ERIC-1003
`Ericsson v IV
`Page 1 of 11
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`U.S. Patent
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`1n.aJ
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`1B0555,71/09SU
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`U.S. Patent
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`Jan. 16, 2001
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`Sheet 2 013
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`US 6,175,550 B1
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`FIG . 2
`
`TS=T+TG
` —
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`BTS
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`FIG. 3
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`10
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`son
`501
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`60h
`803
`sec
`eoe
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`50p
`600
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`60m 600
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`-20
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`-15
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`-10
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`-5
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`5
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`10
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`15
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`20
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`1.2 MHZ/CARRIER
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`U.S. Patent
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`Jan. 16, 2001
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`Sheet 3 013
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`US 6,175,550 B1
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`FIG. 5
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` STATION
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`US 6,175,550 B1
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`1
`ORTHOGONAL FREQUENCY DIVISION
`MULTIPLEXING SYSTEM WITH
`DYNAMICALLY SCALABLE OPERATING
`PARAMETERS AND METHOD THEREOF
`
`BACKGROUND OF THE INVENTION
`
`1. Field of The Invention
`
`This invention relates to communication systems and,
`more particularly, OFDM (Orthogonal Frequency Division
`Multiplexing) modulation schemes which are suitable to
`provide a wide range of information transfer rates in a wide
`range of physical environments.
`2. Description of Related Art
`OFDM is a block-oriented modulation scheme that maps
`N data symbols into N orthogonal carriers separated by a
`distance of 1/T, where T is the block period. As such,
`multi-carrier transmission systems use OFDM modulation
`to send data bits in parallel over multiple, adjacent carriers
`(also called tones or bins). An important advantage of
`multi-carrier transmission is that inter-symbol interference
`due to signal dispersion (or delay spread) in the transmission
`channel can be reduced or even eliminated by inserting a
`guard time interval between the transmission of subsequent
`symbols, thus avoiding an equalizer as required in single
`carrier systems. This gives OFDM an important advantage
`over single carrier modulation schemes. The guard time
`allows delayed copies of each symbol, arriving at
`the
`receiver after the intended signal,
`to die out before the
`succeeding symbol
`is received. OFDM’s attractiveness
`stems from its ability to overcome the adverse effects of
`multi-channel transmission without the need for equaliza-
`tion. A need exists for a flexible OFDM system which
`provides the advantages of OFDM to a variety of commu-
`nication environments.
`
`SUMMARY OF THE INVENTION
`
`The scaleable OFDM system according to the principles
`of the present invention provides increased flexibility and
`adaptability by providing scaling of the operating param-
`eters and/or characteristics for
`the OFDM system. For
`example, control circuitry can scale the transmission rate by
`scaling of the OFDM symbol duration,
`the number of
`carriers and/or the number of bits per symbol per carrier.
`Scaleability permits the scaleable OFDM system to operate
`in various communications environments requiring various
`operating parameters and/or characteristics. By scaling the
`operating parameters and/or characteristics of the OFDM
`system when control circuitry determines that different
`operating parameters and/or characteristics are necessary or
`advantageous, the control circuitry can dynamically change
`the operating parameters and/or characteristics, thereby pro-
`viding compatibility or
`the desired performance. For
`example, by dynamically scaling the bit rate, widely varying
`signal bandwidths, delay spread tolerances and signal-to-
`noise ratio (SNR) requirements can be achieved. As such, a
`scaleable OFDM system is particularly suitable for applica-
`tion in mobile, wireless communication devices, which
`support a variety of services, in a variety of environments,
`indoor as well as outdoor and in radio channels with
`
`differing bandwidths.
`In accordance with aspects of certain embodiments of the
`scaleable OFDM modulation system, a coded OFDM modu-
`lation system can be designed with an upper limit on the
`number of carriers and a variable symbol duration. The
`control circuitry can dynamically scale the number of car-
`riers below the upper limit on the number of carriers to
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`decrease the signal bandwidth and the transmission rate
`while delay spread tolerance remains the same. The control
`circuitry can also dynamically increase the symbol duration
`to decrease the transmission rate and the signal bandwidth
`and provide an increase in delay spread tolerance. In accor-
`dance with other embodiments, the scaleable OFDM modu-
`lation system achieves variable transmission rates using
`adaptive coding where different coding schemes are used to
`improve the link reliability and/or to decrease the peak-to-
`average power ratio.
