5. 1 Introduction
This document describes the Synchronization function and its main benefits and
impacts in the LTE RAN.
Synchronization in the LTE RAN offers a calendar clock for O&M events and a
radio equipment clock for synchronization of the Uu interface. The function
synchronizes events in a Baseband node with events in other nodes. The radio
equipment clock also acts as timing reference for baseband processing in the
Baseband node.
This document describes the synchronization of the radio equipment clock. For
information on the calendar clock for O&M events, see Manage Transport
Network.
1.1 Additional Information
For further information about these functions and related topics, refer to the
following documentation:
— 3GPP TS 36.104
— 3GPP TS 36.133
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6. 2 Overview of LTE Synchronization
Synchronization is an LTE basic function that refers to clock alignment. This
section describes synchronization benefits and impacts in the LTE RAN.
The Baseband node contains two types of clocks:
— Radio equipment clock for synchronization of the Uu interface
— Calendar clock for O&M events, described in Manage Transport Network.
The functional requirements for clocks and synchronization of the clocks are
different for the Uu interfaces and O&M.
2.1 Uu Interface Synchronization
The radio equipment clock provides reference for Uu interface synchronization. It
also acts as timing reference for baseband processing in the Baseband node.
When two or more Baseband Radio Nodes share a radio in mixed mode, node
group synchronization must be used.
The basic functions of the mobile network depend on synchronization, including
the following functions:
— Cell search
— Radio link layer 1 supervision
— Cell downlink transmission
The radio transmission from Baseband nodes in a network can operate in
synchronous or asynchronous mode.
— In asynchronous mode, the radio equipment clock provides reference for
carrier frequency generation.
— In synchronous mode, the Baseband nodes use time and phase
synchronization in addition to carrier frequency synchronization. Time and
phase synchronization aligns the radio equipment clock to a common time
base with absolute time for the network. Based on the common time base,
radio frame transmission is aligned.
To support these functions, the radio equipment clock requires stable
synchronization references. The planning and configuration of these
synchronization references is discussed in Manage Network Synchronization. The
relationships between the different clock functions are shown in Figure 1 for FDD,
or Figure 2 for TDD.
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7. L0000305F
Cell search
Synchronous
FDD
Asynchronous
FDD
Clock Source
over SASE
Synchronous
Ethernet
Clock Source
over NTP
IEEE 1588
Frequency
IEEE 1588
Time/Phase
Clock Source
over GPS
CDMA2000
system time
distribution
System clock with
frequency, time, and
phase synchronization
System clock with
frequency
synchronization only
Radio Link L1
supervision
Figure 1 FDD: Relationship Among Uu Interface Synchronization Functions
L0000741A
Cell search
Synchronous
TDD
Clock Source
over GPS
CDMA2000
system time
distribution
System clock with
frequency, time and
phase synchronization
Radio Link L1
supervision
IEEE 1588
Time/Phase
Figure 2 TDD: Relationship Among Uu Interface Synchronization Functions
2.1.1 Cell Search
Frame structure type 1 is used for Frequency Division Duplex mode. The length of
the radio frame is 10 ms, and it consists of 10 subframes. Each subframe consists
of two slots, so one radio frame consists of 20 slots. Each slot contains, three, six,
or seven Orthogonal Frequency-Division Multiplexing (OFDM) symbols
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8. depending on the length of cyclic prefix and subcarrier spacing as shown in
Figure 3.
L0000306A
0 1 2 3 4 5 6 7 8 9 10 11 19
0 1 2 3 4 5
18
Subframes
Slots
Radio frame SFN = N
9
10ms
1ms
0.5ms
Primary
synchronization
signal
Secondary
synchronization
signal
OFDM
symbols
Primary
synchronization
signal
Secondary
synchronization
signal
Figure 3 Frame Structure Type 1
The last OFDM symbol in slots 0 and 10 contains the primary synchronization
signal. The secondary synchronization signal is mapped to the OFDM symbol
preceding the OFDM symbol for the primary synchronization signal. The primary
and the secondary synchronization signals define the physical layer cell identity.
The 504 cell identities are grouped into 168 unique cell-identity groups. Each
group contains three unique identities. The UE must perform cell search to find a
potential cell to connect to by identifying the cell, learning cell timing, and so on.
The physical layer cell search procedure is divided into three stages, described in
Table 1.
Table 1 Physical Layer Cell Search Procedure
Stage Description
1 Stage 1 is applicable for the initial cell search.
The UE identifies the carrier frequency. The carrier frequency is
known for the intra-frequency neighbor cell search.
