Too late handover: RLF occurs in serving cell before handover or during the handover procedure, then UE reconnect to the target cell (different from the serving cell).
Three types of RLF related to MRO are defined in LTE SON: too late handover, too early handover, and wrong handover.
In this section, basic handover procedure and related parameters of femtocell handover are introduced, then RLF scenarios caused by too late handover, Too early handover, wrong handover, and unnecessary handover are described respectively.
where N1, N2, N3, N4, and N5 denote the numbers of ping-pong handover, continuous handover, too late handover, too early handover and wrong handover, respectively, and α i denotes the weight factor of N i.
In this article, we develop a handover parameter adjusting-based mobility robustness optimization scheme, which aims to reduce unnecessary handovers and RLFs caused by too late handover, too early handover, or wrong handover in open access femtocells.
The RLF ratio is defined as the ratio of the number of RLFs caused by too late handover, too early handover, and wrong handover to the total handover attempts.
The characteristics of the mobility related unsuccessful handovers and the unnecessary handovers are described in Figure 4[2]: (b) Too late handover: RLF occurs in serving cell before handover or during the handover procedure, then UE reconnect to the target cell (different from the serving cell).
Therefore, more Too Late Handovers happen in Algorithm 1, which results in higher RLF ratio.
Therefore, a bigger hysteresis results in less handover attempts, but the too late handovers may still happens with Algorithm 1, resulting in increased RLF ratio.
Since Non-Uniform MRO has a heavy weighting in Too Late Handovers, the proposed non-uniform scheme results in a lower RLF ratio.
