Elevation spatial characteristic investigation based on 3D massive MIMO channel measurements at 6 GHz

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As the key technology of the 5th generation (5G), 3dimensional (3D) massive multi-input and multi-output (MIMO) is expected to be widely used in small cell network (SCN). In this paper, in order to investigated the tradeoff between limited size in SCN and the capacity gain from increasing antenna elements, the spatial performances of 3D massive MIMO based on a 512 -16 MIMO channel measurements at 6 GHz in urban microcell (UMi) scenario are studied. Enormous channel impulse responses (CIR) are collected and reconstructed, which enables us to present comparative results of the capacity and the eigenvalue spread (ES). Furthermore, the impacts of antenna element number and spacing on system performance are investigated, i. e. , 32, 64, 128 elements are selected from the 512 transmitter (Tx) array with elevation interval spacing being 0-5, 1 and 2 wavelengths for each. Interestingly, the capacity gap can be obviously observed on the comparison between the 1 and 2 wavelength antenna spacing cases, which implies that correlation cannot be ignored when the antenna spacing is larger than 1 wavelength when massive antennas are equipped. The contrast results show that the capacities are enlarged with the increasing of antenna elements number, and larger antenna spacing will lead to higher channel capacity as expected. However, the capacity gains brought by the increasing of antenna spacing will descend to certain degrees as the antenna number increases. Collectively, these results will provide further insights into 3D massive MIMO utilization. © 2018 Beijing University of Posts and Telecommunications. All rights reserved.

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... But when SNR increases, the gap decreases gradually. From [17] we can know that the correlation between array elements still exists when the antenna spacing is greater than 1 . Therefore, considering the antenna structure with reasonable spacing in the antenna designing can reduce the spatial correlation and improve the system performance of multiuser MIMO. ...
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In order to understand how many antennas are needed in a multiuser massive MIMO system, theoretical derivation and channel measurements are conducted; the effect of a finite number of base station (BS) antennas on the performance capability of Zero-forcing (ZF) precoding in a rich scattering channel is quantified. Through the theoretical analysis, the needed number of the transmit antennas for ZF precoder to achieve a certain percentage of the broadcast channel (BC) capacity will monotonically decrease with the increase of the transmit signal-to-noise ratio (SNR), and the lower bound of the needed transmit antennas is derived with a simple expression. Then the theoretical derivation is verified by simulation results, and the transmission performance is evaluated by channel measurements in urban microcell (UMi) scenario with frequencies of 3.5 and 6 GHz. From the measurement results, the ZF capability can be enhanced by improving the SNR and enlarging the antenna array spacing when the massive MIMO channel does not under a favorable propagation condition. Furthermore, because of the lower spatial correlation, the performance of ZF precoding at 6 GHz is closer to the theoretical derivation than 3.5 GHz.
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