# Self-Optimizing Traffic Steering for 5G mmWave Heterogeneous Networks
## 3 Self-Optimizing Traffic Steering
### Self-Optimizing Traffic Steering for mmWave HetNet
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Types** & **Nodes** & **Motivations** \\ \hline \multirow{8}{*}{Inter-network traffic steering} & LTE \(\rightarrow\) LTE-LAA & LTE resource limitation. \\  & LTE \(\rightarrow\) WiFi & Megabit-level or even lower data rate requirement. \\ \cline{2-3}  & WiFi \(\rightarrow\) LTE & Overcrowded traffic in WiFi network. \\  & WiFi \(\rightarrow\) LTE-LAA & High QoS requirements. \\ \cline{2-3}  & LTE-LAA \(\rightarrow\) LTE & LTE-LAA resource limitation. \\  & LTE-LAA \(\rightarrow\) WiFi & QoS requirement changes. \\ \cline{2-3}  & mmWave \(\rightarrow\) LTE-LAA & High propagation loss and easy-to-block channels. \\  & mmWave \(\rightarrow\) LTE & High user mobility. \\  & mmWave \(\rightarrow\) WiFi & Energy saving. \\ \cline{2-3}  & LTE \(\rightarrow\) mmWave & Gigabit-level data rate requirement. \\  & LTE-LAA \(\rightarrow\) mmWave & Stable user movement. \\  & WiFi \(\rightarrow\) mmWave & High user density. \\ \hline \multirow{8}{*}{Intra-network traffic steering} & LTE-LAA \(\rightarrow\) LTE-LAA & Overcrowded traffic in one LTE-LAA cell. \\ \cline{2-3}  & WiFi \(\rightarrow\) WiFi & The collision probability of one WiFi cell is too high. \\ \cline{1-1} \cline{2-3}  & mmWave \(\rightarrow\) mmWave & Overcrowded in one mmWave cell. \\ \cline{1-1} \cline{2-3}  & mmWave \(\rightarrow\) mmWave & Small mmWave coverage. \\ \cline{1-1} \cline{2-3}  & Energy saving. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Traffic steering modes in the proposed mmWave HetNet. mmWave communications are suitable for users with relatively low mobility, and thus it is better to steer low-mobility users to the mmWave network to release the traffic burden in other networks. * In the hot-spot area, interference is the main bottleneck for data rate enhancement. In this situation, some users can be steered to the mmWave network to alleviate interference. **Intra-network traffic steering:** Within the same network, traffic can be further steered among different cells and frequency bands. In intra-network traffic steering, actions are triggered based on traffic loads, available spectrum resources, channel capacities, and other factors. Intra-network traffic steering among LTE or WiFi networks is very common and has been widely studied, whereas the traffic steering strategy among different mmWave networks is quite different, as elaborated upon in the following:  
* As mentioned above, objects in daily environments cause blockage to mmWave channels. Hence, overcrowded users in an mmWave AP may lead to severe blockage and propagation loss. In this case, traffic steering is needed to realize load balance among mmWave APs. * Due to the small coverage of mmWave APs, handover among mmWave APs occurs frequently for moving users. Therefore, traffic steering should be applied to select the most suitable mmWave AP according to users' trajectories. * Since traffic service varies significantly with time, geographic location, and social events, resource utilization might be inefficient in some mmWave APs. Therefore, intra-network traffic steering can be used to reconfigure users in lightly loaded mmWave APs and switch off those idle mmWave APs to save energy. **Mathematical model:** Define binary variable \(x_{k}^{ij}\) as the traffic steering indicator, where \(x_{k}^{ij}=1\) indicates that the traffic of user \(k\) is steered to a nearby SBS/AP \(j\) in network \(i\) and \(x_{k}^{ij}=0\) otherwise. Here, the superscript \(i=\{0,1,2\}\) indicates the LTE, LTE-LAA, and mmWave networks, respectively. To ensure efficient and reliable network connectivity, we enable the traffic of user \(k\) to be steered to multiple networks. Furthermore, define \(y_{ku}^{ij}\) as the binary indicator that decides whether the channel \(u\) at SBS/AP \(j\) in network \(i\) is assigned to user \(k\), and define \(p_{k}^{ij}\) as the corresponding transmit power allocated to user \(k\). Let \(R_{k}^{ij}\) denote the achievable data rate of user \(k\) at SBS/AP \(j\in\mathcal{J}^{i}\).