Experimental Verification of Degrees of Freedom for Colocated Antennas in Wireless Channels

It has been postulated that six colocated orthogonally polarized electrical and magnetic dipoles can offer up to six degrees of freedom (DOFs) in a multipath scattering environment. In other words, a sixfold increase in channel capacity can be achieved in the context of multiple-input-multiple-output (MIMO) systems. However, due to the complexity in designing and measuring such a six-port antenna, to our knowledge, no experimental verification has yet been successfully performed. In this paper, the six DOFs hypothesis is experimentally verified at 77 MHz. The experiment involves the design and fabrication of two six-port arrays. In particular, one array is made compact with colocated antenna elements enclosed in a small cubic volume of (0.24λ)³. Using the two arrays, MIMO channel measurements are performed in an RF shielded laboratory in order to measure a multipath scattering environment. Analysis on eigenvalues of the measured channel shows that six DOFs are attained due to the orthogonality in radiation patterns of the two arrays.

I. Introduction

MULTIPLE-INPUT–MULTIPLE-OUTPUT (MIMO) systems employing multiple antenna elements at both the transmit (TX) and receive (RX) ends offer the potential to linearly increase capacity with an increase in the number of antennas [1]. Conventional MIMO systems make use of antenna arrays with spatially separated elements, and they perform best when the spatial correlation among the signals on different antenna branches is low, which typically requires the antenna elements to be separated by one half of a wavelength . On the other hand, low signal correlation can also be obtained by utilizing polarization diversity [2]. The use of two polarization states of plane waves has been known to introduce two degrees of freedom (DOFs) in a wireless communication channel. However, an additional threefold increase in channel capacity is shown in [3] by the use of six distinguishable electric and magnetic field components. Colocated, orthogonally polarized electric and magnetic dipoles are postulated to achieve six DOFs. Nevertheless, the experimental study in [3] demonstrates only three DOFs by means of tripolarized half-wave sleeve dipoles, rather than the postulated sixfold capacity increase. This work has sparked further investigations on the number of DOFs that is theoretically available in wireless channels [4]–[7].