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WO2010138032A1 - Réduction de papr par sélection de tonalité - Google Patents

Réduction de papr par sélection de tonalité Download PDF

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Publication number
WO2010138032A1
WO2010138032A1 PCT/SE2009/050597 SE2009050597W WO2010138032A1 WO 2010138032 A1 WO2010138032 A1 WO 2010138032A1 SE 2009050597 W SE2009050597 W SE 2009050597W WO 2010138032 A1 WO2010138032 A1 WO 2010138032A1
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WO
WIPO (PCT)
Prior art keywords
signal
tone
peak
phase shift
amplitude
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/SE2009/050597
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English (en)
Inventor
Robert Baldemair
Ali Behravan
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
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Telefonaktiebolaget LM Ericsson AB filed Critical Telefonaktiebolaget LM Ericsson AB
Priority to PCT/SE2009/050597 priority Critical patent/WO2010138032A1/fr
Publication of WO2010138032A1 publication Critical patent/WO2010138032A1/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
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2614—Peak power aspects
    • H04L27/2618—Reduction thereof using auxiliary subcarriers

Definitions

  • the invention relates to a method and an arrangement in a communication node, in particular to enable the reduction of the peak to average power ratio of transmitted signals using correction signals .
  • PAPR peak-to-average power ratio
  • OFDM Orthogonal Frequency Division Multiplexing
  • LTE Long Term Evolution
  • WiMAX Worldwide Interoperability for microwave access
  • DAB Digital Audio Broadcasting
  • DVB Digital Video Broadcasting
  • wired data communication standards such as DMT (Discrete MultiTone modulation)
  • the simplest and most commonly used method to reduce the PAPR is clipping a signal 100 at a certain power level in the baseband section, which is illustrated in figure 1.
  • clipping reduces the signal dynamics, i.e. the PAPR, but also causes an increase in the EVM (Error Vector Magnitude) and the ACPR (Adjacent Channel Power Ratio), i.e. causes an uncontrolled in-band and out-of-band distortion.
  • the received signal must satisfy a certain EVM requirement.
  • sharp filtering is needed in order to satisfy the spectrum masks, i.e. to keep the signal within allowed limits concerning frequency and power. Filters with fast decaying in the transition bands introduce a delay, which may be unacceptably long for many systems . Filtering further consumes part of the cyclic prefix, which is used in OFDM to avoid inter-symbol interference. Thereby, the part of the cyclic prefix which is available for delay spread of the propagation channel is reduced.
  • SLM Select Mapping
  • PTS Partial Transmit Sequence
  • PAPR-reduction based on TR either requires solving a complicated optimisation problem, or finding all the correcting subcarriers at once, which may cause peak regeneration .
  • a method in a communication node for enabling reduction of peaks in transmitted signals.
  • a plurality of discrete time samples is obtained by sampling a signal, which is to be transmitted.
  • One of the samples is then selected to represent a peak in the sampled signal, which it is desirable to reduce.
  • a tone frequency is selected from amongst a set of tone frequencies, which are available for use as correction tones, and an amplitude is determined for the selected tone frequency, such that the determined amplitude satisfies a predefined power limit.
  • a phase shift for the selected tone is also determined, such that the selected tone with the determined phase shift will reduce the peak in the signal, when they are used for creating a correction signal.
  • the correction signal is created based on the selected tone, the determined amplitude and the determined phase shift. Then, a comparison is performed, where the maximum magnitude of the original signal comprising the peak is compared to the maximum magnitude of the combined signal which is formed when the correction signal is added to the signal comprising the peak. If the maximum magnitude of the original signal is the largest, a new "original" signal is set to equal the combined signal, and the selected tone frequency is removed from the set of available tone frequencies. If the compared signal magnitudes are equal or the maximum magnitude of the combined signal is the largest, which means that a new peak has been created, the created correction signal is not used and the selected tone frequency is removed from the set of available tone frequencies. As long as a predetermined stop criterion is not fulfilled, the method will be iterated from the operation of selecting a sample representing a peak. If the predetermined stop criterion is fulfilled, the procedure will end with the last combined signal as output.
