United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,914,933
`
`Cimini et al.
`
`[45] Date of Patent:
`
`Jun. 22,1999
`
`USt'105914933A
`
`[54] CLUSTERED OFDM COMMUNICATION
`SYSTEM
`
`S,h?5,572
`5.681105
`
`....................... 370x206
`lUr‘lUlJ'r’ Hidejima el al.
`11.31997 Daffara et al.
`.......................... BTUIZIB
`
`[75]
`
`Inventors: Leunm'd Joseph Cimini, Howell. N..l.;
`Babak Daneshrad. Los Angeles, Calif.;
`Nelson Ray Sollenberger, Tlnlon Falls,
`NJ.
`
`[73] Assignee: Lucent Technologies Inc., Murray Hill.
`N-J‘
`
`[2]] Appl. No: 08,030,430
`_
`Filed:
`
`0°" 15’ 1996
`
`[22]
`
`OTHER PUBLICAHONS
`Casas et a]. “(JFDM for Data Communication Over Mobile
`Radio FM Channels" IEEE Transactions on Communica-
`lions. vol. 39. No. 5. pp. 783—?93. May 1991.
`Sakaku ra et al.. "Pm—Diversity using Coding, Mu lti—carriers
`and Multi—antennas". paper presented at 1995 Fourth lEEE
`International Conference on Universal Personal Communi-
`cations, pp. 605—609. (Tokyo, Japan, Nov. 1995).
`Leonard J. Cimini. Jr.. “Performance Studies for High—
`Speed Indoor Wireless Communications," Wireless Personal
`Communications 2: pp. (17—85 (1995).
`
`Related U.S. Application Data
`
`Primary Examiner—Chan Nguyen
`
`[60]
`
`Provisional applicalion No. otiffillxnfll. Mar. 8. 1996.
`
`[5?]
`
`ABSTRACT
`
`Int. CL“ ..................................................... H04L 27(26
`[51]
`[52] U.S. C]. .......................... 370908; 370,210; 370,343;
`3753299; 375547
`370MB 706
`Field of Search
`37(h’208. 491mg“) 482. 204 480. 343,
`484; 375961, 364. 344, 355: 347’ 299;
`4551,101404
`
`[58]
`
`[551
`
`References Cited
`
`US PATENT DOCUMENTS
`moms
`5:19th Koppelaar et al.
`..
`
`455,133
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`2,-"1997 Scki ct al. ............ 399,20!)
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`...................... 3701210
`3:199? Shelswcll ct at.
`
`5.415.757
`5501035
`5.548.532
`Fatal 12.835
`intoms
`
`A mullicarrier communication system [9" wireless transmis-
`sin" 0“ block“ 01' dam “3"ng *1 Plural“? of digital dam
`symbols in each block. The communication system includes
`a device for distributing the digital data symbols in each
`blocli over a plurality of clusters, each of the clusters
`receiving one or more‘digital data symbols. The digital data
`symbols are encoded In each ot the cluster; and modulated
`in each cluster to produce a signal capable of being trans-
`mitted over the sub-channels associated with each cluster. A
`transmitter thereafter transmits the modulated signal over
`the sub-channels. By distributing the modulated signal over
`" Plulalily "felumqs' “V°rall.p‘iak'l°'avmgc P‘mcl (PAP)
`ratio is reduced during transmission and transmitter diversuy
`is iFIItJl'm’I-‘d-
`
`42 Claims, 7 Drawing Sheets
`
`Ha
`
`
`13
`
`35
`
`N STlEULS
`[ll'EFlSMPLED
`
`
`
`come
`”I ,
`FDR PAP
`SHnPING-
`
`
`
`SEHIAL mu
`HEDUETIO“
`wan mm
`
`
`
`3.5 Hugs
`nu sums Ls
`
`
`.
`9' [Matt
`25
`25,
`253
`mmsnmtn
`
`“‘1
`
`19
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 1
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 1 of 7
`
`5,914,933
`
`FIG.
`
`1A
`
`{/10
`
`273
`
`
`
`
`
`ENEODER
`
`41M
`
`52M
`
`FIG. 13
`
`SINGLE
`RECEIVE
`ANTENNA
`
`40
`
`/'
`
`1E
`
`
`
`DEEDDEH
`
`55
`
`i3
`
`32
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 2
`
`

