Research

Part 1: Array factors of BS antennas in mmWave MU-MIMO systems

03/2016-12/2016

Millimeter wave (mmWave) communication is a significantly enabling technology in the fifth-generation (5G) cellular system to promote the capacity. In recent years, people have focused on studying the propagation characteristics of mmWave. However, the impact of mmWave channel on the system capacity, especially in multi-user multi-input and multi-output (MU-MIMO) scenarios, has not been fully investigated.

In our research, we establish channel models for different base station (BS) deployment positions, and propose a low-complexity numerical calculation method of the sum-capacity in MU-MIMO downlink scenarios. Based on these work, we investigate the effect of BS antenna configurations on the system capacity. The antenna spacing, deployment position and array shape are respectively studied.

Since the relationship between each factor and the system capacity is complicated, it is intractable to derive a rigorous mathematical formula. Hence, Monte Carlo simulations are employed to observe cumulative distribution function (CDF) of channel correlation coefficients and the sum-capacity. Through simulation results, We find insights to achieve higher channel capacity and to guide the deployment of BS antennas in practical cellular scenarios.

*I made presentation about this research on Wireless Communications Systems Symposium of IEEE ICCC'17 (Qingdao, China, Oct. 2017).

Part 2: Tensor-based hybrid precoding design for 3D MIMO systems

01/2017-08/2017

In the next generation cellular system, combining mmWave with massive number of antennas, the throughput can be greatly improved but the computation complexity and power consumption can also be incredibly high, which means that the full digital precoding is not practical. As an alternative, the hybrid precoding structure is generally used.

The channel response of a 3D-massive-MIMO system can be represented by a large matrix, as the conventional representation of MIMO channels. But considering its unique feature, it is more natural to describe it with a tensor. Then the channel in azimuth and elevation dimensions can be thought as slices of the tensor with different modes, and tensor decomposition can be used to analyze the signal subspace and null subspace in different dimensions.

In our research under this topic, we study the hybrid precoding design in mmWave 3D-MIMO systems. To exploit the characteristic of planar antenna arrays, we represent the 3D-MIMO channel response with tensor, and find the null space of interference users with Tucker decomposition. The null space can be well approximated by the Kronecker product of azimuth and elevation directional array vectors, and thus the designed analog precoder can eliminate inter-user interference. Combined with the baseband digital precoding, which find the maximal projection direction of the desired channel on the null space, the conventional zero-forcing block-diagonalization (ZF-BD) precoding method is extended to tensor context with constant-modulus constraint of null space elements. Since there are massive antennas and only limited RF chains, the proposed method has larger freedom to suppress interference. Simulation results verify its superiority.

*I made presentation about this research on Signal Processing for Communications Symposium of IEEE ICCC'17 (Qingdao, China, Oct. 2017).

In the mmWave communication system, although the path loss is fiercer and the direction of the beam is more accurate, the interference will not become insignificant since the inter-cell distance become closer. As can be seen from the channel model, the user needs to estimate and feed the mmWave channel with fewer parameters than the microwave channel. Therefore, in mmWave system, we can reduce the complexity and overhead of interference coordination, if using the feedback from users. At the same time, for that the size of antenna arrays is greatly narrowed, we can deploy the mobile terminal with antenna arrays, which provides more possibilities for the mmWave system to deal with interference in the space domain. In short, considering all these aspects, we should find some innovational methods to solve the inter-cell interference in mmWave communication systems.

In our proposal, the user sets the beam direction before the data transmission, based on the CSI of the serving BS and neighboring BSs. The user then feeds the beam direction and CSI to the serving BS which shares these information to neighboring BSs using X2 interface between BSs. Finally, based on the feedback information from users in self and neighboring cells, each BS optimizes its own precoding (beam direction) to maximize desired signals and avoid interference. Note that the beam direction of users and BSs should be calculated in the same reference coordinate system. We consider implementation of this proposal in heterogeneous and homogeneous cells respectively.