In the asynchronous call, used to set up a link between two stations when one or both of them is scanning asynchronously, the WALE Request PDU must be preceded by a scanning call, the "capture probe", which similarly to 2G/3G ALE is aimed to capture asynchronously scanning receiver(s).
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| Asynchronous two-way point-to-point link setup example (from MIL 188-141D App. G) |
The capture probe consists of repeated blocks of known symbols that will be recognized by the scanning receivers so that they will stop scanning and wait to receive the WALE Request PDU (Protocol Data Unit) which immediately follows the capture probe section. The capture probe must last long enough so that every scanning receiver will have a chance to receive it, thus its duration depends on the number of channels (N) in the scan set and on the minimum dwell time Dmin (200 ms as default setting) and shall be ≥ Dmin * (N+2).
In the sample being analyzed the "Fast" WALE waveform [1] is used to convey the WALE Request PDU which follows the capture probe (figure 1).
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Fig. 1
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Each capture probe block consists of the following 96 PSK8 symbol sequence:
1,1,4,6,5,5,5,2,4,7,6,7,7,3,3,0,5,5,7,4,2,7,2,5,2,7,2,6,6,6,0,4,
6,1,0,7,1,2,1,6,3,2,0,2,5,5,2,7,3,1,0,4,7,7,7,7,2,6,5,6,6,7,0,2,
0,1,2,5,6,1,7,7,4,0,4,7,4,5,4,4,1,4,2,2,2,4,6,7,3,4,5,5,1,0,3,0
and, since it's transmissed at the symbols rate of 2400 symbols/s, each block has a duration of 40 ms. Notice that the number of the known symbols in each block (96) is the
same of the number of the unknown symbols sent in the call PDU: most
likely this choice was adopted to facilitate the recognition and
acquisition of the ending call.
Figure 2 shows the 40 ms ACF spikes of the scanning call portion.
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Fig. 2 - ACF of the capture block section
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It's interesting to compare the way 3G-ALE (STANAG-4538) and 4G-ALE (MIL 188-141D WALE) implement the async calls:
• the 3G-ALE async call consists of 1.35 N "011-type" Request PDUs, where N is the number of frequencies in the scan list, and 1.35 is the duration of the dwell period (in seconds); the call ends with a single "000-type" PDU Request. Since the address of the called station is contained in each of the "011-type" PDU Requests (as in 2G style), all stations that are not included in the call are free to resume scanning;
• since the 4G-ALE async call is not addressed, a station must wait for the Request PDU (sent at the end of the capture probe) to find out if it is the recipient of the call.
It might seem like a disadvantage, but 4G is actually much more faster that 3G; indeed, assuming a 10 channels scan set, the maximum wait duration is approximately equal to 13.5s for 3G and 2.4s for 4G (with minimum dwell time of 200 ms).
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