2 Stage 2 applies to both initial and neighbor cell search.
The UE identifies the cell:
— Timing on 5 ms based on the primary synchronization signal
— Identity within the cell identity group as one of three sequences
used by the primary synchronization signal in the cell
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9. Stage Description
— Partial knowledge of the reference signal structure.
The UE can also lock its oscillator frequency to the Baseband node
carrier frequency.
3 Stage 3 also applies to both initial and neighbor cell search, based on
secondary synchronization signal sequences transmitted in two
different slots within the radio frame.
The UE can identify the cell:
— Timing, that is, achieve downlink synchronization
— Cell identity group
The cell identity group also provides reference to the pseudo-random
sequence used for reference signal in the cell.
2.1.2 Radio Link Synchronization
As described previously, the UE achieves the downlink synchronization during the
cell search procedure. The UE, in RRC-CONNECTED mode, evaluates in the
downlink the quality of the radio link of the serving cell. The UE bases its
evaluation on the cell-specific reference signal relative to thresholds Qout and Qin.
Qout is defined as the level at which the downlink radio link cannot be reliably
received. Qin is defined as the level at which the downlink radio link can be
received with significant reliability.
Qout and Qin are defined relative to the block error rate of a hypothetical PDCCH
transmission and the thresholds are defined in 3GPP TS 36.133. The quality of
the radio link is evaluated in every radio frame if the UE is not in Discontinuous
Reception (DRX) mode.
If the radio link quality is below the threshold Qout over the period of 200 ms,
timer T310 is started, assuming N310=1. See 3GPP TS 36.133.
At the expiration of the T310 timer, the UE switches off uplink transmission. If the
radio link quality is above the threshold Qin over the period of 100 ms, timer T310
is stopped, assuming N311 is set to 1. See 3GPP TS 36.133.
If the UE is in DRX mode, the DRX cycle-length dependent evaluation period is
used instead, see 3GPP TS 36.133.
Uplink synchronization is achieved using the random access procedure, see
Random Access for further information.
Radio link quality evaluation is performed by the Baseband node for the uplink
physical layer radio link synchronization evaluation.
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10. 2.1.3 Frequency Synchronization in LTE and NR
LTE and NR operates over an Ethernet network. Depending on the backhaul
capabilities, the eNodeB or gNodeB can receive a frequency reference from NTP,
PTP, or Synchronous Ethernet sources. A source from GNSS receiver is also valid
option.
Note: NR networks require phase synchronization in all practical cases.
The frequency error requirements are ± 50 ppb over a period of 1 millisecond, as
specified in the 3GPP TS 36.104 standard.
The easiest and most common means of providing frequency synchronization in
LTE is to deploy a PTP grandmaster. This uses the PTP protocol with the ITU-T
G.8265.1 telecom profile.
2.1.4 Phase Synchronization in LTE and NR
Frequency synchronization is mandatory to meet the 3GPP accuracy
requirements of the air interface. Phase synchronization can also be used to
support TDD access and enable features, such as Carrier Aggregation, running on
FDD systems.
Due to synchronization coordination between NR and LTE in non-standalone
operation and between NR nodes in standalone operation, the involved FDD
nodes always have to be time and phase synchronized.
Solutions that enable phase synchronization between adjacent cells are GNSS
and PTP. The phase error requirement between two Antenna Reference Points
(ARP) is 3 μs.
Both DU and Baseband nodes have their own Basic Frame Number (BFN). Cells
have System FrameNumbers (SFN) that are related to the BFN.
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11. L0000307D
BFN = 0 BFN = 1 BFN = 2 BFN = N
SFN = 0 SFN = 1 SFN = 2 SFN = N
SFN = 0 SFN = 1 SFN = 2 SFN = N
time
BFN initialisation time for RBS A
frameStartOffset
frameStartOffset
Cell A2
Cell A1
RBS A
Figure 4 Relationship between BFN and SFN
Without phase synchronization, the initialization time of the BFN is defined
locally in the baseband unit. The BFN offset, that is the BFN difference between
two baseband units, can drift. The SFN offset does not drift between two cells
that are controlled by the same baseband unit.
