WORLD INTELLECTUAL PROPERTY ORGANIZATION
`Intematronal Bureau
`
`
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(51) Imernafionfil Patent C1a53ifiC3“°n 6 3
`H04L 5 06
`I
`
`(11) International Publication Number:
`
`WO 97/30531
`
`(43) International Publication Date:
`
`21 August 1997 (2l.08.97)
`
`(21) International Application Number:
`
`PCT/SE97/00109
`
`(81) Designated States: NO. European patent (AT, BE. CH. DE,
`DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, s13).
`
`(22) International Filing Date:
`
`23 January 1997 (23 .O1.97)
`
`(30) Priority Data:
`9600537-6
`
`14 February 1996 04.02.96)
`
`SE
`
`With international search report.
`
`Published
`
`(71) Applicant: TELIA AB [SE/SE]; Marbackagatan ll, S-123 86
`Farsta (SE).
`
`(72) Inventor: LARSSON, Roger; Blidvagen 53, S-951 48 Lulea
`(SE).
`
`Telia Research AB,
`KARLSSON, Beme;
`(74) Agent:
`Rudsjoterrassen 2, S-136 80 Haninge (SE).
`
`(54) Title:
`
`IMPROVEMENTS IN OR RELATING TO OFDM SYSTEMS
`
`(57) Abstract
`To ensure that the performance of an OFDM system
`is adequate,
`it
`is necessary to provide a guard space
`between symbols so that the effects of mutual interference
`between adjacent symbols is avoided.
`In a conventional
`OFDM system the length of this guard space must be
`equal to, or greater than, the maximum delay spread in the
`system. Bearing in mind that under some conditions the
`delay spread in a mobile system may be large, while under
`other conditions the delay spread may be low, the use of
`a fixed guard space corresponding to the longest delay
`spread, imposes an overhead on the transmission capacity
`of a communications channel. This conflict between
`channel traffic capacity and the need for long guard spaces
`between symbols can be overcome by using a variable
`length guard space, i.e. a guard space of variable duration.
`A receiver is able to instruct a transmitter to increase the
`duration of the guard space if the system perfonnance is
`degraded.
`
`amplitude
`
`0 6
`'
`
`5
`
`4
`
`0 7
`‘
`0
`
`0
`
`2
`
`3
`
`4
`
`5
`6
`7
`fime(s)x70'5
`
`Facebook's Exhibit No. 1048
`
`Page 1
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`Facebook's Exhibit No. 1048
`Page 1
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`

`
`FOR THE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international
`applications under the PCT.
`
`United Kingdom
`Georgia
`Guinea
`Greece
`Hungary
`Ireland
`Italy
`Japan
`Kenya
`Kyraystan
`Democratic People’s Republic
`of Korea
`Republic of Korea
`Kazakhstan
`Liechtenstein
`Sri Lanka
`Liberia
`Lithuania
`Luxembourg
`Latvia
`Monaco
`Republic of Moldova
`Madagascar
`Mali
`Mongolia
`Mauritania
`
`Malawi
`Mexico
`Niger
`Netherlands
`Norway
`New Zealand
`Poland
`Portugal
`Romania
`Russian Federation
`Sudan
`Sweden
`Singapore
`Slovenia
`Slovakia
`Senegal
`Swaziland
`Chad
`Togo
`Tajikistan
`Trinidad and Tobago
`Ukraine
`Uganda
`United States of America
`Uzbekistan
`Viet Nam
`
`Facebook's Exhibit No. 1048
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`
`Amienia
`Austria
`Australia
`Barbados
`Belgium
`Burkina Faso
`Bulgaria
`Benin
`Brazil
`Belarus
`Canada
`Central African Republic
`Congo
`Switzerland
`Cote d'lvoire
`Cameroon
`China
`Czechoslovakia
`Czech Republic
`Germany
`Denmark
`Estonia
`Spain
`Finland
`France
`Gabon
`
`AM
`AT
`AU
`BB
`BE
`BF
`BG
`
`MB
`
`R
`BY
`CA
`CF
`CG
`CH
`CI
`CM
`CN
`CS
`CZ
`DE
`DK
`EE
`ES
`Fl
`FR
`GA
`
`Facebook's Exhibit No. 1048
`Page 2
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`

`
`WO 97/30531
`
`PCT/SE97/00109
`
`_ 1 _
`
`Improvements in or Relating to OFDM Systems
`
`The present
`
`invention relates to OFDM (orthogonal
`
`frequency
`
`division multiplex)
`
`telecommunications
`
`systems,
`
`and methods of operating such systems, more
`
`particularly to the use of variable guard spaces in such
`
`systems.
