. For Firmware version 1.3.1 or higher, the problem has been been eliminated.

Example 1: A Firmware Version prior to 1.3.1 is being used, and the user selects 64 kbps QPSK, uncoded. The symbol rate will be half of this rate, or 32 ksymbols/second. One quarter of this equals 8 kHz. Therefore, the absolute maximum acquisition sweep range which should be considered is ± 8 kHz. If there is any frequency uncertainty on the incoming carrier, this should be subtracted from the sweep width. The problem becomes progressively better with increasing symbol rate.

Example 2: A Firmware Version of 1.3.1, or higher is used. There is no limitation on acquisition sweep width.

Comtech EF Data cannot be held responsible for incorrect operation if the user does not adhere to these guidelines when using uncoded operation.

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