[论文解读] LOS-based Conjugate Beamforming and Power-Scaling Law in Massive-MIMO Systems

Dian‐Wu Yue, Geoffrey Ye Li|arXiv (Cornell University)|Apr 7, 2014

Advanced MIMO Systems Optimization参考文献 29被引用 17

一句话总结

本文提出了一种在Rician衰落信道上的大规模MIMO系统中基于视 Line-of-Sight (LOS) 的共轭波束成形方案，仅利用LOS分量以避免 pilot contamination 并降低CSI开销。推导出一种功率扩展律，表明在特定条件下，随着基站天线数量的增加，用户间LOS干扰和快衰落可被消除，从而在降低发射功率的同时实现高 spectral efficiency。

ABSTRACT

This paper is concerned with massive-MIMO systems over Rician flat fading channels. In order to reduce the overhead to obtain full channel state information and to avoid the pilot contamination problem, by treating the scattered component as interference, we investigate a transmit and receive conjugate beamforming (BF) transmission scheme only based on the line-of-sight (LOS) component. Under Rank-1 model, we first consider a single-user system with N transmit and M receive antennas, and focus on the problem of power-scaling law when the transmit power is scaled down proportionally to 1/MN. It can be shown that as MN grows large, the scattered interference vanishes, and the ergodic achievable rate is higher than that of the corresponding BF scheme based fast fading and minimum mean-square error (MMSE) channel estimation. Then we further consider uplink and downlink single-cell scenarios where the base station (BS) has M antennas and each of K users has N antennas. When the transmit power for each user is scaled down proportionally to 1/MN, it can be shown for finite users that as M grows without bound, each user obtains finally the same rate performance as in the single-user case. Even when N grows without bound, however, there still remains inter-user LOS interference that can not be cancelled. Regarding infinite users, there exists such a power scaling law that when K and the b-th power of M go to infinity with a fixed and finite ratio for a given b in (0, 1), not only inter-user LOS interference but also fast fading effect can be cancelled, while fast fading effect can not be cancelled if b=1. Extension to multi-cells and frequency-selective channels are also discussed shortly. Moreover, numerical results indicate that spacial antenna correlation does not have serious influence on the rate performance, and the BS antennas may be allowed to be placed compactly when M is very large.

研究动机与目标

• 通过仅利用LOS信道分量，解决大规模MIMO系统中的pilot contamination和高CSI反馈开销问题。
• 分析在Rician衰落信道下共轭波束成形的性能，重点关注遍历可实现速率和干扰抑制能力。
• 推导出在天线数量增加时保持频谱效率的功率扩展律。
• 评估空间相关性和阵元间距对系统性能的影响。
• 将分析扩展至单小区和多小区场景下的上行/下行链路，考虑有限和无限用户数。

提出的方法

• 建立一个秩-1的Rician衰落模型，将信道分解为LOS分量和散射分量。
• 设计一种共轭波束成形方案，将散射分量视为干扰，仅使用LOS分量进行波束成形。
• 推导该波束成形方案下信干噪比（SINR）的统计表达式。
• 应用大系统分析方法，研究当基站天线数（M）和用户天线数（N）趋于无穷大时的渐近行为。
• 提出一种功率扩展律，其中发射功率按 1/(MN) 缩放，可在特定渐近条件下实现干扰消除。
• 采用指数空间相关模型，评估阵元相关性对速率性能的影响。

实验结果

研究问题

• RQ1 仅基于LOS分量的共轭波束成形能否在大规模MIMO系统中实现高频谱效率？
• RQ2 何种功率扩展律可实现干扰消除，并在天线数量增加时维持速率性能？
• RQ3 基于LOS的波束成形性能与在Rayleigh衰落信道中依赖完整CSI的常规方案相比如何？
• RQ4 空间相关性和阵元间距对基于LOS的波束成形可实现速率有何影响？
• RQ5 在何种条件下，用户间LOS干扰和快衰落可在多用户大规模MIMO系统中同时被消除？

主要发现

• 当发射功率按 1/(MN) 缩放时，基于LOS的共轭波束成形方案的遍历可实现速率超过Rayleigh衰落信道中MMSE估计波束成形的速率，尤其在 MN 极大时更为显著。
• 在单用户情况下，当 MN → ∞ 时，散射干扰消失，系统可实现接近理论极限的速率。
• 在有限用户数 K 的多用户上行/下行链路场景中，当 M → ∞ 时，每个用户可实现与单用户情况相当的速率性能，尽管存在用户间LOS干扰。
• 对于无限用户，当 K 和 M^α 同时趋于无穷大且保持固定比例（α ∈ (0,1)）时，用户间LOS干扰和快衰落均可被消除；但当 α = 1 时该条件不成立。
• 数值结果表明，即使在高相关性水平下，基站或用户设备处的空间相关性对速率性能的影响也可忽略不计。
• 在 M 较大时，紧凑阵列（如 d = 0.05λ）与宽间距阵列（如 d = 24λ）的系统速率性能相近，表明紧凑阵列在M较大时具有可行性。