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WO2011141048A1 - Détection non linéaire basée sur des constellations d'ordre supérieur - Google Patents

Détection non linéaire basée sur des constellations d'ordre supérieur Download PDF

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Publication number
WO2011141048A1
WO2011141048A1 PCT/EP2010/056371 EP2010056371W WO2011141048A1 WO 2011141048 A1 WO2011141048 A1 WO 2011141048A1 EP 2010056371 W EP2010056371 W EP 2010056371W WO 2011141048 A1 WO2011141048 A1 WO 2011141048A1
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WO
WIPO (PCT)
Prior art keywords
component
receiver
demodulated
estimated
transmitter
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/EP2010/056371
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English (en)
Inventor
Björn GÄVERT
Dan Weinholt
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Telefonaktiebolaget LM Ericsson AB
Original Assignee
Telefonaktiebolaget LM Ericsson AB
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Priority to PCT/EP2010/056371 priority Critical patent/WO2011141048A1/fr
Publication of WO2011141048A1 publication Critical patent/WO2011141048A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • 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
    • H04L27/36—Modulator circuits; Transmitter circuits
    • H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator
    • H04L27/367—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion
    • H04L27/368—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion adaptive predistortion
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
    • H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits
    • H03F1/3247—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits using feedback acting on predistortion circuits

