WO2005018184A1

WO2005018184A1 - Modulation and encryption of a digital date stream, method of decoding, multi-carrier transmission system

Info

Publication number: WO2005018184A1
Application number: PCT/IB2004/051364
Authority: WO
Inventors: Francesc Dalmases; Joachim Kahlert
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2003-08-13
Filing date: 2004-08-03
Publication date: 2005-02-24
Also published as: TW200511788A; WO2005018184A8

Abstract

Method of modulating a digital data stream in MC technology, in which the constellation points (Xo, X1 ... XN-I) change their sequence in accordance with a mapping pattern (Ψi) during modulation. This dynamic mapping represents an encryption, since it is not readily possible for an unauthorized subscriber to restore the original sequence. The encryption level is further increased by the use of a set of multiple mapping patterns (Ψi), which is implemented during transmission in the form of a loop. The encryption level is further increased by defining a hop interval (Ihop), indicating the validity of an individual mapping pattern, after the handshake. The encryption key defines a set of mapping patterns (Ψi), a permutation function (Si), which indicates the order of use of the individual elements of the set of mapping patterns, and a hop interval (Ihop).

Description

MODULATION AND ENCRYPTION OF A DIGITAL DATE STREAM, METHOD OF DECODING, MULTI- CARRIER TRANSMISSION SYSTEM

The invention relates to a method of modulating a digital data stream and to an appliance for implementing a method of this kind. It relates, in particular, to a multi-carrier transmission system for a network, in which the digital data stream is scrambled during the course of the modulation. In multi-carrier (MC) technology, a single data stream is 5 transmitted at a low data rate via a number of carriers. Multi -carrier systems are used in both wireless and wired standards to increase robustness against frequency-related fading. The OFDM (Orthogonal Frequency Division Multiplex) multiple access method is a particular variant of multi-carrier transmission, which uses orthogonal carriers to enable spectral overlap one with another. 10 Multi-carrier modulation converts the N constellation points of an individual carrier-modulated signal into N coefficients in the frequency domain (carrier values). Examples of carrier-modulated methods are Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). The association between the constellation points of the single-carrier modulation and the carrier coefficients 15 is normally defined. For example, in the case of a transmitter of a multi-carrier transmission system, the signal transmitted in the complex baseband is as follows: ^) = ∑ -_o ^X^ -nT) (1) wherein t is the current time, e.g. the system time, T is the cycle duration of 20 the signal to be transmitted, N is the number of modulators, X_n is the constellation point of the n-th modulator (QAM in this example) and g_n(t) is the waveform of the multi-carrier modulator used. The multi-carrier receiver comprises a filter array with complex-conjugate waveforms g _n(t). Since the waveforms are orthogonal, the modulated data can be recovered. This example shows that every constellation point X_n is modulated with the same waveform 25 g_n(t) and demodulated with its complex-conjugate waveform g _n(t). This means that the mapping between the constellation points and the carrier frequencies is established for the duration of the transmission. As is apparent from equation (1), the number of modulators N, and thereby the number of constellation points X also, is limited. Examples of the prior art are shown in the following drawings. Fig. 1 shows an MC transmitter with QAM modulation. Fig. 2 shows an MC receiver with QAM modulation. Fig. 3 shows a Table representing both the association between the carriers and the transmitting-end constellation points and the association between the complex- conjugate carriers and the receiving-end constellation points.

