ES2244305B1 - DEVICE AND METHOD FOR THE EQUALIZATION OF THE DISTORSION OF THE TRANSMISSION CHANNEL IN COMMUNICATION SYSTEMS BY MULTIPLEXATION IN TIME THROUGH SETS OF COMPLEMENTARY SEQUENCES. - Google Patents

Abstract

The estimation of the frequency spectrum of a transmission channel allows obtaining information of high interest to analyze its characteristics and extract important information from it, or correct the distortion effects that it introduces in a communication system. This invention presents a method, see drawing, for estimating the distortions produced in the phases of a communication system and optimally correcting them. For this purpose, a preamble pulse s [n] for the identification of the encoded medium is transmitted by convolving said signal with a set of complementary sequences (2). These sequences, when received simultaneously in the receiver (11) allow to extract the temporal characteristics of the medium (13 and 14) that affect the transmitted data. Using the data obtained, it is postqualified (15) and prequalified (16) a given number of times (17) until an ideal Kronecker delta (18) is obtained. The accuracy of the distortion equalization depends on the length of the sequences used in the coding and the number of times this technique is applied to the received signal.

Description

Device and method for equalizing the transmission channel distortion in communication systems by time multiplexing through sequence sets complementary.

Object of the invention

The present invention relates to a device and a distortion equalization method produced by the transmission medium in a data transmission by sequence coding techniques complementary; that is, it refers to a device emitter-receiver that obtains the equalization of the distortions by transmitting a Krönecker delta.

Background of the invention

Communication systems, analysis spectral, RADAR and SONAR transmit a signal that arrives, reflected or not, to the receiver after passing through a transmission medium. This medium behaves like a linear filter with a response to pulse in frequency H (\ varpi) or temporal h [n].

To enable the recovery process of the information issued in most of the systems of communication is essential to eliminate the effects produced by the transmission means in the emitted signal s [n]. This process is known as equalization. The frequency response can also be used to make a spectral analysis of the medium and thus obtain information about its physical properties.

The channel acts as a filter and distorts the signal. To this we must add the noise, n [n], due to channel disturbances, thermal noise or other signals that interfere with those issued. In conclusion, the received signal, r [n], can be modeled as:

(1) r [n] = s [n] * h [n] + n [n]

where * denotes one convolution

To eliminate the distortion introduced by the medium in the signal a filter with an answer is needed impulsional, f [n], such that:

(2) r [n] * f [n] \ approx s [n]

that is, that the received signal be as similar as possible to the one issued. This is never fully accomplished. because the equalization does not eliminate noise, n [n], nor distortion completely.

To ensure that the equalization is the best possible, it is necessary to know the medium a priori . That is, it is essential to analyze the h [n] of the medium to be able to counteract the distortion effects. There are two methods to achieve that goal:

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Static equalizers: their Properties do not change over time.

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Adaptive Equalizers: se adapts to the temporal variations of the distortion of the means, medium.

The main problem of the first is that they are  more generic and do not solve the particular problems of each situation.

Adaptive equalizers respond better before variations of the environment, but its implementation is more complicated and are very sensitive to noise.

For some as for others it remains indispensable knowledge of the transmission medium. How much The better the modeling, the better accuracy can be achieved at the time of restoring the emitted signal.

The ideal method for the analysis of the medium it consists of transmitting a delta and analyzing what is received, it is Say get the impulse response. Digitally this is get issuing a delta from Krönecker, \ delta [n]:

\hskip6.42cm

s[n] = \delta[n]

 \ hskip6.42cm 

s [n] = \ delta [n]

(3) r [n] = h [n] + n [n]

As noted, the received signal has impulse response information, h [n], contaminated with additive noise.

That information can be obtained through the use of complementary sequences as explained in the Spanish patent P200201151, "Optimal estimation method of transmission spectrum by simultaneous modulation of complementary sequences. "However, it does not explain how to use such information to correct distortions of Medium over the data.

From all of the above the need for a technique that allows using the information obtained from the medium to correct the effects of distortion in the data transmitted.

The existence of a patent or model is unknown. of any utility, whose characteristics are the object of the present invention

Description of the invention

The invention presented uses the result obtained by the method described in P200201151 for equalize the data received in a communication system based in GCM / OTDM (Golay Coding Modulation / Orthogonal Time Division Multiplexing) as described in the Spanish patent P200002086 of August 16, 2000, "Method, transmitter and receiver for spread spectrum communications via Golay complementary sequence modulation. "

In short, the process of obtaining the information and its subsequent treatment, from the signal received, described below.

