8 January 2021

4G-ALE Fast WALE (188-141D) and WBHF traffic (188-110D)

A necessary note due to the multiplicity of abbreviations and acronyms:
WBHF: Wideband HF waveforms, as per MIL 188-110D App.D;

3G-ALE: third generation ALE as per 3G-HF STANAG-4538 (FLSU and RLSU protocol);
3GWB: 3G-ALE extensions to the FLSU protocol for wideband operations;
WBALE: (or WB ALE) Harris implementation of 3GWB;
WALE: (or 4G-ALE) wideband ALE as per MIL 188-141D App.G, fourth generation ALE;
(thus, WBALE is not WALE since they use different waveforms)

Thanks to a reporting of my friend Martin G8JNJ, on 4744.0 KHz (the "assigned" ALE frequency) - mostly in the morning -  it is possible to receive transmissions which use 4G-ALE Fast WALE (MIL 188-141D App.G) and WBHF (MIL 188-110D App.D) waveforms: it's the first time for me that I have the canche to "see" and analyze 4G-ALE signals. 

The WALE (4G-ALE) system uses waveforms derived from the WBHF waveforms for its transmissions, and draws ideas from both second- and third-generation ALE for its protocols. The WALE waveforms operate in 3 kHz and provide two interoperable modes for sending PDU – the “Fast” WALE waveform (intended for very fast link setup in voice-quality channels) and the “Deep” WALE waveform (designed for operation in the most challenging channels, including SNR < 0dB). The choice between Fast or Deep WALE can be made on a call-by-call basis as receivers listen to both types of WALE calls, as well as 3G & 2G ALE calls for simultaneous operation with existing narrowband circuits.[1]

In the recorded session shown in Figure 1, the transmissions consist of two-way 4G-ALE handshakes followed by data transfers using ARQ method and WBHF waveforms: the bursts following the last ACQs are probably an EOM signaling given that the following session begins with a 4G-ALE handshake. Since the strong signals in the analyzed sample, I can't say if it's a bidirectional link. 

Fig. 1

The WALE waveforms employ PSK8 modulation of an 1800 Hz subcarrier at a rate of 2400 symbols per second. The Fast WALE waveform is designed to set up links quickly in relatively good channels (voice quality or better). The two more dense states in the phase plane of Figure 2 are due to the fact that each bit of WALE data is sent using PSK2 (transcoded to PSK8 symbols and then scrambled by modulo 8 addition).

Fig. 2 - Fast WALE bursts

Quoting 188-141D "Each Fast WALE transmission shall begin with zero or more TLC blocks, or a Capture Probe in an asynchronous-mode call or termination, followed by the Fast WALE acquisition preamble, followed by one or more coded and interleaved WALE PDUs. The coded and interleaved bits of each WALE PDU shall be sent in alternating blocks of unknown (PDU) symbols and known (probe) symbols as shown in Figure G-9."
In the analyzed samples (synchronous calls) there are zero TLC blocks and no Capture Probe but, although Fast WALE uses a preamble consisting of nine 32-symbol Walsh sequences, according to my measurements, the preambles consist of seven sequences for a total of (7x32) + 32 + 96 +32 + 96 + 32 = 512 PSK symbols.

Fig. 3 - Fast WALE waveform

The traffic segments are PSK8 modulated at symbol rate of 9600Bd, ACF value is 120ms that makes a 3456-bit length period or 1152 PSK8 symbols (Figure 4): the frame structure (Figure 5) matches the waveform #7 of 188-110D App.D ie, 1024 Unknow symbols (3072 bit) + 128 Known symbols (384 bit). 

Fig. 4 - WBHF waveform #7

Fig. 5

The bursts I termed as "EOM" also employ PSK8 modulation at symbol rate of 9600 Baud, but they show a frame length of 1504 symbols (Figure 6), maybe due to the modulation method used for those messages (PSK8 is as they appear on-air).

Fig. 7

[1] https://www.rapidm.com/wp-content/uploads/2018/10/RM10_WBHF_EN.pdf


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