Millimeter wave (mmWave) communications have attracted extensive interest from academia, industry, and government as they can make full use of abundant frequency resources at the high-frequency band to achieve ultra-high-speed data transmission. The mmWave communication systems are usually equipped with large antenna arrays, known as mmWave massive multiple-input multiple-output (MIMO), to generate highly directional beams and compensate for the severe path loss in the high frequency band. However, the performance of directional beamforming largely relies on the accuracy of channel state information (CSI) acquisition. Compared to the traditional MIMO systems, the CSI acquisition in mmWave massive MIMO systems is challenging. On one hand, the large antenna arrays form a high dimension channel matrix, whose estimation consumes more resources, e.g., pilot sequence overhead, sounding beam overhead, and computational complexity. On the other hand, the mmWave massive MIMO typically employs a hybrid beamforming architecture, where the radio frequency (RF) chains are much fewer than the antennas. Therefore, we can only obtain a low-dimension signal from the RF chains instead of directly getting a high-dimension signal from the frontend antennas, which makes CSI acquisition much more challenging than usual.
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Source: Phys.org