18 November 2015

HFDVL modem tests: sync. preamble

Spanish Universities are still conducting intensive tests of the HFDVL modem (hfdvl-modem-ofdm-73-tone) on 14350.0 KHz on USB, I not tried  to listen on the other 14825.5 KHz test-frequency. While the previous tests consisted of transmission of about 30 and 10 seconds duration, in these samples (at least) a different "test format" is used: it seems to me that one side of the link just sends data (long signals) while the other peer seems just send a sort of ACK (shorter signals). The durations are fixed, ie 1'06'' and 0'11 secs. 

The tests involve two transmitting modems, more likely located at Las Palmas - Canary Islands (14350.0 KHz just belongs to the University of Las Palmas) and Madrid, since the HFDVL waveform is developed by the groups "CeTIC" from the Universidad de Las Palmas de Gran Canaria and "GAPS" from Universidad Politécnia de Madrid, other than Aeropuertos Españoles y Navegación Aérea. As far as I know, the modem manufacturer is MM CICOM Telecomunicaciones from Spain.

Prior to the transmission of data, a three part preamble is transmitted.
Part one consists of an initial unmodulated tone more likely used for Doppler correction. This tone is slighty shifted in the two signals (Pic.1) but just a few hz, so pretty meaningless though some distortion visible in ACK signals which could be due to a falut of modem (Pic. 1b). Looking at the Pic. 2, in 17-Nov recording the initial tone lasts for 
- 633.1/3 ms (19 signal element periods) in ACK signals 
433.1/3 ms (13 signal element periods) in DATA signals 
while in the 24-May recording the tone has a duration of 433.1/3 ms (13 signal element periods). Part two consists of  2 signal element periods of (KNOWN) data and part three consists of 1 signal element period of only even carriers followed by 3 signal element periods of only odd carriers: part two and three sum to 6 signals and make a 200ms duration (Pic. 3). 
My friend KarapuZ pointed my attention on the part two: according to his analysis, this is BPSK 2400Bd modulated and carrier is 60 Hz down-shifted in respect of the unmodulated tone frequency. The 60 Hz shift is probably added for the initial synchronization. Following his tips I replicated his results (Pic. 4).
Pic. 1

Pic. 1b
Pic. 2

Pic. 4

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