29 April 2016

MS188-110: probe designation symbols D1-D2 and 200ms ACF

in a previous post I already talked about the way MS188-110 scrambler length affects the value of the ACF at certain data rates. Now I want verify the typical 200ms ACF  of a 2400 bps data rate (short interleaver): as seen in the above post, this ACF value is also met at other data rates along with the 66ms value.  Summarizing:
  • In case of low data rates (from 150 up to 1200 bps) four groups of the pairs probe+data count 160 symbols (4 x 40) and they are just in sync with the scrambler length (160 symbols) causing the strong 66.67mS ACF.
  • In case of the lowest speed (75bps data rate) the channel probes are not sent so the 66.67mS ACF is just due to the scrambler length (160 symbols).
  • In case of 2400 bps the pair probe+data counts 48 symbols (32 symbols for data +16 symbols for probe) and this value is not correlated to the the scrambler length, so the "visible" ACF in this wavfeorm, and in 4800 uncoded, is the known 200 mS or 480 symbols.
pic. 1

(Since we are talking about MS188-110 Serial Tone, I refer to "symbols" as over-the-air PSK-8 symbols).
But, why the 480 symbols ACF rather than the expected 48?

pic. 2 - strong 200ms ACF spikes in a MS188-110 signal
The short interleaver matrix dimension consists of 40 rows and 72 columns (= 2880 bit room) and is loaded in 600 ms. Indeed, for the 2400 bps data rate the FEC encoder results in 4800 bps coded rate, and so (4800 x 600)/1000 = 2880 are the bits transferred into the interleaver matrix during the 600ms short interleave load. At 2400 bps data rate the bits fetched from the interleaver matrix are grouped together as three bit entities that will be referred to as channel symbols and processed by the Modified-Gray Decoder (MGD). Then, the Symbol Formation function maps the three bit channel symbols from the MGD output into tribit numbers compatible with transmission using an 8-ary modulation scheme.
This means that from the short interleaver matrix we get 960 symbols that in turn will be transmitted into 30 data-blocks (unknown-data) each of 32 symbols (pic. 1). According to the framing table (pic. 1), each data-block is followed by a 16 known symbols sequence (probe). That said, the lenght of a interlever block will be 1440 channel symbols or 30 x (32+16).

The period of this waveform (pic.3) exibiths a 1440 bit lenght (not symbols!), or else 1/3 of the interlever block length: just ten data+probe frames, which are well visible in the bistream. Looking closely at the patterns of the last two probes (yellow circled in the picture) is visible a sort of discontinuity that is not present in the patterns of the middle probes.

pic. 3 - 1440-bit (or 480 symbols) period
The reason is the way those two probe patterns are formed (pic. 4):
 
MIL-STD 188-110 - 5.3.2.3.7.1.2 Known data”
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 the two known symbol patterns preceding the transmission of each new interleaver block are transmitted, the 16 tribit symbols of these two known symbol patterns shall be set to D1 and D2, respectively, as defined in table XV of 5.3.2.3.7.2.1 and table XVII of 5.3.2.3.7.2.2. The two known symbol patterns are repeated twice rather than four times as they are in table XVII to produce a pattern of 16 tribit numbers.
(The three bit values of D1 and D2 also designate the bit rate and interleave setting of the transmitting modem during the Sync preamble sequence)
 
pic. 4
This is more clear when looking at the the probe patterns in the middle positions of each interleaver block (pic. 5)

pic.5 - middle probe patterns
and at the last two probe patterns P(n-1) and P(n) preceeding the transmission of one interleaver block. In pic. 6 these two probe patterns are shown as isolated.

pic. 6 - the two last probe patterns
The three rows, each of 1440 bit, identify one interleaver  block of 1440 channel symbols: the last two probe patterns are clearly visible (pic.7).
In my opinion the patterns of the last two probes of each interleaver block - or better the Designation Symbols D1, D2 - cause the 1400-bit/200ms ACF and they act as a sort of autocorrelation sequence.
 
pic. 7
Its worth noting that the 66.67ms ACF visible at lower data rates, and due to the scrambler length, corresponds to 480 bits and this value is a submultiple, exactly 1/3, of the 1440 bit period (200ms) that is due to the Designation Symbols D1,D2 in the last two probe patterns of the interleaver blocks. These two matters concurr to the two ACF spikes visible, for example, in the 150bps data rate waveform (pics. 8,9).

pic. 8
pic. 9
The Find Period function, running at the same level, prints out one unique 1440 bit value for the 2400bps waveform (pic. 10)

pic. 10

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