Abstract—With deploying LTE (Long-Term Evolution) network
operators are faced with managing up to three partly overlaying
network deployments of different radio access technologies
(RATs), for instance, GSM, UMTS and LTE. A concerted
management of all three RATs requires self-organizing networks
(SON) functionalities which have become an essential part of
LTE. One prominent use case is mobility robustness optimization
(MRO) and, recently, inter-RAT MRO has been discussed and
specified in 3GPP as part of the Rel.11 SON framework. Hereto,
a network deployment scenario was assumed where the existing
legacy cellular network (for instance UMTS) provides full
coverage, while LTE network deployment is limited to areas with
high user penetration. Consequently, 3GPP focused on two inter-
RAT failure cases, namely too late handover from LTE to 3G and
too early handover from 3G to LTE, where the latter case refers
to a handover triggered to LTE while the quality offered there is
too poor to maintain the call. All other inter-RAT handover
failure cases are not considered in Rel.11. In this paper, we
investigate the performance of an inter-RAT MRO algorithm
based on the Rel.11 capabilities with the reduced set of key
performance indicators (KPIs) being applied to deployment
scenario where 3G is not ubiquitous and LTE provides better
coverage in some areas. The simulation results show that the
algorithm with reduced KPI set performs very well for the
considered limited LTE coverage scenario, but show expected
deficiencies if deployment scenario changes.