To illustrate the prior art, Fig. 1 shows an MC transmitter with QAM modulation. The digital data stream is scrambled to multiple constellation points X_o to X_N-_I. Precisely one coefficient go(t) to gN-ι(t) with a specific waveform is assigned to each constellation point, according to its subscript index. A summator generates an output signal s(t) from the data stream modulated in this manner. To illustrate the prior art, Fig. 2 again shows an MC transmitter with QAM modulation. The complex-conjugate, time-dependent waveform g*(t) is applied to the received, time-dependent signal r(t). The same coefficient, g(t) or g*(t), is used with a specific waveform at the transmitting end and the receiving end. The partial signals are then passed on to a filter array, which produces the corresponding output signals Yo, Yj ... Y_N-_I - The disadvantage of this transmission method is that a signal received by a third party can be demodulated offline, since the association, or mapping, between the constellation points and the carrier coefficients is fixed during the transmission. To illustrate the prior art, Fig. 3 shows a Table, illustrating both the association between the carriers go to g_N-i and the transmitting-end constellation points Xo to X_N-_I and the association between the complex-conjugate carriers g*o to g*_N-ι and the receiving-end constellation points Yo to Y_N-_I . It can be seen that the mapping and recovery take place in a specified sequence, i.e. the subscript index is static. Once it has been intercepted and stored, a data stream can be recovered using suitable aids and by trying out various waveforms for g*(t). The transmitted data stream, which is transmitted either wirelessly or by wire, has to be capable of being received and further processed only by the authorized receiver or subscriber. Encryption techniques are used to prevent an unauthorized receiver from receiving the data stream and then determining its content. The encryption hereby takes place in the data link layer (layer 2), in the network layer (layer 3) or the application layer (layer 7) of the OSI 7-layer model by using a key for the encryption that is known only to the transmitter and the authorized receiver. There are a limited number of keys for the encryption, which are stored in both the transmitter and the receiver. At the start of the data transmission, the key valid for this transmission is sent, e.g. in the handshake message. Since the encryption takes place in a layer above the physical layer (layer 1), it is possible to isolate an individual data connection on the physical medium with means suitable for the task. This isolated or copied data stream can then be logged and stored using suitable appliances. This stored data stream can then be decoded offline. It is thus possible for an unauthorized receiver to determine even the content of a data stream that has been transmitted in encrypted form. A known transmission system of this kind is therefore not protected from interception. French patent 2 808 361 claims a method of encrypting a digital signal that is intended to be transmitted, which is composed of consecutive binary elements and is achieved by mapping onto a set of points of a predetermined constellation having a whole number C of points, which are different from a power of 2. With this encryption method, the binary elements of the said digital signal are firstly regrouped into binary, successive code words of length L, wherein the length L is obtained from the equation: L = N + C - 2^N, wherein N is an integer, such that: 2^N < C < 2^N+1. Mapping of the N binary elements of the binary code word onto a point among 2^N points of the constellation is then undertaken for each binary code word, wherein this point among the 2^N points represents N binary elements in an absolute manner, and wherein the values of C - 2^N other binary elements of the binary code word are deductible from the former utilization or non-utilization of C - 2^N other points of the constellation, designated special points, wherein each of them represents in a differential manner a binary element among C - 2^N other binary elements of the binary code word. In accordance with one embodiment of the French patent 2 808 361, the C - 2^N binary elements of the digital signal are suppressed and replaced by markers, which are designed to indicate at least one event relative to the value of the C - 2^N binary elements of the digital signal, wherein the markers are encrypted by the special points. Once the occurrence relative to the value of the element or of multiple binary elements in the digital signal has been recognized by utilizing the corresponding special point, it is not necessary to utilize this special point again if this event is not