|||||||||||
`USOO5802044A
`11
`Patent Number:
`5,802,044
`45 Date of Patent:
`Sep. 1, 1998
`
`United States Patent (19)
`Baum et al.
`
`54
`
`MULTICARRIER REVERSE LINK TMNG
`SYNCHRONIZATION SYSTEM, DEVICE
`AND METHOD
`
`75
`
`Inventors:
`
`Kevin Baum, Rolling Meadows; James
`Robert Kelton, Oakpark; Philip David
`Rasky, Buffalo Grove, all of Ill.
`73) Assignee: Motorola, Inc., Schaumburg, Ill.
`
`"Multicarrier Modulation for Data Transmission: An Idea
`Whose Time Has Come". John A. C. Bingham, May 1990
`IEEE Communications Magazine, pp. 5-8, 11-14.
`"Analysis and Simulation of a Digital Mobile Channel
`Using Orthoganal Frequency Division Multiplexing",
`Leonard J. Cimini, Jr IEEE Transactions on communica
`tions, vol. Con-33, No. 7, Jul. 1985, pp. 665-675.
`"Frequency Synchronization Algorithms for OFDM Sys
`tems suitable for Communication over Frequency Selective
`Fading Channels", Ferdinand Classen, Heinrich Meyer,
`1994 IEEE, p. 16551659.
`Primary Examiner-Benedict V. Safourek
`Assistant Examiner-Seema S. Rao
`Attorney, Agent, or Firm-Darleen J. Stockley
`57
`ABSTRACT
`The present invention provides a communication system,
`device and method of reverse link symbol timing synchro
`nization of transmited signals to facilitate reverse link timing
`synchronization. A base unit transmits a forward link signal,
`receives areverse link signal, and determines atiming offset
`for signals received on a reverse link timing synchronization
`channel, wherein the reverse link synchronization channel
`comprises a plurality of adjacent reverse link carrier fre
`quencies that are utilized by a multicarrier subscriber unit to
`facilitate reverse link symbol timing synchronization. A
`plurality of multicantier subscriber units receive a forward
`link signal and transmit a reverse link symbol timing syn
`chronization burst on a reverse link timing synchronization
`channel wherein the reverse link timing synchronization
`channel comprises a plurality of adjacentreverse link carrier
`frequencies that are utilized by each subscriber unit, and
`adjusts a timing reference for transmitting signals to facili
`tate reverse link timing synchronization.
`
`25 Claims, 4 Drawing Sheets
`
`500
`
`-
`
`
`
`21
`22
`51
`52
`
`58
`
`56)
`
`5.434,905
`5509,016
`5,617,410
`5,638,361
`5,640,396
`5,684,794
`
`Appl. No.: 639,155
`Filed:
`Apr. 26, 1996
`Int. Cl. H04J 3/16
`U.S. C. .................... 370/330; 370/337; 370/344;
`370/347; 370/350; 370/447; 455/69; 37.5/362
`Field of Search ................................. 370/310,321,
`370/324,337,344, 345, 347, 348,350,
`330; 375/316,340,344,354, 362,365,
`371, 375; 379/56, 58-61; 455/39, 49.1,
`53.1, 54.1, 56.1, 67.1, 68, 69
`References Cited
`U.S. PATENT DOCUMENTS
`7/1995 Maeda et al. .......................
`4/1996 Muller ...
`4/1997 Matsumoto ...
`6/1997 Ohlson et al.
`400
`6/1997 Cudak et al. .
`11/1997 Lopez et al. ............................
`OTHER PUBLICATIONS
`"Mobile Radio Communications", Raymond Steele (Ed),
`Pentech Press Limited, London 1992, pp. 43-45, 68, 69.
`"Digital Cellular Radio", George Calhoun, Artech House,
`Inc. Norwood, MA, 1988, pp. 277, 286, 288.
`
`BASE
`
`SU
`RECEIVER
`SU
`TRANSITTER
`BASE
`RECEIVER
`
`SU
`TRANSITTER
`BASE
`
`RCIER
`
`-a-2Al
`( - A
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 1
`
`

