US007133352B1
`
`(12) United States Patent
`Hadad
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,133,352 B1
`Nov. 7, 2006
`
`(54) BI-DIRECTIONAL COMMUNICATION
`CHANNEL
`
`2/2001 Kaiser et a1. ............. .. 375/148
`6,188,717 B1 *
`6,381,236 B1* 4/2002 Miyashita et a1. ........ .. 370/343
`
`(76) Inventor: Zion Hadad, 48 Haalmogim St.,
`R' h L '
`IL
`15 on eZlOn (
`)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 0 days.
`
`( * ) Notice:
`
`* cited by examiner
`Primary ExamineriHuy D. Vu
`Asslsmm ExammeriDuc Duong
`
`(57)
`
`ABSTRACT
`
`(21) App1_ NO; 09/399,109
`
`(22) Filed:
`
`Sep. 20, 1999
`
`(51) Int. Cl.
`(2006.01)
`H04J 11/00
`(52) US. Cl. .................................................... .. 370/208
`(58) Field of Classi?cation Search .............. .. 370/208,
`370/209, 210, 281, 3101, 329, 335, 342,
`370/343, 344, 441, 479; 45/65, 506
`See application ?le for Complete Search history
`_
`References Clted
`
`(56)
`
`US. PATENT DOCUMENTS
`
`4/ 1996 choqly et a1~ ------------ -- 375/205
`5,504,775 A *
`3/1998 Frodlgh et a1~ ~~~~~~~~~~~ ~~ 370/252
`5,726,978 A :
`8/1998 Gudmuneson et a1‘
`370/210
`5,790,516 A
`8/1999 Alamouti et a1. ......... .. 370/330
`5,933,421 A *
`9/1999 Davies et a1. ............. .. 370/210
`5,953,311 A *
`5,995,483 A * 11/1999 Marchok et a1.
`370/207
`6,088,398 A *
`7/2000 Wahlqvist et a1. ........ .. 375/260
`
`A communication system using OFDM transmission from a
`base station to subscriber units, includes means for achiev
`ing a bi-directional channel. These means comprise trans
`mitting means in the subscriber units for the transmission of
`signal synchronous With the guard time interval in the
`OFDM transmission, and receiving means in the base station
`for the reception of the transmitted signals. The system also
`includes signal Shaping means in the receiver Of the base
`station and/or the subscriber unit for the application of a
`Window in time to signals received therein. A communica
`tion system includes a combination of CDMA modulation
`codes and OFDM coding/decoding means to achieve
`orthogonality between signals from the various users in the
`uplink. A communication system includes a combination of
`OFDM and channeling means for achieving orthogonality
`between signals from the various users in the uplink. Auto
`matic Frequency Control is achieved With means in the
`mobile unit for achieving a frequency-lock to the base
`Station
`'
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`US 7,133,352 B1
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`1
`BI-DIRECTIONAL COMMUNICATION
`CHANNEL
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`The present application is related to the application No.
`129186 filed on Mar. 25, 1999 in Israel and entitled “Bi-
`Directional Communication Channel”.
`
`STATEMENT AS TO RIGHTS TO INVENTIONS
`MADE UNDER FEDERALLY SPONSORED
`R&D
`
`There was no Federal sponsoring for the present inven-
`tion, therefore there are no rights deriving therefrom.
`
`FIELD OF THE INVENTION
`
`This invention relates to video, voice and data transmis-
`sion in wireless or cable communications, and more par-
`ticularly to improvements in links between base stations and
`subscriber units.
`
`BACKGROUND OF THE INVENTION
`
`Heretofore, Advanced communications today use the
`Orthogonal Frequency Division Multiplex (OFDM) modu-
`lation for efficient transmission of digital signals. These
`signals may include video, voice and/or data.
`OFDM is a commonly used implementation of Multi-
`Carrier Modulation (MCM).
`The Orthogonal Frequency Division Multiplex (OFDM)
`is a modern advanced modulation method, that achieves
`better use of the frequency spectrum, as detailed below.
`OFDM has been used in recent years in many applications
`where robustness against severe multipath and interference
`conditions is required, together with high system capacity,
`flexibility in providing variable bit rate services, scalability
`and a capability to perform well in Single Frequency Net-
`works (SNF). OFDM forms the basis for various commu-
`nication standards, including for example the Digital Ter-
`restrial Television Broadcasting, Digital Audio Broadcasting
`(DAB), wireless LANs and Wireless Local Loops.