repeated, which enables the number of constellation points utilized during a large number of consecutive code words of the digital signal to be reduced. Reducing the number of constellation points utilized for the same quantity of transmitted information in this manner enables the quality of the transmission to be still further improved. The encryption method from French patent 2 808 361 uses markers that are encrypted by the special points to suppress special binary elements of the digital signal. These encrypted markers intervene in the modulation to increase the yield of the transmitted information whilst retaining the quality of transmission. It is therefore an object of the invention to specify a method of modulating a digital data stream in a multi-carrier transmission system that increases the protection of the data stream from interception. It is further an object of the invention to specify a method of encrypting a digital data stream. One further object is to specify a method of decoding a digital data stream that has been transmitted in encrypted form. It is further an object of the invention to specify an appliance for implementing a method of this kind. It is also an object of the invention to specify a multi-carrier transmission system for a digital data stream. As regards the modulation method, the object is achieved in accordance with the invention by a method for modulating a digital data stream in a multi-carrier transmission system, wherein, in the course of the modulation: - the digital data stream is divided into N partial data streams and scrambled onto N constellation points, - mapping of the N constellation points onto at least N carrier frequencies is undertaken, - through application of a mapping function, encryption is undertaken in that the position of the N constellation points, and thus of the partial data streams, is changed in respect of their sequence in the frequency domain in accordance with a mapping pattern. The digital data stream is divided into N partial streams in a multi-carrier transmission system, wherein N is an integer. These partial streams are assigned to constellation points according to the sequence of their indices. However, they are not assigned in their sequence to the carriers in the frequency domain. As a result, encryption takes place, since any third party who attempts to intercept this connection will be unaware of the sequence of the original partial data streams and therefore cannot piece them together. In accordance with one embodiment of the invention, a set of mapping patterns is available, and during modulation of a data stream during a connection, the mapping function uses individual elements or mapping patterns from the set of mapping patterns. The encryption level is increased thereby. Known deciphering methods work on the basis that the individual letters of a language occur with a certain frequency. This approach of taking frequency into account is not successful if, with this embodiment of the invention, the assignments are changed during the course of a connection. In accordance with a further embodiment of the invention, the set of mapping patterns is implemented in the form of a loop, from the beginning to the end and then starting at the beginning again. A compromise can thereby be found as regards the dynamics of the mapping, i.e. the changing of mapping patterns and the memory space needed for storing the mapping patterns. In accordance with a preferred embodiment of the invention, the contents of a set of mapping patterns used by a subscriber to the network is not constant for each connection. This means that a different set of mapping patterns is compiled for each connection. This is done by a permutation function, which defines the content. The object is also achieved in accordance with the invention by a method for encryption of a digital data stream to be transmitted, which comprises the following steps: - transmitting of an encryption key and thereby: - defining a permutation function, - defining a set of mapping patterns, - defining a hop interval, wherein the last three steps may be performed in any order. Before the start of data transmission, a handshake is established between two subscribers to a network. Once the handshake has been established, a protocol or encryption key is transmitted, making the information necessary for the actual transmission available to the receiving subscriber. In accordance with the invention, a permutation function is defined with the encryption key, indicating which mapping patterns are to be used in which order during the actual transmission. In addition, a set of mapping patterns is defined, the content of which is the assignment of the individual constellation points to arbitrary carriers.

