Estimating Statistical Eigen-Beamforming Gains Using Spatial Channel Correlation

Statistical eigen-beamforming (EBF) is a widely used multiple antenna transmission technique in which one antenna weight is used over potentially a large bandwidth (e.g., all subcarriers in OFDM). With N transmit antennas, the maximum gain for EBF at a receiver is 20 log N decibels over a single antenna transmission (assuming a power unfair comparison where each transmit antenna has a fixed power so the addition of new antennas brings a beamforming gain as well as a total transmit power increase). Based on statistical analysis of field data and observations, this gain is nearly achievable with line-of- sight transmissions. While useful for characterizing the benefits of EBF, the statistical analysis does not indicate the realizable gain for a particular location. This paper presents a relationship between the EBF gain, spatial correlation of the channel, and the number of transmit antennas. The conjectured gain is compared to the expected gain based on an analysis of field data. The results of this comparison show that the spatial correlation as defined in this paper can be used to predict the EBF gain accurately for a variety of antenna spacings and polarizations.