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(11) EP 2 704 388 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.: 05.03.2014 Bulletin 2014/10 H04L 27/26 (2006.01)

(21) Application number: 13305958.4

(22) Date of filing: 05.07.2013

(84) Designated Contracting States: (71) Applicant: ST-Ericsson SA AL AT BE BG CH CY CZ DE DK EE ES FI FR GB 1228 Plan-les-Ouates (CH) GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR (72) Inventor: Dhayni, Achraf Designated Extension States: 06220 VALLAURIS (FR) BA ME (74) Representative: Hirsch & Associés (30) Priority: 04.09.2012 EP 12306052 58, avenue Marceau 75008 Paris (FR)

(54) Reduction of peak-to-average ratio in ofdm systems

(57) An emitter for modulating and emitting an or- comprising: thogonal frequency division multiplexing signal through - Means for clipping said time symbols; a transmission channel (TC), comprising a frequency- to- - Time-to-frequency convertor for converting said time time converter for converting symbols to be transmitted symbols; and into time symbols, and means for serializing and ampli- - Means for applying a set of of the outputs fying said time symbol so as to emit it as an OFDM signal of said time-to-frequency converter as inputs of said fre- through said transmission channel, said emitter further quency-to-time converter EP 2 704 388 A1

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Description tages are not negligible like a reduced efficiency of the frequency amplifier. FIELD OF THE INVENTION [0008] Despite these negative aspects, OFDM re- mains very interesting when weighted with its advantag- [0001] The invention relates to the field of multicarrier 5 es, notably because of a much higher spectral efficiency. communications systems and more particularly to Or- [0009] According to the IEEE (Institute of Electrical and thogonal Frequency-Division Multiplexing (OFCM) sys- Electronics Engineers) 802.11a/g standards, the theo- tems including wireless OFDM systems. retical maximum PAR is around 17 dB. In other words, the peak amplitude excursion of an 802.11a/g- compliant BACKGROUND OF THE INVENTION 10 OFDM signal can be up to seven times larger than the average signal. [0002] Orthogonal Frequency-Division Multiplexing [0010] In order to prevent bad effects of a PAR at (OFDM), also referred to as "multi-carrier " around 17 dB (i.e. to prevent distortion and to be able to (MCM) or "Discrete Multi-Tone Modulation" (DMTM), reproduce the amplified output signal faithfully), the splits up and encodes high-speed incoming serial data, 15 transmitter would need to avoid any undesired clipping, modulating it over a plurality of different carrier frequen- even during the peak excursions of the signal. cies (called "subcarriers") within a communication chan- [0011] This requires the power amplifier to be designed nel to transmit the data from one user to another. The so as to have minimal compression at the peak power. serial information is broken up into a plurality of sub- sig- We can assume that a 1dB compression is acceptable nals that are transmitted simultaneously over the subcar- 20 at peak power. However, most of the time, the power riers in parallel. amplifier operates around the average amplitude (and [0003] By spacing the subcarriers frequencies at inter- not at the peak amplitude). This thus means that, most vals of the frequency of the symbol to transmit, the peak of the time, the amplifier operates at a power 17 dB lower power component of each modulated subcarriers line up than the 1 dB peak compression point, i.e. 7 times lower. exactly with zero power components of the other modu- 25 [0012] For instance, an inductively loaded class-A lated subcarriers, thereby providing orthogonality (inde- transmitter and power amplifier can achieve a maximum pendence and separability) of the individual subcarriers. power efficiency of 50% (achieved when transmitting the This allows a good spectral efficiency (close to optimal) maximum output swing). When it functions at 17 dB lower and minimal inter- channel interference (ICI), i.e. interfer- power below the 1 dB compression point, the best case ences between the subcarriers. 30 achieved efficiency would be only 50/7=7%. [0004] For these reasons, OFDM is used in many ap- [0013] Of course, a transmitter amplifier with a power plications. Many digital transmission systems have efficiency of 7% is not acceptable. adopted OFDM as the modulation technique such as dig- [0014] Some solutions try to improve the situation by ital video broadcasting terrestrial TV (DVB- T), digital au- optimizing the transmitter chain in order to achieve a bet- dio broadcasting (DAB), terrestrial integrated services 35 ter linearity and to obtain higher efficiency. The 1dB com- digital broadcasting (ISDB- T), digital subscriber line (xD- pression point of the amplifier is increased and the SL), WLAN systems, e.g. based on the IEEE 802.11 amount of backoff required to achieve a particular error standard, cable TV systems, etc. vector magnitude (EVM) is reduced. The backoff is de- [0005] An OFDM signal is a signal with varying ampli- fined as how much the signal level must be below the 1 tude envelop, i.e. which carry information both in the am- 40 dB compression point in order to reach a certain specified plitude and in the phase of the transmitted signal. In gen- EVM. eral, such a signal makes more complex the design of [0015] These solutions are however not sufficient as the transmitter according to the extent by which the am- they do not lead to efficient enough amplification. Fur- plitude varies. This extent is usually captured by the PAR thermore, they do not simplify the design of the transmit- parameter, defined as the peak-to-average power ratio. 45 ter chain and, on the contrary, generally make it worse. High PAR corresponds to modulation schemes with largely-varying amplitude envelop, whereas low PAR SUMMARY OF THE INVENTION corresponds to modulation scheme where the amplitude envelop varies to a small extent. [0016] This is achieved with a method for modulating [0006] High PAR modulation schemes are problematic 50 and emitting an orthogonal frequency division multiplex- to handle by transmission systems. For instance, in some ing signal through a transmission channel, comprising systems, high peaks may be clipped by non-linear de- performing a frequency- to-time conversion of symbols to vices at the transmitter sides, causing undesirable effects be transmitted to generate time symbols, and serializing such as high out-of-band activity ("regrowth") and in- and amplifying said time symbol so as to emit it as an band distortion. 55 OFDM signal through said transmission channel, said [0007] To prevent this phenomenon, the transmitter method further comprising design should be carefully adapted, especially the ana- log-to-digital converter (ADC), but still some disadvan- - clipping said time symbols;

