MIMO Reference Antennas Performance in Anisotropic Channel Environments

The new generation of cellular wireless communications, long term evolution (LTE), requires multiple antennas at both the transmit and receive ends of the radio link to improve system performance. This use of the radio link is termed multiple input-multiple output (MIMO), and in this work, the focus is on the downlink (base station transmit and mobile receive). Efforts to define over-the-air (OTA) downlink MIMO measurement methodologies are still being carried out within several standards bodies and are based on the evaluation of so-called good, nominal, and bad MIMO reference antennas. The reference antennas were necessarily defined independently of the channel environment, which is still under discussion. The reference antenna figures of merit, for an isotropic channel environment, were chosen as efficiency, branch power ratio (BPR), and correlation. The objective of this work is to evaluate the radiated performance of the MIMO reference antennas using anisotropic (nonuniform power angular spread) channel environments. The performance of the reference antennas in anisotropic environments is different to that in an isotropic environment, due to the antenna orientation in the three-dimensional space relative to the channel. This leads to a statistical description of the branch power ratio (BPR), correlation, and efficiency, which is expressed in terms of mean effective gain (MEG). The outcome of this new anisotropic evaluation is that the reference antennas are seen to vary significantly for BPR and mean effective gain (MEG) and somewhat less for correlation, relative to the isotropic environment.