17 February 2021

4G-ALE async two-way point-to-point link setup

The asynchronous two-way point-to-point link setup protocol (LSU) is used to set up a link between two stations when one or both of them is scanning asynchronously. The asynchronous link setup protocol is the same as the synchronous one except for the following:
a. the call may be transmitted at any time.
b. the call is preceded by the “capture probe” 

The capture probe, sent after the the LBT (Listen Before Transmit) interval, comprises repeated blocks of known symbols that will be recognized by the scanning receivers so that they will stop scanning and wait to receive the real call PDU (Protocol Data Unit) which immediately follows the capture probe section; the called station will reply sending a request confirm PDU. 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 frequencies (N) in the scan set and on the minimum dwell time dmin: more precisely, it shall be  ≥  dmin(N+2).

Each capture probe block consists of the following 96 PSK8 symbol sequence:
and, since it's transmissed at the symbols rate of 2400 symbols/s, it has a duration of 40 ms. Figure 2 shows the strong ACF spikes just due to the capture probe blocks length. 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.

Fig. 2 - ACF of the capture block section

It's interesting to compare the way 3G-ALE (STANAG-4538) and 4G-ALE (MIL 188-141D WALE) perform async calls. The 3G async call begins with the LBT followed by the transmission of 1.35N "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, then the called station may reply sending the Confirm PDU to the caller.
Since the address of the called station is contained in each of the "011" Requests (as in 2G style), all stations that are not included in the call are free to resume scanning while in 4G-ALE a station must wait for 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).

Asynchronous two-way point-to-point link setup example (from MIL 188-141D App. G)

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