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arXiv 2102.05211cs.ITmath.IT
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宽带THz大规模MIMO的延迟相位预编码

Delay-Phase Precoding for Wideband THz Massive MIMO

Linglong Dai, Jingbo Tan, H. Vincent Poor

备注：
Accepted by IEEE TWC. We first reveal a fundamental problem called the beam split effect in THz massive MIMO. To solve this problem, we then propose a new precoding architecture named delay-phase precoding. Simulation codes are provided: http://oa.ee.tsinghua.edu.cn/dailinglong/publications/publications.html
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摘要：

受益于数十GHz带宽，太赫兹（THz）通信被认为是为未来的6G无线系统提供超高速数据率的有前景的技术。为了补偿太赫兹信号严重的传播衰减，可以利用混合预编码的大规模多输入多输出（MIMO）来产生具有高阵列增益的定向波束。然而，基于频率无关移相器的标准混合预编码架构无法应对太赫兹大规模MIMO系统中的波束分裂效应，其中定向波束将在不同的子载波频率处分裂成不同的物理方向。波束分裂效应将导致整个带宽上严重的阵列增益损失，这在太赫兹大规模MIMO系统中还没有得到很好的研究。本文首先通过分析波束分裂效应对阵列增益的影响，揭示并量化了太赫兹大规模MIMO系统中波束分裂效应的严重性。然后，我们提出了一种新的预编码架构称为延迟相位预编码（DPP），以减轻这种影响。具体而言，所提出的DPP引入时间延迟网络作为标准混合预编码架构中的射频链和移相器之间的新预编码层。通过这种方式，传统的相位控制模拟波束形成可以转换为延迟相位控制模拟波束形成。与频率无关相移不同，DPP中引入的时间延迟网络可以实现频率相关相移，其可以被设计为在整个THz带宽上产生朝向目标物理方向的频率相关波束。由于延迟和相位的联合控制，所提出的DPP可以显着减轻波束分裂效应引起的阵列增益损失。此外，我们提出了一种硬件结构，通过使用真正的时间延迟器来实现DPP的概念。

英文摘要：

Benefiting from tens of GHz bandwidth, terahertz (THz) communication is considered to be a promising technology to provide ultra-high speed data rates for future 6G wireless systems. To compensate for the serious propagation attenuation of THz signals, massive multiple-input multiple-output (MIMO) with hybrid precoding can be utilized to generate directional beams with high array gains. However, the standard hybrid precoding architecture based on frequency-independent phase-shifters cannot cope with the beam split effect in THz massive MIMO systems, where the directional beams will split into different physical directions at different subcarrier frequencies. The beam split effect will result in a serious array gain loss across the entire bandwidth, which has not been well investigated in THz massive MIMO systems. In this paper, we first reveal and quantify the seriousness of the beam split effect in THz massive MIMO systems by analyzing the array gain loss it causes. Then, we propose a new precoding architecture called delay-phase precoding (DPP) to mitigate this effect. Specifically, the proposed DPP introduces a time delay network as a new precoding layer between radio-frequency chains and phase-shifters in the standard hybrid precoding architecture. In this way, conventional phase-controlled analog beamforming can be converted into delay-phase controlled analog beamforming. Unlike frequency-independent phase shifts, the time delay network introduced in the DPP can realize frequency-dependent phase shifts, which can be designed to generate frequency-dependent beams towards the target physical direction across the entire THz bandwidth. Due to the joint control of delay and phase, the proposed DPP can significantly relieve the array gain loss caused by the beam split effect. Furthermore, we propose a hardware structure by using true-time-delayers to realize the concept of DPP.

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