First, a preamble is generated that will allow us to synchronize and then obtain the information of the means necessary for the correction of the received data. The preamble is structured as it appears in the figure
ra 1.

Interval 1 corresponds to the interval of synchronization, during which the reception system is synchronize with the transmission Once synchronized, the interval 2 corresponds to the extraction of information from media distortion on the data received later during interval 3. For this, at least one pulse is sent δ [n] encoded by a pair of sequences complementary by convolution with at least a couple of complementary sequences (A, B).

The simplified result of this process, in the  frequency domain, is as follows:

(4) Tx (\ omega) = A (\ omega) * cos \ omega_ {c} t + B (\ omega) * sen \ omega_ {c} t

The effect is to modulate both sequences in quadrature at the center frequency \ omega_ {c}. To be transmitted to H (\) medium, at reception we obtain:

(5) Rx (\ omega) = [A (\ omega) * cos \ omega_ {c} t + B (\ omega) * sen \ omega_ {c} t] \ cdot H (\ omega) + N (\ omega)

Since the demodulation of this stage is consistent, because the system is synchronized, we extract the following quadrature phases:

Rx_ {1} (\ omega) = [A (\ omega) * cos \ omega_ {c} t + B (\ omega) * sen \ omega_ {c} t]  \ cdot H (\ omega) * cos \ omega_ {c} t + N (\ omega) * cos \ omega_ {c} t

(5) Rx_ {Q} (\ omega) = [A (\ omega) * cos \ omega_ {c} t + B (\ omega) * sen \ omega_ {c} t] \ cdot H (\ omega) * sen \ omega_ {c} t + N (\ omega) * sen \ omega_ {c} t

In the demodulation process, assuming that we are already synchronized, a component in 2 \ omega_ {c} appears that is filtered low, obtaining only the original base bands convolved with two new media, H_ {Q} (\ omega) and
H_ {I} (\ omega):

D_ {RI} (\ omega) = A (\ omega) \ cdot H_ {I} (\ omega) + N_ {I} (\ omega)

(6) D_ {RQ} (\ omega) = B (\ omega) \ cdot H_ {Q} (\ omega) + N_ {Q} (\omega)

where N_ {Q} (\ omega) and N_ {I} (\ omega) modulated input noise to the center frequency Now we apply the correlation with the sequences transmitted:

D_ {RI} (\ omega) = A (\ omega) \ cdot A (- \ omega) \ cdot H_ {I} (\ omega) + N_ {I} (\ omega) \ cdot A (- \omega)

(7) D_ {RQ} (\ omega) = B (\ omega) \ cdot B (- \ omega) \ cdot H_ {Q} (\ omega) + N_ {Q} (\ omega) \ cdot B (- \ omega)

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Obviously, the objective of this stage would be extract the coefficients of H_ {I} (\ omega) and H_ {Q} (\ omega), and equalize both phases independently, however, since the data is coded so that the sum of both phases should give a delta From Krönecker, we must equalize the following sum expression:

(8) D_ {R} (\ omega) = A (\ omega) \ cdot A (- \ omega) \ cdot H_ {I} (\ omega) + N_ {I} (\ omega) \ cdot A (- \ omega) + B (\ omega) \ cdot B (- \ omega) \ cdot H_ {Q} (\ omega) + N_ {Q} (\ omega) \ cdot B (- \ omega)

To understand the following process we must indicate that the main property of the sequences used in this invention is that they possess an autocorrelation feature ideal, that is, it corresponds to a perfect Krönecker delta of way they meet:

       \ vskip1.000000 \ baselineskip
    

one

where \ phi_ {ii} are the individual autocorrelations of each of the M complementary sequences, of length L , chosen. Particularized in the case of pairs of complementary Golay sequences (A, B):

2

The generation of such sequences is done at from the so-called basic kernel known to date of 2, 10 and 26 bits (Golay sequence generation rules are discuss in the article entitled "Complementary Sequences" of M. J. E. Golay, published in IRE Transactions on Information Theory, vol. IT-7, p.p. 82-87, April from 1961).

Once the previous expression has been sampled symbol frequency (figure 2), and given the properties of the complementary sequences described, we will consider that (8) can rewritten in discrete time as:

(11) d_ {R} [n] = 2L \ delta [n] * h_ {T} [n] + n_ {T} [n] \ cdot

where h_ {T} [n] is a new one media transfer function that distorts data transmitted.

