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.
For 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.
The UE is required to detect an E-UTRA FDD or TDD intra-frequency target cell within 800 ms
Typical LTE gap length is 6 ms which accommodates 5 ms measurement time (PSS and SSS are transmitted once every 5 ms) and RF re-tuning time of 0.5 ms before and after the measurement gap.
The measurement gap repeats with a periodicity of either 40 ms or 80 ms.
Actually mostly interviewer ask this to verify if he or she has concept of Gap or not.
We need to clear the concept behind it ,it is not a big issue to measure the signal quality of the target cell if the target cell is at the same frequency as the current cell (Intrafrequency measurement).
But there would be an issue when the target cell is at a different frequency from the current cell (Interfrequency measurement).
So they come out with a special technique called Measurement GAP.
This is the same concept as Compressed Mode in UMTS.
The idea of the Measurement GAP is to create a small gap during which no transmission and reception happens.
Since there is no signal transmission and reception during the gap, UE can switch to the target cell and perform the signal quality measurement and come back to the current cell.
Autonomous GAP periods are periods where the UE will pause/stop the Tx/Rx of the serving Cell to perform measurements or read MIB/SIB of the neighboring Cell. If Autonomous Gaps are not configured then UE will use IDLE periods.
It’s just google…I am not aware of word autonomous gap…
In CONNECTED MODE, Ue does the intrafrequency measurement when there is no data transmission/reception. As mentioned the 3gpp spec below, any time the receiver is active is an opportunity for nbr CRS detection and if enough time cgi identification by decoding MIB/SIB. The latter can be achieved in DRX mode when more measurement time is available.
In case of measurement gap, measurement gap has high priority than data transmission/reception, but in DRX data transmission or handling UL grant has higher priority than DRX.
Since CRS is frequency shifted as per PCI mode 6. Ue has to detect CRS which could be in any of all 12 subcarriers due to PCI based freq shift.
In terms of bandwidth, Ue will measure max BW of current cell and minimum target cell BW. Since CRS is averaged (independent of measurement bandwidth)
I think Ue also can also measure in connected mode when no data tx/rx and inactivity timer has not expired yet. Since tinactivity (usu set around 10 seconds max 62) this could be in opportunity for measurement.
3GPP TS 36.133
Eutra Requirements for support of radio resource management
TIntra : This is the minimum time that is available for intra frequency measurements, during the measurement period with an arbitrarily chosen timing. Time is assumed to be available for performing intra frequency measurements whenever the receiver is guaranteed to be active on the intra frequency carrier.