WHITE PAPER

Timing Accuracy for eCPRI Fronthaul and Open RAN
Michael Rupert, Principle System Architect, Infrastructure Mixed-Signal Division

Introduction
Open RAN is an eCPRI-based open fronthaul interface between the radio unit (RU) and distributed unit (DU) [1]. This white paper discusses
timing accuracy for eCPRI fronthaul transport networks.
The eCPRI interface is supported by packet switched fronthaul transport networks. When the fronthaul network is used to synchronize an
eCPRI node, it must provide timing at the user network interface according to the relevant 3GPP timing accuracy category. Time error budgets
for 4 timing accuracy categories are given by the eCPRI Transport Network Requirements Specification [2].
Time synchronization for eCPRI nodes can be accomplished using the precision time protocol (PTP) per IEEE 1588 [3]. The ITU-T maintains a
set of recommendations including G.8271.1 [4] and G.8273.2 [5] that cover PTP based synchronization. The ITU-T recommendations define thecharacteristics of telecom boundary clocks (T-BC) and telecom time slave clocks (T-TSC) that are used for PTP based synchronization; and theydescribe methods to estimate time error accumulation in networks.
The time error characteristics of the clocks within DUs, RUs, and switches and routers have a direct bearing on how many T-BC hops can besupported by the time error budget of a fronthaul network. The greater the number of T-BC hops the lower the number of primary referencetime clocks (PRTC) that will be required and the lower the deployment cost. Further, the greater the number of allowed T-BC hops the greater
the flexibility in the deployment.
Renesas time synchronization hardware and software products meet the tight time error requirements of eCPRI and Open RAN equipment.
The product line includes system synchronizers for DUs, switches and routers used in fronthaul networks, and radio synchronizers for RUs.
Linux oriented PTP software is available for both families of synchronizer products.
Renesas also offers a wide range of jitter attenuators, clock generators, clock synthesizers and clock buffers for wireline and wireless timing
applications.

Acronyms
3GPP

3rd Generation Partnership Project

RU

Radio unit

CPRI

Common Public Radio Interface

TAE

Time alignment error

DU

Distributed unit

T-BC

Telecom boundary clock

FFO

Fractional frequency offset

TETime error

RAN

Radio access network

T-GM

Telecom grand master

PPS

Pulse per second

T-TSC

Telecom time slave clock

PRTC

Primary reference time clock

UNI

User network interface

PTP

Precision time protocol

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Definitions
eCPRI
An interface defined by the CPRI industry cooperation for connecting eCPRI radio equipment control and eCPRI radio equipment via a fronthaultransport network

Maximum time error
The maximum of the absolute values of a set of time errors

Relative time error
The absolute value of the difference between time errors represented at two reference points

Time alignment error
The relative time eror between transmitter antenna ports

Time error
The difference between the time represented at a reference point and the time represented by an agreed common time reference [e.g., theTemps Atomique International (TAI)]Time error budget
The maximum allowed for the relative time error between the input and output of a network or network element

Specifications for Timing Accuracy in Fronthaul Networks
O-RAN.WG4.CUS.0-v07.00
O-RAN.WG4.CUS.0-v07.00 [6] includes the specification of the O-RAN synchronization plane (S-Plane); it is available from the O-RAN Alliance.
The O-RAN specification is eCPRI based [1].

eCPRI Transport Network Requirements Specification
The eCPRI Transport Network Requirements Specification [2] (eCPRI specification) defines the timing accuracy requirements for eCPRI
transport networks.

IEEE 802.1CM™
IEEE 802.1CM™ [7] defines parameters that enable Ethernet bridged networks to transport time sensitive fronthaul streams.
IEEE 802.1CM™ includes time error budget calculations for the eCPRI timing categories and T-TSC deployment cases.

ITU-T G.8271.1
ITU-T G.8271.1 [4] (G.8271.1) defines network limits for time synchronization in packet networks with full timing support from the network.
G.8271.1 describes network reference models and sets limits for time error at various reference points within networks; this recommendation
also defines the synchronization deployment cases. G.8271.1 includes informative appendices that describe simplified, and simulation based
methods to analyze time error accumulation.

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ITU-T G.8273.2
ITU-T G.8273.2 [5] (G.8273.2) defines the time characteristics of T-BCs and T-TSCs for use with full time support from the network.
G.8273.2 specifies the time error characteristics for T-BCs and T-TSCs, class B and class C.

