(12) United States Patent
`Cimini, Jr. et al.
`
`USOO6928O84B2
`US 6,928,084 B2
`Aug. 9, 2005
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54)
`
`(75)
`
`(73)
`(*)
`
`(21)
`(22)
`(65)
`
`(60)
`
`(51)
`(52)
`(58)
`
`(56)
`
`OFDM COMMUNICATION SYSTEMAND
`METHOD HAVING A REDUCED PEAK-TO
`AVERAGE POWER RATIO
`
`Inventors: Leonard Joseph Cimini, Jr., Howell,
`NJ (US); Nelson Ray Sollenberger,
`Farmingdale, NJ (US)
`Assignee: AT&T Corp., Bedminster, NY (US)
`Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 799 days.
`
`Appl. No.: 09/778,254
`Filed:
`Feb. 7, 2001
`Prior Publication Data
`
`US 2001/0036151 A1 Nov. 1, 2001
`Related U.S. Application Data
`Provisional application No. 60/192,708, filed on Mar. 28,
`2000.
`Int. Cl................................................. H04Q 11/02
`U.S. Cl. ..................
`... 370/430; 370/481; 375/222
`Field of Search ................................. 370/430, 481,
`370/203,342, 206–210,445, 480; 375/220-222,
`260-262,346
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`6,125,103 A * 9/2000
`6,175,551 B1 * 1/2001
`6,301.268 B1 * 10/2001
`6,314,146 B1 * 11/2001
`6,363,060 B1 * 3/2002
`6,501,747 B1 * 12/2002
`
`Bauml et al. ............... 370/203
`AWater et al. .............. 370/210
`Laroia et al. ............... 370/481
`Tellado et al. .............. 375/346
`Sarkar ........................ 370/342
`Friedlander et al. ........ 370/342
`
`OTHER PUBLICATIONS
`Muller et al., “A Novel Peak Power Reduction Scheme For
`OFDM”, pp 1090–1094, Sep. 1997.*
`Lewrey et al., “Peak To Average Power Ratio Reduction of
`OFDM Signals Using Peak Reduction Carriers”, Parsa
`22–25, August 1999.*
`Zekri et al., “DMT Signals with Low Peak-to-Average
`Power Ratio”, Parsa. 362-368, Jul. 1999.*
`Goeckel et al., “Increasing Diversity with Non-Standard
`Signal Sets in Wireless OFDM Systems”, Parsa. 20–24, Sep.
`1999.*
`U. Reimer; “Digital Video Broadcasting”, IEEE Commun.
`Mag., vol. 36, No. 6, Jun. 1998; pp. 104-110.
`L.J. Cimini, Jr., J.C. Chuang, and N.R. Sollenberger,
`“Advanced Cellular Internet Service, IEEE Commun.
`Mag., vol. 36, No. 10, Oct. 1998; pp. 150–159.
`R. O'Neill and L.N. Lopes, “Envelope Variations and Spec
`tral Splatter in Clipped Multicarrier Signals.” Proc. Of
`PIMRC '95; pp. 71-75.
`X. Li and L.J. Cimini, Jr., “Effects of Clipping and Filtering
`on the Perfomance of OFDM, IEEE Commun. Letts., vol.
`2, No. 5, May 1998; pp. 131-133.
`(Continued)
`Primary Examiner John Pezzlo
`(57)
`ABSTRACT
`An OFDM system embeds sequence information in the
`transmitted Signal that reduces peak average power ratio
`(PAP) with minimal impact on the overall system efficiency.
`A marker is embedded onto the transmitted information that
`is used to identify the combining (inversion) sequence at the
`receiver. In one embodiment, Selected tones in a cluster are
`rotated when the corresponding phase factor rotates the
`cluster.
`
`10 Claims, 6 Drawing Sheets
`
`200
`
`204
`
`206
`
`X
`
`IFFT
`(LENGTH N)
`
`"..."
`
`S/P AND
`PARTITION
`SUBSYSTEM
`
`202
`
`DATA X
`SOURCE
`
`
`
`
`
`V
`
`PTIMIZER
`O
`
`208
`
`VWGoA EX1023
`U.S. Patent No. 8,467,366
`
`