  • an arrangement in a communication node is adapted to enable the reduction of a peak of a transmit signal.
  • the arrangement comprises a sampling unit, which is adapted to obtain a plurality of discrete time samples by sampling a signal, which is to be transmitted.
  • the arrangement further comprises a peak finding unit, which is adapted to select one of the samples, which represents a peak in the sampled signal.
  • the arrangement further comprises a frequency selecting unit, which is adapted to determine which tone frequency to select from a set of tone frequencies, which are available for use as correction tones.
  • the arrangement further comprises an amplitude determining unit, which is adapted for determining an amplitude level for the selected tone k that satisfies a predefined power limit, and a phase shift determining unit, which is adapted to determine a phase shift for a selected tone, so that the selected tone with the determined phase shift will, when used for creating a correction signal, reduce the peak in the signal.
  • the arrangement further comprises a correction signal creating unit, which is adapted to create a correction signal based on the selected tone, the determined phase shift and the determined amplitude.
  • the arrangement further comprises a signal comparison unit, which is adapted to compare the maximum magnitude of the original signal comprising the peak to the maximum magnitude of the combined signal which is formed when the correction signal is added to the signal comprising the peak.
  • the arrangement further comprises a signal setting unit, which is adapted to set a new "original" signal to equal the combined signal if the maximum magnitude of the original signal is the largest.
  • the arrangement further comprises a tone removing unit, which is adapted to remove the selected tone frequency from the set of available tone frequencies when the new signal has been set or when the compared signal magnitudes are equal or the maximum magnitude of the combined signal is the largest, which means that a new peak has been created.
  • the arrangement further comprises an iteration unit, which is adapted to repeat the method from the operation of selecting a sample representing a peak, if a predefined stop criterion is not fulfilled, and otherwise to output the last combined signal.
  • the proposed peak-reducing procedure is mainly targeted at OFDM signals, but it can be used for any kind of signal, i.e. for single carrier signals as well as for multicarrier signals, as no property of multicarrier signals is used to find the correcting tones.
  • the procedure can be used in single-standard radio systems or in multi- standard radio systems, in which the signal comprising a peak to be reduced is generated by a radio access technology such as GSM, WCDMA, LTE, or WiMAX or any combination of these radio access technologies .
  • the proposed procedure may provide optimisation of both tone frequency and phase shift, which allows for the best PAPR- reduction possible with any potential constraints applied to the frequency of the tones or their phase shift.
  • the procedure has low computational complexity. For example, no FFT operation is needed in finding the correction tones.
  • the optimum frequency of the tone can be found by a simple comparison of only R real values, and in case of a constraint applied to the frequency of the tone, the optimum phase is found using a simple formula.
  • the PAPR of the resulting signal samples can be checked for each correcting tone candidate. This control results in that the tones which are found to create new peaks are discarded, wherefore no peak regeneration will occur.
  • the algorithm may be implemented only at the transmitter side of a communication link, while the receiver side does not need to be changed in any way
  • - Figure 1 illustrates a signal of which the peaks have been reduced by clipping according to prior art.
  • - Figure 2 is a flow chart illustrating a procedure for peak- reduction according to one example embodiment.
  • - Figure 3 illustrates a transmit signal.
  • - Figure 4 illustrates a multi -standard system, in which the described embodiments may be used.
  • - Figure 5 illustrates allowed areas for correction tones according to one example embodiment .
  • - Figure 6 illustrates allowed areas for constellation extension according to one example embodiment.
  • - Figure 7 is a table containing simulation parameters for one example embodiment .
  • FIG. 11 is a schematic view illustrating an arrangement in a communication node according to one example embodiment.
  • the invention can be used to enable reduction of the PAPR of a signal.
  • the PAPR can be reduced by using a procedure with a relatively low computational complexity, which only needs to be implemented in the transmitter.
  • the procedure does not require any side information to be sent to the receiver, nor does the receiver need to have any "knowledge" of the procedure. Further, the procedure does not regenerate peaks.
