Channel Extrapolation for FDD Massive MIMO: Procedure and Experimental Results

Abstract

Application of massive multiple-input multipleoutput (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.

Performance Analysis of Channel Extrapolation in FDD Massive MIMO Systems

“…[20] proposes to acquire DL CSI through UL pilots in combination with a limited feedback from DL pilots. In [21], channel extrapolation performance is experimentally evaluated, in terms of MSE of the extrapolated channel and beamforming efficiency.…”

Cluster Extrapolation for FDD Downlink MIMO Precoding

“…We used the conventional MRT precoder, consistent with previous work in [6]. From the extrapolated channel distribution structure provided in this paper, new precoding strategies can however be designed to outperform MRT.…”

“…The extrapolation error is most of the time quantified by the Mean Square Error (MSE) criterion [6], [7]. The linear extrapolation approach provides a simple expression for the MSE at subcarrier k…”

“…Recently, papers [6], [7] proposed to fully extrapolate the downlink channel from the uplink measurements. The principle consists in retrieving all the reciprocal parameters of a ray-based model (complex path gains, delays, directions of departure) using high resolution estimation techniques.…”

“…Due to the finite resolution of practical antenna arrays, those intra-cluster rays remain unresolvable. This issue could be mitigated by first separating the clusters constituting the channel (for instance using high resolution techniques [6], [7]) then using frequency domain extrapolation of the frequency-dependent cluster gains.…”