Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal 1 Third COST 289 Workshop Multiple Access Techniques for the Uplink in Future Wireless Communications

Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal

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Third COST 289 Workshop

Multiple Access Techniques for the Uplink in Future Wireless Communications Systems

Cristina Ciochina(1),(2), David Mottier(2), and Hikmet Sari(1)

(1) SUPELEC, Plateau du Moulon, 3 rue Joliot-Curie F-91192 Gif sur Yvette, France

(2) Mitsubishi Electric ITE-TCL, 1 allée de Beaulieu F-35780 Rennes Cedex 7, France


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PAPER OUTLINE

Introduction

The uplink problems in wireless systems

Presentation of OFDMA, IFDMA and DFT-Spread OFDM

System model with a nonlinear power amplifier

Performance analysis

Conclusions


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Introduction

One of the basic questions for the uplink in wireless One of the basic questions for the uplink in wireless communications systems is whether multicarrier or communications systems is whether multicarrier or single-carrier transmission must be used.single-carrier transmission must be used.

Multicarrier transmission suffers from a high peak-to-average power ratio (PAPR), but it opens the way to OFDMA, which concentrates the transmitted signal OFDMA, which concentrates the transmitted signal power in a fraction of the channel bandwidth.power in a fraction of the channel bandwidth.

These considerations indicate that a single-carrier These considerations indicate that a single-carrier technique with an OFDMA-like multiple access would technique with an OFDMA-like multiple access would combine the desired features of both transmission combine the desired features of both transmission techniques. techniques.

This is achieved by Interleaved Frequency-Division This is achieved by Interleaved Frequency-Division Multiple Access (IFDMA).Multiple Access (IFDMA).


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Introduction (cont’d)

IFDMA interleaves different user signals in the IFDMA interleaves different user signals in the frequency domain without having to make any frequency domain without having to make any transformations between the time domain and the transformations between the time domain and the frequency domain.frequency domain.

The basic principle of IFDMA consists of splitting the user signals into symbol blocks and repeating these blocks a certain number of times with a user-specific phase ramp..

The 3GPP LTE Group has favored a frequency-The 3GPP LTE Group has favored a frequency-domain implementation of IFDMA, which coincides domain implementation of IFDMA, which coincides with Distributed OFDMA that includes a precoding with Distributed OFDMA that includes a precoding operation by a Discrete Fourier Transform (DFT). operation by a Discrete Fourier Transform (DFT).

The major argument in favor of this implementation, The major argument in favor of this implementation, which is referred to as DFT-Spread OFDM(A), is its which is referred to as DFT-Spread OFDM(A), is its flexibility.flexibility.


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Multiple Access techniques

One of the basic requirements for the multiple access One of the basic requirements for the multiple access technique to be used on the uplink is to use technique to be used on the uplink is to use efficiently the power transmitted by the user efficiently the power transmitted by the user terminal.terminal.

One possibility is to use OFDMA, which was recently adopted by the WiMAX Forum for mobile broadband wireless access..

OFDMA consists of assigning different carrier groups OFDMA consists of assigning different carrier groups to different users. Since the user transmit power is to different users. Since the user transmit power is concentrated in a fraction of the channel bandwidth, concentrated in a fraction of the channel bandwidth, OFDMA significantly increases cell coverage. OFDMA significantly increases cell coverage.

But OFDMA shares the PAPR problem of OFDM. But OFDMA shares the PAPR problem of OFDM. Although many PAPR reduction algorithms are Although many PAPR reduction algorithms are available today, they all fall short of giving significant available today, they all fall short of giving significant gains in practical applications. gains in practical applications.


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Distributed OFDMA: Uniform Carrier Spacing


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Localized (Clustered) OFDMA


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Interleaved FDMA (IFDMA)

IFDMA is based on compression and repetition of the IFDMA is based on compression and repetition of the user data blocks. user data blocks.

The spectrum of the compressed and Q times The spectrum of the compressed and Q times repeated signal has the same shape as that of the repeated signal has the same shape as that of the original signal with the difference that it features Q-1 original signal with the difference that it features Q-1 zero-valued spectral components between two zero-valued spectral components between two adjacent data subcarriers. adjacent data subcarriers.

This feature can be exploited to interleave Q different user signals in the frequency domain. All that is . All that is needed is to shift the user signals in the frequency needed is to shift the user signals in the frequency domain so that their useful spectral components do domain so that their useful spectral components do not overlap. not overlap.

The signal keeps its single-carrier nature and The signal keeps its single-carrier nature and amplitude variations remain low. amplitude variations remain low.


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Time-Domain Generation of IFDMA


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Frequency-Domain Generation of IFDMA


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DFT-Spread OFDM(A)

DFT-Spread OFDMA consists of sending the user data DFT-Spread OFDMA consists of sending the user data block of length M to an M-point DFT and passing the block of length M to an M-point DFT and passing the DFT output to an N-point IDFT input in some way. DFT output to an N-point IDFT input in some way.

It the M-point DFT output is uniformly distributed to It the M-point DFT output is uniformly distributed to the N-point IDFT input, DFT-Spread OFDMA is the N-point IDFT input, DFT-Spread OFDMA is mathematically equivalent to IFDMA. mathematically equivalent to IFDMA.