`
`In
`
`an
`
`OFDM system data
`
`is modulated
`
`onto a
`
`broadband signal comprising a large number of individual
`
`frequency carriers which
`
`form a
`
`frequency division
`
`multiplex.
`
`The bandwidths of
`
`the individual
`
`frequency
`
`channels, are small and arranged so that the maximum of
`
`sin(x)/x), power spectrum, of
`(sinc(x)
`the sinc(x),
`one channel, corresponds with the first minimum in the
`
`sinc(x) power spectrum of
`
`the adjacent channels.
`
`In
`
`other words,
`
`the channel separation equals
`
`l/(symbol
`
`length), for rectangular symbols.
`
`It is for this reason
`
`that adjacent channels are described as "orthogonal".
`
`OFDM systems normally use a FFT (fast fourier transform)
`
`process
`
`to
`
`demodulate
`
`the
`
`data
`
`signal
`
`from the
`
`transmitted signal.
`
`Convolution error coding and FFT
`
`may be employed at
`
`the modulator (transmitter) stage.
`
`In
`
`the
`
`receiver,
`
`complementary
`
`FFT processing
`
`is
`
`combined with Viterbi decoding, at
`
`the demodulator
`
`stage. This ensures that the overall bit error rate is
`
`very low.
`
`This particular variant of OFDM is known as
`
`COFDM (Coded Orthogonal Frequency Division Multiplex).
`
`In recent years, COFDM systems have been developed for
`
`a variety of broadcasting applications, e.g. for digital
`
`audio broadcasting
`
`and
`
`high definition TV.
`
`For
`
`convenience,
`
`in this specification the term OFDM is used
`
`to refer to both OFDM and COFDM.
`
`The next
`
`generation of mobile
`
`communications
`
`systems may, with advantage, employ OFDM. However,
`
`in
`
`10
`
`15
`
`20
`
`25
`
`30
`
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`
`W0 97/30531
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`PCTISE97/00109
`
`_ 2 _
`
`mobile communications
`
`systems
`
`the distance between a
`
`receiver
`
`and
`
`transmitter
`
`is not
`
`constant
`
`and,
`
`in
`
`addition,
`
`the terrain between receiver and transmitter
`
`may vary.
`
`This results in signal dispersion in the
`
`frequency domain, or delay spread in the time domain,
`
`in
`
`a communications channel employing OFDM.
`
`To ensure that the performance of an OFDM system is
`
`adequate,
`
`it
`
`is necessary to provide a guard space
`
`between
`
`symbols
`
`so
`
`that
`
`the ‘effects
`
`of mutual
`
`interference between adjacent symbols is avoided.
`
`In a
`
`conventional OFDM system,
`
`the length of this guard space
`
`must be equal
`
`to, or greater than,
`
`the maximum delay
`
`spread in the system. Bearing in mind that under
`
`some
`
`conditions the delay spread in a mobile system may be
`
`large, while under other conditions the delay spread may
`
`be low,
`
`the use of a fixed guard space corresponding to
`
`the longest delay spread,
`
`imposes an overhead on the
`
`transmission capacity of a communications channel.
`
`Any
`
`compromise in the selection of the length of guard space
`
`will,
`
`in some circumstances, cause a loss of transmitted
`
`data.
`
`It
`
`is
`
`an object of
`
`the present
`
`invention to
`
`minimise
`
`the
`
`overhead
`
`imposed
`
`on
`
`channel
`
`traffic
`
`capacity in an OFDM system by the use of a guard space
`
`between symbols, without degrading system performance.
`
`The
`
`use
`
`of
`
`guard
`
`spaces
`
`in
`
`mobile
`
`telecommunications
`
`systems
`
`is well known.
`
`European
`
`Patent Application EP 0,295,227 describes a digital TDMA
`
`(time division multiple access) mobile telephone system
`
`in which time slot assignment
`
`is made in dependence on
`
`the propagation time between different mobile stations
`
`and a base station.
`
`Each time frame includes a guard
`
`space used to avoid overlap between transmissions to, or
`
`from, different mobile stations.
`
`10
`
`15
`
`20
`
`25
`
`30
`
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`
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`

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`WO 97130531
`
`PCT/SE97/00109
`
`_ 3 _
`
`US Patent 5,371,548 discloses the use of guard
`
`spaces with OFDM systems.