`

`US 6,928,084 B2
`Page 2
`
`OTHER PUBLICATIONS
`
`A.E. Jones, TAZ Wilkinson, and S.K. Barton, “Block Cod
`ing Scheme for Reduction of Peak to Mean Envelope Power
`Ratio of Multicarrier Transmission Scheme.” Elec. Letts.,
`vol. 30, No. 25, Dec. 1994; pp. 2098–2099.
`R.D.J. van Nee, “OFDM Codes for Peak-to-Average Power
`Reduction and Error Correction,” Proc. Of Globecom 96;
`pp. 740–744.
`
`S.H. Muller and J.B. Huber, “OFDM with Reduced
`Peak-Average Power Ration by Optimum Combination of
`Partial Transmit Sequences,” Elec. Letts., vol. 33, No. 5,
`Feb. 1997; pp. 368–369.
`L.J.Cimini, Jr., and N.R. Sollenberger, “Peak-to-Average
`Power Ratio Reduction of an OFDM Signal Using Partial
`Transmit Sequences.” Proc. Of ICC 99. (Also, to appear
`IEEE Commun. Letts., Mar. 2000.) pp. 511–515.
`* cited by examiner
`
`

`

`U.S. Patent
`
`Aug. 9, 2005
`
`Sheet 1 of 6
`
`US 6,928,084 B2
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`FIG.
`
`1
`
`120
`
`118
`
`102
`
`104
`
`106
`
`108
`
`110
`
`114
`
` QoCo
` DAC
`al
`
` 112
`
`
`
`yuajeg°S'n
`
`S007‘6‘sny
`9JO|waYSg
`
`
`
`ADD CYCLIC
`PARALLEL
`oO
`PILOT
`CODING
`NUT
`10.
`EXTENSION
`INTERLEAVINGH->4
`MAPPING
`
`DATA INSERTION||PARALLEL
`SERIAL
`AND WINDOWING
`
`
`
`SERIAL|_|REMOVE cYCLIC
`
`wT
`CHANNEL
`PARALLEL
`PARALLEL
`EXTENSION
`DATA
`SERIAL
`
`CORRECTION
`
`130
`
`SYMBOLy
`TIMING
`
`FREQUENCY
`CORRECTION
`SIGNAL
`TIMING AND
`FREQUENCY
`SYNCHRONIZATION
`
`=
`
`RF RK
`
`ADC
`
`7A£80'876'9SA
`
`

`

`FIG. 2
`
`150
`
`AMP
`
`M
`
`N (W) FREQUENCY
`
`600 160b 160c 3.
`
`

`

`U.S. Patent
`
`Aug. 9, 2005
`
`Sheet 3 of 6
`
`US 6,928,084 B2
`
`FIG. 4
`
`202
`
`204
`
`
`
`206
`
`DATA X
`SOURCE
`
`S/P AND
`PARTITION
`SUBSYSTEM
`
`

`

`U.S. Patent
`
`Aug. 9, 2005
`
`Sheet 4 of 6
`
`US 6,928,084 B2
`
`FIG. 5
`
`SET PHASE FACTORS TO
`(bm= 1)
`
`COMPUTE PAP (PAP)
`OF COMBINED SIGNAL
`
`SET m
`
`INVERT bim (bm= -1)
`COMPUTE NEW PAP
`
`300
`
`502
`
`504
`
`306
`
`108
`
`310
`
`314
`
`NO
`
`bm
`
`NEW
`PAP LESS THAN
`PAP2
`YES
`RETAIN bm= -1 FOR
`FINAL SEQUENCE
`
`316
`
`
`
`
`
`
`
`
`
`
`
`
`
`m m+
`
`518
`
`

`

`U.S. Patent
`
`Aug. 9, 2005
`
`Sheet 5 of 6
`
`US 6,928,084 B2
`
`
`
`Pr (PAP × PAP CCDF
`
`O
`
`,005
`
`.00
`
`Mct 6
`ITERATIVE
`
`ME 16
`OPTIMUM
`
`

