La imagen pública y las reglas conversacionales de Lakoff en 2º curso de la

In later stages of LTE deployment, it is expected that the 3G and LTE networks will be fully overlaying. The 3GPP Case 3 network layout with Inter-Site Distance (ISD) of 1732 [3GP10] is considered as shown in Fig. 2.5 for fully overlaying inter-RAT deployment. A large ISD has been chosen since it is difficult to obtain coverage holes with small ISD of 500 corresponding to 3GPP Case 1 scenario. The total number Nbs

−3000 −2000 −1000 0 1000 2000 3000 −3000 −2000 −1000 0 1000 2000 3000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 X[m] Y[m]

Figure 2.5. Fully overlaying LTE (blue) and 3G (red) co-sited networks. The street grid is shown in black.

of BSs is 14 among which 7 LTE and 3G BSs are co-sited. Each BS serves tri-sectored homogeneous and hexagonal cells. The cell indices 1 to 21 (blue) are used for LTE cells and 22 to 42 for 3G cells (red).

2.8 Deployment Scenarios 33

Similar to the partially overlaying LTE and 3G scenario, some of the UEs move ran- domly and others move on a street grid which is shown in black in Fig. 2.5. The streets are placed in a specific way that covers most of the areas which are interesting for inter-RAT MRO study. Two streets pass through the same boundary of cells 8, 9, 11 and 12. Another street passes through an area which is directly below the antenna of cell 6. Two perpendicular streets pass through a three cell area which is common for cells 1, 6 and 8. Two parallel and perpendicular streets pass through the same boundaries of cells 3 and 11. A single street is perpendicular to the boundary of cell 16. Another single street passes along the boundary of cell 2. Finally, a single street passes through a three cell area which is common for cells 2, 6 and 19.

This scenario comprising two fully overlaying LTE and 3G co-sited networks is used to study radio-driven inter-RAT handovers from both directions, i.e., to exclude the traffic steering policy from 3G to LTE which was adopted for partially overlaying network scenario, described in Section 2.8.2. A radio-driven inter-RAT handover from the 3G to the LTE network can be only triggered if coverage holes exist in the 3G network and at the same time there is a good coverage from LTE. This case can only happen if the shadowing values of two fully co-sited LTE and 3G BSs are uncorrelated, i.e., ζnw= 0 which is rather an aggressive assumption. However, setting ζnw = 0 provides a proper scenario where the stationarity of UEs in each network is generated without the use of any traffic steering policy. This is clearly seen in Fig. 2.6 which shows the number of UEs in each network for ζnw = 1 in Fig. 2.6(a) and ζnw = 0 in Fig. 2.6(b).

0 1000 2000 3000 4000 5000 0 200 400 600 800 1000 Time [s]

Number of UEs in each network

3G LTE

(a) Correlated shadowing values of co-sited LTE and 3G BSs, ζnw=1. 0 1000 2000 3000 4000 5000 0 200 400 600 800 1000 Time [s]

Number of UEs in each network

3G LTE

(b) Uncorrelated shadowing values of co-sited LTE and 3G BSs, ζnw=0.

Figure 2.6. The number of UEs in each network as a function of time in s.

According to Fig. 2.6(a), the number of UEs in LTE decays as time passes. This is because the UEs are kept in the 3G network and are not handed over to LTE since any 3G coverage hole corresponds also to an LTE coverage hole assuming that LTE

34 Chapter 2: System Model

and 3G operate at carrier frequencies of 2.6 GHz and 2.1 GHz, respectively. On the other hand, if ζnw is set to 0, the number of UEs remains more or less the same in each network which is necessary for inter-RAT MRO investigation. In addition, the assumption ζnw= 0 makes the scenario more challenging for studying inter-RAT MRO since in this case, radio driven inter-RAT handovers are triggered from both directions, LTE to 3G network and vice versa.

35

Chapter 3

Inter-RAT Handover Parameters and

Mobility Failure Types

3.1

Introduction

The inter-RAT handover decisions rely basically on the measurement reports which are sent by UEs to the serving BS. These reports are triggered by the handover parameters and reporting criteria which are configured by the BS [HSS12]. The most relevant inter-RAT handover parameters are the two handover thresholds Q(1)c and Q(2)c corresponding to the measurements of the serving and target cells, respectively. Other parameters such as the time interval Q(3)c which refers to TTT and the filter coefficient kL3 used for L3 filtering of the measurements have also an impact on the robustness of handovers. The two handover thresholds are currently configured cell-specifically, i.e., can be set differently in each cell.

The setting of the handover parameters in a cell is mainly affected by three factors. The first factor is the radio propagation condition which varies in each cell. As long as a cell does not have any coverage holes, there is no need for inter-RAT handovers and in turn no need to optimize its corresponding handover parameters. On the contrary, other cells have coverage holes and their corresponding handover parameters may need to be properly configured to avoid any inter-RAT mobility problems. The second factor is the user path in the cell. The coverage holes in a cell are not critical unless the users are passing through them. In this case, the users need to be handed over to another RAT early enough before they approach these coverage holes. The third and last factor is the user velocity. Fast moving UEs are typically more vulnerable to mobility problems due to the rapid changes in their signal measurements.

The mobility failure types which are defined for inter-RAT scenarios can be divided into two categories: The first consists of inter-RAT RLFs and the second includes the undesired costly inter-RAT handovers which should be avoided [3GP12c]. The author has proposed to differentiate between two types of TLHs in inter-RAT scenarios: 1) A TLH due to the misconfiguration of serving cell threshold and 2) A TLH due to the misconfiguration of target cell threshold. This proposal has been accepted and specified by the 3GPP Rel. 11 standard [3GP12c]. In this study, all inter-RAT mobility failure types are considered though 3GPP standard has focused only on a subset of

36 Chapter 3: Inter-RAT Handover Parameters and Mobility Failure Types

them [3GP12f] which are relevant for a partially overlaying LTE deployment as defined in Section 2.8.2.

As in many other optimization problems, inter-RAT MRO underlies trade-offs. Since the considered handover thresholds are currently cell-specifics, it might be challenging in some cells to reduce all mobility problems. It can happen that a reduction in one type of mobility failure is possible only at the expense of an increase in the values of other types. Typically, resolving most critical failure types is prioritized over others. These trade-offs in inter-RAT MRO are discussed in detail in this chapter along with the inter-RAT handover parameters and mobility failure types.

This chapter is organized as follows. The inter-RAT handover parameters are discussed in Section 3.2. The factors affecting the setting of the handover parameters are elab- orated in Section 3.3. The different types of mobility failures which are defined for inter-RAT scenarios are explained in Section 3.4. Finally, the trade-offs in inter-RAT MRO problem are highlighted in Section 3.5.