Patent Application
20050157817
1. Field of the Invention
The present disclosure relates generally to the field of telecommunications and in particular but not exclusively to a method for receiving two decorrelated signals transmitted on a single channel.
2. Description of the Related Art
Transmission band requirements increase with the development of telecommunications.
Generally, the transmission band of a given communication channel is increased by increasing bit loading on a digital data transmission channel according to a given modulation pattern defining a finite set of complex points or information-carrying symbols. In Phase Shift Keying modulation, a set of complex points distributed on the same circle is defined. While the simplest version, known as Binary Phase Shift Keying (BPSK), makes it possible to transmit 1 bit per symbol, QPSK can simultaneously transmit two bits, and 3 bits can be obtained with 8QPSK modulation and 4 bits with 16QPSK modulation.
Quadrature Amplitude Modulation makes it possible to further increase transmission rates by means of a bigger constellation by operating at the same time a phase and shift modulation. A 16QAM or 64QAM modulation makes it possible to increase the transmission rate up to 4 or 6 information bits respectively, especially when associated with an efficient error correcting code system.
However, QAM techniques cannot be implemented when a data transmission channel suffers from latent nonlinearity as occurs in some situations, and in particular in satellite transmissions. As everybody knows, the electronics loaded on to a satellite are subjected to strict constraints of operation, and in particular to minimal power consumption as they are powered by solar cells. Thus, traveling wave tubes that are loaded onto satellites operate in saturated mode in order to minimize electronic current consumption. Nonlinearity is then very important, as illustrated in
In the presence of such nonlinearity, it is riot possible to resort to constellations and performances of QAM-type constellations. The choice is then confined to simpler modulation patterns, compatible with the nonlinearity affecting the data transmission channel, and in particular to modulation phase shift keying or MPSK. In practice, for satellite transmission, one hardly exceeds the performances of an 8PSK modulation, which corresponds to loading of three information bits.
To increase rate, other alternative techniques should be considered.
A purpose of an embodiment is to overcome the limitations arising from the nonlinearity of some data transmission channels that, in addition, is likely to vary in time.
An aspect provides a technique making it possible to increase the flow in a nonlinear data transmission channel and to provide an alternative solution to QAM-type modulations.
Another aspect includes carrying a method for receiving, via a single channel affected by nonlinearity, first and second data flows transmitted according to a MPSK-type modulation pattern, with a device for automatically adjusting parameters according to the nonlinearity of the channel.
An aspect includes a method for receiving a composite signal transmitted via a nonlinear data transmission channel according to a modulation pattern using a given constellation comprising a first signal UL and a second signal LL.
The process comprises the following:
In one embodiment, the nonlinearity function is realized by means of a look-up table comprising p Cn(k) coefficients, with k=1 to p, which are updated according to an adaptive process based on a correlation calculation between the result E of said subtraction and said continuous time waveform.
In this manner, both information UL and LL can be decoded without knowing precisely the nonlinearity that affects the data transmission channel. This also allows the method to work even when the internal characteristics of the channel vary in time.
The coefficients of the look-up table will be arranged more densely around the saturation point of the data transmission channel in order to benefit from the greatest precision around this point.
In a particular embodiment, said Cn(k) coefficients of the nonlinear function are applied by means of a double Cartesian-polar-Cartesian transformation comprising the following:
In one embodiment, the current coefficient Cn (k) used for said nonlinearity function is updated according to the following mechanism:
Cn+1(k)=Cn(k)+μE z*
where
Alternatively, the calculating time could be reduced by directly addressing the look-up table from value x2+y2 instead of the value of the module of z.
The process finds direct application in a satellite transmission of a composite signal comprising primary information UL that can be received and processed by a great number of television receivers and secondary information LL that can be received and processed by a subcategory of receivers.
An aspect also provides a digital communication system receiver for receiving and processing a composite signal comprising a first signal UL and a second signal LL. The receiver comprises:
Like for the method, the receiver comprises a set of coefficients that are updated according to calculation of the correlation between the results E of said subtraction and said continuous time waveform, according to the mechanism defined below:
Cn+1(k)=Cn(k)+μE z*
where
The receiver can be used with any communication system intended for a nonlinear channel, and in particular for a television receiver transmitted by satellite way by traveling wave tubes.