Definitions

  • the present invention relates to a receiver for receiving digital communication, the receiver being arranged to estimate a transmitted signal, where the receiver comprises demodulator means for demodulating a received signal such that a first demodulated component and a second demodulated component are obtained.
  • the present invention also relates to a method in a receiver for obtaining a transfer function for at least one amplifier in a transmitter, the receiver and the transmitter being comprised in a digital wireless communication system, the method comprising the step of demodulating received mapped data signal symbols such that a first demodulated component and a second demodulated component are obtained.
  • the output signal is usually transmitted at relatively high output powers.
  • the transmitter comprises power amplifiers which have non-linear characteristics, which will degrade the performance of the system by means of primarily spectrum and in-band signal degradation. This means that special care must be taken when designing these power amplifiers.
  • Power amplifier (PA) non-linearity is a well known problem in most microwave radio equipment utilizing quadrature modulation methods like M- QAM, where M > 4.
  • a power amplifier is required to be linear, and is normally of a so-called class A type. This usually results in a power amplifier design with a relatively low efficiency, which in this context means that the output power is low or very low compared to the power consumption of the power amplifier.
  • High power consumption leads to added cost on both designing the power amplifier and cooling it. High power consumption and heat also leads to increased failure rate of the system.
  • predistorsion may be performed, for example by means of coupling a part of the output from the power amplifier or amplifiers in a transmitter to a detector, which detects the distortion that is inflicted by said power amplifier.
  • the detected distortion is used to update parameters in a transmit pre-distorter.
  • a direct feedback in the transmitter leads to higher system complexity. Increased system complexity usually leads to higher power consumption and cost.
  • the signal envelope amplitude exceeds the constellation peak power.
  • a problem is to connect which part of a detected error that is due to constellation point envelope and which is due to the signal envelope between the constellation points.
  • a traditional detector utilizes the error signal obtained from comparing the input and output from the detector. In this case, it becomes more difficult since the signal envelope amplitude exceeds the constellation peak power between the constellation points.
  • basic and simple constellation based detection can minimize the error within the peak power of the constellation.
  • the region between the peak power of the constellation and the peak power of the modulated signal is not easily handled and the error is not explicitly visible. This region can be referred to as a blind region. If for example polynomial or spline compensation is used, the lack of reliable detection in the blind region will result in faulty higher order compensation (higher order terms in polynomial compensation) and thereby severely distort the blind region. Detecting nonlinear distortion using only the constellation does not guarantee correct extrapolation in the blind region Table based compensations will suffer from the same problems, i.e. no reliable errors driving compensation in the blind region. This will affect the performance of the DPD and during bad conditions, when there is a significant contribution to the non linear distortion in the blind region, totally destroy the effect of the compensation. Thus, it is essential to control the behavior and the ability to detect non linear errors in the blind region.
  • the object of the present invention is to provide enhanced pre-distortion for a transmitter comprising at least one power amplifiers which, due to non-linear behaviour, inflicts distortion.
  • a receiver for receiving digital communication, the receiver being arranged to estimate a transmitted signal, where the receiver comprises demodulator means for demodulating a received signal such that a first demodulated component and a second demodulated component are obtained.
  • the receiver further comprises estimation means, the first demodulated component and a second demodulated component being arranged to be input into the estimation means, the estimation means further being arranged to estimate and output a first estimated transmitted component and a second estimated transmitted component, where the receiver further comprises first calculating means, the first demodulated component, the second demodulated component, the first estimated transmitted component and th e second estimated transmitted component being arranged to be input into the first calculating means, where the first calculating means is arranged to estimate and output a transfer function.
  • the receiver is intended to be a part of a digital communications system comprising a transmitter, where the transmitter comprises at least one amplifier which is arranged for amplifying a transmitted signal.
  • the transmitter comprises mapping means for mapping a data signal to a digital communication signal constellation in two dimensions comprising a number of symbols, where the mapped data signal has a first component and a second component.
  • the transmitter further comprises a first transmitter filter arranged for filtering the first component, and a second transmitter filter arranged for filtering the second component, and where said amplifier is arranged for amplifying the combined and modulated first and second components.
  • the transfer function is a transfer function related to the amplifier.
  • the receiver is arranged to receive the data signal having the first component and the second component.
  • the demodulator means is arranged for separating, demodulating and filtering the received mapped data signal symbols such that the first demodulated component and the second demodulated component are obtained.
  • the receiver further comprises a detector arranged for estimating th e first demodulated component and the second demodulated component such that an estimated mapped data signal having a first estimated component and a second estimated component, is obtained.
  • the receiver further comprises a first receiver filter arranged for filtering the first estimated component such that a first estimated transmitted component is obtained, and a second receiver filter arranged for filtering the second estimated component such that a second estimated transmitted component is obtained.
  • the detector, the first receiver filter and the second receiver filter are comprised in the estimation means.
  • the first receiver filter is equal to the first transmitter filter
  • the second receiver filter is equal to the second transmitter filter
  • the object of the present invention is also obtained by means of a method in a receiver for obtaining a transfer function for at least one amplifier in a transmitter, the receiver and the transmitter being comprised in a digital wireless communication system, the method comprising the steps of:
  • Figure 1 schematically shows a transmitter
  • Figure 2 schematically shows a receiver according to the present invention
  • Figure 3 schematically shows an example of an envelop in a 16QAM constellation
  • Figure 4 shows a flow chart of a method according to the present invention.
  • a transmitter 1 comprises mapping means 3 for mapping a data signal to a digital communication QAM, Quadrature Amplitude Modulation, signal constellation 4.
  • the mapped data signal has a first component I and a second component Q, where the transmitter 1 comprises a first transmitter filter 9 arranged for filtering the first component I, and a second transmitter filter 10 arranged for filtering the second component Q.
  • the filtered signal components I, Q are fed to a predistorsion unit 18 and further to a respective first mixer 19 and second mixer 20, and the modulated outputs are combined in an adder 21 .
  • the transmitter also comprises an amplifier 8, which may be constituted by a number of amplifiers, which is arranged for amplifying the combined and modulated first and second components I, Q.
  • the amplified signal is transmitted to a receiver 2, which will be described below with reference to Figure 2.
  • the receiver 2 comprises a first mixer 22 and second mixer 23, where the received signal is splitted into the modulated first and second components I, Q in a splitter 24 and fed to the respective mixer 22, 23.
  • the demodulated signals are fed to a respective first demodulator filter F1 and second demodulator filter F2, and further to a Digital Signal Processing block (DSP) 29.
  • the splitter 24, the first mixer 22 and second mixer 23, the demodulator filters F1 , F2, and the DSP 29 are in this example regarded as being comprised in a demodulator means 1 1 for separating, demodulating and filtering the received mapped data signal symbols such that the first demodulated component l ou t and the second demodulated component Q ou t are obtained.
  • the demodulator means 1 1 is thus arranged to output a first demodulated component l ou t and a second demodulated component Q ou t-
  • the receiver 2 further comprises a detector 12 arranged for estimating the first demodulated component l ou t and the second demodulated component Qout such that an estimated mapped data signal having a first estimated component 13 and a second estimated component 14 is obtained.
  • the detector 12 is comprised in estimation means 6, where the first demodulated component l ou t and the second demodulated component Q ou t are arranged to be input into the estimation means 6.
  • the estimation means 6 is further arranged to estimate and output a first estimated transmitted component ⁇ ⁇ and a second estimated transmitted component Q in .