Furthermore, a hop interval is defined, defining the time of change from one mapping pattern to the next. The defining of the permutation function, the set of mapping patterns and the hop interval serves for alignment of the transmitting and receiving subscribers to the network, in order to ensure that both subscribers are using the same encryption method. The defining of the encryption method may take place in any order. As regards the encryption method, the object is also achieved in accordance with the invention through the implementation of a permutation procedure, which comprises a loop with the following steps: - set an interval to 1 , - wait for the end of a predefined hop interval, - increase the interval by the value of 1 , - undertake a comparison of whether the current value of the interval is greater than the total number of elements in a permutation function, which indicates the positions of the mapping patterns to be used for a modulation of the digital data stream, wherein one or the other of the following then takes place: - if the result of the comparison is positive: reset the interval to 1, - if the result of the comparison is negative: equate the current mapping function with the mapping pattern located at the position specified by the permutation function. The advantage of the encryption method in accordance with the invention is that no extra bandwidth is necessary for the data transmission. In a further embodiment, however, redundancy is deliberately generated in that the number of N constellation points is greater than the number m of usable carriers. Usable carriers are those that transmit a partial data stream. The remaining (N-m) carriers are occupied with a pseudo random code. The encryption level is thereby further increased. The permutation procedure enables the mapping pattern serving for the encryption to be changed again and again during a connection without any prior knowledge of the actual length of the connection. As regards the appliance for implementing a modulation method or encryption method, the object is achieved in accordance with the invention in that it is equipped with a first controller, which controls the execution of the mapping function. As regards the decoding method for a received digital data stream that has been transmitted in encrypted form, the object is achieved in accordance with the invention through the implementation of a second permutation procedure, which comprises a loop with the following steps: - set an interval to 1 , - wait for the end of a predefined hop interval, - increase the interval by the value of 1 , - undertake a comparison of whether the current value of the interval is greater than the total number of elements in a permutation function, which indicates the positions of the recovery patterns to be used for a demodulation of the encrypted digital data stream, wherein one or the other of the following then takes place: - if the result of the comparison is positive: reset the interval to 1, - if the result of the comparison is negative: equate the current recovery function with the recovery pattern located at the position specified by the permutation function. This method also begins with the resetting of an interval to 1, in order that a defined starting point is present, from which point counting takes place on expiry of the hop interval. The receiving subscriber is notified of the hop interval and permutation function after the handshake. The content of the recovery pattern to be used currently is derived through creation of the complex-conjugate waveform of the relevant mapping pattern. It is thereby ensured that the authorized receiving subscriber is using the same procedure as the transmitting subscriber. As regards the decoding appliance, the object is achieved in accordance with the invention in that it is equipped with a second controller, which controls the execution of the recovery function. As regards the multi-carrier transmission system, the object is achieved in accordance with the invention by a multi-carrier transmission system with an appliance for modulating a digital data stream that is scrambled during the modulation, and with an appliance for decoding a digital data stream that has been transmitted in encrypted form, which exhibits means for: - undertaking an encrypted modulation, - undertaking the recovery of a digital data stream that has been transmitted in encrypted form. This multi-channel transmission system exhibits an encryption mechanism, which takes place in the physical layer of the network (layer 1 of the OSI layer model). The above-described methods in accordance with the invention may be used in either a wireless network, such as the mobile radio network or a WLAN (Wireless Local Area Network), or a wired network, such as a LAN (Local Area Network).

