Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/06—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/38—Demodulator circuits; Receiver circuits

Definitions

• the invention relates to the improvement of the demodulation of such signals, and in particular the optimization of the attachment of the synchronization system] and the reduction in the probability of the release of this synchronization system.
• demodulation consists in placing the values
• This space is divided into decision regions, defined by decision boundaries. Each region is assigned to one of the states of the constellation, which is considered most likely, and which is retained as the result of the demodulation, when a received value is found in this region.
• the reception space is then the Fresnel plane (I / Q plane).
• I / Q plane the Fresnel plane
• Digital modulation techniques of the MAQ type therefore rely, in mono-sensor receivers, on the implementation of a modulation constellation, conventionally represented in the I / Q plane in the form illustrated in FIG. 1 in the particular case of 'A 16 MAQ modulation (only the first quadrant is represented. The three others are deduced directly by symmetry).
• the points 11 of the modulation are distributed at equal distance from each other, in a regular manner.
• the modulation then consists in choosing one of the points
• the demodulation operation therefore consists in associating the received value 12 with the most probable transmitted point 14.
• demodulation boundaries 15 are defined, parallel to the axes I and Q, maximizing the distances (the value received 12 is considered to correspond to the nearest point 14).
• an objective of the invention is to provide a demodulation technique making it possible to fight more effectively against the effects of frequency shift, compared to the conventional technique.
• an objective of the invention is to provide such a technique, allowing faster attachment of the synchronization system, in particular in the presence of frequency offset.
• Another objective is, of course, to provide such a technique making it possible to reduce the probability of the synchronization system going off hook.
• Another objective of the invention is to provide such a technique, which is easy and inexpensive to implement, in particular in consumer receivers, without requiring modifications to the microwave oscillators.
• the invention also aims, according to a particular aspect, to provide such a technique, which is adaptive, and which takes account of all the disturbances induced by the transmission channel (phase noise and Gaussian additive noise).
• this method comprises the following stages: association with at least one of said points of said modulation constellation of at least one generating area encompassing said point, representative of the potential effect of additive Gaussian noise;
• the invention proposes to modify the conventional modulation boundaries (generally minimizing distances from the points; of the modulation constellation) by taking into account on the one hand the fact that a phase error can, under certain conditions , greatly distance a j received signal point from the corresponding transmitted point, and on the other hand the fact that the received signal can be disturbed by additive Gaussian noise (white noise and / or colored noise).
• additive Gaussian noise white noise and / or colored noise
• the invention does not suppose, according to this aspect, of a particular treatment on transmission (although an advantageous method of modulation is proposed subsequently).
• the same signal can therefore be processed on the one hand by conventional receivers, and on the other hand, more effectively, in terms of error rate and / or hooking, by receivers implementing the demodulation method of the invention.
• the invention takes into account both aspects relating to the transmission (the structure of the constellation implemented) and to the reception (the Gaussian noise) .
• the j i decision boundaries are plotted in the I / Q plane, so as to associate with acun i of said points of the modulation constellation a decision region corresponding to a portion of said I / Q plan.
• the same approach can of course be adapted to other representations.
• said boundaries are variable as a function of variations of said additive Gaussian noise. It is thus possible to optimize the demodulation as a function of the reception conditions. !
• said generating zone forms a disc, nt the radius can for example be proportional to the standard deviation of said additive Gaussian noise.
• At least one of said discs is centered on the corresponding point of said modulation constellation.
• At least two concentric generating zones are taken into account, in order to draw at least one border for at least one of said points of said modulation constellation.
• At least one of said borders is a combination of at least one border portion corresponding substantially to an edge of said scanned surface and at least one linear portion corresponding to an axis of symmetry defined by said constellation of
• At least one of said generating zones is not centered on the corresponding point of said modulation constellation, so as to simulate a modification of the constellation on transmission.
• the points which are associated with at least one border adapted as a function of the potential effect of a phase shift include at least, preferably, the points of the constellation most distant from the center of said I / Q plane.
• said constellation of modulation corresponds to an amplitude quadrature modulation (MAQ) j.
• MAQ amplitude quadrature modulation
• onj advantageously implements boundaries as illustrated in FIGS. 5 or 11 or 13 (it is not easy and ineffective to describe these boundaries mathematically, while the figures allow a direct understanding. For this reason, reference is made exceptionally to the figures in the corresponding claim).
• said received signal can be a multi-carrier signal or a single-carrier signal. It can in particular be an ignal $ transmitted by bursts, for which the invention proves to be very interesting.
• the demodulation method of the invention is advantageously implemented during an attachment phase of a phase locked loop, j
