Fifth International Symposium on Signal Processing and its Applications,
`ISSPA ‘99, Brisbane, Australia, 22-25 August, 1999
`Organised by the Signal Processing Research Centre, QUT, Brisbane, Australia
`
`ABSTRACT
`This paper outlines some of the potential advantages of
`multiuser OFDM. An overview of several new techniques
`that improve system reliability and spectral efficiency are
`presented. These include adaptive modulation, adaptive
`frequency hopping, and multiple transmitter cells. A low
`bandwidth hardware implementation is also presented.
`
`MULTIUSER OFDM
`E. Luwrey
`Electrical and Computer Engineering
`James Cook University, Douglas Campus, Townsville, Queensland, 48 14, Australia
`Email: Eric.Lawrey @jcu.edu.au,
`modulation scheme can be used including BPSK, QPSK,
`8PSK, 16QAM, 64QAM.. . Each modulation scheme
`provides a trade off between spectral efficiency and the bit
`error rate. The spectral efficiency can be maximised by
`choosing the highest modulation scheme that will give an
`acceptable Bit Error Rate (BER). In a multipath radio
`channel, frequency selective fading can result in large
`variation in the received power of each carrier. For a
`channel with no direct signal path this variation can be as
`much as 30 dB in the received power resulting in a similar
`variation in the SNR. Using adaptive modulation the
`carrier modulation is matched to the SNR, maximising the
`overall spectral efficiency.
`In systems that use a fixed modulation scheme the carrier
`modulation must be designed to provide an acceptable
`BER under the worst channel conditions. This results in
`most systems using BPSK or QPSK. These give a poor
`spectral efficiency (1-2 bits/s/Hz) and provide an excess
`link margin most of the time. Using adaptive modulation,
`the remote stations can use a much higher modulation
`scheme when the radio channel is good. Thus as a remote
`station approaches the base station the modulation can be
`increased from 1 bits/s/Hz (BPSK) up to 4-6 bits/s/Hz
`(1 6QAM - 64QAM), significantly increasing the spectral
`efficiency of the overall system. Preliminary results show
`that for a cellular network the system capacity can
`potentially be doubled using adaptive modulation.
`There are several limitations with adaptive modulation.
`Overhead information needs to be transferred, as both the
`transmitter and receiver must know what modulation is
`currently being used. Also as the mobility of the remote
`station is increased, the adaptive modulation process
`requires regular updates, further increasing the overhead.
`There is a trade off between power control and adaptive
`modulation. If a remote station has a good channel path
`the transmitted power can be maintained and a high
`modulation scheme used (i.e. 64 QAM), or the power can
`be
`reduced
`and
`the modulation
`scheme
`reduced
`accordingly (i.e. QPSK).
`Distortion, frequency error and the maximum allowable
`power variation between users
`limit
`the maximum
`modulation scheme that can be used. The received power
`for neighbouring carriers must have no more than 15-30
`dB variation at the base station, as large variations can
`result
`in strong signals swamping weaker carriers.
`Intermodulation distortion (IMD) results from any non
`linearites in the transmission. This IMD causes a higher
`noise
`floor
`in
`the
`transmission band,
`limiting
`the
`maximum SNR to typically 30-60 dB. Frequency errors in
`the transmission due to synchronisation errors and Doppler
`shift result in a loss of orthogonality between the carriers.
`A frequency offset of only 1% of the carrier spacing
`results in the effective SNR being limited to 30 dB [4].
`
`1 INTRODUCTION
`OFDM has been widely used in broadcast systems. It is
`being used for Digital Audio Broadcasting (DAB) [l] and
`for Digital Video Broadcasting (DVB) in Europe and
`Australia. It was selected for these systems primarily
`because of its high spectral efficiency and multipath
`tolerance. OFDM transmits data as a set of parallel low
`bandwidth (100 Hz - 50 kHz) carriers. The frequency
`spacing between the carriers is made to be the reciprocal
`of the useful symbol period. The resulting carriers are
`orthogonal to each other provided correct time windowing
`at the receiver is used. The carriers are independent of
`each other even though their spectra overlap. OFDM can
`be easily generated using an Inverse Fast Fourier
`Transform (IFFT) and received using a Fast Fourier
`Transform (FFI). High data rate systems are achieved by
`using a large number of carriers (i.e. 2000-8000 as used in
`DVB). OFDM allows for a high spectral efficiency as the
`carrier power, and modulation scheme can be individually
`controlled for each carrier. However in broadcast systems
`these are fixed due to the one way communication.
