## 2 State-of-the-Art Overview and Problem Formulation
### State-of-the-Art in the Location-Aware Communications and Handover Schemes
However, the proposed schemes may suffer severe degradation when GNSS-based positioning is disabled or provides low-accuracy location estimates, which can be the practical cases especially in the urban macro scenario. With the proxy mobile IPv6 (PMIPv6) in mind, the authors of [10] investigated several handover schemes, such as fast handover for PMIPv6 (FPMIPv6), smart buffering, and low-latency handover, and compared the corresponding handover performance with the proposed location-aware fast PMIPv6 (LA-FPMIPv6). In particular, the FPMIPv6 aims at reducing the packet loss by scanning and detecting the network status of all APs around the source AP. The handover is thus prepared once the RSSI of the source AP falls below a certain threshold. Similarly, the smart-buffering scheme, which also predicts the handover based on the RSSI, was proposed in [10] to reduce the packet loss. The main difference between smart-buffering scheme and FPMIPv6 lies in the fact that in the FPMIPv6 the UE scans the surrounding APs once handover decision is made, whereas for the smart-buffering scheme, the target AP, with which the UE is connected, searches for the source AP to retrieve the buffered packets. The resulting handover latency is the same as with the PMIPv6, but smart buffering enjoys a lower packet loss. Furthermore, by omitting the UE's authentication procedure, low-latency handover was also proposed in [10] to reduce the packet loss. However, due to the fact that only RSSI has been considered as the parameter, the handover cannot be predicted precisely whenever the measured RSSI is considerably different from the actual values. Hence, the LA-FPMIPv6 was proposed to perform a handover based on both the location information of the UE as well as the RSSI. Hence, both the handover timings and the next AP to which the UE will connect can be predicted more precisely than in the aforementioned algorithms, and therefore it achieves enhanced performance in terms of the reduced signaling cost, buffering cost, and handover latency. As key components in reducing the network latency and increasing the link robustness, control/user plane separation (CUPS) [36] together with MR-DC have been employed to improve the handover performance in mmWave HetNets. The authors of [11] proposed a prevenient handover scheme with radio resource control signalling duplication and master-secondary switch in order to reduce the handover failure rate and service interruption time. Similarly, for a CUPS network, the authors of [12] presented a seamless handover scheme by introducing a handover-assisted micro evolved NodeB (HO A-eNB) in the overlapping area. Such HO A-eNB maintains a continuous connectivity with the UE throughout the handover process, resulting in improved success probability of handover. However, deploying such HO A-eNBs in all the possible overlapping areas may incur prohibitive costs as well as posing a stringent requirement on the backhaul link capacity of the network. By integrating the mmWave networks with the sub-6 GHz networks for better link robustness and higher throughput, the benefit of performing handover with location awareness of the UE has been investigated in [13; 14; 15]. In [13], the location and velocity information (mobility information) of the UE was exploited to optimize the overall network performance, such as reducing the number of handovers as well as enhancing the user QoS. However, similar to the aforementioned works, the way of acquiring a perfect knowledge of the mobility information of the UE was not clearly specified. Moreover, the importance of location information has been investigated in [14; 15]. By exploiting the available CSI at sub-6 GHz bands, the authors in [14] proposed to apply a machine learning method to predict the location information of the UE, which is then utilized to achieve faster handover. The results showed that with location awareness, the UEs enjoy higher spectral efficiency than with the conventional handover scheme.