The invention will be further described with reference to examples of embodiments shown in the drawings, to which, however, the invention is not restricted. Fig. 4 shows, in a schematic representation, an appliance in accordance with the invention for modulating a digital data stream to be transmitted. Fig. 5 shows, in a schematic representation, an appliance in accordance with the invention for demodulating and decoding a received digital data stream. Fig. 6 shows a Table, in which examples of mapping patterns are entered. Fig. 7 shows a Table giving examples of possible permutation functions. Fig. 8 shows schematically, in a flowchart, a method in accordance with the invention for encrypting a digital data stream. Fig. 9 shows schematically, in a flowchart, a method in accordance with the invention for decoding a scrambled data stream.

Fig. 4 shows, in a schematic representation, an appliance 1 in accordance with the invention for modulating a digital data stream to be transmitted, comprising the partial streams do, di ... dN-i. In this example, the partial streams of the data stream are scrambled onto the constellation points Xo, Xi ... X_N-_I by quadrature amplitude modulation. The encryption of the scrambled data stream then takes place in that the constellation points X_o, Xi ... X_N-_I are mapped onto the carriers go(t), gι(t) ... gN-ι(t) according to a mapping function ψ_n , wherein they are regrouped in respect of their subscript indices. The content of a constellation point thereby has a different position in the frequency domain. The mapping of the constellation points in accordance with the invention thus differs from the prior art, i.e. it does not take place in the order of the predetermined positions, but in accordance with a mapping function ψ_n. The subscript index n hereby indicates that an individual mapping pattern Ψ, has a validity of one interval of length n, and then a change takes place. The interval n may hereby be defined in relation to time and correspond to a cycle duration T_h0p or defined in relation to data packets and correspond to a specific number Q of data packets. The mapping is therefore dynamic. The current mapping function ψ_n is then part of a quantity totaling H mapping patterns Ψ,: ^ e {Ψ, , Ψ₂ , ... Ψ„ } (2) wherein H is the integral quantity of mapping patterns Ψ, and n is the interval that indicates the validity of the respective current mapping function ψ_n. In one embodiment, the mapping patterns Ψ, are applied successively, wherein the position of the mapping pattern is determined by the permutation function S,={ρ_l, p_2 ... p_M}, so that: Ψ_n = Ψ_p__π (3) The current mapping function ψ_n determines the mapping between the constellation points Xo, Xi ... X_N-_I and the mapping points ao, ai ... aN-i, and thereby also the carrier frequencies go(t), gι(t) ... gN-ι(t), since, with this embodiment, the mapping points and the carrier frequencies are assigned to each other in accordance with their subscript indices. This definition is shown in the following equation (4): [a₀ a .... a_N_ ^T = ψXx, X. ... X_N- ) (4) It can also be seen from Fig. 4 that, with the appliance 1 in accordance with the invention for modulating a digital data stream in a multi-carrier transmission system, an encryption method is added to a modulation method that is known per se. The combination of modulation and encryption in accordance with the invention takes place in the physical layer, i.e. layer 1, of the OSI 7-layer model. A dynamic mapping controller 2 controls the assignment of the constellation points to the mapping points and carriers in accordance with the mapping function. The mapping patterns are stored in a memory, e.g. a RAM (Random Access Memory). The encryption key transmitted after the handshake defines inter alia the mapping patterns to be used and their sequence and validity duration. These definitions are stored or at least put into intermediate storage, so that the dynamic mapping controller has access to them. Fig. 5 illustrates, in a schematic representation, an appliance 3 in accordance with the invention for recovery, i.e. for demodulation and decoding, of a scrambled, encrypted data stream. The digital data stream has been scrambled and encrypted during the modulation. In this example, the complex-conjugate, time-dependent waveform g*(t) is applied to the received, time-dependent signal r(t). The partial signals are then sent to a filter array, which creates the corresponding output signals bo, bi ... b -i as intermediate values. The receiving-end constellation points Y_o, Yi ... Y_N-_I are created from the intermediate values by applying a dynamic recovery function φ_n. This also sends the indices into the correct, successive order. The encryption key transmitted after the handshake, or its content, is stored, or put into intermediate storage, e.g. in a RAM, at least for the duration of the transmission, demodulation and decoding. A dynamic recovery controller 4 controls the assignment of the complex- conjugate carriers to the receiving-end constellation points. Fig. 6 shows a Table in which examples of mapping patterns Ψ, are entered. In this example, according to a first mapping pattern Ψ„ the constellation point X₀ is combined with the waveform go, the constellation point X_\ with the waveform g -i and the constellation point X_N-_I with the waveform gj. Each of the individual mapping patterns Ψi, Ψ₂ ... Ψ_H> wherein H is an integer, represents an encryption, since the constellation points have changed position as regards their sequence in the frequency domain. A further encryption is achieved by using successive different mapping patterns during one transmission. The number H of elements of the mapping function ψ_n is hereby restricted, as is apparent from equation (2). The current mapping function ψ_n is, however, variable, as is apparent from equation (3). This is achieved because the different mapping patterns Ψ, are used again and again in a