The distorted response can be expressed mathematically according to the expression:

(12) and [k] = s [k] h_ {T} [0] + \ sum \ limits ^ {- 1} _ {i = - \ infty} h_ {T} [i] s [k-i] + \ sum \ limits ^ {\ infty} _ {i = 1} h_ {T} [i] s [k-i] + n_ {T} [i]

In this expression the first term represents the symbol at instant k, the second term corresponds to the interference between adjacent "precursor" symbols, the second term corresponds to interference between symbols adjacent "postcursor" and, finally, n_ {T} corresponds to noise obtained at the exit of the demodulation process

To equalize the channel we will divide it into two stages (figure 2 and 3):

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Postcursor Equalization (after of the maximum of the sampled response of the channel).

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Precursor Equalization (before of the maximum of the sampled response of the channel).

The postcursor stage is the elimination full of interference between adjacent symbols (ISI) that are they find to the "right", in time, of the symbol k. This cancellation corresponds to a linear equalizer approximation by "zero-forcing", or all zeros. Here the desired symbol is s [k] where k represents the sample k-th of an infinite sequence of symbols.

If we consider the response to the causal impulse of finite duration M, the ideal "postcursor" equalizer will correspond to:

       \ vskip1.000000 \ baselineskip
    

3

From the expression (11), it is evident that the acquired samples d_ {R} [n] correspond to the function of  transfer of the medium h_ {T} [n] multiplied by a factor dependent on the length of the sequences, added a given noise, and therefore match an estimator of the filter coefficients modeling the transmission medium h_ {T} [n]. Substituting and applying transformed z, the Equalizer response corresponds to the following expression:

4

So, the equalization process "postcursor" will apply the previous filter to the signal processed received, which will allow us to eliminate interference between "postcursor" symbols. The filter can be implemented digitally in a simple way using any of the existing direct structures. The result of filtering the signal d_ {E} [n] using the filter (14) will correspond to a signal that we will call d_ {E} [n] and that, ideally, it will not possess "postcursor" interference

Once the interference is eliminated "postcursor" we must eliminate the interference "precursor". Assuming the duration of the "precursor" is N, and that d_ {E} [0] matches the maximum of the signal (which must be 1 when applying the filter (14)). The equalization is carried out through a FIR filter. Filter coefficients, h_ {F} [n]. are extracted from the resolution of an N system equations with N unknowns defined by:

5

The value of N is always greater than or equal to the length of the sequences used. The resolution of this system of equations is direct, because below its main diagonal all values are zero and, in addition,
h_ {F} [NI] = 1.

Since the result of both processes is not ideal due to noise, quantification effects, rounding, etc., if we apply this technique iteratively you get a equalization that improves each iteration.

This technique allows to equalize any channel linear, including any distortion effect such as, effects of due to multiple paths, reflections, circuits and filters not ideals, etc. Obviously, the selection of the length of the N and M filters will depend on the length of the channel response, and of the length of the sequences used.

In conclusion, it can be stated that the invention  described is a powerful equalization system for distortion in communication systems, especially oriented to GCM / OTDM techniques and their variants.

Brief description of the drawings

Figure 1.- Shows the preamble generated for  Extraction of distortion information:

one.

In this phase the receiver synchronizes with the transmitter in phase, time and symbol, so that in the next stage the receiver know exactly where the data received from equalization

2.

In This phase performs synchronous or coherent demodulation, due to to the knowledge in the previous phase of the information of synchronism, so that the coefficients of the equalizer filters

3.

In this last phase, the equalizer filters obtained in the previous phase, and the data is free from interference between symbols (ISI) and reduced noise.

Figure 2.- Shows the signal received in the ADC sampling stage d_ {R} [n] and its subsequent sampling at the symbol frequency, from which the distortion caused by adjacent symbols h_ {F} [n], ISI

Figure 3.- Shows the block diagram of a system that explains a possible application for estimating the distortion of the medium and its subsequent correction using a single pair of complementary sequences.

The different parts that compose it are detailed  then:

one.

Signal digital to emit s [n]: to estimate the medium the ideal is a Krönecker delta.

2.

Encoder with a pair of sequences Golay complementary.

3.

Signals resulting from coding I [n] and Q [n].

Four.

QASK modulator Modulate the signal I [n] in phase and Q [n] in quadrature.

5.

Signal resulting from the QASK Tx modulation [n].

6.

Radio Frequency Modulator

7.

Antenna.

8.

Antenna.

9.

Demodulator of radiofrequency

10.

Signal resulting from demodulation in  radiofrequency Rx [n].

eleven.

