WO2009071589A2

WO2009071589A2 - Synchro-frame method based on the discrete logarithm

Info

Publication number: WO2009071589A2
Application number: PCT/EP2008/066722
Authority: WO
Inventors: Eric Garrido; Guillaume Fumaroli; Xavier Bertinchamps
Original assignee: Thales
Priority date: 2007-12-04
Filing date: 2008-12-03
Publication date: 2009-06-11
Also published as: EP2220799A2; KR101459176B1; KR20100099251A; FR2924549A1; WO2009071589A3

Abstract

Method for transmitting data having a first format or format 1 in a datastream complying with a second format, the second format or format 2 consisting of a stream of data symbols incorporating in a regular manner a symbol dedicated to synchronization and placed every r data symbols, the symbol dedicated to synchronization being the current term of a series S(t) satisfying a linear recurrence, the data being split in format 1 into data blocks of fixed size including kr symbols, the data symbols being considered to be elements of a finite field GF(q) where q is the number of elements of the field, the series S(t) satisfies a linear recurrence over GF(q) and admits as characteristic polynomial a primitive polynomial P of degree n over GF(q), and is periodic with a period T=qn-1, α being a root of P in the field GF(qn).

Description

SYNCHRO-FRAME METHOD BASED ON DISCRETE LOGARITHM

The invention relates to a method for transmitting data in a first format or format 1 in a data stream respecting a second format or format 2.

The invention applies to data flow transmission systems, especially in the case where these transmissions are simplex and synchronous. It finds applications for example in very low frequency radiocommunication systems or Very Low Frequency (VLF) or Low Frequency (LF) in which the data are broadcast according to the format (in French). defined by the STANAG 5065 (MSK LF mode) or the STANAG 5030. These systems allow the broadcast of messages to surface vessels for the STANAG 5065, to submarines for the STANAG 5030. It applies for various forms of for example, MSK (Minimum frequency-shift keying) and CPFSK (Continuous-phase frequency-shift keying).

FIG. 1A represents an example of a VLF / LF broadcasting system broken down into three distinct entities, the different functionalities of which have been represented in the figure. It includes a command center 1, a VLF / LF radio transmission station 2, receiving platforms 3 such as surface ships, submarines receiving messages broadcast by the VLF / LF transmission station. . The command center 1 can be positioned at a remote site connected to the VLF / LF transmission station via an inter-site link. The command center has the particular function of ensuring the generation 4 of the messages to be transmitted, then their transfer 5 to the VLF / LF transmission station. The VLF / LF 2 transmitting station receives messages from the command center and provides broadcast on the very low frequency / low frequency channel 6, designated by the abbreviation VLF / LF for Very Low Frequency / Low Frequency. To perform these functions, the station contains an interface gateway 7 with the network, one or more encryptors 8 and a VLF / LF modulator 9, as well as a transmission system 10 or broadcasting equipment. Depending on the architecture chosen for the implementation, the encryptors can be located in the command center or in the transmitting station. In order to illustrate the object of the present invention, the description aims at the second solution. The broadcasting message reception system mainly comprises a reception antenna 11, a receiver 12, a VLF / LF demodulator 13, the message reception terminal 14 and one or more decoders 15.

The transmitting stations broadcast continuously. In the absence of messages to be transmitted, stuffing messages must be injected into the data stream.

Among the receiving platforms, submarines are not constantly listening to transmitted messages. Therefore, a mechanism is needed for receiving equipment to synchronize with the data stream. For example, the stream of data broadcast in Stanag format 5030/5065 incorporates a synchronization sequence corresponding to a so-called Fibonacci sequence transmitted continuously with the useful data. The Fibonacci sequence is recognized by reception equipment - demodulator and decipherer. It allows these devices to synchronize with the data flow. The principle is robust enough to tolerate transmission errors induced by the channel. Moreover, by means of having clocks quite precise in transmission and reception, once acquired the synchronization can be maintained, this even in the absence of reception of the signal because the flow of data is synchronous. Encrypted encryptors protect the data flow in confidence. An encryptor ensures (see Figure 1 B), in this case: • Protection of messages before transmission using the encryption function 20,

Protocol adaptation between messages received from the inter-site network and the modulator, including asynchronous / synchronous conversion;

• Stuff generation in the absence of a message at its input, 22.