`

`US. Patent
`
`mJ
`
`9
`
`2w.
`
`n0w410v5
`
`
`
`%4,.mzszfima225$
`
`.EEEsigma;@238
`
`s
`
`m:mm
`
`n.mm
`
`
`m6E
`
`1m@532M:2é_.
`
`
`285m:2
`
`2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 3
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 3
`
`
`
`
`

`

`US. Patent
`
`Jun.22, 1999
`
`Sheet 3 0f 7
`
`5,914,933
`
`FIG. 3
`
`T0 IHAGINARY
`DFT TABLES:
`
`rst_fft_cntr
`FROM FIG.
`4
`
`
`T0 FIG. 3
`
`
`
`LOOP FILTER
`Hisi
`
`rsi_fit_cntr
`
`105
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 4
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 4
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 4 of 7
`
`5,914,933
`
`225$“:an
`
`magi
`
`zSZwEo
`
`
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 5
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 5
`
`

`

`US. Patent
`
`Jun.22, 1999
`
`Sheet 5 0f 7
`
`5,914,933
`
`FIG. 6
`
`1
`
`PHARGIN=10 ”3
`
`DUTAGE
`
`DUTABE
`
`.01
`
`.0001
`
`.001
`
`0
`
`5
`
`10
`
`15
`
`NUHBER 0F FREQUENCIES CORRECTED
`
`‘01
`
`.001
`
`.0001
`
`0
`
`5
`
`10
`
`15
`
`NUMBER OF FREQUENCIES CORRECTED
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 6
`
`

`

`US. Patent
`
`Jun.22, 1999
`
`Sheet 6 0f 7
`
`5,914,933
`
`FIG. 3
`
`DUTABE
`
`.01
`
`0
`
`5
`
`10
`
`15
`
`NUMBER OF FREDUENCIES CORRECTED
`
`FIG. 9
`
`RED. NUMBER
`OF CORRECTED
`
`FREQUENCIES
`
`0
`
`0.5
`
`1.0
`
`1.5
`
`2.0
`
`2.5
`
`3.0
`
`NDRNALIZED RHS DELAY SPREAD
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 7
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 7 0f 7
`
`5,914,933
`
`FIG. 10
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 8
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 8
`
`