• the method of the invention comprises the following steps: j
• the invention also relates to a method of modulating a digital signal implementing a modulation constellation, according to which the position of at least one of the points of said modulation constellation is chosen taking into account the potential effect d 'a phase rotation on the latter, so as to increase the probability of correct demodulation of the corresponding received value, after transmission via a transmission channel capable of inducing said phase rotation.
• At least one of said boundaries is adapted taking into account on the one hand the potential effect of a phase shift on at least one of said points of the modulation constellation, and on the other hand potential effect of additive Gaussian noise applied to said point, said additive noise Gaussian being represented by a generating surface associated with said point, and said phase shift by a rotation over an angular range as a function of symmetries defined by said modulation, so as to define a surface swept by said generating zone, said boundary being chosen so that said scanned surface essentially belongs to the decision region associated with the point of the corresponding modulation constellation.
• the invention also relates to a system for transmitting at least one digital signal, from at least one transmitter to at least one receiver, using means for modifying the modulation constellation on transmission and / or on reception, and / or means for modifying the corresponding decision boundaries, taking into account on the one hand the potential effect of a phase shift on at least one of said points of the modulation constellation, and on the other hand of the potential effect of an additive Gaussian noise applied to said point, said additive Gaussian noise being represented by a generating surface associated with said point, and said phase shift by a rotation over an angular range as a function of symmetries defined by said modulation, of so as to define a surface swept by said generating area, said border being chosen so that said swept surface essentially belongs to the decision region ass associated with the point of the corresponding modulation constellation. !
• the invention also relates to a digital signal implementing a modulation constellation, in which the position of at least one of the points is chosen taking into account the potential effect of a phase rotation on the latter, so increasing the probability of correct demodulation of the corresponding received value, after transmission via a transmission channel capable of inducing said phase rotation.
• FIG. 1 already discussed in the preamble, illustrates a constellation of MAQ16 modulation, and the principle of its demodulation according to the prior art
• Figure 2 shows, in the form of a block diagram, a digital synchronization system, known per se
• Figure 3 illustrates the characteristic of the detector from 2 to
• FIG. 6 illustrates an example of implementation of demodulation using the borders of FIG. 5
• FIG. 7 illustrates the characteristic of a phase detector implementing the decision boundaries of FIG. 5,
• FIG. 10 illustrates the characteristic of a phase detector at
• FIG. 12 illustrates the characteristic of a phase detector at
• FIG. 13 shows the first quadrant of reception of an MAQ 16 constellation, using modified boundaries according to the invention and a simulation of modification of the constellation on transmission; ! - Figure 14 illustrates the characteristic of a phase detector to
• a digital carrier synchronization system of a receiver is presented in FIG. 2 implementing a Directed Decision (DD) algorithm derived from the application of the maximum likelihood criterion. (ML for "Maximum Likelihood” in Anglo-American) based on a looped structure (“Feedback”: FB) and knew a prior recovery of the rhythm (T). ;
• DD Directed Decision
• the structure of the system results from the derivation from the phase error of the criterion of Maximum likelihood A Posteriori [l] j (for simplification, all the documents cited in this patent application are grouped in the appendix 1).
• This system is called DDML i FBT and is composed of three elements: a phase detector 21, a loop filter 22 and an integrator 23, as illustrated in FIG. 2.
• the solutions of the invention can apply in to JS digital carrier synchronization systems based on a Directed Decision algorithm, which uses an estimation of the symbols received.
• the transmitted signal s (t) is received in the form r (t), after transmission via a transmission channel 24.
• This received signal is sampled (25) then demodulated, using a multiplier 26 controlled by the integrator 23.
• the real (27) and imaginary (28) parts are separated from the demodulated signal w (k). They allow a comparison with the original constellation (29, 210), and supply the phase detector 21.
• the role of the phase detector 21 which is of particular interest in the context of the present invention is to provide) "information representative of the phase error.
• This information is then filtered (22) and then integrated (23) in the loop in order to generate the phase correction ⁇ to be made to the signal.
• the decision making it possible to generate the estimated symbols d (k) uses the conventional decision boundaries F 0 of the constellation C 0 relating to the MAQ16. ; This characteristic reveals the intrinsic properties of the following phase detector: -
• the gain of the detector is defined as the slope of the linear range at the origin. The higher K d is, the more the value of ⁇ () represents unequivocal information representative of the phase error.
• the phase detector is sensitive to the noise level of the input signal. As the noise increases, its linearity range decreases as does its gain. On the other hand, noise in certain cases minimizes the probability of the presence of false attachment points. Characterization of the loop Under the assumption of normalization of the gain K d of the detector and of the gain K 0 of the integrator, the update relationship of the estimated phase is written:
• carrier recovery systems use a second order looped structure [3]. For this reason, and once again without this being restrictive, it is this structure which is retained in the examples described below.
• the structure of the second-order loop can be defined by ' two more significant parameters than ⁇ and ⁇ .
• the equivalent monolateral noise band of the loop B t is used as a parameter, which is normalized with respect to the duration of the symbols T s . The higher the value of B ⁇ T S , the greater the hooking speed but the more the loop generates a noisy estimate ⁇ (k).
• the expression of BT s is defined by:
• Modification of decision boundaries It is possible to improve the tolerance to a phase error at least for certain symbols of the constellation C 0 by modifying the boundaries of decision.
• any modification of the decision boundaries results from a compromise between the tolerance with respect to Gaussian noise and with respect to a phase error.
• Figure 4 is a simplified block diagram illustrating the general principle of an embodiment of the invention.
• This generating area 55 can be a circle, but other shapes can also be envisaged.
• the radius of the circle is advantageously a function of the standard deviation ⁇ of the Gaussian additive noise 42.
• the system is adaptive, as a function of the level of Gaussian noise
• Information on additive noise can be obtained by various known methods, for example by analyzing the signal received during a period during which no signal is transmitted or during which a reference signal (known to the receiver) is transmitted .
• generating zones 56, 57 (FIG. 5) (for example two, corresponding to circles of radius ⁇ and 2 ⁇ ) are advantageously taken into account, for at least some of the points, to optimize the borders. They can be concentric or not.
• the generating zones can be centered on the point of the constellation or offset from it (third embodiment).
• a rotation 58 is applied to them (43) so as to define a swept surface 59 representative of the potential effect of a phase rotation.
• This rotation being applied to the; zoned generating, the swept surface is therefore representative on the one hand of the effect of additive Gaussian noise and on the other hand of the effect of a phase rotation.
• the range of rotation applied to each of the generating zones is a function of the symmetries induced by the constellation.
• the points 51 and 52 will undergo a rotation of ⁇ ! 2-
• the points 53 and 54 which are two on the same radius, will undergo a rotation of ⁇ / 4. ;
• the borders are formed from circular arcs 5101, 5102 and] 5103 of straight portions 5131, 5132 corresponding to mediating planes; between points, or symbols, in the constellation.
• BBAG Gaussian additive
• ⁇ is the standard deviation of the additive Gaussian noise (other values of type . ⁇ can be used).
• the probability that a symbol affected by Gaussian noise is in the circle of radius ⁇ ⁇ st on the order of 90%.
• modified boundaries more particularly affect the decision-making relating to the external symbols of the constellation which are the most sensitive to phase errors.
• the maximum value of the standard deviation of the Gaussian noise must be less than ⁇ / 2 (if 2a is the minimum distance between symbols). This application limit results in the case of a QAM16 by a minimum signal to noise ratio of 16 dB.
• the first step consists of a conventional demodulation 61 (according to FIG. 1) which, with a received symbol w (k), associates the symbol d (k) of the nearest constellation C 0 : this is equivalent to decision-making with respect to the classical boundaries F 0 .
• the second step consists in applying an algorithm 62, which i will annotate M A , making a second decision from the result of the classical demodulation d (k) and the received symbol w (k).
• This algorithm uses as parameters the mapping 63 of the constellation as well as the signal to noise ratio 64. Thanks to these two parameters, it is then possible to take a second decision on the symbol received w (k) by applying the modified decision rrontièfes relating to the first estimated symbol d (k) denoted FôM A and represented in solid line in FIG. 5 (5103, 5132, 5101, 5131, 5102); .
• FIG. 9 represents the tolerances for phase errors of the various symbols of a conventional constellation C 0 and of the modified constellation C ⁇ . It highlights better tolerances in the case of the constellation C ⁇ .
• a first possible variant of the modified demodulation is the combination of a modified constellation C, decision boundaries F, and a modified boundaries algorithm M A.
• the resulting decision boundaries 111 will then be annotated F ⁇ M A.
• the second variant uses a constellation C 1 combined with an algorithm which we will annotate M B.
• This algorithm differs from the algorithm M A in the sense that it takes as a parameter not the constellation used but a virtual constellation.
• the effect of this virtual constellation is to center the circles of radii ⁇ and 2 ⁇ on virtual symbols, which induces a modification of the decision boundaries compared to those obtained using the algorithm M A.
• the virtual constellation provided as a parameter is com] Dosed with the following symbols: (+ a, + a), (+ 2.8a, + a), (+ a, + 2.8a) and (+ 3.2a, + 3.2a).
• the decision boundaries 131 used are illustrated in Figure 13.
• the modified decision bodies have the best performance
• the symbols of the first quadrant are (+ a, + a), (+ 3a, + a), (+ a, + 3a).

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• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Power Engineering (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

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• 2001
  • 2001-05-15 FR FR0106411A patent/FR2824977A1/fr active Pending
• 2002
  • 2002-05-15 JP JP2002590609A patent/JP3971706B2/ja not_active Expired - Fee Related
  • 2002-05-15 EP EP02740791A patent/EP1388242A1/de not_active Withdrawn
  • 2002-05-15 KR KR10-2003-7014894A patent/KR20040008179A/ko not_active Application Discontinuation
  • 2002-05-15 US US10/477,810 patent/US7447288B2/en not_active Expired - Fee Related
  • 2002-05-15 WO PCT/FR2002/001641 patent/WO2002093862A1/fr active Application Filing

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