AWADA et al.: SON-BASED ALGORITHM FOR OPTIMIZATION OF INTER-RAT HANDOVER PARAMETERS 1909
Fig. 1. Four different cases for inter-RAT TLH. (a) Case A where the entering condition of the measurement event is not fulfilled. The misconfiguration o f
T
thr
is the root cause for the TLH. (b) Case B where the entering condition of the measurement event is not fulfilled. The misconfiguration of S
thr
is the root
cause for the TLH. (c) Case C where the entering condition of the measurement event is not fulfilled. The misconfiguration of the threshold correspondingtothe
smallest value between Δ
S
and Δ
T
is identified as the root cause for the TLH. (d) Case D where the entering condition of the measurement event is fulfilled. The
misconfiguration of one of the two thresholds, which is reached later, is identified as the root cause for the TLH.
on the differences between the values of the thresholds and
their corresponding measured signal levels evaluated at t
RLF
.
Let Δ
S
= S
thr
− S
u, c
(t
RLF
) and Δ
T
= T
u, c
0
(t
RLF
) − T
thr
be the differences corresponding to thresholds S
thr
and T
thr
,
respectively. The root cause for the TLH in Case C is identified
as the misconfiguration of the threshold of which the difference
is the smallest. The rule determines the threshold that has to
be adjusted first by comparing the two negative values Δ
S
and Δ
T
. Once the threshold corresponding to the smallest
difference is correctly adjusted in subsequent steps, i.e., its
corresponding value of Δ
S
or Δ
T
becomes positive, the rule
detects that the other threshold having Δ
S
< 0orΔ
T
< 0 has
to be adjusted. As a result, the rule needs multiple steps to
detect that both thresholds have to be adjusted and consequently
resolve the TLH. The proposed routine for classifying a TLH
as either TLH(S
thr
) or TLH(T
thr
) inCasesA,B,andCis
summarized in pseudocode 1.
Pseudocode 1: Routine for classifying a TLH as either
TLH(S
thr
) or TLH(T
thr
).
1: Input Parameters: S
u, c
(t
RLF
), T
u, c
0
(t
RLF
), S
thr
,
and T
thr
.
2: Calculate Δ
S
= S
thr
− S
u, c
(t
RLF
).
3: Calculate Δ
T
= T
u, c
0
(t
RLF
) − T
thr
.
4: if Δ
S
< Δ
T
5: TLH is classified as TLH(S
thr
).
6: else
7: TLH is classified as TLH(T
thr
).
8: end if
As for Case D, the root cause for the TLH cannot be
determined using the aforementioned pseudocode. In this case,
the root cause for the TLH is the misconfiguration of one
of the two thresholds, which is reached later. For clarity, an
example is shown in Fig. 1(d), which shows Case D. According
to the figure, the entering condition of the measurement event
is fulfilled; nevertheless, an RLF occured before the T
T
time
interval is completed. The TLH could be resolved if the entering
condition would have been fulfilled earlier. To this end, the
threshold that delayed the fulfillment of the entering condition
needs to be determined and adjusted. In this example, T
thr
is
reached before S
thr
, and the root cause for this RLF is the
misconfiguration of S
thr
. Decreasing T
thr
would not solve t he
TLH as the entering condition would not be fulfilled earlier
since S
u, c
(t) is greater than S
thr
. However, if S
thr
is reached
earlier, the entering condition of the measurement event would
have been fulfilled earlier, and the RLF would have been
avoided. We note that to resolve TLH(S
thr
), S
thr
should be
increased, whereas TLH(T
thr
) is resolved by decreasing the
T
thr
threshold.
2) TEH: The UE is successfully handed over from cell A
to another cell B of a different RAT. Shortly after, an RLF
happens, and the UE reconnects to the previous RAT, either to
the same cell A or to a different one. Moreover, the inter-RAT
handover failure, occurring when the UE fails during the