Patent Application
20150189334
This application claims priority to foreign French patent application No. FR 1303116, filed on Dec. 31, 2013, the disclosure of which is incorporated by reference in its entirety.
The subject of the invention relates to a method for securing DVB-S2 transmissions (ETSI EN 302 307 standard, available on the ETSI website), notably by defining a TRANSEC mode enciphering all the DVB-S2 symbols, including the “PLHeader” header.
The DVB-S2 standard was developed for very high speed civil applications, for example for television broadcasting. Once configured, a DVB-S2 carrier operates with a fixed symbol tempo, i.e. a fixed passband. The only possible adaptation is to change frame by frame the modulation and coding or MODCOD pair, which makes it possible to resist changes in propagation conditions with a dynamic swing of above 15 dB. This ACM (Adaptive Coding & Modulation) is intrinsic to the standard, but requires a return channel to inform the DVB-S2 sender of the propagation conditions seen by the receiver. MODCODs may differ from one frame to the next and so the spectral efficiency may too. However, in DVB-S2, a frame always transports 64800 bits, and it does so independently of the MODCOD pair used. Table I below indicates for “normal” frames (FECFRAME of 64800 bits), the number of symbols following modulation and the presence or absence of a pilot symbol:
The result is that a DVB-S2 frame does not have a fixed number of symbols and therefore a fixed duration. This absence of temporal synchronicity prevents the implementation of efficient techniques for securing transmission (TRANSEC), signalling (NETSEC), or communication (COMSEC). Indeed, a frame S2 cannot be enciphered as a function of an implicit marker such as its number or the time, because of its aperiodic structure. The receiver cannot know:
The prior art known to the Applicant does not describe how to encipher a DVB-S2 frame on a synchronous mode. Various systems exist for enciphering a part of the frame only. The header of the physical frame is then passed in clear form and sometimes with the time marker. There is also a TRANSEC option for DVB-S2, but this TRANSEC is actually only a COMSEC because it does not protect from interception, or from scrambling.
Most of the systems described in the prior art have a low level of security, with analysis of the traffic being possible by reading the headers transmitted in clear form, and easier scrambling due to the temporal position of the header being known. Indeed, it is enough to scramble this part for the rest of the signal to be unusable.
There is therefore a need for a method making it possible to secure the entire DVB-S2 frame, headers (PLHEADER and BBHEADER) and data included, in order to avoid the problems of poor security encountered in systems of the prior art.
In the remainder of the description, the following definitions will be used:
One of the aims of the present invention is to propose a method making it possible to construct a DVB-S2 super-frame of fixed duration in order to be able to define a TRANSEC mode adapted for enciphering all the DVB-S2 symbols, including the header or PLheader. The definition of a new temporal structure must notably meet the following requirement: the structure of the DVB-S2 frames must not be modified, so as to have a minimal effect on the standard, and consequently on the existing technological DVB-S2 building blocks.
If one wishes to define a period structure for a DVB-S2 carrier and implement the TRANSEC over all the symbols of the frame in the physical layer PLFrame (Physical Layer Frame), it is necessary to define a structure allowing all types of DVB-S2 frame.
The invention relates to a method for enciphering a DVB-S2 frame or super-frame including at least a first type of frame modulated with a first modulation M1 and a second type of frame modulated with a second modulation M2, characterized in that it comprises at least the following steps:
In a variant embodiment a super-frame ST comprises a number a of frames modulated with a first modulation M1, b frames modulated with a second modulation M2, and c frames modulated with a kth modulation Mk, and x dummy frames “n” of length n are introduced for the frames of modulation M1, y dummy frames “p” of length p are introduced for the frames of modulation M2, and z dummy frames “q” of length q are introduced for the frames of modulation Mk, in order to obtain a given length of super-frame T whatever the modulations used for several configurations.
A super-frame is, for example, composed of a QPSK frames and b 8PSK frames, and a number x of dummy frames “n” of length n and a number y of dummy frames “n−1” of length n+1 are introduced in order for the length of the super-frame ST to correspond to a given length or length of time T.
In the case where the super-frame is composed of 10 QPSK frames with pilots and 15 8PSK frames with pilots, a dummy frame “1” of a length of 1 slot is introduced for the QPSK frame.
In a variant, it is possible to distribute the dummy frames “n” regularly in the super-frame.
In another variant, a single dummy frame “n” is introduced at the end of the super-frame, according to the principle mentioned previously “n”+“p”=“n+p”.
In a variant embodiment, the type of dummy frame “n” to be introduced is determined according to the frame type, frame with pilot or pilotless frame.
The PLS signalling comprising the modulation, encoding and type features of a frame can be modified to indicate the size “n” of a dummy frame “n” and a type 00 (dummy frame “1”) corresponding to one slot to align the pilotless frames, a type 01 (dummy frame “2”) having two slots, a type 10 (dummy frame “9”) of 9 slots and a type 11 (dummy frame “10”) of 10 slots to align the types of frames with pilots are defined.
For example, a dummy frame “n” is introduced at the start of the super-frame ST in order to facilitate synchronization.
According to one embodiment of the method, super-frames ST are constructed having a duration of a few hundred ms, 250 ms.