`

`5,914,933
`
`1
`CLUSTERED OFDM COMMUNICATION
`SYSTEM
`
`RELATED APPLICATIONS
`
`This application claims the benefit of provisional US.
`patent application Ser. No. 60f011.601, filed Mar. 8, 1996.
`
`FIELD OF INVENTION
`
`The present invention relates generally to communication
`systems and more specifically, to a clustered multicarrier
`wireless communication system.
`
`DESCRIPTION OF THE PRIOR ART
`
`5
`
`ID
`
`15
`
`In a radio environment, multipath delay spread can
`severely limit the maximum transmission rate. Multicarrier
`transmission such as OFDM (orthogonal frequency division
`multiplexing) and single-carrier systems with equalization
`are often proposed as techniques for overcoming these
`limitations. However, both techniques present practical dif-
`ficulties which can restrict their application.
`the
`For instance.
`in a wireless LANEATM application.
`desire to transmit short packets requires fast start-up, espe-
`cially in a peer-to-peer architectures and this requirement
`could limit the usefulness of an equalized system, especially
`one using the LMS (“least mean square") algorithm to
`acquire the equalizer coetficients. because of the typically
`long convergence time of this algorithm. Algorithms which
`converge faster, such as RLS (“recursive least square”), may
`be too complex for applications requiring transmission rates
`of 20 Mb/s or more. On the other hand, a multicarrier
`transmission scheme has. the advantage of requiring very
`little training since equalization can usually be avoided.
`However, a multicarrier signal with a large number of
`sub-channels is burdened with a large peak-tu-average ‘
`power ratio. Hence. highly linear (and inetlicient) amplifiers
`must be used to avoid distortion and spectral spreading. In
`addition. with either approach,
`a
`technique which can
`exploit the potential of diversity without requiring multiple
`receivers is desirable.
`
`40
`
`45
`
`50
`
`00
`
`2
`The clustered multicarrier communication system
`includes a receiver for receiving and demodulating the
`modulated signal. The system further includes a device
`located at the receiver for measuring frequency response of
`each sub-channel of the plurality of clusters, and for pro-
`viding feedback representing a frequency response charac-
`teristic of each transmission sub-channel to the transmitter.
`
`The transmitter includes a device for switching each of the
`one or more symbols to be transmitted to an optimum
`transmission sub-channel according to the frequency
`response of the optitnum transmission sub-channel.
`The clustered multicarrier communication system further
`includes a device for synchronizing data to enable simulta-
`neous transmission of data over all the sub-channels of the
`plurality of clusters to efiect simultaneous transmission of
`data over an entire transmission bandwidth.
`
`The device for modulating the encoded one or more
`digital data symbols includes a device for providing OFDM
`modulation.
`
`The various features of novelty which characterize the
`invention are pointed out with particularity in the claims
`annexed to and forming a part of the disclosure. For a better
`understanding of the invention, iLs operating advantages, and
`specific objects attained by its use. reference should be had
`to the drawing and descriptive matter in which there are
`illustrated and described preferred embodiments of the
`invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. IA and 1B are general block diagrams illustrating
`respectively the transmitter and receiver portions of the
`improved OFDM communication system of the invention.
`FIG. 2 is a detailed block diagram illustrating one trans-
`mitter portion of the improved OFDM communication sys-
`tem of the invention.
`
`FIG. 3 is a schematic block diagram illustrating a single
`cluster of the transmitter.
`
`FIG. 4 is a schematic block diagram of a clock generation
`circuit for the transmitter.
`
`FIG. 5 is a detailed block diagram illustrating the receiver
`portion of the improved OFDM communication system of
`the invention.
`
`FIG. 6 is a diagram showing outages versus the number
`of frequencies which are being corrected in the decoder in
`the case of flat fading.
`FIG. 7 is a diagram showing outages versus the number
`of frequencies which are being corrected in the decoder in
`the case of a two-ray power delay profile.
`FIG. 8 is a diagram showing outages versus the number
`of frequencies which are being corrected in the decoder in
`the case of an exponential power delay profile.
`FIG. 9 is a diagram showing outages versus the number
`of frequencies which need to be corrected to achieve a 1%
`outage for a RMS delay spread.
`FIG. 10 showing frame synchronization across all clusters
`and utilizing the entire transmitted bandwidth.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`SUMMARY OF THE INVENTION
`
`The invention improves the performance of a wireless
`data communication system by employing a clustered
`approach to a multicarrier modulation technique.
`According to a preferred embodiment of the invention,
`there is provided a multicarrier communication system for
`wireless transmission ofblocks ofdata having a plurality of
`digital data symbols in each block, the system comprising:
`a device for distributing the plurality of digital data symbols
`in each block over a plurality of clusters, each of the
`plurality of clusters capable of receiving one or more digital
`data symbols; a device for encoding the one or more digital
`data symbols in each of the plurality of clusters; a device for
`modulating the encoded one or more digital data symbols in
`each cluster to produce a modulated signal capable of being
`transmitted over sub-channels associated with each respec-
`tive cluster; and a transmitter for transmitting the modulated
`signal over the sub-channels.
`Advantageously. by distributing the modulated signal
`over a plurality of clusters,
`the overall peak-to-average
`power (PAP) ratio is reduced during signal transmission and
`transmitter diversity is improved.
`A non-linear coding technique may be implemented by
`the encoding device to encode the one or more symbols in
`each cluster to reduce the peak-to-average power ratio.
`
`The clustered wireless communication system 10 includes
`a transmitter portion 13 shown conceptually in FIG. 1A, and
`.‘ a receiver portion 16 shown conceptually in FIG. 1B. In the
`transmitter portion, an input digital data stream of informa-
`tion 17 is input at a predetermined data rate, MT,
`to an
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 9
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 9
`
`

