22 September 2017

TE-204 (Rockwell AN/USC-11) 75Bd in-band frequency and time diversity HF modem

TE-204 (AN/USC-11) is most commonly used by Allied Air Forces as an air-to-ground messaging system as well as in ground and naval applications. The transmission was heard on 11220.0 KHz/USB, HF-GCS frequency, at 2145 UTC.
The modem converts the incoming serial binary data stream into an FSK audio signal at baseband and appears on-air as MFSK-4 150Bd/440Hz (Figure 1) although the real transfer speed is 75 Baud: the reason is in the "in-band frequency and time diversity" mode used by this modem.

Fig. 1 Over-the-air parameters
Four data subcarriers are used at 935 Hz through 2255 Hz with tones spaced every 440 Hz: TE-204 transmits a "mark" on 935 Hz for 6.67 msec period followed by another 6.67 msec period at 1815 Hz. Similarly, a "space" is transmitted at 1375 Hz for 6.67 msec period followed by another 6.67 msec period at 2255 Hz (Fig. 2).

Fig. 2 in-band frequency and time diversity mode
This "mode" provides an in-band frequency diversity function for each data bit while also transmitting the same data bit over two(!) separate 6.67 msec periods of time, thus achieving a frequency and time diversity function  (thus the speed is the half of the measured one). As for above, from the perspective of the data-transfer, the modem works as a FSK-2 75Bd/880Hz modem.
No special preamble or SOM/EOM codes are employed, decoding the signal as in the depicted mode will be possible to get the 128-bit KG-84 Message Indicators (Figs. 3,4)
By the way, the TE-204 modem is embedded in the  Rockwell Collins MDM2001 Multimode HF Modem.

Fig. 3
Fig. 4

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