`

`U.S. Patent
`
`Aug. 9, 2005
`
`Sheet 6 of 6
`
`US 6,928,084 B2
`
`
`
`.5
`
`.
`
`WORD
`ERROR
`
`RATE 03
`
`N
`N
`
`AWGN CHANNEL
`N Y
`\, y N=256
`N \ u-16
`
`V
`
`.01
`
`005
`
`W
`
`\ \
`DECODING:
`SIMPLE
`- - - - - HAMMING \
`- - - EUCLIDEAN
`
`V
`
`

`

`US 6,928,084 B2
`
`1
`OFDM COMMUNICATION SYSTEMAND
`METHOD HAVING A REDUCED PEAK-TO
`AVERAGE POWER RATIO
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`This application claims the benefit of U.S. Provisional
`Application Ser. No. 60/192,708, filed on Mar. 28, 2000.
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH
`Not Applicable.
`
`FIELD OF THE INVENTION
`The present invention relates generally to communication
`Systems and, more particularly, to Orthogonal Frequency
`Division Multiplexing (OFDM) wireless communication
`Systems.
`
`15
`
`2
`mined number M of Statistically independent Sequences are
`generated from the Same information and the Sequence with
`the lowest PAP is chosen for transmission. However, this
`introduces additional complexity for providing improved
`PAP statistics for the OFDM signal. In addition, the receiver
`must have knowledge about the generation process of the
`transmitted OFDM signal in order to recover the informa
`tion. The Sequence information is sent as Side information,
`resulting in Some loSS of efficiency.
`It would, therefore, be desirable to provide an OFDM
`system having a reduced PAP with optimal efficiency.
`
`SUMMARY OF THE INVENTION
`The present invention provides an OFDM system that
`embeds PAP-reducing inversion Sequence information in the
`transmitted signal with no additional overhead. With this
`arrangement, the PAP ratio of the transmitted Signals is
`reduced with little or no impact on the overall System
`efficiency. While the invention is primarily shown and
`described in conjunction with an OFDM system, it is under
`stood that the invention is applicable to other Systems in
`which it is desirable to detect embedded Sequence informa
`tion.
`In one aspect of the invention, in an OFDM system a
`block of Symbols is partitioned into a predetermined number
`of clusters. A respective phase factor is generated for each
`cluster to form an inversion sequence that reduces the PAP
`of the transmitted Signals. A variety of techniques can be
`used to generate the inversion Sequence including Subopti
`mal iterative algorithms and optimum approximations,
`which can correspond to Walsh Sequences for example. The
`inversion Sequence is embedded onto the transmitted data by
`rotating Selected ones of the tones in a cluster based upon
`whether the cluster phase factor rotates the cluster. In one
`embodiment, binary phase factors, i.e., plus/minus one, are
`used. If the inversion Sequence does not rotate the cluster,
`i.e., the cluster phase factor is plus one, then none of the
`tones in the cluster are rotated. If the inversion Sequence
`does rotate the cluster, i.e., the cluster phase factor is minus
`one, then every other tone in the cluster is rotated by a
`predetermined amount, e.g., TL/4 radians.
`To detect the inversion Sequence, the receiver first
`removes the data modulation. A test Statistic is then gener
`ated for each cluster. The test Statistics can be used to make
`decisions in a variety of ways including quantizing the test
`Statistic and making independent decisions for each cluster,
`quantizing the test Statistics and decoding the entire
