US 20050054313A1
`
`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2005/0054313 A1
`
`Gummadi et al.
`(43) Pub. Date:
`Mar. 10, 2005
`
`(54)
`
`(75)
`
`SCALABLE AND BACKWARDS
`COMPATIBLE PREAMBLE FOR OFDM
`SYSTEMS
`
`Inventors: Srikanth Gummadi, Rohnert Park, CA
`(US); Srinath Hosur, Plano, TX (US);
`Peter Murphy, Santa Rosa, CA (US)
`
`Publication Classification
`
`(51)
`
`Int. Cl.7 ............................ H04B 17/00; H04B 1/02;
`H03C 7/02; H04B 7/02
`
`(52) us. Cl.
`
`..................................... 455/2201; 455/6711
`
`Correspondence Address:
`TEXAS INSTRUMENTS INCORPORATED
`
`(57)
`
`ABSTRACT
`
`P 0 BOX 655474, M/S 3999
`DALLAS, TX 75265
`
`(73)
`
`Assignee: Texas Instruments Incorporated, Dal-
`las, TX
`
`(21)
`
`Appl. N0.:
`
`10/811,519
`
`(22)
`
`Filed:
`
`Mar. 29, 2004
`
`Related US. Application Data
`
`(60)
`
`Provisional application No. 60/500,438, filed on Sep.
`5, 2003.
`
`Amethod comprising encoding a plurality of signals accord-
`ing to a predetermined negation scheme and transmitting the
`plurality of signals, wherein each signal is transmitted by
`way of a wireless channel. The method further comprises
`receiving a signal, wherein the received signal is a combi-
`nation of the plurality of transmitted signals, and interpo-
`lating between data in the received signal to generate a
`plurality of systems of equations. The method further com-
`prises solving the plurality of systems of equations to
`determine a gain and phase shift applied to each of the
`plurality of transmitted signals by a corresponding wireless
`channel.
`
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`Patent Application Publication Mar. 10, 2005 Sheet 1 0f 3
`
`US 2005/0054313 A1
`
`92
`
`MULTIPLE
`WIRELESS
`CHANNELS
`
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`FIG. 1b
`
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`
`180
`
`AT LEAST TWO TRANSMITTERS EACH
`TRANSMIT A PREAMBLE TO A RECEIVER
`
`184
`
`186
`
`182
`
`THE RECEIVER RECEIVES ONE SIGNAL
`
`PROCESSOR OF RECEIVER COMPARES
`
`
`RECEIVED SIGNALS TO TRANSMITTED SIGNALS
`
`
`TO DETERMINE MATHEMATICAL EXPRESSIONS
`
`PROCESSOR DETERMINES ADDITIONAL
`
`
`EQUATIONS BY WAY OF INTERPOLATION
`
`
`
`RECEIVER DETERMINES PHASE AND GAIN
`IMPARTED ON TRANSMISSIONS BY EACH
`CHANNEL; CHANNELS SUCCESSFULLY ESTIMATED
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`Patent Application Publication Mar. 10, 2005 Sheet 2 0f 3
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`Patent Application Publication Mar. 10, 2005 Sheet 3 0f 3
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`US 2005/0054313 A1
`
`Mar. 10, 2005
`
`SCALABLE AND BACKWARDS COMPATIBLE
`PREAMBLE FOR OFDM SYSTEMS
`
`PRIORITY CLAIM
`
`[0001] This application claims priority to US. Patent
`Application Ser. No. 60/500,438, filed on Sep. 5, 2003,
`entitled “SCALABLE AND BACKWARDS COMPATIBLE
`
`PREAMBLE FOR 11n,” incorporated herein by reference.
`
`BACKGROUND
`
`[0002] Wireless local area networks (“WLAN”) allow
`electronic devices, such as computers,
`to have network
`connectivity without the use of wires. Network connections
`may be established via, for example, radio signals. Awire-
`less access point (“AP”) may comprise a wired Internet or
`Ethernet connection and radio communication circuitry
`capable of transmitting data to and receiving data from any
`compatible wireless device. The AP may provide Internet
`and/or network connectivity to such wireless devices (e.g.,
`portable computers) called receiver stations (“STA”) by
`transmitting and receiving data via radio signals.