`
`European Patent Application
`
`EP
`
`0,589,709
`
`A2
`
`discloses an OFDM system employing quadrature amplitude
`
`modulation, or modified phase shift keying,
`
`in which
`
`long guard times and/or wider
`
`channel
`
`spacings are
`
`employed.
`
`US Patent 4,937,819 discloses a
`
`'TDMA
`
`system in
`
`which ranging, with respect
`
`to time,
`
`is employed to
`
`10
`
`reduce guard times between successive transmissions.
`
`The present
`
`invention
`
`overcomes
`
`the
`
`conflict
`
`between channel traffic capacity and the need for long
`guard spaces between symbols by using a variable length
`
`guard space,
`
`i.e.
`
`a guard space of variable duration.
`
`15
`
`A receiver is able to instruct a transmitter to increase
`
`the
`
`duration
`
`of
`
`the
`
`guard
`
`space
`
`if
`
`the
`
`system
`
`performance is degraded.
`
`20
`
`25
`
`30
`
`The use of variable duration guard spaces in OFDM
`
`systems is not disclosed in the prior art.
`
`According
`
`to
`
`a
`
`first
`
`aspect
`
`of
`
`the
`
`present
`
`invention,
`
`there is provided an OFDM system comprising
`
`a
`
`receiver and a
`
`transmitter,
`
`said transmitter being
`
`adapted to transmit data in OFDM symbol bursts in which,
`
`in the time domain,
`
`a portion of’ a
`
`symbol burst
`
`is
`
`repeated within said symbol burst as a guard space, said
`
`receiver
`
`having
`
`a
`
`time window with which,
`
`after
`
`demultiplexing,
`
`said symbol burst must
`
`be aligned,
`
`characterised in that said repeated portion of a symbol
`
`burst has
`
`a duration which is variable and which is
`
`adjusted, during system operation,
`
`to a minimum duration
`
`necessary to prevent data loss caused by delay spread.
`
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`W0 97/30531
`
`PCT/SE97/00109
`
`_ 4 _
`
`Preferably said receiver is adapted to vary said
`
`time window to produce, on demultiplexing, a guard space
`
`of a minimum duration necessary to prevent data loss
`
`caused by delay spread.
`
`The portion of said symbol burst which is repeated
`
`may be copied from a leading edge of said symbol burst.
`
`Said repeated portion of said symbol burst may be
`
`repeated at a trailing edge of said symbol burst.
`
`10
`
`to said transmitter
`
`indicative of
`
`a
`
`level of delay
`
`Said receiver may be adapted to transmit a signal
`
`spread in a
`
`received signal,
`
`and said transmitter is
`
`adapted to adjust said guard space duration to a minimum
`
`value necessary to prevent data loss caused by delay
`r
`
`spread.
`
`15
`
`20
`
`25
`
`Preferably said receiver
`
`is adapted to detect
`
`a
`
`transmitted signal when said time window is incorrectly
`
`adjusted.
`
`Said OFDM system may be adapted to operate as a
`
`mobile telecommunications system.
`
`According to a
`
`second
`
`aspect of
`
`the present
`
`invention there is provided a receiver, for use with an
`
`OFDM system including a transmitter adapted to transmit
`
`data in OFDM symbol bursts,
`
`in which,
`
`in the
`
`time
`
`domain, a portion of a symbol burst
`
`is repeated within
`
`said symbol burst as a guard space, said receiver having
`
`a
`
`time window with which, after demultiplexing,
`
`said
`
`symbol burst must be aligned, characterised in that said
`
`repeated portion of a symbol burst has a duration which
`
`is variable and said receiver is adapted to vary said
`
`'30
`
`time window to produce, on demultiplexing, a guard space
`
`of a minimum duration necessary to prevent data loss
`
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`W0 97/30531
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`PCT/SE97/00109
`
`caused by delay spread.
`
`Preferably
`
`said
`
`receiver
`
`includes
`
`performance
`
`measuring means
`
`for measuring received signal quality
`
`and reception window adjusting means for adjusting said
`
`time window duration in response to a signal
`
`from said
`
`performance measuring means.
`
`Preferably said receiver is adapted to transmit a
`
`signal
`
`to said transmitter
`
`indicative of a
`
`level of
`
`delay spread in a received signal, and said transmitter
`
`10
`
`is adapted to adjust said guard space duration to a
`
`minimum value necessary to prevent data loss caused by
`
`delay spread.