`

`Sep. 1, 1998
`
`Sheet 1 of 4
`
`5,802,044
`
`os!
`
`NTT3S83A34
`
`YNI1
`
`09t
`
`U.S. Patent
`
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`CTISVEuyAYOd
`(T3NNYHOS1STHS30NT0NI)091JONIUIIININNS
`
`
`
`
`NIT3S¥3A34
`
`
`
`
`
`(TANNVHOSESTHS3QNTONI)LINSNVYLTWNOTSS1STY
`
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`IANieea_l
`|LININ3MISNOY][YOLYYINGD
`|LINNNOTIVATY30
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`
`
`LINNJSVE3ATL4OLVYINID
`
`
`
`JONIYIIININVYI
`
`LINNNOTLVAIYIG
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 2
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 2
`
`
`
`

`

`U.S. Patent
`
`Sep. 1, 1998
`ssRBERN
`
`Sheet 2 of 4
`
`5,802,044
`
`|
`
`TRANSI
`IIMEBASE UNI
`RAVE REFERENCE
`DERIVATION UNIT
`
`- - - - - -
`
`- - - - - - BAs
`DSP/ASIC/PROC
`TRANSIT
`ADUSTMENT
`|ESSAGE GENERATOR
`
`
`
`
`
`TTTT -
`
`
`
`-204
`
`200
`
`320-BASE UNIT
`TIVE REFERENCE
`BASE H DHD
`TRANSITTER
`
`BASE
`RECEIVER
`
`- a -
`3101
`
`300
`
`40
`A
`-
`a+2A
`/
`420
`
`-
`
`ls
`-
`
`-
`
`410
`-
`
`400
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 3
`
`

`

`U.S. Patent
`
`Sep. 1, 1998
`
`Sheet 3 of 4
`
`5,802,044
`AVC 6
`
`BASE
`TRANSITTER
`SU
`REcivER
`SU
`TRANSITTER
`BASE
`RECEIVER
`
`BASE
`TRANSITTER
`
`500
`
`H
`
`S H REcivER
`SU wins R
`B.S.
`RECEIVER
`
`-a-2Al
`| (- A -
`
`510
`
`
`
`ATCHED FILTER
`(SLIDING INTEGRATOR)
`
`
`
`
`
`
`
`SYBOL TIVING
`ESTIVATOR
`
`SYBOL
`TING
`OFFSET
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 4
`
`

`

`U.S. Patent
`AVC. 27
`700
`
`Sep. 1, 1998
`
`Sheet 4 of 4
`
`5,802,044
`
`710
`
`- - - -- or drum sur unar w
`
`a m - - - - -
`
`GDAED
`DATA CARRIER
`DOWN-MIXER
`
`FRON
`RECEIVER
`
`
`
`ODULATED DATA
`CARRIER FREQUENCY
`
`!----- St St.
`AVC 6
`
`
`
`TTE -->
`
`SYMBOL
`TIVING
`offs,
`
`800
`
`
`
`TRACKING WODE SINGE
`
`Eifiotto:
`
`GUARD
`CUARD
`cuARD
`GUARD
`Trf
`
`
`
`
`
`820
`
`Tr/Trf
`Triff
`Tr/Trf
`Triff
`Tr/Tri
`Triff
`Trf
`If I
`810 INITIAL ACCESS
`RSTS CHANNE
`AAC.9 900
`
`
`
`Trf Tr/Trf
`Trf Tr/Trf
`Triff
`Triff
`Trf
`Tri
`
`Triff
`Tr/Trf
`Triff
`Trf
`
`GUARD
`CUARD
`GUARDN
`Trf
`
`RSS CHANNEL
`
`810
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 5
`
`