Time Error Budgets for eCPRI Fronthaul Networks
The eCPRI specification sets time error (TE) budgets for the user network interface (UNI) for fronthaul networks so that the time alignment
error (TAE) requirements for four categories of 3GPP features and RANs are met. The eCPRI specification also defines the TE budgets for the
T-TSCs that synchronize the RUs in two time synchronization deployment cases. The TE budget for RUs is not explicitly given, but it can bederived from the eCPRI specification.
This white paper focuses on eCPRI timing accuracy categories A, B and C, and time synchronization deployment cases 1.1 and 1.2 because
these are most relevant to Open RAN applications.

Reference Points for eCPRI Fronthaul Networks
Figure 1 illustrates the |TE| and |TAE| reference points for eCPRI. Some of the reference points are named differently vs. the eCPRI specification
for consistency with other sections of this document1. The synchronization source could be a PRTC+T-GM, or a DU that is directly or remotelysynchronized by a PRTCUNI|TEUNI|

Antenna|TERU|
RU

PRTC

|TAE|
Synchronization
sourceFronthaul
network|TEUNIR|RU

Figure 1: Fronthaul |TE| and |TAE| Reference Points

The terms used in Figure 1 are defined as follows:• TERU refers to the time error budget for an RU• TEUNI refers to the maximum time error for a UNI• TEUNIR refers to the relative time error budget between two UNIs

1

For the purposes of this white paper, the eCPRI radio equipment time error budget is applied to the radio unit.

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Time Synchronization Deployment Cases for eCPRI Fronthaul Networks
Figure 2 shows two fronthaul time synchronization deployment cases; these cases are defined in G.8271.1.

Deployment case 1• The T-TSC is embedded in the RU (end application equipment)• The T-TSC time error budget is included on the RU side of the UNI

Deployment case 2• The T-TSC is external to the RU (end application equipment)• The T-TSC time error budget is included on the fronthaul network side of the UNI

UNI

Antenna
RU

Embedded T-TSC
G.8271.1 deployment
case 1

T-TSC
T-BC

Synchronization
sourceFronthaul
network

RU

T-BC
T-TSC

Measurement port

External T-TSC
G.8271.1 deployment
case 2Time sync. interface

Figure 2: Fronthaul Time Synchronization Deployment Cases

An RU with embedded T-TSC, per deployment case 1, does not need to bear the cost of supporting a time synchronization interface (PPS and
TOD) as required for an RU in deployment case 2.
The RU clock at the transmitter antenna port needs to meet 3GPP |TAE| and 3GPP fractional frequency offset (FFO) requirements. The output ofa T-TSC might require significant additional low-pass filtering to meet 3GPP FFO requirements. For some embedded T-TSCs, the time clock andthe frequency clock might need to be coherent (i.e., from the same source) in which case the T-TSC and additional low-pass filtering functions
might need to be implemented by one integrated clock.
If an integrated T-TSC and RU clock uses a low-pass filter with a bandwidth below 50 mHz then the clock will not behave the same as astandalone T-TSC. Further, integrated implementations might not provide a measurement port to observe T-TSC behavior. This situation is
acknowledged by G.8273.2 App. IV.

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Radio Unit Time Error Budgets for eCPRI Fronthaul Networks
Figure 3 illustrates a cluster of RUs using synchronization deployment case 1. The T-TSC is on the RU side of the UNI so the RU time error
budget from Figure 1 is redefined to include |TET-TSC| as shown.
The terms used in Figure 3 are defined as follows:• TET-TSC refers to the time error budget for a T-TSC• TERU refers to the time error budget for an RU
Time error budgets in the eCPRI network specification accumulate linearly along a synchronization path.
The eCPRI specification describes two sub-cases for deployment case 1.

Deployment case 1.1• Regular T-TSC with |TET-TSC| budget according to G.8273.2 class B (60 ns)

Deployment case 1.2• Enhanced T-TSC with |TET-TSC| budget equal to 15 ns
Using this information and the |TE| and |TAE| budgets from the eCPRI specification, EQ1. can be used to solve for the |TERU| budget as shownin Table 1.
Table 1 can be compared to Table 9-3 in [6].

UNI

Antenna|TET-TSC||TERU|
RU

T-TSC

|TAE|
Synchronization
sourceFronthaul
network|TEUNIR|
RU
T-TSC|TET-TSC||TERU|

Figure 3: |TE| and |TAE| Reference Points for Synchronization Deployment Case 1

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EQ1.