  • the receiver 2 further comprises first calculating means 7, where t h e first demodulated component l ou t, t h e second demodulated component Q ou t, the first estimated transmitted component i in and the second estimated transmitted component Q in are arranged to be input into the first calculating means 7, the first calculating means 7 further being arranged to estimate and output a transfer function.
  • the first calculating means 7 comprises data regarding the transfer function of the demodulator means 1 1 .
  • a part of the first demodulated component l ou t and the second demodulated component Q ou t are coupled to the first calculating means 7 by means of corresponding first coupling means 25 and second coupling means 26.
  • the estimation means 6 further comprises a first receiver filter 15 arranged for filtering the first estimated component 13, such that the first estimated transmitted component i in is obtained, and a second receiver filter 16 arranged for filtering the second estimated component 14 such that the second estimated transmitted component Q in is obtained.
  • a part of the first estimated component 13 is coupled to the first receiver filter 15 by means of a first coupling means 27, and a part of the second estimated component 14 is coupled to the second receiver filter 16 by means of a second coupling means 28.
  • the parts of the first estimated component 13 and the second estimated component 14 which are not coupled to the receiver filters 15, 16 are fed to a de-mapper 30.
  • the first receiver filter 15 and the second receiver filter 16 are connected to the first calculating means 7, enabling the first estimated transmitted component i in and the second estimated transmitted component Q in to be input into the first calculating means 7.
  • the first receiver filter 15 is equal to the first transmitter filter 9
  • the second receiver filter 16 is equal to the second transmitter filter 10.
  • the transfer function is a transfer function related to the amplifier 8.
  • the first calculating means 7 comprises an envelope calculator 17, where furthermore the first calculating means 7 is in the form of a non-linear distortion detector arranged for calculating non-linear distortion as a function of envelope.
  • the transfer function comprises for amplitude and phase, between, on one hand, the first demodulated component l ou t and the second demodulated component Q ou t and , on the other hand, the first estimated transmitted component i in and the second estimated transmitted component Q in .
  • the first calculating means 7 is arranged to estimate the transfer function for the amplitude according to
  • the first calculating means 7 is further arranged to estimate the transfer function for the phase according to
  • Cartesian representation may be used such that
  • the transfer function constitutes predistortion information for distortion that occurs between the symbols in the signal constellation 4, and is fed back to the predistorsion unit 18 at the transmitter 1 .
  • the detector 12 is arranged for estimating the first demodulated component l ou t and the second demodulated component Q ou t by means of so- called hard decisions.
  • the present invention is a part of a feedback power amplifier (PA) predistortion concept, where the actual pre-distorter is situated on the transmit side and the detector is situated on the other side of the hop, the receiver side.
  • the distorting amplifier is the transmitter amplifier 8 described above.
  • a detector in a feedback pre-distortion architecture measures non linear distortion in the form of an error vector as a function of envelope or power.
  • the detected baseband signal containing residual errors from linear and non linear distortion, is the only source of information without any aid from a known transmitted pilot signal.
  • non linear distortion can only be seen as detected errors in the received signal space, and the power dynamics are limited to the one of the transmitted constellation.
  • the transmitted pulse-shaped signal has a larger dynamic range then t h e mapped constellation.
  • a PA will distort the mapped pulse shaped transmitted signal.
  • a traditional radio receiver re-constructs the transmitted constellation without ISI (Inter Symbol Interference) using matched filters and equalizers (adaptive filtering).
  • Adaptive filters converge to the inverse of the frequency response of the channel and the result will be a flat channel without any ISI in the received symbols. Any non linear error in the received signal will however also be filtered by the receive filtering, and the linear adaptive filters are unable to compensate for non-linear distortion.
  • the invention utilizes the ISI minimizing effect of the filter and equalizer.
  • Non linear distortion affects the complete pulse-shaped transmitted signal, by adding a continuous error vector to it. Due to the receiver filtering with the aim to minimize ISI, the complete error signal will be visible in the received symbols.
  • the detected errors are both a function of the envelope of the constellation points and the signal envelope between the constellation points.
  • the main problem is to connect which part of the detected error is due to constellation point envelope and which is due to the signal envelope between the constellation points. This can easily be done by re-creating the transmitted signal using the detected symbols using filtering.
  • the filter impulse response is the same as the one used in the transmitter/receiver baseband filtering.
  • the observed error can be split into parts connected to the correct transmitted envelope.
  • errors and corresponding envelopes much larger than the constellation can be observed, and a non linear error detector can be designed, driving a compensation of the transmitted pulse shaped signal.
  • the technique of detecting non linear errors for high envelopes is quite simple. Symbol patterns observed in the constellation can be used to predict high modulated peaks outside the constellation, in a blind region. This is performed by re-creating a fractional spaced (>2 samples per symbol) transmitted signal in the receiver using the detected symbols and an equivalent transmit filter. Signal peaks larger then the constellation will occur between two symbols and the corresponding distortion will be visible in the detected symbols after the receiver baseband filtering since the receiver filter impulse response is designed for zero ISI at symbol distance. Peaks outside the constellation, contributing with non linear distortion, can be predicted. The corresponding non linear distortion error can be calculated from the detected symbol errors and correlated with the correct transmitted envelope.
  • the non linear distortion can be considered as a continuous correlated error signal added to the modulated signal.
  • AD conversion there will be time discrete samples of a modulated signal and an error signal coming from non linear distortion. High peaks only occur in the transition between two symbols. If significant non-linear distortion is present, the samples coming from high peaks contains large errors from non linear distortion. When the sampled signal is filtered in the receive baseband filters, the impulse response from these large error signals will be present in resulting symbol samples.
  • the distortion of peaks outside the constellation becomes visible and easily measured after the receiver baseband filters. This is because of the fact that the receiver filter impulse response from the error signals is non zero in the symbol samples.
  • the error vectors in the symbol samples are strongly related to the error vector from modulated peaks situated between the symbols.
  • the transmitted peak envelope between two symbols can easily be re-created by filtering the detected symbols with a baseband filter using the same roll -off factor as in the transmitter with an oversampling of >2. Both the error and the corresponding envelope outside the constellation can therefore both be determined and can be used for driving an adaptive non linear compensation loop.
  • the present invention also relates to a method in a receiver 2 for obtaining a transfer function for at least one amplifier 8 in a transmitter 1 , the receiver and the transmitter being comprised in a digital wireless communication system, the method comprising the steps:
  • 102 estimating th e first demodulated component l ou t and the second demodulated component Q ou t such that a first estimated transmitted component i in and a second estimated transmitted component Q in are obtained; and 103: calculating a transfer function that enables the transmitted signal to be re-created by using the first estimated transmitted component i in and the second estimated transmitted component Q in .
  • the present invention is not limited to the examples above, but may vary freely within the scope of the claims. For example, either a table or a polynomial may be used for the predistorsion.
  • the receiver may be made in many different ways known to the skilled person, the shown arrangement is only one example of many.
  • the main elements are the demodulator means 1 1 , the estimation means 6 and the first calculating means 7, where these may have any suitable configuration.
  • the DSP 29 may be placed within the demodulator means 1 1 our outside the first calculating means 7.
  • the DSP 29 may output one sample per symbol or >1 sample per symbol.
  • the present invention relates to the receiver 2, although the receiver is intended to be used together with the transmitter 1 . It should nevertheless be understood that the present invention only lies in the receiver.
  • the present invention is applicable for any type of modulation that may be represented by means of an I component and a Q component, for example PSK, Phase Shift Keying.
  • the mixers are schematically shown to comprise an oscillator and a multiplicator. This is only an example of a mixer arrangement.

