188-110A, ACF lengths and interleaved blocks boundaries

MIL 188-110A, unlike other waveforms such as STANAG-4539  or STANAG-4285, does not always show the same ACF value as the data rate changes: it is mainly due to the variations of the frame structure (Table XIX) and the way it interacts with the length of the scrambling sequence. The corresponding periods lengths of the bit streams then make it possible to identify the interleaved block boundaries which in turn depend on the bit rate and the interleave delay (Long or Short). 

Fig. 1 - 188-110A Serial Tone blocks

Table X lists the interleaver matrix dimensions (rows and columns) that shall be allocated for each required bit rate and interleave delay.

The bits obtained from the interleaver matrix are grouped together as one-, two-, or three-bit entities that will be referred to as "channel symbols" (or - more simply - "symbols"): the number of bits that must be fetched per symbol is a function of the bit rate. 
The scrambler is feed with a number from 0 to 7 supplied by the data sequence randomizing generator, a 12 bit shift register with the initial state 101110101101 (0xBAD, hexadecimal). After 160 transmit symbols, the shift register is reset to 0xBAD and this sequence produces a periodic pattern 160 symbols in length.  

The interleaved blocks boundaries can be identified by looking carefully at the probes in the demodulated bit streams. As per MIL-STD 188-110: during the periods where known (channel probe) symbols are to be transmitted, the channel symbol formation output shall be set to 0 (000) except for the two known symbol patterns preceding the transmission of each new interleaved block. When those two known symbol patterns are transmitted, the 16 tribit symbols are set to Dl and D2, respectively, as defined in table XV and table XVII. 

2400 bps
Each data frame has a length of 48 symbols and consists of a data block consisting of 32 data symbols, followed by a probe consisting of 16 symbols of known data. Although the expected ACF is 20 ms, the actual value is 200 ms ie corresponding to a block of 10 frames (Figure 2). Since 10 frames contain 10×16 = 160 probe symbols, the 200ms ACF spikes are likely due to a kind of "resonance"  between the 160 probe symbols and the 160 symbols of the scrambing sequence.

Fig. 2 - 200 ms ACF for 110A 2400 bps

The short interleaver matrix for 2400 bps consists of 40 rows and 72 columns, ie a length of 2880 bit: that means that 960 tribit symbols will be fetched and transmitted as 30 data blocks (960:32); thus, one interlever block needs 30 frames to be transmitted. As you may see in Figure 3,  each three rows the patterns of the last two probes exhibit a discontinuity that is not present in the other probes: that's what we were looking for. Indeed - as seen abvove - 3 period rows contain 3×10 = 30 frames that just identify a single short interleaved block for the 2400 bps speed. The interleaver matrix is fetched  in 600 ms, ie  200 ms  × 3.

Fig. 3 - 2400 bps Short interleaver

A similar calculation can be verified for the long interleaver. In this case the interleaver matrix consists of 40 rows and 576 columns, ie a length of 23040 bit or 7680 tribit channel symbols. Since the 32 symbols length frame, one long interleaved block will be sent into 7680:32 = 240 frames, thus 24 rows (Figure 4). The interleaver matrix is fetched  in 4.8 s, ie  200 ms  × 24.

Fig. 4 - 2400 bps Long interleaver

1200 bps
In case of low data rates (from 150 up to 1200 bps) the data frames are structured as a 40-symbol pattern: each frame consisting of a data block consisting of 20 data symbols, followed by a probe consisting of 20 symbols of known data. The expected ACF value is then 16.67 ms, but the actual one is 66.67 ms ie four times greather (Figure 5). The reason is that four groups of the pairs data + probe  count 160 symbols (4×40) and they are just "in sync" with the scrambler length (160 symbols) causing the strong 66.67 ms ACF spikes of Figure 5.

Fig. 5 - 66.67 ms ACF for 110A 1200 bps

The short interleaver matrix consists of 40 rows and 36 columns, ie a length of 1440 bit: that means that 720 dibit symbols will be fetched and transmitted as 36 data blocks (720:20); thus, one interlever block needs 36 frames to be transmitted.  Nine period rows are indeed the boundary of a single short interleaved block (Figure 6).

Fig. 6 - 1200 bps Short interleaver

The long interleaver matrix consists of 40 rows and 288 columns, ie a length of 5760 dibit symbols that will be fetched and transmitted as 288 data blocks (5760:20); thus, 288:4 = 72 period rows is the boundary of each long interleaved block (Figure 7).

Fig. 7 - 1200 bps Long interleaver

600-150 bps
The short interleaver matrix for 600-150 bps consists of 40 rows and 18 columns. Given that only one-bit per channel symbol is fetched, a single short interleaved block will be trasmitted as (40×18):20 = 36 frames. The boundaries of the short interleaved blocks are clearly visible each 9 rows in the usual 480-bit/4-frame period (Figure 8). 

Fig. 8 - 600-150 bps Short interleaver

The long interleaver matrix for 600-150 bps consists of 40 rows and 144 columns. Given that only one-bit per channel symbol is fetched, a single long interleaved block will be trasmitted as (40×144):20 = 288 frames. By grouping the bit stream in a 8-frame period, the boundaries of the long interleaved blocks are clearly visible each 36 rows (Figure 9).

Fig. 9 - 600-150 bps Long interleaver

As a last thought consider that since the bit streams come from demodulating the on-air signals, we see the "coded" data rate and not the actual "input" data rate... FEC coding is still there.