Transcript Multiple Access Techniques for the Uplink in Future

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
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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Introduction

One of the basic questions for the uplink in wireless
communications systems is whether multicarrier or singlecarrier 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 power in a fraction of the
channel bandwidth.

These considerations indicate that a single-carrier technique
with an OFDMA-like multiple access would combine the
desired features of both transmission techniques.

This is achieved by Interleaved Frequency-Division Multiple
Access (IFDMA).
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Introduction (cont’d)

IFDMA interleaves different user signals in the frequency
domain without having to make any transformations
between the time domain and the 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-domain
implementation of IFDMA, which coincides with Distributed
OFDMA that includes a precoding operation by a Discrete
Fourier Transform (DFT).

The major argument in favor of this implementation, which
is referred to as DFT-Spread OFDM(A), is its flexibility.
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Multiple Access techniques

One of the basic requirements for the multiple access
technique to be used on the uplink is to use efficiently the
power transmitted by the user 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 to
different users. Since the user transmit power is
concentrated in a fraction of the channel bandwidth, OFDMA
significantly increases cell coverage.

But OFDMA shares the PAPR problem of OFDM. Although
many PAPR reduction algorithms are available today, they all
fall short of giving significant gains in practical applications.
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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 user
data blocks.

The spectrum of the compressed and Q times repeated
signal has the same shape as that of the original signal with
the difference that it features Q-1 zero-valued spectral
components between two adjacent data subcarriers.

This feature can be exploited to interleave Q different user
signals in the frequency domain. All that is needed is to shift
the user signals in the frequency domain so that their useful
spectral components do not overlap.

The signal keeps its single-carrier nature and 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 block
of length M to an M-point DFT and passing the DFT output
to an N-point IDFT input in some way.

It the M-point DFT output is uniformly distributed to the Npoint IDFT input, DFT-Spread OFDMA is 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 the
increase of PAPR which results when the DFT output signal is
not mapped on equidistant subcarriers.
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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DFT-Spread OFDM(A): General Principle
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DFT-Spread OFDM(A)
Version A: Distributed subcarriers
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DFT-Spread OFDM(A)
Version 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:
vOUT  a (1  e
 b vIN
)  c vIN e
2
 d vIN
,
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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:
(vOUT )  max(0, f (1  e

 g ( vIN h)
)),
The simulated values are f = 0.7311, g = 0.3266 and h =
0.3891.
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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:
IBO dB  10 log10
E

2
PSAT , IN
and
OBO dB  10 log10

v IN (t )
E

vOUT (t )
PSAT ,OUT
,
2
.
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.
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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
 b 2

i
2
CCDF( INP ( v IN ))  Pr 
 
 Pavg , IN



was used to illustrate PAPR performance.
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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.
Third COST 289 Workshop, 12 – 13 July 2006, Aveiro, Portugal
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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 singlecarrier 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.
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