`

`5,914,933
`
`3
`encoder 21 to allow for errorferasure correction in the
`receiver portion 16. As shown in FIG. 1A. the encoder 21
`(and modulator) produces a multicarrier (or multitone) sig»
`nal 19 comprising a quantity ofNM symbols that is demul-
`tiplexed by demultiplexer circuit 26 to separate the serial
`encoded signal 19 into M blocks or clusters 27a. .
`.
`. 27M
`with each cluster being transmitted in parallel over a sepa-
`rate sub-channel. Preferably, each sub-channel is of narrow
`bandwidth for carrying N tones separated in frequency by
`UN'I'. For each cluster, Orthogonal Frequency Division
`Multiplexing (OFDM) is implemented by respective Fast
`Fourier Transform devices 41a. .
`.
`.
`. 41M for converting
`each digital stream into N tones for transmission over the
`respective sub-channel. In each of the M clusters,
`the N
`tones are carrier modulated and amplified by devices
`520, .
`. .
`. 52M for transmission over respective separate and
`ideally independent antennas 60a. .
`.
`.
`, 60M.
`At the receiver portion 16 shown in FIG. 1B, 3 single
`receive antenna 40 and demodulator devices 65 and 73 are
`
`JD
`
`15
`
`.
`
`4
`narrow bandwidth channel carrying N tones for transmission
`at a sub-channel symbol rate of UNT. Ideally. the bandwidth
`ofthe sub-channels will be narrow enough so there is no 181
`and that the only effect of multipath is flat fading in each
`sub-channel. As will be explained, the clustering of tones in
`this manner has several advantages. First.
`the peak-to-
`average power ("PAP") ratio is reduced by It} log(M);
`second. the size of the table needed for non-linear coding as
`will be explained below, is significantly reduced; and third,
`the transmission of difierent clusters on separate antennas
`results in independent fading on each cluster. With the use
`of error correction coding across all
`frequencies and a
`minimal amount of information from the receiver regarding
`the relative performance of the clusters,
`the clustering
`approach can result
`in an efi‘ective means for realizing
`transmit diversity.
`In the clustered OFDM communication system, the input
`digital data stream 25a.
`.
`.
`.
`, 25M carrying N tones for
`transmission over each respective M clusters 27o, .
`.
`. 27M
`is subject to Orthogonal Frequency Division Multiplexing
`(OFDM). As shown in cluster 27a of FIG. 2. the digital data
`stream 25:: is input to a serial-to-parallel converter 31 that
`produces a data word in parallel that is suitable for coding
`in the PAP reduction coder 35 for the purposes of PAP
`reduction. Preferably, the PAP reduction coder 35 imple-
`ments a non-linear code or mappirlg of signal 25:: based on
`PAP-ROM table lookup techniques which guarantees the
`PAP ratio reduction. As an example, a sequence ofseven (7)
`tones (14 bits each) may be mapped into eight 8 tones (16
`bits each) requiring 214 16-bit entries in the transmitter
`PAP-ROM 35. As seventy-five percent (75%) of the PAP-
`ROM table look-up entries may be avoided, in this instance.
`then those table entries forming large power peaks may be
`avoided.
`It should be understood that other mathematical
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 10
`
`coding and decoding techniques such as Complementary
`Golay sequences may be utilized instead of PAP reduction
`look-up tables. Having accomplished the non-linear
`mapping, the resulting non-linear coded signal 39 is then
`modulated onto the tones (subcarriers) by performing a
`Discrete Fourier Transform implemented by DFT element
`41, shown in cluster 27:? in FIG. 2. As will be explained
`herein, the receiver portion 16 of the OFDM communication
`system is capable of receiving pilot tunes from the trans-
`mitter to measure frequency response characteristics of
`particular transmission sub-channels. The receiver will pro—
`vide the transmitter portion with sub-channel frequency
`response information in the form of a feedback signal, to
`enable optimal switching of particular tones to a particular
