# Multi-RAT Orchestration Method for Heterogeneous Wireless Networks
## 2 State-of-the-Art
Analysis related to multi-RAT related to traffic steering cannot ignore the proposed functional solutions developed by the 3 GPP group, allowing for simultaneous data transmission via Wi-Fi and LTE networks, as well as only via LTE networks using both licensed and unlicensed bands, owing to aggregating all available bands. The use of wireless network deployment in unlicensed bands gives some freedom in network expansion by both operators and users. The former have the ability to increase the network capacity without the need to acquire new spectrum resources, while the latter may take measures to increase the coverage and/or capacity of the network regardless of mobile network operators. These rationales prompted work by 3 GPP standardization organizations, which resulted in a series of technological solutions to enable the shared use of the bands. During simultaneous transmission via Wi-Fi [21] and LTE, in such a way where the LTE RAN network does not need to know about parallel data transmission in Wi-Fi, there are limitations in maintaining IP mobility between networks. Moreover, difficulties arise from the need to select a particular WLAN network and to determine the conditions on which WLAN networks should be selected. For these needs, 3 GPP has specified the Access Network Discovery and Selection Function (ANDSF), where a dedicated server in the operator's network controls and manages the connection [22; 23]. Another approach is to implement functionality at the RAN level through LTE-WLAN aggregation (referred to as LWA). The data stream splitting with subsequent aggregation then occurs in the LTE PDCP layer, thus the data are transmitted via LTE and Wi-Fi. LAA (Licensed-Assisted Access) is a certain standardization approach close to LWA in the unlicensed 5 GHz band where transmission is carried out via the LTE with dedicated mechanisms to avoid collisions with Wi-Fi networks. It should be emphasized, however, that the LAA does not allow transmission in stand-alone mode, i.e., a primary cell (Pcell) working in the licensed band is always required, a cell in the unlicensed band is configured as the second (Scell--secondary cell) and is used only in conjunction with Carrier Aggregation (CA). The cell serves the user and aggregates the bandwidth of divided packets at the MAC layer level. A 4G/LTE network that can operate without the need to cooperate with cells in the licensed band is \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Software** & **eNB** & **gNB** & **EPC** & **5 GC** \\ \hline OpenAirInterface [13] & Yes & Yes & Yes & Yes \\ srsRAN [14] & Yes & In development & Yes & No \\ Open5 GS [15] & No & No & Yes & Yes \\ OMEC [16] & No & No & Yes & No \\ free5 GC [17] & No & No & No & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Available open source solutions for mobile networks using SDR. the product solution developed by MulteFire Alliance [24]. Moreover, in 5G/NR-U, no aggregation with the cell in the licensed band is required. Details are presented in Table 2.  
Figure 1 shows schematically the places of the so-called data split. It can be seen that the mechanisms described above perform splitting at the Access Network (RAN) layer. In the case of E2E network orchestration, decisions about switching (or dividing) the data path may occur outside the decision space embedded in the architecture of a given system. Listed short summary of data transmission in the unlicensed band (both in the single-RAT and multi-RAT models), at least two basic goals should be noted: using the maximum needed throughput from the perspective of the end-user device and releasing resources in the licensed band (so-called offloading). These solutions, referring to the developments undertaken within the 3 GPP, are used in places where it is necessary to achieve the goals mentioned above. However, they require a connection to the LTE network and they should be under the control of the LTE network (operator).