An antenna-independent approach to the capacity of a wireless system

by 1979- Kishore, Pinak

KISHORE, PINAK. An Antenna-Independent Approach to the Capacity of a Wireless System. (Under the direction of Professor Brian L. Hughes.) Information theory (IT) promises a huge increase in capacity for systems equipped with multiple antennas at both the transmitter and receiver (called Multiple-Input Multiple-Output or MIMO) in a rich multipath scattering environment relative to single antenna systems (called Single-Input Single-Output or SISO). Since the first results on capacities for MIMO systems were published by Telatar [2] and Foschini et al. [3], there has been an extensive research effort to develop techniques and algorithms to realize the gains promised by MIMO systems. Most of these studies have focussed on particular antenna systems such as uniform linear arrays and have derived channel capacities using a statistical signal-space approach under ideal fading conditions. The number and locations of the antennas in such systems, however constrains the information that is extracted from the underlying electromagnetic field thereby giving antenna-dependent capacity results. To overcome these constrains imposed by particular antenna systems and to find the true capacity of systems limited only by their volume, we need to look at continuous-time systems in an antenna-independent way. In this thesis, we look at one such system which consists of a spherical volume at the transmitter having some arbitrary current distribution and radiating an electric field around it. The receiver consists of a spherical shell located in the far-field of the transmitter capturing all the signals radiated by it. We then use electromagnetic theory to derive the input-output equation for this system and use an orthonormal series expansion to reduce it to the form of a MIMO channel. We then calculate the capacity and the spatial degrees of freedom for such a system and look at how they vary with the size of the transmitting volume and the available transmitter power. We show that the spatial degrees of freedom grow linearly with the effective aperture of the spherical volume and the capacity grows even faster. We also investigate the increase in spatial degrees of freedom and capacity that can be achieved by using three-dimensional (tri-polarized) current distributions at the transmitter instead of one-dimensional (uni-polarized) current distributions. We show that capacity gains of 3 times can be achieved for a sufficiently large transmitter. Lastly, we look at a more realistic complete transmit-receive system with an identical spherical volume at the receiver and an ideal fully-scattered channel connecting the transmitter to the receiver. We model the channel as a Rayleigh fading channel and compare the results on capacity and spatial degrees of freedom with previous results. An Antenna-Independent Approach to the Capacity of a Wireless System by Pinak Kishore A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science Department of Electrical and Computer Engineering Raleigh August 12, 2005