`
`[0003] Architects of WLAN systems and devices must
`take various factors into account. One such factor is multi-
`
`path interference. In multipath interference, a signal trans-
`mitted from a source (e.g., an AP) may take multiple paths
`through a wireless medium and thus reach the intended
`destination as more than one version of the same signal.
`FIG. 1a illustrates this phenomenon, in which a signal from
`an AP is transmitted directly to a STA and also bounces off
`the walls 10, 12 before reaching the STA. The lengths of the
`different paths may vary,
`thereby causing a phase/time
`difference in the received signals. Accordingly, multipath
`interference may cause distortion in the signal. Thus, the
`signal received by the STA may be a distorted version of the
`signal that was originally transmitted by the AP. The tech-
`nique of “channel estimation” may be implemented in a STA
`or AP receiver to eliminate such distortion and generate a
`version of the signal which is nearly identical to the signal
`that was originally transmitted by the AP.
`
`[0004] Channel estimation comprises transmitting a pre-
`determined signal (described below) from a transmitter to a
`receiver, where the transmitted predetermined signal
`is
`known to both the transmitter and the receiver prior to
`transmission. Due to multipath interference, the predeter-
`mined signal received by the receiver will generally be
`different from the predetermined signal transmitted by the
`transmitter. Upon receiving the signal,
`the receiver may
`compare the received signal to the transmitted signal to
`determine how multipath interference has distorted the sig-
`nal. The receiver may use such information to synchronize
`the receiver to the transmitter(s) and eliminate distortion
`present in future received signals.
`
`[0005] A signal used specifically for channel estimation
`comprises a preamble. Apreamble comprises, among other
`things, a short sequence of data and a long sequence of data.
`The short sequence may be used to perform basic synchro-
`nization, including determining whether a packet is en route
`to the receiver, estimating frequency offset, and other vari-
`ous synchronization operations. The long sequence is the
`sequence actually used in channel estimation. Standard
`IEEE 802.11 protocols, such as 802.11a, comprise long
`sequence designs that enable channel estimation for standard
`
`in a
`single input, single output (“SISO”) systems. Thus,
`system comprising a single transmitter and a single receiver,
`the receiver is able to successfully estimate the channel
`between the transmitter and the receiver, thereby eliminating
`distortions present in a received signal. However, multiple-
`input, multiple-output (“MIMO”) signaling systems com-
`prising a plurality of transmitters and receivers present
`unique problems for existing channel estimation techniques.
`
`the rate at which data is
`In a MIMO system,
`[0006]
`transferred (“data rate”) between a transmitter and a receiver
`may be raised by increasing the number of antennas asso-
`ciated with each wireless device in the system. For instance,
`a system comprising a transmitter with multiple antennas
`and a receiver with multiple antennas may have a higher data
`rate than a system comprising a transmitter with a single
`antenna and a receiver with a single antenna. The MIMO
`antennas are part of a design that attempts to achieve a linear
`increase in data rate as the number of transmitting and
`receiving antennas linearly increases.
`
`[0007] MIMO systems present unique problems for exist-
`ing channel estimation techniques due to difficulties intro-
`duced by signal overlapping, wherein a receiver receives a
`mixture of signals instead of a single signal. For example, in
`a system comprising two transmitters and two receivers,
`each receiver receives a signal that is a combination of the
`signals transmitted by each of the transmitters. In order to
`estimate the four channels (i.e., one channel from a first
`transmitter to a first receiver, a second channel from a first
`transmitter to a second receiver, a third channel from a
`second transmitter to a first receiver, a fourth channel from
`a second transmitter to a second receiver), a receiver must
`mathematically analyze the received signal to determine a
`plurality of equations describing distortion imparted by each
`channel on a signal transmitted through the channel. Each
`receiver then may successfully estimate all four channels.
`Channel estimation information subsequently may be used
`by a receiver to eliminate distortion present in future signals.
`Thus, a technique to separate mixed signals and eliminate
`signal distortion in MIMO systems is desirable.