Other features of embodiments will appear when reading the following description and accompanying drawings, only given by way of nonrestrictive examples. In the drawings:
Embodiments of a method for receiving two decorrelated signals transmitted on a single channel and a receiver for implementing the method are described herein. In the following description, numerous specific details are given to provide a thorough understanding of embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
In reference to
It will be noted that the receive circuit described hereafter is particularly adapted for receiving a composite signal transmitted by satellite. In a given embodiment, the composite signal is composed of a first signal UL (Upper Layer)—having a higher amplitude—and a second signal LL (Lower Layer) having a lower amplitude. Typically, signal UL is the predominating signal of the first information such as a televised message intended to be received and processed by all categories of television systems whereas second signal LL carries a second information that only a particular subcategory of the television systems will be able to decode.
Generally, the first and second information signals UL and LL are unspecified information signals that are however selected so that they are not related.
As can be seen in
Thus, the demodulation and error correction chain 2-3 makes it possible to eliminate this “noise” and, consequently, to regenerate the first information UL that is available on electrode 11 for example, and can be forwarded to any system receiving information UL.
The receive circuit of
The continuous time signal generated by block 4 is then transmitted to a block 5 realizing a nonlinearity function and more specifically a nonlinear processing that is comparable with the one that affects the considered data transmission channel and, consequently, composite signal 1.
Generally, there are many different possible ways to model the channel's nonlinearity, and in particular when the characteristic curves of the traveling wave transponders are known from manufacturers.
In one embodiment, such nonlinearity function is realized by means of a look-up table used to associate a given complex coefficient with a signal input amplitude value (that is to say the module of a complex number |z|=|x+j y|). As a result, nonlinearity is modeled by means of a successive approximation curve, as shown in
Index n takes into account the coefficient variation in time by means of an adaptive process that will be described hereafter.
The various “plateaus” are distributed non-uniformly to reach maximum precision around the saturation point of the amplifier used in the satellite, as shown in
The variation of coefficient Cn (k) as a function of k, then as a function of n, will now be further developed.
1) Variation of Coefficients Cn (k) with Index k
Coefficients Cn(k) are corrected in such a way that the complex value of the nonlinearity output signal is equal to the input value multiplied by the Cn(k) coefficient corresponding to the input module.
In
X+jY=C(x+jy).
Alternatively, coordinates can be polar-transformed, then the new module can be directly calculated by tabulating curve F(ρ), and θ(ρ) is added to the argument (or multiplication by exp(jθ(ρ) is carried out). Then, it is still possible to return to Cartesian coordinates but with a more important implementation cost because of the many calculations involved.
It will be noted that one can also work with first order development: instead of storing only C a coefficient C′ is also kept, thus one has X+jY=(C+C′ dr.) (x+jy) where dr is the difference between the measured squared module compared to the squared module of the center of the C(k) class.
The update of C is identical, and the update of C′ is
C′n+1(k)=C′n(k)+μ′dr. E z*.
In this way, it is possible to consider less table inputs, however at the price of a greater complexity.
The receive circuit further includes a subtracter 7 that receives the rebuilt, recoded and processed output signal from block 5, and the composite signal delayed by means of a delay line 6.
The output of subtracter 7 then provides a signal that, in theory, is free of the primary information UL and that can thus be used in a second demodulation and decoding chain to regenerate secondary information LL. This second chain comprises an UL demodulator 8 associated with a LL FEC block 9 comprising an error-correcting code similar to unit 2-3 of primary information UL demodulation chain.
2) Variation of Coefficients Cn (k) with Index n
In one embodiment, the set of coefficients stored in the look-up table is constantly updated by means of an adaptive mechanism that makes it possible not to know the values of the internal characteristics of the nonlinear transponders used in satellites.
To this end, a servo loop is inserted in the adaptive mechanism for minimizing the correlation between two signals present in the receive circuit, and in particular the signal output from subtracter 7 and the signal output from nonlinear modeling block 5.