The decoder of the receiving platform of the messages ensures in this case:

The decryption of the messages transmitted on the VLF / LF radio channel and demodulated by the demodulator,

Protocol adaptation between the demodulator and the receiving data terminal, in particular synchronous / asynchronous conversion,

• Clearing the received jam 25.

It should therefore be noted that the encryptors and decryptors integrate both an encryption function and other functions generally designated in this document by "coding" and "decoding" (see FIG. 1B).

In the case of the aforementioned STANAG format, the data transmission is carried out in the form of a data stream of a telegraphic channel, according to FIG. 2A, organized in a 7-bit frame (frame t), comprising 6 data bits. and a current bit of the so-called Fibonacci suite, used for synchronization.

In reception, the demodulator supports the following processes:

- demodulation of symbols,

- Recognition of the continuation of Fibonacci and synchronization, These various treatments are known to those skilled in the art and will not be detailed in the present description.

The so-called Fibonacci sequence used is a non-zero binary sequence which satisfies a linear recurrence (in the Galois body with two elements GF (2)): S (t) = S (t-3) + S (t-31) , where '+' denotes the modulo 2 addition. It is generated for example by a Linear Feedback Shift Register (LFSR) of characteristic polynomial P (X) = 1 + X ²⁸ + X ³¹ .

Let E (t) = (S (t), S (t + 1), ..., S (t + 30)), t> 0, the current state (31-bit vector) of the LFSR register that delivers S (t). The period of the non-zero sequence (S (t), t> O) is also the period of the sequence of the states of the register (E (t), t> O) and is equal to T = 2 ³¹ -1, because the polynomial P is primitive. In transmission, this register is implemented in the encryptor and advances one step to each frame. Each 7-bit current frame, denoted frame (t), includes the current bit S (t) of the Fibonacci sequence. The Fibonacci suite thus incorporated provides frame sync and sync digit as described below.

The current state E (t) of the LFSR shift register can be used as the initialization vector for encrypting the data bits of the current frame. The 6 bits of useful data in the current frame are encrypted, for example, by a bitwise xor with 6 pseudo-random bits computed with a cryptographic algorithm from a traffic key K and the state of the current LFSR. And). The Fibonacci suite is emitted in clear according to the format of the Stanag 5030/5065, for example. In reception, a test of the three-term recurrence relation (S (t) = S (t-3) + S (t-31)) on a sufficiently large window allows its detection in the cryptogram stream received and then fixed. cut into 7-bit word unambiguously, this process is known as the "frame sync". Each frame is then deciphered with the state of the register correctly found and maintained in the decryptor (a transition from the LFSR to each new frame).

The existence of a continuously incorporated linear sequence in the transmission of a synchronous and simplex data stream enables the demodulation and decryption equipment to synchronize in a robust and reliable manner. However, this technique freezes the format of the transmitted data and restricts this format to a few bits, typically a character. The invention makes it possible, while retaining this mechanism, its advantages and the advantages already existing infrastructures, to efficiently transmit other types of data and thus to widen the scope of applications. This processing is implemented at the level of the encrypting equipment in transmission and at the level of the deciphering equipment in reception. If the above description only mentions the sequence of Fibonacci S (t) = S (t-3) + S (t-31), the present invention applies in any equipment implementing a continuous synchronization method based on a sequence checking a linear recurrence.

