1) A Tier 1 mobile network operator conducted a field trial of a passive centralized-RAN (C-RAN) architecture to evaluate performance, costs, and challenges. 2) Initial fiber inspection using EXFO's probe found most connectors were dirty, increasing optical loss. After cleaning, optical time domain reflectometry characterized the fiber span and found a missing connection. 3) Using real-time OTDR and a visual fault locator, technicians identified and corrected the missing connection and mislabeled fiber within the span. Characterization then verified the full fiber path with reduced optical losses.

1. case
study
Tier 1 MNO
C-RAN field trial

2. case
study
© 2017 EXFO Inc. All rights reserved. 2
Context
In 2015 alone, over 1.4 billion smartphones were sold worldwide. With the proliferation
of smartphones all around the world, mobile bandwidth demands are increasing at an
exponential rate. On top of the never ending demand for bandwidth, the fierce competition
between mobile network operators (MNOs) is forcing them to constantly upgrade and
improve their mobile networks.
As more and more mobile cell sites are being installed and commissioned, the need to
reduce operating expenditures to maintain profitability remains a top priority for MNOs.
Cell site location rental and power are two of the most important operational costs for MNOs.
Therefore, MNOs are continually seeking possibilities to reduce these costs by modernizing
their mobile networks. This is where fiber to the antenna (FTTA) comes into play.
The radio access network (RAN) evolution
FTTA
FTTA is the initial phase of the RAN network evolution that began a few years ago in most
regions of the world. Among the many benefits of FTTA, the main advantage is power
savings. In FTTA, the radio and radio frequency (RF) power amplifier are placed very close
to the antenna at the top of the tower (or roof top). Furthermore, copper cables used in legacy
mobile installations are replaced with fiber. In these legacy installations, the power loss in
the copper cables can account for more than 50% of the total power consumption. Today,
with FTTA, this RF power loss is virtually eliminated, providing tremendous cost savings for
MNOs. New communications protocols were created to transport the digital RF signals on
the optical link between the base band unit (BBU) and the remote radio heads (RRH). This
optical link is referred to as fronthaul. The protocols running on the optical links are either
Common Public Radio Interface (CPRI) or Open Base Station Architecture Initiative (OBSAI).
Copper/coax
top to bottom
Cell-site cabinet
D-ROF
BBU
RRU
RRU
RRU
FTTA:
CPRI/OBSAI protocol
Cell-site cabinet
RRH
RRH
CSG
BBU
Tier 1 MNO C-RAN field trial
With the proliferation
of smartphones all
around the world,
mobile bandwidth
demands are
increasing at an
exponential rate.
Figure 1. Traditional coaxial-based systems on cell
towers with large overhead (copper cabling, large
footprint, power, A/C and high power consumption)
Figure 2. Next-generation fiber-based cell tower with
lower overhead (reduced power consumption, fiber
replaces copper cabling, RRH at top of tower, digital
radio over fiber, CPRI/OBSAI protocol)

3. case study
© 2017 EXFO Inc. All rights reserved. 3
Centralized radio access networks (C-RAN)
The next phase in the RAN evolution is C-RAN. In C-RAN, the BBUs are centralized in a
common location such as a central office or data center, providing additional cost savings
for MNOs. This concept is the reach extension of the local fiber network at the cell site.
With C-RAN, the remote sites hosting the RRHs can now be located 15 km away in a small
non-ventilated location, thus greatly reducing site rental cost and power consumption. Since
the BBUs are pooled in one central location owned by the MNO, lower maintenance cost and
increased ease of access are also realized.
RRH
RRH
Fronthaul: CPRI
(up to 15 km)
Central office /
data center
(BBU hotel)
D-RoF
D-RoF
RRH
RRH
RRH
RRH
IP/MPLS
network
Optical
distribution
network
Backhaul
Figure 3. C-RAN architecture (BBU hotel)
There are many C-RAN architectures that are being evaluated in the lab and tested in the field.
The two main categories are active and passive fronthaul C-RAN architectures.
Active fronthaul C-RAN
In the active fronthaul network, the CPRI/OBSAI traffic is encapsulated and transported by
an optical transport network (OTN). Active fronthaul networks usually include an automatic
fiber protection system that provides network scalability. However, these added features
tend to increase the system cost and complexity by requiring traffic engineering to improve
latency and jitter—key metrics that need to be controlled in mobile networks.
Fiber provider
Mobile
operator
Central
office
Active
equipment
Working
Mobile
operator
RRH
RRH
RRH
RRH
Protection
RRH
RRH
Figure 4. Active fronthaul C-RAN
With C-RAN, the
remote sites hosting
the RRHs can now
be located 15 km
away in a small
non-ventilated
location, thus
greatly reducing
site rental cost and
power consumption.