The interesting feature of DFT-Spread OFDMA is that the mapping of the DFT output signal onto subcarriers can be made arbitrarily, and this leads to increased flexibility. .

However, that flexibility has to be traded off against However, that flexibility has to be traded off against the increase of PAPR which results when the DFT the increase of PAPR which results when the DFT output signal is not mapped on equidistant output signal is not mapped on equidistant subcarriers. subcarriers.


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DFT-Spread OFDM(A): General Principle


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DFT-Spread OFDM(A)

Version A: Distributed subcarriersVersion A: Distributed subcarriers


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DFT-Spread OFDM(A)

Version B: Localized subcarriersVersion B: Localized subcarriers


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Some Comments

From the frequency diversity perspective, it is better to use distributed carriers rather than clustered carriers in OFDMA. Alternatively, a group of clustered carriers can be frequency hopped according to some hopping sequence

Another key concept is precoding, which is an efficient way of dispersing the energy of transmitted symbols over the channel bandwidth and solving the frequency diversity problem of OFDM-based systems.

The conventional way of precoding in OFDM-based systems is to use Walsh-Hadamard (WH) sequences. The DFT matrix represents an alternative to the WH matrix for precoding in order to spread energy uniformly.


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Some Comments (cont’d)

The beauty of the precoding by a DFT matrix is that it is mathematically equivalent to a single carrier system (and the PAPR problem disappears) when the precoder output is mapped on clustered or uniformly-spaced carriers.

Conventional IFDMA is a single-carrier approach in which the signal is generated in the time domain. The energy of transmitted symbols is naturally distributed on all carriers.

DFT-Spread OFDM(A) aims at keeping some flexibility in terms of mapping while dispersing the energy of transmitted symbols and reducing PAPR. But the PAPR problem is actually solved only when the DFT output is mapped onto clustered or uniformly spaced carriers.


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Investigated System Model

We consider OFDMA and DFT-Spread OFDMA. In both cases, we use N = 512 subcarriers, among which 300 are data carriers, 1 is DC, and the remaining 211 are guard carriers.

We use the QPSK, 16-QAM and 64-QAM signal constellations, Gray mapping, and a (753, 531)8 convolutional code with rate 1/2.

For the transmit power amplifier, we use the Honkanen model corrected by a Rayleigh-type factor. The AM-AM characteristic of this model is given by:2

(1 e ) e ,ININd vb v

OUT INv a c v


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System Model (cont’d)

The parameters simulated are a = 1.36, b = 1.813, c = -1.003 and d = 1.97.

The AM-PM characteristic of the amplifier is given by:

The simulated values are f = 0.7311, g = 0.3266 and h = 0.3891.

( )( ) max(0, (1 )),INg v hOUTv f e


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AM-AM and AM-PM Characteristics


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Amplifier Back-Off Definitions

The input back-off (IBO) and output back-off (OBO) are defined as follows:

and

In the simulations, an AWGN channel and the COST 259 urban channel model was used. Soft Viterbi decoding and, for the second channel, frequency-domain equalization was used.

2

10,

( )10log ,

IN

dBSAT IN

E v tIBO

P

2

10,

( )10log .

OUT

dBSAT OUT

E v tOBO

P


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Computer Simulations

Simulations were performed to compare OFDMA to SC/FDMA in its frequency domain version (DFT-Spread OFDMA).

The performance analysis included SNR degradation in the presence of HPA nonlinearity and BER performance on frequency-selective channels.

The cumulative complimentary distribution function (CCDF) of the instantaneous normalized power (INP) defined as

was used to illustrate PAPR performance.

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,

CCDF( ( )) Pr iIN

avg IN

bINP v

P


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CCDF of INP vs. CCDF of PAPR

For performance evaluation, it is more appropriate to use the CCDF of INP than the CCDF of PAPR.

The reason is that the former takes into account all signal samples, which fall in the nonlinear region of the power amplifier characteristics, and not only the peak values.

The difference between the two measures is small on an ideal (clipper type) amplifier, but it gets large on a practical amplifier with a significant nonlinear zone.

With practical amplifiers, the SNR degradation and spectral spreading are not caused by the peak signal samples only, but instead by all signal samples that fall in the nonlinear zone of the amplifier characteristics.


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CCDF of INP Results, QPSK, L = 4


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Spectrum Plots, QPSK, Ideal Clipper


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Spectrum Plots, QPSK, HPA Model of [10]


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Total SNR Degradation at BER = 10-4 vs. OBO


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BER Performance on COST 259 Channel


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SUMMARY AND CONCLUSIONS

We have given a review and a performance comparison of multiple access techniques proposed for future wireless communications systems.

OFDMA is compared to SC/FDMA, which relies on single-carrier transmission. The latter can be implemented in the time domain (IFDMA) or in the frequency domain.

DFT-Spread OFDM(A) is identical to IFDMA when the DFT output is mapped onto equi-spaced subcarriers.

The single-carrier structure makes DFT-Spread OFDM(A) less sensitive to amplifier nonlinearities than is OFDMA, but OFDMA was found to have slightly better performance on the COST 259 channel when used with rate = ½ code.

Documents

Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal 1 Third COST 289 Workshop Multiple Access Techniques for the Uplink in Future Wireless Communications