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- performing a time-to-frequency conversion of said cy-to-time converters have a length of 128 bits. time symbols; and - clipping said time symbols consists in using the min- - applying a set of subcarriers of the result of said time- imum possible clipping level leading to the minimum to-frequency conversion as inputs of said frequency- possible peak-to-average power ratio. to-time conversion. 5 - said clipping level is -7dB. - The emitter is adapted to emit 802.11 standard- com- [0017] According to embodiments of the invention the pliant OFDM signal. method may comprise one or several of the following features, taken alone or in partial or full combinations: BRIEF DESCRIPTION OF THE DRAWINGS 10 - said set of subcarriers corresponds to data subcar- [0022] riers; - out-of-band zero subcarriers are applied as input of Fig. 1 depicts a spectral mask according to the frequency-to-time conversion, outside of the 802.11a/g standards and a typical OFDM signal. bandwidth of said data subcarriers; 15 Fig. 2 shows a schematic functional architecture of - said time-to-frequency conversion and said frequen- a OFDM emitter according to an embodiment of the cy-to-time conversion are performed with a 128-bit invention. algorithm; Fig. 3 shows a detail view of a part of the figure 2. - clipping said time symbols consists in using the min- Fig. 4a and 4b show comparison of schematic spec- imum possible clipping level leading to the minimum 20 tral shapes according to prior art solutions and to the possible peak-to-average power ratio. invention. - this clipping level is -7dB. DETAILED DESCRIPTION OF EMBODIMENTS OF [0018] Anotheraspect of theinvention relatesto a com- THE INVENTION puter program comprising program instructions and be- 25 ing loadable into a data-processing unit and adapted to [0023] The figure 1 depicts a spectral mask M accord- cause execution of the method according previously de- ing to 802.11a/g standards and a typical OFDM signal fined, when the computer program is run by the data S. The transmitted spectral density of the signal S should processing unit. fall below this spectral mask M. [0019] Another aspect of the invention relates to a data 30 [0024] The signal S is a combination of multiple sub- storage medium having recorded thereon the computer carriers signals separated by 312 kHz, and around a cen- program previously described. tral frequency Fc. A total of 52 subcarriers are used, 48 [0020] Another aspect of the invention relates to an of which are used for carrying data and 4 are so-called emitter for modulating and emitting an orthogonal fre- "pilot subcarriers". The numbers of the subcarriers are quency division multiplexing signal through a transmis- 35 depicted by figures on top of the mask M. In total, the sion channel, comprising a frequency-to-time converter bandwidth of transmitted signal is 52x312 kHz = 16.25 for converting symbols to be transmitted into time sym- MHz. bols, and means for serializing and amplifying said time [0025] The mask M aims at extracting this bandwidth symbol so as to emit it as an OFDM signal through said by rejecting the frequencies outside of it, i.e. upper the transmission channel, said emitter further comprising: 40 26th and below the -26th subcarrier, corre- sponding respectively to Fc+9 MHz and Fc-9 MHz. Ac- - Means for clipping said time symbols; cordingly, the mask drops to a -20dB value at Fc+11 MHz - Time-to-frequencyconvertor for convertingsaid time and Fc-11 MHz (this -20 dB value is a value relative to symbols; and the maximum spectral power density of the transmitted - Means for applying a set of subcarriers of the outputs 45 signal). of said time- to-frequency converter as inputs of said [0026] The spectral mask is relaxed within the interval frequency-to-time converter [Fc-20 MHz; Fc-11 MHz] and the interval [Fc+11 MHz; Fc+20 MHz] to tolerate the out- of-band spectral regrowth [0021] According to embodiments of the invention the of OFDM-coded signals. Beyond, Fc+20 MHz and Fc- 20 method may comprise one or several of the following 50 MHz, the mark M rejects more strongly the signal. features, taken alone or in partial or full combinations: [0027] As it has been explained above, the higher is the PAR, the more important is the spectral regrowth. - said set of subcarriers corresponds to data subcar- Accordingly, the maximum acceptable PAR at the input riers of the transmitter amplifier would be the one that does - out-of-band zero subcarriers are applied as input of 55 not lead to a spectral regrowth that violates the specified the frequency- to-time converter, outside of the band- spectral mask M of the transmitted signal S. As explained width of said data subcarriers. earlier, in the case of 802.11a/g standards, this corre- - said time-to-frequency converter and said frequen- sponds to a maximum acceptable PAR of 17 dB that