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

Abstract

La présente invention se rapporte à un récepteur (2) comprenant un moyen de démodulation (11) pour démoduler un signal reçu de telle sorte qu'un premier composant démodulé (lout) et un second composant démodulé (Qout) soient obtenus. Le récepteur comprend en outre un moyen d'estimation (6), le premier composant démodulé (lout) et un second composant démodulé (Qout) étant agencés pour être entrés dans le moyen d'estimation (6). Le moyen d'estimation (6) est en outre agencé pour estimer et transmettre un premier composant transmis estimé (îin) et un second composant transmis estimé (Qin). Le récepteur (2) comprend en outre un premier moyen de calcul (7), le premier composant démodulé (lout), le second composant démodulé (Qout), le premier composant transmis estimé (îin) et le second composant transmis estimé (Qin) étant agencés pour être entrés dans le premier moyen de calcul (7). Le premier moyen de calcul (7) est agencé pour estimer et transmettre une fonction de transfert. La présente invention se rapporte également à un procédé correspondant.
PCT/EP2010/056371 2010-05-10 2010-05-10 Détection non linéaire basée sur des constellations d'ordre supérieur Ceased WO2011141048A1 (fr)

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PCT/EP2010/056371 WO2011141048A1 (fr) 2010-05-10 2010-05-10 Détection non linéaire basée sur des constellations d'ordre supérieur

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/EP2010/056371 WO2011141048A1 (fr) 2010-05-10 2010-05-10 Détection non linéaire basée sur des constellations d'ordre supérieur

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1280272A1 (fr) * 2001-07-27 2003-01-29 Siemens Mobile Communications S.p.A. Procédé de linéarisation d'un étage de puissance d'un émetteur, système et récepteur correspondant
EP2117115A1 (fr) * 2008-05-06 2009-11-11 Nokia Siemens Networks S.p.A. Amélioration des procédés de contrôle à distance de pré-distorsion adaptative de signaux de transmission

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1280272A1 (fr) * 2001-07-27 2003-01-29 Siemens Mobile Communications S.p.A. Procédé de linéarisation d'un étage de puissance d'un émetteur, système et récepteur correspondant
EP2117115A1 (fr) * 2008-05-06 2009-11-11 Nokia Siemens Networks S.p.A. Amélioration des procédés de contrôle à distance de pré-distorsion adaptative de signaux de transmission

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