Vector channel modeling and prediction for downlink beamforming in frequency-division duplex systems

Researchers have studied the behavior of the vector channel in order to study the quality of beamforming methods and to increase downlink signal power and signal-to-interference ratio. Jeng (see IEEE Trans. Antennas Propagat., vol.46, p.749-57, 1998) showed that knowledge beyond simply directions of arrival of the vector channel at one frequency may not be useful at another. We propose that if the ratio between two frequencies, f₁ and f₂, is close to one, then the autoregressive (AR) coefficients which model the behavior of the vector channel at f₁ are essentially the same AR coefficients as those which model the vector channel at f₂. This implies that accurate predictions of the vector channel at f₂ can be performed knowing only the channel at f₁, as well as enough knowledge of the channel at f₂ in order to provide initial conditions. Ray-tracing simulations of downtown Austin, TX, show that this hypothesis holds for f₁ = 1.85 GHz and f₂ = 1.93 GHz, which correspond to uplink and downlink frequencies, respectively, in US personal communications systems.