`In accordance with yet other embodiments of the scale-
`able OFDM modulation system, scaleable transmission rates
`permit asymmetric data rates between mobile units and base
`stations. For example, the mobile units can have lower data
`rates than the base stations by allocating only a fraction of
`the total number of carriers to each mobile, while the base
`stations transmit at all carriers simultaneously. Additionally,
`during data downloading for example, a mobile unit could
`have a larger downlink data rate than uplink data rate. In
`accordance with other aspects of a scaleable OFDM system,
`mobile units and base stations using the same antennas for
`both transmit and receive can benefit from adaptive antennas
`with any additional processing done at
`the base station,
`thereby keeping the mobile as simple as possible. The
`scaleable OFDM modulation system can use an adaptive
`antenna at the base by sending feedback through the uplink,
`for example, when channel characteristics of uplink and
`downlink are not identical.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other aspects and advantages of the present invention
`may become apparent upon reading the following detailed
`description and upon reference to the drawings in which:
`FIG. 1 shows a block diagram of an embodiment of an
`OFDM transmitter according to certain principles of the
`present invention;
`FIG. 2 shows a diagram for explaining the windowing of
`OFDM symbols;
`FIG. 3 shows a plot of an OFDM power spectrum for
`explaining the effects of changes to certain parameters of an
`OFDM transmitter;
`FIG. 4 shows a block diagram of an embodiment of an
`OFDM receiver according to certain principles of the
`present invention; and
`FIG. 5 shows an OFDM system using OFDM transmitters
`and receivers according to the principles of the present
`invention.
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`Illustrative embodiments of the improved OFDM system
`with scaleable operating parameters and/or characteristics
`according to the principles of the present
`invention are
`described below as the improved OFDM system might be
`implemented to provide a flexible communications system
`for use in a variety of communication environments. Scale-
`ability permits the scaleable OFDM system to operate in
`various communications environments requiring various
`operating parameters and/or characteristics. By scaling the
`operating parameters and/or characteristics of the OFDM
`system when control circuitry determines that different
`operating parameters and/or characteristics are necessary or
`advantageous, the control circuitry can dynamically change
`the operating parameters and/or characteristics, thereby pro-
`viding compatibility or
`the desired performance. For
`example, by dynamically scaling the bit rate, widely varying
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`signal bandwidths, delay spread tolerances and signal-to-
`noise ratio (SNR) requirements can be achieved.
`The scaleable OFDM systems can be characterized by
`various operating parameters, including the following:
`number of carriers (N);
`symbol duration (T5);
`number of bits per symbol per carrier
`forward error correction coding scheme;
`coding rate; and
`the fraction of the symbol duration that is used as guard
`time.
`
`By varying these parameters, various operating charac-
`teristics can be scaled, including the following:
`transmission rate (bit rate or data rate);
`signal-to-noise ratio (the larger the SNR, the lower the bit
`error rate);
`delay-spread tolerance;
`signal bandwidth; and
`implementation complexity
`The scaleable OFDM system can scale operating param-
`eters and/or characteristics in various ways. For example, to
`dynamically scale the transmission rate,
`the scaleable
`OFDM system can dynamically adjust the symbol duration,
`coding rate, the number of bits per symbol per carrier and/or
`the number of carriers depending upon the required or
`desired operating parameters and/or characteristics. In this
`particular example, depending upon how the control cir-
`cuitry scales transmission rate, the scaleable OFDM system
`scales delay spread tolerance, signal to noise ratio, and
`signal bandwidth in different ways, making the scaleable
`OFDM system an attractive scheme for the implementation
`of flexible, (dynamically) scaleable communication systems.
`For example,
`to double the transmission rate of the
`scaleable OFDM system the following operating parameters
`and/or characteristics of the system can be dynamically
`scaled or adjusted:
`1. The coding rate. In general, a channel code is applied
`to reduce the rate of bit errors caused by OFDM-
`specific channel impairments, such as multipath among
`the carriers. The rate of such a code can be varied to
`
`trade off bit rate against bit error rate.
`2. The carrier modulation scheme. By doubling the num-
`ber of bits per symbol per carrier, the bandwidth and
`delay spread tolerance does not change, but the SNR is
`reduced, thereby resulting in a higher bit error rate.