3 5 EP 2 704 388 A1 6 leads to a maximum class- A transmitter efficiency of only - A frequency domain, upstream 7%. - A time domain, downstream. [0028] One aspect of the invention consists in taking the problem according to a reverse approach. Instead of [0037] According to the invention, at the output of the optimizing the transmitter chain to improve the amplifier’s 5 IFFT unit 6, the high peaks of the time domain OFDM efficiency for a given PAR (e.g. the PAR specified by symbol are clipped in order to reduce the peak-to-aver- 802.11a/g standards), according to the invention, the age ratio PAR (sometimes also called "peak-to-average PAR is reduced as much as needed to achieve accept- power ratio", PAPR). able transmission amplifier power efficiency. [0038] According to an embodiment of the invention, [0029] In other words, the constraint for the PAR is the 10 the clipping unit 12 uses the minimum possible clipping linearity of the transmission chain (e.g. the power ampli- level that leads to the minimum possible PAR (at the input fier) and not (or not only) the spectral mask M. As a result, of the amplifier). In this way, the power efficiency reduc- greater amplification efficiency can be achieved tion (imposed by the PAR) is minimized as much as pos- [0030] The figure 2 depicts a high-level functional ar- sible. chitecture of an emitter according to an embodiment of 15 [0039] Taking the example of 802.11a/g standard, in the invention. order to achieve the highest system efficiency, it is com- [0031] The principle of the OFDM system consists in mon to operate the power amplifier in class AB mode and transmitting several symbols (or signal) in parallel by as- back-off by about 7 dB from the 1 dB compression point signing to each of them a different subcarrier, each sub- (instead of the 17 dB with respect to the spectral mask carrier being orthogonal to the others. The number of 20 as explained above). subcarriers depends on the total bandwidth and on the [0040] The stages prior to the amplifier are typically duration of the symbol. In WLAN 802.11 a/g implemen- operated at a larger back-off to make sure that the non- tation the number of subcarriers is equal to 52, as previ- linearity is not dominated by these stages. Since these ously mentioned. stages bum a relatively small power, this trade- off allows [0032] The symbols to be transmitted, initially in the 25 achieving the highest possible overall efficiency. frequency domain, are transposed into the time domain [0041] According to the above explanation, the maxi- and modulated for transmission over the transmission mum possible PAR must be limited to 7dB. It thus means channel TC. The receiver transposes the received signal that the clipping occurs at 17-7=10 dB lower than the back to the frequency domain to extract the transmitted maximum possible peak at the output of the IFFT unit 6. symbols. 30 It should be noted that the maximum possible peak oc- [0033] The signal So to be transmitted is inserted in a curs randomly and that its value is related to the modu- transmission chain made of several chained functional lation scheme. For instance, higher-order modulation units, namely an encoder unit 1, an interleaver unit 2, a schemes like 64-QAM or 256-QAM have higher peaks mapper unit 3, a pilot insertion unit 4, a serial- to-parallel than lower-order modulation schemes like BPSK or 4- transform unit 5, a frequency-to-time transform unit (or 35 QAM. Inverse Fourier Transform) 6, a parallel-to-serial trans- [0042] As a result, the power efficiency is three times form unit 7, a cyclic extension addition unit 8, a digital- better than with the state-of-the art techniques. to-analog converter (DAC) unit 9, a low- pass filter 10 and [0043] The clipped OFDM symbol, outputted by the an antenna 11. The antenna transmits a transmitted sig- clipping unit 12, is provided to a time- to-frequency trans- 40 nalS T (representative of the source signal S o). This trans- form unit 13. This unit is generally implemented by a Fast mitted signal ST is sent over a communication channel Fourier Transform FFT. to the antenna of an OFDM receiver. [0044] This FFT unit 13 can be an added circuit in the [0034] The transmitted OFDM symbol signal spectrum OFDM emitter. However, in the common situation where is the sum in the frequency domain of the orthogonal the emitter is coupled with a receiver (to form an OFDM subcarrier sinc functions that are superposed over each 45 transceiver), the FFT unit 13 can be the one of the re- other. The individual symbols can be independently mod- ceiving circuitry. As a result, the hardware circuitry of the ulated by using different types of tech- transceiver is let unchanged, and no additional chips are niques, like for instance QAM (Quadrature Amplitude required. Modulation) or PSK (Phase-Shift Keying). [0045] The result of the FFT unit 13 on the clipped [0035] This functional architecture can be compliant 50 OFDM signal corresponds to the original OFDM frequen- with the state-of-the-art functional architectures and the cy-domain signal plus the spectral regrowth on the out- above-mentioned functional unit will not be described of-band subcarriers. with full details here as the man skilled in the art is knowl- [0046] The spectral regrowth caused by the clipping is edgeable about them. then rejected by applying back only the data subcarriers [0036] The frequency-to-time transform unit 6 is gen- 55 as inputs to the IFFT unit 6. erally implemented by an inverse Fast Fourier Transform [0047] Keeping the example implementation of IFFT. This IFFT unit 6 splits the transmission chain into 802.11a/g standards depicted in fig. 1, the critical part of two subparts: the spectral regrowth is gathered at 16.25/2 MHz at the

