While handover controls the mobility of UE in Connected (EMM-Registered, ECM-Connected and RRC-Connected) state, cell reselection controls the same while in Idle (EMM-Registered, ECM-Idle and RRC-Idle)
state. During a handover, it is the network (MME or source eNB) that
decides which cell to handover to. During cell reselection, however, it
is UE that decides which cell to camp on2.
A cell reselection procedure can be one of
the two types as seen below and in Figure 1. In Figure 1, the UE is
camping on Cell 5 that belongs to the TA list of {TA1, TA2} previously
assigned by the MME at the time of network attach, and is staying in
Idle state.
- Cell Reselection without TAU (❼:
EMM Case 7): UE moves to a TA that IS registered at MME (i.e. listed in
the TAI list of the UE), for example TA2 in Figure 1. Cell reselection
is performed but no Track Area Update (TAU) is required.
- Cell Reselection with TAU (❾:
EMM Case 9): UE moves to a TA that is NOT registered at MME (i.e. not
listed in the TAI list), for example TA3 in Figure 1. After cell
reselection, a TAU procedure is performed.
Figure 1. Two Cell Reselection Types
UE may reselect a cell i) that uses the same
LTE frequency as its current serving cell that it is camping on
(intra-frequency), ii) that does not use the same frequency
(inter-frequency), or iii) that uses other Radio Access Technology (RAT3) (inter-RAT). In the example cases of cell reselection shown in Figure 1, we only discuss intra-frequency cell reselection in a single LTE network environment that uses only a single LTE frequency.
This document describes cell reselection case ❼ only, and the other case (❾) will be covered in our next document “EMM Procedure 8 & 9. Handover & Cell Reselection with TAU”[6].
In Chapter II below, we will review the preliminary information
required to understand the cell reselection procedure. In Chapter III, a
detailed description of the cell reselection without TA procedure will
be given.
II. Cell Reselection: Required System Information and Criteria
Before we learn the detailed cell
reselection procedure in Chapter III, we will provide a brief overview
of a cell reselection procedure in Section 2.1 first. Then, we will look
into what system information is required for the procedure and the
criteria for the cell reselection in Sections 2.2 and 2.3, respectively.
2.1 Overview of Cell Reselection Procedure
It is UE who is in control of cell
reselection. UE obtains information needed for cell reselection (e.g.
threshold values used to decide whether to measure the signal strength
of neighbor cells or not, parameters used for calculating rank of the
serving cell and neighbor cells, etc.) from the system information
broadcasted by eNB. In case of intra-frequency cell reselection,
required information is delivered through System Information Block (SIB)
3 and SIB 4, which will be discussed further in Section 2.2.
■ Cell Reselection Triggering
Serving Cell Measurement: UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Qrxlevmeas) and calculate the received signal level (Srxlev)
of the serving cell to decide whether it should stay or move to another
cell. Here, the UE’s transmission and reception conditions are
reflected in the calculation, for example by applying minimum received
signal level Qrxlevmin, allowed maximum TX power level PEMAX, etc., (see Section 2.2 for details).
Cell Reselection Triggering: If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch),
the UE stays in the current serving cell. If not, it triggers a cell
reselection procedure. The threshold value that works as triggering
criterion is delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.
UE in Idle state wakes up at the end of every DRX cycle to measure the received signal level of its serving cell (Srxlev)
when it has stayed in the same location for a while. Let’s assume a UE
that has camped on the same serving cell for a while because the
received signal level of the cell has remained lower than the set
threshold (s-IntraSearch).
If it leaves the serving cell, the received signal level of the cell
decreases gradually. Finally when the received signal level becomes
lower than the threshold (s-IntraSearch),
a cell reselection procedure is triggered. Then the UE begins to
measure the signal strength of the neighbor cells (i.e. non-serving
cells).
■ Cell Reselection Criteria
Cell-Ranking Criterion: The UE ranks each cell (Rs, Rn) based on the measured signal strength of the serving cell (Qmeas,s) and neighbor cells (Qmeas,n).
Parameters required for cell ranking are delivered through SIBs 3 and 4
(see Section 2.2). The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.
Cell Reselection:
Once the serving cell and neighbor cells (non-serving cells) are
ranked, the UE checks whether the cell reselection criterion is
satisfied (Rn > Rs)
or not. If there are neighbor cell(s) that satisfy the criterion, the
UE selects the best satisfying cell, and then camps there. Cell
reselection is performed only when the criterion is satisfied for a
certain period of time (t-ReselectionEUTRA).
