Case: RF Improved for 4G and 3G reselection

In this post we will go through a common case of RF reselection between 4G and 3G networks using huawei equipment. We will review the main parameters involved to ensure subscriber keeps a wireless connection while it moves.

Reselection process is about changing serving cell based on coverage, priorities, service etc. One important thing to keep in mind is that reselection happens when the UE is in idle mode. When the UE is in connected mode (on a call, browsing internet, watching videos online etc.) the mobility process is called Handover.

For this exercise, we will review the reselection between 3G and 4G networks using the next frequency spectrum example:

For this scenario we will go with the following idle mobility strategy:

L2100 <-> L2100 cell reselection (Intra-Freq)
L2100 -> U1900 cell reselection (Coverage Based)
U1900 -> L2100 cell reselection (Priority Based)

Case 1: L2100 <-> L2100 Cell Reselection

Also known as Intra-RAT Intra-Band cell reselection. For this example, we can set the L2100 as priority 7 (maximum value possible). This means that this band will have the highest priority (higher the priority, more likely the UE will try to move/remain in this band).

The highest reselection priority is forthe LTE high bands, because usually they have bigger bandwidth and higher throughput.

Low bands are set with low priority to ensure better coverage.

Intra-Frequency reselection triggers when UE moves away from the coverage area of the serving cells and start receiving signal from a neighbor cell with better RF levels.
Usually the RSRP is the measurement that triggers the reselection process.

In order to set the highest reselection priority, we should run the following commands in every LTE 2100 cell.

MOD CELLRESEL: LocalCellId=X, CellReselPriority=7;

With this command, all UEs connected to the cell will know from SIB messages that the cell is configured with priority 7. In the next table we can see some of the most important parameters involved in this kind of reselection

LTE Parameter	MO	Recommended Value	Description
SIntraSearchCfgInd	CELLRESEL	CFG	Whether to set the measurement triggering threshold for reselection to intra-frequency cells
SIntraSearch	CELLRESEL	28 (2 dB) = -70 dBm	Measurement triggering threshold for intra-frequency cell reselection
Qhyst	CELLRESEL	4(dB)	Hysteresis for the measured RSRP of the serving cell
QRxLevMin	CELLRESEL	-64 (2 dB) = -128 dBm	Minimum required RX level
TReselEUTRAN	CELLRESEL	1s	Cell reselection timer value for EUTRAN

Case 2: L2100 -> U1900 Coverage-Based Cell Reselection

For this scenario, the 3G network should have a lower priority than LTE. This means the UE will move from LTE to UMTS only in the case where the LTE RF signal is bad enough and the 3G RF signal is good. This way the UE remains as long as possible in the LTE network which provides a better performance.

UMTS 1900 can be configured with any priority between 0 and 6 (lower priority than LTE 2100). Select priority 4 so we can have room for future changes (for example new bands added). Once LTE 2100 coverage is weaker than certain value and there are no LTE 2100 neighbors where the UE can reselect to, the InterRAT measurements starts. The UE will perform reselection to UMTS once the RF signal is better than certain threshold.

Following the previous picture, reselection occurs when following both conditions happen:

LTE RSRP is below -114 dBm (ThreshServLow)
UMTS RSCP is above -103 dBm (ThreshXLow) for the time specified by TReselUTRAN.

The MML command to configure the priority in the LTE eNodeB is:

MOD UTRANNFREQ: LocalCellId=X, UtranDlArfcn=123, CellReselPriorityCfgInd=CFG, CellReselPriority=4;

LTE Parameter	MO	Recommended Value	Description
SNonIntraSearchCfgInd	CELLRESEL	CFG	Whether to configure the measurement triggering threshold for cell reselection to inter-frequency or inter-RAT cells
SNonIntraSearch	CELLRESEL	9 (2 dB) = -110 dBm	Measurement triggering threshold for reselection to an inter-frequency or inter-RAT cell
ThrshServLow	CELLRESEL	7 (2 dB) = -114 dBm	Threshold used in the evaluation of reselection to a cell on a lower priority E-UTRAN frequency or on an inter-RAT frequency
CellReselPriority	CELLRESEL	7 for LTE 2100	Absolute cell reselection priority of the serving frequency
QRxLevMin	CELLRESEL	-64 (2 dBm) = -128 dBm	Minimum required RX level
CellReselPriorityCfgInd	UTRANNFREQ	CFG	Whether to set the priority of a cell assigned with the UARFCN used in cell reselection to UTRAN
CellReselPriority	UTRANNFREQ	4 for UMTS 2100	Cell reselection priority of the neighboring UTRAN frequency
ThreshXLow	UTRANNFREQ	6 (2 dB) = -103 dBm	Minimum RX level required for a cell on the UTRAN frequency to become a candidate for cell reselection
QRXLevMin	UTRANNFREQ	-58 (2 dBm) = -115 dBm	RX level required for a neighboring cell on the UTRAN frequency to become a candidate for selection
UTRANDLARFCN	UTRANNFREQ	This is the UMTS ARFCN	Downlink UARFCN of the neighboring cell on the UTRAN frequency
TReselUTRAN	CELLRESELUTRAN	1s	Evaluation period for a UE to determine whether to reselect a neighboring UTRAN cell to camp on

Case 3: U1900 -> L2100 Priority-Based Cell Reselection

UEs will reselect to/prefer to camp on LTE due to its higher cell priority than UMTS.

Once LTE RSRP is equal or above -110 dBm (ThdToHigh) during time specified in seconds by TResel, UE will reselect back to LTE 2100 no matter the signal levels in 3G.

In this case, the configuration is in the 3G RNC side.

UMTS Parameter	MO	Recommended Value	Description
EARFCN	UCELLNFREQPRIOINFO	This is the LTE EARFCN	E-UTRA Absolute Radio Frequency Channel Number
NPriority	UCELLNFREQPRIOINFO	7 for LTE 2100	Absolute priority level of the E-UTRA frequency
EMeasBW	UCELLNFREQPRIOINFO	D50 (This depends on LTE BW)	Measurement bandwidth information common for all cells on the target frequency
ThdToHigh	UCELLNFREQPRIOINFO	9 (2 dB) = -110 dBm	RSRP above which the UE performs cell reselection from the serving cell to a cell
Eqrxlevmin	UCELLNFREQPRIOINFO	-64 (2 dB) = -128 dBm	Minimum required RSRP of the cell on the target frequency
SPriority	UCELLSELRESEL	4	Absolute priority level of the serving cell when a UE performs cell reselection
SibCfgBitMap	UCELLSIBSWITCH	SIB19-1	Whether SIB19 is broadcast in a cell

As you can see, in the 3 cases there is no need to configure neighbor relations, this is because cell reselection happen in the UE itself, network is not aware of the UEs actions because the UE is in idle mode, hence the UE does not need to report measurement report.