`Sequence by nearest Hamming distance, and decoding the
`Sequence by nearest Euclidean distance.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The invention will be more fully understood from the
`following detailed description taken in conjunction with the
`accompanying drawings, in which:
`FIG. 1 is a top level diagram of an OFDM system having
`reduced PAP in accordance with the present invention;
`FIG. 2 is a pictorial representation of OFDM Subcarriers
`that can be generated by the OFDM system of FIG. 1;
`FIG. 3 is a pictorial representation of orthogonal OFDM
`Subcarriers that can be generated by the OFDM system of
`FIG. 1;
`FIG. 4 is a Schematic representation of a portion of an
`OFDM system that embeds inversion sequence information
`in the transmitted Signals in accordance with the present
`invention;
`
`25
`
`BACKGROUND OF THE INVENTION
`Orthogonal frequency division multiplexing (OFDM)
`wireleSS communication Systems have desirable character
`istics for high-bit-rate transmission in a radio environment.
`For example, by dividing the total bandwidth into many
`narrow Subchannels, which are transmitted in parallel, the
`effects of multipath delay spread can be minimized. OFDM
`Systems have been adopted or proposed for Digital Audio
`Broadcasting, Digital Terrestrial Television Broadcasting,
`wireleSS LANs, and high-speed cellular data.
`One disadvantage of using OFDM techniques for wireless
`applications is the potentially large peak-to-average power
`ratio (PAP) characteristic of a multicarrier signal with a large
`number of subchannels. For example, a baseband OFDM
`35
`signal with N subchannels has a PAP equal to the number of
`Subchannels Squared divided by the number of Subchannels,
`i.e., PAP=N*/N=N. For N=256, the PAP-24 dB. When
`passed through a nonlinear device, Such as a transmit power
`amplifier, the Signal may Suffer Significant spectral Spreading
`and in-band distortion.
`Conventional solutions to reducing the PAP for OFDM
`Systems include using a linear amplifier and using a non
`linear amplifier while backing off the amplifier operating
`point. However, these approaches result in a significant
`power efficiency penalty.
`Another attempt to reduce the PAP includes deliberately
`clipping the OFDM signal before amplification to improve
`the PAP at the expense of Some performance degradation.
`Another technique uses nonlinear block coding, where the
`desired data Sequence is embedded in a larger Sequence and
`only a Subset of all the possible Sequences are used, Spe
`cifically those with low peak powers. Using this approach,
`a 3 dB PAP can be achieved with a relatively small band
`width penalty. However, to implement Such a coding
`Scheme, large look-up tables are required at the transmitter
`and the receiver, thereby limiting its usefulness to applica
`tions with a small number of subchannels. Progress has been
`made toward coding schemes that reduce the PAP and can be
`implemented in a Systematic form with Some error-correct
`ing capabilities. Nevertheless, these methods are difficult to
`extend to Systems with more than a few Subchannels and the
`coding gains are relatively Small for adequate levels of
`redundancy.
`Additional techniques for improving the Statistics of the
`PAP of an OFDM signal include selective mapping (SLM)
`and partial transmit sequence (PTS). In SLM, a predeter
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`