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12. L0000365A
BFN = 0 BFN = 1 BFN = 2 BFN = N
SFN = 0 SFN = 1 SFN = 2 SFN = N
BFN = 0 BFN = 1 BFN = 2 BFN = N
SFN = 0 SFN = 1 SFN = 2 SFN = N
Common time base from GPS
Common BFN initialisation time for RBSs
frameStartOffset
frameStartOffset
Cell B1
RBS B
Cell A1
RBS A
Figure 5 Time and Phase Synchronization in LTE
When the BFN is locked to an external time base, the value (set to 0) is aligned in
time between different baseband units and can drift only within a limited range.
As a result, the SFN initialization time for radio frame transmission is correlated
to the external synchronization reference.
Time and Phase synchronization can be achieved with the following methods:
— Connecting a GNSS receiver to the baseband unit to use it locally in the
eNodeB.
— Connecting a GNSS receiver to the baseband unit and activating the
grandmaster function to distribute synchronization information to other
eNodeBs through PTP.
— Synchronizing each baseband unit to an external grandmaster clock through
PTP according to the ITU-T G.8275.1 or the ITU-T G.8275.2 profiles.
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13. 2.1.5 Loose Time Synchronization
Loose Time Synchronization is introduced as a low accuracy time and phase
synchronization mechanism in asynchronous LTE radio access network. Loose
time synchronization is built on existing time and phase synchronization
mechanisms.
Loose time synchronization is used to secure that the hyper system frame
number is aligned between the nodes in the radio access network. This is
required to correct Idle Mode eDRX operation. The loose time synchronization
mechanism is not applicable in a synchronous LTE radio access network, because
another, more accurate synchronization mechanism is already deployed there.
The loose time synchronization mechanism is configured using MO attributes.
The threshold for the synchronization mechanism is provided. The threshold is
typically set to about 1 s.
2.1.6 Clock States
The radio equipment clock works as a slave clock locked to the network
synchronization references, see Manage Network Synchronization.
The operational state and the availability status of the cell depend on
synchronization mode and the clock state as described in the following table:
Table 2 Radio Equipment Clock States and Cell Status
ClockState Synchronization Mode Cell Operational State
( OperState )
Cell
AvailabilityStatus
FREQUENCY_LOCKED
TIME_OFFSET_LOCKED
Asynchronous or
synchronous
ENABLED NO_STATUS
FREQUENCY_HOLDOVER
TIME_OFFSET_HOLDOV
ER
Asynchronous ENABLED NO_STATUS
RNT_TIME_LOCKED Synchronous ENABLED NO_STATUS
RNT_TIME_HOLDOVER Synchronous ENABLED
See explanations in
the note after this
table.
NO_STATUS
Any other state Asynchronous or
synchronous
DISABLED DEPENDENCY_FAILED
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14. Note: The following applies to ClockState RNT_TIME_HOLDOVER and
synchronous synchronization mode.
When timeAndPhaseSynchCritical has the value TRUE, if the clock
accuracy is worse than the value specified by timePhaseMaxDeviation,
the following conditions apply:
— OperState is DISABLED
— AvailabilityStatus is DEPENDENCY_FAILED
When timeAndPhaseSynchCritical is FALSE and
timeAndPhaseSynchAlignment is TRUE, the following conditions apply:
— OperState is ENABLED or DISABLED.
— AvailabilityStatus has one of the following values:
• NO_STATUS
• DEGRADED
• DEPENDENCY_FAILED
If the timeAndPhaseSynchAlignment attribute is TRUE, the value of
OperState and AvailabilityStatus depends on the used threshold:
— timePhaseMaxDeviationMbms
— timePhaseMaxDeviationOtdoa
— timePhaseMaxDeviationSib16
— timePhaseMaxDeviationTdd
— timePhaseMaxDeviationTdd(1-7)
— timePhaseMaxDeviationCdma2000
2.1.7 GNSS Reference Holdover Readiness
If the synchronization reference to GNSS is lost, the system clock enters
RNT_TIME_HOLDOVER mode. While in this mode, the radio equipment clock keeps
the synchronization accuracy for some time, before the system clock indicates a
synchronization problem. This time, that is, the guaranteed holdover time,
depends on the holdover readiness of the radio equipment clock.
The holdover readiness is determined by how long the radio equipment clock has
been locked to the synchronization reference. For examples on required holdover
readiness for guaranteed holdover time, see Table 3. The radio equipment clock
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15. tolerates occasional loss of the synchronization reference without degrading the
holdover readiness. The following periods of unavailability apply:
— One interrupt of 12 minutes length per one hour in an asynchronous network.
— Three interrupts of 12 minutes length per 24 hours in a synchronous network.
