13 May 2016

Link-11 SLEW: scrambler length and ACF value

TADIL-A/Link 11 is a secure half-duplex TADIL radio link used by US and NATO that receives or transmits --but not both simultaneously-- a sequential data exchange digital link.  It exchanges digital information among airborne, land-based, and ship-board tactical data systems. It is the primary means to exchange data such as radar tracking information beyond line of sight.
LINK-11 comes in two waveroms: the original Conventional  Link  Eleven  Waveform (CLEW) and Single (serial) tone Link Eleven Waveform (SLEW).

Single tone Link Eleven Waveform (SLEW) is one of the modes defined within the Link 11 NATO standard. For SLEW, a single analog waveform is generated for the upper side band, the PSK-8 modulation process is achieved by assigning the tri-bit numbers from the scrambler to 45-degree phase increments of a 1800 Hz carrier. Symbols rate is 2400 Bd while the user data rate is 1800 bps (pic. 1).

pic. 1
The SLEW waveform transmission format consists of an acquisition preamble followed by two or more fields. Each 45 symbols field is followed by a 19 symbols reinsertion probe. The first field  after the preamble is the header field and contains information that is used by the Combat Data System (CDS) and the encryption device. If there are data to transmit, successive data fields follow the reinsertion probe of the preceding fields (pic. 2,3).

pic. 2 SLEW waveform structure
pic. 3
Running the Cross Correlation or Auto Correlaton functions, a 64 symbols or 192 bits frame are expected, but in contrast the CCF output exhibits clear and strong 320 symbols spikes corresponding to a period of 960 bits. Note that five data and reinsertion-probe pairs are arranged inside the period window (pic. 4).

pic. 4 - SLEW waveform CCF result (133.33ms)
So, why the 133.33ms, or 320 symbols, period?
As in pic. 5, the 45 phase encoded pairs (values 0, 1, 2, 3) are mapped into tri-bit numbers (by multiplying by 2).  The tri-bit numbers (0, 2, 4, 6) are used for symbol generation and scrambled  to take on all 8 phase states. During the reinsertion probe, 19 tri-bits (set all to "000") are used for known symbol formation and scrambled.
pic. 5 - SLEW wavefrom formation (reinsertion probe and data field)
Since the scrambler could be an important factor in ACF generation,
let's give it a close look: it's worth to noting that the data sequence randomizing generator is the same 12-bit shift register used in MS188-110 serial tone!

"The tri-bit numbers supplied for the symbols (both data and probe) are modulo-8 added to a three-bit value supplied by the data sequence randomizing generator. At the start of the data phase, the shift register is loaded with the initial pattern 101110101101 (binary) or BAD (hex) and advanced 8 times. The resulting three bits are used to supply the scrambler with a number from 0 to 7 which is modulo-8 added to the data/probe symbol. The shift register is shifted eight times each time a new three-bit number is required (every transmit symbol period). After 160 transmit symbols, the shift register is reset to BAD (hex) prior to the eight shifts."

As seen in MS188-110 low data rates, this 12-bit randomizing generator is the cause of the Link-11 SLEW ACF.
In fact, since the scramble length of 160 symbols coincides with 2.5 frames, we get that each five frames - or just two scramble cycles(!) - the same probe value "000" is scrambled exactly after the same number of shifts and hence produce the same probe patterns (pic. 6). These same patterns repetion produces the 320 symbols (or 960 bit) spikes in CCF and ACF function.

pic. 6 (qualitative rapresentation, not in scale)

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