`BRIEF SUMMARY
`
`[0008] The problems noted above are solved in large part
`by a method and apparatus for efficiently estimating chan-
`nels in a MIMO system and using the channel estimations to
`eliminate signal distortion. One exemplary embodiment may
`comprise encoding a plurality of signals according to a
`predetermined negation scheme and transmitting the plural-
`ity of signals, wherein each signal is transmitted by way of
`a wireless channel. The method further comprises receiving
`a signal, wherein the received signal is a combination of the
`plurality of transmitted signals, and interpolating between
`data in the received signal to generate a plurality of systems
`of equations. The method also comprises solving the plu-
`rality of systems of equations to determine a gain and phase
`shift applied to each of the plurality of transmitted signals by
`a corresponding wireless channel.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0009] For a detailed description of the preferred embodi-
`ments of the invention, reference will now be made to the
`accompanying drawings in which:
`
`[0010] FIG. 1a illustrates a block diagram describing the
`multipath interference phenomenon in accordance with
`embodiments of the invention;
`
`
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`

`US 2005/0054313 A1
`
`Mar. 10, 2005
`
`[0011] FIG. 1b illustrates a block diagram of a receiver in
`accordance with various embodiments of the invention;
`
`[0012] FIG. lc illustrates a flow diagram in accordance
`with embodiments of the invention;
`
`[0013] FIG. 1d illustrates a block diagram of a MIMO
`system in accordance with embodiments of the invention;
`
`[0014] FIG. 16 illustrates a block diagram of a wireless
`channel distorting preambles en route to a receiver in
`accordance with embodiments of the invention;
`
`[0015] FIG. 2 illustrates a block diagram comprising two
`preambles in accordance with embodiments of the inven-
`tion;
`
`[0016] FIG. 3 illustrates a block diagram of a MIMO
`system comprising three transmitters in accordance with
`embodiments of the invention; and
`
`[0017] FIG. 4 illustrates a block diagram comprising three
`preambles in accordance with embodiments of the inven-
`tion.
`
`NOTATION AND NOMENCLATURE
`
`[0018] Certain terms are used throughout the following
`description and claims to refer to particular system compo-
`nents. As one skilled in the art will appreciate, various
`companies may refer to a component by different names.
`This document does not
`intend to distinguish between
`components that differ in name but not function. In the
`following discussion and in the claims, the terms “includ-
`ing” and “comprising” are used in an open-ended fashion,
`and thus should be interpreted to mean “including, but not
`limited to .
`.
`. ” Also, the term “couple” or “couples” is
`intended to mean either an indirect or direct electrical
`
`connection. Thus, if a first device couples to a second device,
`that connection may be through a direct electrical connec-
`tion, or through an indirect electrical connection via other
`devices and connections. Furthermore,
`the notation “X”
`denotes a variable number. For example, “L1.x” may rep-
`resent “L1.1,”“L1.32,” or any other number. Further still, the
`terms “series of frequency tones” and “series of numbers”
`are used interchangeably throughout this document.
`
`DETAILED DESCRIPTION
`
`In a general MIMO system comprising multiple
`[0019]
`transmitters, a signal received by a receiver is a combination
`of several transmission signals emitted from the various
`transmitters. To eliminate distortion present in the signals,
`all wireless channels existing between transmitters and
`receivers in a MIMO system are estimated. FIG. 1b illus-
`trates a receiver 104, comprising an antenna 94, a processor
`98 and a memory 96, coupled to a plurality of wireless
`channels 92. The receiver 104 may estimate all wireless
`channels 92 to which the receiver 104 is coupled by deter-
`mining how each channel 92 affects data that passes through
`that channel 92. Specifically, the receiver 104 determines the
`phase and gain added to data passing through the channel 92
`by way of the process illustrated in the flow diagram of FIG.
`IC. The process may begin with the transmission of a
`preamble from each of at
`least
`two transmitters to the
`receiver 104, wherein each preamble comprises a different,
`predetermined series of numbers known to the receiver 104
`prior to transmission (block 180). In block 182, the receiver
`
`is a
`that
`104 receives by way of antenna 94 a signal
`combination of all signals transmitted in block 180. The
`processor 98 may compare the received signal to the trans-
`mitted, predetermined series of numbers to determine a set
`of mathematical expressions describing each channel’s
`effects on the transmitted series of numbers (block 184).
`
`[0020] Using interpolation, the processor 98 determines
`additional sets of expressions (block 186). All sets of expres-
`sions are then solved using known mathematical calcula-
`tions to determine the phase and gain applied to each number
`of each series of numbers by the channel 92 through which
`the series was transmitted, thereby concluding the channel
`estimation process (block 188). The processor 98 stores the
`channel estimations in the memory 96 and is able to use such
`channel estimations in the future to effectively reverse
`distortions (i.e., phase and gain) added to a signal during
`transmission.
