In this paper, spatial and polarization diversities are simultaneously implemented in an ultra-wideband (UWB) multiple-input-multiple-output (MIMO) antenna to reduce the correlation between the parallel-placed radiators. The keystone of the antenna is systematically modified coplanar ground planes that enable excitation of circular polarization (CP). To realize one sense of circular polarization as well as ultra-wideband operation, an extended rectangular slot is etched on the left-hand-side of the coplanar waveguide (CPW) feed. This is combined with the asymmetrical ground plane geometry on the right-hand-side of the feeding line. The current flowing on the slotted ground plane forms a quasi-loop and generates CP, whereas the combination of the vertical current on the feedline and the horizontal current on the asymmetric ground plane adds to the axial ratio (AR) bandwidth. To implement the MIMO design with polarization and spatial diversity, the position of the coplanar ground planes is switched with respect to the feedline, and placed in a parallel formation with the edge-to-edge distance of 0.29λ0 . All geometrical parameters are optimized at the full-wave level of description before prototyping and experimental characterization. Simulation and measured results indicate that the proposed MIMO antenna features approximately 82% impedance bandwidth from 2.9 GHz to 7.1 GHz and 68.5% (3.1 GHz- 6.35 GHz) AR bandwidth. Moreover, the peak envelop correlation coefficient (ECC) is below 0.003, which corresponds to almost no correlation between the radiators. The antenna can be operated with either bidirectional or unidirectional characteristics, covering multiple commercial application bands including WLAN and WiMax.