What is measurement gap in LTE?

To measure the cell power (signal quality) of neighbour cells, target cell and current cell frequency by UE is called Measurements. Measuring the signal quality of the cells at the same frequency (Intra frequency measurement) is not the difficult task. But it is difficult to measure signal quality of cells at the different frequency (Inter frequency measurement). So to resolve this issue measurement gaps comes in picture where idea is to create a gap in which UE will not receive or transmit anything from/to serving cell and switch its frequency to target cell and perform the signal quality measurement and come back to the current cell. To make this work seamlessly, there should be a well-established agreement between UE and Network about the gap definition (e.g, Starting Position of the Gap, Gap length, number of Gaps etc) and this agreement is established by a couple of RRC messages (e.g, Measurement Control, Physical Channel Reconfiguration etc) In the active state the eNB provides measurement gaps configuration (MeasGapConfig RRC message IEs ) in the scheduling of the UE where no downlink or uplink scheduling occurs. Ultimately the network makes the decision, but the gap provides the UE sufficient time to change frequency, make a measurement, and switch back to the active channel. This can normally occur in a few TTIs.

MeasGapConfig ::=                           CHOICE {

       release                                              NULL,

       setup                                                 SEQUENCE {

             gapOffset                                          CHOICE {

                          gp0                                                    INTEGER (0..39),

                          gp1                                                    INTEGER (0..79),

                          …

             }

       }

}