Extremely large-scale multiple-input-multiple-output (XL-MIMO) with hybrid precoding is a promising technique to meet the high data rate requirements for future 6G communications. To realize efficient hybrid precoding, it is essential to obtain accurate channel state information. Existing channel estimation algorithms with low pilot overhead heavily rely on the channel sparsity in the angle domain, which is achieved by the classical far-field planar wavefront assumption. However, due to the non-negligible near-field spherical wavefront property in XL-MIMO systems, this channel sparsity in the angle domain is not available anymore, and thus existing far-field channel estimation schemes will suffer from severe performance loss. To address this problem, in this paper we study the near-field channel estimation by exploiting the polar-domain sparse representation of the near-field XL-MIMO channel. Specifically, unlike the classical angle-domain representation that only considers the angle information of the channel, we propose a new polar-domain representation, which simultaneously accounts for both the angle and distance information. In this way, the near-field channel also exhibits sparsity in the polar domain. By exploiting the channel sparsity in the polar domain, we propose the on-grid and off-grid near-field channel estimation schemes for XL-MIMO. Firstly, an on-grid polar-domain simultaneous orthogonal matching pursuit (P-SOMP) algorithm is proposed to efficiently estimate the near-field channel. Furthermore, to solve the resolution limitation of the on-grid P-SOMP algorithm, an off-grid polar-domain simultaneous iterative gridless weighted (P-SIGW) algorithm is proposed to improve the estimation accuracy, where the parameters of the near-field channel are directly estimated. Finally, numerical results are provided to verify the effectiveness of the proposed schemes.
翻译:超大型多投入-多输出量( XL- MIMO ), 且具有混合编码, 是满足未来 6G 通信高数据率要求的一个很有希望的技术 。 要实现高效混合预解, 就必须获得准确的频道状态信息 。 现有的低试点间接率的频道估算算法, 在很大程度上依赖于角域的频道偏小度, 这是古老的远方平流波端假设所实现的 。 然而, 由于 XL- MIMO 系统中不可忽略的近地球级平流波边属性, 角域的频道偏移性已经不再可用, 因此现有的远地频道估算计划将因严重的业绩损失而受损。 为了解决这一问题,我们在本论文中研究近地端的 XL- MP- MIMIM 频道的极地( 离地平流层平流层平流层平流数据), 与传统的角域域域域网路段平面平面平面平面平面平面图不同, 我们提议一个新的极地平面平面平面代表, 。