Table 1: Radio Unit Time Error Budget|TAE| [a]|TEUNIR|(Local cluster) [a]|TET-TSC||TERU||TET‑TSC| + |TERU|

1.2

130 ns

60 ns

15 ns [b]

20 ns

35 ns

1.1

260 ns

100 ns

60 ns [c]

20 ns

80 ns

1.2

260 ns

190 ns

15 ns

20 ns

35 ns

1.1

60 ns

20 ns

80 ns

1.2

15 ns

20 ns

35 ns

eCPRI Category

Synchronization
Deployment Case

Category A

1.1

Category B
Category C[d][a] eCPRI network specification, Table 2.
[b] Enhanced T-TSC per eCPRI network specification, Table 2.
[c] Regular T-TSC; G.8273.2 Class B, |cTE| + |dTEL|; the peak-to-peak value of |dTEL| is used.
[d] For Category C, the respective case 1.1 and case 1.2 values are copied from category A / category B.

Timing Accuracy Category C Time Error Budget Examples
For eCPRI category C, the base stations are not necessarily arranged in clusters and the |TAE| requirement is relaxed compared to the othercategories. Figure 4 shows an example of a network that supports eCPRI category C.
The DU is synchronized via network distribution; G.8271.1 reference point C is located at the synchronization input to the DU.
The |TE| terms used in Figure 4 are defined as follows:• G.8271.1 reference point C, the maximum absolute value time error at this point is 1100 ns• TEDU refers to the time error budget for a DU
• TEFN refers to the time error budget for a fronthaul network
The overall |TAE| limit for Figure 4 is 3000 ns according to the eCPRI specification. Assuming the network branches have identical time error
budgets, EQ3. applies and can be used to work out time error budgets, as shown in the examples in Table 2.
Table 2 can be compared to Table 9-3 in [6].

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G.8271.1
referencepoint C

DU-UNI|TEDU|

1100 ns

PRTC
T-GM

RU-UNIDU|TEFN|

Antenna|TET-TSC||TERU|
RU

Fronthaul
network

|TAE|

T-TSC

Midhaul
network

DU

1100 ns

RU
Fronthaul
network

T-TSC|TEFN||TET-TSC||TEDU||TERU|

Figure 4: eCPRI Category C Fronthaul |TE| and |TAE| Reference Points

Time error budgets in the eCPRI network specification accumulate linearly along a synchronization path.

EQ2

EQ3.

Table 2: Timing Accuracy Category C Time Error Budget Examples for Deployment Cases 1.1 and 1.2|TE| for Reference Point C|TEDU||TEFN||TET-TSC| + |TERU|

Notes

Example 11100 ns

225 ns

95 ns

80 ns

95 ns = 190 / 2 ns; like one branch of a fronthaul network
capable of eCPRI category B.
Deployment case 1.1

Example 21100 ns

225 ns140 ns

35 ns

An enhanced T-TSC leaves more budget for the fronthaul network
vs, Example 1.
Deployment case 1.2.

Example 31100 ns

320 ns

0 ns

80 ns

Point-to-point connection between the DU and RU with nointervening T-BC hops.
Deployment case 1.1.

Models for Time Error Accumulation in Fronthaul Networks
The time error budgets in the eCPRI specification accumulate linearly along a synchronization path. Efficient synchronization planning requires
more sophisticated methods to estimate time error accumulation for a chain of T-BCs. To this end, G.8271.1 describes simplified and simulation
models for time error accumulation.