`sub-channels (in a particular cluster) having a matching
`frequency response characteristic.
`The OFDM mu lticarrier signal generated in each of the M
`clusters is then multiplexed into serial form as shown by the
`para11el to serial converter 45, converted to analog form by
`DEA converter 4?, carrier modulated by RF mixer 52.
`amplified by RF amplifier 57. and transmitted over a sepa-
`rate and ideally independent antenna 60.
`FIG. 3 illustrates a schematic diagram of a non-limiting,
`example embodiment of a datapath for single transmit
`cluster 27a of the transmitter portion 13 of HG. 2, that is
`simple to implement with a minimization of hardware and
`complexity. The system operates at a maximum clock rate of
`10 MHz and requires three difl'erent clock signals which are
`related to each other through the implementation of PLU
`clock generation circuit such as shown in FIG. 4, described
`.‘ below. Although the components used in the embodiment
`were chosen to support data rates of up to 22 Mbps. it should
`be understood that higher speed versions of the transmit
`
`used for demodulating the OFDM signal using conventional
`techniques such as coherent or differential detection.
`Decoder 82 is used as the erasuret’error correction decoder.
`It is understood that
`the receiver portion may consist of
`multiple receivers, and. that the system 10 may be provided
`with full duplex transmission capability.
`The clustered OFDM communication technique is advan-
`tageous in that minimal or no training is required. enabling
`short packets to be accommodated more efficiently in peer-
`to-peer architectures. The usual benefit of using OFBM in a
`frequency-selective environment
`is that by dividing the
`transmitted bandwidth into many narrow sub-channels M
`which are transmitted in parallel, the efl'ects of delay spread
`are minimized, eliminating the need for an equalizer.
`Additionally.
`the peak-to-average power ratio is reduced
`since there are l'ewertones transmitted per transmitter result-
`ing in less spectral spreading when subjected to a non-
`linearity andt'or smaller required power amplifier hacked". or
`equivalently. better power efiiciency. The average power for
`an individual amplifier is also reduced as the amplifier
`requirements are distributed across the M clusters. The
`proposed architecture allows for a
`flexible, parallel
`implementation. with lower complexity than equalizer
`approaches.
`A more detailed block diagram of the transmitter portion
`13 of the improved clustered wireless communication sys-
`tem 10 is shown in FIG. 2. In FIG. 2. the input digital data
`stream 19 that has been previously encoded using conven-
`tional techniqUes, e.g.. Reed-Solomon coding. to allow for
`eventual errorierasure correction at the receiver. The input
`encoded digital data stream 19 is represented as a multitone
`signal comprising a quantity of NM symbols. each symbol
`having a period of '1', and a symbol rate UT which is usually
`limited by the amount of mu ltipath fading as experimentally
`realized in the particular environment. e.g.,
`indoor or
`outdoor. that the communication system is operating. It is .
`understood that the coding is provided across all frequencies
`of the multitone signal, with Reed-Solomon coding being an
`example coding technique. The encoded digital data stream
`19 is then input to a demultiplexer or equivalent circuit 26
`that separates the serial encoded signal 19 into M parallel
`signals, 25o. .
`.
`. , 25M, for transmission over separate M
`blocks or clusters 27a,
`.
`.
`.
`, 27M, with each cluster
`constituting a separate sub-channel. In the diagram shown in
`FIG. 2.
`the input digital data stream multitone signal 19
`comprises a quantity ofNM tones (e.g.. NM-ZS) for distri-
`bution over M clusters {'M=4), with each cluster 27o trans-
`mitting N tones (N=7). Each sub-channel 27:1, .
`.
`. 27M is a
`
`40
`
`45
`
`50
`
`00
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 10
`
`