`
`Said
`
`receiver may
`
`be
`
`adapted
`
`to
`
`detect
`
`a
`
`transmitted signal when said time window is incorrectly
`
`15
`
`adjusted.
`
`According
`
`to
`
`a
`
`third aspect
`
`of
`
`the
`
`present
`
`invention there is provided a method of operating an
`
`OFDM system comprising a receiver and a transmitter, by
`
`transmitting data as a series of OFDM symbol bursts,
`
`each symbol burst,
`
`in the time domain, having a portion
`
`thereof repeated therein as a guard space, said receiver
`
`having a time window with which, after demultiplexing,
`
`said symbol burst must be aligned, characterised by the
`
`repeated portion of
`
`a
`
`symbol burst having a variable
`
`duration,
`
`and
`
`adjusting said variable duration to
`
`produce a guard space of a minimum duration necessary to
`
`prevent data loss caused by delay spread.
`
`Preferably said receiver time window is adjusted to
`
`produce, on demultiplexing, a guard space of a minimum
`
`duration necessary to prevent data loss caused by delay
`
`spread.
`
`20
`
`I5
`
`30
`
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`
`W0 97/30531
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`PCT/SE97/00109
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`_ 5 _
`
`The repeated portion of said symbol burst may be
`
`copied from a leading edge thereof.
`
`The repeated portion of said symbol burst may be
`
`repeated at a trailing edge of said symbol burst.
`
`Said receiver may measure received signal quality
`
`and may adjust said time window duration so that said
`
`received
`
`signal
`
`quality
`
`exceeds
`
`a
`
`predetermined
`
`threshold.
`
`10
`
`by way of example, with reference to the accompanying
`
`Embodiments of the invention will now be described,
`
`drawings,
`
`in which:
`
`Figure 1 shows a typical impulse response for hilly
`
`rural terrain.
`
`15
`
`terrain.
`
`Figure 2 shows a typical impulse response for urban
`
`Figure 3 shows a typical impulse response for the
`
`interior of a building.
`
`Figure 4
`
`shows
`
`a
`
`typical
`
`impulse response for
`
`a
`
`micro cell
`
`in a cellular digital mobile telephone
`
`20
`
`system.
`
`Figure 5 is a diagrammatic illustration of an OFDM
`
`transmission burst.
`
`Referring to Figure 1.
`
`to 4,
`
`there is shown the
`
`effect of terrain on delay spread,
`
`in an OFDM system.
`
`25
`
`As can be seen from the drawings delay spread can vary
`
`from 0.2115 to 50115.
`
`If a guard space is to be used
`
`which will avoid inter—symbol
`
`interference, caused by
`
`delay spread,
`
`in all of the different environments for
`
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`PCT/SE97/00109
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`_ 7 _
`
`which impulse responses are illustrated in Figures 1
`
`to
`
`A, it will need to be equal, or greater than, 50us.
`the case where delay spread is equall to 0.2us
`
`In
`this
`
`imposes an unnecessary loss of channel capacity.
`
`It
`
`should be noted that the delay spread in the time domain
`
`transforms
`
`to frequency dispersion in the frequency
`
`domain. whether
`
`the effect
`
`is viewed as a
`
`frequency
`
`dispersion, or
`
`a
`
`time dispersion,
`
`the effect
`
`is the
`
`same, i.e.
`
`loss of channel capacity.
`
`The use of a guard space in an OFDM system,
`
`to
`
`overcome the effect of delay spread,
`
`is illustrated in
`
`Figure 5.
`
`The guard space takes the form of a cyclic
`
`repetition of the first part of an OFDM burst, i.e.
`
`the
`
`beginning of
`
`the OFDM burst
`
`is copied and repeated at
`
`the end of
`
`the burst.
`
`The OFDM symbol duration is
`
`denoted by Ts and the duration of
`
`the guard space is
`
`denoted by Tg.
`which it
`can
`
`An OFDM receiver has a time window in
`receive zul OFDM symbol.
`The minimum
`
`duration of
`
`this
`
`time window is equal
`
`to TV
`
`receiver can,
`
`therefore, only see that part of
`
`The
`
`the
`
`signal which is not destroyed by delay spread in the
`
`communications channel.
`
`A receiver must,
`
`therefore,
`
`The
`+ Tg.
`time window which corresponds to TS
`have a
`loss of performance imposed by the use of a guard space
`
`By using a variable
`l0log(TflTs)dB.