loop. At the receiving end, the corresponding recovery function φ_n assigns the constellation points Y_o, Yi ... Y_N-_I by application of the complex-conjugate waveform g o, g _I ... g _N-i- Before the start of data transmission, the sequence and the interval duration n for which the mapping patterns Ψ_ls Ψ₂ ... Ψ_H are to be used are defined during the handshake between the two partners to a connection. The Table in Fig. 6 illustrates that, in accordance with the invention, the mapping of the constellation points onto the carriers takes place dynamically, i.e. in accordance with a mapping function. In this example, four mapping functions Ψi to Ψ_H are shown. It is apparent that the number of constellation points and of carriers is the same and that every constellation point Xo to X_N-_I is mapped on one of the carriers go to gN-i- The example shows possible assignments for four mapping patterns Ψ. In one embodiment of the invention, the mapping patterns Ψi to Ψ₄ are used successively. If the transmission is not yet complete when the last mapping function has finished, a start is again made at the first mapping function for the encryption. In accordance with a preferred embodiment of the invention, the mapping patterns Ψi to Ψ_H and the recovery patterns Φj to Φ_H are used not successively, but in a sequence that is specified by one of the permutation functions S,. Fig. 7 shows a Table which, for explanatory purposes, gives examples of possible permutation functions Sι, S and S . Here, L is an integer which indicates the entire number of available, predefined permutation functions S,. In this example, the length M of each individual permutation function is 5. This means that every individual permutation function S, indicates the sequence of utilization for 5 mapping patterns Ψ,. The same mapping pattern may occur multiple times. Individual mapping patterns from the "stock", i.e. individual predefined assignments of constellation points to carriers, may also be omitted. The number M of positioning information p_i is an integer and may be greater or smaller than the number H of mapping patterns Ψ, or equal to it. Preferably: M>=H, so individual mapping patterns are used multiple times during one loop. At the start of a transmission, which of the three permutation functions Si to S_L possible in this example is to be used is defined between transmitter and receiver. The change from one mapping pattern Ψ, to the next takes place on expiry of a hop interval I_hop. A hop interval I_h0p is alternatively defined by: a) a time T_hop or b) a number Q of data packets. An appropriate appliance is present, both at the transmitting end and the receiving end, to measure the time Ih_op in accordance with alternative a) or to count the number Q of data packets in accordance with alternative b). It is obvious that, in the transmission system, both the transmitter and the receiver have the same time, e.g. t is the system time of the network, to which all subscribers are synchronized. Fig. 8 shows schematically, in a flowchart, a method in accordance with the invention for encrypting a digital data stream. Following on from the handshake 100, at step 200 the encryption key is transmitted. This initiates the following, in any order: - the defining of a permutation function S, 210, - the defining of mapping patterns Ψ, 220, - the defining of a hop interval Ih_op 230. The permutation function S, indicates the order in which the mapping patterns Ψ, are to be used. Definition 210 of the permutation function valid for the current transmission may alternatively take place by: c) Transmitting a vector S„ which contains the specific permutation sequence {p_l, p_2 ... p_M} or d) Transmitting only the name of an individual permutation function S,. Alternative c) enables an unauthorized third party to intercept the permutation sequence and therefore comprises an aid to decoding the transmitted digital data stream. However, this method has the advantage that space is saved, both at the transmitting and the receiving end, since the permutation sequence valid for the current transmission need only be put into intermediate storage and may be deleted on termination of the transmission. Alternative d) presupposes that, both at the transmitting and the receiving end, all possible permutation functions Si, S₂ ... S_L have to be permanently stored in order that the permutation function S, valid for the transmission can be invoked. The advantage of this variant is that an unauthorized third party cannot determine the sequence of mapping functions ψ, implied by the permutation function S, used, since it is not transmitted. Step 220, defining mapping patterns, may alternatively take place by either: e) Transmitting the specific, individual mapping pattern Ψ, in the form of vectors containing the assignments of constellation points to mapping points or carriers. or f) Transmitting the name of the mapping pattern Ψ, to be used . The advantages and disadvantages of alternatives e) and f) are, as with the defining of the permutation function S„ that the transmission of the specific information reduces the protection against interception, and the storage of predefined mapping patterns occupies space at both the transmitting and receiving ends. Step 230, defining the hop interval I_h0p means, alternatively: g) Specifying a cycle duration Th_op or h) Specifying a number Q of data packets. The dynamic mapping 300 then begins. The first permutation procedure 400 is as follows: at step 410, the interval n is set to 1 and the current mapping function ψi is equated with the mapping pattern Ψ_{p t} whose position p i is specified by the permutation function S,. At step 420, there is a wait for the expiry of the hop interval Ih_op- Measurement of the time for determining the end of the cycle duration, or the counting of the transmitted data packets, takes place by means of appropriate appliances, such as a counter or a flip-flop. When the end of the hop