`A particular example of the use of OFDM is in DVB
`systems.
`Digital video broadcasting (DVB) systems are now being
`developed, based on several standards, in Europe, U.S.A.
`and Japan. Each standard addresses cable broadcasting as
`well as satellite and terrestrial/area broadcasting.
`In Europe, various standards define the digital video
`broadcasting (DVB) system, including DVB-T (DVB-Ter-
`restrial), DVB-C (DVB-Cable) and DVB-S (DVB-Satellite).
`For example, the European standard EN 300 744 defines
`the DVB-T.
`The OFDM modulation method has been chosen, for
`example, for the digital television broadcasting (MPEG-2)
`per standard DVB-T.
`A disadvantage of presently used DVB-T systems is their
`unidirectional operation. That is, information is only trans-
`mitted from a base station (the transmitter) to subscribers
`(the receivers). A system may contain many base stations
`and subscribers, however there is always an unidirectional
`flow of information.
`
`An interactive system can be achieved by using a tele-
`phone line as a return channel from the set top box, however
`this method is slow and inconvenient.
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`A fast, flexible link from subscriber to the base station is
`required for the multitude of advanced services that are in
`demand today.
`To achieve a bi-directional link is a difficult task, that
`requires an innovative approach as detailed below. The
`problem is further aggravated by the requirement that the
`solution should cause no deterioration in performance;
`moreover, updated subscriber units capable of bi-directional
`operation should coexist with older subscriber units that do
`not have these transmission capabilities.
`To understand the difficulty of complying with these
`requirements, one should delve into the intricacies of con-
`temporary digital video broadcasting standards. These stan-
`dards specify advanced signal processing, to achieve higher
`quality communications at very high bit rates.
`Thus, in the Orthogonal Frequency Division Multiplex
`(OFDM) modulation method, a block of information is
`divided among N frequency channels, so that a portion of the
`information is transmitted in each of the abovementioned
`
`channels or frequencies. Since each charmel is orthogonal to
`the others, a better utilization of the frequency spectrum is
`achieved.
`
`The OFDM method achieves lower Inter-Symbol Inter-
`ference ISI, since the distribution of the information over N
`carriers allows each bit of information to be sent for a longer
`time period (N times longer). For a low ISI, the overlap
`between adjacent symbols should be lower than 10%. The
`ISI increases as the percentage of overlap between adjacent
`symbols increases. In OFDM, since each symbol is N times
`longer,
`the percent overlap between adjacent symbols
`decreases, hence the Inter-Symbol Interference ISI is lower.
`Still better spectrum utilization is achieved by QAM
`(Quadrature Amplitude Modulation) on each of the N car-
`riers.
`
`An IFFT (Inverse Fourier Transform) is performed on the
`modulated carriers, to form the signal in the time domain
`that corresponds to the above modulated carriers. The signal
`is transmitted as a frame that contains the block of infor-
`mation to be transmitted.
`
`A possible problem with the above modulation method is
`multipath, that may result in interference between adjacent
`transmitted frames. To address this problem, a guard time
`period is inserted between adjacent frames. The guard time
`is especially important in QAM, that is more sensitive to
`interference. In DVB-T systems, the guard time is chosen as
`either M1, %, 1/is or 1/32 of the symbol time.
`A disadvantage of presently used OFDM channels is the
`need to reserve a guard interval in order to battle multipath
`and to enable operation in SFN networks. The guard inter-
`val, which is up to 25% of the symbol duration, is in effect
`a wasted time, because no information is transmitted during
`that time interval.
`
`Although the guard time is used to address the multipath
`problem, it is a costly solution, since it reduces the capacity
`of the communication system. It would be highly desirable
`to use other means for solving the multipath problem, that
`would allow charmel operation at full speed.
`Therefore,
`it
`is a formidably difficult task to try and
`improve or change these complex communications stan-
`dards.
`
`Several methods are now used to separate signals trans-
`mitted over a common charmel, including:
`TDMA—Time Division Multiple Access
`FDMA—Frequency Division Multiple Access
`DS-CDMA—Direct Sequence/Code Division Multiple
`Access
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`US 7,133,352 B1
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`3
`CDMA systems may use either DS/CDMA or FH/CDMA.