`

`5,802,044
`
`10
`
`15
`
`1
`MULTICARRIER REVERSE LINK TMING
`SYNCHRONIZATION SYSTEM, DEVICE
`AND METHOD
`FIELD OF THE INVENTION
`The present invention relates generally to communication
`systems and, in particular, to bandwidth efficient, multiple
`user two-way communication systems.
`BACKGROUND OF THE INVENTION
`Communication systems are known to comprise a plural
`ity of subscriber units that communicate with one or more
`base or headend units via signals communicated over the air
`or over a wireline network. One such communication system
`is a two-way wireless communication system. In a two-way
`wireless communication system, a service access point is
`provided by a base unit which commonly includes a trans
`mitter and receiver, or transceiver. The base unit may
`provide connectivity to another network such as the Public
`Switched Telephone Network, commonly referred to as the
`PSTN. Remote service connection is provided by a device
`referred to as a subscriber unit (Su), since service access is
`often subscription-based. These subscriber units may be
`mobile transceivers often consisting of handheld
`"telephone-like” devices which communicate with the base
`units via the RF spectrum. Each subscriber unit conveys
`information to the base unit by transmitting a signal to the
`base unit. The signal transmitted by a subscriber unit to the
`base unit may be referred to as a reverse link signal, or
`uplink signal. The base unit conveys information to each
`subscriber unit by transmitting a signal which may be
`referred to as a forward link signal or downlink signal.
`As the use of wireless communication systems continues
`to expand, more of the available RF spectrum is becoming
`occupied. Therefore, it is desirable for modern communica
`tion systems to be bandwidth-efficient. Orthogonal Fre
`quency Division Multiplexing (OFDM) is a multicarrier
`modulation method known in the art which allows a high
`rate digital data stream to be divided into a set of lower rate
`digital data streams, each of which are modulated onto a
`separate data carrier signal. The modulated data carrier
`signals have distinct carrier frequencies, but the carrier
`frequencies are closely spaced such that the spectra of
`adjacent modulated data carrier signals have significant
`overlap, as is known in the art.
`Because of good bandwidth efficiency potential and
`robustness to certain types of channel impairments, OFDM
`aS
`is currently utilized in broadcast and wireline applications,
`including but not limited to Digital Audio Broadcasting
`(DAB) and wireline modems. Moreover, OFDM is usable
`for the forward link of multiple user two-way communica
`tion systems. In these applications, the relationship between
`the multiple modulated data carrier signals can be controlled
`easily (using the Discrete Fourier Transform, for example)
`since they are all generated within a single transmitter unit.
`In the case of DAB, the modulated data carrier signals are
`generated within a single transmit source, summed, and
`broadcasted simultaneously. In the case of a communication
`system forward link, the modulated data carrier signals are
`generated within a single base unit, summed, and transmit
`ted simultaneously.
`A need exists for a reverse link timing synchronization
`method which is well suited for systems having subscriber
`units which transmit (Tx) multicarrier reverse link signals.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic representation of a two-way com
`munication system utilizing a reverse link symbol timing
`synchronization scheme in accordance with the present
`invention.
`
`55
`
`2
`FIG. 2 is a block diagram of a DSPIASIC/microprocessor
`for a base unit and a subscriber unit of a two-way commu
`nication system utilizing a reverse link symbol timing syn
`chronization scheme in accordance with the present inven
`tion.
`FIG. 3 is a schematic representation of a signal transmis
`sion and reception timing diagram of a two-way communi
`cation system in the absence of propagation delay.
`FIG. 4 is a schematic representation of a signal transmis
`sion and reception timing diagram of a two-way communi
`cation system in the presence of propagation delay.
`FIG. S is a schematic representation of a comparison of
`the signal transmission and reception timing diagram of
`FIG. 3 and the signal transmission and reception timing
`diagram of a communication system employing reverse link
`symbol timing synchronization in the presence of propaga
`tion delay.
`FIG. 6 is a block diagram illustrating a base unit which
`includes areverse linksymbol timing synchronization single
`carrier demodulator in accordance with the present inven
`tion.
`FIG. 7 is a block diagram illustrating a base unit which
`includes a tracking mode single carrier demodulator in
`accordance with the present invention.
`FIG. 8 is a diagram illustrating one embodiment of an
`initial access reverse link symbol timing synchronization
`channel format and one embodiment of a tracking mode
`reverse link symbol timing synchronization channel format
`in accordance with the present invention.
`FIG. 9 is a diagram illustrating another embodiment of of
`an initial access reverse link symbol timing synchronization
`channel format in accordance with the present invention.
`DETALED DESCRIPTION OF APREFERRED
`EMBODMENT
`The present invention provides reverse link symbol tim
`ing synchronization method well Suited for communication
`systems having bandwidth efficient spectrally overlapping
`transmissions on the reverse link, where multiple reverse
`link transmissions occur simultaneously from a plurality of
`subscriber units, hereinafter referred to as overlap band
`width subscriber units because their reverse link signal
`frequency spectra may overlap.
`Mutual interference between the multiple transmitting
`overlapbandwidth subscriber units is avoided by controlling
`and coordinating the parameters of the transmit signals from
`the separate overlap bandwidth subscriber units. By allow
`ing OFDM-like spectrally overlapping transmissions in the
`reverse link of the two-way communication system as in
`FIG. 1, numeral 100, the present invention enables the
`reverse link (150) of the two-way communication system to
`obtain bandwidth efficiency similar to that of the forward
`link (160) of the two-way communication system assuming
`OFDM is used in the forward link of the two-way commu
`nication system.
`In the present invention, a reverse link symbol timing
`synchronization method which can be used in a system
`having a plurality of transmitting overlap bandwidth sub
`scriber units on an OFDM-like spectrally overlapping
`reverse link is provided. Begin by considering the general
`case of a overlap bandwidth subscriber unit which is trans
`mitting a reverse link signal. The reverse link signal is a
`digitally modulated signal that includes one or more modu
`lated data carrier signals. A modulated data carrier signal is
`a carrier which is modulated by a digital information source
`
`35
`
`40
`
`55
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 6
`
`

