In LTE E-UTRAN measurements to be performed by a UE for mobility are classified as below

• Intra-frequency E-UTRAN measurements
• Inter-frequency E-UTRAN measurements
• Inter-RAT measurements for UTRAN and GERAN
• Inter-RAT measurements of CDMA2000 HRPD or 1xRTT frequencies

For each measurement type a measurement identity is used by E-UTRAN when configuring measurements as well as by the UE when reporting results of the measurements. Measurement quantities and reporting events are considered separately for each measurement type. Measurement commands are used by E-UTRAN to order the UE to start measurements, modify measurements or stop measurements. Three reporting criteria are used: event triggered reporting, periodic reporting and event triggered periodic reporting.

For measurements within E-UTRAN at least two basic UE measurement quantities shall be supported:

• Reference symbol received power (RSRP)
• E-UTRA carrier received signal strength indicator (RSSI)

In LTE E-UTRAN measurements performed by a UE for intra/inter-frequency mobility can be controlled by eNB, using broadcast or dedicated control. In RRC_IDLE state, a UE follows the measurement parameters defined for cell reselection specified by the E-UTRAN broadcast. In RRC_CONNECTED state, a UE follows the measurement configurations specified by RRC directed from the eNB (e.g. as in UTRAN MEASUREMENT_CONTROL).

Intra-frequency neighbour (cell) measurements are performed by the UE when the current and target cell operates on the same carrier frequency. The UE should be able to carry out such measurements without measurement gaps. See below some intra frequency scenarios without measurement gaps

Inter-frequency neighbour (cell) measurements are performed by the UE are  when the neighbour cell operates on a different carrier frequency, compared to the current cell. The UE should not be assumed to be able to carry out such measurements without measurement gaps. UE may need to perform neighbour cell measurements during DL/UL idle periods that are provided by DRX or packet scheduling (i.e. gap assisted measurements). See below some inter frequency scenarios with measurement gaps

The UE determines whether a particular cell measurement needs to be performed in a transmission/reception gap and the scheduler needs to know whether gaps are needed. Measurement gaps patterns are configured and activated by RRC.

Fo Intra-frequency neighbouring cells, there is no need to indicate neighbouring cell in the serving cell system information to enable the UE to search and measure a cell i.e. E-UTRAN relies on the UE to detect the neighbouring cells. Where as for the search and measurement of inter-frequency neighbouring cells, only the carrier frequencies need to be indicated.

For a UE to search and measure neighbouring GERAN cells, the ARFCNs of the BCCH carriers need to be indicated in the serving cell system information . For a UE to search and measure neighbouring UTRAN cells, the serving cell indicates a list of carrier frequencies and scrambling codes.

Source : 3GPP TS 36.300