`Tg is A, where: A
`guard space, as opposed to a fixed guard space having a
`
`duration corresponding to the maximum needed to overcome
`
`the effects
`
`of delay spread,
`
`the
`
`loss
`
`of
`
`system
`
`10
`
`15
`
`20
`
`25
`
`performance is reduced to the minimum extent necessary
`
`30
`
`to avoid loss of data.
`
`In order
`
`to take account of
`
`known delay spread in different environments, Tgmust be
`capable of being varied from 0.2us to 50us.
`
`The concept of the present
`
`invention, namely the
`
`use of a variable guard space can be implemented in an
`
`35
`
`OFDM system as explained below.
`
`The guard space is
`
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`W0 97/30531
`
`PCT/SE97/00109
`
`_ 3 _
`
`inserted into
`
`the
`
`transmitted data
`
`stream after
`
`modulation and before multiplexing by the transmitter.
`In the multiplexed OFDM signal,
`the guard space
`is
`
`transformed into an increase in the carrier
`
`spacing
`
`present in the OFDM burst,
`
`in the frequency domain. At
`
`the receiver,
`
`the guard space is regenerated when the
`
`OFDM burst
`
`is
`
`demultiplexed by
`
`FFT
`
`(fast
`
`fourier
`
`transform) processing. The receiver can detect the OFDM
`
`burst even if its time window is incorrectly set, and
`
`can adjust
`
`its time window to the correct value for
`
`proper reception of subsequent OFDM bursts.
`
`when
`
`the
`
`system is
`
`adapted for
`
`the
`
`two way
`
`transmission of data,
`
`i.e. when the receiver has
`
`a
`
`transmission capability,
`
`the receiver may measure the
`
`delay spread present in a received signal and transmit
`
`a control signal
`
`to the transmitter which causes
`
`the
`
`transmitter to adjust the duration of T
`
`to the minimum
`
`3
`
`value needed to overcome
`
`loss of data through delay
`
`spread.
`
`In this way,
`
`the system uses the minimum guard
`
`space necessary to overcome the effects of delay spread,
`
`thereby maximising the channel traffic capacity without
`
`signal degradation in the receiver.
`
`A transmitter for use in the present invention must
`
`be able to insert a guard space in the transmitted data
`
`bursts and be able to adjust
`
`the duration of the guard
`
`space, so that it is just sufficient to eliminate signal
`
`degradation
`
`by
`
`delay
`
`spread.
`
`In
`
`addition,
`
`the
`
`transmitter may be adapted to receive a control signal,
`
`from a
`
`receiver,
`
`indicative of
`
`the amount of delay
`
`spread present
`
`in a received signal, and to use this
`
`signal
`
`to adjust
`
`the duration of the guard space, on a
`
`continuous basis. This ability to continuously vary the
`
`duration of the guard space, during system operation, is
`
`particularly valuable where the system is used in mobile
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`telephony.
`
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`

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`wo 97/30531
`
`PCTISE97/00109
`
`_ 9 _
`
`A receiver for use in the present invention must be
`
`able to adjust the reception window for OFDM bursts to
`
`match the size of
`
`the transmitted burst, as
`
`received.
`
`In particular,
`
`the FFT frame must be synchronised with
`
`the received OFDM burst, if the received signal is to be
`
`demultiplexed correctly. However, a receiver can still
`
`detect a received burst, even if its reception window is
`
`incorrectly set, and can adjust the reception window to
`
`match the correct value for
`
`the received signal.
`
`In
`
`addition,
`
`the receiver may measure the amount of delay
`
`spread present
`
`in the received signal and transmit a
`
`control
`
`signal,
`
`to
`
`the
`
`transmitter,
`
`causing
`
`the
`
`transmitter to adjust
`
`the duration of the guard space
`
`employed.
`
`The present invention has particular application in
`
`mobile telephony,
`
`in which a plurality of base stations
`
`transmit data Ix) a plurality of mobile stations.
`
`In
`
`such systems, delay spread may vary during the course of
`
`transmission, between a given base station and a given
`
`mobile, as the mobile's position continuously changes.
`
`Furthermore,
`
`in such systems,
`
`traffic capacity is at a
`
`premium.
`
`The ability to continuously measure the delay
`
`spread present and adjust guard spaces,
`
`in OFDM bursts,
`
`to ensure undegraded transmission with a minimum guard
`
`space
`
`duration,
`
`conserves
`
`traffic _capacity
`
`in
`
`a
`
`transmission channel.