Exceeding any of the listed values begins to decrease gradually the holdover
readiness. This affects the duration that the system-clock can maintain required
accuracy while in holdover. The required accuracy includes the following:
— timePhaseMaxDeviation
— timePhaseMaxDeviationMbms
— timePhaseMaxDeviationOtdoa
— timePhaseMaxDeviationSib16
— timePhaseMaxDeviationTdd
— timePhaseMaxDeviationTdd(1-7)
— timePhaseMaxDeviationCdma2000
Examples of guaranteed holdover times that apply when the required holdover
readiness is obtained are shown in, Table 5.
If the radio equipment clock, in a synchronous network, is powered-off for longer
than five days it must be powered up again for at least five days to obtain
holdover readiness for guaranteed holdover time. For short power-off periods,
less than five days, the radio equipment clock must be powered up at least as
long as it was powered off to obtain readiness for guaranteed holdover time.
When time reference is recovered after an outage and the time offset is less than
10us, the time is aligned with the reference within 30 minutes.
Table 3 Holdover Readiness Required for Guaranteed Holdover Time
Synchronization Type Total Hours Locked to
GPS Reference (1)
Total Time Powered up
after Period of Power
Outage (2)
Asynchronous 1 hour No requirement
Synchronous 24 hours 5 days
(1) Occasional periods of unavailability (less than 12 minutes) in the GPS Reference do not affect
holdover readiness. One unavailability period per 1 hour for Asynchronous-Network and three
unavailability periods per 24 hour for Synchronous-Network. More unavailability periods slowly
decrease the holdover readiness.
(2) The time depends on whether the power outage of the radio equipment clock lasted longer
than five days.
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16. 2.1.8 Assisted Time Holdover
Assisted Time Holdover is a network synchronization feature that allows the
extension of the time holdover period when the primary time synchronization
reference is lost.
The following synchronization references are supported as an assisting reference
source:
— PTP over IP (ITU-T G.8275.2), also referred to as Assisted Partial Timing
Support (APTS)
— Synchronous Ethernet (SyncE)
— PTP over Ethernet (ITU-T G.8275.1)
For more information, see the Assisted Time Holdover and Manage Network
Synchronization documents.
2.1.9 Mixed Mode
For Baseband nodes with sectors running in Mixed Mode, the radio equipment
clock in the LTE node must be synchronized with the radio equipment clock in the
node sharing its radios.
The synchronization in Mixed Mode is described in Manage Node Group
Synchronization, and the general description of Mixed Mode is available in Mixed
Mode Radio.
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17. 3 Configure the LTE RAN Synchronization
3.1 Configuration Parameters for Radio Equipment Clocks
Parameters for radio equipment clocks are described in Manage Network
Synchronization. This section gives guidance to the use of these parameters in
LTE RANs, and describes the specific LTE parameters associated with network
synchronization.
Note: Traffic can be affected if these parameters are changed. It can cause
malfunction of the corresponding feature because of the time and phase
deviation for time and phase synchronization of the eNodeB.
Table 4 Synchronization Parameters Introduced in MO ENodeBFunction
Parameter Description
timeAndPhaseSynchCritical Controls whether the Baseband node can or cannot transmit over the Uu interface
when the Baseband node is not time and phase locked to GPS time.
Setting this parameter to true activates time and phase synchronization. (The cell
must be locked before modifying this attribute.)
When timeAndPhaseSynchCritical is set to true and the deviation of time and
phase synchronization exceeds the threshold parameter timePhaseMaxDeviation,
all cells in the eNodeB are disabled.
timePhaseMaxDeviation Defines the maximum allowed time and phase deviation of the radio equipment
clock from the reference clock source if time and phase synchronization is used (that
is, if the timeAndPhaseSynchCritical parameter is set to true).
The minimum holdover time of the radio equipment clock is determined by the
acceptable time and phase deviation, that is the timePhaseMaxDeviation
parameter setting. For examples of guaranteed holdover times, see Table 5.