  • Figure 3 illustrates a signal s 300 with a high PAPR.
  • the peak 302 illustrates a peak with a high power magnitude, which may be desirable to reduce in order to improve the amplifier efficiency or reduce distortion.
  • the signal may for example have been generated in a wireless multicarrier system, such as an LTE system; a wired multi-carrier system, such as a DMT system, or in a multi-standard system. If the signal has been generated in a multi- standard system, it may be a combination of signals generated by different multi-carrier or single- carrier radio access technologies (RATs) .
  • RATs single- carrier radio access technologies
  • the different RATs can be any multi- carrier or single-carrier RATs, such as WiMAX, LTE, WCDMA, MC- CDMA, CDMA, SC-FDMA, GSM, etc.
  • the baseband signals from different RATs are created and upsampled to a common sampling rate.
  • the signals are then combined, and in order to reduce the PAPR of the combined signal, correction tones are added at the common radio part .
  • tone will be used as indicating a signal that is describes as A k Qxp(j2 ⁇ f k t+ ⁇ k ) , where A k is the amplitude of the tone, f k is the frequency of the tone, and ⁇ . is the phase (or phase shift) of the tone.
  • This signal is normally considered within a limited time interval, O ⁇ t ⁇ T. Also since the operations are done in a discrete-time domain, a tone can
  • tone index is also used to refer to any of these indices in the discrete frequency domain.
  • the nth sample of the signal is described by
  • the nth sample of the correcting signal can then be defined as:
  • C k A k exp (j ⁇ k ) is the complex-valued correcting symbol.
  • the phase shift ⁇ k can take any value in the range of [0,2 ⁇ ] .
  • the amplitude A k is determined/specified by the maximum allowed power for the kth correction tone, which is specified by the EVM for the in-band tones, see figure 5, and by the spectral mask for the out-of-band tones. Thereby, no further optimisation of the amplitude A k needs to be done.
  • the amplitude A k could optionally be set to some smaller value, within the allowed power limit.
  • the operation of determining the amplitude A k may also involve to set the amplitude A k to a predetermined value or to verify that an amplitude A k , which was set during a previous iteration, is valid.
  • PAPR-reduction may be achieved by finding a correction signal c[n], such that
  • the correcting symbols C k A k exp ij ⁇ k ) , which causes the greatest possible reduction of the PAPR of the signal s, should be found.
  • the sample s [n 0 ] having the largest instantaneous power, is identified and selected to represent the peak.
  • other samples could be selected to represent the peak, e.g. samples in a limited interval placed around the sample (s) having the largest instantaneous power, or samples depending on this sample in some other way.
  • the frequency and the phase shift of the tone which minimises the resulting instantaneous power at n 0 should be determined.
  • the optimal tone index k * and the optimal phase shift ⁇ k * where "optimal” implies "the one that minimises the resulting instantaneous power at n 0 " , can be determined mathematically by solving
  • the frequency of the tone or tone index k and the corresponding phase shift ⁇ k may be selected from the following predefined sets of frequencies and phase shifts
  • the values which the tone frequency or tone index k can take are restricted to a certain, predefined set.
  • tone-reservation where only certain tone frequencies are available for PAPR-reduction use.
  • tone-inj ection with a given target EVM the set of available tone indices is restricted, while there is no restriction on the phase shift of each tone.
  • the possible correcting vectors i.e. the correcting tones which are allowed in order to satisfy the target EVM requirement, are illustrated as small areas around the constellation points of a 16QAM constellation.
  • the target EVM is illustrated as the dashed circumferences around the areas. It can also be seen from the figure that for a given tone index k, the phase shift ⁇ jc of the correcting tones can take any value in the interval [0,2 ⁇ ] .
  • the optimum phase shift for tone index k can be determined such that the correction signal c [n] has an opposite phase relative to the signal s [n] at n 0 ,
  • the values which the phase shift of the tones can take are restricted by a predefined condition.
  • a predefined condition is active constellation extension for OFDM symbols, where the phase shift ⁇ k has to be within a confined set or range. This will be described further below.
  • the optimum tone is the tone that has the opposite phase relative to the signal phase at n 0 .
  • FIG. 6 illustrates some possible phase values of the correcting vector A k exp(j' ⁇ ) in a 16 QAM constellation when active constellation extension is used for OFDM symbols.