`

`5,802,044
`
`4
`
`3
`in the transmitting unit, where the modulation method may
`comprise M-ary Quadrature Phase Shift Keying (M-PSK),
`M-ary Quadrature Amplitude Modulation (QAM), or other
`digital modulation method which may be known in the art.
`In OFDM, each of the carriers is commonly referred to as a
`subcarrier or a tone. A modulated data carrier signal has an
`associated carrier frequency, symbol time reference, and
`symbol pulse-shape function. The symbol time reference
`determines the symbol timing when a reverse link transmis
`sion occurs. The pulse-shape function for the modulating
`symbols on a modulated data carrier signal is selected from
`any of the known OFDM compatible pulse-shapes. The most
`common pulse-shape functions have a constant value over
`the pulse-shape function duration. If the pulse-shape func
`tion holds a constant value over the entire pulse-shape
`
`10
`
`Because all of the modulated data carrier signals com
`prising the OFDM signal have identical symbol time refer
`ences and are spaced in frequency by 1?ts, any overlap
`bandwidth subscriber unit is able to detect the data on any
`of the modulated data carriers without interference from the
`other modulated data carriers. This detection is performed
`by integrating the received (Rx) OFDM signal over a
`constant time window of length ts. The equation below
`shows the detection of the n" symbol in time by the p"
`overlap bandwidth subscriber unit given the reception of the
`OFDM signal, x(t).
`
`i.-:
`J (n-1)T,
`
`o, as
`
`(n-1)T, +,
`N-1
`A(t)ex) 's
`(n-1)T,
`n=-oot=0
`
`--
`get - (n- 1?
`
`(n-1), +,
`
`= SJ
`
`O (n-1)T,
`
`p-co
`
`Alexit-(n-1)T,” at
`
`--
`
`m
`
`( - 1)T, -
`
`11" Asex, as
`to (-1)T,
`
`--
`
`=
`
`(n-1)T,+t,
`Aer Xalt
`(r-1),
`= Aes n
`
`duration, the function is commonly referred to as a rectan
`gular pulse-shape function. A preferred embodiment of a
`pulse shape function, g(t), is defined in the following
`equation.
`
`1 0 is fi (T = (1+A)
`O else
`
`45
`
`In this pulse shape definition, the portion of the pulse
`shape from 0 totsis hereinafter referred to the useful symbol
`portion, and tis is hereinafter referred to as the useful symbol
`duration. The portion of the pulse shape from t to Ts is a
`symbol extension portion commonly referred to as a cyclic
`extension, periodic extension, or guard interval. The cyclic
`extension portion may be placed before the useful symbol
`portion instead of after the useful symbolportion, and in this
`case may also be referred to as a cyclic prefix. A cyclic
`extension is sometimes used in OFDM to improve perfor
`mance in the presence of a multipath channel.
`Consider an OFDM signal based on the defined pulse
`shape. In a preferred embodiment, this signal may transmit
`ted on a forward link by a single base unit to a plurality of
`overlap bandwidth subscriber units. The following equation
`shows an OFDM signal, x(t), based on the defined pulse
`shape function, g(t). The number of subcarriers used for
`transmission is given by N. Transmitted symbols are given
`by x, and channel attenuation and phase rotation are given
`by A(t)e.
`
`SO
`
`55
`
`60
`
`65
`
`Now consider the case where the modulated data carrier
`signals are from two separate overlap bandwidth subscriber
`units, each of which simultaneously transmits a reverse link
`signal that has a modulated data carrier signal, with each
`modulated data carrier signal having a distinct carrier
`frequency, but with overlapping signal spectra. In this case
`since the overlap bandwidth subscriber units are physically
`separated, they contain separate local frequency, time, and
`phase references. These local references are commonly
`based on a local oscillator contained within each overlap
`bandwidth subscriber unit. Also note that the attenuation and
`phase rotation of the second overlap bandwidth subscriber
`unit reverse link signal may differ from the first due to
`propagation path differences. A composite signal received at
`the base unit that includes the sum of the reverse link signals
`from the two overlap bandwidth subscriber units may be
`written as shown in the following equation. The portion of
`the equation identified by k=0 corresponds to the signal
`transmitted on the first reverse link and the portion of the
`equation identified by k=1 corresponds to the signal trans
`mitted on the second reverse link. Time references and local
`frequency references are represented by 8, and v respec
`tively. Note that any propagation delay differences between
`the base unit and the two subscriber units are a part of the
`variable representing the Time reference since the equations
`represent the signals received at the base unit. Attentuations
`due to the channel are represented by A(t) and phase
`rotations due to either the channel or local phase reference
`offsets are represented by e.
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 7
`
`

