How LTE/5G UE measure Intra frequency neighbor cells?

Hi All.

We know that UE need measurement gap for scanning inter-frequency cell, UE tune its receiver to new freq and during that time serving frequency is not seen by UE at all.

So question is: how UE measure Intra frequency neighbor cells? Assume UE downloading from serving cells, how does simultaneously measure neighboring intra frequency cells?

In CDMA/WCDMA there was concept of RAKE fingers, where each finger could measure individual neighbor cell of same frequency.

What is the case in LTE/5G UE?

Admin note: this post was updated with image below.

For 5G, is the answer being looked for about UE autonomous gaps or SMTC for SSB measurement?

It is just a few subframes as per my understanding, so not too big gap required for intrafreq.

SMTC require only in ENDC.

We are talking about SA LTE or NR.

SMTC is for 5G SA too, right?

For any measurements within the BWP, SMTC can work, outside the BWP, it still needs gap.

Sorry my bad. Yes SMTC used in NR SA also.

But if SSB is within active BWP, then no gap should be required.

I think SMTC is used for measuring SSB outside UEs active BWP.

For 4G, there’s no measurement for intra frequency, it’s offset/hyst based.

Yes thanks, that I know.

Question is how simultaneously UE measure multiple intra freq cells?

I think there is not simultaneity, the UE only enter into a measurement gap in certain subradioframes defined (each 40 or 80 ms) and then switch to measure Intrafreq neighbors.

For intra frequency measurement?

I dont think so. In LTE, mgap is not required for intra freq measurement.


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.

Yes the mgap LTE or compressed mode concept is from UMTS.

I think that is if UE is in 4G & ENDC enabled in NSA mode.

But if @sadanandk2 is looking for an answer than it is much more serious. :blush:

Not for Intrafreq. Only for Interfreq and InterRat the measurement gap.

And only monitor other frequencies during this time.

Measurement gap not required for inter-freq HO from FR1 to FR2.

Thank you all dears for the comments.

But this is my query again:
How does UE measure intra frequency nbr cells while it is connected and downloading data from serving cell?

Let us not talk about inter freq, inter RAT things now. That part is pretty clear to everyone here. :wink:

And lets talk about LTE only to avoid confusion…


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…

I think all the time UE measures PCI of all neighbors based on CRS position for same frequency.

There is a hard coded parameter T_measure separately for TDD and FDD in case connected mode DRX is not used and if conn mode DRX is used then based on DRX cycle, 2-3 values are hard coded.

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.

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