  • the possible phase values for the correcting vector are shown as dashed lines and shaded areas.
  • the phase shift can only be O 1 while for tone number k+ ⁇ , the phase shift has to be - ⁇ /2 in order to maintain good performance. The reason for this is that by allowing other phase shifts, the constellation points would be moved closer to each other, which would make signal detection more difficult at the receiver, since a detected constellation point could thereby more easily be mistaken for an adjacent constellation point.
  • the PAPR of these new signal samples -? [n] may be lower than for the signal samples s [n] , although, it could also be the same or higher due to peak regeneration. This means that even if the large peak, which was identified in s [n] , is reduced in s [n] , a new, possibly even larger peak could have been created in another part of the signal s[n] .
  • a test could be performed, which determines whether i.e. it is determined if a new peak, being larger than the previously identified peak, has been created when the correction signal c [n] has been added to the signal s [n] .
  • the condition (10) above is not met, that indicates that such a new peak has been created.
  • FIG. 2 is a flow chart, which illustrates the procedure steps of one embodiment of using the above described peak- reducing procedure.
  • a signal s is sampled in step 200.
  • a sample representing a peak in the sampled signal is selected.
  • a frequency index k, a phase shift ⁇ k for the frequency index k, and an amplitude A k are determined in the steps 204-206. These determined values are then used in order to create a correction signal C n in step 210.
  • the steps 204 and 208 can be performed using different formulas depending on if there are restrictions associated with the tones or their phase shifts, or not. Further, the steps 204 and 208 can in some cases be performed in the opposite order.
  • the amplitude can also be determined in other positions than the one illustrated in figure 2.
  • the frequency k is removed from the set of available frequencies in step 216.
  • step 218 it is determined if a stop criterion is fulfilled or not.
  • the stop criterion could for example be one or more of: that all tones available for use as correction tones are used; that some maximum allowed complexity or delay is reached; or that the Peak to Average Power Ratio of the signal S n has a value which is below a predefined threshold. If the stop criterion is not fulfilled, the procedure is repeated from the operation of selecting a sample representing a peak. It is also possible to have an optional procedure step 220, which determines if it is the same peak which is to be reduced as during the preceding iteration.
  • step 204 the procedure steps could be repeated from step 204 instead of from step 202, since no new sample needs to be selected.
  • the procedure steps could also be repeated from step 204 based on the knowledge that a direct transition from step 212 to step 216 has been performed, i.e. when the signal S n has not been corrected. If the stop criterion is fulfilled, the procedure is completed and will be ended with the output S n .
  • One embodiment of the peak-reducing procedure presented above has been applied in a simulated PAPR-reduction in an LTE-DL with a bandwidth of 10 MHz.
  • the correcting tones are placed in the guard band.
  • the bandwidth of the signal is 9 MHz, and a set of R tones are placed on the two sides of the signal bandwidth.
  • TR tone-reservation
  • Figure 8 shows a CCDF (Complementary Cumulative Distribution Function) of the PAPR of a signal, with and without correcting tones, for the simulated system with the parameters described in figure 7.
  • the PAPR of the signal is reduced by almost 2 dB at a PAPR probability of 10 "3 , when using a single resource block, i.e. 12 subcarriers, on each side of the signal bandwidth as correcting tones.
  • Figure 9 shows a CCDF of the instantaneous power of a signal with and without the correcting tones, for the simulated system with the parameters described in figure 7.
  • the correcting tones are placed at the edge of the signal bandwidth, and thereby, the gain in PAPR-reduction comes at the price of extra correcting tone power at the edge of the signal bandwidth.
  • High power at the edge of the bandwidth is undesired, since it can cause a high out-of-band radiation.
  • the power of the correcting tones can be controlled in the above described peak-reducing algorithm by choosing a proper value for A k in equation (9) . In the simulation, the correcting tones and the data tones are assumed to have equal power.
  • Figure 10 shows the PSD (Power Spectral Density) of the signal with and without correcting tones, for the simulated system with the parameters described in figure 7.
  • PSD Power Spectral Density