Mobile operators can prevent too frequent
cell reselection and make sure reselection is performed in accordance
with the cell status by controlling the UE’s dwelling time on the
serving cell, based on the hysteresis and cell-specific offset values.
In addition, they can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSF, t-ReselectionEUTRA-SF) depending on the traveling speed of the UE.
2.2 System Information
System Information (SI) refers to the
information broadcasted by eNB and consists of MIB (Master Information
Block) and SIBs (System Information Blocks; SIBs 1 ~ 16) [4]. MIB, SIB 1
and SIB 2 are mandatory, but the rest SIBs are optional. All SI is
delivered to UE through an RRC message like MIB, SIB 1, or SI message4. An SI message consists of a group of SIBs (SIBs 2 ~ 16), excluding MIB and SIB 1.
UE performs a cell reselection procedure
based on the SI broadcasted by eNB. MIB, SIB 1 and SIB 2 are applied to
all the UEs, either in Connected state (EMM-Registered, ECM-Connected, RRC-Connected) or in Idle state (EMM-Registered, ECM-Idle, RRC-Idle).
Conversely, SIBs 3 ~ 8 are only used in cell reselection by those in
Idle state. Table 1 describes different types of SI and their parameters
(see our One-Shot Gallery for more information about for a selected
list of SIB information5).
Table 1. Cell Reselection-related System Information
Let’s assume two mobile operators (A and B) who operate the network as follows:
•
|
A:
|
-
|
has a LTE-only nationwide network |
|
|
-
|
LTE frequency: only one channel of 10 MHz in 1.8 GHz band (lteFA1) |
|
|
|
|
•
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B:
|
-
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has nationwide 3G (UMTS) AND LTE networks |
|
|
-
|
3G frequency: six channels of 5 MHz in 2.1 GHz band (3gFA 1/2/3/4/5/6) |
|
|
-
|
LTE frequency: one channel of 10 MHz in 1.8 GHz band, and another channel of 10 MHz in 850 |
|
|
|
MHz band (lteFA1, lteFA2) |
In case of Operator A, its eNB needs information relating to
intra-frequency for cell reselection (eNB broadcasts SIBs 3 and 4, but
not SIBs 5, 6, 7 and 8. On the other hand, Operator B’s eNB needs
information relating to all three types of reselection: intra-frequency,
inter-frequency, and inter-RAT (3G UTRA) (eNB broadcasts SIBs 3, 4, 5
and 6).
The SI broadcasted by eNB is commonly
applied to all the UEs, but each UE may receive different type of SI
depending on its capacity (e.g. Release 9 UE or Release 11 UE).
Figure 1 displays a UE camping on Cell 5 at
eNB 2 along with its neighbor eNBs. Figure 2 however shows eNB 2 only,
along with its mandatory and cell reselection-related SI.
Figure 2. Cell Reselection-related SIBs Broadcasted by eNB2
The network used in Figure 1 is an LTE-only
network that uses a single frequency (lteFA1) with home PLMN only. This
document is about the intra-frequency cell reselection procedure. The SI
related to intra-frequency cell reselection is broadcasted through SIBs
3 and 4. Table 2 below lists the parameters used in the two SIBs, but
not those related to other frequency, RAT or VPLMN.6
Table 2. Cell Reselection Parameters (SIBs 3 and 4)
2.3 Cell Reselection Criteria
Though not within the scope of this
document, below we will review briefly the criteria used in cell
reselection after UE is turned on. One of the following criteria is
applied (i.e. criterion (1-1) for Release 8 UE and (1-2) for Release 9
or later UE). Table 3 provides a description of parameters used in the
criteria.
■ Cell Selection Criteria
•
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Release 8: |
Srxlev > 0
|
(1-1)
|
•
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Release 9: |
Srxlev > 0 and Squal > 0
|
(1-2)
|
|
|
where, Srxlev = Qrxlevmeas - Qrxlevmin - PCompensation [dB]
|
|
|
|
Squal = Qqualmeas – Qqualmin [dB]
|
|
Table 3. Cell Selection Criteria Parameters (TS 36.304 [5])
During the initial cell selection, a cell must have the cell RX level (Srxlev) greater than the sum of Qrxlevmin and PCompensation to be a serving cell. The Srxlev must be greater than Qrxlevmin
because, in order for UE to correctly receive messages from its serving
cell, the measured signal strength must be at least greater than Qrxlevmin. UE’s Tx power (PPowerClass) lower than the value allowed in the cell (PEMAX) will result in greater PCompensation value, making it hard to select the cell. When selecting a cell, UE’s transmission and reception conditions are considered.