QASK demodulator. Results r_ {i} [n] and r_ {Q} [n].

12.

Signals resulting from demodulation QASK r_ {I} [n] and r_ {Q} [n].

13.

Decoder with a couple of sequences  Golay complementary and obtaining d_ {R} [n].

14.

Symbol rate sampling for the extraction of the coefficients of h_ {F} [n].

fifteen.

Equalization "postcursor".

16.

Equalization "precursor".

17.

Iteration, if necessary, to the block 16.

18.

Equalized final result.

Figure 4.- Shows the diagram of the block that  solve the system of equations and get the coefficients of pre-equalizer filter It is composed of a basic structure formed by a register, change of sign and divisor that is repeated regularly based on the order of the system of equations. The data captured "precursor" serially invested in the time d_ {R} [- n]. For the lower output they are calculated, at each clock cycle, pre-equalizer filter coefficients h_ {F} [n].

Preferred Embodiment of the Invention

Below is a possible implementation of this technique applied to the equalization of a communication system that uses OTDM as a modulation technique of data.

For clarity, in figure 3 appears Schematized implementation. This implementation is based on the Application of this method to radio frequency systems.

For a better understanding, start from the diagram  of blocks of figure 3, which show:

one.

Signal digital to emit s [n]: to estimate the medium the ideal is a Krönecker delta.

2.

Encoder with a pair of sequences Golay complementary.

3.

Signals resulting from coding I [n] and Q [n].

Four.

QASK modulator Modulate the signal I [n] in phase and Q [n] in quadrature.

5.

Signal resulting from the QASK Tx modulation [n].

6.

Radio Frequency Modulator

7.

Antenna.

8.

Antenna.

9.

Demodulator of radiofrequency

10.

Signal resulting from demodulation in  radiofrequency Rx [n].

eleven.

QASK demodulator. Results r_ {I} [n] and r_ {Q} [n].

12.

Signals resulting from demodulation QASK r_ {I} [n] and r_ {Q} [n].

13.

Decoder with a couple of sequences  Golay complementary and obtaining d_ {R} [n].

14.

Symbol rate sampling for the extraction of the coefficients of h_ {F} [n].

fifteen.

Equalization "postcursor".

16.

Equalization "precursor".

17.

Iteration, if necessary, to the block 16.

18.

Equalized final result.

To simplify the explanation, recourse has been made to the particular case of pairs of complementary sequences Golay modulated in QASK (Quadrature Amplitude Shift Keying). The system It consists of two distinct blocks: the transmission system and the reception.

The transmission system is responsible for:

?

Convolve for any convolution method the input signal with each of the two sequences that form the pair of complementary sequences of the following features

-

any length L.

-

be emit using any symbol width, T, with any amplitude and with any level of oversampling.

-

be broadcast in parallel with other sets of sequences complementary or not to the previous ones.

-

be emit simultaneously using a frequency modulation, phase or amplitude, or combinations thereof.

and generate the equalization preamble.

?

Encode one or more deltas of Krönecker with identical or different amplitudes and any temporal and frequency combination in order to implement in the device.

?

Modulate the two signals in QASK resulting from coding with the sequences complementary.

?

Modulate the quadrature signal via QASK (Quadrature Amplitude Shift Keying) or QAM (Quadrature Amplitude Modulation) to transmit it on the corresponding area of the radio spectrum.

?

Transmit it through the medium using any type of transducer or antenna.

System Reception takes care of:

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Synchronize with the transmitter and demodulate the signal received by the antenna.

?

Get the components r_ {I} [n], in phase, and r_ {Q} [n], in quadrature, by QASK demodulation.

?

Detect the beginning of equalization preamble and perform the decoding process through sums of correlations, as presented in this document.

?

Get the correlation or adapted filtering, using any method, of the signals received at the decoder input.

?

The sum of the results of the resulting correlations to obtain the characteristics medium.

?

Storage in a memory of Said result for further processing.

?

Get the sum of the coefficients of the initial equalizer filter (DISTORTION).

?

Perform the steps of "Post-update" and "Pre-update" described.

?

The introduction of the data recovered, after the post-qualification phase, invested in the time, to a bank of N records, depending on the order of the system of equations to solve.

?

The cascading of the records of data that make up the record bank and its setting to initial zero.

?

The use of records auxiliaries to store the N-2 coefficients obtained in the successive cycles of the process, and its zeroing initial.

?

The realization at each cycle of clock, k, of the sum of all products corresponding to the values calculated in the previous cycle, h_ {E} [k-1], found in the auxiliary records, for the data d_ {R} [- k + 1], which found within the data records, changed sign, thus obtaining the value of the coefficient h_ {E} [k].