4 7 EP 2 704 388 A1 8 left of the OFDM band and 16.25/2 at the right of the the spectral mask because it is getting clipped by the OFDM band. In other words, the spectral regrowth occu- power amplifier. pies a frequency band which is equal to the OFDM band [0063] On figure 4b, the PAR reduction from 17dB to (split into 2 subparts). 7dB implies that no clipping occurs at the power amplifier, [0048] In order to be able to reject the clipping spectral 5 so that the spectral mask is not violated. The spectral regrowth, the FFT unit 13 and the IFFT unit 6 should each efficiency is reduced by only 3dB, i.e. it is 2 times lower have a length of at least 52*2=104. As the length should instead of 7 times lower according to prior art solutions. be a power of 2, this number should be set to 128 (the [0064] Among other advantages, the invention can ap- higher power of 2 of 104). ply to any OFDM system. It is very simple to implement [0049] Compared to most-common implementation, 10 as it makes use of already- existing on-chip resources. It this embodiment of the invention requires then to shift does not add analog silicon overhead, and only limited from 64 FFT and IFFT algorithms to 128 FFT and IFFT and reasonable digital silicon overhead. It provides very algorithms. Extra hardware resources are however con- good performance: the PAR versus power amplification sidered as negligible and anyhow far less than any ex- efficiency and nonlinearity problem is completely solved. isting solution of the prior art. 15 [0065] Further, although this technique is aiming at [0050] Other embodiments can be deployed in con- solving the PAR versus amplification efficiency issue in nection with other standards and transmission technol- OFDM transmitters, it can also be used to reduce the ogies. spectral growth much lower than the specified spectral [0051] The figure 3 shows with greater details how the mask. The solution therefore improves the error vector FFT unit 13, the IFFT unit 6 and the clipping unit articulate 20 magnitude (EVM). together. [0066] The invention has been described with refer- [0052] The FFT unit and the clipping unit are depicted ence to preferred embodiments. However, many varia- bya singlefunctional block 14, performing both functions. tions are possible within the scope of the invention. [0053] This block 14 has 128 inputs in the frequency domain, corresponding to 128 subcarriers spaced by Δf. 25 These inputs are provided by the serial-to-parallel unit Claims (corresponding to reference 5 in figure 2, not depicted in this figure 3). 1. A method for modulating and emitting an orthogonal [0054] The inputs correspond to data subcarriers DC frequency division multiplexing signal through a (or in-band subcarrier) and out- of-band zero subcarriers 30 transmission channel (TC), comprising performing a ZC. There are 52 data subcarriers DC (including the pilot frequency-to-time conversion of symbols to be trans- subcarriers), ranging from -26xΔf to 26xΔf. mitted to generate time symbols, and serializing and [0055] The output in the time domain of the block 14 amplifying said time symbol so as to emit it as an is provided to a parallel-to-serial unit (corresponding to OFDM signal through said transmission channel, reference 7 in figure 2, not depicted in this figure 3) as 35 said method further comprising well as to the FFT unit 13. [0056] At the output of this FFT unit 13, the out- of-band - clipping said time symbols; subcarriers OC are not set to zero anymore but are im- - performing a time-to-frequency conversion of pacted by the spectral regrowth due to the clipping. said time symbols; and [0057] Some of the outputs are connected back as in- 40 - applying a set of subcarriers of the result of puts to the block 14. The looped backed outputs corre- said time-to-frequency conversion as inputs of spond to the inband subcarriers (or data subcarriers, in- said frequency-to-time conversion. cluding pilot subcarriers). [0058] However clipping spectral regrowth subcarriers 2. A method according to claim 1, wherein said set of are not connected to the out-of-band zero subcarriers. 45 subcarriers corresponds to data subcarriers In other words the remaining inputs (i.e. outside of the bandwidth of these data subcarriers) are provided with 3. A method according to claim 1 or 2, wherein out- of- out-of-band zero subcarriers ZC only, as explained band zero subcarriers (ZC) are applied as input of above. the frequency-to-time conversion, outside of the [0059] More concretely, zero subcarriers ZC are ap- 50 bandwidth of said data subcarriers. plied on the inputs [-64xΔf; -27xΔf] and [27xΔf; 64xΔf] of the block 14. 4. A method according to any of the previous claims, [0060] In this way, the spectral regrowth due to the wherein said time- to-frequency conversion and said clipping is rejected. frequency-to-time conversion are performed with a [0061] The figures 4a and 4b show a comparison of 55 128-bit algorithm. schematic spectral shapes according to prior art and ac- cording to the invention. 5. A method according to any of the previous claims, [0062] On figure 4a, the 17dB PAR signal is violating wherein clipping said time symbols consists in using