More specifically, the following correction mechanism is applied to calculate the new value Cn+1(k) of a coefficient used by the mechanism at a given moment:
Cn+1(k)=Cn(k)+μE z*
where
If the input signal of block 5 is: z=x+j y, block 5 outputs a value Z=X+j Y, with Z=Cn(k).z where Cn(k) is the coefficient of the coefficient table corresponding to the value of the input module of z.
Generally, it is noted that the time-constant specific to this regulation mechanism is proportional to μ*|z|2. Faster convergence of the mechanism is observed if μ decreases with the module of z.
An embodiment of the operation of the adaptive mechanism is more particularly illustrated in the synoptic diagram of
|z|={square root}(x2+y2) with z=x+j y.
The value of the module is then used to select the value of k to be used to choose the complex coefficient Cn (k).
A block 501 then multiplies this complex coefficient 501 by signal z in order to produce value Z according to the formula:
Z=Cn(k) x z.
This value is then subtracted from the composite signal provided by block 508 by means of a subtracter 504 that then provides an error signal E that can be transmitted to LL decoder.
The same signal E is also transmitted to a multiplier 505 that receives the conjugated complex of signal z in order to carry out a correlation calculation:
E x z*.
The result is transmitted to an attenuator block 509 for multiplication by coefficient μ, which result is then added to the current value Cn(k) of the coefficient that was used in the servo loop. Thus, one carries out adaptive control of the value of the current coefficient Cn(k) in order to calculate the new value Cn+1(k) that will be then be loaded into the table.
In step 61, module |z| is calculated.
In step 62, the process then selects the Cn(k) coefficient corresponding to the calculated module |z|.
In step 63, the process calculates Cn (k) x z that is carried out by block 5.
In step 64, the process calculates the difference between the received composite signal and the output signal of block 5.
In step 65, the process calculates the correlation between signal z and error signal E: E x z*.
In step 66, the coefficient Cn+1(k) is updated according to the formula:
Cn+1(k)=Cn(k)+μ E z*.
With the adaptive mechanism according to one embodiment of the present invention, one can operate an automatic adjustment of the parameters of the coefficients Cn (k) calculated to minimize the correlation between two signals E and z, which ensures the adaptation of the mechanism even when the internal characteristics of the transponders vary with time.
In an embodiment, the mechanism of parameter value adjustment simultaneously corrects two or three consecutive parameters Cn(k) and Cn(k+/−1), with possibly different weightings within the look-up table implemented in block 5.
In another embodiment, instead of calculating module |z|, value x2+y2 is calculated directly, and this value is directly used to select the adequate coefficient Cn(k). Thus, extraction of the square root is avoided, which reduces the processing time.
There is described now an improvement of the adaptive control of the coefficients Cn(k) which was described above in relation with
Indeed, it has been discovered that by applying non linearities in the adaptive control of coefficients Cn(k) of
It is possible to improve that convergence process by inserting a low pass filter before applying the correlation as illustrated in
Filter 510 introduces a low-pass filtering effect on the Error signal E before the latter is multiplied by the conjugate complex z* (as performed by step 65 of
By means of filter 510, the correlation calculation is only performed on the part of the error signal E which is within the band of frequency which is used and this results in a convergence of the adaptive control of coefficients Cn(k) to a set of more accurate values.
Filter 510 introduces an additional delay into the process of calculating the correlation and such additional dealy has to be compensated for.
To achieve this, there is inserted a first delay element 511 between bloc 507 providing the reconstructed signal z and the second input of mutliplier 505 receiving z*.
A second delay element 512 is inserted on the coefficients Cn(k) before the latter is inserted into the second path of adder 506.
The numerical values of delay elements 511 and 512 are selected in order to match the detay inserted by low-pass filter 510. Such determination is straightforward for the skilled man and will not be further developped.
All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.
The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments and examples are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention and can be made without deviating from the spirit and scope of the invention.
These and other modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03 05749 | May 2003 | FR | national |