`

`5
`
`5,802,044
`
`6
`the local frequency references goes to zero as shown in the
`following equation:
`
`--oo
`E
`re-co
`
`E
`O
`
`skiv)--
`
`When the base unit attempts to detect the modulated data
`carrier from the first reverse link, there is generally inter
`ference from the second reverse link. This interference is
`caused by a mismatch between either the time references or
`the local frequency references of the two overlap bandwidth
`subscriber units. The following equation shows the detection
`of the n" symbol transmitted on the first reverse link
`assuming perfect synchronization of the time references and
`a mismatch between the local frequency references.
`Specifically, the two reverse link signals arrive at the base
`unit with identical symbol timing but are no longer spaced
`in frequency by an integer multiple of 1?t Mathematically,
`6–0, 6=0, vo-0, and v7.0.
`
`5
`
`O
`
`15
`
`io
`
`n)dit
`
`(n-1)T,
`
`(n - 1) + 1,
`
`SE
`re-se
`
`pathive
`A(teko Kaga- (n-1)Te
`
`(n - 1) + i
`
`pathw)--
`E Aes Xgit- (n-1)Te
`
`p-co
`
`-; i. O
`
`Ashw)--
`(n-1)+ 1,
`Atekee
`(n-1),
`
`1
`
`(n-1)T,
`
`Aoor xoat
`
`pelvio
`(n-1)T, + 1,
`Aelure
`(n-1),
`
`1.
`
`
`=Aoarx+ Aelu Xu F2(1-v) +vi)
`
`ac-1)--
`e
`
`= Aeon Xo, + Alele * 2n(v) .
`
`t
`
`--
`
`(-1)
`(-1),
`-
`
`act
`
`= Aeon X + Alois * 2n(v)
`-
`
`2.
`e
`
`= Aoets X + Aleis X
`
`plwi)
`arry
`
`(1-ori)
`
`Assuming perfect synchronization between the symbol
`timing of the signals received at the base unit, the interfer
`ence term shown above disappears if the mismatch between
`
`A.
`pictivo
`IX.--LA-X. Alox.- civil-(1-or)
`-
`Archo
`= Aoetor Xo + AleX1 2C1 +0)
`-
`As
`= Aeonxo. + Alexi- - (1-1)
`= Aer X,
`
`1-episo
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 8
`
`