`3. The symbol duration. By halving the symbol duration,
`the delay spread tolerance is halved, signal bandwidth
`is doubled, but
`implementation complexity is only
`increased by a factor of 2 (due to the speed-up by a
`factor of two).
`4. The number of carriers. By doubling the number of
`carriers, delay spread tolerance remains the same, the
`signal bandwidth doubles and the implementation com-
`plexity is quadrupled (both number of operations and
`speed are doubled) for an IDFT implementation or by
`2(n+1)/n if an IFFT implementation is used.
`An additional scaling parameter which can be changed is
`the ratio of guard time and symbol time. Changing this ratio
`affects SNR (a larger relative guard time claims energy that
`would otherwise go into the signal) and transmission rate (a
`larger relative guard time reduces the bit rate) and the
`delay-spread tolerance (a larger
`relative guard time
`improves the resistance against delay-spread).
`FIG. 1 shows an OFDM transmitter 10 having signal
`circuitry 11 which receives a data stream of data bits from
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`a data source 12. The coding block 14 receives the data
`stream and partitions the data stream into successive groups
`or blocks of bits. The coding block 14 introduces redun-
`dancy for forward error correction coding.
`In certain
`embodiments according to other aspects of the present
`invention, variable data rates with OFDM are achieved by
`using different forward error correction coding schemes
`and/or variable modulation schemes for each carrier as
`controlled by dynamic control circuitry 15. For example, if
`a mobile unit is at the edge of a coverage zone, the dynamic
`control circuitry can decrease the coding rate to lower the
`data rate with the advantage of increased delay spread
`tolerance and better SNR performance. Such a decrease in
`coding rate is followed by a decrease in spectral efficiency
`(amount of bits per second which can be transmitted in a
`certain bandwidth) proportional to the decrease in coding
`rate.
`
`In accordance with the principles of the present invention,
`the dynamic control circuitry 15 can be responsive to any of
`a number of possible inputs to set the coding block 14 to the
`appropriate coding rate. For example,
`in a transceiver
`embodiment,
`the dynamic rate control circuitry 15 can
`detect transmission errors, such as through feedback from an
`OFDM receiver (FIG. 4) and dynamically reduce the coding
`rate. Alternatively, each data packet could have a fixed code
`indicating the appropriate coding rate, or in a transceiver
`application, the coding scheme could mirror the coding rate
`of the received input from another transmitter (not shown).
`Finally,
`the dynamic rate control circuitry 15 could be
`responsive to external settings to set the coding rate.
`In similar fashion, the control circuitry 15 can respond to
`a variety of inputs by scaling the number of bits per symbol
`per carrier (for example, by changing the constellation size
`in embodiments using phase shift keying (PSK)
`modulation). By increasing the number of bits per symbol
`per carrier, the bandwidth and delay spread tolerance do not
`change, but the SNR is reduced resulting in a higher bit error
`rate. To scale the number of bits per symbol per carrier, for
`example, the dynamic rate control circuitry 15 can change
`from QPSK (quaternary or 4-PSK) modulation to other
`phase modulations, such as 8-PSK, or to other modulation
`schemes, such as QAM (quadrature amplitude modulation,
`e.g., 16-QAM).
`The blocks of coded data bits are input into an N-points
`complex IFFT (Inverse Fast Fourier Transform) 16, where N
`is the number of the OFDM carriers. In this particular
`embodiment, the IFFT 16 is performed on blocks of 2N
`coded data bits received from the coding block 14.
`In
`practice,
`the transmitter 10 has to use oversampling to
`produce an output spectrum without aliasing which intro-
`duces unwanted frequency distortion due to (intended or
`unintentional) low pass filtering in subsequent stages of the
`transmitter or in the transmission channel. Thus, instead of
`an N-points IFFT 16, an M-points IFFT 16 is actually done
`where M>N to perform the oversampling. These 2N bits are
`converted into N complex numbers, and the remaining M-N
`input values are set to zero.
`A clock 17 provides a time base for the IFFT 16, and the
`output of the IFFT 16 is parallel-to-serial converted to
`produce an OFDM symbol.
`In particular embodiments
`according to the principles of the present invention, the
`control circuitry 15 scales operating parameters and
`characteristics, such as transmission rate, by changing the
`symbol duration T5 while keeping the number of carriers N
`constant. In this particular embodiment, the control circuitry
`15 accomplishes this by controlling the clock 17 to adjust the
`time base to the IFFT 16. By decreasing the symbol
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`duration, an inversely proportional increase in the transmis-
`sion rate is achieved. At the same time, the delay spread
`tolerance is decreased. However,
`this is usually not a
`problem, because the higher data rate also means a decrease
`in range, and lower range means lower delay spread values.