`
`[0021] FIG. 1d illustrates a WLAN system comprising a
`first transmitter TX1100 and a second transmitter TX2102,
`each wirelessly coupled to a receiver RX1104 by way of
`wireless channels 108, 110, respectively. The TX1100 and
`the TX2102 each transmit to the RX1104 a preamble com-
`prising a series of predetermined numbers L1 and L2,
`respectively. The predetermined series L1 and L2 are known
`to the RX1104 prior to transmission.
`
`[0022] Upon transmission of the preambles comprising L1
`and L2, the RX1104 receives a single series of numbers,
`where the single series of numbers is a linear combination of
`the transmitted preambles, as shown in FIG. 16. FIG. 16
`illustrates a series L1 .k transmitted from the TX1100 and a
`
`series L2.k transmitted from the TX2102. During transmis-
`sion through the wireless channel 108,
`the series L1.k
`experiences the effects H1.k of the wireless channel 108.
`Similarly, during transmission through the wireless channel
`110,
`the series L2.k experiences the effects H2.k of the
`wireless channel 110. In turn, the RX1104 receives a signal
`r(k) which is a linear combination of L1.k and L2.k, where
`L1.k and L2.k have been altered by the wireless channels
`108, 110 to a degree H1.k, H2.k, respectively. Specifically,
`
`Abdjkfiflkfllkfiflk
`
`[0023] where H1.k and H2.k represent the effects of the
`wireless channel 108, 110 each of the transmitted preambles
`experiences prior to being received at the RX1104.
`
`[0024] The received signal r(k) comprises L1.k and L2.k,
`where L1.k is defined as:
`
`[0025] L1.k: {L1.0, L11, L12 .
`.
`. L149, L150, L151},
`
`.
`
`. L125, 0, L126 .
`
`[0026] where the “0” value is used to differentiate negative
`frequencies from positive frequencies and is not actually
`transmitted. L2.k is identical to L1.k, except the odd tones
`(e.g., L2.1, L249, L251) are negated:
`
`[0027] L2.k: {L1.0, —L1.1, L12 .
`.
`.
`. —L1.49, L1.50, —L1.51}
`
`.
`
`. —L125, 0, L126
`
`[0028] The series of numbers representing L1.k and L2.k
`above are exemplary of one embodiment of the invention
`and do not limit the scope of this disclosure. Any series of
`numbers and any negation scheme may be used to represent
`L1.k and L2.k.
`
`
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`HUAWEI VS. SPH
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`000006
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`

`

`US 2005/0054313 A1
`
`Mar. 10, 2005
`
`In the signal r(k) received by the RX1104, the odd
`[0029]
`have
`effectively been
`subtracted
`(i.e.,
`r(k)=
`tones
`L1.k*H1.k+L2.k*H2.k=L1.k*H1.k—L1.k*H2.k=
`
`L1.k*(H1.k—H2.k)) and all even tones have been added (i.e.,
`r(k)=L1.k*H1.k+L2.k*H2.k=L1.k*
`H1.k+L1.k*H2.k=
`L1.k*(H1.k+H2.k)) to produce the received signal r(k).
`Thus, for a portion of the received signal defined as:
`
`[0030]
`
`r(k): {r(0), r(l), r(2) .
`
`.
`
`. r(51)},
`
`the RX1104 may generate a system of equations to
`[0031]
`solve for all values of H1.k and H2.k, as illustrated in Table
`1 below.
`
`TABLE 1
`
`System of 52 equations relating all values of L1.k= H1.k= H2.k and r k .
`
`Expression
`
`Expression No.