`US 6,928,084 B2
`
`3
`FIG. 5 is a flow diagram of an exemplary Sequence of
`steps for providing an optimal PAP for an OFDM system in
`accordance with the present invention;
`FIG. 6 is a graphical depiction showing the probability of
`error in detecting an inversion Sequence embedded in the 5
`transmitted Signals in accordance with the present invention;
`and
`FIG. 7 is a graphical depiction of detection performance
`for an OFDM system in accordance with the present inven
`tion.
`
`1O
`
`4
`rotated by respective phase factors b-b, which are embed
`ded into the transmitted data, as described below in detail.
`An optimizer subsystem 208 can facilitate selection of the
`phase factors that reduce the PAP ration of the transmitted
`OFDM signals.
`Initially, a block of N symbols {X, n=0,..., N-1} is
`formed with each symbol modulating one of a set of N
`Subcarriers, f, n=0, 1,..., N-1}. The N Subcarriers are
`chosen to be orthogonal, i.e., f=nAf, where Af=1/NT and T
`is the original symbol period, as shown in FIG. 3. The
`resulting Signal after D/A conversion can be expressed as Set
`forth below in Equation 1:
`
`W
`
`x(t) = X. Xejini, () # 1 # NT
`
`=0
`
`Eq. (1)
`
`The PAP of the transmitted signal from Equation (1) can be
`defined as shown in Equation (2) below:
`
`Eq. (2)
`
`To obtain the partial transmit sequence (PTS), the input
`data block is partitioned into disjoint Sub-blockS or clusters
`by Subsystem 204 which are combined to minimize the PAP.
`A data block is defined as {X, n=0, 1,...,N-1}, which can
`be represented as a vector X=XX . . . X", where T is
`the symbol period. The vector X is partitioned into a
`predetermined number M of disjoint sets represented by
`vectors {X, m=1,2,... M. The partial transmit sequence
`technique forms a weighted combination of M clusters as Set
`forth below in Equation 3:
`
`i
`
`X = Xbox,
`
`Ed. (3
`q. (3)
`
`where {b, m=1,2,..., M are phase or weighting factors,
`which can be pure rotations. In the time domain, this
`relationship can be represented as shown in Equation 4
`below:
`
`Eq. (4)
`
`The vector X, which is referred to as the partial transmit
`sequence, is the Inverse Fast Fourier Transform (IFFT) of
`vector X. The phase factors b, are chosen to minimize the
`PAP of x', as described below.
`The phase factors can be generated in a variety of ways to
`minimize the PAP of the transmitted OFDM signals, includ
`ing optimization, iteration, and random generation. For
`example, a predetermined number of Walsh Sequences can
`be generated.
`In one particular embodiment, the phase factors b, are
`binary phase factors, i.e., t1. In an alternative, more com
`plex embodiment, the phase factors include t1 and it. After
`the input data block is divided into a predetermined number
`M of clusters, the M N-point partial transmit Sequences are
`formed. For example, an OFDM system having 256 Subcar
`riers can include Sixteen (M=16) data clusters each having
`Sixteen Subcarriers.
`FIG. 5 shows an exemplary Sequence of Steps for deter
`mining binary phase factors for the partial transmit
`
`15
`
`25
`
`40
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The present invention provides a technique for achieving
`high-bit-rate wireleSS data transmission in an orthogonal
`frequency division muliplexing (OFDM) system with a
`relatively low peak-to-average power ratio (PAP). The sys
`tem utilizes partial transmit Sequences for providing favor
`able PAP statistics with combining sequence information
`embedded in the transmitted data. With this arrangement, no
`overhead is required to provide the combining Sequence
`information to the receiver.
`FIG. 1 shows an exemplary OFDM system 100 having
`Sequence data embedded in the transmitted data in accor
`dance with the present invention. The system 100 includes
`components for transmission and reception of data. A coding
`Subsystem 102 encodes binary data from a data source. The
`coded data is interleaved by an interleaving Subsystem 104
`and then mapped onto multi-amplitude multi-phase constel
`lation Symbols by a mapping Subsystem 106. In one par
`ticular embodiment, the multi-amplitude multi-phase con
`Stellation Symbols include quadrature phase shift keying
`(QPSK) symbols. Pilot signals can then be inserted by a pilot
`insertion Subsystem 108 to estimate the channel at the
`remote Subscriber unit receivers. A Serial-to-parallel conver
`sion Subsystem 110 converts the serial data stream to a
`parallel data Stream that is provided to an inverse fast fourier
`transform (IFFT) subsystem 112.
`The transformed data is converted to Serial data Stream by
`a parallel-to-Serial converter 114. Cyclic extension and win
`dowing can be added by a Subsystem 116 prior to digital
`to-analog conversion by a DAC 118 and transmission by an
`antenna 120 system. A receive portion 130 of the OFDM
`System includes corresponding components for extracting
`45
`the data from the received OFDM signal.
`As shown in FIG. 2, the OFDM system 100 utilizes an
`overlapping orthogonal multicarrier modulation technique
`having a plurality of Subcarriers 150. FIG. 3 shows the
`orthogonal relationship of the Subcarriers. More particularly,
`each of four Subcarriers 160a–160d of one OFDM data
`Symbol has an integral number of cycles in the interval T.
`The number of cycles between adjacent Subcarriers differs
`by one.
`FIG. 4 shows a portion of an OFDM system 200 that
`embeds PAP-reducing inversion Sequence information
`within the transmitted data with no overhead in accordance
`with the present invention. With this arrangement, the need
`to dedicate reference Subcarriers, e.g., one for each cluster,
`to transmit phase factor information is eliminated.
`The OFDM system 200 includes a data source 202
`generating a data Stream X that is converted from a Serial
`Stream to a plurality of parallel data streams X-X and
`partitioned by a subsystem 204 as described below. The
`partitioned data Streams are transformed by respective
`inverse fast Fourier transform systems 206-206, in a
`conventional manner The clusters of transformed data are
`
`50
`
`55
`
`60
`
`65
`
`