`As an example, consider an OFDM system which has to
`support applications ranging from mobile telephony, with
`raw data rates in the order of 270 kbps, to indoor wireless
`LANs, with data rates up to 20 Mbps. Maximum delay
`spread requirements are 16 ys for mobile telephony down to
`about 200 ns for wireless LANs. Further, we require the
`OFDM signal to occupy a bandwidth of 200 kHz for the
`mobile telephony case, in order to be compatible with GSM
`channel spacing. All these requirements can be met by using
`OFDM with 32 carriers and a variable symbol duration T5 of
`200 ys down to 2 ys. For a symbol duration of 200 ys, a
`guard time of 20 ys is included to deal with the delay spread.
`This gives a carrier spacing of 1/(180 ys):55.6 kHz. This
`means there are exactly 36 carriers possible in a bandwidth
`of 200 kHz. By using 4 carriers as guard band, in order to
`fulfill spectrum requirements, 32 carriers remain for data
`transmission. Using QPSK with 2 bits per carrier per
`symbol, this gives a raw data rate of 32.2/(200 ys)=320 kpbs.
`By decreasing the OFDM symbol duration in the above
`described example, the data rate can be increased at the cost
`of a decreased delay spread tolerance. The maximum allow-
`able delay spread is proportional to the OFDM guard time.
`Hence, for wireless LANs with a maximum tolerable delay
`spread of 200 ns, the symbol duration can be decreased to
`2.5 ys, including a guard time of 250 ns. These parameters
`give an occupied bandwidth of 16 MHZ and a raw data of
`25.6 Mbps.
`Table 1 lists several parameter options for various scale-
`able transmission or data rates. The first three options are for
`32 carriers, the next three for 64 carriers, showing larger
`delay spread tolerance and a slightly smaller occupied
`bandwidth.
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`additional advantage of such an approach is that the peak-
`to-average power ratio per user is reduced. This means a
`better power efficiency can be achieved, which is very
`important
`for battery-driven devices. Alternatively,
`the
`dynamic control circuit 15 can scale the number of carriers
`by directing the modulation of only part of the phases onto
`adjacent carriers. Such a result
`is advantageous if the
`encoder has to operate in a channel with a smaller available
`bandwidth.
`
`In accordance with certain embodiments of the present
`invention, the dynamic control circuitry 15 can dynamically
`change N to vary the number of carriers. For example, the
`N-points IFFT 16 can be dynamically reduced to a X-points
`IFFT 16 where X<N. In this particular example, the IFFT 16
`is designed to handle the N carriers as the maximum number
`of carriers and dynamically scaled to less than N carriers by
`performing an X-point IFFT 16 according to the control
`signals from the dynamic rate control circuitry 15.
`Alternatively, the dynamic control circuitry 15 can dynami-
`cally direct the OFDM transmitter 10 to transmit fewer than
`N carriers by calculating the IFFT for less than 2N input bits,
`leaving the other values zero and thereby permitting mul-
`tiple access.
`To decrease the sensitivity to inter-symbol interference,
`the cyclic prefixer and windowing block 18 copies the last
`part of the OFDM symbol and augments the OFDM symbol
`with the copied portion of the OFDM symbol. This is called
`cyclic prefixing. The control circuitry 15 can control the
`cyclic prefixer and windowing block 18 to adjust the guard
`time and/or the fraction of the guard time to symbol
`duration, for example, to the values listed for the above
`OFDM system example. To reduce spectral sidelobes, the
`cyclic prefixing and windowing block 18 performs window-
`ing on the OFDM symbol by applying a gradual roll-off
`pattern to the amplitude of the OFDM symbol. The OFDM
`symbol is input into a digital-to-analog converter after which
`it is sent to the transmitter front-end 22 that converts the
`
`TABLE 1
`
`Examples of parameter options for scaleable data rates, assuming
`QPSK modulation of all carriers.