`
`The design and implementation of OFDM systems are
`
`well
`
`known
`
`to
`
`those
`
`skilled
`
`in
`
`the
`
`art
`
`of
`
`telecommunications,
`
`and the manner
`
`in which an OFDM
`
`system,
`
`implementing the present
`
`invention,
`
`can
`
`be
`
`realised will be readily apparent
`
`to those skilled in
`
`the art,
`
`from the foregoing description.
`
`10
`
`l5
`
`20
`
`25
`
`30
`
`Facebook's Exhibit No. 1048
`
`Page11
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`Facebook's Exhibit No. 1048
`Page 11
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`

`
`WO 97/30531
`
`PCT/SE97/00109
`
`CIJKIJHS
`
`1.
`
`An
`
`OFDM system comprising
`
`a
`
`receiver
`
`and
`
`a
`
`transmitter, said transmitter being adapted to transmit
`
`data in OFDM symbol bursts in which,
`
`in the time domain,
`
`a portion of
`
`a
`
`symbol burst
`
`is repeated within said
`
`symbol burst as a guard space, said receiver having a
`time window with which, after demultiplexing,
`said
`
`symbol burst must be aligned, characterised in that said
`repeated portion of a symbol burst has a duration which
`is variable
`and which
`is
`adjusted,
`during system
`
`operation,
`
`to a minimum duration necessary to prevent
`
`data loss caused by delay spread.
`
`2.
`
`An OFDM system as claimed in claim 1, characterised
`
`in that said receiver‘
`
`is adapted to vary‘ said time
`
`window to produce, on demultiplexing, a guard space of
`a minimum duration necessary to prevent data loss caused
`
`by delay spread.
`
`3.
`
`An OFDM system as claimed in either claim 1, or
`
`said
`the portion of
`characterised in that
`claim 2,
`symbol burst which is repeated is copied from a leading
`
`edge of said symbol burst.
`
`4.
`
`An OFDM system as claimed in any previous claim
`
`characterised ix:
`that said repeated portion of said
`symbol burst
`is repeated at
`a trailing edge of said
`
`10
`
`15
`
`20
`
`25
`
`symbol burst.
`
`5.
`
`An OFDM system as claimed in any previous claim,
`
`characterised in that said receiver includes performance
`measuring means
`for measuring received signal quality
`and reception window adjusting means for adjusting said
`
`30
`
`time window duration.
`
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`Facebook's Exhibit No. 1048
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`

`
`W0 97/30531
`
`PCT/SE97/00109
`
`_ 11 _
`
`6.
`
`An OFDM system as claimed in claim 5, characterised
`
`in that said receiver is adapted to transmit a signal to
`said transmitter indicative of a level of delay spread
`in a received signal, and said transmitter is adapted to
`
`adjust said guard space duration to a nunimum value
`
`necessary to prevent data loss caused by delay spread.
`
`7.
`
`An OFDM system as claimed in any previous claim,
`
`characterised in that said guard space can be adjusted
`
`to a duration of between 0.2ps and 50us.
`
`10
`
`8.
`
`An OFDM system as claimed in any previous claim,
`
`characterised in that said receiver is adapted to detect
`
`a
`
`transmitted
`
`signal
`
`when
`
`said
`
`time window is
`
`incorrectly adjusted.
`
`9.
`
`An OFDM system as claimed in any previous claim,
`
`15
`
`characterised in that said system includes a plurality
`
`of mobile stations and a plurality of base stations, and
`
`is adapted to operate as a mobile telecommunications
`
`system.
`
`10.
`
`A receiver,
`
`for use with an OFDM system including
`
`20
`
`a transmitter adapted to transmit data in OFDM symbol
`
`bursts,
`
`in which,
`
`in the time domain,
`
`a portion of
`
`a
`
`symbol burst
`
`is repeated within said symbol burst as a
`
`guard space,
`
`said receiver having a
`
`time window with
`
`which, after demultiplexing, said symbol burst must be
`
`25
`
`aligned, characterised in that said repeated portion of
`
`a symbol burst has a duration which is variable and said
`
`receiver is adapted to vary said time window to produce,
`
`on demultiplexing,
`
`a guard space of a minimum duration
`
`necessary to prevent data loss caused by delay spread.
`
`.30
`
`11.
