Coding and Resource Scheduling in Packet Oriented Adaptive TDMA/OFDMA Systems.

Tommy Svensson , Chalmers U. of Technology,
Sorour Falahati , Ericsson AB and
Mikael Sternad , Uppsala University.

IEEE 63d Vehicular Technology Conference, VTC2006-Spring , Melbourne, Australia, May 2006. © IEEE

Outline:

Adaptive systems allocate (schedule) time, frequency and antenna resources based on channel quality and user requirements. They enable efficient resource utilization and multi-user scheduling gains, when channels to different terminals fade independently.

In systems based on time division multiple access/ adaptive OFDM (TDMA/OFDMA), time-frequency resources (chunks) are allocated to the individual users and link adaptation is performed individually in each chunk. The chunk size is chosen such that the channel is essentially flat in time and frequency. This provides a flexible small-scale granularity of the resources for multi-user scheduling and link adaptation, which makes it possible to obtain large multi-user diversity gains

Based on the results obtained within the Swedish Wireless IP (WIP) project, we are currently evolving and assessing the feasibility of adaptive TDMA/OFDMA in novel broadband radio interfaces within the EU FP6 Integrated Project WINNER.

We here investigate adaptive downlinks and uplinks based on fast scheduling and link adaptation, also for users at vehicular speeds, with a non-adaptive diversity-based fall-back mode.

In the proposed downlink, each terminal predicts the signal-to- interference-and-noise ratio (SINR) over a major part of the total bandwidth. All active terminals report source coded SINR values or source coded suggested modulation formats over a shared uplink control channel. A resource scheduler, located close to one or several radio access points, allocates the downlink resources.

An open question raised here is how to best combine this fine-grained resource allocation and link adaptation with efficient channel coding schemes and retransmission protocols for network layer packets. An adequate combination preserves the multi-user scheduling and link adaptation gains regardless of various sizes of packets and different reliability requirements. This study proposes some approaches to tackle this problem, and initial results are presented.

Abstract:

Within the EU FP6 Integrated Project WINNER, adaptive transmission is investigated as a key technology for boosting the spectral efficiency of a new radio interface for 4G systems.

Adaptive allocation and link adaptation of time-frequency chunks based on channel prediction in an OFDM-based system offers a significant potential to design a spectrally efficient system. The chunk size is typically defined based on the minimum coherence time and coherence bandwidth of the targeted channels. It is important to allow efficient channel coding and link retransmission schemes without restricting the resource scheduler, even for systems using small chunk sizes, to achieve multi-user diversity gains.

In this paper we introduce some possible approaches to implement FEC coding and Hybrid ARQ and analyze their interplay with resource scheduling in packet oriented adaptive TDMA/OFDMA.

Related publications:

Proc. of the IEEE (Dec. 2007) invited paper on adaptive transmission in beyond-3G wireless systems.

IST Mobile Summit 2005 paper on the WINNER phase 1 results this paper improves upon.

IST Mobile Summit 2006 paper describing the WINNER phase 1 MAC concept for cellular transmission.

Later WINNER II design (ICT Mobile Summit 2008).

Downlink control overhead in WINNER II design (VTC-Fall 2008).

Wireless IP project Paper 1 at VTC2003, on adaptive modulation, multiuser diversity and channel variability within bins in downlinks.

Wireless IP project Paper 2 at VTC2003, on the OFDM downlink and cell planning for high SIR.

Wireless IP project Paper 3 at VTC2003, on OFDM downlink channel estimation and channel prediction.

Wireless IP project Paper 4 at VTC2003, on the impact of prediction errors on the adaptive modulation.

Source:

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