Inter-RAT MRO in 3GPP Rel.11: What Works, and What Does Not

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.