  Application of massive multiple-input multiple-output (MIMO) systems to
frequency division duplex (FDD) is challenging mainly due to the considerable
overhead required for downlink training and feedback. Channel extrapolation,
i.e., estimating the channel response at the downlink frequency band based on
measurements in the disjoint uplink band, is a promising solution to overcome
this bottleneck. This paper presents measurement campaigns obtained by using a
wideband (350 MHz) channel sounder at 3.5 GHz composed of a calibrated 64
element antenna array, in both an anechoic chamber and outdoor environment. The
Space Alternating Generalized Expectation-Maximization (SAGE) algorithm was
used to extract the parameters (amplitude, delay, and angular information) of
the multipath components from the attained channel data within the training
(uplink) band. The channel in the downlink band is then reconstructed based on
these path parameters. The performance of the extrapolated channel is evaluated
in terms of mean squared error (MSE) and reduction of beamforming gain (RBG) in
comparison to the ground truth, i.e., the measured channel at the downlink
frequency. We find strong sensitivity to calibration errors and model mismatch,
and also find that performance depends on propagation conditions: LOS performs
significantly better than NLOS.