`

`S
`Sequence. In Step 300, the phase factors b, are Set to be one
`for all m and in step 302 the PAP (PAP) of the combined
`Signal with all phase factorS Set to one is computed. In Step
`304, the phase factor index m is set to 1.
`In step 306, the first phase factorb, is inverted, i.e., b=-1,
`and the PAP is re-computed with inverted phase factor in
`step 308. In step 310, is it determined whether the new PAP
`value is lower than the original PAP. If it is lower, then in
`Step 312 the first phase factor b remains minus one as part
`of the final phase sequence {b, m=1,..., M. If it is not
`lower, in Step 314 the first phase factor is reset to one, i.e.,
`b=1. In step 316, the index value M is examined and in step
`318 the index value is incremented until each phase factor is
`determined to be a one or a minus one.
`Alternatively, a predetermined number of random
`Sequences, which can be Walsh Sequences, are Selected. The
`information Sequence is multiplied by a predetermined num
`ber of the sequences. The result providing the best PAP
`characteristics is then Selected. This approach approximates
`an optimum PAP as described in L. J. Cimini. Jr. and N. R.
`Sollenberger, “Peak-to-Average Power Ration Reduction of
`an OFDM Signal Using Partial Transmit Sequences.” IEEE
`Commun. Letts., Vol. 4, No. 3, March 2000, pp. 390-393,
`which is incorporated herein by reference.
`FIG. 6 shows the PAP versus CCDF simulated results for
`an OFDM system having 256 subcarriers with the transmit
`ted signal oversampled by a factor of four. QPSK signal
`modulation is assumed with the energy normalized to unity.
`Results are shown for the case of a single OFDM block
`(M=1) and for 16 clusters (M=16) each including 16 Sub
`carriers. The unmodified OFDM signal has a PAP that
`exceeds 10.4 dB for less than 1% of the blocks. For the
`Suboptimal algorithm using 16 Walsh Sequences of length 16
`as the inversion Sequence, a value of about 8 dB is obtained.
`By using the PTS approach with the optimum binary phase
`sequence for combining, the 1% PAP reduces to 6.8 dB.
`While a degradation of about 1 dB is encountered using the
`Suboptimal approach, the optimization proceSS has been
`reduced to 16 Sets of 16 additions, a Significant Savings as
`compared to finding the optimum set of phase factors.
`To recover the data, the OFDM system receiver deter
`mines the inversion Sequence that was embedded in the
`transmitted Signals. In contrast to known Systems that Send
`inversion Sequencing information as explicit Side informa
`tion (via Subcarriers) at the expense of Some loss in
`efficiency, an OFDM system in accordance with the present
`invention embeds a marker onto the transmitted data that can
`be used to uniquely identify the inversion Sequence at the
`receiver. The detection of the inversion Sequence should be
`Sufficiently reliable So as not to have a significant effect on
`the overall System performance.
`As described above, the OFDM system embeds markers
`on the transmitted Signals. In an exemplary embodiment
`described above, if the inversion Sequence does not rotate
`the cluster, e.g., b=1, then no tones in the cluster are
`rotated. If the inversion Sequence rotates the cluster, e.g.,
`b=-1, then every other tone in that cluster is rotated by JL/4
`radians. This arrangement is equivalent to using two signal
`constellations for the data Symbols in a cluster: one for the
`unrotated clusters and another, rotated by JL/4, for the
`modified clusters. This algorithm puts an embedded marker
`on rotated clusters that can be reliably detected even in the
`presence of noise and multipath fading with a minimal
`impact on the overall System performance.