`

`5,914,933
`
`5
`
`cluster can be realized depending upon the components and
`the printed circuit board fabrication technique used.
`In the non-limiting example embodiment of a single
`transmit cluster 27:? shown in FIG. 3, the serial data bit
`stream input
`to the cluster is at a rate, e.g., 1.875 MHz,
`which would imply that the digital data stream input to the
`transmitter demultiplexer 26 (FIG. 2) was at a bit rate equal
`to 7.5 Mbps for distribution to each of the four clusters
`(M-4) at the same rate (1.875 Mhpsicluster). In each cluster.
`the serial-to-parallel converter 31 produces a 12-bit word at
`a reduced rate of 156.25 kHz. In a conventional OFDM
`
`system, this 12-bit word could have been used to modulate
`6 complex tones, however. in the improved OFDM system.
`the 6 tones are coded (mapped) into 7 complex tones for
`purposes of PAP reduction. Anon-linear code is used for this
`purpose which guarantees the PAP ratio of the 7 tones to be
`no more than 3.2 dB. Due to in; non-linear nature, the PAP
`coding needs to be implemented via table lookup and in the
`embodiment shown in FIG. 3, a 4 le4 ROM based table
`35 and lookup technique is provided to implement
`the
`non-linear mapping. The 14-bit PAP-ROM output word 39
`represents the encoded complex symbols of a QPSK
`constellation. which modulate a cluster of seven (7) complex
`tones. It should be noted that the speed with which these PAP
`ROM tables are accessed is equal to the speed with which
`the OFDM blocks are generated and in the implementation
`shown in FIG. 3. this rate is 156.25 kHz.
`In OFDM, modulation onto the tones (subcarriers) is
`performed by way of a Discrete Fourier Transform imple-
`mented by (DFT). Given a desire to transmit M (M=4)
`clusters of N (N-7) tones each.
`the modulator on each
`section must realize the following equation (I):
`
`n
`
`art!
`
`I’m“ =
`
`_ Wind-Eel
`_
`txy. +;.r:,..1t4' ’
`1'!
`2
`
`55
`It = t1.
`m :0. I. 3. 3
`
`[It
`
`v.
`is the output
`where “m is the cluster number, Ymtk)
`sequence which is fed to the DEA for transmission and x,I
`represents the n”I bit of the 14-bit word appearing at
`the
`output of the PAP ROM 35. In the embodiment shown in
`FIG. 3, the even hits were assigned to the real part of the
`symbol and the odd bits were assigned to the imaginary part.
`Acloser look at equation (1) reveals that the output sequence
`consists of 56 complex sa mples. twice of what is required
`for a typical 28-point DFT. This is due to the desire to
`oversample the DFT output sequence by a factor of two.
`which requires the one-half (123) multiplier introduced into
`the exponential function in equation (I). The oversampling
`guarantees a separation of f.f2 between the baseband signal
`and the first
`image of the signal output
`from the DEA
`converter 47. The separation results in a significant relax-
`ation of the specification for the image canceling lowpass
`filters following the DEA converter 47.
`As the OFDM transmit block typically consists of an
`original N-point block (N=56 in the embodiment of FIG. 3),
`a cyclic prefix or extension block, and possibly a guard
`interval block, a total of eight (8) samples were allowed for
`the combination of the cyclic prefix and the guard interval
`and. in the particular implementation described, the contents
`of these eight samples may be freely chosen. Consequently.
`for every 14-bit word that appears at the output of the PAP
`ROMs 35 (FIG. 3), sixty-four (64) samples need to be read
`from the DFT ROMs 41 and presented to the DEA converter
`47. These 64 samples constitute a complete OFDM symbol
`(block). In the embodiment shown in FIG. 3,
`the cyclic
`
`ID
`
`15
`
`40
`
`45
`
`6
`prefixing, windowing, and the DFT operation was consoli-
`dated into a ROM lockup table 41 in order to avoid the use
