United States Patent
`Ciminiet al.
`
`{15
`
`US005914933A
`(1) Patent Number:
`(45) Date of Patent:
`
`5,914,933
`Jun. 22, 1999
`
`[75]
`
`[54] CLUSTERED OFDM COMMUNICATION
`SYSTEM
`Inventors: Leonard Joseph Cimini, Howell, N_J.;
`Babak Daneshrad, Los Angeles, Calif.;
`Nelson Ray Sollenberger, Tinton Falls,
`N.J.
`
`[73] Assignee: Lucent Technologies Inc., Murray Hill,
`N.J.
`
`5,675,572
`5,687,165
`
`10/1997 Hidejima et al. wo...ccs 370/206
`11/1997 Daffara et al.
`..........cccceeeerseee 370/208
`OTHER PUBLICATIONS
`Casas et al, “OFDM for Data Communication Over Mobile
`Radio FM Channels” IEEE Transactions on Communica-
`tions, vol. 39, No. 5, pp. 783-793, May 1991.
`Sakakuraet al., “Pre—Diversity using Coding, Multi—carriers
`and Multi-antennas”, paper presented at 1995 Fourth IEEE
`International Conference on Universal Personal Communi-
`cations, pp. 605-609. (Tokyo, Japan, Nov. 1995).
`Leonard J. Cimini, Jr, “Performance Studies for High—
`[21] Appl. No.: 08/730,430
`Speed Indoor Wireless Communications,” Wireless Personal
`-
`Communications 2: pp. 67-85 (1995).
`Filed:
`[22]
`Oct. 15, 1996
`Primary Examiner—Chau Nguyen
`Related U.S. Application Data
`[57]
`ABSTRACT
`Provisional application No. 60/011,601, Mar. 8, 1996.
`[60]
`A multicarrier communication system for wireless transmis-
`Ite Che cc ccccccssssssnnsssesecsssccesesssensnnnnes HO4L 27/26
`[ST]
`Sion of blocks of data having a plurality of digital data
`Ch:er 370/208; 370/210; 370/343;
`symbols in each block. The communication system includes
`375/299; 375/347
`a device for distributing the digital data symbols in each
`sessseeseeee
`370/203, 206
`............
`Field of Search.
`[58]
`370/208, 491.210, 482, 204, 480, 343,_Dlock over a plurality of clusters, each of the clusters
`484: 375/261, 364, 344, 355, 347, 299:
`receiving one or more digital data symbols. The digital data
`455/101—104
`symbols are encoded in each of 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
`® Plurality ofclusters, overallpeak-to-average power (PAP)
`ratio is reduced during transmission andtransmitter diversity
`‘1S improved.
`
`[56]
`
`References Cited
`
`5,416,767
`5,507,035
`5,548,582
`§,602,835
`5,610,908
`
`U.S. PATENT DOCUMENTS
`5/1995 Koppelaaretal. ..
`. 370/206
`
`. 455/133
`.....
`4/1996 Bantz et al.
`
`we 370/206
`.
`8/1996 Brajal et al.
`..sssccssscrrsseseressee 370/206
`2/1997 Sekiet al.
`
`3/1997 Shelswell et al. cscs 370/210
`
`42 Claims, 7 Drawing Sheets
`
`
`
`13
`
`4
`
`‘1
`
`
`
`OVERSAMPLED
`FFT +
`CODING
`FOR PAP
`SHAPING-
`
`
`
`REDUCTION
`GUARD.
`INTVL
`SERIAL DATA
`
`
`7.5 ioe
`NH SYMBOLS
`
`;
`el (exfet)
`
`5 253
`TRANSMITTER
`
`13
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 1
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 1
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 1 of 7
`
`5,914,933
`
`FIG.
`
`1A
`
`("
`
`e/a
`
`
` N SYMBOLS
`
`FIG.
`
`1B
`
`SINGLE
`RECEIVE
`ANTENNA
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 2
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`
`
`e/2
`
`c9IH
`
`)
`
`09~~
`1bseEI
`
`Sheet 2 of 7
`
`5,914,933
`
`9TAINImnsoo$1*
`UILLIWSNVELEGa9 (134x2)(°
`
`'NOTLING3HVIVOWIulsaedd10sall
`
`STO8WASWN
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 3
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 3
`
`
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 3 of 7
`
`5,914,933
`
`FIG. 3
`
`TO IMAGINARY
`DFT TABLES:
`
`rst_fft_entre
`FROM FIG.
`4
`
`
`
`TO FIG. 3 LOOP FILTER
`
`
`rst_fft_cntr
`
`H{s)
`
`105
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 4
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 4
`
`