`To detect the inversion Sequence, the data modulation
`must first be removed. In an illustrative embodiment, the
`frequency Symbols are raised to the fourth power, which is
`
`6
`a Standard approach for removing QPSK modulation.
`Higher-order PSK modulations can be removed in a similar
`fashion, as is well known to one of ordinary skill in the art.
`With the modulation removed, the data symbols (in the
`frequency domain) can be differentially detected by
`computing, for each cluster, a test Statistic as Set forth below
`in Equation 5:
`
`Zn =
`
`N M-1 X (in ;...)"
`
`Eq. (5)
`
`where Y represents the jth tone in the mth cluster and *
`denotes conjugation. Thus, in the absence of noise, if cluster
`m was not altered by the inversion Sequence, then the mth
`test statistic Z is +(N/M-1). If b--1, then Z, is -(N/M-
`1). Therefore, a relatively simple binary detection Scheme
`can recover the inversion Sequence. The Summation over the
`tones in a cluster averages the noise and provides a signifi
`cant performance improvement.
`Given the decision Statistic in Equation 5, a variety of
`detection Schemes can be used. In one embodiment, the test
`Statistic is quantized to t1 and decisions are made indepen
`dently for each cluster. While this approach is relatively
`Simple, there is no Straightforward mechanism for correcting
`COS.
`In another embodiment, detection performance is
`improved by quantizing the individual test Statistics Z, and
`then decoding the entire Sequence to the nearest Sequence,
`e.g., Walsh Sequence. Specifically, the System first generates
`the Sequence {ReZ), m=1,2,..., M. and quantizes each
`component to +1 or -1. The system then chooses the Walsh
`Sequence of length M that is closest, in Hamming distance,
`to the resulting Sequence. This technique provides error
`correction Since the received Sequence is mapped into one of
`only M possible Walsh sequences.
`In a further embodiment that provides further perfor
`mance improvements, all of the information in the decision
`Statistics is retained. Therefore, one preferred Strategy is to
`compute the sequence {Z, m=1,2,..., M. and then choose
`the Walsh sequence of length M that is closest, in Euclidean
`distance, to the resulting Sequence.
`The performance of an OFDM system in accordance with
`the present invention was simulated. The simulated OFDM
`system includes 256 subcarriers (N=256), which are divided
`into 16 clusters (i.e., M=16), each having 16 Subcarriers.
`QPSK is used to modulate the tones. Performance is mea
`sured by the word error rate (WER), where word corre
`sponds to one OFDM block or, equivalently, the length of
`one inversion Sequence. Initially, results on the detection
`performance are based upon the probability that the inver
`Sion Sequence is received in error.
`FIG. 7 shows the probability of error in detecting the
`inversion Sequence as a function of the Signal-to-noise ratio
`(SNR) in an additive, white, Gaussian noise (AWGN) envi
`ronment. The simple cluster-by-cluster detection Scheme
`requires the highest SNR for a desired WER. By using
`minimum distance decoding, whether based on Hamming or
`Euclidean distances, Significant improvements are obtained.
`The benefit comes from the error correction that is possible
`with minimum distance decoding. The 16 Walsh Sequences
`of length 16 have a minimum distance of 8 and, as Such, can
`correct up to 4 errors. Using the Hamming (Euclidean)
`distance, a 1% WER can be achieved with an SNR of about
`3.2 dB (2.3 dB).
`The present invention provides an OFDM system that
`provides enhanced PAP statistics with minimal loss in
`
`US 6,928,084 B2
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`