?

Storage, at each cycle clock, of the calculated results h_ {E} [k], in the corresponding auxiliary records, to be used in the next cycle

?

The first record connection, record bank entry, with a multiplexer that allows control the change of sign of the signal with which it processes the system described.

?

Repeat iteratively the previous process until obtaining an equalized signal within the limits set by the system, which will correspond to a Krönecker delta in the ideal case.

The resulting filters together form the reverse filter of the distortion suffered by the data in the process of transmission, propagation and reception.

Claims (10)

1. Device and method of equalization of the distortion of the transmission channel in time-multiplexing communication systems by means of a set of complementary sequences, characterized in that the device, emitter-receiver, allows to emit signals through a physical means comprising the set generation of complementary sequences whose main property is that the sum of the autocorrelations \ phi_ {II}, of the sequences that form the set is a Krönecker delta. 2. Device and method of equalization of the transmission channel distortion in time multiplexing communication systems by means of a set of complementary sequences, characterized by the first claim and also because the complementary sequences used have the following characteristics:

?

Any length L.

?

They are issued using any symbol width, T, with any amplitude and with any oversampling level.

?

They are issued in parallel with others sets of sequences complementary or not to the previous.

?

They are issued simultaneously using a frequency, phase or amplitude modulation, or combinations of these.

3. Device and method of equalization of the transmission channel distortion in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because the complementary sequences are emitted and received, after propagating through of the medium, using any type of transducer or antenna. 4. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because the method is based on the extraction of information obtained through the transmission of a delta of Krönecker encoded by complementary sequences transmitted simultaneously to a physical medium that, when received in a receiver, it obtains data of the distortion produced and which are used to design the equalizer filters that compensate for said distortion. 5. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because the complementary sequences are used to transmit signals to a medium with the in order to obtain an impulse response h [n] or response in frequency H (ome). 6. Device and method for equalizing the transmission channel distortion in time multiplexing communication systems by means of a set of complementary sequences, characterized by claims 1 and 5 and also because the method encodes one or more Krönecker deltas with identical or different amplitudes and any temporal and frequency combination in order to implement in the device. 7. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because the method for generating the coding with sequences complementary to the object of implementation includes at least:

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The convolution, using any method of the input signal with each of the complementary sequences that make up the set.

?

The emission of the signals resulting from the convolution.

8. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because the method for obtaining the coefficients of the temporal or frequency distortion, by decoding with complementary sequences it comprises at least:

?

The correlation or filtering adapted, using any method, of the signals received to the decoder input with each of the sequences complementary that make up the set used in the issue.

?

The sum of the results of the  resulting correlations to obtain the characteristics medium.

?

Storage in a memory of Said result for further processing.

9. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by the preceding claims and also because to equalize the received data it comprises at least:

?

The decoding of the claim 8, and its application as filter coefficients inverse "postqualizer" that corrects the distortion or inter-symbol interference (ISI) "postcursor" due to subsequent symbols at the moment of the decision.

?

The resolution of a system of equations and their application as inverse filter coefficients "pre-equalizer" that corrects distortion or interference between symbols (ISI) "precursor" due to the symbols previous to the instant of the decision.

?

The application iteratively, is say as many times as necessary, of the two sections previous until you get an improvement in equalization within certain limits defined by the system.

10. Device and method of equalization of the distortion of the transmission channel in time multiplexing communication systems by means of a set of complementary sequences, characterized by claim 9 and also because to solve the system of equations set forth in the previous claim and obtain The coefficients of the "pre-equalizer" filter are needed at least:

?

The introduction of the data recovered, after the post-qualification phase, invested in the time, to a bank of N records, depending on the order of the system of equations to solve.

?

The cascading of the records of data that make up the record bank and its setting to initial zero.

?

The use of records auxiliaries to store the N-2 coefficients obtained in the successive cycles of the process, and its zeroing initial.

?

The realization at each cycle of clock, k, of the sum of all products corresponding to the values calculated in the previous cycle, h_ {E} [k-1], found in the auxiliary records, for the data d_ {R} [- k + 1], which found within the data records, changed sign, thus obtaining the value of the coefficient h_ {E} [k].

?

Storage, at each cycle clock, of the calculated results h_ {E} [k], in the corresponding auxiliary records, to be used in the next cycle

?

The first record connection, record bank entry, with a multiplexer that allows control the change of sign of the signal with which it processes the system described.