`In DS/CDMA multicode or DS/Multicode/CDMA, the sepa-
`ration of signals transmitted over a common channel
`is
`achieved using orthogonal codes.
`At present, a problem in DS/CDMA is how to generate
`these orthogonal codes. It is possible to have N charmels
`using orthogonal Walsh codes to multiply each charmel,
`wherein each user has a different Walsh code. In the down-
`
`link charmel (DL), that is the channel from the base station
`to subscribers, the orthogonality is preserved, since trans-
`mission to all users is prepared and transmitted at the same
`time. Each user receives all the encoded messages at the
`same time.
`
`In the uplink, however, each user has a different timing
`because of a different propagation time delay.
`Thus, each Walsh code (corresponding to a specific user)
`may be shifted in time relative to the other codes (that
`correspond to the other users). This effect creates interfer-
`ence between charmels.
`
`The problem is further aggravated by multipath, that may
`cause the phase shift in each channel to change in time.
`In prior art CDMA, alignment up to a portion of one chip
`was required to maintain orthogonality between signals.
`This is a severe requirement,
`that affects the cost and
`complexity of the communication equipment.
`A reduced orthogonality may cause a higher level of
`inter-user interference, caused by cross-correlation effects.
`Undesired frequency deviations pose a difficult problem
`in the uplink channel (from subscribers to base), since the
`base has to concurrently receive and process signals from a
`plurality of subscribers: each subscriber may have a different
`deviation, from Doppler Or other causes.
`If part of the signals from one user overlap with another’s
`because of a frequency deviation, these signals cannot be
`separated in the receiver—the damage carmot be repaired,
`and it may cause interference between users.
`Various attempts in prior art at solving this problem are
`not effective.
`
`In one prior art system, there are a plurality of receivers
`at the base station, each receiver is tuned to one subscriber
`transmitter, with a closed loop to track and correct frequency
`deviations in that transmitter.
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`Such an approach may be highly complex and expensive
`when there are a large number of subscriber units.
`The above solution carmot be used where subscribers
`
`45
`
`share a common charmel, rather frequency multiplexing is
`used, a different band for each subscriber.
`effective
`The present disclosure presents
`a more
`approach, which allows higher performance and flexibility
`at a lower cost, using a common receiver at the base, for
`processing signals from all the subscriber transmitters.
`In this approach, however, performance degrades rapidly
`if there are frequency deviations in the received signals. As
`frequency variations may have different values for the
`various subscribers, the common receiver carmot track them.
`Wideband Systems, and more so broadband systems, are
`very sensitive to such frequency deviations, which may
`cause a deterioration in the orthogonality of subcarriers and
`information in the received signals, resulting in errors, thus
`reducing charmel quality.
`A Doppler frequency shift may result from a mobile
`subscriber’s motion. For example, at a frequency of 2 GHz
`and a vehicle velocity of 100 km/h, a Doppler of about 185
`Hz is expected. The received signal may have any value of
`deviation, between -185 Hz and -185 Hz; the deviation will
`usually change with time; the same applies to the other
`subscribers.
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`At a higher velocity and/or frequency, a larger Doppler
`deviation is expected (the Doppler deviation is proportional
`to velocity and frequency). There are other sources of
`undesired frequency deviations, such as multiplexers in a
`fiber-optic charmel, etc.
`In another prior art system, the subscriber locks on a
`signal from the base and transmits at that signal. One or
`more pilots may be used. This, however, will not correct for
`frequency deviations due to multiplexers in the charnel, both
`in the downlink and the uplink: as the signal from base
`deviates because of various causes, the signal received back
`at the base accumulates all the deviations on the way.
`This system will not solve the Doppler frequency shift:
`assuming a moving subscriber receiving a signal from the
`base at a frequency deviation of +Fd, if it transmits at the
`received frequency, then the signal received at the base will
`have double the deviation or +2 Fd, since the Doppler acts
`the same way in both directions.
`Another problem in prior art uplink channels is the need
`for signals from all the subscribers to arrive at about the
`same time window at the base, to allow their processing
`together. Due to different distances to base, the subscribers
`will be received each at a different time delay—a subscriber
`farther from base will have a longer time delay.