`

`7
`efficiency. The System embeds combining or inversion
`Sequence information without additional overhead. In
`addition, the embedded inversion Sequence can be reliably
`detected by the receiver. Performance can be further
`improved by increasing the number of tones per cluster.
`One skilled in the art will appreciate further features and
`advantages of the invention based on the above-described
`embodiments. Accordingly, the invention is not to be limited
`by what has been particularly shown and described, except
`as indicated by the appended claims. All publications and
`references cited herein are expressly incorporated herein by
`reference in their entirety.
`What is claimed is:
`1. A method for reducing the PAP ratio in an OFDM
`System, comprising:
`15
`dividing a data block into a plurality of clusters,
`determining a respective phase factor for each of the
`plurality of clusters to form an inversion Sequence for
`reducing the PAP ratio of transmitted data correspond
`ing to the plurality of clusters,
`embedding the inversion Sequence onto the transmitted
`data;
`rotating at least one tone in a first one of the plurality of
`clusters when the corresponding phase factor rotates
`the first one of the plurality of clusters; and
`rotating every other tone in the first one of the plurality of
`clusters.
`2. A method for reducing the PAP ratio in an OFDM
`System, comprising:
`dividing a data block into a plurality of clusters,
`determining a respective phase factor for each of the
`plurality of clusters to form an inversion Sequence for
`reducing the PAP ratio of transmitted data correspond
`ing to the plurality of clusters,
`embedding the inversion Sequence onto the transmitted
`data;
`detecting the inversion Sequence, and
`computing a test Statistic for each of the plurality of
`clusters to determine the inversion Sequence.
`3. A method for reducing the PAP ratio in an OFDM
`System, comprising:
`dividing a data block into a plurality of clusters,
`determining a respective phase factor for each of the
`plurality of clusters to form an inversion Sequence for
`reducing the PAP ratio of transmitted data correspond
`ing to the plurality of clusters,
`embedding the inversion Sequence onto the transmitted
`data;
`detecting the inversion Sequence, and
`quantizing the test Statistics.
`4. The method according to claim 3, further including
`decoding the inversion Sequence to a nearest Walsh
`Sequence.
`
`25
`
`8
`5. The method according to claim 3, further including
`decoding the inversion Sequence to a nearest Walsh
`Sequence based upon Hamming distance.
`6. The method according to claim 3, further including
`decoding the inversion Sequence to a nearest Walsh
`Sequence based upon Euclidean distance.
`7. A method of embedding PAP-reducing inversion
`Sequences onto transmitted data, comprising:
`determining an initial PAP value for a block of symbols;
`partitioning the block of Symbols into a predetermined
`number of clusters,
`Selecting a respective phase factor for each of the clusters
`So as to form an inversion Sequence that reduces a PAP
`of transmitted data corresponding to the block of Sym
`bols;
`embedding the inversion Sequence onto the transmitted
`data by rotating Selected tones in each of the clusters
`based upon a value of the associated phase factor; and
`Selecting the inversion Sequence from predetermined
`Walsh Sequences.
`8. A method of embedding PAP-reducing inversion
`Sequences onto transmitted data, comprising:
`determining an initial PAP value for a block of symbols;
`partitioning the block of Symbols into a predetermined
`number of clusters,
`Selecting a respective phase factor for each of the clusters
`So as to form an inversion Sequence that reduces a PAP
`of transmitted data corresponding to the block of Sym
`bols;
`embedding the inversion sequence onto the transmitted
`data by rotating Selected tones in each of the clusters
`based upon a value of the associated phase factor; and
`rotating every other tone in each cluster having an asso
`ciated phase factor that rotates the cluster.
`9. A method of embedding PAP-reducing inversion
`Sequences onto transmitted data, comprising:
`determining an initial PAP value for a block of symbols;
`partitioning the block of Symbols into a predetermined
`number of clusters,
`Selecting a respective phase factor for each of the clusters
`So as to form an inversion Sequence that reduces a PAP
`of transmitted data corresponding to the block of Sym
`bols;
`embedding the inversion Sequence onto the transmitted
`data by rotating Selected tones in each of the clusters
`based upon a value of the associated chase factor;
`detecting the inversion Sequence; and
`computing a test Statistic for each cluster.
`10. The method according to claim 9, further including
`Selecting the inversion Sequence from a nearest one of
`predetermined Walsh Sequences.
`
`k
`
`k
`
`k
`
`k
`
`k
`
`US 6,928,084 B2
`
`35
`
`40
`
`45
`
`50
`
`55
`
`