3GPP. Mobility issues are quite different for stationary GEO and fast-moving LEO satellites. While handover for GEO satellites may still be triggered by signal strength-based measure- ments as in terrestrial networks, LEO satellites may benefit from additional handover deci- sion criteria. There are different realizations to provide coverage for LEO satellites: earth moving beams and earth fixed beams. While earth fixed beams could reduce the number of handovers and mobility issues, earth moving beams as shown in Figure 5.3.8 are prevalent in satellite networks nowadays. Unlike terrestrial networks, where cells are stationary, in LEO systems, cells and beams move quickly over the coverage area on the earth causing a high frequency of handovers. In order to support LEO, 3GPP will have to define mobility enhancements to minimize the handover failure rate and to reduce interruptions of the user-plane data transmission. Fortunately, LEO satellite constellations are known and LEO satellite movements are deter- ministic. If the UE location is known, handover triggers based on satellite ephemeris could Satellite #1 10:15:00 Satellite #1 Satellite #1 10:30:00 10:00:00 PLMN ID #1 TAC #2 PLMN ID #1 TAC #3 PLMN ID #1 TAC #1 foorprint geographical area #2 geographical geographical Figure 5.3.8 Moving satellites with moving beams with earth fixed tracking area codes. (Source: Reproduced by permission of © 3GPP). 5.3 Non-terrestrial Networks be used (3GPP R2-1818050). This can be used for UEs that support GNSS and therefore know their location; however, for other types of UEs a different method must be considered. Similar approaches considering UE positioning information can be considered for idle mode mobility, that is,tracking area updates. Large Tracking Areas and Tracking Areas linked to a geographical area (see Figure 5.3.8) are preferred to avoid regular updates for stationary UEs. However, the size will still be limited by the overall paging capacity of the NR standard, which should not be exceeded. Other areas of study