`

`5,802,044
`
`7
`Pertinent to the present invention, the equation below
`shows the detection of the n" symbol transmitted on the first
`reverse link assuming perfect synchronization between the
`two local frequency references and a mismatch between the
`time references. Specifically, the two reverse link signals 5
`arrive at the base unit spaced in frequency by an integer
`multiple of 1/ts, but without identical symbol timing.
`Mathematically, vo-0, v=0, 8-0, and 8a0.
`
`8
`
`1
`= ,
`
`(n - 1)T + i.
`
`e.
`X.
`
`Ask
`1
`S. A(t)00Xgt- (n-1)T,-6e's di
`
`-
`
`1.
`(n-1)T, +,
`, (n-1)T,
`
`- A.
`
`--
`"f Alexit-(n-1)7,-8", at
`
`(n-1)7,+t,
`(n-1)T, + a
`All-xt-(n-1)T-8,
`g A.-----(n-1)Taif
`=
`" J (n-1)T,
`m-->
`(n-1)T, m-ce
`
`--
`
`at
`
`1.
`=
`
`1.
`(n-1)T, + 1,
`Adelaxed +.
`(n-1)T,
`
`px
`(n-1)T + 1, -e-.
`S. Aeluxgf-(n-1)7,-8ke a dit
`J (n-1)T, PP
`
`=Aoetox. +
`
`pac--
`(n-1)T, +, +
`2. Aelaxga- (n-1)T,-8ke a de
`
`Depending on the value of 6, the equation for the detection
`of the of the n" symbol takes on one of three forms.
`
`CASE 1: -7, C8, C-T +
`
`1.
`A.
`x=Analoxo. +
`
`As--
`(n-1)T + ,
`S. Aeluxgf-(n-1)T,-8ke
`dit
`(n-1)T, mP
`
`= Aoeta + ,
`
`1
`ps -
`nT+8,
`1)T Aolve a dt +
`(n m
`
`px--
`(1)T, to
`T+ S
`Aelur-HXie "a dit
`F.T. b
`
`f
`=Aoetox. + Aelux j2st
`
`ps-
`e
`is
`
`ni
`
`1.
`+ Alientix, f2t
`in-1)T,
`
`(w-)h
`
`as--
`
`arts
`
`= Aoetomo Aelux fi
`
`n
`8
`pr- - -
`
`g
`- - As
`
`=Aoetox. +A1X-A - (-5. fir- -------- |-- ar,
`
`51
`
`5
`
`n
`
`J2E
`2.
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 9
`
`

`

`5,802,044
`
`10
`
`CASE 2: -T, + ¢,< 5, <0
`
`i =ApoOaX _ (2-1)T,+t,
`in =
`ry
`ty
`(n-1)T;
`
`f
`pat
`400
`EZ AlleXimgt —(m—1)T.— ile dt
`mac
`
`a4
`
`dt
`
`pe
`Apo?aX)ne
`
`i,
`
`ta
`2 3
`
`(DTstts
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`the first overlap bandwidth subscriber unit may be summa-
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`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 10
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`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 10
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`€
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`