`of elaborate and costly signal processing ICs and to provide
`a flexible mechanism in which the relative size of the cyclic
`prefix and guard intervals of the OFDM symbol can be
`varied. The implementation of the ex ample embodiment
`shown in FIG. 3. also enable 5 the user to realize any
`windowing function on the OFDM symbol.
`In the embodiment described, the DFT ROM 41 has a
`total oftwenty (20) input address bits. fourteen (14) from the
`PAP ROM 35 output signal 39 and six (6) bits for the output
`signal 43 of a 64-bit counter 46 that reads off the 64 samples
`of the OFDM symbol. This results in a total ROM address
`space of one million words and a ROM access speed equal
`to a preferred DEA converter rate of 10 MHz. In order to
`avoid the use of high-end memory modules when higher
`DKA data rates (e.g. 30 MHz) are implemented, the DFT
`lookup table 41 is partitioned into two DFT ROMS, 41a,b_.
`each for modulating Ni? complex tones. e.g., 3.5 tones in the
`embodiment shown in FIG. 3. Taking into account sampling
`quantization and the result ing in-band interference. an 8-bit
`representation was used for the DFT samples stored in each
`of the two DFT ROMS. The outputs ofthese ROMs 410 and
`41!) are then added together by adder element 43 in a
`programmable logic device 45 ("PLD") to realize the
`desired total of N tones for the channel, e.g., seven (7) tones.
`Thus, partitioning of the DFT task enables the replacement
`of a 1 Mbyte ROM with a pair of 8 kBer ROMS.
`To enable cluster switching, i.e.. to enable each transmit-
`ter cluster board to transmit tones over any one of the four
`clusters,
`the two 8 kbyte DFT ROMs 4In.4lb, may be
`replaced by two 32 kbyte ROMs having two additional
`address lines 42 that enable selection between one of the
`four ditferent clusters as shown in FIG. 3.
`As funher illustrated in the example embodiment of the
`transmit cluster shown in FIG. 3, a synch-word ROM 53 is
`provided that contains a synchronization word 56 that is sent
`at the beginning of each packet (a packet consists of many
`OFDM symboLs) to enable the receiver to identify the start
`of the incoming packet of OFDM symbols. In the embodi»
`ment of FIG. 3,
`the synchronization word is stored in a
`separate ROM 53 and its samples are sent
`to the DIA
`converter by way of the multiplexer device 56 built into PLD
`45.
`The operation of the datapaths described in connection
`with the transmit cluster 27a of FIG. 3 is governed by three
`clocks and a control signal. In the schematic diagram of the
`clock generation circuit 90 shown in FIG. 4, it is observed
`that the serial data coming into the cluster is clocked at a
`1.875 MHZ rate. that is a clock reference signal 91 is input
`at
`the rate that data is input
`to each cluster. The clock
`reference signal 91 is then be divided by twelve (12) to
`provide the clock signal 92 at, e.g.. 156.25 kHz. for timing
`operation of the serial-to-parallel converter 31, as. well as the
`PAP ROM 35 of transmit cluster 270. As shown in FIG. 4,
`a voltage controlled oscillator (VCXO) 95 centered at 10
`MHZ is used to generate the clock signal 93 for the WA
`converter. As described above. for each 14-bit word pro-
`duced by the PAP ROM 35, sixty-four (64) samples have to
`be read from the DFT ROMs 410,!) and processed through
`to the DEA converter 47 which is being clocked at 10 MHz
`(64x156.25 kHz) with the BIA clock signal 93. To provide
`for the sampling of the DFT ROMs, the DEA clock signal 93
`is input to a six-bit counter 10? to divide the DEA clock
`.‘ signal 93 in frequency by sixty-four (64) and provide a
`divide-by-64 DIA clock signal 97 that is phase locked with
`serial
`to parallel converter clock signal 92. A simple
`
`50
`
`60
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 11
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 11
`
`

