Spatial-wideband effect in massive MIMO systems

For massive multiple-input multiple-output (MIMO) systems, the furthest distance between two antennas may be very large compared with the carrier wavelength. Therefore, the physical propagation delay of electromagnetic wave across the array aperture cannot be ignored, which will cause spatial-wideband effect and make the system design much different from the conventional one that only considers the frequency-wideband effect. Taking mmWave-band communications as an example, we demonstrate the spatial- and frequency-wideband effects, called the dual-wideband effects, in massive MIMO systems. We first discuss a new dual-wideband channel model. By exploiting the channel sparsity in the angle and the delay domains, we then develop a simple yet effective channel estimation algorithm. Thanks to the angular-delay reciprocity, the proposed channel estimation strategy is suitable for both TDD and FDD communication systems. The subsequent numerical results clarify that the proposed transmission design can well address the dual-wideband effects and significantly outperform the existing designs that only consider the frequency-wideband effect.