Performance Indicators 121
is not beneficial as it may yield to high number of RACH failures which occur when the UE fails to access the target cell threshold during the handover. This can be illustrated using Fig. 6.1 which shows a serving cell c overlaying with inter-RAT neighboring cell i1 and three other neighboring cells i2, i3, i4.
Figure 6.1. Serving cell c and its four corresponding neighboring inter-RAT cells i1, i2, i3 and i4.
Decreasing only the TTT of serving cell c with respect to neighboring cell i4 may trigger faster the handover of UEs to neighboring cell i4, including those which might be far from i4. Accordingly, there is a risk that the UEs moving on the other two streets passing through i2 and i3 try to hand over first to neighboring cell i4. However, these far UEs would most probably fail to access the target cell i4 during the handover and in turn they would detect RACH failures.
The RACH failures could occur as well when the TTT is increased cell-pair specifically. For instance, assume that the TTT of serving cell c is increased only with respect to overlaying cell i1. As the other three neighboring cells i2, i3 and i4 have now smaller values of TTT, the UEs may first try to handover to one of these three neighboring cells instead of i1. However, these UEs are far from the BSs serving cells i2, i3 and i4, and consequently they would fail to access these cells during the handover. As a result, the UEs detect RACH failures and drop the call.
6.3
Classification of the Inter-RAT Mobility Fail-
ure Events into Two Sets of Key Performance
Indicators
In order to make use of the TTT, the mobility failure events that can be resolved by adjusting the TTT should be differentiated from those that can be resolved by the
122Chapter 6: Joint Automatic Optimization of Handover Thresholds and Time-to-Trigger
handover thresholds. For this reason, the mobility failure events are classified into three categories as shown in Fig. 6.2:
1. Category I comprises the mobility failure events that can be resolved only by handover thresholds.
2. Category II comprises the mobility failure events that can be resolved only by TTT.
3. Category III comprises the mobility failure events that can be resolved either by handover thresholds or TTT.
Figure 6.2. The classification of the inter-RAT mobility failure events into two sets of KPIs.
Using this classification, the mobility failure events of Category I that can be resolved only by the handover thresholds are isolated from those of Category II and III that can be resolved by adjusting the TTT. Accordingly, two sets of KPIs are proposed as shown in Fig. 6.2: The set of KPIs that counts the mobility failure events of Category I is denoted by Set 1 whereas the set of KPIs that counts the mobility failure events of Category II and Category III is denoted by Set 2. The KPIs of Set 1: TLH\− 1,
\
TLH− 2, [TEH, \HWC, cPP and dUH are used for TLH of type 1, TLH of type 2, TEH, HWC, PP and UH, respectively. Similarly, the KPIs of Set 2 are indicated by ^TLH− 1,
^
TLH− 2, ]TEH, ^HWC, fPP and gUH. Currently, it is not possible in 3GPP standard to differentiate between these two sets of KPIs.
Category II consists of only one case of TLH that can be resolved exclusively by TTT. This case is more probable to occur for high values of TTT and it is depicted in Fig. 6.3.
6.3 Classification of the Inter-RAT Mobility Failure Events into Two Sets of Key
Performance Indicators 123
The figure shows the measurement MQu,c(tn) and MQu,ik(tn) of a UE u corresponding
to serving cell c and kth inter-RAT neighboring cell i
k, respectively. The UE connected
Figure 6.3. A special case of TLH which can be resolved exclusively by TTT.
to serving cell c fails to handover to neighboring cell ik and experiences an RLF at time step tRLF before the handover is initiated. This mobility failure event is counted as TLH if the UE was not handed over from the previously serving cell. Otherwise, the mobility failure event is counted as TLH only if the UE has stayed in cell c without an RLF for more than TTE time interval which is defined in Section 3.4.1 to differentiate between TEHs and TLHs.
In order to resolve this case of TLH, the handover should be completed before the UE detects an RLF. Increasing the serving cell threshold Q(1)c or decreasing the target cell threshold Q(2)c does not help since the entering condition of the measurement event is already fulfilled from the first time instant the UE is connected to serving cell c. The only solution for this case of TLH is to decrease the TTT parameter Q(3)c so that the handover is triggered prior to the RLF. The automatic algorithm optimizing only the handover thresholds cannot react to this special case of TLH. However, it can react to all the other mobility failure events of Category I and III.
The classification of mobility failure events into Set 1 or Set 2 of KPIs depends on whether the mobility event can be resolved by TTT or not. In this study, TLHs of type 1 and type 2 are called missed inter-RAT handovers whereas TEHs, HWCs, PPs and UHs are called fast inter-RAT handovers. A missed inter-RAT handover is classified into Set 2 of KPIs if the handover could be triggered prior to RLF by decreasing the value of TTT. On the other hand, a fast inter-RAT handover is classified into Set 2 of KPIs if the handover could be avoided by increasing the value of TTT.
An example of classifying a missed and a fast inter-RAT handover into Set 2 of KPIs is shown in Fig. 6.4(a) and Fig. 6.4(b), respectively. In Fig. 6.4(a), the UE detects
124Chapter 6: Joint Automatic Optimization of Handover Thresholds and Time-to-Trigger
an RLF at time step tRLF before the handover is initiated. The entering condition of the measurement event is fulfilled for the first time at time step t1. This missed
(a) Missed inter-RAT handover classified by Set 2.
(b) Fast inter-RAT handover classified by Set 2.
Figure 6.4. An example of a missed and fast inter-RAT handover which can be resolved by decreasing and increasing the TTT parameter Q(3)c , respectively.
handover corresponds to a TLH of type 1 since the target cell threshold is crossed first, see Section 3.4.1. Moreover, it is classified in TLH^− 1 of Set 2 if there exists a TTT value gQ(3)c which is smaller than Q(3)c and satisfies the following condition
g
Q(3)c + Thp(inter)< tRLF− t1 (6.1)
where Thp(inter)is the inter-RAT handover preparation time. The same condition applies for TLHs of type 2. By decreasing the value of TTT, the inter-RAT handover would be completed before the RLF occurs.
An example of a fast handover is shown in Fig. 6.4(b) and is generalized for TEH, HWC, PP and UH. For all these failure types, the UE is successfully handed over from cell c to inter-RAT neighboring cell ik at time step tHO. However, shortly after 1) the UE will detect an RLF in case of TEH and HWC, 2) the UE is handed over to the previous RAT in case of PP and 3) the UE stays in the 3G inter-RAT neighboring cell in case of UH. The time step t2 indicates the instant when the entering condition of the measurement event of the previously serving and target cells is not fulfilled for the first time after the inter-RAT handover is executed. All the aforementioned types of fast handovers are classified into Set 2 of KPIs if there exists a TTT value gQ(3)c which is higher than Q(3)c and satisfies
g
Q(3)c > Q(3)c + Thp(inter)+ (t2− tHO). (6.2) By increasing the value of TTT, the fast inter-RAT handover would not be triggered. The checking of (6.1) and (6.2) require the UE to log and send the signal measurements of the serving and target cells to the BS, even after the handover in case of (6.2) which