  • Figure 11 illustrates an arrangement 1100, which is adapted to enable use of one embodiment of the peak-reducing procedure, and which comprises a set of logical units.
  • a sampling unit 1102 samples a signal s.
  • a peak finding unit 1104 selects a sample representing a peak, which is desirable to reduce.
  • a frequency selecting unit 1106 determines which tone frequency k to select amongst a set of possible tones, in order to reduce the peak.
  • An amplitude determining unit 1108 determines an amplitude A k for the tone k, which amplitude should be kept within certain power limits.
  • a phase shift determining unit 1110 determines a phase shift of the selected tone k, so that the tone k with the phase ⁇ k will reduce the peak in the sampled signal S n , when used for creating a correction signal.
  • the frequency selection and the phase shift determination could also in some cases be performed in the opposite order.
  • the amplitude determination may also be performed in other positions than the one illustrated in figure 11.
  • a correction signal c n is created in the correction signal creating unit 1112, based on the determined frequency k, amplitude A k and phase shift ⁇ k .
  • a signal comparison unit 1114 determines if the correction signal C n regenerates peaks when added to the signal S n .
  • a signal setting unit 1116 sets a new signal to equal the corrected signal. Thereafter, the selected tone k is removed from the set of possible tones by a tone removing unit 1118.
  • the iteration unit 1120 determines whether a stop criterion is fulfilled or not, and either repeats the procedure from step 1104, or ends the procedure with the output S n .
  • the procedure could optionally be repeated from step 1106 if it is the same peak which is to be reduced as during the last iteration, or, if the phase shift should be determined before the tone k, the procedure could be repeated from the step 1110, which then would be placed before step 1106.
  • figure 11 merely illustrates various functional units in the arrangement 1100 in a logical sense. However, the skilled person is free to implement these functions in practice using any suitable software and hardware means. Thus, the invention is generally not limited to the shown structure of the arrangement 1100.
  • the performance improvement accomplished by applying the described peak-reducing procedure depends on the number of correction tones as well as their position.
  • the procedure is potentially more effective when using spread correcting tones as compared to when using localised tones, since adjacent tones have substantially the same characteristics, which could be unsuitable for peak-reduction.
  • the tones are considered at the edge of the band, as mentioned previously.
  • the optimal frequencies and phase shifts of a set of tones used for PAPR-reduction of a signal i.e. the frequencies and phase shifts which maximises the PAPR-reduction, can be found using the described peak-reducing procedure.
  • the peak-reducing procedure is mainly targeted at OFDM signals, as in the example above, but it can be used for any kind of signal, i.e. for single carrier signals as well as for multicarrier signals, as no property of multicarrier signals is used to find the correcting tones.
  • the proposed procedure may provide optimisation of both tone frequency and phase shift, which allows for the best PAPR-reduction possible with any potential constraints applied to the frequency of the tones or their phase shift.
  • the procedure has low computational complexity. For example, no FFT operation is needed in finding the correction tones.
  • the optimum frequency of the tone can be found by a simple comparison of only R real values, and in case of a constraint applied to the frequency of the tone, the optimum phase is found using a simple formula.
  • the PAPR of the resulting signal samples can be checked for each correcting tone candidate. This control results in that the tones which are found to create new peaks are discarded, wherefore no peak regeneration will occur.
  • the procedure may be implemented only at the transmitter side of a communication link, while the receiver side does not need to be changed in any way.
  • the described procedure can be applied to find the correcting tones in both TR and TI methods.
  • standards such as LTE, where there are no free tones available within the signal bandwidth, TI within the signal bandwidth or TR outside the signal bandwidth can be used.
  • WiMAX there are extra tones available within the bandwidth of the signal, and therefore, both TR and TI can be used across the entire frequency axis, as long as the spectrum mask is not violated.
  • the node(s) in which the described procedure could be applied could for example be an eNB, a repeater node, a terminal node, in a single- or multi-standard radio system, or the corresponding nodes in a wired system.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
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Abstract