`Yet another problem in prior art communications is the
`need for dynamic allocation of charmels: The radio spectrum
`is a precious resource; allocating it wisely is of paramount
`importance.
`Various users need each a different channel capacity—
`some say want to send or receive pictures, which require
`large amounts of data; others transmit voice or music, which
`requires less data; others are idle, thinking or waiting for
`something. Each user’s needs may change at a moment’s
`notice. It would be highly desirable to provide a system and
`method for allocating bandwidth in the amount required by
`each user—to each user to allocate bandwidth in the system,
`dynamically, according to their momentary needs.
`Prior art systems apparently do not disclose a system
`similar to that detailed in the present disclosure.
`Thus, Seki et al., U.S. Pat. No. 5,771,224, discloses an
`orthogonal frequency division multiplexing transmission
`system and transmitter and receiver therefor. It transmits an
`OFDM transmission frame, with null symbols and reference
`symbols being placed in the beginning portion of the frame
`and QPSK symbols are placed in an information symbol
`data region in the frame, with equal spacing in time and
`frequency.
`The carrier amplitude and phase errors are corrected by a
`correction information producing section on the amplitude
`and phase variations of the received signal detected by the
`variation detector to produce corrected information.
`Apparently, Seki does not address the problem of reverse
`link transmissions. Moreover, Seki performs a different type
`of signal processing.
`Baum et al., U.S. Pat. No. 5,802,044, discloses a multi-
`carrier reverse link timing synchronization system. A center
`station transmits a forward link signal, receives a reverse
`link signal, and determines a timing offset for signals
`received on a reverse link timing synchronization charmel. A
`reverse link symbol timing synchronization can be used in a
`system having a plurality of transmitting overlap bandwidth
`subscriber units on an OFDM-like spectrally overlapping
`reverse charmel. The modulation method may comprise
`M-ary Quadrature Phase Shift Keying(M-PSK), M-ary
`Quadrature Amplitude Modulation (QAM) or other digital
`modulation method.
`
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`US 7,133,352 B1
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`5
`Gudmundson et al., U.S. Pat. No. 5,790,516, discloses a
`method and system for pulse shaping for data transmission
`in an orthogonal frequency division multiplexed (OFDM)
`system.
`
`SUMMARY OF THE INVENTION
`
`The present disclosure relates to improvements in OFDM-
`based digital communications. The scope and spirit of the
`invention are better described with the inclusion of specific
`applications thereof.
`One application of the invention relates to the conversion
`of a DVB-T unidirectional
`link to a bi-directional
`link
`wherein enhanced subscriber units can coexist with stan-
`
`dard, limited performance units.
`A second application relates to a higher performance
`bi-directional OFDM link. A third application relates to
`improvement in the performance of an OFDM unidirectional
`broadcast system.
`A further improvement is achieved with a combination of
`OFDM in the uplink and CDMA on OFDM.
`the
`According to one aspect of the present invention,
`guard time interval during DVB broadcasting is advanta-
`geously used for transmission in the other direction, that is
`from subscribers to base station.
`
`An interactive system is thus achieved, using the existing
`bandwidth so that no additional frequency allocation is
`required. The guard time, that was hitherto wasted, will now
`be utilized to achieve a bi-directional interactive system.
`Especially shaped windows in the time domain are
`applied in order to extend the transmission time of each end,
`that is the base station and the subscriber unit. This allows
`
`to shorten the reception time, to leave more time for trans-
`mission back to the base station. A truncated transmission
`
`results in sidelobes in the frequency domain, that is a ripple
`signal that may cause inter-symbol interference. To correct
`for this effect, a window is applied that results in lower
`sidelobes. Additionally, equalizer means (for example a
`transversal filter) may further reduce the remaining side-
`lobes in the frequency domain.
`A shortened receive time has the additional benefit that
`transmission to the base station can start earlier in time, so
`that enough time is left for transmission in the other direc-
`tion.
`
`For example, a DVB-T or DAB unidirectional link can be
`converted to a bi-directional link. Enhanced subscriber units
`
`can coexist with standard, limited performance units. This
`preserves full compatibility with subscriber units that do not
`have the new transmit capability and require a constant
`transmit power. The window is only applied in the receiver
`of subscriber units that are converted to bi-directional opera-
`tion.
`
`According to the present invention, various unidirectional
`point-multipoint systems may be converted to bi-directional.
