CiM-550 IP Enabled Satellite Modem

Rev. 2

Forward Error Correction

CD/CIM550.IOM

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.

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.

Comtech EF Data strongly cautions users when using uncoded operation. If the acquisition sweep width exceeds one quarter of

CAUTION the symbol rate, there is a very high probability that the demodulator will false lock.

As an example, if 64 kbps QPSK, uncoded is used, the symbol rate will be half of this rate, or 32 ksps. 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.

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