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 data + probe (Unknown + Known) 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+16 UK) and this value is not correlated to the the scrambler length, so the "visible" ACF in this wavfeorm, and in 4800 uncoded, is the 200 mS or 480 symbols.
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| pic. 1 |
So,
why the 480 symbols ACF rather than the expected 48?
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| pic. 2 - strong 200ms ACF spikes in a MS188-110 signal |
The short interleaver matrix dimension for 2400 bps speed consists of 40 rows and 72 columns i.e. 2880 bits and is loaded in 600 ms. 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 consisting of 32 symbols and followed by a known 16 symbols sequence (probe or Known data). That said, the lenght of one transmitted interlever block is 1440 symbols or 30 UK frames (32+16).
The period of the demodulated stream (pic.3) is 1440 bit lenght (not symbols!), just 10 UK frames, which are well visible in pic. 3. It's worth noticing that the patterns of the last two probes (yellow circled) exhibits a certain discontinuity that is not present in the patterns of the middle probes. Also notice that 3 rows identify 1 interleaver block!
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| pic. 3 - 1440-bit period |
The reason is in the way the last two probe are formed (pic. 4). Quoting 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)
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| pic. 4 |
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 since they act as a
kind of autocorrelation sequence (pic. 5)
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| pic.5 - the 10 interleaver blocks |
Its worth noting that the 66.67ms ACF visible at lower data rates (due to the scrambler length) corresponds to 480 bits and this value is a submultiple, exactly 1/3, of the 1440 bit period that is due to the Designation Symbols D1,D2. These two matters concurr to the two ACF spikes which are visible, for example, in the 150bps data rate waveform (pics. 6,7).
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| pic.6 |
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| pic.7 |
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