[论文解读] Efficient Beam Alignment for mmWave Single-Carrier Systems with Hybrid MIMO Transceivers

Xiaoshen Song, Saeid Haghighatshoar+1 more|arXiv (Cornell University)|Jun 17, 2018

Millimeter-Wave Propagation and Modeling参考文献 35被引用 8

一句话总结

本文提出了一种针对毫米波单载波单载波MIMO系统的时域波束赋形（BA）方案，采用非负最小二乘法（NNLS）从稀疏信道测量中高效估计最强的到达/离开角（AoA-AoD）对。该方法在高多普勒扩展下表现出鲁棒性能，并在波束对准后形成频率平坦的单抽头信道，从而无需时域均衡。

ABSTRACT

Communication at millimeter wave (mmWave) bands is expected to become a key ingredient of next generation (5G) wireless networks. Effective mmWave communications require fast and reliable methods for beamforming at both the User Equipment (UE) and the Base Station (BS) sides, in order to achieve a sufficiently large Signal-to-Noise Ratio (SNR) after beamforming. We refer to the problem of finding a pair of strongly coupled narrow beams at the transmitter and receiver as the Beam Alignment (BA) problem. In this paper, we propose an efficient BA scheme for single-carrier mmWave communications. In the proposed scheme, the BS periodically probes the channel in the downlink via a pre-specified pseudo-random beamforming codebook and pseudo-random spreading codes, letting each UE estimate the Angle-of-Arrival / Angle-of-Departure (AoA-AoD) pair of the multipath channel for which the energy transfer is maximum. We leverage the sparse nature of mmWave channels in the AoA-AoD domain to formulate the BA problem as the estimation of a sparse non-negative vector. Based on the recently developed Non-Negative Least Squares (NNLS) technique, we efficiently find the strongest AoA-AoD pair connecting each UE to the BS. We evaluate the performance of the proposed scheme under a realistic channel model, where the propagation channel consists of a few multipath scattering components each having different delays, AoAs-AoDs, and Doppler shifts.The channel model parameters are consistent with experimental channel measurements. Simulation results indicate that the proposed method is highly robust to fast channel variations caused by the large Doppler spread between the multipath components. Furthermore, we also show that after achieving BA the beamformed channel is essentially frequency-flat, such that single-carrier communication needs no equalization in the time domain.

研究动机与目标

• 解决由于大全向路径损耗导致的毫米波系统中快速波束赋形挑战，尤其是在路径损耗大且信噪比（SNR）低的场景下。
• 在粗略同步假设但缺乏精细定时与多普勒补偿的毫米波网络中，实现高效的初始接入。
• 利用毫米波信道的稀疏角度结构，降低训练开销并提升对准鲁棒性。
• 确保波束对准后，有效信道变为频率平坦，从而支持无需均衡的单载波单载波传输。
• 开发一种与使用有限射频链路和大阵列天线的混合MIMO架构兼容的实用波束赋形方案。

提出的方法

• 基站定期使用伪随机波束成形码本和伪随机扩频码发送导频信号。
• 用户设备通过观测接收信号能量，估计对应最强多径分量的AoA-AoD对。
• 将波束赋形问题建模为角度域中的稀疏非负向量估计问题，利用信道在AoA-AoD空间中的稀疏性。
• 应用非负最小二乘法（NNLS）以高效识别具有最大信道增益的主导AoA-AoD对。
• 该方法引入二阶信道统计特性，包括随机增益和多普勒频移，以建模真实的毫米波传播环境。
• 对准成功后，有效信道近似为频率平坦，且主要由单个抽头主导，从而支持使用标准同步机制的单载波传输。

实验结果

研究问题

• RQ1 在低SNR和高多普勒扩展条件下，如何在使用混合MIMO架构的毫米波单载波系统中实现高效的波束赋形？
• RQ2 能否利用毫米波信道的稀疏角度结构来降低训练开销并提升对准鲁棒性？
• RQ3 波束赋形在多大程度上可将频率选择性、时变信道转换为频率平坦、准静态信道？
• RQ4 波束对准后，单载波传输的可实现频谱效率是多少？与理论上限的差距如何？
• RQ5 与穷举搜索相比，所提出的基于NNLS的方法在训练开销和抗多普勒频移能力方面表现如何？

主要发现

• 所提出的波束赋形方案即使在多径分量间存在大范围多普勒扩展时，仍表现出鲁棒性能，且训练开销极低。
• 波束对准后，信道近似为频率平坦，仅存在一个主导延迟抽头，从而无需时域均衡。
• 即使在低预波束成形SNR（例如-30至-20 dB）下，该方案的遍历可实现速率也与理论上限仅存在微小差距。
• 在波束成形后，使用标准定时与频率同步的单载波传输在低SNR环境下仍可实现高谱效率（1–2 bit/s/Hz）。
• 由于采用二阶统计特性和基于NNLS的估计，该方法对快速信道变化具有高度鲁棒性。
• 仿真结果证实，波束成形后的信道可良好近似为确定性的高增益单抽头信道，从而在移动室外场景中实现高效通信。