`
`L .0 * (H1.0 + H2.0) = r(0)
`L .1 * (H1.1 + (—H2.1)) = r(1)
`L .2 * (H1.2 + H2.2) = r(2)
`L .3 * (H1.3 + (—H2.3)) = r(3)
`L .4 * (H1.4 + H2.4) = r(4)
`L .5 * (H1.5 + (—H2.5)) = r(5)
`L .6 * (H1.6 + H2.6) = r(6)
`L .7 * (H1.7 + (—H2.7)) = r(7)
`L .8 * (H1.8 + H2.8) = r(8)
`L .9 * (H1.9 + (—H2.9)) = r(9)
`L .10 * (H .10 + H2.10) = r(10)
`L .11 * (H .11 + (—H2.11))= r(11)
`L .12 * (H .12 + H2.12) = r(12)
`L .13 * (H .13 + (—H2.13)) = r(13)
`L .14 * (H .14 + H2.14) = r(14)
`L .15 * (H .15 + (—H2.15)) = r(15)
`L .16 * (H .16 + H2.16) = r(16)
`L .17 * (H .17 + (—H2.17)) = r(17)
`L .18 * (H .18 + H2.18) = r(18)
`L .19 * (H .19 + (—H2.19)) = r(19)
`L .20 * (H .20 + H2.20) = r(20)
`L .21 * (H .21 + (—H2.21)) = r(21)
`L .22 * (H .22 + H2.22) = r(22)
`L .23 * (H .23 + (—H2.23)) = r(23)
`L .24 * (H .24 + H2.24) = r(24)
`L .25 * (H .25 + (—H2.25)) = r(25)
`L .26 * (H .26 + H2.26) = r(26)
`L .27 * (H .27 + (—H2.27)) = r(27)
`L .28 * (H .28 + H2.28) = r(28)
`L .29 * (H .29 + (—H2.29)) = r(29)
`L .30 * (H .30 + H2.30) = r(30)
`L .31 * (H .31 + (—H2.31)) = r(31)
`L .32 * (H .32 + H2.32) = r(32)
`L .33 * (H .33 + (—H2.33)) = r(33)
`L .34 * (H .34 + H2.34) = r(34)
`L .35 * (H .35 + (—H2.35)) = r(35)
`L .36 * (H .36 + H2.36) = r(36)
`L .37 * (H .37 + (—H2.37)) = r(37)
`L .38 * (H .38 + H2.38) = r(38)
`L .39 * (H .39 + (—H2.39)) = r(39)
`L .40 * (H .40 + H2.40) = r(40)
`L .41 * (H .41 + (—H2.41)) = r(41)
`L .42 * (H .42 + H2.42) = r(42)
`L .43 * (H .43 + (—H2.43)) = r(43)
`L .44 * (H .44 + H2.44) = r(44)
`L .45 * (H .45 + (—H2.45)) = r(45)
`L .46 * (H .46 + H2.46) = r(46)
`L .47 * (H .47 + (—H1.47)) = r(47)
`L .48 * (H .48 + H2.48) = r(48)
`L .49 * (H .49 + (—H2.49)) = r(49)
`L .50 * (H .50 + H2.50) = r(50)
`L .51 * (H .51 + (—H2.51)) = r(51)
`
`
`
`
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`16
`17
`18
`19
`20
`21
`22
`23
`24
`25
`26
`27
`28
`29
`30
`31
`32
`33
`34
`35
`36
`37
`38
`39
`40
`41
`42
`43
`44
`45
`46
`47
`48
`49
`50
`51
`52
`
`predetermined, transmitted values. With knowledge of r(k)
`and L1.k, H1.k+H2.k may be estimated for even values of k
`using any appropriate method of estimation (e.g., a simple
`division method, a least-squares (“LS”) method, a minimum
`mean squared error (“MMSE”) method). Similarly, with
`knowledge of r(k) and L1.k, H1.k—H2.k may be estimated
`for odd values of k. As a result, there will exist 26 equations
`for H1.k+H2.k estimations and 26 equations for H1.k—H2.k
`estimations.
`
`[0033] Because there exist only 26 equations for H1.k+
`H2.k and 26 equations for H1.k—H2.k, estimating H1.k and
`H2.k for all values of k is impossible. To determine H1.k and
`H2.k for all values of k, additional equations may be
`necessary. Accordingly, H1.k+H2.k is interpolated for even
`values of k to provide an estimate of H1.k+H2.k for odd
`values of k. Similarly, H1.k—H2.k is interpolated for all odd
`values of k to provide estimates of H1.k—H2.k for even
`values of k. There now exist 104 equations (i.e., 52 equa-
`tions for H1.k+H2.k for all values of k and 52 equations for
`H1.k—H2.k for all values of k) and 104 unknown values.
`These equations may be solved to determine the 104
`unknown values. After all values of H1.k are obtained, the
`RX1104 has effectively determined the change imparted on
`the preamble transmitted through the wireless channel 108.
`Likewise, once all values of H2.k are obtained, the RX1104
`has effectively determined the change imparted on the
`preamble transmitted through the wireless channel 110.