In the rest of the description, the useful data are encoded in the format 1 or 2. The encoding operation generates data comprising the useful data and / or data related to an error correction code and / or any other information. technique conventionally used in coding / decoding methods (see FIG. 2B). a) In the format 1, the data are grouped into blocks of fixed size including kr symbols. b) In the format 2, the data are grouped in blocks of fixed size called super-frames, themselves constituted of frames:

a frame is a window of the stream including r data symbols and 1 symbol dedicated to synchronization, the symbol dedicated to synchronization being distributed all the r data symbols. This dedicated symbol is a common term of a sequence S (t) satisfying a linear recurrence,

a super-frame is a window of the stream consisting of k consecutive frames, which therefore includes k data symbols and k synchronization symbols.

The invention relates to a method for transmitting data having a first format or format 1 in a data stream respecting a second format, the second format or format 2 consisting of a stream of data symbols regularly incorporating a symbol dedicated to the synchronization and arranged all the r data symbols, the dedicated symbol the synchronization being the current term of a sequence S (t) satisfying a linear recurrence, the data being cut in the format 1 into blocks of data of fixed size including kr symbols, the data symbols being considered as elements of a finite field GF (q) where q is the number of elements of the body, the sequence S (t) satisfies a linear recurrence on GF (q) and admits for characteristic polynomial a primitive polynomial P of degree n on GF (q) , and is periodic with a period T = q ⁿ -1, α being a root of P in the body GF (q ⁿ ), characterized in that it comprises at least the following steps: - in transmission the sequence S (t ) is generated by a linear automaton whose current state E (t) is α ^f written in a particular base of GF (q ⁿ ), during the transmission phase of the data stream,

the input data are formatted in several data blocks according to the format 1, the k data symbols constituting a block in format 1 are positioned in the given part of k consecutive frames in format 2, which corresponds to a super- frame, where a frame corresponds to a window of the format stream 2 including r data symbols and 1 symbol dedicated to synchronization, and a superframe corresponds to a window of the stream in format 2 consisting of k consecutive frames, which includes k data symbols and k synchronization symbols,

the different data blocks in format 1 are successively placed in superframes,

the rank t of the first frame (t) in a super frame including a block is chosen so that the modulo value k of t is equal to a fixed value "a"; at the reception phase of a super frame:

the synchronization at the format level 1 is determined on reception by completing the conventional process of the format 2 synchronous frame making it possible to recognize the synchronization sequence S (t) transmitted in the data stream, by performing the following steps: reconstituting E (t) the current state of the automaton generating the sequence from the symbols S (t),

- E (t ₀ ) the recognized state and associated with the first frame of the stream processed in reception, determine from the value y = E (t ₀ ) the single integer t belonging to the interval [O, T -1] which satisfies the relation α ^f = y, using a discrete logarithm calculation on GF (q ⁿ ),

once the rank t _{0 is} recognized, deduce the rank of all the other following frames and the position of the super-frames including the blocks of the format 1, the first frame of a super frame including a data block has for rank a integer whose remainder modulo k is equal to the arbitrary value "a" chosen.

The method can comprise at least the following steps: to transmit a stream of data blocks in format 1, at the level of the initialization step, the elements of the counter CPT and the linear automaton LFSR are initialized in the following way: before the transmission of the first frame, the current counter CPT is in any state X, that imposed after the powering up of the equipment or obtained after the transmission of a previous traffic, the transmitter then calculates d = X modulo k and realizes elementary transitions of elements LFSR and CPT where u is equal to:

- 0 if d = a modulo k;

- 1 if d = a + k-1 modulo k;

- 2 if d = a + k-2 modulo k;

- k-1 if d = a + 1 modulo k.

According to one embodiment, the residues are treated in the following manner: in the case where the period T = (q ⁿ -1) is not multiple of k and is of the form Qk + h, 0 <h <k, a complete cycle of the T powers of α absorbs Qk frames thus Q super frames and there remains h frames at the end of this cycle which are included in a special super frame called residue including only h frames instead of k, associated with the last h powers of α in the cycle: α ^{τ "h} , α ^{τ" (h "1)} , ..., α ^{τ" 1} .