In case of Release 9, in addition to Srxlev, Squal is added as a cell selection criterion. Qrxlevmeas is the cell’s Reference Signal Received Power (RSRP) while Qqualmeas
is the cell’s Reference Signal Received Quality (RSRQ). Compared to
RSRP which only indicates the strength of received signal, RSRQ provides
more accurate information for radio link quality because it indicates
the signal to interference and noise ratio (SINR).
■ Cell Reselection Triggering
UE camping on the serving cell may continue
to camp on there without having to measure other cells if the serving
cell’s RX level fulfills the criterion (2-1) for Release 8 UE or (2-2)
for Release 9 or later UE.
•
|
Release 8: |
Srxlev > SintraSearch
|
(2-1)
|
•
|
Release 9: |
Srxlev > SintraSearchP and Squal > SintraSearchQ
|
(2-2)
|
|
|
where, Srxlev = Qrxlevmeas - Qrxlevmin - PCompensation [dB]
|
|
|
|
Squal = Qqualmeas – Qqualmin [dB]
|
|
In the above inequalities, values of SintraSearch, SintraSearchP and SintraSearchP are
given by SIB 3 (see Table 2). If the serving cell does not fulfill
either of the foregoing criteria ((2-1) or (2-2)), i.e. if it fulfills
the criterion (3-1) in case of Release 8 UE or (3-2) in case of Release 9
or later UE below, the UE begins measuring the neighbor cells for
reselection.
Cell Reselection Triggering (Neighbor Cell Measurement Triggering)
•
|
Release 8: |
Srxlev ≤ SintraSearch
|
(3-1)
|
•
|
Release 9: |
Srxlev ≤ SintraSearchP or Squal ≤ SintraSearchQ
|
(3-2)
|
■ Cell Reselection Criteria
Cell Ranking Criterion
If the measured Srxlev of the serving cell
fulfills the foregoing criterion ((3-1) or (3-2)), the UE begins to
measure the neighbor cell’s RSRP. Then based on the resulting
measurements, it ranks all the cells by applying the criteria (4-1) and
(4-2) below [5]. Table 4 shows the parameters used in cell ranking.
•
|
Serving Cell Rank: |
Rs = Qmeas,s + Qhyst
|
(4-1)
|
•
|
Neighbor Cell (non-serving cell) Rank: |
Rn = Qmeas,n - Qoffset
|
(4-2)
|
Table 4. Cell Reselection Criteria Parameters (TS 36.304 [5])
Cell Reselection
If there are multiple neighbor cells that fulfill the criterion (5) below, the UE selects the best cell and camps on it.
As seen in the foregoing criteria (4-1) and (4-2), Rs and Rn are calculated using different criteria. The greater Qhyst and Qoffset values are, the longer the UE can stay in the serving cell.
III. Procedure for Cell Reselection without TAU
Figure 3 illustrates the EMM Case 7. Cell Reselection without TAU (❼)
procedure. In Figure 1, the UE selected Cell 5 that fulfills the
criterion (1-1) or (1-2), and was allocated a TA list of {TA1, TA2} by
the MME after its initial attach to the network through Cell 5. Then
later while being served in Cell 5, the UE transited to Idle state, and
now is camping on Cell 5.
Figure 3 shows how UE camping on Cell 5
performs an intra-frequency cell reselection procedure as it switches to
eNB 4, and camps on Cell 10. Here, the UE’s mobility state is “Normal”,
and hence scaling factors are not considered. Neighbor cells to be
measured are Cells 4, 6, 10 and 13, but Figure 3 displays Cells 10 and
13 only for the sake of convenience. Chapter III will explain the
procedure for intra-frequency cell reselection that is successfully
performed as seen in Figure 1, and thus satisfies the following
conditions:
- Camping on the serving cell: The UE is camping on Cell 5.
- Cell Reselection Triggering: As UE moves away from the serving cell, cell reselection is triggered.
- Serving cell measurement: The serving cell is measured to decide whether to measure neighbor cells or not.
- Neighbor cells measurement: Neighbor cells (Cells 4, 6, 10 and 13) are measured for cell reselection.
- Cell Reselection Criteria
- Cell ranking: Cells are ranked based on the measurement results for the serving cell and neighbor cells.
- Cell reselection: Cells that satisfy the criteria are identified, and the best satisfying cell (Cell 10) is selected.
- Camping on the new cell: The UE camps on Cell 10.
Figure 3. Intra-frequency Cell Reselection Procedure (UE Moving to a registered TA)
1) [UE] UE Camping on the Serving Cell
The UE is camping on its serving cell (i.e. Cell 5 in eNB 2) while staying Idle.
