7 July 2017

STANAG-4538, optimized Asynchronous FLSU call ?

I copied these 3G-HF transmissions very often and, since the "extended" duration of the first burst, I always thinked to Asynchronous FLSU calls; but, apart from the un-expected ending BW5 burst, a careful examination of the first burst in some cases may reserve an interesting aspect as in the present sample.

Asynchronous FLSU calls consist of a sequence of Async_FLSU_Req PDUs sent consecutively on the channel: FLSU protocol use burst waveform 5 so an Async FLSU call can be easily detected in the ACF output screen just thanks to the BW5 spikes (Figs. 1,2).

Fig. 1 - BW5 timing
Fig. 2 - ACF output of an asynchronous FLSU request
Analyzing the recording in question, my analysis tool reveals only one initial BW5 burst, instead of the expected n-bursts (!), which is then followed by a block consisting of 2400Bd PSK-8 modulated symbols. I had a look at the on-air symbols and found that they have a 768-bit period length which corresponds to 256 PSK-8 symbols (Fig. 3).

Fig. 3
BW5 uses a Psuedo Noise spreading sequence that is generated using a table that just contains 256 values [S-4538 #13.9.6] thus the whole first burst match the BW5 waveform (also note the repeated patterns that characterize the bitstream).

Looking at the BW5 timing (Figure 1) and the ACF output screen of the signal (Fig. 4), it seems that the TLC (Transmit Level Control) section is sent only in the first BW5 burst while the following bursts are a bit shorter and consist only of the preamble and data sections, as shown in Figure 5.

Fig. 4
Fig. 5 - the Async FLSU request in question
This is in contrast with what I seen so far in Async FLSU requests in which the BW5 bursts are contiguously transmitted, each with its own TLC section (Figs. 2,6) 
Fig. 6 - ASsync FLSU request as depicted in STANAG-4538
Indeed, STANAG-4538 does not specify this point "The Asynchronous call begins with the LBT (for at least one dwell period), followed by the transmission of about 1.35N Async_FLSU_Request PDUs on the requested link frequency, where N is the number of channels in the scan list, and 1.35 is the duration of each dwell (in seconds)." [STANAG-4538 #5.1.3].

The initial TLC section is used for transmitter level control and receiver AGC settling [1], thus sending TLC sections in contiguous BWn bursts make a poor sense: it's my guess that probably the removal of the redundant TLCs is an optimized implementation adopted by this manufacturer.
By the way, my analysis tool fails because most likely uses a burst-duration approach in order to identify the waveform.

[1] Johnson, Koski, Furman, Jorgenson, "Third Generation and Wideband HF Radio Communications"
Existing HF radios were generally not designed with burst waveforms in mind. For example, MIL-STD- 188-141 military radios are allowed 25 ms to reach full transmit power after keying. While the transmitter radio frequency stages are ramping up, the input audio signal level is adjusted by a transmit level control (TLC) loop so that it fully modulates the transmit power. At the receiver, an automatic gain control (AGC) loop must also adjust to a new receive signal. To accommodate these characteristics of existing radios, the 3G burst waveforms begin with a TLC section of “throwaway” 8-ary PSK symbols that are passed through the system while the transmitter’s and receiver’s level control loops stabilize. 

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