`In most communication systems two-way communications
`is required and multiple users must be supported. OFDM
`can be applied in a multiuser application producing a
`highly flexible, efficient communications system. Little
`work has been previously done on multiuser OFDM. It
`was first presented by Wahlqvist [2] who suggested one
`possible implementation. The system design of a multiuser
`OFDM system is dependent on the intended application
`and hardware complexity. This paper presents multiuser
`OFDM in a more general form and outlines some of the
`potential techniques that could be used to make it a highly
`efficient and reliable communication system. Additionally
`a test hardware solution is presented using SHARCB
`Digital Signal Processors
`(DSP) demonstrating
`the
`feasibility of a simple multiuser OFDM system.
`
`2 ADAPTIVE MODULATION
`Most OFDM systems use a fixed modulation scheme over
`all carriers for simplicity. However each carrier in a
`multiuser OFDM system can potentially have a different
`modulation scheme depending on the channel conditions.
`Any coherent or differential, phase or amplitude
`
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`GM 1012
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`Fifth International Symposium on Signal Processing and its Applications,
`ISSPA ‘99, Brisbane, Australia, 22-25 August, 1999
`Organised by the Signal Processing Research Centre, QUT, Brisbane, Australia
`
`5
`
`Remote
`Station 2
`
`o $
`
`-5 g
`a
`-10% 2
`-15 2
`-20 2 U
`
`m
`
`-25
`
`-30
`
`0
`
`25
`
`50
`
`75
`
`Figure 1. Single user Adaptive Frequency Hopping
`
`-1
`
`V
`
`Remote
`
`Remote
`Station 3
`
`I u L
`
`Transmit
`Spectrum
`Spectrum
`Figure 3. Reverse link of a multiuser OFDM system
`
`carriers are transmitted in short time blocks. These blocks
`were randomly frequency hopped to ensure that the time
`period spent
`in a null would
`is relatively short,
`approximately 11 symbols. To recover data lost during a
`null, time interleaving and forward error correction was
`used. These come at the cost of reduced capacity and an
`increased delay.
`
`3.2 Adaptive Frequency Hopping
`A new adaptive hopping technique is proposed such that
`carrier block hopping is based on the channel conditions.
`After the radio channel has been characterised each user is
`allocated carriers which have the best SNR ratio for that
`user. Since each user will be in a different location the
`fading pattern will be different for each remote station.
`The strongest carriers for one user are likely to be different
`from other users. Thus most users can be allocated the best
`carriers for them with minimal clashes.
`Preliminary studies have shown that adaptive hopping can
`provide a dramatic improvement in received power (5-
`20dB) in a frequency selective channel. Adaptive hopping
`virtually eliminates frequency selective fading.
`Figures 1 and 2 show results for a single user adaptive
`frequency hopping system. The radio channel model used
`for this experiment was measured for a link between two
`rooms
`in
`the Electrical and Computer Engineering
`building at JCU. The transmitter and receiver were spaced
`24 m apart. Figure 1 shows simulated adaptive frequency
`hopping for this measured radio channel. For the No
`Hopping case the frequency used was fixed and thus a
`straight line on figure 1. The bandwidth allocated to the
`user was 1% of the system bandwidth. The user was
`allocated the strongest frequency band and the frequency
`allocation was updated every 5 cm. Figure 2 shows for the
`same measured channel the received power for the
`adaptive frequency hopping receiver and for one which
`has no hopping. The adaptive hopping receiver suffers
`much less fast fading and has a much greater average
`power level than when no hopping was.used.
`In a multipath channel the frequency response of the
`channel will change significantly in 15 cm at 1 GHz. It is
`therefore important that the frequency update rate is much
`faster than every 15 cm moved. Typically an update
`distance of 5 cm is sufficient. At a velocity of 60 km/Hr
`
`2b
`
`40
`
`120
`
`140
`
`160
`
`$0
`80
`100
`Distance (cm)
`Figure 2. Received power verses distance travelled for
`Adaptive Frequency Hopping and for a fixed frequency
`The limited SNR restricts the maximum spectral efficiency
`to approximately 5-7 bits/s/Hz.
`Adaptive modulation requires accurate knowledge of the
`radio channel. Any errors in this knowledge can result in
`large increases in the BER, due to the small link margin
`used. For a multiuser OFDM system, transmitting pilot
`tones or
`reference
`symbols can perform channel
`characterisation. Transmitting a symbol with known data
`allows the phase error to be estimated, giving the SNR of
`each carrier. This SNR can then be used to select the
`modulation scheme.
`
`3 USER ALLOCATION
`There are several methods for allocating carriers to users.
`The main three groups are grouped carriers, spread out
`carriers and adaptive camer allocation.
`
`3.1 Grouped carriers
`The simplest scheme is to group the carriers allocated to
`each user. Grouping carriers minimises
`inter-user
`interference due to distortion, power level variation and
`frequency errors. However, grouping the carrier makes the
`transmission susceptible to fading, as the whole group of
`carriers can be lost in a null in the spectrum. This problem
`can be partly overcome by frequency hopping the carriers.