`
`A receiver as claimed in claim 10, characterised in
`
`that said receiver includes performance measuring means
`
`for measuring received signal quality and reception
`
`Facebook's Exhibit No. 1048
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`Facebook's Exhibit No. 1048
`Page 13
`
`

`
`W0 97/30531
`
`PCT/SE97/00109
`
`_ 12 -
`
`window adjusting means for adjusting said time window
`
`duration in response to a signal
`
`from said performance
`
`measuring means.
`
`12.
`
`A receiver as claimed in claim ll, characterised in
`
`that said receiver is adapted to transmit a signal
`
`to
`
`said transmitter indicative of a level of delay spread
`
`in a received signal, and said transmitter is adapted to
`
`adjust. said guard space duration to a
`
`rninimunn value
`
`necessary to prevent data loss caused by delay spread.
`
`13.
`
`A receiver as claimed in any of claims 10 to 12,
`
`characterised in that said guard space can be adjusted
`
`to a duration of between 0.2ps and 50us.
`
`14.
`
`A receiver as claimed in any of claims 10 to 13,
`
`characterised in that said receiver is adapted to detect
`
`a
`
`transmitted
`
`signal when
`
`said
`
`time window
`
`is
`
`incorrectly adjusted.
`
`10
`
`15
`
`15.
`
`A receiver as claimed in any of claims 10 to 14,
`
`characterised in that said receiver is a mobile radio
`
`telephony transceiver adapted for use with a mobile
`
`20
`
`telecommunications system.
`
`16.
`
`A method of operating an OFDM system comprising a
`
`receiver and a transmitter, by transmitting data as a
`
`series of OFDM symbol bursts, each symbol burst,
`
`in the
`
`time domain, having a portion thereof repeated therein
`
`as a guard space,
`
`said receiver having a time window
`
`with which, after demultiplexing, said symbol burst must
`
`be aligned, characterised by the repeated portion of a
`
`symbol burst having a variable duration, and adjusting
`
`said variable duration to produce a guard space of a
`
`minimum duration necessary to prevent data loss caused
`
`by delay spread.
`
`25
`
`30
`
`Facebook's Exhibit No. 1048
`
`Page14
`
`Facebook's Exhibit No. 1048
`Page 14
`
`

`
`W0 97/30531
`
`PCT/SE97/00109
`
`- 13 _
`
`17.
`
`A method as claimed in claim 16 characterised in
`
`that said receiver time window is adjusted to produce,
`
`on demultiplexing, a guard space of a minimum duration
`
`necessary to prevent data loss caused by delay spread.
`
`18.
`
`A method as claimed in either claim 16, or claim
`
`17, characterised by copying the repeated portion of
`
`said symbol burst from a leading edge thereof.
`
`19.
`
`A method as claimed in any of claims
`
`16 to 18,
`
`characterised by repeating the repeated portion of said
`
`symbol burst at a trailing edge of said symbol burst.
`
`20.
`
`A method as claimed in any of claims 16 to 19,
`
`characterised by said receiver measuring received signal
`quality and adjusting said time window duration so that
`
`said received signal quality exceeds
`threshold.
`
`a predetermined
`
`21.
`
`A method as claimed in any of claims
`
`16
`
`to 20,
`
`characterised in that said guard space can be adjusted
`
`to a duration of between 0.2us and 50us.
`
`22.
`
`A method as claimed in any of claims 16
`
`to 21,
`
`characterised by said receiver detecting a transmitted
`
`signal when said time window is incorrectly adjusted.
`
`10
`
`15
`
`20
`
`Facebook's Exhibit No. 1048
`
`Page15
`
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`

`
`W0 97/30531
`
`PCT/SE97/00 109
`
`7/2
`
`amplifuc/e
`
`0.8
`
`0.7
`
`0.6
`
`05
`
`0.4
`
`0.3
`
`0.2
`
`0.7
`
`0
`
`0
`
`7
`
`2
`
`3
`
`4
`
`7_5
`6
`5_
`fIme(s)x70
`
`amplifucle
`
`0.5
`
`7
`
`25
`
`2
`time (5):: 70’5
`
`Facebook's Exhibit No. 1048
`
`Page 16
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`Facebook's Exhibit No. 1048
`Page 16
`
`

`
`W0 97/30531
`
`PCT/SE97I00109
`
`2/2
`
`amp//‘rude
`
`0
`
`2. 5
`v‘.‘.‘.-- u n- --‘.'. a nu \-A
`fime {she 70 '7
`
`OFDM symbol fime (Ts)
`
`Tg
`
`Facebook's Exhibit No. 1048
`
`Page 17
`
`Facebook's Exhibit No. 1048
`Page 17
`
`

`
`INTERNATIONAL SEARCH REPORT
`
`lnmnafional appucafion No,
`
`PCT/SE 97/00109
`
`A. CLASSIFICATION OF SUBJECT MATTER
`
`1
`
`IPC6: HO4L 5/06
`According to International Patent Classification (IPC) or to both national classification and IPC
`B. FIELDS SEARCHED
`
`Minimum documentation searched (classification system followed by classification symbols)
`
`IPC6: H04L
`
`Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched
`
`SE,DK,FI,NO classes as above
`
`Electronic data base consulted during the international search (name of data base and, where practicable, search terms used)
`
`C. DOCUMENTS CONSIDERED TO BE RELEVANT
`
`A
`
`Citation of document, with indication, where appropriate. ofthe relevant passages
`DE 4425713 C1 (INSTITUT FOR RUNDFUNKTECHNIK GMBH),
`20 April 1995 (20.04.95),
`claims 1-3,
`abstract
`
`Relevant to claim No.
