Transparent User-Specific 3D MIMO in FDD Using Beamspace Methods

In typical macro-cell deployments for 3GPP LTE the base station employs an array of cross-polarized or co-polarized antennas that are linearly spaced in azimuth. In many panel array designs used in cellular deployments, each antenna port is actually formed by co-phasing some number of vertically-arranged physical sub-elements to achieve a desired elevation pattern and gain. If these vertically-arranged sub-elements could be individually and adaptively controlled then the antenna array could adapt its transmission in both the elevation and azimuth dimensions on a per-user basis (aka 3D MIMO) while maintaining the same form factor as the original array. In this paper we consider the problem of incorporating user-specific 3D-MIMO into the LTE FDD OFDMA downlink in a way that is transparent to the existing mobile devices. We propose using the beamspace concept along with uplink transmissions to control the elevation dimension while using the existing LTE codebook feedback methods to control the azimuth dimension. We show that the ability to adapt in both the elevation and azimuth dimensions on a per-user basis can provide significant gains in performance.