Series in Broadband Communications

Carlo Mutti

Link Adaptation Schemes for
Wireless Multicarrier Transmission Systems

1. edition 2005; VIII, 170 pages/Seiten, 64,00. ISBN 3-89649-995-5

The demand for enhancing the data rate of existing mobile communication systems like e.g. the universal mobile telecommunication system and wireless systems beyond the third generation is still growing. Over the last years, wireless services like voice, data and multimedia had an unexpected and spectacular growth fueling the need for higher capacity in wireless systems. Following the motto "be connected everywhere, everytime and with everyone," the market demands for new wireless and mobile systems that can satisfy the future needs of broadband communications.

In this context, emerging technologies that improve wireless systems' spectrum efficiency are becoming necessary, especially in broadband applications. Some popular examples include smart antennas, in particular multiple-input multiple-output technology, coded multicarrier (MC) modulation, adaptive modulation and coding techniques.

MC transmission in the form of orthogonal frequency-division multiplexing (OFDM) is regarded as a very promising technique for future broadband wireless systems. In this thesis, two powerful techniques for the physical layer of a conventional OFDM system are investigated. Both techniques exploit available channel state information (CSI) at the transmitter side.

First, a set of new algorithms adapting the power, the modulation alphabet on the subcarriers, as well as the channel coding are presented. They are often referred to as link adaptation. The presented adaptation policies aim at minimizing the bit-error rate at the decoder output of hard decision based receivers, assuming either perfect or outdated CSI at the transmitter. The performance gain obtained using these adaptive schemes with respect to nonadaptive transmission systems is investigated by means of computer simulations for an OFDM system using bit-interleaved coded modulation (BICM).

In the second part, multiple antenna systems -- at the transmitter only and at both sides of the link -- in combination with BICM-OFDM are studied. The use of adaptive schemes that track channel variations and adapt transmission parameters in multiple antenna OFDM systems is very promising. The performance of the combinations of these two techniques is demonstrated by simulation results and compared to an existing transmit diversity scheme such as space-time block coding. It is found that if we assume to have perfect CSI, the best transmission strategy is to use adaptive schemes. With decreasing knowledge of the channel, diversity becomes more and more important and, at some point, a diversity transmission technique like e.g. the Alamouti scheme will yield better performance.

Keywords: OFDM, BICM, MIMO, MISO, CSI, adaptive power loading, adaptive bit, loading and adaptive coding.

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