`5,802,044
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`10
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`Note that the interference term in the above equation
`disappears as the mismatch in symbol timing approaches
`zero. Note that the use of a cyclic extension eases the
`equality requirement for the symbol timing of the two
`reverse link transmissions. The timing references may differ
`by an amount as large as the cyclic extension duration. In
`other words, the requirement on the symbol timing of the
`second reverse link signalis-Ats-8<0 for it not to interfere
`with the first reverse link signal.
`While the preceding analysis was shown for a single
`modulated data carrier from each of two overlap bandwidth
`subscriber units with a carrier spacing near 1/ts, the analysis
`is also applicable to scenarios with more than two overlap
`bandwidth subscriber units and carrier spacings near any
`integer multiple of 1?ts.
`In order to avoid mutual interference, the local frequency
`reference and symbol time reference of each overlap band
`width subscriber unit must be controlled in a predetermined
`manner by the communication system. The frequency ref
`erence requirement is that the modulated data carrier fre
`quencies be spaced by integer multiples of 1?t The time
`reference requirement is that all reverse link signals arrive at
`the base unit with the same symbol timing, which means that
`the beginning of a received symbol is at the same timing
`phase relative to a base unit symbol clock regardless of
`which subscriber unit transmitted the symbol. Note that as
`the previous equations show, differences in the amplitudes
`and phases of the reverse link signals arriving at the base
`unit do not cause interference if the frequency and symbol
`timing parameters are properly controlled. The prior analy
`sis indicates that a bandwidth-efficient multiple user reverse
`link may be implemented by utilizing a frequency spacing of
`as little as 1?t between the modulated data carriers from
`separate overlap bandwidth subscriber unit reverse link
`signals, and that mutual interference can be avoided even
`though the spectra of the reverse link signals overlap.
`The present invention addresses the symbol timing syn
`chronization aspect by providing a reverse link symbol
`timing synchronization (RLSTS) method. Although the pre
`vious set of equations indicate the importance of reverse link
`symbol timing synchronization in a system with overlap
`bandwidth subscriber units, the present invention is also
`very beneficial in systems with multicarrier reverse link
`signals even when the reverse link signal spectra from
`different subscriber units do not overlap. The present
`invention, by providing timing synchronization of all
`reverse link signals, enables the base unit to be simplified
`significantly by enabling the base unit to use the same
`symbol sampling phase for detecting reverse link signals
`from all subscriber units. In a system which combines
`multicarier modulation such as OFDM with Time Division
`Multiple Access (TDMA), the present invention allows the
`symbol sampling phase for detecting a reverse link signal to
`remain the same for all received TDMA time slots.
`Note that in a TDMA based system, a subscriber unit
`typically obtains a received signal symbol, slot, and frame
`timing by monitoring a received forward link signal which
`is transmitted by the base unit. See FIG.3, numeral 300. The
`reverse link slot and frame timing may be defined such that
`a reverse link frame begins at fixed time offset, or and
`numeral 310, from the beginning of a forward link frame
`where time is referenced to the base unit timebase or timing
`reference 320. In a situation where the subscriber unit is
`located relatively far from the base unit, a significant propa
`gation delay can result as shown in FIG.4, numeral 400. If
`the subscriber unit uses the timing of the received forward
`link signal to determine the timing of its reverse link signal
`
`12
`transmission, the propagation delay will result in a delay in
`the reverse link transmit timing reference of the subscriber
`unit as compared to the base unit timing reference. From
`reciprocity, the transmission path from the subscriber unit to
`the base unit will experience the same propagation delay.
`This propagation delay, A and numeral 410, will be given by
`A=de
`where d is the distance of the subscriber unit from the base
`unit, c is the speed of light free space (3x10 m/s), and A is
`the propagation delay in seconds. Therefore, if a subscriber
`unit simply transmits a reverse link signal at a fixed offset
`from a reference timing obtained from the forward link
`signal, the time of arrival of the reverse link signal at the
`base unit will be ot-2A (420) seconds compared to or (320)
`for a subscriber unit co-located with the base unit. The
`quantity 2A is commonly referred to as the round trip
`propagation delay or simply the round trip delay. As an
`example, consider a subscriber unit located 3 km from the
`base unit. The roundtrip propagation delay for such a system
`would be 20 uSec. For an OFDM communication system in
`which the subscriber units may be relatively far from the
`base unit, any significant delays will cause a difference in the
`symbol timing of signals received at the base unit from the
`distant and the co-located subscriber units. As shown in the
`previous equations, this misalignment will cause mutual
`interference. While it is possible through the use of large
`cyclic extensions to mitigate the effect of such delays, this
`is not desirable in many cases because the use of a longer
`cyclic extension will result in reduced system bandwidth
`efficiency. The present invention provides, for a multicarrier
`communication system, a method for adjusting the symbol
`timing reference of each subscriber unit such that the reverse
`link signals from different subscriber units arrive at the base
`unit with the same symbol timing. Having the same symbol
`timing means that the beginning of a received symbol is at
`the same timing phase relative to a base unit timebase/timing
`reference regardless of which of a plurality of subscriber
`unit transmitted the symbol.
`More specifically, the present invention provides a mul
`ticarier reverse link symbol timing synchronization
`(RLSTS) method. In a preferred embodiment of a system
`using the RLSTS method, a subscriber unit advances the
`transmission time of its reverse link signal by 2A to remove
`the effect of propagation delay on the signal received by the
`base unit. This advance of the transmit timing will cause a
`reverse link signal transmitted from a distant subscriber unit
`to arrive at the base unit with the same symbol timing as one
`transmitted from a nearby subscriber unit as shown in FIG.
`5, numeral 500. Note that the RLSTS method must acquire
`an estimate of the round trip propagation delay. Further,
`once known, the round trip propagation delay is preferably
`be tracked or updated periodically to maintain an accurate
`round trip delay estimate as the subscriber unit position
`changes relative to the base unit. Periodic updating of the
`RLSTS may also be beneficial when the subscriber unit is
`stationary because a drift of the subscriber unit timing
`reference may be perceived as a change in propagation
`delay, and hence may be corrected by the present invention.
`The problems of initially acquiring and then tracking the
`round trip propagation delay are treated separately in a
`preferred embodiment of the present invention. A subscriber
`unit will initially acquire symbol and frame level timing
`from a forward link signal transmitted by a base station, as
`known in the art. After this timing is obtained, the subscriber
`unit will perform an initial RLSTS channel access, which
`includes formatting a RLSTS message for a RLSTS channel,
`
`45
`
`55
`
`65
`
`Petitioner Sirius XM Radio Inc. - Ex. 1010, p. 11
`
`

