Measurement Gap in LTE

What is measurement GAP?

Measurement GAP is introduced to measure the different frequencies of neighbor cell and inter RAT from the current cell in that mentioned GAP, so when neighboring inter frequency Cell has better signal than serving Cell then UE will measure the RSRP in that GAP and informs to current cell in measurement report. During this gap ue will not listen or transmit from/to serving cell.

In the active state, the eNB provides measurement gaps 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. This has to be coordinated with the DRX, which also causes the system to shut off the radio for periods of time to save power.

UE will get the Measurement GAP configuration  from  RRC_Connection_Reconfiguration message while attaching to network and the Information Element(IE) is MeasGapConfig.

The MeasGapConfig consists of gap pattern type (gp0 or gp1) The gap patterns are to identify the periodicity and gapOffset

Figure 1:Reference from 36.331

Figure 2:Reference from 36.133

Each Measurement gap starts at an SFN and subframe meeting the following condition:

SFN mod T = FLOOR (gapOffset/10);

subframe =  gapOffset mod 10;

with T  = MGRP/10 

where MGRP is 40 for gp0 and 80 for gp1

1. For FDD, the UE shall not transmit in the subframe occurring immediately after the measurement gap.
2. For TDD, the UE shall not transmit in the uplink subframe occurring immediately after the measurement gap.
3.  If the subframe occurring immediately before the measurement gap is a downlink subframe