`1-22
`
`NO 9406231 A1 (BRITISH BROADCASTIG CORPORATION),
`17 March 1994 (17.03.94), page 7,
`line 37 - page 8,
`line 9, abstract
`
`IEEE TRANSACTIONS ON BROADCASTING, Volume 41, No 1,
`March
`1995, William Y. Zou et al,
`"COFDM: AN
`OVERVIEW", see especially abstract and chapter 3.3.
`
`1-22
`
`1-22
`
`I] See patent family annex.
`D Further documents are listed in the continuation of Box C.
`In:
`Pub
`date or priority
`Special categories of cited documents:
`"I" later document
`lished alter the international I'll’
`date and not in oonflict with the application but cited to understand
`A’ document defining the general state of the art which is not considered
`the principle or theory underlying the invention
`to he of particular relevance
`eriier document but published on or after the international filing date
`document which my “now doubu on Priority dam“) of which is
`cited to establish the publication date of anotha citation or other
`‘P9951 333°“ (33 'P°°i“°d)
`'0' document referring to an oral disclosure. use, exhibition or othgr
`means
`docummt published prior to the international filing date but later than
`"39 POOP“? 33“ 5131111011
`
`‘X’ dounnent of particular relevance: the claimed invention cannot be
`considered novel or cannot be considaed to involve an inventive
`“P "5" "" “°°‘-“““" “ ""‘“‘ “‘°“°
`‘Y’ document of particular relevance: the claimed invention cannot be
`considered to involve an inventive step when the document is .
`I
`combined with one or more other such documents. such combination
`“ins °bVi°“‘ t° 3 P===°n *i“°‘ in W‘ 5"
`'&' document rnanher ofthe same patent family
`
`I ‘
`
`‘E’
`.L.
`
`Date of the actual completion of the international search
`
`Date of mailing of the international search report
`
`Swedish Patent Office
`
`Box 5055, S-102 42 STOCKHOLM
`Facsimile No. +46 8 666 O2 86
`Form PCT‘/ISA/210 (second sheet) (July 1992)
`
`Authorized officer
`
`MIKAEL s0[_|_ERHE[)
`Tel hone No.
`+46 8 782 25 00
`
`Facebook's Exhibit No. 1048
`
`Page18
`
`Facebook's Exhibit No. 1048
`Page 18
`
`

`
`INTERNATIONAL SEARCH REPORT
`Information on patent family members
`
`02/04/97
`
`
`
`International application No.
`
`PCT/SE 97/00109
`
`member(s)
`
`AU
`W0
`
`3162795 A
`9602989 A
`
`
`
`date
`
`16/02/96
`01/02/96
`
`
`
`
`
`
`cited in search report
`
`DE
`
`4425713
`
`C1
`
`
`
`dam
`
`20/04/95
`
`NO
`
`9406231 A1
`
`17/03/94
`
`
`
`
`
`
`
`
`
`
`
`29/03/94
`4975493 A
`AU
`21/06/95
`0658295 A
`EP
`23/03/94
`2270819 A,B
`GB
`
`
`06/02/96
`8501195 T
`JP
`
`
`11/03/97 US 5610908 A
`
`———.._____—___—————..—_———————_——————-———--—_-_—___—____——————————————_——__
`
`
`
`Form PCT/ISA/210 (patent family annex) (July 1992)
`
`
`
`Facebook's Exhibit No. 1048
`
`Page 19
`
`Facebook's Exhibit No. 1048
`Page 19