interval Ih_op has been reached, the interval n is increased by a value of 1 at step 430. At step 440, a comparison is made of whether the current value for the interval n is greater than the total number M of elements of the permutation vector. If the result of the comparison is "yes", the loop starts again at step 410 and the interval n is reset to the value of 1. If the result of the comparison is "no", step 450 invokes the mapping pattern located at the n-th position p_n of the permutation function S, , i.e. ψ_n = Ψ_{p n} , and this is applied continuously until the end of the hop interval I_hop is reached in the course of the loop at step 420, after which the interval n is increased by a value of 1 at step 430. Fig. 9 shows schematically, in a flowchart, a method in accordance with the invention for decoding a scrambled data stream. Once the handshake has taken place at 500, the encryption key is transmitted at step 600. This initiates the following: - the defining of a permutation function S, 610, - the defining of recovery patterns Φ, 620, - the defining of a hop interval Ih_op 630. The permutation function S, is the same as that used for the transmitting-end modulation and encryption, and indicates the order in which the recovery patterns Φ, are to be used. The definition of the permutation function S, valid for the current transmission may alternatively take place by: i) Storing, or putting into intermediate storage, a vector S,, which contains the specific permutation sequence {p_l, p_2 ... p_M} or j) Storing, or putting into intermediate storage, an individual permutation function S,. Which of the alternatives i) or j) is selected depends on the transmitter's operating mode. The disadvantages specified for alternatives c) and d) apply here. The step of defining recovery patterns, 620, may alternatively take place by: k) Storing, or putting into intermediate storage, the specific, individual recovery pattern Φ, in the form of vectors that have been transmitted by the transmitter or 1) Storing, or putting into intermediate storage, the recovery pattern Φ, corresponding to the name of the mapping pattern Ψ, used or m) Generating the recovery pattern Φ, corresponding to the transmitted mapping pattern Ψ,. The advantage of alternative k) is that only the patterns necessary for the current data transmission are stored. The disadvantage is that, in some circumstances, these could be intercepted during the transmission of the encryption key. The advantage of alternative 1) is that the encryption key contains no specific data necessary for decoding. The disadvantage is that all possible recovery patterns have to be stored, including those that are not necessary for the current transmission. Alternative 1) has the disadvantage that a time delay may possibly occur as a result of the generation of the recovery pattern not yet being completed before the encryption key is fully transmitted and the data stream begins. Step 630, defining the hop interval Ih_op may alternatively take place by: n) Storing a received cycle duration T_hop or p) Storing a received number Q of data packets. Alternatives n) and p) are governed by the transmitter's operating mode during this connection. Only then does dynamic recovery begin at 700. The second permutation procedure 800 is comparable with the first permutation procedure 400 of the transmitter, since the decoding has to take place in steps corresponding with encryption. At step 810, the interval n is set to "1" and the current recovery function φi is equated with the recovery pattern φ_p , located at position p i of the permutation function S,. At step 820, there is a wait for the expiry of the hop interval Ih_op. Measurement of the time for determining the end of the cycle duration T_h0p, or the counting of the transmitted data packets for determining the number Q, takes place by means of appropriate appliances, such as an oscillator, a counter or a flip-flop. When the end of the hop interval I_hop has been reached, the interval n is increased by a value of 1 at step 830. At step 840, a comparison is made of whether the current value for the interval n is greater than the total number M of elements of the permutation vector. If the result of the comparison is "yes", the loop starts again at step 810 and the interval n is reset to the value of 1. If the result of the comparison is "no", step 850 invokes the recovery pattern located at the n-th position p_n of the permutation function S, , i.e. φ_n = φ_{p n} , and this is applied continuously until the end of the hop interval Ih_0p is reached in the course of the loop at step 820, after which the interval n is increased by a value of "1" at step 830. The encryption key 600 initiates, with three steps, the defining of different variables that are valid for this current connection. The order of defining the variables may, without affecting the invention, differ from that shown as an example in Fig. 9: definition of a permutation function 610, definition of recovery patterns 620 and definition of a hop interval 630. The invention may be summarized by the phrase "encrypted modulation". A first encryption level is achieved in that the constellation points change their order in the frequency domain. A further encryption level is achieved in that different mapping patterns are used during a connection, i.e. the assignment of the transmission channels is changed and the modulation is therefore dynamic. The encryption level is further increased by means of a permutation function, which defines the order of the mapping patterns to be used. The encryption level is further increased through the use of a hop interval, which differs in length for different connections. The encryption level may be further increased if redundancy is accepted, in which the number of usable carriers that transmit a partial data stream is smaller than the number of constellation points, and the remaining carriers are occupied with a pseudo random code.

