Wavecom W-SPECTRA (Automatic Monitoring System) V1.1.0 - Manual (Page 158)

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Transmission Modes WAVECOM Monitoring System W-SPECTRA, W-SPEED Manual V1.1.0
the center cursor, the center frequency may be set more precisely. The precision of the center fre-
quency is very important and directly influences the performance of the decoding.
To ensure, that the system can handle drifting signals or a coarsely adjusted center frequency, AFC may
be enabled in the Demodulator menu. The carrier tracking function is now activated. If the deviation be-
tween the tracked and the adjusted center frequency is too large, the center frequency should be correct-
ed. The carrier tracking only works over a range of the center frequency ± 10 Hz.
The deviation of the adjusted center frequency and the tracked frequency is shown in the tuning display.
If a PACTOR-II signal was not found, the bar of the tuning display moves randomly. If a valid PACTOR-II
signal was detected, the bar is frozen during the entire frame and shows the deviation. Now the center
frequency may be manually adjusted to fit the correct frequency if necessary.
If a valid PACTOR-II signal was found the detected modulation type, frame length and the polarity is dis-
played. If it was an error free frame, the detected compression type, the status and the decoded text is
displayed.
Options | CRC Recognition. The detected CRC is displayed in the decoder status bar, see “ CRC Recogni-
tion ” on page 43.
Using Options | CRC Table a certain CRC mask can be defined, see “ CRC Table ” on page 43.
Options | Auto Decrypt. If enabled, the mode tries to calculate the key of encrypted connections. It may
take a few seconds until the key has been determined correctly. This function can not be used together
with CRC Recognition.
PACTOR-II-AUTO
Fully automatic switching between PACTOR-II (ARQ) and PACTOR-II-FEC may be achieved by selecting
PACTOR-II-AUTO mode.
PACTOR-II-FEC
With PACTOR-II-FEC, a modern, narrow-band and reliable two-channel PSK mode is available, which co-
vers the needs of professional users for a secure and fast data transmission system. The good perfor-
mance during disturbances and a small bandwidth of 500Hz are achieved by the use of a two channel
DPSK modulation.
Parameter Value
Frequency range HF
Operation modes PSK, Simplex FEC
Modulation Two channel DQPSK
Symbol rate 100.0 Bd
Automatic frequency control (AFC): ± 50 Hz
Receiver settings DATA, CW, LSB or USB
Input format(s) AF, IF
Additional Info ITA-5 with block coding
HEX (Binary Output)
In contrast to PACTOR-II, modulation and block length in PACTOR-II-FEC is fixed to "DQPSK LONG
BLOCK". Where PACTOR-II uses ARQ, i.e., after each block sent, an acknowledge message from the re-
ceiving station is required, PACTOR-II-FEC has no acknowledge return channel. Thus, the receiving station
must be able to do all the error correction. This is achieved by using a Viterbi decoder with k = 9 and code
rate = ½. Interleaving protects the transmission against fading. Using a DQPSK modulation is a good
compromise when looking at the amount of bits transmitted and the necessary signal-to-noise-ratio.
To extract the data from a PACTOR-II-FEC signal, the signal is received and demodulated with a two
channel DPSK-demodulator. The center frequency must be adjusted to the midpoint between the two
channels. To achieve the best performance, the AFC (automatic frequency control) should be enabled. Af-
ter the demodulation and the bit synchronization, the symbols of the two channels are combined to one bit