`

`5,914,933
`
`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 4 of 7
`
`X001J1eV1
`
`WI1N3H35510
`
`ASVHd
`
`NOI193130
`
`
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 5
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 5
`
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 5 of 7
`
`5,914,933
`
`OUTAGE
`
`OUTAGE
`
`FIG. 6
`
`Pyangin’ 10 48
`
`01
`
`0
`
`9
`
`10
`
`15
`
`NUMBER OF FREQUENCIES CORRECTED
`
`0001
`
`001
`
`01
`
`NUMBER OF FREQUENCIES CORRECTED
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 6
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 6
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 6 of 7
`
`5,914,933
`
`QUTAGE
`
`FIG. 8
`
`01
`
`NUMBER OF FREQUENCIES CORRECTED
`
`FIG. 9
`
`30
`
`20
`
`10
`
`REQ. NUMBER
`OF CORRECTED
`FREQUENCIES
`
`= =. =. - -
`
`===
`=<...
`
`"=
`= =e
`Tee eeeee,
`
`=
`
`"=<...
`
`
`
` Puarcin’ 10 08
`
`0
`
`09
`
`10
`
`%15
`
`#20
`
`25
`
`43.
`
`NORMALIZED RMS DELAY SPREAD
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 7
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 7
`
`

`

`U.S. Patent
`
`Jun. 22, 1999
`
`Sheet 7 of 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 U.S.
`patent application Ser. No. 60/011,601, 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
`
`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 offen 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 LAN/ATM 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 coefficients, 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-to-average ~
`power ratio. Hence, highly linear (andinefficient) amplifiers
`must be used to avoid distortion and spectral spreading. In
`addition, with cither approach,
`a
`technique which can
`exploit the potential of diversity without requiring multiple
`receivers is desirable.
`
`40
`
`SUMMARYOF 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 ofthe 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.
`
`45
`
`50
`
`60
`
`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 ofthe 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 optimum transmission sub-channel.
`The clustered multicarrier communication system further
`includes a device for synchronizing data to enable simulta-
`neous transmission ofdata overall the sub-channels of the
`plurality of clusters to effect 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
`understandingof the invention, its 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. 1A and 1B are general block diagramsillustrating,
`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 ofthe invention.
`
`FIG, 3 is a schematic block diagram illustrating a single
`cluster ofthe transmitter.
`
`FIG. 4 is a schematic block diagram ofa clock generation
`circuit for the transmitter.
`
`FIG, $ 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 acrossall clusters
`and utilizing the entire transmitted bandwidth.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`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, 1/T,
`to an
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 9
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 9
`
`