`

`5,802,044
`
`s
`
`15
`
`35
`
`40
`
`13
`and transmitting a RLSTS burst which contains the RLSTS
`message at a predetermined time on the reverse link. The
`predetermined time is preferably at a predetermined time
`offset ot relative to a timing reference point in the signal
`received from the base unit. The RLSTS message is pref
`erably a random or unique data code to differentiate a given
`subscriber unit from others which may attempt initial access
`at the same time. Because of round trip propagation delays,
`the initial RLSTS channel access burst will arrive at the base
`unit at a time 2A seconds later than it would if the subscriber
`unit was co-located with the base unit. The base unit
`measures the time offset between the known moment that an
`RLSTS burst would arrive from a co-located subscriber unit
`and the actual time of arrival to determine the round trip
`propagation delay. After determining the round trip delay,
`the base unit performs an RLSTS acknowledgment. The
`RLSTS acknowledgment process includes transmitting a
`RLSTS acknowledgment burst on the forward link contain
`ing a RLSTS acknowledgment message. The RLSTS mes
`sage preferably includes the measured timing offset and the
`unique code which was received in the reverse link RLSTS
`burst. The unique code is preferably used to confirm the
`identity of the subscriber unit which transmitted the RLSTS
`burst. The subscriber unit, subsequent to receiving the
`RLSTS acknowledgment burst, advances subsequent trans
`mission times by shortening the duration of Otto O-2A (S10),
`causing subsequent transmissions to arrive at the base unit
`with the same symbol timing as would be observed if the
`subscriber unit where co-located with the base unit.
`The first step in the initial acquisition process is for the
`subscriber unit to ascertain the symbol timing of the signal
`received from the base unit using a method known in the art.
`This timing will be delayed by A relative to the time the
`signal was transmitted from the base unit. The subscriber
`unit must then derive the frame level timing of the forward
`link signal. This may also be accomplished by one of several
`methods known in the art, one such method being correla
`tion to a predetermined pattern transmitted by the base unit
`at a predetermined point within each frame.
`As described earlier, after the symbol and frame timing of
`the signal transmitted by the base unit has been determined
`by the subscriber unit, the subscriber unit performs an initial
`RLSTS channel access. The RLSTS burst is preferably
`transmitted at a predetermined time relative to the forward
`link frame timing.
`Note that the RLSTS burst may not arrive at the base unit
`with proper symbol timing due to the round trip delay. To
`avoid interfering with modulated data carriers on nearby
`carrier frequencies several carriers on either side of the
`RLSTS burst are made a part of the RLSTS channel in the
`system. The particular number of such carriers will depend
`on system parameters such as modulation pulse shaping,
`carrier spacing, the susceptibility of the chosen modulation
`to interference, and the maximum tolerable e