4. case study
© 2017 EXFO Inc. All rights reserved. 4
Passive fronthaul C-RAN
In passive fronthaul networks, the CPRI/OBSAI traffic is transported end-to-end without being
altered. The most common types of passive architectures include:
RRH
RRH
RRH
Central
office
RRH
RRH
RRH
RRH
RRH
RRH
RRH
RRH
Central
office
Single
fiber
Building the foundation—the road to 5G
A major factor driving the development of 5G is the exponential growth for mobile
bandwidth. The expected performance objectives of 5G networks are impressive, as shown
in the diagram.
MISSIONCRITICALSERVICES
INTERNET OF THINGS
Mobile data volume
10 Tbit/s/km2
Energy efficiency
10% of current
consumption
End-to-end
latency
5 ms
Reliability
99.999%
Service deployment time
90 minutes
Number of devices
1 m/km²
Mobility
500 km/h
Peak data rate
10 Gbit/s
25 ms
100 Mbit/s
99.99%
10 Gbit/s/km2
5G
4G
1 k/km290 days
USER EXPERIENCE CONTINUI
TY
Figure 7. 5G performance objectives
Figure 5. Point-to-point passive
fronthaul C-RAN
Figure 6. CWDM or DWDM networks with passive
optical MUX and DEMUX

5. case study
© 2017 EXFO Inc. All rights reserved. 5
5G mobile networks will be designed to support a massive amount of devices. Connected
devices ranging from self-driving cars to traffic safety control and many more will
be supported by 5G mobile networks. Many of these new connected devices will be
mission-critical and require a combination of extreme reliability and ultra-low latency.
These stringent requirements provide a major technical challenge for MNOs. Although there
are still many unknowns regarding the inner workings of 5G, putting in place a solid fiber
network including BBU centralization will be crucial for building tomorrow’s 5G networks.
It is therefore critical for MNOs to properly install and validate these C-RAN architectures
today. Today’s fronthaul interface as well as next generation fronthaul architectures based
on Ethernet will require multiple links at 10 Gbit/s or more, such as 25 Gbit/s, 50 Gbit/s or
even 100 Gbit/s. These higher rates over the fiber infrastructure are designed to support the
demanding capacity and low latency in a “pay as your RAN grows” topology.
Setting the stage—passive C-RAN field trial
The objective of this C-RAN field trial for the Tier 1 MNO RAN team was first, to evaluate the
operational performance and cost savings of the selected C-RAN architecture and secondly,
uncover any potential challenges with the installation and commissioning of this new type
of RAN architecture. Realizing the benefits of building a solid RAN foundation today, this
forward-thinking MNO decided to implement a passive C-RAN field trial.
“We have to consider the evolution of fronthaul for tomorrow’s 5G networks. We don’t know
what the interfaces will be. It could be CPRI with a high bit rate or something completely
different, but we want to be sure that our fronthaul network choice today will be compatible”,
explained a senior manager of the MNO RAN team.
“We consider that our technology choices to achieve this network segment with fiber (based
on passive infrastructure, with optionally low latency active equipment) and microwave are
natively compatible with any future 5G implementation making the solution futureproof”.
To realize this objective, the MNO RAN team worked with EXFO to support them during
this C-RAN field trial installation and validation. We will discuss the different challenges
encountered during the field trial and present the test solutions that facilitated the
troubleshooting and accelerated the installation process.
RRH
8.8
dB
<
Totalpow
er
loss
<
14.7
dB
M
U
X2
dB
<
<3
dB
M
inim
um
2
connectors
0.6
dB
<
<1.4
dB
D
EM
U
X
2
dB
<
<3
dB
M
inim
um
4
connectors
1.2
dB
<
<2.8
dB
(average
0.5
dB
/connector)
Fiber
3
dB
<
<4.5
dB
(0.3
dB
/km
at
1310
nm
)
(0.2
dB
/km
at
1550
nm
)
C
W
D
M
fiber
20
km
15
km
BBU
BBU
BBU
2
dB
<
<3
dB
M
inim
um
2
connectors
0.6
dB
<
<1.4
dB
M
inim
um
2
connectors
BBU
BBU
BBU
B
B
U
hotel
Figure 8. Budget-loss variation in a passive C-RAN installation
Although there are
still many unknowns
regarding the inner
workings of 5G,
putting in place a
solid fiber network
including BBU
centralization will be
crucial for building
tomorrow’s 5G
networks.

