Multi-band Wireless Networks: Architectures, Challenges, and Comparative Analysis

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 define the two potential MBN configurations, i.e., (i) Stand-alone MBN where different base stations (BSs) can be flexibly deployed independent of each other, and (ii) Integrated MBN where transceivers can operate on multiple frequencies. Relevant key performance indicators (KPIs) in this context will be defined. Then, we highlight the fundamental challenges of MBNs at the PHYsical (PHY) and Medium Access (MAC) layer, such as unique propagation characteristics, transceiver design, resource management, traffic offloading, mobility management, etc. Later, we perform a comparative analysis of two MBN architectures considering a co-existing RF/THz network in terms of achievable data rate, handoff probability, and deployment cost efficiency. Our results show that stand-alone deployment requires a higher number of BSs compared to integrated deployment in order to achieve a given data rate. Stand-alone deployment, however, offers flexibility and enables controlling the number of access points in different transmission bands. Also, we propose a simplified metric for user offloading decisions in the coexisting RF/THz network. Finally, open research directions will be presented.