Capacity Evaluation of MIMO Antenna Systems Using Spherical Harmonics Expansion

Multiple input multiple output (MIMO) wireless communication systems have been intensely investigated in the past years. In order to understand the fundamental limitations of antennas and propagation models on the performance of MIMO systems, the use of spherical harmonics theory allows to decouple the effects of the antennas and the propagation medium. In this work, we make use of spherical harmonics expansion to perform a spherical mode analysis of MIMO systems including the effect of different transmit/receive antennas. Two antenna models are used for evaluating the capacity behavior as a function of their associated spherical harmonics expansion. The first antenna is a theoretical dipole antenna, whose spherical harmonics coefficients have uniform distribution. The second is a a real-world patch antenna, whose coefficients were obtained from practical RF measurements. Both antennas are simulated in a rich-scattering propagation scenario (i.i.d. channel). Our results illustrate how channel capacity is dependent on the number and distribution of the antennas´ spherical modes.