`

`5,914,933
`
`20
`
`40
`
`3
`encoder 21 to allow for error/erasure 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 of NM 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
`L/NT. 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
`52a, ...,52M for transmission over respective separate and
`ideally independent antennas 60a, .
`.
`.
`, 60M.
`At the receiver portion 16 shown in FIG, 1B,a single
`receive antenna 40 and demodulator devices 65 and 73 are
`used for demodulating the OFDM signal using conventional
`techniques such as coherent or differential detection.
`Decoder 82 is used as the erasure/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 OFDM in a
`frequency-selective environment
`is that by dividing the ;
`transmitted bandwidth into many narrow sub-channels M
`which are transmitted in parallel, the effects of delay spread
`are minimized, eliminating the need for an equalizer.
`Additionally,
`the peak-to-average power ratio is reduced
`since there are fewer tones transmitted per transmitter result-
`ing in less spectral spreading when subjected to a non-
`linearity and/or smaller required power amplifier backoff, or
`equivalently, better power efficiency. 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.
`Amore 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 error/erasure correction at the receiver. The input
`encoded digital data stream 19 is represented as a multitone
`signal comprising a quantity of NM symbols, cach symbol
`having a period of T, and a symbolrate 1/T whichis usually
`limited by the amount of multipath fading as experimentally
`realized in the particular environment, ¢.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, 25a, ..., 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 of NM tones (e.g., NM=28)fordistri-
`bution over M clusters (M=4), with each cluster 27@ trans-
`mitting N tones (N=7). Each sub-channel 27a, ... 27M is a
`
`45
`
`50
`
`60
`
`4
`narrow bandwidth channel carrying N tonesfor transmission
`at a sub-channel symbol rate of 1/NT. Ideally, the bandwidth
`of the sub-channels will be narrow enough so there is no ISI
`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 10 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 different 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 effective 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 27@,...27M
`is subject to Orthogonal Frequency Division Multiplexing
`(OFDM). As shownin cluster 27a ofFIG. 2, the digital data
`stream 25a 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 mapping ofsignal 25a based on
`PAP-ROM table lookup techniques which guarantees the
`PAP ratio reduction. As an example, a sequence of seven (7)
`tones (14 bits each) may be mappedinto eight 8 tones (16
`bits cach) requiring 214 16-bit entries in the transmitter
`PAP-ROM 35. Asseventy-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
`
`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 27a in FIG. 2. As will be explained
`herein, the receiver portion 16 of the OFDM communication
`system is capable of receiving pilot tones 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 tonesto a particular
`sub-channels (in a particular cluster) having a matching
`frequency response characteristic.
`The OFDM multicarrier signal generated in each of the M
`clusters is then multiplexed into serial form as shown by the
`parallel to serial converter 45, converted to analog form by
`D/A converter 47, 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 FIG. 2, that is
`simple to implement with a minimization of hardware and
`complexity. The system operates at a maximum clockrate of
`10 MHz and requires three different clock signals which are
`related to each other through the implementation of PLL/
`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
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 10
`
`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 27a shown in FIG, 3, the serial data bit
`Stream input
`to the cluster is al 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 Mbps/cluster). 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. A non-linear code is used for this
`purpose which guarantees the PAPratio of the 7 tones to be
`no more than 3.2 dB, Due to its non-linear nature, the PAP
`coding needs to be implemented via table lookup and in the
`embodiment shown in FIG. 3, a 4 Kx14 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
`shownin 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 (1):
`
`6
`prefixing, windowing, and the DFT operation was consoli-
`dated into a ROM lookuptable 41 in order to avoidthe use
`ofelaborate and costly signal processing ICs and to provide
`a flexible mechanism in which the relative size ofthe 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 s the user to realize any
`windowing function on the OFDM symbol.
`In the embodiment described, the DFT ROM 41 has a
`total of twenty (20) input address bits, fourteen (14) from the
`PAP ROM 35output signal 39 and six (6)bits for the output
`signal 43 ofa 64-bit counter 46that readsoff 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 D/A converter rate of 10 MHz. In order to
`avoid the use of high-end memory modules when higher
`D/A data rates (e.g. 30 MHz) are implemented, the DFT
`lookup table 41 is partitioned into two DFT ROMS, 41a,6,
`each for modulating N/2 complex tones, ¢.g., 3.5 tones in the
`embodiment shown in FIG. 3. Taking into account sampling
`quantization and the resulting in-band interference, an 8-bit
`representation was used for the DFT samples stored in each
`of the two DFT ROMS. The outputs of these ROMs41a and
`41b are then added together by adder element 48 in a
`programmable logic device 45 (“PLD”) to realize the
`desired total of N tones for the channel, ¢.g., seven (7) tones.
`Thus, partitioning of the DFT task enables the replacement
`of a 1 Mbyte ROM with a pair of 8 kByte ROMS.
`To enable cluster switching, Le., to enable each transmit-
`ter cluster board to transmit tones over any one of the four
`clusters,
`the two 8 kbyte DFT ROMs 41a,41b, may be
`replaced by two 32 kbyte ROMs having two additional
`address lines 42 that enable selection between one ofthe
`four different clusters as shown in FIG, 3.
`k=0,...,55
`_-ptehlacrTon
`;
`Yk
`ie a2
`Xin + JXInet
`mth) =
`As further illustrated in the example embodiment ofthe
`
`
`" Vin+Panel m=, 1,2,3
`nO
`transmit cluster shown in FIG. 3, a synch-word ROM 83 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 D/A
`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 MHzrate, 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 35of transmit cluster 27a. 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 D/A
`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 ROMs41a,6 and processed through
`to the D/A converter 47 which is being clocked at 10 MHz
`(64x156.25 kHz) with the D/A clock signal 93. To provide
`for the sampling of the DFT ROMs,the D/A clocksignal 93
`is input to a six-bit counter 107 to divide the D/A clock
`signal 93 in frequency by sixty-four (64) and provide a
`divide-by-64 D/A clock signal 97 that is phase locked with
`serial
`to parallel converter clock signal 92. A simple
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 11
`
`6
`
`”
`
`(1)
`
`is the output
`is the cluster number, Y,,(k)
`where “m”
`sequence which is fed to the D/A for transmission and x,,
`represents the n” bit of the 14-bit word appearing at
`the
`output of the PAP ROM 35. In the embodiment shown in
`FIG, 3, the even bits were assigned to the real part ofthe
`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 samples, 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 (!4) multiplier introduced into
`the exponential function in equation (1). The oversampling
`guarantees a separation of{,/2 between the baseband signal
`and the first
`image of the signal output
`from the D/A
`converter 47, The separation results in a significant relax-
`ation of the specification for the image canceling lowpass
`filters following the D/A 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
`ROMs35(FIG. 3), sixty-four (64) samples need to be read
`from the DFT ROMs41 and presented to the D/A converter
`47. These 64 samples constitute a complete OFDM symbol
`(block). In the embodiment shown in FIG. 3,
`the cyclic
`
`20
`
`40
`
`45
`
`50
`
`60
`
`Petitioner Sirius XM Radio Inc. - Ex. 1003, p. 11
`
`