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the minimum possible clipping level leading to the minimum possible peak-to-average power ratio.

6. A method according to claim 5, wherein said clipping level is -7dB. 5

7. A computer program comprising program instruc- tions and being loadable into a data- processing unit and adapted to cause execution of the method ac- cording to any of claims 1 to 6, when the computer 10 program is run by the data processing unit.

8. A data storage medium having recorded thereon the computer program of claim 7. 15 9. An emitter for modulating and emitting an orthogonal frequency division multiplexing signal through a transmission channel (TC), comprising a frequency- to-time converter for converting symbols to be trans- mitted into time symbols, and means for serializing 20 and amplifying said time symbol so as to emit it as an OFDM signal through said transmission channel, said emitter further comprising:

- Means for clipping said time symbols; 25 - Time-to-frequency convertor for converting said time symbols; and - Means for applying a set of subcarriers of the outputs of said time-to-frequency converter as inputs of said frequency-to-time converter. 30

10. An emitter according to claim 9, wherein said set of subcarriers corresponds to data subcarriers

11. An emitter according to claim 9 or 10, wherein out- 35 of-band zero subcarriers (ZC) are applied as input of the frequency-to-time converter, outside of the bandwidth of said data subcarriers.

12. An emitter according to any of the claims 9 to 12, 40 wherein said time-to-frequency converter and said frequency-to-time converters have a length of 128 bits.

13. An emitter according to any of the claims 9 to 12, 45 wherein clipping said time symbols consists in using the minimum possible clipping level leading to the minimum possible peak-to-average power ratio.

14. An emitter according to claim 13, wherein said clip- 50 ping level is -7dB.

15. Anemitter according toany of claims 9 to 14,adapted to emit 802.11 standard-compliant OFDM signal. 55

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