`
`Symbol duration Guard time
`[,1/,s]
`Lus]
`200
`20
`10
`1
`2.5
`0.25
`400
`40
`20
`2
`5
`0.5
`
`Number of Bandwidth
`carriers
`32
`32
`32
`64
`64
`64
`
`0.2
`4
`16
`0.19
`3.78
`15.11
`
`Raw data
`rate [Mbps]
`0.32
`6.4
`25.6
`0.32
`6.4
`25.6
`
`The advantage of this OFDM modulation system over the
`existing GMSK modulation of GSM is higher spectrum
`efficiency and better spectrum properties in terms of adja-
`cent channel interference. OFDM can have relatively large
`peak-to-average power ratio, but dynamically scaling the
`number of carriers can reduce the peak-to-average power
`ratio.
`
`In this particular embodiment, the control circuitry 15 can
`provide variable transmission rates as well as other operat-
`ing features by scaling the number of carriers. By transmit-
`ting a subset of the maximum number of carriers designed
`for the particular OFDM system, the decrease in data rate is
`proportional to the decrease in the number of transmitted
`carriers. Decreasing the number of transmitted carriers can
`also combine modulation technique and Medium Access
`Control (MAC), since multiple users can transmit simulta-
`neously in the same band, using different sets of carriers. An
`
`baseband wave form to the appropriate RF carrier frequency
`in this particular embodiment for transmission over antenna
`24.
`
`40
`
`FIG. 2 shows a basic representation of the windowing of
`an OFDM symbol where T5 is the total symbol duration, T
`is the FFT time, i.e., there are N samples in T seconds. The
`carrier spacing is 1/T in Hz, and T6 is the guard time which
`helps reduce the intersymbol interference caused by multi-
`path. The roll-off time is represented by [3 T5 where [3 is the
`roll-off factor. FIG. 3 shows an OFDM power spectrum in
`dB. The x-axis is normalized to carrier spacing, and the three
`(3) dB bandwidth has 16 carriers 60a—60p. Changing the
`FFT time T will change the spacing between the carriers
`60a—p. Increasing the number of carriers N for a constant
`sampling rate 1/T will increase the number of carriers 60a—p
`while keeping the carrier spacing fixed, thereby increasing
`the width of the transmitted OFDM power spectrum.
`Decreasing the number of carriers N will similarly lead to
`decreasing the width of the transmitted OFDM power spec-
`trum. Decreasing the sampling rate 1/T will increase T and
`decrease the carrier spacing, thereby decreasing the width of
`the transmitted OFDM symbol.
`the transmitted
`With particular reference to FIG. 4,
`OFDM signal is received by an OFDM receiver 30 having
`signal circuitry 31 through a selected antenna 32. The
`OFDM signal
`is processed (down converted) using the
`receive circuitry 34 and automatic gain control (AGC) block
`36. The processed OFDM signal is input into an analog-to-
`digital converter 38. The digital OFDM signal is received by
`a level detector 40 to provide a gain estimate feedback signal
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`to the AGC 36. The digital OFDM signal is also received by
`a frequency compensation block 42 and a timing and fre-
`quency synchronization block 44. The timing and frequency
`synchronization block 44 acquires the OFDM symbol tim-
`ing and provides a frequency estimate signal to the fre-
`quency compensation block 42 to correct for initial fre-
`quency offset and a timing signal to a Fast Fourier Transform
`(FFT) block 46.
`invention,
`In accordance with aspects of the present
`dynamic control circuitry 47 provides scaleable operating
`parameters and/or characteristics at the receiver 30. The
`dynamic control circuitry 47 can receive inputs from the
`transmitter 10 (FIG. 1), from external settings and/or from
`the data destination block 51. In response, the dynamic rate
`control circuitry 47 controls the operation of the FFT 46
`which is driven by a time base provided by clock 49. The
`dynamic control circuitry 47 can dynamically change the
`symbol duration by altering the time base from the clock 49
`to the FFT 46. Additionally, the dynamic control circuitry 47
`can respond to its inputs by controlling the operation of the
`FFT 46. The FFT 46 is designed to perform an N-point fast
`fourier transform on the OFDM symbol, but depending on
`the control signals from the dynamic control circuitry 47,
`can perform an X-point FFT where X<N to dynamically
`change the number of carriers.