`Because the change (i.e., phase and gain) values for each
`wireless channel 108, 110 has been computed for each
`frequency tone, the channels 108, 110 have been estimated.
`These channel estimations may be used by the RX1104 to
`eliminate signal distortion in incoming signals caused by
`multipath interference or any other factor.
`
`[0034] Different preamble designs may be used to estimate
`channels in various MIMO systems. For example, one
`MIMO system may comprise two transmitters, as shown in
`FIG. 1d. FIG. 2 illustrates preambles that may be transmit-
`ted from each transmitter TX1100, TX2102. Specifically,
`preamble 200 represents a transmission from transmitter
`TX1100 and comprises, among other
`things,
`a short
`sequence 202, a long sequence 204, a legacy signal field 206
`and a new signal field 208. Preamble 250 represents a
`transmission from transmitter TX2102 and comprises,
`among other things, a short sequence 252, a long sequence
`254, a legacy signal field 256 and a new signal field 258. The
`long sequences 204 and 254 each comprise, among other
`things, two series of numbers (i.e., two series of frequency
`tones).
`
`[0035] Each of the two series of frequency tones LS 210
`in the preamble 200 may be structurally similar to L1 above
`and each of the two series of frequency tones LSl260 in the
`preamble 250 may be structurally similar to L2 above. Thus,
`LS 210 may be defined as:
`
`[0032] Because the series r(k) is received by the RX1104,
`all values of r(k) (e.g., r(0), r(l) .
`.
`. r(51)) are known to the
`RX1104. The L1.k values in Table 1 above represent the
`
`[0036] LS:
`{1,1,—1,—1,1,1,—1,1,—1,1,1,1,1,1,1,—1,—1,1,1,—1,1,—1,—
`1,1,1,1,0,1,—1,—1,1,1,—1,1,—1,1,—1,—1,—1,—1,—1,1,1,—
`1,—1,1,—1,1,—1,1,1,1,1}
`
`
`
` AZX-I
`%
` T 1011
`HUAW 4'.
`
`
`
`VS.
`SPI—I
`HUAW 4'.
`
`
`
`000007
`
`HUAWEI EXHIBIT 1011
`HUAWEI VS. SPH
`
`000007
`
`

`

`US 2005/0054313 A1
`
`Mar. 10, 2005
`
`[0045] Accordingly, FIG. 3 illustrates a MIMO system
`comprising three transmitters TX1300, TX2302, TX3304 in
`communications with one receiver RX1306 by way of
`wireless channels 308, 310, 312, respectively. FIG. 4 illus-
`trates preambles 400, 402, 404 that may be transmitted from
`the transmitters TX1300, TX2302, TX3304, respectively,
`during a channel estimation process. The channel estimation
`process for the MIMO system of FIG. 3 is similar to the
`channel estimation process for the MIMO system of FIG.
`1d, but the structures of the preambles 400, 402, 404 may
`slightly differ from the structures of the preambles 200, 250.
`Specifically, because the MIMO system of FIG. 3 comprises
`three transmitters, the preambles 400, 402, 404 must com-
`prise additional long sequences 414, 424, 434, respectively,
`for reasons described above.
`
`[0046] Long sequences 408, 414, 418 may comprise a
`series of frequency tones LS 436, 438, 440, respectively,
`defined as:
`
`[0047] LS:
`{1,1,—1,—1,1,1,—1,1,—1,1,1,1,1,1,1,—1,—1,1,1,—1,1,—
`1,1,1,1,1,0,1,—1,—1,1,1,—1,1,—1,1,—1,—1,—1,—1,—1,1,—
`1,—1,—1,1,—1,1,—1,1,1,1,1}.
`
`[0048] Long sequences 428, 434 may comprise a series of
`frequency tones LSl444, 446, respectively, defined as:
`
`[0049] LSl:
`{1,—1,—1,1,1,—1,—1,—1,—1,—1,1,—1,1,—1,1,1,—1,—1,1,1,—
`1,1,1,—1,1,—1,0,1,1,—1,—1,1,1,1,1,1,1,—1,1,—1,1,1,—1,—
`1,1,1,1,1,1,1,—1,1,—1}.