Other features and advantages of the device according to the invention will become more apparent on reading the following description of exemplary embodiments, given for the purposes of illustration and without being in any way limiting, appended with figures representing:
To be able to define a DVB-S2 TRANSEC, the method determines a temporal structure called the super-frame ST which exhibits a fixed length of time T for several configurations and for a band B of the system. The defined super-frame will be compliant with the structure of a DVB-S2 frame. Its duration TST is for example in the order of a few milliseconds, less than 500 ms, for example, in order to allow rapid synchronization, and its duration will also be chosen as a function of the ACM modulation mechanisms. The ACM mechanisms must indeed be able to be applied at a faster tempo than the super-frame. The super-frame is composed of several DVB-S2 frames for a given configuration, and a DVB-S2 frame is modulated with a given modulation, for example QPSK or 8 PSK.
“Normal” DVB-S2 frames are of different lengths depending on the type of modulation:
In the DVB-S2 standard, the dummy frames (dummy frame “37” of the present invention) have no real use unless it is to send stuffing. The method will use dummy frames existing in the standard, while modifying their length, and the new length will be indicated in the PLHeader header. The receiver terminal will not need to know in advance the size n of the dummy frames “n” because this item of information, in this case “n”, will be indicated in the PLHeader.
The method will proceed, for example, in the following manner. Let T be the length of time that an ST super-frame must observe, a super-frame being defined for a band B of the system. Let a be the number of frames modulated with QPSK modulation and b the number of frames modulated with 8 PSK modulation, for example. The method will introduce regularly, for example, several dummy frames “1” in order for the super-frame ST to always preserve the same length for all the desired configurations (modulation configurations). It is also possible to insert the dummy frame “1” at the end of the super-frame.
Table II below gives an example of configurations for multi-ACM QPSK/8PSK super-frames. According to the implementation of the method, frames having different modulations are combined within one and the same super-frame ST, in order to have an optimal multi-ACM.
For each configuration, it is possible to compose the structure of the STs by uniformly distributing the frames of same modulation in order to limit jitter as illustrated in
The method relies on the possibility of transmitting as many dummy slots as the ST structure requires. An extensive use of the PLS signalling included in the PLHeader of the DVB-S2 frame would make it possible to indicate to the receiver the size n of the current dummy frame “n”.
In the prior art, a DVB-S2 receiver can decode any DVB-S2 carrier, without a priori knowledge of the modulation and the encoding of the carrier, since each frame indicates its features (MODCOD, TYPE) via the PLS. The method according to the invention will preserve this property.
Table III below gives an example for modifying the PLS field according to the steps of the method.
In this example four types of dummy frames “n” are defined in the field 29 of MODCOD. Types 00 and 01 will be used to align pilotless frames. In this example, TYPE 00 corresponds to one slot (PLHEADER alone), TYPE 01 to 2 slots including the slot of the header in the case of pilotless frames. The types 10 and 11 are reserved for pilotless frames. TYPE 10 corresponds to adding 9 slots and TYPE 11 to adding 10 slots to align pilotless frames.
Table IV below gives an example of the type of “dummy” frame that is required to complete super-frames composed of frames of the same modulation in a system that only requires pilotless frames, typically a fixed system with equipment with low phase noise. The example is given for a super-frame the modulation of which is at best 16 APSK modulation.
For 16 APSK modulation, no dummy frame is introduced into the super-frame, for 8PSK modulation, dummy frames “1” are introduced, and for QPSK modulation, dummy frames “2” are introduced. In general, it is possible to write for a predetermined value n, that dummy frames of length n, n−1, n+2 with n=0 for 16 APSK, 8PSK, and QPSK modulation respectively are introduced.
With ACM systems, “pilot” symbols can be inserted into the physical layer frame structure to facilitate the synchronization and also for channel estimation purposes. At the demodulator, with the specified phase noise, the phase recuperation appears very difficult without a pilot for 8 PSK and higher-order modulations. Moreover, in the ACM system, a receiver is generally capable of decoding a part of the total stream only, and more precisely only the sent frames whose MODCO are compatibles with the conditions of the user channel. In this context, the pilot symbols also allow the recovery of carriers without knowledge of frame data, even in cases where certain PLHeaders are not correctly decoded, because the pilots are regularly spaced.
The principle disclosed above is applicable to frames with a pilot. However, the number of pilot symbols per frame is not multiple of 90:
To have a multiple of 90 symbols per super-frame, a minimum of 5 frames with pilot is required (5*36=2*90= . . . ).
By applying the rules given for a system which only requires frames with pilots, the size n of the dummy frame “n” that is necessary to complete the super-frames is deduced therefrom.
The following Table V gives an example of a type of “dummy” frame that will be used to complete the super-frame.
An ST with a pilot therefore requires more symbols than a pilotless ST. In order not to affect the ACM functionality inherent to the DVB-S2, it is preferable to have an ST in the order of a few 100 ms (typically 250 ms).
For an ST with a pilot of 250 ms this enforces a minimum bitrate of 1.33 Mbauds.
For mobile applications, 16 APSK modulations will not be used. The structure of the ST can therefore be optimized by reducing the size of the “dummy” frames by additions as indicated in Table VI below:
The introduction of a dummy frame “n” of variable size n makes it possible to structure a DVB-S2 carrier in a super-frame of fixed duration and thus to implement a TRANSEC with an implicit marker based on the time or the number of the ST for example.
The method according to the invention notably increases the security of the transmissions, and resistance to interception. It allows temporal alignment of a super-frame ST2 and makes it possible to add time markers to the DVB-S2 structure and consequently to implicitly define an initial enciphering vector. The “n” modified dummy frames have no effect on the DVB-S2 standard. The invention makes it possible to encipher the whole DVB-S2 frame.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1303116 | Dec 2013 | FR | national |