2) [UE] Obtaining SI from the Serving Cell
The UE obtains SI required for cell
reselection from the serving cell. If each neighbor cell has different
offset values with the serving cell, the serving cell then provides the
UE with the list of neighbor cells through SIB 4. Then the UE acquires
the following parameters through SIBs 3 and 4:
- Parameters required for deciding on cell reselection triggering: q-RxLevMin, p-Max, s-IntraSearchP, s-IntraSearchQ, t-ReselectionEUTRA, q-QualMin (SIB 3)
- Parameters required for ranking the serving cell: q-Hyst (SIB 3)
- Parameters required for ranking the neighbor cells: q-OffsetCell (SIB 4)
3) [UE] Measuring the Signal Strength of the Serving Cell
At the end of the every DTX cycle, the UE wakes up and measure the signal of the serving cell (RSRP and RSRQ) to get Qrxlevmeas and Qqualmeas. Then, based on them, it computes the cell reselection received level (Srxlev) and cell reselection quality level (Squal).
The UE, by applying the criterion (3-1) or (3-2), whichever is
applicable depending on its release, checks whether it should reselect a
new cell or it may continue to camp on the current serving cell. For
example, if Srxlev and Squal does not satisfy the criterion, the UE continues to camp on the current cell. If either of them does, then it performs Step 4).
4) [UE] Measuring Neighbor Cells
The UE measures RSRP of the neighbor cells that are in the same frequency as the serving cell (Qmeas,n, n=4,6,10,13).
5) [UE] Cell-Ranking Criterion
Once the RSRPs are measured, the UE ranks
the serving cell (Cell 5) and neighbor cells (Cells 4, 6, 10 and 13).
The rank of the serving cell, Rank R5, can be computed by applying the criterion (4-1), and those of neighbor cells, R4/R6/R10/R12, can be determined by applying the criterion (4-2).
6) [UE] Cell Rank Comparison
Now, the UE compares Rank R5 and Rank Rn (n=4,6,10,13),
and checks if the criterion (5) is satisfied. If no cell satisfies it,
then the UE continues to camp on Cell 5. In the figure, the criterion
(5) was satisfied by Cells 10 and 13.
7) [UE] Selecting a New Cell
The UE compares the two satisfying cells, R10 and R13, and selects R10, the best satisfying cell, as its new serving cell.
8) [UE] Camping on the New Cell
The UE now camps on Cell 10. After receiving
SIB 1 broadcasted by Cell 10, it learns that TAI at Cell 10 is in the
TAI list. Since the new serving cell belongs to the UE’s registered TA
list, no TAU is performed. Thereafter, the UE wakes at the end of every
DRX cycle, to monitor the system and paging information of Cell 10, and
measure the signal of Cell 10 (RSRP, RSRQ).
IV. EPS Entity Information: Before/After Cell Reselection without TAU
This chapter will describe how information
elements in the EPS entities are different before and after the cell
reselection procedure. Since the UE stays in Idle state (EMM-Registered, ECM-Idle, RRC-Idle) before and after the procedure, MME also stays in Idle state (EMM-Registered, ECM-Idle).
The UE moves from Cell 5 to Cell 10, and hence no TAU procedure was
performed. Thus, after cell reselection, the information elements in the
EPS entities remain unchanged, and will be the same as those stored
after S1 release[10], as seen in Figure 4.
Figure 4. Information in EPS entity before/after Cell Reselection without TAU
V. Closing
We have learned how a UE in Idle state moves
to TA where the UE is registered, and reselects a new cell without TAU.
This document covers only the intra-frequency cell reselection
procedure where cell reselection is performed within the same frequency.
Most LTE operators might have more than one LTE carrier frequency in
their commercial networks, and they usually operate their LTE network
along with their 2G/3G networks. So, not only intra-frequency cell
reselection that we discussed here, but also inter-frequency and
inter-RAT cell reselections are considered in actual cell reselection.
In the next document, we will discuss the procedure for cell reselection
with TAU required when UE moves to TA where the UE is not registered
The T301 timer is started when the UE sends an RRCConnectionReestablishmentRequestfollowing a radio link failure. If the timer expires before the UE has received an RRCConnectionRestablishmentor an RRCConnectionReestablishmentReject, the UE enters Idle mode.
The T310 timer is started when Physical layer problems are detected whereby the UE receives N310 out-of-sync indicators. The timer is stopped if N311 in-sync indicators are received. If T310 expires, the UE either enters Idle mode or initiates the connection reestablishment procedure, depending on whether security is activated.
The T311 timer is started when the RRC reconnection procedure is started. If the timer expires before the selection of a suitable LTE (or other RAT) cell, the UE enters Idle mode.