The incorrect setting of this parameter affects the synchronization quality (if the
value is too high) or the cell can be automatically disabled too early (if the value is
too low). The recommended value is:
— FDD only: 10 µs
— TDD only: 3 µs
timePhaseMaxDeviationEdrx The maximum allowed deviation for the loose time synchronization mechanism
used by Idle Mode eDRX. When the max deviation is reached and loose time sync is
active, a BFN jump is performed. The maximum recommended value is 5 s. The
default value is 1 s. An accuracy below 300 ms can not be guaranteed.
timeAndPhaseSynchAlignment Setting this parameter to true activates time and phase synchronization. (The cell
must be locked before modifying this attribute.)
timePhaseSyncStateEdrx Read-only attribute. Set to false when the Idle Mode eDRX feature is degraded due
to time and phase synchronization accuracy requirements not fulfilled, otherwise it
is set to true.
timePhaseMaxDeviationMbms The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB. It is the deviation that is allowed for Multimedia
Broadcast Multicast Service (MBMS) to function. When the limit is exceeded, the
service is deactivated.
timePhaseMaxDeviationOtdoa The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB. It is the deviation that is allowed for Observed Time
Difference of Arrival (OTDOA) Support to function. When the limit is exceeded, the
service is deactivated.
timePhaseMaxDeviationSib16 The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB. It is the deviation that is allowed for SIB16 Time
Information Broadcast feature to function. When the limit is exceeded, the feature is
deactivated.
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18. Parameter Description
timePhaseMaxDeviationTdd The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB for TDD cells. When the limit is exceeded, TDD cells
that use this limit are disabled.
(1)
timePhaseMaxDeviationTdd(1-7) The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB for TDD cells. When the limit is exceeded, TDD cells
that use this limit are disabled.
(1)
timePhaseMaxDeviationCdma2000 The maximum allowed time and phase deviation for time and phase
synchronization of the eNodeB. This is the deviation allowed for CDMA2000 related
features to function, and for SIB8 to include system time. When the limit is
exceeded, features are deactivated, and system time is not included.
(1) The following eight parameters are available for TDD cells regardless of the maximum number of TDD cells:
— ENodeBFunction.timePhaseMaxDeviationTdd
— ENodeBFunction.timePhaseMaxDeviationTdd1
— ENodeBFunction.timePhaseMaxDeviationTdd2
— ENodeBFunction.timePhaseMaxDeviationTdd3
— ENodeBFunction.timePhaseMaxDeviationTdd4
— ENodeBFunction.timePhaseMaxDeviationTdd5
— ENodeBFunction.timePhaseMaxDeviationTdd6
— ENodeBFunction.timePhaseMaxDeviationTdd7
This makes it possible to have separate time and phase synchronization accuracy for each TDD cell in the same
eNodeB. When one limit is exceeded, only the affected TDD cell is disabled.
Table 5 Guaranteed Holdover Time for Synchronous Mode Based on Time
Phase Max Deviation Parameter Setting
Synchronization Method Time Phase Max
Deviation (1)
Minimum Holdover Time
GPS 1.5 µs 15 minutes
GPS 3 µs 1 hour
GPS 10 µs 8 hours
IEEE 1588 Time and
Phase(2)
1.5 µs No Holdover Capability
IEEE 1588 Time and
Phase
3 µs 15 minutes
IEEE 1588 Time and
Phase
10 µs 7 hours
(1) Time Phase Max Deviation is a general term for the entire group of possible parameters:
timePhaseMaxDeviation, timePhaseMaxDeviationMbms, timePhaseMaxDeviationOtdoa,
timePhaseMaxDeviationSib16, timePhaseMaxDeviationTdd,
timePhaseMaxDeviationTdd(1-7), or timePhaseMaxDeviationCdma2000
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19. (2) The holdover time for IEEE 1588 Time and Phase is not relevant when the Ethernet connection
is lost since the connection to the core network is also lost.
MO class EUtranCellFDD or EUtranCellTDD administers the cell timing
configuration by defining the SFN offset, relative to BFN.
The following tables describe the parameters in MO EUtranCellFDD or
EUtranCellTDD:
Table 6 FDD Only: Synchronization Parameters in MO EUtranCellFDD
Parameter Description
frameStartOffset Offset of frame start of the cell
eDRXAllowed Set to true when eDRX shall be allowed in the cell. False by default.
Please note that eDRX becomes active only when an eDRX S1AP paging is received
and eDRX allowed is true in the cell.
Table 7 TDD Only: Synchronization Parameters in MO EUtranCellTDD
Parameter Description
frameStartOffset Offset of frame start of the cell
eDRXAllowed Set to true when eDRX shall be allowed in the cell. False by default.
Please note that eDRX becomes active only when an eDRX S1AP paging is received
and eDRX allowed is true in the cell.
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20. 4 Performance Management
No significant KPIs are directly associated with the synchronization function.
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