ANR in LTE

ANR: Automatic Neighbour Relation is one of the SON features where SON banks on UE to detect unknown cells and report them to eNB. These unknown cells can be Intra-frequency, Inter-frequency and Inter-RAT.

Intra-frequency: Same carrier frequency, can be of different bandwidths
Inter-frequency: Different carrier frequency as compared to the serving cell
Inter-RAT: UTRAN, GERAN

In LTE, the operator is relieved from the duty of handling neighbour information. The ANR functionality resides with the eNB and it is responsible for creation/updation/deletion of NEIGHBOUR RELATION. 

eNB:

1. The procedure of building the NRT lies with eNB. eNB initiates the procedure by asking the UE to start and send measurement of nCell (NEIGHBOUR CELL)
2. Here, UE plays the role of identifying the neighbour cell, measuring the signal strength (RSRP/RSRQ) and informing the serving eNB
3. eNB receives the Measurement Report of the nCell which contains the E-UTRAN CELL GLOBAL IDENTITY of the neighbour cell

MME:

1. The serving eNB gets in touch with MME (if an X2 connection doesn’t exist) to assist it-self for creating a X2 tunnel with the target eNB. Once when this tunnel (IPSec and SCTP) is established, the serving cell eNB forwards the CGI-info to the target eNB. Thus, both the eNB's updates its own respective NRT (NEIGHBOUR RELATION TABLE).
2. Now, an INTRA or INTER FREQUENCY HO can take place:
   
Let's continue with CGI-info in this post:

Thought:
Why does UE have to get involved in building the Neighbour table in the eNB's?
Answer: In order to reduce the signaling over-head among eNB's. It will be a tedious task for the operator to manually manage Neighbour tables. Even if there is an automatic procedure for eNB to update NRs, it would cause enough signaling (talking to LTE/UTAN/GERAN) overhead for eNB. 

UE:

1. UE is in RRC Connected state
2. The eNB sends RRCConnectionReconfiguration message to UE which configures necessary parameters for nCell measurements

3. 
Query_1: UE is already connected to serving cell then how will UE read measurement of the nCell?
For INTER-frequency nCell measurements and INTRA-frequency nCell measurements are defined as follows:

a. Intra-frequency neighbour (cell) measurements: nCell measurements performed by the UE are intra-frequency measurements when the current and target cell operates on the same carrier frequency. The UE shall be able to carry out such measurements without measurement gaps.
b. Inter-frequency neighbour (cell) measurements: Neighbour cell measurements performed by the UE are inter-frequency measurements 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.

Query_2: What are Measurement Gaps?
Measurement Gaps are periods where UE switches off its Rx and TX from the serving Cell and starts listening to other. This GAP period has to be in sync with the eNB because the eNB should know when the UE will enter the GAP state. To configure the GAP period in UE, eNB uses measGapConfig IE which is included in RRCConnectionReconfiguration message.

Therefore, the UE has now the required parameters for nCell measurements and GAP periods. Remember, for Intra-frequency measurements, GAPS are not required as stated above. The above call flow is for Intra-frequency as the EAFCN for both the cells are 5780.
If the EARFCN were differents then Meaurement Gaps would have been required. This would be the case ofInter-frequency measurements. 

4. UE measures the power level (RSRP/RSRQ) of the nCell and informs eNB in MEASUREMENT REPORT. This report consists of measResults of sCell and nCell.
5. eNB receives this report and decides whether a HO is required or not.
6. If HO is required, then eNB checks whether Physical Cell Identity of nCell is present in the MR.
  a. If PCI is not there then eNB proceeds with ANR procedure 
7. eNB constructs another RRCConnectionReconfiguration message and includes the "cellForWhichToReportCGI" IE. This IE contains the cellID of the nCell for which CGI-INFO is requested.  
8. Now, UE has to report the CGI-INFO for the Cell so that eNB will be able to contact the nCell-eNB and exchange necessary configurations for building the NRT

CGI-INFO consists of: 
i. eCGI (EUTRAN CELL GLOBAL IDENTIFIER)
ii. TAC (TRACKING AREA CODE)
iii. PLMN Identity List

9. 
Query_3: UE is in state RRC-connected to sCell so how does UE get the CGI-Info from nCell?
1. Measurement Gaps are used for the purpose of measurements only. To get the CGI-Info, UE have to read SIB-1 of the nCell. To read the SIB1, UE have to decode MIB as well as the PSS and SSS of the nCell. To achieve this, UE would need quite a amount of time as compared to performing measurements of the nCell; and, measurement Gaps does not provide such a long time for this activity.
Therefore, to get the SIB1, UE uses IDLE periods to read SIB1. These idle periods are configured using drx-config IE embedded in the RRCConnectionReconfiguration message.

drx_Config  
            setup  DRX_Config
             onDurationTimer sf50
             drx_InactivityTimer sf2560
             drx_RetransmissionTimer psf1  

Therefore, when the inactivity timer expires, UE enters IDLE period and it is during this idle period, UE will sync with nCell and read MIB/SIB of the nCell.

2. 
  a. There is one more alternate way to read the CGI-info of the nCell which is -AUTONOMOUS GAPS-. If the UE supports autonomous gaps then it sends this information in the UECapabilityInformation message.
  b. eNB knows that UE supports autonomous gaps and asks the UE to use autonomous gaps for reading CGI-info of the nCell. A new IE is introduced in Rel9 of 3GPP "si-RequestForHO". If this bit is set to TRUE in the measConfig then UE will use AUTONOMOUS GAPS. If autonomous gaps are used then there is a time limit prescribed by 3GPP to complete the reading of cgi-info.

Query_4: What are AUTONOMOUS GAPS?
These are GAP periods where UE will halt the Tx/Rx of the sCell for a moment to perform measurements or read MIB/SIB of the nCell. If Autonomous Gaps are not configured i.e. si-RequestForHO is set to FALSE  then UE will use IDLE periods as stated in Query_1 above. 

Important Points to use:

1. An interesting point to note is that if the DRX configurations are not optimized correctly then eNB will not allow the UE to breathe, which means UE will not be able to enter IDLE periods. If this is the case, then UE will not be able to read SIB1, but it will still send MeasurementReport. However, this MR will not contain the CGI-info, instead it will carry only the signal strengths of sCell and nCell.
 2. When DRX IDLE period is on-going, eNB shouldn't send any data or UL grant because:
   a. Handling of UL grant has higher priority than DRX, which means, if UE has any UL grant or DL data during the ON time of DRX cycle then UE does not enter the IDLE period
   b. If eNB keeps on sending UL grant or DL data then UE will never enter IDLE period which is needed for switching to target cell for reading CGI detection.

3. In case of Measurement GAPS, GAPS have higher priority than data Tx/Rx.