23 July 2016

PacTOR-II, modulation changes

In this examples (recordered on 16285.0 KHz/USB) the PacTOR-II modem exhibits the ability to adapt its modulation method (and data rate) dynamically based on channel conditions or on current needs (ie broadcast/PtP). Since PacTOR is not an auto-baud waveform, this routine (knows as 'PacTOR-II AUTO') is usually implemented in the controller/modem firmware.
Looking at this case, Tunisian Minister of Interior network, the used mode seems to be reported in the addresses of the email (HFARQ STAT11, HFARQ STAT151) sent after the 188-141 2G-ALE handshake (fig. 1): most likely, their adopted messaging system is implemented on ARQ just per deafult.

fig. 1 - 188-141 2G-ALE followed by PacTOR-II email
These records (figs 2,3) refer to two separate email transmissions: in the former (1137 local time) the modem use the DPSK-8 FEC mode while in the latter (1213) the modem switched to the more reassuring DBPSK ARQ. Modulation rates remain constant (100 symbol/sec).

fig. 2 - PacTOR-II FEC PSK-8 100Bd
fig. 3 - PacTOR-II ARQ PSK-2 100Bd
According to the two spectrograms, the channel conditions degrade since the strength of the received signal decreases and this could justify the change of modulation. However it is fair to say that the tx antenna may use different beamings.

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