`These may include TD, TDD or FDD modulation systems.
`OFDM systems may be used without a guard time inter-
`val, using equalization and pulse shaping techniques. This
`increases the channel capacity.
`Where the system retains the guard time interval, this
`interval is now used for the transmission of useful informa-
`tion.
`
`For example, in a FDD modulation communication sys-
`tem with only a downlink charmel, an uplink charmel may be
`added having orthogonal properties with respect
`to the
`downlink. In this example, the uplink may use a different
`frequency band, to achieve orthogonality in the frequency
`domain.
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`A second application relates to a higher performance
`OFDM bi-directional
`link. This is implemented with a
`window being applied both in the transmitter (the base
`station) and the receiver. Amatched filter system is achieved
`that has better signal to noise performance. An equalizer
`may be used to improve the performance of the channel.
`A further improvement is achieved with a combination of
`OFDM in the uplink and FH/CDMA on OFDM.
`According to the present invention, the base station fur-
`ther includes means for coordinating the operation of the
`subscriber units, including:
`Automatic Synchronization Control (ASC)
`Automatic Power Control (APC)
`Automatic Frequency Control (AFC).
`ASC signals sent from the base station are used to bring
`all the subscriber units in synchronization with the base
`station, thus maximizing the transmission time and prevent-
`ing transmission overlap between subscriber units and the
`base station.
`
`APC signals sent from the base station to each subscriber
`unit are used to control the transmit power of each subscriber
`unit so that signals received at the base station from the
`various users have about the same power. This achieves
`more efficient processing at
`the base station and helps
`minimize ISI to nearer subscribers.
`
`locks onto the
`the subscriber unit
`The AFC loop at
`frequency of a signal received from the base station. The
`AFC then generates a transmit signal having a frequency
`derived from that of the signal received from the base
`station. The various clocks in the subscriber unit are also
`
`derived from the signal that is locked to the base frequency.
`Thus, the subscriber will transmit a signal to the base
`station, at a frequency that is equal or close to the correct
`value of a signal to be received in the base station.
`The use of AFC in the mobile stations helps achieve
`nearly orthogonality between subscriber units transmitting
`to the same base station.
`
`Frequency diversity may be used to enhance the link with
`a particular subscriber unit that has lower power or weaker
`gain. By allowing that unit to simultaneously transmit the
`same data over more than one channel, the lower gain or
`power is corrected for, to achieve the desired performance of
`the communication charmel.
`In the receiver, a coherent
`maximum ratio combiner may be used to combine the
`channels so as to achieve best reception.
`A dynamic frequency allocation to subscribers may be
`implemented, responsive to the needs of each user at any
`given time. This implements a bandwidth on demand
`scheme.
`
`An OFDM uplink charmel comprising N frequency car-
`riers may be shared between several simultaneous subscriber
`units, with each unit utilizing part of the frequency carriers
`to transmit its own data. As each subscriber unit utilizes Mi
`
`carriers, the sum of all the carriers utilized Mi equals the
`total number N of carriers in the channel.
`
`Each subscriber unit may use a different number Mi of
`carriers, according to its bandwidth demand.
`Furthermore, by allocating to each subscriber unit several
`carriers that are separated from each other (not adjacent to
`each other), a frequency hopping effect
`is achieved that
`reduces frequency selective fading.
`Canceling the guard time interval, and the use of espe-
`cially shaped windows in the time domain, together with
`signal processing in the receiver, will imitate the guard time
`interval so as to protect against ISI.
`
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`US 7,133,352 B1
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`7
`This third application of the invention relates to improve-
`ments in the performance of the unidirectional OFDM
`broadcast system.
`The performance of existing standards like DVB-T, DAB,
`Hiperlan (ETSI BRAN), Hiperaccess (IEEE 802.11) may be
`improved using the abovedetailed methods and systems
`according to the present invention.
`To preserve the separation between users in a CDMA
`communication system, an OFDl\/I/CDMA system is dis-
`closed. The system includes carrier and timing correction
`means to achieve improved performance.
`This system is effective even in the uplink charmel, where
`there may be a time difference between signals received
`from the various users.
`
`This method and system achieve better performance with
`respect to prior art CDMA. CDMA is based on orthogonal
`codes being allocated to the various users. The codes,
`however, are orthogonal only if they are received at the same
`time.