`
`[0050] Long sequence 424 may comprise a series of
`frequency tones —LS 442, defined as:
`
`[0051] LS:
`{—1,—1,1,1,—1,—1,1,—1,1,—1,—1,—1,—1,—1,—1,1,1,—1,—
`1,1,—1,1,—1,—1,—1,—1,0,—1,1,1,—1,—1,1,—1,1,—1,1,1,1,—
`1,1,—1,—1,1,1,—1,1,—1,1,—1,—1,—1,—1,—1},
`
`where the values of —LS are the values of LS,
`
`[0052]
`negated.
`
`[0053] Continuing with this example, the preambles 400,
`402, 404 are transmitted through the wireless channels 308,
`310, 312 by the transmitters TX1300, TX2302, TX3304,
`respectively. The RX1306 receives a signal that is a com-
`bination of the preambles 400, 402, 404. In this example, the
`received signal comprises two series of frequency tones
`defined as:
`
`{r(1.0),
`r(k.0,k.1):
`[0054]
`r(52.0)}{r(1.1),r(2.1),r(3.1) .
`
`r(3.0)
`r(2.0),
`.
`. r(52.1)}.
`
`[0037]
`
`and LSl260 may be defined as:
`
`[0038] LSl:
`{1,—1,—1,1,1,—1,—1,—1,—1,—1,1,—1,1,—1,1,1,—1,—1,1,1,—
`1,1,1,—1,1,—1,0,1,1,—1, —1,1,1,1,1,1,1,—1,1,—1,1,1,—1,—
`1,1,1,1,1,1,1,—1,1,—1,},
`
`[0039] where LSl260 is nearly identical to LS 210, except
`the odd tones in LSl260 are negated.
`
`[0040] Continuing with this example, the preambles 200,
`250 are transmitted through the wireless channels 108,110
`by the TX1100 and the TX2102, respectively. The RX1104
`receives a signal that is a combination of the preambles 200,
`250.
`In this example, a portion of the received signal
`representing the combination of LS 210 and LSl260 is
`defined as:
`
`[0041]
`
`r(k): {r(l), r(2), r(3) .
`
`.
`
`. r(52)}.
`
`[0042] The received signal r(k) is a sum of LS 210 and
`LSl260, where LS 210 and LSl260 have been altered by the
`channels 108,110. Thus, based on the definitions of LS 210
`and LSl260 above, r(l) is a sum of two positive tones, r(2)
`is a sum of a positive tone and a negated tone, r(3) is a sum
`of two positive tones, and so forth. The RX1104 effectively
`generates a first system of equations describing such rela-
`tionships, similar to expressions (1)-(52) in Table 1 above.
`The RX1104 then may simplify the equations by estimating
`the H1.k+H2.k terms for all even values of k and H1.k—H2.k
`
`terms for all odd values of k by way of any appropriate
`estimation method, comprising a simple division method, a
`least-squares method or a minimum mean squared error
`method.
`
`the
`[0043] To determine all values of H1.k and H2.k,
`RX1104 may generate a second system of equations by way
`of interpolation, as previously described. By generating a
`second system of equations wherein each equation may be
`combined with a corresponding equation in the first system
`of equations to solve for two unknown values, the RX1104
`is able to solve for all values of H1.k and H2.k,
`thus
`determining for each transmitted frequency tone a complex
`number comprising the phase and gain imparted by the
`wireless channels 108, 110. Thus, the RX1104 has estimated
`both the wireless channels 108,110 and is able to use the
`computations to eliminate signal distortion in future signals.
`
`[0044] Preamble structures for MIMO systems comprising
`three, four or more transmitters may differ from preamble
`structures for MIMO systems comprising two or fewer
`transmitters. For example, in the system of FIG. 1d above,
`a single long sequence in each transmitted signal generates
`two equations for each frequency tone, which are sufficient
`to calculate all values of H1.k and H2.k. However,
`the
`addition of a third transmitter and a third transmitted signal
`comprising a single long sequence would introduce a third
`unknown series of frequency tones. In such a case, for each
`frequency tone, two equations would be insufficient to solve
`for three unknown values H1.k, H2.k and H3.k. Thus, in a
`system with three or four transmitters, it would be necessary
`to structure transmitted preambles such that each preamble
`comprises an additional long sequence. An additional long
`sequence would provide additional equations. Thus, for a
`system with three transmitted signals, there would exist a
`sufficient number of equations to solve for all three unknown
`values H1.k, H2.k and H3.k.
`
`. r(52.0)}
`.
`[0055] The first frequency tone series {r(1.0) .
