Towards Systems Beyond 3G Based on Adaptive OFDMA Transmission

Mikael Sternad, Uppsala University,
Tommy Svensson , Chalmers University of Technology
Tony Ottosson , Chalmers University of Technology,
Anders Ahlén, Uppsala University,
Arne Svensson, Chalmers University of Technology and
Anna Brunstrom Karlstad University.

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)

Outline:

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:

• 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.

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

Among others, the following problems will be addressed:

• What is the potential gain obtained by using the frequency variability of broadband channels?
• 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 supported?
• How can link adaptation, multi-antenna transmission 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?

Abstract:

High data rates, high spectral efficiency, flexibility and low delays over the air interface will be important features in next-generation wireless systems. 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 feasible solutions. The basic simplifying assumptions used here are clustering of antennas into cells, orthogonal 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 resources, the 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 IST-WINNER project. 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

Keywords:

Link adaptation, challen-aware scheduling, channel estimation, channel prediction, OFDM, OFDMA, channel prediction, SDMA.

By choosing to view this document, you agree to all provisions of the copyright laws protecting it.