Correlation and channel capacity of MIMO systems employing multimode antennas

A novel way of exploiting higher modes of antennas as diversity branches in multiple-input-multiple-output (MIMO) systems is introduced. Essentially, antennas employing multiple modes offer characteristics similar to an antenna array, through multiple modes and using only a single element. The physical mechanism that yields different received signals is the fact that each mode has a different radiation pattern. Analytical expressions for the correlation between signals received by different modes are presented for a biconical and a circular microstrip antenna that employs higher order modes. It is found that the correlation is low enough to yield a significant diversity gain. Furthermore, the channel capacity of a MIMO system using a multimode antenna, i.e., an antenna employing multiple modes, is found to be comparable to the capacity of an array. Since only one element is needed, the multimode antenna offers several advantages over traditional arrays, and is an interesting antenna solution for future high capacity MIMO systems.