`of received signal r(k.0,k.1) is a sum of LS 436, LS 440 and
`LSl444, where LS 436, 440 and LSl444 have been altered
`by channels 308, 310, 314, respectively. Thus, r(1.0) is a
`sum of three positive tones, r(2.0) is a sum of two positive
`tones and a negated tone, r(3.0) is a sum of three negated
`tones, r(1.1) is a sum of two positive tones and a negated
`tone, and so forth. The RX1306 effectively generates a first
`system of equations based on the long sequences 408, 418,
`428. In a fashion similar to the technique described for Table
`1 above, the RX1306 manipulates the first system of equa-
`tions to produce a system of equations relating values of
`H1.k, H2.k and H3.k. The RX1306 then generates a second
`system of equations by way of interpolation.
`
`
`
`
`
` T 1011
`HUAW“.
`*1X-I
`%
`
`
`VS.
`SPI—I
`HUAW 4'.
`
`
`
`
`
`000008
`
`HUAWEI EXHIBIT 1011
`HUAWEI VS. SPH
`
`000008
`
`

`

`US 2005/0054313 A1
`
`Mar. 10, 2005
`
`[0056] All values of H1.k, H2.k and H3.k cannot yet be
`determined, because three equations are required in order to
`determine three unknown values, for reasons previously
`discussed. Thus, the RX1306 may further generate a third
`system of equations using the long sequences 414, 424, 434
`in a fashion similar to that used to generate the first system
`of equations. The RX1306 manipulates the third system of
`equations to produce a system of equations relating values of
`H1.k, H2.k and H3.k (in systems comprising four transmit-
`ters, this third system of equations relating H1.k, H2.k and
`H3.k would be interpolated to generate a fourth system of
`equations). The second frequency tone series {r(1.1) .
`.
`.
`r(52.1)} of r(k.0,k.1) is a sum of LS 438, —LS 442 and
`LSl446, where LS 438, —LS 442 and LSl446 have been
`altered by the channels 308, 310, 314, respectively.
`
`the
`[0057] By generating three systems of equations,
`RX1306 is able to solve for each value of H1.k, H2.k and
`H3.k for all k, computing for each frequency tone in each
`signal a complex number comprising the phase and gain
`imparted by the appropriate wireless channel 308, 310, or
`314. Thus, the RX1306 has estimated the wireless channels
`308, 310, 314 and is able to use the computations to
`eliminate signal distortion in future transmissions.
`
`[0058] The subject matter disclosed herein may be applied
`to
`any
`orthogonal
`frequency
`division multiplexing
`(“OFDM”) based system. While illustrative embodiments
`comprising two, three and four transmitters were discussed,
`the techniques described above are scalable and may be
`implemented in a wireless local area network (“WLAN”)
`system comprising any number of transmitters and receiv-
`ers. The above subject matter also is backwards compatible
`with pre-eXisting technology. The above discussion is meant
`to be illustrative of the principles and various embodiments
`of the present invention. Numerous variations and modifi-
`cations will become apparent to those skilled in the art once
`the above disclosure is fully appreciated. It is intended that
`the following claims be interpreted to embrace all such
`variations and modifications.
`
`What is claimed is:
`
`1. A method, comprising:
`
`encoding a plurality of signals according to a predeter-
`mined negation scheme;
`
`transmitting said plurality of signals, each signal trans-
`mitted by way of a wireless channel;
`
`receiving a signal, wherein said signal is a combination of
`the plurality of transmitted signals;
`
`interpolating between data in the received signal to gen-
`erate a plurality of systems of equations; and
`
`solving the plurality of systems of equations to determine
`a gain and phase shift applied to each of the plurality of
`transmission signals by a corresponding wireless chan-
`nel.
`
`2. The method of claim 1, further comprising using the
`gain and phase shift
`to eliminate distortion in received
`signals.
`3. The method of claim 1, wherein encoding a plurality of
`signals comprises negating odd tones of negative frequency
`and even tones of positive frequency.
`
`4. The method of claim 1, wherein encoding a plurality of
`signals comprises negating even tones of negative frequency
`and odd tones of positive frequency.
`5. The method of claim 1, wherein encoding a plurality of
`signals comprises generating a plurality of signals with
`different contents.
`
`6. A system, comprising:
`
`a receiver adapted to generate a plurality of equations
`based on data in a received signal and by interpolating
`between data in said recei