`the
`In the case of the maximum number of carriers,
`resulting N complex carriers are input into a phase estima-
`tion block 48 and a phase compensation block 50. The phase
`estimation block 48 tracks the phases of the N carriers and
`provides phase estimates to the phase compensation block
`50 which compensates the N carriers accordingly. The
`compensated carriers are input into decoding block 52 which
`decodes the forward error correcting code of the transmitter
`10 (FIG. 1) and provides the data signals to the data
`destination block 51. Depending on its inputs, the dynamic
`control circuitry 47 can control the decoding block 52 to
`dynamically change the decoding rate and/or the demodu-
`lation scheme, thereby dynamically changing the operating
`parameters and/or characteristics, such as the data rate.
`FIG. 5 shows an improved OFDM system 70 consisting
`of a base station 72 and a number of remote stations 74
`
`which use dynamically scaleable OFDM transmitters 10
`(FIG. 1) and receivers 30 (FIG. 4) according to the principles
`of the present invention to provide a dynamically scaleable
`OFDM system 70. The dynamic control circuitry 15 (FIG. 1)
`and 47 (FIG. 4) provides scaleability of operating param-
`eters and/or characteristics between the base station 72 and
`the remote units 74. In the case of dynamically scaling the
`data rate, the improved OFDM system starts with low data
`rate between the base station 72 and a remote unit 74. Then,
`the dynamic control circuitry 15 (FIG. 1) of the transmitting
`station increases the data rate as the system design and signal
`quality permits. If the signal quality degrades, the dynamic
`control circuitry 15 (FIG. 1) decreases the data rate. The
`signal quality can be measured by one of the following:
`received signal strength, received signal to noise plus inter-
`ference ratio, detected errors (CRC),
`the presence of
`acknowledgments (lack of acknowledgments the link for
`communication signals is bad). Additionally, other operating
`characteristics and/or parameters can be similarly monitored
`and scaled.
`
`The OFDM receiver 30 (FIG. 4) of the receiving station
`72 or 72 can perform these measurements on received
`signals, after which the dynamic control circuitry 47 deter-
`mines what data rate or other operating characteristics
`and/or parameters to use and what data rate or other oper-
`ating characteristics and/or parameters to use in the reverse
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`direction. Accordingly, the receiver 30 provides feedback to
`the dynamic control circuitry 15 of the transmitter 10 of the
`receiving station 72 or 74 to dynamically scale the operating
`characteristics and/or parameters, such as the data rate,
`between the two stations. Alternatively, the receiver 30 (FIG.
`4) of the receiving station 72 or 74 can perform the signal
`quality measurements and send back the quality information
`or a request for particular operating characteristics and/or
`parameters, such as data rate, through its transmitter10 to the
`receiver 30 of the transmitting station 72 or 74. The receiver
`30 of the transmitting station 72 or 74 then can provide
`feedback to the dynamic control circuitry 15 at the trans-
`mitting station 72 or 74 to dynamically scale the operating
`characteristics and/or parameters, such as the data rate,
`between the stations 72 or 74. Although this particular
`embodiment of the OFDM system 70 has a base station 72
`and remote stations 74, the scaling features according to
`aspects of the present invention extend to a network of
`non-centralized OFDM transceivers.
`
`Furthermore, in certain embodiments, the OFDM system
`70 according to the principles of the present invention, can
`be used to implement multiple access of multi-rate systems
`by dynamically scaling the number of carriers. One remote
`station 74 could be sending on just one carrier, another
`remote station 74 on 4 other carriers, while a third remote
`station 74 could be sending on yet another 2 carriers, all at
`the same time. For proper decoding it is mandatory that the
`signals of all carriers (from different remote stations 74) are
`received with roughly the same relative delays by the base
`station 72.
`
`In the case of certain embodiments of centralized systems
`which dynamically scale the number of carriers, the base
`station 72 receives from and transmits to all remote stations
`
`(mobile units 74 in this embodiment) within its range. Thus,
`the base station 72 of this particular embodiment should be
`capable of receiving and transmitting at all carriers simul-
`taneously. This implies that the base station 72 faces a larger
`peak-to-average power ratio than the mobiles 74, but that is
`not really a drawback, since the base 72 is not battery-
`driven.
`
`Transmitting on subsets of carriers provides the possibil-
`ity of asymmetric data links, meaning that data rates can be
`different for uplink and downlink. Asymmetric links often
`occur in practice, e.g., downloading data. The OFDM sys-
`tem 70 can support such asymmetric links by dynamically
`providing remote stations 74 with a different number of
`carriers for uplink and downlink. Also, since in a centralized
`system the base station 72 can transmit at higher power
`levels than the mobiles 74, it is possible to use higher level
`modulation schemes on the carriers (e. g. 16 QAM), such that
`the downlink capacity is larger than the uplink capacity.
