Optimizing Wireless Communication Systems
Optimizing Wireless Communication Systems presents the underlying technological breakthroughs that allowed the current state of wireless technology development to evolve. The focus is on the two lower layers of the ISO/OSI layered model, specifically the
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Link Adaptation for MIMO-OFDM Systems Darlan C. Moreira, Walter C. Freitas Jr., Cibelly A. de Ara´ujo, and Charles C. Cavalcante
10.1 Introduction The paradigm of the design of a wireless system has changed. Since the use of the dimensioning for the “worst case”, which means to design the system to work on the fading margin available when the channel has its poorest behavior, the driver of the optimization has evolved to a more suitable use of the available resources for performing a reliable communication. This approach is then called link adaptation (LA), when the system chooses the parameters which are the most suitable for usage in a certain channel condition. The always increasing demand for higher data rates, lower energy consumption, etc., requires that the system resources are utilized as efficiently as possible and LA techniques are already a reality in any modern wireless communication systems to achieve that goal. While many aspects of LA, such as usage of different modulations and code rates for providing better “protection” to data streams according to the channel condition, have already been understood, each system has a different set of “interesting parameters” to be adapted in multiple dimensions and the trade-off between LA gains and signaling overhead still provides challenges to be answered. Typical dimensions used in LA procedure are modulation and coding. The choice of the modulation allows the system to improve/decrease the spectral efficiency and the code rate impacts the amount of redundancy inserted for error protection into data frames. However, it is possible to envisage the exploitation of other features of the wireless system, for instance the spatial and frequency domains. This chapter describes the use of transmission modes considering parameters which are important to the performance of a wireless system, in particular the extension of LA to the MIMO-OFDM case in fourth generation (4G) systems. The rest of the chapter is organized as follows. The fundamentals of multipleinput multiple-output (MIMO) systems are presented in Section 10.2, where classical MIMO schemes are described. Section 10.3 discusses the trade-off between the diversity and multiplexing gains that can be extracted from the MIMO channel F. Cavalcanti, S. Andersson (eds.), Optimizing Wireless Communication Systems, c Springer Science+Business Media, LLC 2009 DOI 10.1007/978-1-4419-0155-2 10,
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D. C. Moreira, W. C. Freitas Jr., C. A. de Ara´ujo, and C. C. Cavalcante
and describes some hybrid MIMO schemes that are able to achieve both gains simultaneously. The fundamentals of LA are described in Section 10.4 for a softinput/soft-output (SISO) system and then extended to the MIMO-OFDM case. The summary of the chapter and envisaged research directions are discussed in Section 10.5.
10.2 Fundamentals of MIMO Transceiver Architectures Higher transmission rates and better reliability are always desirable in communication systems. However, according to information theory, to increase one, the other has to be de
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