The relict super-frame incorporates, for example, stuffing and / or data in a particular coding, in transmission and reception, it is identified as the super-frame positioned on the rank frame (T-h). The method can integrate in the super frame:

redundancy bits linked to an error correction code,

- Explicit stuffing information as flags indicating whether the data transported in the super frame are useful data or correspond to stuffing.

Other characteristics and advantages of the present invention will appear on reading the following description of an example of embodiment given by way of illustration and in no way limiting, appended figures which represent: • Figure 1 A, an example of architecture a VLF / LF broadcasting system,

• Figure 1B, the operation of an encryptor and a decryptor in the process,

FIG. 2A, the organization of a bit frame using a synchronization bit,

• Figure 2B, the frame, super-frame and data block cut-out in format 1 and format 2,

FIG. 3, a representation of a linearly looped shift register, FIG. 4, a linear code control matrix used in an example of the coding principle, and

FIG. 5, an example of implementation of the shift register according to the method of the invention. In order to better understand the method according to the invention, the following example is given for a system in which the data format must check the one defined in the stanag 5030/5065. The data terminal interfacing with the encryptor delivers a data stream in a format that is not necessarily directly adapted to the raster cut of r = 6 bits of the Stanag 5030/5065.

The example of the method given without limitation relates to a way of transporting an n-ary data stream in the frames of the Stanag 5030/5065 in an optimized manner in the following sense: the bits transmitted in the data portion of the frames of the Stanag

5030/5065 are all used to encode useful symbols or stuffing symbols and possibly redundancy bits associated with an error correction code,

no bit of data in the frames is explicitly used to recognize the cut into n-ary symbols or packets of n-ary symbols in reception (symbol synchro), in normal operation of the method according to the invention.

For the implementation of the method according to the invention, the transmitter of the system comprises for example an LFSR type automaton for generating the sequence S (t) satisfying the linear recurrence, having a current state E (t) and a counter modulo T, CPT (t). The receiver is equipped in the same way with an LFSR type PLC and a CPT (t) modulo T counter. When the data stream is sent, the input data is formatted as a data block according to the format 1. According to an implementation example, at the level of the system encoder, the method performs a step of encoding the user data in format 1. It is considered according to FIG. 2B, that the useful data consist of L packets of N bits. These L packets are positioned in the data portion of k consecutive frames of the format 2, which corresponds to a super-frame. The format 2 frames correspond, for example, to the STANAG 5030/5065 format. The rank t of the first frame (t) in a super-frame including a block of format 1 is chosen so that the value of t is equal to 0 modulo k and more generally equal to an arbitrary value modulo k. In addition to the useful L-packets (LN bits), a super-frame may include: - L bits to indicate for each packet whether it actually corresponds to useful data or whether it corresponds to a stuffing,

- possibly redundancy bits to add a correction service and / or FEC (Forward Error Correction) which takes into account the constraints of TEB (Bit Error Rate abbreviation) of the radio channel.

By way of example, the description specifies two possible modes of implementation for cutting useful data:

a cutoff favoring the bit rate where the super-frame consists of k = 3 frames carrying L = 2 useful octets (N = 8) without any redundancy;

a cutout where the superframe consists of k = 4 frames carrying L = 2 bytes (N = 8) and incorporating an error correction code.

The coding of the useful data in the super-frames is all the more effective when no additional bit is allocated to ensure the super-frame sync (detection of the new super-frame cut). Like the synchro-frame, the super-frame sync will be deduced from the Fibonacci sequence imposed by the Stanag 5030/5065 as it is explained below. Example 1: Mode without redundancy on byte stream In this mode, the superframe consists of 3 frames. It is used to transmit a block of L = 2 bytes (N = 8) useful (Oi, O ₂ ) in 3 elementary frames of STANAG 5030/5065 (k = 3, r = 6). This super-frame thus includes k ^* r = 3 ^* 6 = 18 bits of useful data and k = 3 bits for the sync (Fibonacci sequence). The 18 bits of data are precisely: -8 bits (ao, ai, a2, a _3, a _4, a5, _6, a7) of the useful byte Oi, -8 bits (b _o, bi, b2, b3 , b ₄ , b5, b6, b ₇ ) of the useful octet O ₂ , -U: 1 bit indicating whether Oi is a useful byte or stuffing, -f ₂ : 1 bit indicating whether O ₂ is a useful byte or stuffing. Example 2: Mode with redundancy on byte stream

