## 5 Conclusions  
In this paper, unlike previous works (e.g., [9; 10; 11; 12]) that exploited the microwave channel model, by exploiting the 28 GHz mmWave channel model, we have presented a technique for 3D spatial reuse of multi-millimeter-wave (multi-mmWave) spectra in ultra-dense in-building small cells. In doing so, we first present an analytical model for the 28 GHz mmWave spectrum to characterize co-channel interference (CCI) between small cells enabled with dual transceivers operating in 28 and 60 GHz millimeter-wave (mmWave) bands in a multistory building. The 28 GHz mmWave spectrum is considered to model CCI and define a minimum distance between co-channel small cells in both intra- and inter-floor levels, such that finding an optimal size of a 3D cluster of small cells in a building is applicable for both 28 and 60 GHz spectrum bands, due to less indoor propagation loss in the 28 GHz band than in the 60 GHz band. We then define the corresponding 3D spatial reuse factor, such that the entire 28 and 60-GHz spectra can be reused by each 3D cluster in each building. Considering a system architecture where outdoor macrocells and picocells operate in the 2 GHz microwave spectrum, we derive the system-level average capacity, spectral efficiency (SE), and energy efficiency (EE) performance metrics and develop an algorithm for the proposed technique.  
Extensive system-level numerical and simulation analysis has been carried out to evaluate the performance of the proposed technique. It has been shown that with an increase in the number of small cell buildings \(L\) for a given value of \(\varepsilon_{\text{RF}}>1\), SE increases significantly when employing 3D spatial reuse of spectra by small cells within each building as compared to when no reuse is considered (i.e., \(\varepsilon_{\text{RF}}=1\)). This implies that employing horizontal 2D spatial reuse of mmWave spectra only once per small-cell building (i.e., by increasing \(L\)) is not sufficient to gain high SE, since \(L\) is limited by the macrocell coverage. To achieve ultra-high SE for 6G mobile networks, in addition to the horizontal spatial reuse of spectra, the vertical spatial reuse of mmWave spectra within each building needs to be exploited. In short, the average SE improves by the product of horizontal spatial reuse of mmWave spectrums by varying the number of buildings \(L\) and vertical spatial reuse of mmWave spectrums by varying the 3D spatial reuse factor \(\varepsilon_{\text{RF}}\), i.e., \((\varepsilon_{\text{RF}}\times L)\).  
On the other hand, the average EE improves exponentially with an increase in \(L\) and becomes almost steady for large values of \(L\). However, with an increase in \(\varepsilon_{\text{RF}}\), EE improves noticeably for low values of \(L\) (e.g., \(1\leq L\leq 5\)). As \(L\) gets larger (e.g., \(L>50\)), no significant improvement in EE can be achieved, irrespective of the values of \(\varepsilon_{\text{RF}}\). Hence, \(\varepsilon_{\text{RF}}\) mainly impacts SE enhancement, irrespective of the values of \(L\), whereas \(\varepsilon_{\text{RF}}\) contributes to enhancing EE only for low values of \(L\). Finally, we have shown that in contrast to considering \(\varepsilon_{\text{RF}}=1\), which requires \(L=30\), the expected requirements for 6G mobile networks of 270-370 bps/Hz average SE and \(0.3\times 10^{-6}\) Joules/bit average EE can be satisfied with \(\varepsilon_{\text{RF}}>1\) for low values of \(L\) (e.g., \(\varepsilon_{\text{RF}}\) values of 4, 9, and 16 require \(L\) values of just 3, 2, and 1, respectively), with each containing 180 small cells. Hence, 3D spatial reuse of multi-mmWave spectra by small cells deployed within multistory buildings is a promising technique to address the expected average SE and EE requirements for 6G mobile networks.  
This research received no external funding.  
This paper is partly submitted to the 2020 IEEE 92 nd Vehicular Technology Conference (VTC2020-Fall), Victoria, BC, Canada, [27].  
The author declares no conflict of interest.