In this section, the procedure of handing over a UE to a neighboring intra- or inter-RAT cell is explained.
In LTE and 3G networks, the handover of a UE u is triggered by the serving BS, typically when a measurement report is received from this UE. The reporting of the
26 Chapter 2: System Model
UE measurements to the serving BS can be either periodic or event triggered. In the latter case, the measurement report is sent when a certain condition, called the entering condition of the measurement event, is fulfilled for a certain time interval. The parameters of the entering condition are configured by the serving BS and are called handover parameters although they do not necessarily lead to a handover. The evaluation of the entering condition requires the UE to perform signal measurements for the serving cell and intra- or inter-RAT neighboring cells.
The entering condition of the measurement event used for intra-RAT handover expires at time step t0 when the measurement of an intra-RAT neighboring cell c0 ∈ C exceeds the measurement of serving cell c by an offset ϕ for a certain time interval denoted by TT(intra) [3GP12b, 3GP12g], i.e.,
MQu,c0(tn) > MQu,c(tn) + ϕ for t0−
l
TT(intra)/Tn m
< tn < t0 (2.31) where ⌈.⌉ is the ceiling operator. In (2.31), the time interval TT(intra), called also TTT, is converted from absolute number in ms to a number of simulation time steps. For instance, TT(intra) = 128 ms corresponds to a duration of three time steps if Tn = 50 ms. Both handover parameters, ϕ and TT(intra), should guarantee the reliability of the measurement event and the following handover decision.
For inter-RAT handover, a dual threshold measurement event is configured by the serving BS [3GP12b,3GP12g]. The measurement event which is used to hand over UEs from 3G to LTE and vice-versa is called measurement event 3A and B2, respectively. The entering condition of this measurement event expires at time step t0 when the measurement of the serving cell c falls below a threshold Q(1)c and the measurement of an inter-RAT neighboring cell exceeds another threshold Q(2)c for a certain time interval TTT denoted by Q(3)c , i.e., MQu,c(tn) < Q(1)c ∧ MQu,ik(tn) > Q (2) c for t0− Q(3)c /Tn < tn< t0. (2.32) The three inter-RAT handover parameters in (2.32) are the threshold Q(1)c correspond- ing to the serving cell, the threshold Q(2)c corresponding to neighboring cell and the TTT parameter Q(3)c . The index for the three handover parameters is denoted by m = 1, . . . , M, M + 1, where M is the total number of handover thresholds, i.e., M = 2 in inter-RAT case. That is, the index m = 1, 2 and 3 are used for serving cell thresh- old, target cell threshold and TTT, respectively. The cell-specific value of the mth inter-RAT handover parameter is denoted by Q(m)c .
After the entering condition of the measurement event is fulfilled for TTT time interval, the following steps [VWL+11] are executed to hand over the UE to an intra- or inter- RAT cell:
2.6 Modeling of Handover Procedure 27
1. The UE sends a measurement report to the serving BS. The measurement report can contain the measurements of many neighboring cells. The transmission of the measurement report has some probability to fail, especially if the UE is already far from the serving BS.
2. Upon receiving the measurement report, the BS sends a handover request to the neighboring cell corresponding to the strongest measurement and waits for either an acknowledgment or a rejection. This induces an additional delay which is typically called handover preparation time, and is denoted by Thp(intra) and Thp(inter) for intra- and inter-RAT handover, respectively.
3. The BS informs the UE to connect to the prepared target neighboring cell by sending a handover command. The transmission of the handover command can fail, in particular if the UE was moving during preparation and is already in the coverage area of another cell. This failure is modeled by checking, after the handover preparation time, the SINR of the UE with respect to the serving cell if it is below a certain threshold QHC in dB.
4. Once the handover command is successfully received, the UE will try to access the target cell of handover using the Random Access Channel (RACH) [3GP12f]. This random access can fail as well and is modeled by checking, after the handover preparation time, the SINR of the UE with respect to the target cell if it is below a certain threshold QRACH in dB [MZMT12].
5. The UE is finally handed over upon a successful access of the target cell.
Thus, the handover of the UE is executed after the handover preparation time if the SINRs of the UE with respect to the serving and target cells are high enough. Other- wise, the handover fails and the UE stays in the previously serving cell where it might experience later an RLF. In particular, if the handover failure is due to a weak SINR with respect to the target cell, an RLF is directly detected and the UE selects a new cell.
The cell c serving a UE u at time step tnis given by the connection function xu(tn) = c. An intra-RAT handover of UE u is executed from cell c to a neighboring cell c0 of the same network at time step tHO if the following conditions hold:
xu(tn) = c0 for tn> tHO if MQu,c0(tn) > MQu,c(tn) + ϕ for tHO− l Thp(intra)/Tn m −lTT(intra)/Tn m < tn < tHO− l Thp(intra)/Tn m ,
28 Chapter 2: System Model
γu,c(tHO) > QHC and γu,c0(tHO) > QRACH. (2.33)
Similarly, the UE is handed over to a neighboring cell ik of a different network if the following conditions hold:
xu(tn) = ik for tn > tHO if MQu,c(tn) < Q(1)c ∧ MQu,ik(tn) > Q (2) c for tHO− l Thp(inter)/Tn m −Q(3)c /Tn < tn < tHO− l Thp(inter)/Tn m ,
γu,c(tHO) > QHC and γu,ik(tHO) > QRACH. (2.34)