L'invention porte sur un procédé et un agencement (1100) dans un nœud de communication pour permettre une réduction de crêtes (302) dans des signaux émis. Dans le procédé, un échantillon représentant une crête dans un signal est sélectionné (202), et une fréquence, une amplitude et un déphasage sont déterminés (204-208) et sont utilisés pour créer (210) un signal de correction qui est additionné (214) au signal initial s'il est constaté (212) qu'il réduit la crête. La procédure est répétée à partir de l'opération de sélection d'un échantillon représentant une crête tant qu'un critère d'arrêt n'est pas satisfait (218).
PCT/SE2009/050597 2009-05-26 2009-05-26 Réduction de papr par sélection de tonalité Ceased WO2010138032A1 (fr)

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PCT/SE2009/050597 WO2010138032A1 (fr) 2009-05-26 2009-05-26 Réduction de papr par sélection de tonalité

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PCT/SE2009/050597 WO2010138032A1 (fr) 2009-05-26 2009-05-26 Réduction de papr par sélection de tonalité

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102075484A (zh) * 2011-01-18 2011-05-25 中兴通讯股份有限公司 一种降低信号峰均比的方法和装置
US8817900B2 (en) 2012-04-10 2014-08-26 Telefonaktiebolaget L M Ericsson (Publ) Frequency-domain peak power reduction
WO2014155024A1 (fr) * 2013-03-29 2014-10-02 Orange Procede de transmission d'un signal multiporteuse, dispositif de transmission et programme d'ordinateur correspondants
WO2014188143A1 (fr) * 2013-05-24 2014-11-27 Orange Procede de transmission d'un signal multi porteuse avec reduction du papr du signal émis
US20150271001A1 (en) * 2012-10-05 2015-09-24 Orange Method for the transmission of a multi-carrier signal, and corresponding transmission device and computer program
WO2019027341A1 (fr) * 2017-08-04 2019-02-07 Autonomous Non-Profit Organization For Higher Education "Skolkovo Institute Of Science And Technology" Réservation de tonalité sélective pour la réduction de papr dans des systèmes de communications sans fil

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WO2008005600A2 (fr) * 2006-06-30 2008-01-10 Motorola, Inc. Système et procédé d'écrêtage de pics de puissance

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Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102075484A (zh) * 2011-01-18 2011-05-25 中兴通讯股份有限公司 一种降低信号峰均比的方法和装置
US8817900B2 (en) 2012-04-10 2014-08-26 Telefonaktiebolaget L M Ericsson (Publ) Frequency-domain peak power reduction
WO2014167370A1 (fr) * 2012-04-10 2014-10-16 Telefonaktiebolaget L M Ericsson (Publ) Réduction de la puissance crête dans le domaine fréquentiel
US20150271001A1 (en) * 2012-10-05 2015-09-24 Orange Method for the transmission of a multi-carrier signal, and corresponding transmission device and computer program
US9455858B2 (en) * 2012-10-05 2016-09-27 Orange Method for the transmission of a multi-carrier signal, and corresponding transmission device and computer program
WO2014155024A1 (fr) * 2013-03-29 2014-10-02 Orange Procede de transmission d'un signal multiporteuse, dispositif de transmission et programme d'ordinateur correspondants
FR3004040A1 (fr) * 2013-03-29 2014-10-03 France Telecom Procede de transmission d'un signal multiporteuse, dispositif de transmission et programme d'ordinateur correspondants
WO2014188143A1 (fr) * 2013-05-24 2014-11-27 Orange Procede de transmission d'un signal multi porteuse avec reduction du papr du signal émis
WO2019027341A1 (fr) * 2017-08-04 2019-02-07 Autonomous Non-Profit Organization For Higher Education "Skolkovo Institute Of Science And Technology" Réservation de tonalité sélective pour la réduction de papr dans des systèmes de communications sans fil

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