Towards Systems Beyond 3G Based on Adaptive OFDMA
, Chalmers University of Technology
, Chalmers University of Technology,
Chalmers University of Technology and
Invited Paper, Proceedings of the IEEE ,
Vol. 95, 2007, pp. 2432-2455.
Special Issue on Adaptive Modulation/Adaptive Transmission,
© 2007 IEEE
Paper in Pdf (1.45M)
OFDM has been introduced in
wireless local area network (WLAN)
standards and this technology
is the primary alternative
in newer wireless broadband standard
proposals such as IEEE 802.16
WiMAX and WiBro. An important development
is the ongoing Third Generation
Partnership Project Long-Term Evolution (3GPP-LTE)
or Evolved UTRA
standardization effort where the use of
adaptive OFDMA is proposed for the downlinks.
It is therefore now
appropriate to try to summarize
the design issues that are encountered
when utilizing adaptive transmission
in OFDMA-based systems beyond 3G.
We here focus on some of the challenges involved
in any design that would have the following aims:
Numerous design aspects and tradeoffs are
then encountered. We discuss many of them
in this paper and draw on illustrative
and supporting results that have been
obtained within the
Swedish Wireless IP program
and in the EU WINNER projects
- Multiple data flows are transmitted
over frequency-selective wide-band channels.
Sets of infrastructure-connected antennas
communicate with terminals that each may
have multiple antennas.
- Packet data is to be transmitted
flexibly and adaptively with respect to
the properties and quality-of-service demands
of the different packet flows.
Time, frequency and spatial
(antenna) resources are to be
scheduled and used adaptively
with respect to the channel properties,
whenever this improves the transmission.
- A low latency over the air interface
is desired. This enables
adaptivity with respect to fast channel variations,
it facilitates high-throughput TCP/IP traffic
and it enables fast link retransmission,
which is of advantage for the performance
perceived at higher layers.
Among others, the following problems will
What is the potential gain obtained by
using the frequency variability of
What are the appropriate sizes of the resource
units that are allocated to different users?
What is an adequate level of channel prediction
accuracy and what terminal mobilities can be
How can link adaptation,
and multi-user scheduling be organized
in a computationally efficient way?
What air interface delays are realistically
attainable, and what constraints do these
place on computational delays and
transmission control loop designs?
High data rates, high spectral efficiency,
and low delays over the air interface
will be important features in next-generation
The over-all challenge will be packet
scheduling and adaptive radio transmission
for multiple users, via multiple antennas and over
frequency-selective wideband channels. This problem
needs to be structured to obtain
The basic simplifying assumptions used
here are clustering of antennas into
transmission by use of cyclic-prefix OFDM
(orthogonal frequency division multiplexing) and
a time-scale separation view
of the total link adaptation, scheduling
and inter-cell coordination problem.
Based on these assumptions,
we survey techniques
that adapt the transmission to the
temporal, frequency and spatial
channel properties. We provide a
systematic overview of the
design problems, such as the
dimensioning of the allocated time-frequency
influence of duplexing schemes, adaptation
control issues for
downlinks and uplinks, timing issues and
their relation to the
required performance of channel predictors.
Specific design choices are illustrated by recent
research within the Swedish Wireless IP
program and the EU
The presented results indicate that
high-performance adaptive OFDM transmission
systems are indeed feasible,
also for challenging scenarios that involve
vehicular velocities, high carrier
frequencies and high bandwidths
Link adaptation, challen-aware scheduling,
channel estimation, channel
prediction, OFDM, OFDMA, channel prediction, SDMA.
Wireless IP Project;
entry in list of publications
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