Claims

CLAIMS:

1. A method of modulating a digital data stream in a multi-carrier transmission system, wherein, in the course of the modulation:

- the digital data stream is divided into N partial data streams (do, d] ... dN-i) and scrambled onto N constellation points (X_G, Xi ... X_N-_I), - mapping of the N constellation points (X_θ5 Xi ... X_N-_I) onto at least N carrier frequencies (go(t), gι(t) ... gN-ι(t)) is undertaken, characterized in that, in the course of the modulation, through the application of a mapping function (ψ_n), encryption is undertaken in that the position of the constellation points (X₀, Xi ... X_N-_I), and thus of the partial data streams (do, d] ... dN-i), is changed in respect of their sequence in the frequency domain (g₀(t), gι(t) ... g_N-ι(t)) in accordance with a mapping pattern (Ψ,).

2. A method as claimed in claim 1, characterized in that, during a modulation, the mapping function (ψ_n) uses, in succession, individual elements from a set of mapping patterns (Ψ,).

3. A method as claimed in claim 2, characterized in that the set of mapping patterns (Ψ,) is implemented in the form of a loop, from the beginning to the end and then starting at the beginning again.

4. A method as claimed in claim 2 or 3, characterized in that a permutation function (S,) defines the content of a set of mapping patterns (Ψi).

5. A method of encrypting a digital data stream to be transmitted, characterized by the following steps: - transmitting an encryption key (200) and thereby: - defining (210) a permutation function (S,), - defining (220) a set of mapping patterns (Ψi), - defining (230) a hop interval (I_ho_P), wherein the last three steps (210, 220, 230) may be performed in any order.

6. A method of encrypting a digital data stream characterized by the implementation of a permutation procedure (400), which comprises a loop with the following steps: - setting (410) an interval (n) to 1, - waiting (420) for the end of a predefined hop interval (I_hop), - increasing (430) the interval (n) by the value of 1, - undertaking a comparison (440) of whether the current value of the interval (n) is greater than the total number (M) of elements in a permutation function (S;), which indicates the positions of the mapping patterns (Ψ,) to be used for a modulation of the digital data stream, wherein one or the other of the following then takes place: - if the result of the comparison is positive: resetting the interval (n) to 1, - if the result of the comparison is negative: equating the current mapping function (ψ_n) with the mapping pattern (Ψp n) located at the position (p_n) specified by the permutation function (S,).

7. A method of encrypting a digital data stream, which is distributed in the course of the modulation onto m usable carriers, characterized in that the number N of constellation points used is greater than the number m of carriers that transmit partial data streams.

8. An appliance (1) for implementing a method as claimed in any one of the preceding claims, characterized in that it is equipped with a first controller (2), which controls the execution of the mapping function (ψ„).

9. A method of decoding a received digital data stream that has been transmitted in encrypted form, characterized by the implementation of a second permutation procedure (800), which comprises a loop with the following steps: - setting (810) an interval (n) to 1, - waiting (820) for the end of a predefined hop interval (I_hop), - increasing (830) the interval (n) by the value of 1, - undertaking a comparison (840) of whether the current value of the interval (n) is greater than the total number (M) of elements in a permutation function (S,), which indicates the positions of the recovery patterns (Φj) to be used for a demodulation of the encrypted digital data stream, wherein one or the other of the following then takes place: - if the result of the comparison is positive: resetting the interval (n) to 1, - if the result of the comparison is negative: equating the current recovery function (φ_n) with the recovery pattern (Φ_{p n}) located at the position (p_n) specified by the permutation function (S,).

10. An appliance (3) for implementing a method as claimed in claim 9, characterized in that it is equipped with a second controller (4), which controls the execution of the recovery function (φ_n).

11. A multi-carrier transmission system with an appliance, in particular an appliance (1) as claimed in claim 8, for modulating a digital data stream that is scrambled during the modulation, and with an appliance, in particular an appliance (3) as claimed in claim 9, for decoding a digital data stream that has been transmitted in encrypted form, characterized in that it exhibits means for: - undertaking an encrypted modulation, - undertaking the recovery of a digital data stream that has been transmitted in encrypted form.

12. A use of any one of the above-mentioned methods in a wireless or in a wired network.