In this mode, the super frame consists of 4 frames. This super-frame is used to transmit a block of L = 2 bytes (N = 8) useful ((Oi, O ₂ ) in k = 4 elementary frames of the STANAG 5030/5065 (k = 4, r = 6). -frame thus includes k ^* r = 4 ^* 6 = 24 bits of useful data and k = 4 bits for the sync (Fibonacci sequence) The 24 bits of data are precisely: -8 bits (ao, ai, a ₂ , a3, a ₄ , a ₅ , a ₆ , a7) of the useful byte Oi, -8 bits (bo, bi, b ₂ , b ₃ , b ₄ , b ₅ , b ₆ , b ₇ ) of the useful octet O ₂ ,

-f-i: 1 bit indicating if Oi is a useful byte or stuffing,

-f ₂ : 1 bit indicating whether O ₂ is a useful byte or stuffing,

6 redundancy bits (r ₀ , r ₁ , r ₂ , r ₃ , r ₄ , r ₅ ) derived from a Hamming code with parity bit. The control matrix defining the code is given in FIG. 5. This parity Hamming code used to code the 24-bit word systematically corrects an error and detects two errors. For example: ro = ao + a-ι + a ₃ + a ₄ + aβ + bo + b ₂ + b ₃ + bs + b ₇ + f ₂ r-ι = ao + a ₂ + a ₃ + as + a6 + bi + b ₂ + bi + bs + fi + f ₂ r ₂ = a ₄ + a ₅ + a ₆ + a ₇ + b ₀ + bi + b ₂ r ₃ = a-ι + a ₂ + a ₃ + a ₇ + bo + b-ι + b ₂ + b ₆ + b ₇ + f-ι + f ₂ r ₄ = b ₃ + b ₄ + b ₅ + b ₆ + b ₇ + fi + f ₂ r ₅ = ri + r ₂ + r ₃ + r ₄ + ao + ai + a ₂ + a ₃ + a ₄ + as + a ₆ + a ₇ + bo + bi + b ₂ + b ₃ + b ₄ + bs + b ₆ + b ₇ + f ₁ + f ₂ = ao + ai + a ₂ + a ₄ + a5 + a ₇ + bi + b ₂ + b ₃ + b ₄ + bε + f ₂ (the sign + corresponding to the operation xor binary). Super-frame sync

In the preceding cuts, no bit is allocated to ensure the synchronization at a super-frame, that is to say which allows the unambiguous determination of the super-frame cut on the stream processed in reception. Super-frame sync is determined using the sequence synchronization already used for elementary frames, for example the Fibonacci sequence in the case of the STANAG 5030/5065.

The Fibonacci sequence is generated by a linear automaton of characteristic polynomial P = 1 + X ²⁸ + X ³¹ . Let a be a root of P in the finite field GF (2 ³¹ ).

An equivalent implementation of the LFSR is considered to

(S (t)) in the form of a divisor register which explicitly realizes the multiplication by α in the polynomial base (1, α, α ² , .., α ³⁰ ). This equivalent implementation of the LFSR shows that the current state E (t) = (s (t), s (t + 1), ..., s (t + 30)) is a vector of 31 bits which, to a permutation near the components, can be interpreted as the element α ^f of the finite field GF (2 ³¹ ) decomposed in the polynomial base (1, α, α ² , ..., α ³⁰ ).

More precisely, the current state of the register E (t) = (s (t), ..., s (t + 31)) of the LFSR is associated with the element: α ^f = s (t + 27) α ° + s (t + 26) α ¹ + s (t + 25) α ² + ... + s (t) α ²⁷ + s (t + 30) α ²⁸ + s (t + 29) α ²⁹ + s (t + 28) α ³⁰ . EQ1 - This example is shown in Figure 6.

