Speed Based

The algorithms of this class typically compare the UE speed with absolute speed thresholds to lower the HO probability for medium to high speed users. Note that the vast majority of speed-based algorithms incorporate other HO decision parameters as well, mainly including the RSS, traffic-type and available bandwidth on the target cell. In the following, we discuss three representative speed based HO decision algorithms.

a. Speed and Traffic-type based HO Algorithm for the Macrocell-Femtocell Network

The algorithm in [58] consists of two different HO decision strategies: the proactive and the reactive. In the proactive strategy, a HO is initiated before the RSS of the serving cell reaches an absolute HO hysteresis threshold. To achieve this, the algorithm estimates the residual time prior to the HO execution. In the reactive strategy, the HO execution is initiated when the minimum required RSS is (almost) reached. The proactive strategy aims at minimizing the packet loss and the delay of the HO decision, whereas the reactive strategy aims at lowering the number of unnecessary HOs. The proposed algorithm applies to the multiple- macrocell multiple-femtocell HO decision scenario. Figure 59 illustrates the fundamental operation of the proposed algorithm.

When the UE speed is higher than 10km/h, the proposed algorithm avoids inbound mobility to femtocells and performs normal RSS-based HO decision. On the other hand, if the UE speed varies between 5 km/h and 10 km/h the proposed algorithm uses mobility prediction and employs the proactive strategy for real-time traffic or the reactive strategy for non real-time traffic. A similar approach is followed when the UE speed is lower than 5 km/h; however, without using mobility prediction. The mobility prediction scheme is discussed in [139].

Figure 59: Ulvan et al. HO algorithm [58]

The use of mobility prediction in combination with the UE speed is a strong feature of the algorithm in [58], which is expected to lower the HO probability for medium to high speed users. An improved QoE is also expected for the proposed algorithm, owing to the use of different HO decision strategies depending on the traffic-type. Nevertheless, the motivation for using the specific speed thresholds and the mobility prediction scheme should be discussed in more detail. The impact of the reactive HO decision strategy on the interference and throughput performance of nearby cells should be thoroughly investigated as well.

b. Low-complexity HO Algorithm for the Macrocell-Femtocell LTE-Advanced Network

The authors in [59] propose a low-complexity HO decision algorithm for the macrocell-femtocell LTE-A network. A signaling cost evaluation model accompanies the algorithm based on the work in [109]. The proposed algorithm applies to the single-macrocell single-femtocell HO decision scenario and consists of handing over to the femtocell station whenever a) the RSRP status of the femtocell exceeds over the RSRP status of the macrocell plus a HHM and b) the UE speed is lower than a prescribed speed threshold. The proposed algorithm is depicted in Figure 60.

Figure 60: Zhang et al. speed based HO algorithm [59]

Among the advantages of the algorithm in [59] is that it attains backwards compatibility with the LTE-A system and its signaling performance is validated by a performance analysis. The algorithm is also expected to lower the HO probability for medium to high speed users compared to SC-based HO decision algorithms. Nevertheless, the selection of an appropriate speed threshold is not thoroughly investigated and further numerical results are required to validate the performance of the algorithm in terms of interference, throughput and UE energy consumption.

c. HO Algorithm for the LTE-Advanced Network with Hybrid Femtocells

The algorithm in [60] incorporates a wide range of parameters to reach the HO decision, mainly including the RSS of the serving and the target cells, the UE speed, the interference level at target femtocells, the bandwidth availability on the target cell, the UE membership status and the traffic type. The algorithm can be used to a) remain in the current serving cell, b) handover to the macrocell, or c) handover to a hybrid femtocell. The algorithm is illustrated in Figure 61.

If the serving cell is a HeNB, the proposed algorithm performs a HO to the macrocell only if a) the UE speed exceeds over a prescribed speed threshold , and the macrocell can support the bandwidth requirements of the UE, or b) the RSRP status of the serving HeNB decreases and the macrocell can support the bandwidth requirements of the UE. If the serving cell is not a HeNB and the UE is not member of the CSG supported by the target femtocell, a HO is performed only if a) the interference level at the hybrid femtocell is greater than a prescribed threshold , b) the UE speed is lower than a prescribed threshold , , and c) the target femtocell can support the bandwidth requirements of the UE. On the other hand, if the UE belongs to the CSG of the target femtocell, the algorithm performs an inbound HO by using a) absolute RSS and relative RSS with hysteresis margin, b) UE speed, c) traffic-type, and d) bandwidth-related criteria.

Figure 61: Wu et al. HO algorithm [60]

The algorithm in [60] accounts for a wide range of HO decision criteria, which are expected to minimize the HO failure probability. However, the required signaling and delay overhead for commuting these parameters to the serving cell should be further investigated. The speed and interference thresholds should be specified, whereas system-level simulation results are also required to validate the performance of the algorithm. The HO decision for CSG cells can be further improved by taking into account the operating frequency of the UE and the femtocell, i.e., validate whether the UE and the femtocell operate in the same band.