27 October 2018

STANAG-5066 Annex G, the ancestor to US MIL STD 188-110B and STANAG-4539

The other day I was still trying to figure out something from the S4539 8-ary burst transmissions, posted here, using Harris RF-5710A HF modem. Turning between the various operating modes provided I came across what at first glance seemed a strange waveform: i.e. 5066-G. As far as I know, STANAG-5066 is a protocol standard that does not define waveforms, so I went into detail to see what the hell it is.
STANAG-5066 Annex G presents guidelines and requirements for the use of the STANAG 5066 protocol profile with modems and waveforms at rates above 2400bps. Early draft versions of STANAG 5066 Annex G included a detailed specification for high-speed single-tone waveform and convolutional forward-error-correction coding with data rates up to 9600 bps, and this formed the basis for more than one vendor implementation of a commercial product.
Quoting Edition 3 #G.3.0 Implementation Guidance for STANAG 5066 Operation at Higher Rates : "It is clear that higher throughput will be available for the HF long-haul channel in near future (i.e, post 2000). What is not clear is the final form of the waveform standard or standards that will provide these data rates". Notice that the Edition 3 was promulgated on 30 march 2015 and the Annex G is still "information only", ie it is still not mandatory for the purposes of the communication minimum requirements.

Now look at the words "...in near future (i.e, post 2000)": clearly, Annex G has remained unchanged from the first editions of STANAG-5066 (dated before year 2000). Most likely the NATO groups responsible for the standardization preferred a separate STANAG to define the new waveforms since 5066 is a protocol standard, so Annex G is "frozen" and stands like a kind of ancestor to MIL 188-110B and STANAG-4539 (3200 to 12800 bps): these new waveforms used constellations and much of the waveform structure developed for Annex G and added further enhancements. 
Harris developed its implementation of 5066-G, you may find it among the operational modes of their RF-5710A HF modem:

The STANAG-5066 Annex G waveforms provide the highest possible data rates over conventional 3KHz HF channels. A single 1800 Hz sub-carrier is modulated at a constant rate of 2400 symbols per second. the type of modulation varies from QAM-64 to PSK-4 according to the data rate selected. Known data symbols are periodically inserted in the transmitted signal to allow fro adaptive channel equalization at the receiver. Convolutional coding FEC and Viterbi decoding are combine with interleaving to enhance the performance of the receive modem on fading HF channels. Data rates from 3200 bps to 9600 bps are supported together with long, double long, short, and zero interleaving options. An additional 12800 bps uncoded waveform is supported for line-of-sight applications. Automatic detection of the data rate and interleaver setting are provided in the receive mode.

By the way, I tried to demodulate the S4539 8-ary bursts using the 5066-G mode , despite obsolete. Surprisingly, the modem is responsive and distinguishes four different waveforms: 4800L (PSK-8), 8000L bps (QAM-32), 9600L and 12800 bps uncoded (QAM-64), the last two are detected less frequently (Fig. 1). It should be noted that when the same transmission is demodulated using 4539 or 110B you always get the same waveform 12800U.

Fig. 1

Annex G: Use of Waveforms at Data Rates Above 2400 bps (V1.2) 

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