`

`5,914,933
`
`8
`a particular antenna, i.e., providing cluster switching. First,
`pilot tones are sent over each cluster 27a. .
`.
`.
`. 27M and. at
`the expense of receiver training. the receiver measures the
`frequency response of each sub-channel associated with
`each of the clusters. As shown in FIG. 5, the receiver portion
`16 includes a device 67 for analyzing the frequency response
`ofthe sub-channels. The frequency response information of
`the sub-channels is provided back to the transmitter in the
`form of feedback signal 1.19 shown in FIGS. 2 and 5. Thus,
`it may be determined which tones are bad and a bad cluster
`may be switched to a different antenna by appropriate
`circuitry such as demultiplexethS (FIG. 2) and cluster select
`address lines 42 (FIG. 3) which enable the dynamic assign-
`ment of clusters to antennas.
`resulting in a significant
`improvement in the outage performance. In particular, for a
`20-dB fade margin, cluster switching provides more than
`two orders of magnitude improvement
`in the outage.
`Alternatively, for a fixed outage of 1%, cluster switching can
`provide about a 10 dB reduction in the required fading
`margin. Table 1, shows the a performance comparison
`between systems implementing cluster switching (adaptive)
`versus a system without.
`
`ID
`
`15
`
`TABLE I
`
`PM, (Fixed)
`3.44 x 10-'
`3.94 x 10"
`3.99 x 10""
`3.99 x 10-4
`
`Pm. (Adaptive!
`3.?0 x 10*
`3.91 x 10"
`4.00 x 104'
`4.00 n 10-a
`
`F
`0.9000
`0.9900
`0.9990
`0.9999
`
`7
`exclusive-0R gate 101 is used as a phase detector and the
`phase error signal 103 output from the exclusive-0R gate
`101 is filtered by an active loop filter 105 having a transfer
`function H(s_)-(R2Cs+l)}R1Cs before being fed-back to the
`VCXO oscillator 95.
`It
`is understood that
`the transfer
`
`function 11(5) for the active loop filter 105 was chosen for the
`particular components and frequencies used in the cluster
`transmitter shown in FIG. 3, and may change depending
`upon the frequencies and circuit designs used.
`In addition to synchronizing the frequency of the three
`system clocks 91, 92 and 93, it
`is also important to syn-
`chronize the 6-bit counters that generate the divide-by-64
`DEA clock signal 97 and the 6-bit counter 46 that generates
`the IS LSB‘s of the DFT addresses. As the DFT address
`
`counters 46 (FIG. 3) are started immediater upon system
`start-up and the divide-by-64 DEA clock signal 97 signal
`undergoes some frequency fluctuations until
`the PLL is
`locked, synchronization is required to ensure that the rela-
`tive position of these two 6-bit counters are the same in
`steady state. In order to guarantee synchronization, a reset
`control signal 109 signal is generated by sensing a sixty-two
`(62) count and delaying the signal by one DEA clock period
`93. This signal 109 is then sent to the DFT address counter
`which undergoes a synchronous reset at the next rising edge
`of DEA clock 93.
`The complete clustered-OFDM transmitter illustrated in
`FIG. 2 and described in detail with respect to FIG. 3, would
`require four transmitter boards, and a single clock generation
`board which can be realized using inexpensive, off-the-shelf
`ROMs and PLDs. With a modular approach in providing
`separate clock and control circuitry, the capability is pro-
`vided for varying the number ofclusters at will so that any
`number of transmit cluster board 5 can be plugged into the
`UFDM communication system.
`As the clustering approach applies only to the transmitter.
`the receiver must
`implement a complete N~point [)FI‘ to
`recover the data. However. with the exception of this single
`processing element, the remainder of the receiver can be
`realired using inexpensive, off-the-shelf ROMS and PIJJs in
`the same manner as the transmitter.
`
`As the clustered DFDM system is intended for wireless
`data packet transmission, frame or block synchronization
`between transmitter and receiver portions is necessary. As
`illustrated in Fl ('5. 10, separate frame synchronization words
`120 are transmitted simultaneously on all cluster antennas
`600—60M (FIG. 1) using the entire transmission bandwidth,
`resulting in independent fading. Four conventional correla-
`tcr detectors (not shown) are used at
`the receiver with
`noncoherent combining. The advantages of this approach
`are: (I) using the full bandwidth provides more accurate
`timing and (2) simultaneous transmission on all clusters with
`separate frame synch words (pseudo-random number
`sequence) provides diversity in the synch signal. which is
`extremely important. As shown in FIG. 10. the synchroni-
`zation is obtained at the expense of minimal training 130. for
`e.g., for carrier recovery.
`Assuming N sub-channels, each of bandwidth UT and
`separated by lfl‘, where T is the symbol interval for the
`individual sub-channels. and assuming no 181 and that the
`individual sub-channels are narrow enough so that the only
`effect of multipath is flat fading in each sub-channel, it is
`shown that the peak-to-average power ratio, PAP, for such a
`mu lticarrier signal is equal to N. For example, for 32 tones,
`PAP is 15 dB. The clustered OFDM system 10 of the
`invention reduces PAP since less tones are transmitted
`through a given amplifier. For the same total of 32 tones as
`above but with four antennas transmitting 8 tones each, PAP
`is reduced by 6 dB (PAP=9 dB). This translates into a factor
`of four reduction in the PAP seen by each amplifier, plus a
`factor of four reduction in the average power for an indi-
`vidual amplifier. Of course, four such amplifiers are
`required.
`The combination of clustering and coding may provide
`some performance benefits since now more uncorrelated
`.‘ symbols are presented to the decoder making the coding
`more efiective. An estimate of the benefits is given as
`follows:
`
`.
`
`40
`
`45
`
`50
`
`00
`
`A non—limiting, example embodiment for implementing
`the single receiver portion 16 of the OFl'JM communication
`system is illustrated in FIG. 5. As shown in FIG. 5. a single
`receive antenna 50 receives the ()FDM signal transmitted by
`clusters 27a. .
`.
`. , 27M and an RF demodulator 65 is used
`to demodulate the received signal. The demodulated signal
`is then converted into digital form by MD converter 68 and
`signal processing such as coherent or differential detection is
`implemented to recover the data. To implement a preferred
`method of differential detection. a synchronous detector 70
`and buffer