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Simplified Model for Time Error Accumulation
G.8271.1 App. IV presents the simplified model of time error accumulation in synchronization distribution networks. The analysis decomposes
time error into a constant time error component, denoted cTE; and a dynamic time error component, denoted dTE. The analysis further
decomposes dTE into a low-pass filtered component denoted dTEL, and a high pass filtered component denoted dTEH. The low-pass filter andhigh-pass filter cut-off frequencies are both 0.1Hz.
• Constant TE is unaffected by clock filters, and it accumulates linearly, T-BC by T-BC, along a synchronization path.
• Dynamic TEL tends to pass through clock filters, and it accumulates incoherently, T-BC by T-BC, along a synchronization path. Dynamic TEL
accumulates as the square-root of the sum of squares of the individual dTEL contributions. Depending on the network model, dTEL can beassumed to be between two extremes: fully symmetric about cTE; and fully asymmetric about cTE. In the symmetric case, the peak value(dTEL/2) contributes for each T-BC; in the asymmetric case, the peak-to-peak value (dTEL) contributes for each T-BC.
• Dynamic TEH tends to be filtered by each clock in the chain and only the dTEH generated by the last clock in the chain contributes to thefinal TE. Note that dTEH plays a role in determining frequency error for an RU. Dynamic TEH is not considered further in this document.
Figure 5 illustrates a fronthaul synchronization network for a cluster of two RUs connected to a common DU. The common DU contains a T-BC.
The synchronization chain for each branch is composed of an equal number of T-BCs.2• X represents the number of T-BC hops between the DU and the RU per network branch• cTE represents the constant time error for each T-BC
• dTEL represents the dynamic time error, low-pass filtered, for each T-BC
• dTEDURL represents the relative dynamic time error, low-pass filtered, between DU ports• cTEDUR represents the relative constant time error between DU ports• dTEUNIRL represents the dynamic relative time error, low-pass filtered, between UNIs
• cTEUNIR represents the relative constant time error between UNIs
• TEUNIRL represents the maximum relative time error, low-pass filtered, between UNIs
Applying the analysis in G.8271.1 App. IV, one can estimate |TEUNIRL| using EQ4, EQ5 and EQ6.

EQ4

EQ5

EQ6

2

The T-BC numbering in Figure 5 is intended to ease comparison with Open RAN documentation; this numbering is different from that used in G.8271.1 App. II.1.3.

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UNI

1X

RU

T-BC

DU

Synchronization
sourceT-BC|dTEDURL| |dTEUNIRL||cTEDUR | |cTEUNIR ||TEUNIRL|

1X

RU

T-BC

T-BC

Figure 5: Example Fronthaul Synchronization Network

Table 3 shows results of TE accumulation (|TEUNIRL|) calculations for T-BC classes C and B using the simplified model. For both classes the “X”
shown is the maximum number of T-BCs that can be accommodated by the |TEUNIRL| budget while leaving allowance for uncompensated link
asymmetry. In these examples, the T-BC in the DU is assumed to be the same class as the T-BCs in the fronthaul network.
Table 3: TE Budgets Using the Simplified ModeleCPRI
Category

Deployment |TEUNIRL|
BudgetCase

Cat. B

Case 1.1
Case 1.2

100 ns
190 nsClock Class

Supported
Hops (X) [a]|cTEUNIR| [b] [c]|dTEUNIRL| Calculated [d] [e] [f]|TEUNIRL||TEUNIRL|
Unused Budget [g]

C

2

52 ns

17 ns

69 ns

31 ns

B

0

40 ns

14 ns

54 ns

46 ns

C

7

152 ns

23 ns

175 ns

15 ns

B

2

120 ns42 ns

162 ns28 ns[a] “X” represents the number of T-BC hops between the DU and RU per network branch. G.8271.1 App. II.1.3 uses “M” to represent		 the length of a clock chain; M= X+1.
[b] |cTE| is 20 ns for Class B, and 10 ns for Class C.
[c] |cTEDUR| is 40 ns for Class B (G.8271.1 App. XII.5), and 12 ns for Class C.
[d] Using EQ5.
[e] |dTEL| is 20 ns (peak value) for Class B, and 5 ns (peak value) for Class C.
[f]		 |dTEDURL| is 14 ns class C; assume the same for class B.
[g] The unused budget is available for uncompensated link asymmetry in the fronthaul network.
The simplified model assumes dTE noise generated by clocks in a synchronization path is uncorrelated and that the measurement filters are
ideal. These assumptions trade-off accuracy for simplicity.

Simulation Model for Time Error Accumulation
G.8271.1 App. XII.5 presents the results of a simulation model for dTE accumulation. Using simulation based values for |dTEUNIRL|, one canestimate |TEUNIRL| using EQ4 and EQ6.
Table 4 shows results of TE accumulation (|TEUNIRL|) calculations for T-BC classes C and B using the simulation model. For both classes the “X”
shown is the maximum number of T-BCs that can be accommodated by the |TEUNIRL| budget while leaving allowance for uncompensated link
asymmetry. In these examples, the T-BC in the DU is assumed to be the same class as the T-BCs in the fronthaul network.

