Signal Design

Background

Developing high data rate technology with robust and seamless services is the main motivating factor behind both fundamental and applied research in wireless communications today. A key technique in fulfilling this target is the application of multiple transmit and multiple receive antennas, which provide an additional spatial degree of freedom. This spatial degree of freedom gives unique opportunities, e.g., for enhancing link reliability, for interference suppression, and for supporting high data rates. However, as the symbol data rate increases in broadband wireless applications, the underlying multiple-input multiple-output (MIMO) channel exhibit strong frequency selectivity. As is well known, orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission scheme that comes with low-complexity (de)modulation, equalization, and decoding to mitigate frequency-selective fading effects. Therefore, a combination of MIMO and OFDM (MIMO-OFDM) techniques appears particularly promising for next generation broadband wireless systems. The block diagram of a MIMO-OFDM system is illustrated in Fig. 1.

Various signal design methods for MIMO and MIMO-OFDM systems have been proposed in the literature. A common assumption in the signal design study of MIMO/MIMO-OFDM systems is that the channel coefficients between the pairs of transmit-receive antennas are independent and identically distributed (i.i.d.) complex Gaussian process, which are perfectly known by the receiver. This is an idealistic situation. In practice, however, the channel coefficients are spatially correlated, where the degree of correlation depends mainly on the antenna geometry, the richness of scattering, and the presence of dominant specular components. Moreover, the channel coefficients are in practice not perfectly known, and in some cases they are even unknown. Therefore, it is important to analyze and design signals for MIMO/MIMO-OFDM systems in the presence of correlated channel coefficients on the condition of imperfect channel state information (CSI) or even without any CSI.

Fig. 1: Block diagram of a MIMO-OFDM system.

Planned Research

The general goal of this research project is to develop new MIMO-OFDM techniques for the physical layer of future wireless communication systems enabling the provision of a multitude of services. In addition to the development of practical techniques and solutions, an equally important goal is to achieve a clear understanding of the characteristic of wireless MIMO channels. The most crucial and currently relevant topics include the design of efficient space-time constellation and modulation signal sets for MIMO/MIMO-OFDM systems with imperfect (or without) CSI. Novel signal designs and optimization methods for correlated MIMO-OFDM systems will also be studied by using channel models developed for different propagation environments. Thus, the scope of this research project covers the following three different, but closely related research topics:

• Signal design for MIMO systems with partially known and without CSI.
• Performance analysis and signal design for MIMO systems in the presence of spatially correlated MIMO channels.
• Performance analysis and signal design for MIMO-OFDM systems with non-perfect CSI and/or spatially correlated channels.

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