This paper presents the vision of multi-band communication networks (MBN) in 6G, where optical and TeraHertz (THz) transmissions will coexist with the conventional radio frequency (RF) spectrum. This paper will first pin-point the fundamental challenges in MBN architectures at the PHYsical (PHY) and Medium Access (MAC) layer, such as unique channel propagation and estimation issues, user offloading and resource allocation, multi-band transceiver design and antenna systems, mobility and handoff management, backhauling, etc. We then perform a quantitative performance assessment of the two fundamental MBN architectures, i.e., {stand-alone MBN} and {integrated MBN} considering critical factors like achievable rate, and capital/operational deployment cost. {Our results show that stand-alone deployment is prone to higher capital and operational expenses for a predefined data rate requirement. Stand-alone deployment, however, offers flexibility and enables controlling the number of access points in different transmission bands.} In addition, we propose a molecular absorption-aware user offloading metric for MBNs and demonstrate its performance gains over conventional user offloading schemes. Finally, open research directions are presented.

翻译：本文阐述了6G中多频段通信网络（MBN）的愿景——光通信和太赫兹（THz）传输将与传统的射频（RF）频谱共存。本文首先指出MBN架构在物理层（PHY）和媒体接入控制（MAC）层面临的基本挑战，例如独特的信道传播与估计问题、用户卸载与资源分配、多频段收发器设计与天线系统、移动性与切换管理、回程链路等。随后，我们针对两种基本MBN架构——即{独立式MBN}与{集成式MBN}，考虑可达速率及资本/运营部署成本等关键因素，进行了定量性能评估。{研究结果表明，在预定数据速率要求下，独立式部署更容易产生更高的资本和运营费用；然而，独立式部署提供了灵活性，并允许控制不同传输频段中的接入点数量。} 此外，我们提出了一种分子吸收感知的用户卸载指标，并展示了其相较于传统用户卸载方案的性能优势。最后，本文提出了开放性的研究方向。