## 1 Introduction  
Given that fifth-generation (5G) mobile communication networks are scheduled to be commercially deployed in 2020, researchers are looking forward to the next-generation (i.e., sixth-generation (6G)) mobile communication networks. In line with this, both academics and industry researchers have started discussing the structure of 6G, while several proposals for 6G vision and enabling techniques have already been recommended [1; 2; 3; 4]. Because of the highly diversified nature of these proposals, it is very hard to compile a definite set of requirements and enabling techniques at this moment. However, there is a common expectation regarding the 10-fold improved performance of 6G over its predecessor 5G networks in terms of several key performance metrics, including average user data rate, system capacity, spectral efficiency, and energy efficiency. Since most data is generated in indoor environments, particularly in urban multistory buildings, developing new techniques to utilize spectra in the high-frequency bands (including millimeter-wave (mmWave), terahertz, and visible light bands)to address high user data rate, system capacity, spectral efficiency (SE), and energy efficiency (EE) demands for the future 6G mobile networks has been proposed as one of the key pillars [1; 3; 4]. Spatial spectrum reuse techniques have been considered as effective techniques to improve spectrum utilization. However, traditional approaches to spectrum reuse by base stations in two-dimensional (2D) space are not sufficient to address the envisaged user data rate, system capacity, SE, and EE demands of 6G mobile networks. High-frequency bands are coverage-limited due to associated high propagation loss. Moreover, the penetration losses of high-frequency mmWave bands through external and internal walls and floors in any multistory building are significant compared to low-frequency microwave bands. For these reasons, the reuse of high-frequency mmWave bands are explored in the third dimension (i.e., the height of a multistory building), which results in reusing the same high-frequency band more than once at the inter-floor level. In addition, the conventional spectrum reuse techniques at the intra-floor level in a multistory building are used in order to facilitate extensive reuse of mmWave spectra in ultra-dense deployed small cells within the building. Importantly, the capacity is directly proportional to the available spectrum bandwidth of a channel, which can be extended by increasing either the number of available spectra, such that each small cell can operate in more than one spectrum; or the number of times the same spectrum is reused by small cells through vertical spatial reuse in a multistory building. Hence, techniques for three-dimensional (3D) spatial reuse of high-frequency mmWave spectra with in-building multiband-enabled small cells can achieve the predicted 10-fold increase of the average user data rate, system capacity, SE, and EE of 5G systems to address the expected requirements of the future 6G mobile networks. Applying a multiband cooperative network architecture for 5G heterogeneous networks is mandatory to address seamless coverage [5]. Moreover, numerous studies on multiband-enabled network architectures have been carried out in the literature. The authors of [5] proposed a control/user plane split-based multiband cooperative network architecture where small access points are considered, enabling multiple mmWave bands to serve user plane data and high data rate demands, while macrocells are enabled with sub-6 GHz bands to serve control plane signals and to enable coverage for 5G heterogeneous networks. Likewise, an architecture for long-term evolution (LTE) systems considering unlicensed bands for splitting control and user plane data is proposed in [6], the fundamentals and general structure of a control and user plane split architecture are proposed in [7], and a mobility management scheme for ultra-dense networks is proposed in [8].