The starting point α ° = 1 is associated with the state of the LFSR E (t ₀ ) = (, s (t _o ), s (t _o +1),

..., s (t _o + 30)) where the 31 components s (t ₀ + i) are zero except s (t ₀ + 27) which is 1. A superframe has k frames (in the examples, k = 4 in redundant mode, and k = 3 in non-redundant mode).

In transmission, the encoder imposes that the first frame in a super frame is associated with a state of the register E (t) in correspondence with an element a ^x for which the value of t is equal to a fixed value "a" modulo k. For the rest of the explanation, the arbitrary hypothesis chosen is a = 0, which amounts to taking t multiple of k (t = ku).

For this example (a = 0), the k register states associated with the k frames of a super-frame will therefore be of the form:

E (t) = α ^ku , E (t + 1) = α ^{ku + 1} , E (t + 2) = α ^{ku + 2} , ..., E (t + k-1) = α ^{ku + k " 1} . At the very beginning of the processing of the data stream, the coder selects a current state E (t ^" ) of departure to generate the Fibonacci sequence, associated with the element α ^f with t ^* multiple of k.

In transmission this can be done, for example, by explicitly managing in addition to the LFSR register which generates the Fibonacci sequence S (t) whose current state is E (t), a 31-bit counter which codes t such that E (t) is associated with a \ During the key change, the LFSR register managed by the encryptor is initialized to its initial value E (t ₀ ) previously explained (Eq1), and the counter t is initialized to its value t _o = O. Then, at each new processed frame, the current LFSR register advances by one step and the counter t increments by 1 modulo T to keep at all times the knowledge of 2 data:

the current state E (t) of the LFSR register associated with a

- and its logarithm t in [0, T-1].

In reception, once the frame synchro recovered, the state of the current LFSR register Y = (s (t), ..., s (t + 31)) associated with the first processed "frame (t)" frame is available . The method then considers the element of the finite field GF (2 ³¹ ): y = s (t + 27) α ° + s (t + 26) α ¹ + s (t + 25) α ² + ... + s (t) α ²⁷ + s (t + 30) α ²⁸ + s (t + 29) α ²⁹ + s (t + 28) α ³⁰ .

A discrete logarithmic calculation on the finite field GF (2 ³¹ ) then makes it possible to find the unique value of t in [0.2 ³¹ -2] such that a ^x = y. More generally, the computation can be carried out in a finite field GF (q ⁿ ) with q ^Λ n elements. 1) By observing the remainder modulo k of t, the receiver deduces then the position of the first super-frame which it can treat: For all v, with 0 <= v <k, if t is of the ku form v, the first superframe is [frame (t + v), frame (t + v + 1), ..., frame (t ++ v + k-1)].

Once it has recognized the rank t ₀ , the decoder belonging to the decryptor can deduce the rank of all the other frames that follow and therefore the position of the super frames including the blocks of the format 1. The first frame of a super frame including a block of data has for rank a multiple of k. Of course, once the superframe sync determined initially, it is easily maintained in current mode by processing the frame flow per packet of k consecutive frames.

In order to adjust the positioning of the blocks of the format 1 in the frames of the format 2, or the place where a block of format 1 must be, the transmitter and the receiver of the system according to the invention manage two automata: an automaton of the type LFSR which generates the sequence or sequence S (t) of synchronization and whose current state E (t) is in correspondence with the element α ^f of GF (q ⁿ ), - A counter modulo T, whose current state CPT (t) is the value of rank t.

With each new frame received (in format 2) or transmitted, a transition of

LFSR is executed, this transition calculating the new state E (t + 1) from its old value E (t).

Similarly, the counter CPT increments from 1 to each new frame: CPT (t + 1) = CPT (t) +1 modulo T.

