[论文解读] Preamble-based Channel Estimation in OFDM/OQAM Systems: A Time-Domain Approach.

Eleftherios Kofidis|arXiv (Cornell University)|Jun 11, 2013

PAPR reduction in OFDM参考文献 9被引用 10

一句话总结

本文提出了一种基于时域训练序列的FBMC/OQAM系统信道估计方法，直接估计信道冲击响应，避免了以往研究中常见的平坦子载波假设。通过设计最优且短时长的训练序列（1个或2个符号），该方法在高度频率选择性信道中显著提升了性能，且无误码地板，优于传统的频域方法。

ABSTRACT

Filter bank-based multicarrier (FBMC) systems based on offset QAM (FBMC/OQAM) have recently attracted increased interest in several applications due to their enhanced flexibility, higher spectral efficiency, and better spectral containment compared to conventional OFDM. They suffer, however, from an inter-carrier/inter-symbol interference that complicates signal processing tasks such as channel estimation. Most of the methods reported thus far rely on the assumption of (almost) flat subchannels to more easily tackle this problem, addressing it in a way similar to OFDM. However, this assumption may be often quite inaccurate, due to the high freq. selectivity of the channel and/or the small number of subcarriers employed to cope with frequency dispersion in fast fading. In such cases, severe error floors are exhibited at medium to high SNR values, which cancel the advantage of FBMC over OFDM. Moreover, the existing methods provide estimates of the subchannel responses, most commonly in the frequency domain. The goal of this paper is to revisit this problem through an alternative formulation that focuses on the estimation of the channel impulse response itself and makes no assumption on the degree of frequency selectivity of the subchannels. The possible gains in estimation performance offered by such an approach are investigated through the design of optimal (in the MSE sense) preambles, of both the full and sparse types, and of the smallest possible duration of only one pilot FBMC symbol. Existing designs for flat subchannels are then shown to result as special cases. Longer preambles, consisting of two consecutive pilot FBMC symbols, are also analyzed. The simulation results demonstrate significant improvements from the proposed approach for both mildly and highly frequency selective channels. Most notably, no error floors appear anymore over a quite wide range of SNR values.

研究动机与目标

• 解决现有FBMC/OQAM信道估计方法依赖平坦子载波假设的局限性，该假设在高度频率选择性环境中失效。
• 克服传统方法在中高信噪比下出现的误码地板问题，尤其是在子载波数量较少或信道快速衰落时。
• 提出一种新方法，直接在时域估计信道冲击响应，避免频域子载波简化假设。
• 设计最优训练序列——包括完整和稀疏类型，且时长最短（1个符号），以在一般频率选择性条件下实现精确信道估计。
• 通过仿真证明所提方法优于现有设计，尤其在具有挑战性的高度选择性衰落环境中。

提出的方法

• 通过直接估计信道冲击响应，而非子载波响应，将FBMC/OQAM中的信道估计重新表述为时域问题。
• 在时域设计最小化信道估计均方误差（MSE）的最优训练序列，使用最优训练序列结构的闭式表达式。
• 引入两种训练序列类型：最短的单符号训练序列和更长的双符号训练序列，均针对MSE性能进行优化。
• 在最小持续时间约束下推导最优训练序列结构，表明现有平坦子载波设计可作为该框架的特例。
• 采用考虑信道全部频率选择性的时域信号模型，避免对子载波平坦性的简化假设。
• 应用最小均方误差（MMSE）准则推导最优训练序列，实现对多样化信道条件的鲁棒估计。

实验结果

研究问题

• RQ1 基于时域训练序列的方法能否消除传统频域方法在FBMC/OQAM信道估计中常见的误码地板？
• RQ2 在高度频率选择性信道中，单符号训练序列的性能与更长训练序列相比如何？
• RQ3 在不假设平坦子载波的前提下设计的最优训练序列，在不同信噪比和选择性水平下，能将估计精度提升多少？
• RQ4 所提方法能否被证明可将现有平坦子载波设计作为特例涵盖？
• RQ5 在真实的选择性衰落环境中，训练序列的稀疏性和时长对估计精度有何影响？

主要发现

• 所提时域方法消除了传统FBMC/OQAM信道估计在中高信噪比下通常存在的误码地板。
• 即使仅使用单符号训练序列，也实现了显著的性能增益，表明在轻度和高度频率选择性信道中均具有鲁棒性。
• 在MMSE准则下推导出的最优训练序列设计，优于假设平坦子载波的现有方法，后者被证明是本框架的特例。
• 该方法在广泛的信噪比范围内保持低估计误差，证实其在快速衰落和高选择性场景下的鲁棒性。
• 仿真结果证实，所提方法在均方误差性能上优于以往的频域方法，尤其在具有挑战性的信道条件下。
• 双符号训练序列配置进一步提升了估计精度，尤其在强频率选择性环境中。