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For deployment case 1.2 using class C T-BCs, the simulation model allows for one less T-BC hop compared to the simplified model. For
the other scenarios, the number of T-BC hops is the same as the simplified model, although the unused budgets that are available foruncompensated link asymmetry are different.
Table 4 can be compared to Table H-2 in [6].
G.8271.1 App. XII.5 recommends that the simulation model results should be considered together with the simplified model when studying
noise accumulation.
Table 4: TE Budgets Using the Simulation ModeleCPRI
Category

Deployment |TEUNIRL|
BudgetCase

Clock Class

Supported
Hops (X) [a]|cTEUNIR| [b] [c]|dTEUNIRL| Simulated [d]|TEUNIRL||TEUNIRL|
Unused Budget [e]Cat. B

Case 1.1

C

2

52 ns

14 ns

66 ns

34 ns

B

0

40 ns

14 ns

54 ns

46 ns

Case 1.2[a]

100 ns
190 nsC

6

132 ns

28 ns

160 ns

30 ns

B

2

120 ns

37 ns

157 ns

33 ns“X” represents the number of T-BC hops between the DU and RU per network branch. G.8271.1 App. II.1.3 uses “M” to represent the lengthof a clock chain; M= X+1.

[b]|cTE| is 20 ns for Class B, and 10 ns for Class C.

[c]|cTEDUR| is 40 ns for Class B (G.8271.1 App. XII.5), and 12 ns for Class C.

[d]Using simulation data from G.8271.1 App. XII.5.

[e]

The unused budget is available for uncompensated link asymmetry in the fronthaul network.

Conclusion
The time error characteristics of DUs, RUs, and switches and routers have a direct bearing on how many T-BC hops can be supported by thetime error budget of a fronthaul network. The greater the number of T-BC hops a fronthaul network can support the lower the number of PRTCs
that will be required and the lower the deployment cost. Further, the greater the number of allowed T-BC hops the greater the flexibility in the
deployment.
Class C T-BCs are more efficient with time error budgets than Class B T-BCs. As shown in Table 4, both deployment cases 1.1 and 1.2 canbenefit from a larger number of T-BC hops when class C clocks are used.
RUs for deployment case 1.2 use enhanced T-TSCs that consume 90 ns less of the TAE budget than the regular T TSCs used for deployment
case 1.1. The result, as shown in Table 4, is that enhanced T-TSCs allow up to 4 more T-BC class C hops per branch in category B applications
than regular T-TSCs.
Renesas time synchronization hardware and software products support Class C (and better) T-BCs and T-TSCs to meet the tight time error
requirements of eCPRI and Open RAN equipment. The product line includes system synchronizers for DUs, switches and routers used in
fronthaul networks, and radio synchronizers for RUs.
Renesas synchronizer hardware minimizes constant time error and relative constant time error in single or multi-card systems. The devices
include precision time to digital converters to measure internal and external clock skew with picosecond resolution; and they support input-tooutput skew control, also with picosecond resolution.

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The Renesas synchronizer devices minimize dynamic time error with digitally controlled oscillators that enable high precision phase andfrequency adjustments. In multi-card systems they minimize relative dynamic time error by supporting clocks with embedded pulse per second
allowing wideband DPLLs on separate cards to closely track a central synchronizer.
The Renesas synchronizers are supported by Linux kernel drivers and Linux PTP so that G.8273.2 clocks can be implemented using only open
source software. There is an option to license Renesas proprietary PTP Clock Manager software for additional functionality such as packet
delay variation filtering.
Renesas also offers a wide range of jitter attenuators, clock generators, clock synthesizers and clock buffers for wireline and wireless timing
applications.

References
1.

2.
3.
4.
5.
6.
7.

Nokia Corporation, “Nokia readies O-RAN fronthaul with NTT DOCOMO for multi-vendor 5G operation.,” 07 12 2021. [Online].
Available: https://www.nokia.com/about-us/news/releases/2021/12/07/nokia-readies-o-ran-fronthaul-with-ntt-docomo-for-multi-vendor5g-operation/.
CPRI, “Common Public Radio Interface: Requirements for the eCPRI Transport Network, Version 1.2 (Requirements Specification),” 2018.
IEEE, “Std 1588™, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” 2019.
ITU-T, “Recommendation ITU-T G.8271.1, Network limits for time synchronization in packet networks. Amendment 1,” 2020.
ITU-T, “Recommendation ITU-T G.8273.2, Timing characteristics of telecom boundary clocks and telecom time slave clocks,” 2020.
O-RAN Alliance, “O-RAN.WG4.CUS.0-v07.00, Technical Specification, O-RAN Fronthaul Working Group Control, User and Synchronization
Plane Specification,” 2021.
IEEE, “Std 802.1CM™, IEEE Standard for Local and metropolitan area networks - Time-Sensitive Networking for Fronthaul.,” 2018.

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