To transmit a stream of data blocks in format 1, in format 2 frames, the transmitter initializes the LFSR and CPT PLCs, for example, as follows:

Before the transmission of the first frame in format 2, the current counter CPT is in any state X, in general, that which is imposed by the power on of the equipment or obtained after the transmission of a previous traffic.

The transmitter then calculates d = X modulo k and realizes a number u of elementary transitions of the elements LFSR and CPT where u is equal to: 0, if d = 0 modulo k,

1, if d = k-1 modulo k,

2, if d = k-2 modulo k,

k-1, if d = 1 modulo k. Treatment of backorders

The period T = (2 ³¹ -1) is not divisible by k and is of the form kQ + r, 0 <r <k.

A complete cycle of the T powers of α absorbs kQ frames. There are still frames at the end of this cycle that we include in a special super frame called balance. The remainder comprises only the r frames instead of k, associated with the last r powers of α in the cycle: α ^{τ "r} , α ^{τ" (M)} , ..., α ^{τ "1} .

As an illustrative example, the description specifies two ways of coding the remainder associated with the examples described above. The "remainder" includes only r frames instead of k, associated with the last r powers.

In practice, the processing of the remainders is not done because the cycle is not traveled during the use of the same key.

Reliquat in redundancy mode

In redundant mode, the super frame is composed of k = 4 frames. T = 40 + r, with r = 3.

The remainder is therefore of size 3 instead of 4. This exceptional case is treated by considering the following coding for this remainder:

- 8-bit (a _o, ai, a2, a3, _4, a5, a6, ₇₎ useful Oi byte,

the 8 bits (bo, bi, b ₂ , b ₃ , b ₄ , b ₅ , b ₆ , b ₇ ) of the useful octet O ₂ are set to 0 and not transmitted,

- fi: 1 bit indicating if Oi is a useful byte or stuffing,

- f ₂ : 1 bit set to a random value (stuffing) and transmitted,

- 6 redundancy bits (r _o , r ₁ , r ₂ , r ₃ , r ₄ , r ₅ ) resulting from the same hamming code as for a standard super-frame calculated from the useful data (a _o , ai, a ₂ , a 3, a _4, a5, a6, _7, bo, bi, b _2, b _3, b _4, b 5, b 6, b _7, fi, f ₂₎ taking into account the

In reception, the processing of this exceptional case is reduced to the current processing by filling the bits received with the 8 bits bi to 0. The decoding of hamming is done in a similar way as in the standard case unless an error is detected on a bit bj. In this case, the decoding does not correct the bit and declares the super frame erroneous. Reliquat in non-redundant mode

In non-redundant mode, the super frame is composed of k = 3 frames. T = 3Q + r, with r = 1. The remainder is therefore of size 1 instead of 3. In this case, the 6 data bits of this reliquat frame (which is associated with the state in correspondence with α ^{τ "1} ), are directly filled with stuffing.

The process according to the invention notably offers the following advantages:

1) The robustness of the synchronization is preserved compared to the system on which it is based:

• if frame sync is provided, super frame sync is also,

• it is lost if the sequence of Fibonacci S (t) is not correctly recognized as it is already the case for the frame sync, • in the event of loss of frame sync, its recovery via the continuation of

Fibonacci automatically causes the recovery of the cut in super frame.

2) No additional synchronization information is needed, the super-frame cut is obtained directly using the synchronization sequence already present in the frames.

3) The calculation of the discrete logarithm in a small body GF (2 ³¹ ) is simple and achievable in the decipherer.

4) The format of the superframes adapts to the data to be transmitted and allows the addition of explicit stuffing information and error correcting codes.