`

`5,914,933
`
`7
`exclusive-OR gate 101 is used as a phase detector and the
`phase error signal 103 output from the exclusive-OR gate
`101 is filtered by an active loop filter 105 having a transfer
`function H(s)=(R,Cs+I)/R,Cs before being fed-back to the
`VCXO oscillator 95.
`It
`is understood that
`the transfer
`
`function H(s)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
`D/A clock signal 97 and the 6-bit counter 46 that generates
`the 6 LSB’s of the DFT addresses. As the DFT address
`
`counters 46 (FIG. 3) are started immediately upon system
`start-up and the divide-by-64 D/A 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 D/A 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 D/A 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 of clusters at will so that any
`numberof transmit cluster board s can be plugged into the
`OFDM communication system,
`Asthe clustering approach applies only to the transmitter,
`the receiver must
`implement a complete N-point DFT to
`recover the data. However, with the exception ofthis single
`processing element, the remainder of the receiver can be
`realized using inexpensive, off-the-shelf ROMs and PLDsin
`the same manner as the transmitter.
`A non-limiting, example embodiment for implementing
`the single receiver portion 16 of the OFDM communication
`system is illustrated in FIG. 5. As shownin FIG. 5, a single
`receive antenna 50 receives the OFDMsignal 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 A/D converter 68 and
`signal processing suchas coherent or differential detection is
`implemented to recover the data. To implement a preferred
`method ofdifferential detection, a synchronous detector 70
`and buffer 71 elements provide the parallel digital words to
`a Fast Fourier Transform element 73, the output of which is
`converted back into serial form by parallel to serial con-
`verter clement 76. Using a ROM table phase look-up table
`78 and associated phase detectcircuitry 79, the serial data is ;
`differentially phase detected. Further serial to parallel trans-
`formation of the digital signal
`is performed by serial to
`parallel converter 81, and PAP non-linear decoder 83 is used
`to obtain the original data. Using the PAP non-linear coder
`having 2" described above in connection with FIG. 2, the
`receiver PAP non-linear decoder 83 will contain 2'° 14-bit
`entries. It should be understood that suitable error/erasure
`correction (¢.g., Reed-Solomon) decoding is performed (not
`shown).
`The performance ofthe clustered OFDM communication
`system can be improved (that is, more bit rate achieved for
`a given bandwidth) by optimally assigning a given cluster to
`
`8
`a particular antenna, i.e., providing cluster switching.First,
`pilot tones are sent over each cluster 27a, .
`.
`.
`, 27M and,at
`the expense ofreceiver training, the receiver measures the
`frequency response of each sub-channel associated with
`each of the clusters. As shownin FIG. 5, the receiver portion
`16 includes a device 67 for analyzing the frequency response
`of the sub-channels. The frequency response information of
`the sub-channels is provided back to the transmitter in the
`form of feedback signal 119 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 demultiplexer 26 (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.
`
`TABLEI
`
`Pom (Fixed)
`3.44 x 1071
`3.94 x 1077
`3.99 x 104
`3.99 x 104
`
`Pom (Adaptive)
`3.70 x 10-*
`3.97 x 10-4
`4,00 x 107°
`4,00 x 10-*
`
`P
`0.9000
`0.9900
`0.9990
`0.9999
`
`As the clustered OFDM system is intended for wireless
`data packet transmission, frame or block synchronization
`between transmitter and receiver portions is necessary. As
`illustrated in FIG. 10, separate frame synchronization words
`120 are transmitted simultaneously on all cluster antennas
`60a—60M (FIG. 1) using the entire transmission bandwidth,
`resulting in independent fading. Four conventional correla-
`tor detectors (not shown) are used at
`the receiver with
`noncoherent combining. The advantages of this approach
`are: (1) using the full bandwidth provides more accurate
`timing and (2) simultaneous transmission onall 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 1/T and
`separated by 1/T, where T is the symbol interval for the
`individual sub-channels, and assuming no ISI 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
`shownthat the peak-to-aver