`In user allocation scheme described by [ 2 ] , groups of
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`Fifth International Symposium on Signal Processing and its Applications,
`ISSPA '99, Brisbane, Australia, 22-25 August, 1999
`Organised by the Signal Processing Research Centre, QUT, Brisbane, Australia
`
`T -
`
`Base Station 42-5m w
`Transmitter
`t
`
`I
`
`I
`
`I
`
`\ '- .
`Multiple Transmitter 0 Remote Station
`
`\
`
`/ Boundary
`
`Base Station Repeater5
`Obstruction
`Equivalent Single
`Transrmtter Base Station -b Transmltted Signal
`Figure 4. Reduced shadowing with a multiple
`transmitter cell
`this results in an update rate of 330 times per sec. The
`overhead for the frequency hopping will depend on the
`user data rate, the number of users, and whether the
`system is a full, or half duplex system.
`For a full duplex system the transmitter and receiver
`frequency are offset from each other by A 0 MHz to allow
`for isolation between them. For this type of transmission
`the number of radio channels that must be characterised is
`2N, where N is the number of users. The number of
`reference symbols that must be transmitted is N+1, one
`from the base station in the forward link and one from
`each user. This can be reduced by transmitted a comb
`pattern, allowing typically 20 users to be characterised per
`reference symbol. However for a lOMHz system BW, full
`duplex system with 100 users at 5Okbps each (QPSK
`average) and using a comb characterisation, the overhead
`will be 30-50% of the data capacity.
`This overhead can be reduced by using a time division
`half-duplex system. If both the forward and reverse links
`use the same frequency, then only one reference symbol is
`needed. A single reference symbol transmitted by the base
`station
`is received by all remote stations, allowing
`characterisation of all forward links. Due to the reciprocal
`nature of radio channels the transfer function of the
`reverse link will be the same as the forward
`link.
`Regardless of how the radio channels are characterised the
`number of frequency re-allocations will be proportional to
`the number of users. The overhead for a half-duplex
`system as described above will only be 10-15%.
`Adaptive frequency hopping reverts to normal random
`hopping when the velocity is too high for the hopping to
`keep up with the rate of change of the channel. Fortunately
`this occurs at high velocity and consequently
`the
`coherence
`time of
`the fading
`is very small. Time
`interleaving and forward error correction can be used to
`overcome these fading periods and since the fade time is
`short, the time interleaving length required is also short,
`resulting in minimal delay.
`Adaptive hopping comes at the cost of increasing the
`complexity of the transmitter and receiver. Additionally
`
`10-50m
`
`Base Station
`Receiver
`Figure 5. Simple Multiple Transmitter Set up for
`wireless LAN
`adaptive hopping is less useful at high velocities over 30
`km/Hr.
`3.3 Comb Spread Carriers
`Carriers can be allocated in a comb pattern, spreading
`them over the entire system bandwidth. This improves the
`frequency diversity, preventing all the carriers used by a
`user being lost in a null in the spectrum. However, this
`allocation scheme may be susceptible
`to
`inter-user
`interference. This type of user allocation is useful in
`applications that can not use adaptive hopping. Figure 3
`shows an example of a comb user carrier allocation.
`
`4 MULTIPLE TRANSMITTER CELLS
`ODFM signals are intrinsically multipath robust due to the
`low symbol rate used and the addition of a time domain
`guard period [3]. Multipath reflections that have a delay
`spread less than the guard period cause no inter-symbol
`interference. This allows for Single Frequency Networks
`(SFN) to be used in broadcast OFDM systems [l]. A
`single frequency can be used for all transmitters in a
`country wide broadcast. For DAB the transmitter can be
`spaced up to 75 km apart. Normally each transmitter must
`use a different frequency from neighbouring transmitters,
`as they would appear as strong multipath if the same
`frequency were used. A SFN greatly reduces shadowing as
`multiple signals are received from different directions
`resulting in spacial diversity.
`The concept of a SFN can be applied to a multiuser
`OFDM system. The base station would consist of multiple
`repeaters
`transmitting
`the same signal. Each signals
`received by the repeaters can either be combined and
`decoded with a single central receiver, or each repeater
`could have its own receiver. Using a multiple transmitter
`cell will significantly reduce shadowing due to the
`increased spacial diversity, as shown in figure 4.
`Multiple transmitter cells are particularly suitable for
`wireless LAN applications. Shadowing makes it difficult
`to achieve good coverage of a building. Repeaters are
`often required, except that in conventional systems these
`repeaters cause multipath problems. In a multiuser OFDM
`system repeaters could be added where needed, with no
`additional problems. A multiple transmitter cell could be
`as simple as a coax running the length of a building
`corridor with multiple tap off points (see figure 5).
