## 2 System Model
### Transmission Protocol  
The system adopts a dynamic TDD transmission mechanism to avoid link interference in the HAPS-IMT network effectively. All cells (including a HAPS cell and GBS cells) are configured synchronously. Each radio frame (10 ms duration) consists of 10 subframes (1 subframe is 1 ms duration).  
The asynchronous transmission protocol configuration of different layers will introduce cross-link interference. Cross-link interference includes downlink to uplink interference caused by BS in neighbor cells and uplink to downlink interference caused by users in neighbor cells. Because the transmission power of the HAPS node is much greater than that of GBS, the interference from the HAPS node to neighbor cells is the major reason for the decline of system performance. To effectively support dynamic TDD in an integrated system and mitigate cross-link interference, this paper designs the transmission protocol as shown in Figure 2. The transmission protocol combines power control and time slot resource configuration. HAPS divides each subframe into two parts, one is called a normal subframe, and the other one is called LP-ABS. To mitigate HAPS-to-GBS interference, the HAPS node and GBS configure synchronous uplink and downlink slots on the normal subframe. In addition, to provide better flexibility to support the asymmetry of data flow, each GBS dynamically configures the proportion of uplink and downlink slots on the dynamic subframe. In this case, the asynchronous transmission configuration between GBS may lead to serious interference from the HAPS node to GBS. HAPS configure downlink transmission with low transmission power P on LP-ABS in order to reduce unnecessary interference and utilize spectrum resources efficiently.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Symbol** & **Meaning** \\ \hline \(U^{S}(U^{B})\) & Utility of soft QoS service. (Utility of best effort service). \\ \(\Gamma^{d}(\Gamma^{u})\) & Downlink user set in integrated networks. (Uplink user set in integrated networks). \\ \(P_{o}^{u}\) & Uplink transmission power of user \(i\). \\ \(P_{o}^{u}\) & Downlink transmission power of HAPS node in the LP-ABS. \\ \(P_{o}^{u}\left(P_{o}^{d}\right)\) & Uplink transmission power of HAPS node in the normal subframe. \\ \(P_{o}^{d}\) & (Downlink transmission power of HAPS node in the normal subframe). \\ \(\gamma_{i}^{d}\left(\gamma_{i}^{u}\right)\) & Downlink SNR of user (Uplink SNR of user). \\ \(h_{ij}\) & Channel gain from user \(i\) to BS \(j\). \\ \(k_{j,j}\) & Average interference from Cell k to users in cell \(j\). \\ \(c_{i}^{d}\left(c_{i}^{d,u}\right)\) & Spectrum efficiency of uplink scheduling of user \(i\) in normal/LP-ABS \\ \(c_{i}^{n,d}\left(c_{i}^{n,u}\right)\) & Spectrum efficiency of downlink scheduling of user \(i\) in normal/LP-ABS \\ \(\Phi^{n,d}\left(\Phi^{n,u},\ \Phi^{t}\right)\) & Proportion of normal downlink frames, normal uplink frames, and LP-ABS in each transmission period \\ \(B=\) & \(B\) is the set of all BSs in the system model. 0 is represented to HAPS \\ \(\{0,1,2,\ldots,M-1\}\) & node, \(i\) to M-1 represented to M-15G Ground Base Stations. \\ \(B_{i}\) & Base station for User \(i\). \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameter symbols and physical meaning.  
In each transmission cycle, the proportions of downlink normal frames, uplink normal subframe and LP-ABS is denoted as \(\Phi^{n,d}\), \(\Phi^{n,\mu}\), and \(\Phi^{l}\). Each configuration \(\left(\Phi^{n,d},\Phi^{n,\mu},\Phi^{l}\right)\) corresponds to a transmission protocol configuration.