`In prior art systems the various signals may be
`received with different relative time delay, so they are no
`more orthogonal as intended.
`In the novel system and method, using OFDM to achieve
`synchronous reception of the various signals, the CDMA
`codes are received concurrently. Thus, in the novel approach
`the various transmissions are synchronized.
`This achieves orthogonality between the various CDMA
`transmissions. Thus,
`the above scheme may achieve
`improved performance relative to CDMA.
`A novel architecture is disclosed, of an OFDM system
`with precise frequency setting in each unit.
`A plurality of transceivers are used, each with its own
`frequency reference/timing unit.
`A transmitter includes an IFFT processor operating on a
`plurality of inputs for all the users. Furthermore, a pulse
`shaper may be used to reduce the Guard Interval (GI) time.
`Using a closed loop control, the base may shift the user
`response forward or backward, until it is received at the
`center location in time with all the other uplink users. That
`is, all the users in the uplink channel will be received during
`the same time window.
`
`To implement the above method, Automatic Synchroni-
`zation Control (ASC) signals are sent to the mobile sub-
`scribers. These signals correct the transmit time of each
`subscriber. The signal’s value is stored in the mobile unit for
`subsequent transmissions.
`The performance is further improved with each user
`including means to be frequency-locked to the base (AFC).
`Each mobile user locks its frequency to a transmission from
`the base. The stabilized frequency is then used to transmit
`back to base, at a known, expected frequency.
`An OFDl\/I/TDD system is disclosed that achieves
`improved orthogonality between signals for the various
`users, while allowing for more relaxed timing requirements.
`This achieves a system that is easier to implement. More-
`over, the novel system may be allocated per bit in each
`carrier, to may achieve less sensitivity to interference and
`multipath.
`Moreover, in the novel system the guard interval forms a
`shorter percentage of the time interval. That is, the guard
`interval is shorter relative to the symbol duration. This helps
`achieve a more efficient communication system, where less
`time is wasted for the guard interval.
`Further objects, advantages and other features of the
`present invention will become obvious to those skilled in the
`art upon reading the disclosure set forth hereinafter.
`
`8
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a digital information broadcasting sys-
`tem using OFDM (prior art).
`FIG. 2 details the complex signal processing being imple-
`mented in digital broadcasting with OFDl\/I/DVB-T (prior
`art).
`FIG. 3 details the timing of transmitted signals in OFDM,
`using guard time intervals (prior art).
`FIG. 4 illustrates the principle of a return channel using
`TDD, utilizing the guard time intervals.
`FIG. 5 details the application of windows in the bi-
`directional charmel.
`
`FIG. 6 details various types of windows that may be
`employed.
`FIG. 7 illustrates an OFDM broadcasting system con-
`verted to a bi-directional channel.
`FIG. 8 details the structure of an OFDM base station
`
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`adapted for bi-directional operation.
`FIG. 9 details the structure of an OFDM subscriber unit
`
`adapted for bi-directional operation.
`FIG. 10 details the spectrum of transmitted signals in a
`bi-directional charmel with dynamic spectrum allocation to
`subscribers.
`
`FIG. 11 details a CDMA system using Walsh codes (prior
`art).
`FIG. 12 details an OFDM with precise frequency setting
`in each unit.
`
`FIG. 13 details a CDMA implementation using OFDM for
`transmission from users to a center.
`
`FIG. 14 details the correction in time and frequency in the
`new system.
`FIG. 15 details an implementation of the CDMA decoding
`using FFT.
`FIG. 16 illustrates a processing performed on received
`signal, with the application of a window in the time domain.
`FIG. 17 details a multi-user system with signal processing
`in the time and frequency domains.
`FIG. 18 details the signals in a multi-user system with
`Walsh coding over OFDM.
`FIG. 19 details a functional block diagram of the signal
`processing in a mobile transmitter.
`FIG. 20 details a functional block diagram of the signal
`processing in a central station receiver.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`A preferred embodiment of the present invention will now
`be described by way of example and with reference to the
`accompanying drawings.
`FIG. 1 illustrates a prior art digital information broad-
`casting system using OFDM. The system includes a base
`station 1 that transmits to a plurality of subscriber units 2.
`A data input charmel 11 brings to the base station 1 the
`video, audio