Claims

1 - Method for transmitting data having a first format or format 1 in a data stream respecting a second format, the second format or format 2 consisting of a stream of data symbols regularly incorporating a symbol dedicated to synchronization and arranged all the r data symbols, the symbol dedicated to the synchronization being the current term of a sequence S (t) satisfying a linear recurrence, the data being cut in the format 1 into blocks of data of fixed size including kr symbols, the data symbols being considered as elements of a finite field GF (q) where q is the number of elements of the body, the sequence S (t) satisfies a linear recurrence on GF (q) and admits for a characteristic polynomial a polynomial primitive P of degree n on GF (q), and is periodic with a period T = q ⁿ -1, α being a root of P in the body GF (q ⁿ ), characterized in that it comprises at least the steps ff antes:

in transmission the sequence S (t) is generated by a linear automaton whose current state E (t) is α ^f written in a particular base of GF (q ⁿ ), during the transmission phase of the data stream - the input data is formatted into several data blocks in format 1,

the kr data symbols constituting a block in format 1 are positioned in the given part of k consecutive frames in format 2, which corresponds to a super-frame, where a frame corresponds to a window of the format stream 2 including r symbols of data and 1 symbol dedicated to the synchronization, and a superframe corresponds to a window of the stream in format 2 constituted by k consecutive frames, which includes kr data symbols and k synchronization symbols,

the different data blocks in format 1 are successively placed in superframes, the rank t of the first frame (t) in a super frame including a block is chosen so that the modulo value k of t is equal to a fixed value "a"; at the reception phase of a super frame: the synchronization at the format level 1 is determined on reception by completing the conventional process of the format 2 synchro-frame for recognizing the synchronization sequence S (t) transmitted in the data flow, by performing the following steps:

reconstituting E (t) the current state of the automaton generating the sequence from the symbols S (t),

- E (t ₀ ) the recognized state and associated with the first frame of the stream processed in reception, determine from the value y = E (t ₀ ) the single integer t belonging to the interval [0, T -1] which verifies the relation α ^f = y, using a discrete logarithm calculation on GF (q ⁿ ), - once the rank t _{0 is} recognized, deduce the rank of all the other following frames and the position of the super with the blocks of the format 1, the first frame of a super frame including a block of data has for rank an integer whose remainder modulo k is equal to the arbitrary value "a" chosen,

2 - Process according to claim 1, characterized in that to adjust the positioning of format 1 blocks in the format 2 frames, the method manages two automata:

an LFSR type automaton generating the synchronization sequence S (t) and whose current state E (t) is in correspondence with the element of GF (q ⁿ ) α ^t ,

a modulo counter T whose current state CPT (t) is the value of the rank t and in that,

at each new frame received or to be transmitted, a transition of the LFSR is performed and calculates the new state E (t + 1) from its old value E (t), the counter CPT increments from 1 to each new CPT frame (t + 1) = CPT (t) +1 modulo T.

3 - Process according to claim 2, characterized in that it comprises at least the following steps: to transmit a stream of data blocks in format 1, at the initialization step, the elements of the counter CPT and the Linear PLC LFSR are initialized in the following manner; before the transmission of the first frame, the current counter CPT is in any state X, that imposed after the powering up of the equipment or obtained after the transmission of a previous traffic; the transmitter then calculates d = X modulo k and realizes elementary transitions of the elements LFSR and CPT where u is equal to:

- 0 if d = a modulo k; - 1 if d = a + k-1 modulo k;

- 2 if d = a + k-2 modulo k;

- k-1 if d = a + 1 modulo k

4 - Method according to one of claims 1 or 2, characterized in that the residues are treated in the following manner: in the case where the period T = (q ⁿ -1) is not multiple of k and is of the form Qk + h, 0 <h <k, a complete cycle of the T powers of α absorbs Qk frames thus Q superframes and there remains h frames at the end of this cycle which are included in a special super frame called a remainder comprising only h frames instead of k, associated with the last h powers of α in the cycle: α ^τ - ^h , α ^, .... α ^{τ "1} .

5 - Process according to claim 4, characterized in that the residual super frame incorporates stuffing and / or data according to a coding In particular, in transmission and reception, it is identified as the super-frame positioned on the rank frame (Th).

6 - Method according to one of claims 1 or 2, characterized in that it integrates in the super frame:

redundancy bits linked to an error correction code,

7 - Use of the method according to one of claims 1 to 6 for formatting data format stanag 5065 or format stanag 5030.