Thanks to a reporting of my friend Martin G8JNJ, on 4744.0 KHz - mostly in the morning - it is possible to receive transmissions which use Fast WALE (188-141D App.G) and WBHF (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 WALE 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 WALE handshake. Since the strong signals in the analyzed sample, I can't say if it's a bidirectional link.
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| Fig. 1 |
The WALE LSU protocol use a fixed 96-bit length PDU for both Fast and Deep waveforms with a correction
coding consists of a constraint length 9 (CL-9), half rate convolutional
code producing a 192-bit coded block (ie, for each bit input to the
encoder, two bits are taken from the encoder). Using Fast WALE the coded and interleaved bits of each PDU are sent in
alternating blocks of unknown (data) symbols and known (probe) symbols.
The WALE waveforms employ PSK8 modulation of an 1800 Hz subcarrier at a rate of 2400 symbols per second. The two more dense states in the phase plane of Figure 2 are due to the BPSK modulation of Fast WALE data.
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| Fig. 2 |
In the analyzed samples the portion before the coded and interleaved PDU, according its 83.33 ms duration, consists of 200 PSK8 symbols (figure 3). Since the preamble and the TLC sections use the same 32-element Walsh chips (32 PSK8 symbols), the initial portion consists of 1 TLC block (13.333 ms) followed by 168 PSK8 symbols (70 ms) which do not resolve into an integer number of 32-element Walsh chips! These values, durations and symbols, are not compliant with relevant standard (188-141D #G.5.1.8.2) which requires nine 32-element Walsh chips for the Fast WALE preamble, for a total of 288 PSK8 symbols and a 120 ms duration.
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| Fig. 3 |
The structure of the known/unknown symbols blocks is exactly compliant with 188-141D: ie, 288 PSK8 symbols and a 120 ms duration (figure 4). Notice that according 188-141D, the length of the preamble shall be equal to the sum of the lengths of the known/unknown symbols blocks.
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| Fig. 4 |
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| Fig. 5 - WBHF waveform #7 |
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| Fig. 6 |
The bursts I termed as "EOM" also employ PSK8 modulation at symbol rate of 9600 Baud and have a frame length of 1504 symbols (Figure 7).
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| Fig. 7 |
[1] https://www.rapidm.com/wp-content/uploads/2018/10/RM10_WBHF_EN.pdf
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