`
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`Fifth International Symposium on Signal Processing and its Applications,
`ISSPA ‘99, Brisbane, Australia, 22-25 August, 1999
`Organised by the Signal Processing Research Centre, QUT, Brisbane, Australia
`
`OHz
`
`4kHz . BkHz . 12LHz
`
`16kH:
`
`2OkHz
`
`24kHz
`
`User 1
`
`User 2
`
`0 dB
`
`10 dB
`
`20 dB
`
`30 dB
`
`40 dB
`
`50 dB
`
`60 dB
`
`70 dB
`
`Figure 6. Example Multiuser OFDM transceiver
`
`5 HARDWARE IMPLEMENTATION
`A small-scale test system was made using Analog Devices
`Ez-Kit evaluation boards. This board includes a 40 MHz
`SHARC@ DSP processor, a 16 bit stereo CODEC,
`bootloader kernel, and a serial interface. A baseband
`multiuser OFDM system was implemented using the on-
`board CODECs. The maximum sample rate for the
`CODECs was 48 kHz. A 5 12 point real FFT was used for
`signal generation, of which 196 carriers were active,
`giving a bandwidth of 18 kHz. A guard period of 32
`samples was used. Each transceiver was made using two
`Ez-kit boards. Figure 6 shows one of the multiuser OFDM
`transceivers.
`5.1 User Allocation
`A three user system was made consisting of a base station
`and two remote stations. The forward link transmission
`was subdivided into 2 groups of 96 carriers, one group for
`each user. The reverse link transmission from each remote
`station was a group of 96 carriers. One remote station used
`the lower 96 carriers, the other the upper 96 carriers. No
`adaptive frequency hopping was used.
`5.2 Carrier Modulation
`The input data for each user was an audio signal sampled
`at 8 kHz at 8 bits. This data was modulated with 256 PSK,
`using a linear mapping from the 8 bit audio data. Due to
`the linear mapping errors in reception have only a small
`effect. The modulation was fixed to simplify the system.
`5.3 Time Synchronisation
`Multiuser OFDM requires strict time and frequency
`synchronisation. In the reverse link the signals from all
`users are combined in the channel and are received as a
`complete OFDM signal. All remote stations must be
`in order for the
`frequency and
`time synchronised
`transmitted signals to remain orthogonal to each other. All
`signals in the forward channel originate from the base
`station, and thus synchronisation techniques developed for
`broadcast OFDM can be used [3,4].
`
`-
`. - ~
`-
`80 dB
`Figure 7. Separated spectrum of the reverse link for the
`multiuser OFDM test system.
`The remote stations were synchronised to the base station
`using a null symbol frame synchronisation. The base
`station transmitted a null symbol at the start of each frame
`(36 symbols). The remote stations synchronised to the null
`using a moving average envelope detector.
`5.4 Results
`The mobile system was found to work, with no apparent
`cross talk between the two users. The forward
`link
`synchronisation was found to be stable, with an error of
`-1-2 samples/frame at a high SNR, degrading to 8-32
`samples at a SNR of IdB. Reverse link synchronisation
`was slightly worse caused by difference
`in forward
`synchronisation of the two users.
`
`6 CONCLUSION
`This paper has presented an overview of multiuser OFDM
`and some of the new techniques that can enhance system
`performance. Multiuser OFDM allows for highly flexible
`communications, thus can be made to adapt to radio
`channel conditions. This adaptability results in a high
`spectral efficiency and reliability.
`
`7 REFERENCES
`Thibault L. and Le M.T., “Performance Evaluation of
`COFDM for Digital Audio Broadcasting, Part I:
`Parametric Study”, IEEE Transactions on Broadcasting,
`Vol. 43, No. 1, pages 64-75, March 1997
`Wahlqvist M., Ostberg C., Beek J., Edfors O., Borjesson
`P., “A Conceptual Study of OFDM-based Multiple Acess
`Schemes”, Technical Report Tdoc 117196, ETSI STC
`SMG2 meeting no 18, Helsinki, Finland, May 1996,
`http://www.sm.hth.se/csee/sdpublications/
`Lee D., Cheun K., “A new symbol timing recovery
`algorithm for OFDM systems”, IEEE Transactions on
`Consumer Electronics, Vol. 43, No. 3, pages 766-775,
`August 1997
`Moose P., “A Technique for Orthogonal Frequency
`Division Multiplexing Frequency Offset Correction”,
`IEEE Transactions on Communications, Vol. 42, No. 10,
`pages 2908-2914, October 1994
`
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