`Advantages of using dynamic control circuitry 15 (FIG.
`1) and 47 (FIG. 4) to achieve asymmetric rates are:
`Downlink capacity can be made larger than uplink capac-
`ity.
`Uplink capacity can be shared by dividing total number of
`carriers into subsets.
`
`Mobiles 74 can transmit longer packets at a lower rate
`compared to pure TDMA. This has the advantage that
`the average transmitted power is lower (simpler power
`amplifier) and also that the relative overhead caused by
`training is reduced.
`Mobiles 74 only have to transmit a limited number of
`carriers, which reduces the peak-to-average power of
`the transmitted signal. This means the mobiles 74 can
`achieve a better power efficiency, which is very impor-
`tant for battery-driven devices.
`
`ERIC-1003 I Page 8 of 11
`
`ERIC-1003 / Page 8 of 11
`
`

`
`US 6,175,550 B1
`
`9
`When different mobiles 74 are allowed to transmit simul-
`
`taneously at different carriers, the following can arise:
`Symbol synchronization is necessary between mobiles
`and base station. Such synchronization is already
`present in TDMA systems like GSM. For the described
`OFDM example with a symbol duration of 200 ys, the
`synchronization offset should be limited to about 5 ys.
`Some power control
`is necessary to reduce near-far
`effects. The near-far effect is less serious than in CDMA
`
`systems, because the OFDM carriers are orthogonal,
`while CDMA codes usually have some non-zero cross-
`correlation. In OFDM, power control is only needed to
`reduce the dynamic range of A/D converters in the
`receiver, and to reduce multi-user interference caused
`by frequency offsets, which may introduce some cor-
`relation between carriers of different users.
`
`In the previously described OFDM mobile phone option,
`with 32 carriers delivering 320 kbps in a bandwidth of 200
`kHz, the band can be divided into 8 channels of 4 carriers
`each. Each channel then carries data at a raw rate of 40 kbps,
`which provides about 70% of redundancy for signaling
`overhead and forward error correction coding of a 13 kbps
`speech signal.
`Thus, the OFDM system 70 can provide the advantages of
`asymmetric data rates when needed, such as during the
`downloading of data from the base station 72 to the remote
`station 74, by dynamically altering the number of carriers
`used for downlink to receiver 30 (FIG. 4) of the remote
`station 74 and for uplink from the transmitter 10 of the
`remote station 74. Additionally, the OFDM system 70 can
`dynamically scale various operating characteristics and/or
`parameters for the stations 72 and 74 and can provide
`different operating characteristics and/or parameters
`between the base station 72 and different remote stations 74
`
`or provide varying symmetric operating characteristics and/
`or parameters between the base station 72 and a remote unit
`74. Alternatively, the dynamic scaling of operating param-
`eters and/or characteristics between stations to provide dif-
`ferent operating parameters and/or characteristics between
`the stations can be performed in a non-centralized OFDM
`system of transceivers.
`In certain embodiment of the OFDM system 70 of FIG. 5,
`adaptive antennas 78 can be used at the base station 72 to
`make the antenna pattern adaptive and different for each
`carrier such that the signal-to-noise plus interference ratio is
`maximized for each carrier. In OFDM, the base 72 simply
`measures the amplitude of several carriers to obtain the
`spectrum of incoming signal which provides simultaneous
`adaptive antennas. Adaptive antenna control circuitry 80 can
`control the adaptive antennas 78 in the following manner to
`provide improved performance in the OFDM system 70:
`Base 72 measures uplink channel (N carrier amplitudes,
`SNR/SIR), assuming downlink channel equal to uplink
`channel;
`If downlink and uplink channels are not equal because
`they are at different frequencies for instance (as in
`UMTS), the mobile 74 can send measured downlink
`carrier amplitudes as feedback over the uplink to the
`base station 72;
`In uplink, base station 72 uses adaptive antenna to maxi-
`mize signal-to-noise and interference ratio; and
`In downlink, base station 72 uses measured uplink chan-
`nel or feedback from mobile to select amplitudes and
`phases for each carrier and each antenna of the adaptive
`antennas 78. In this way, the OFDM system 70 benefits
`from improved antenna gain for each carrier. By trans-
`
`5
`
`10
`