6. case study
© 2017 EXFO Inc. All rights reserved. 6
Challenges
The move to FTTA provides many advantages for MNOs but also includes its share of
challenges. The fact that optical fiber is new to many in the wireless industry adds a level
of complexity during the installation and construction of cellular infrastructures. The move
to a passive CWDM C-RAN architecture increases both the benefits but also the installation
and troubleshooting complexities.
Optical challenges
The fiber span between the BBU hotel and the remote RRH location can reach up to 15 km
and contain multiple fiber interconnections. These fiber interconnections, if not properly
cleaned and inspected, can be dirty or damaged, causing high optical loss leading to digital
communication issues, such as bit errors or even optical signal loss. Furthermore, improper
fiber interconnections and fiber mislabeling are often an issue because of the many fibers
present at the junction sites along the fiber span.
Protocol challenges
Both FTTA and C-RAN introduce new optical digital communication protocols between
the BBU and the RRH—CPRI and OBSAI. In many cases, although the fiber inspection and
characterization has been completed, issues at the RRH or the BBU may still be present,
requiring protocol testing for troubleshooting and validation. The most common issues seen
are improperly seated SFPs or SFP fiber connectors in the RRH, dirty or damaged fiber
connectors at the RRH, and SFP mismatch between the BBU and the RRH. In order to detect
and resolve these issues, cell tower technicians must have the right test instruments with
protocol testing capabilities (CPRI or OBSAI) as well as the right method of procedure on
how to test these technologies.
The move to a
passive CWDM
C-RAN architecture
increases both the
benefits but also
the installation and
troubleshooting
complexities.

7. case study
© 2017 EXFO Inc. All rights reserved. 7
Solution 1—fiber connector inspection
Before starting the fiber characterization from the BBU hotel site, EXFO recommended the
inspection of the fiber connectors. During this first step of the C-RAN validation, the team
noticed that most fiber connectors did not pass the automated fiber inspection probe (FIP)
test based on the IEC standard.
Figure 9. Results generated from EXFO’s automated FIP-435B fiber inspection probe
Figure 10. Results generated from EXFO’s automated FIP-435B fiber inspection probe
FIP-400B
wireless fiber
inspection
probe

8. case study
© 2017 EXFO Inc. All rights reserved. 8
With any fiber optic-based network installation, a critical first step is to ensure proper
cleanliness of fiber optic endface connectors. This ensures minimal optical loss along the
fiber path and optimal system performance. This is especially important in passive C-RAN
installations since no optical signal regeneration is performed along the optical path.
Today, RAN installations may be operating at 1.2 Gbit/s or 2.4 Gbit/s but in the near future,
the data rate running on the same optical network will increase to 9.8 Gbit/s, 12.1 Gbit/s or
even 24.3 Gbit/s, providing higher mobile bandwidth. As with any optical communication,
optical impairments such as chromatic dispersion (CD), polarization mode dispersion (PMD)
and intersymbol interference (ISI) become more important as the optical transmission rate
increases. It is therefore critical to minimize optical loss in the optical network by properly
inspecting each interconnection along the fiber span. This will ensure proper operation today
but also guarantees a solid foundation for tomorrow’s higher optical data rates.
Taking into consideration the importance of optimizing the optical network, the MNO RAN
team proceeded to inspect and clean the fiber connectors at the BBU hotel site using EXFO’s
wireless fiber inspection probe (FIP-435B). Verification of the connectors was done quickly
and automatically taking a maximum of 4 seconds per connector, allowing the team to test
multiple fiber connectors within minutes.
Solution 2—fiber network characterization
The second step in this field trial was to characterize the common fiber span between the
BBU hotel and the remote RRH site. According to the installation plans, the MNO RAN team
knew that there was approximately 8 km of fiber between these two sites. The team needed
to use an optical time domain reflectometer (OTDR) measurement tool to ensure that the
entire 8 km fiber span had no optical issues such as high connector loss, macrobends or
even incorrect interconnections.
From the BBU hotel site, the team connected EXFO’s FTB-720G V2 to the common link
and performed the OTDR test to characterize the fiber span. The results obtained from the
OTDR test indicated a missing fiber interconnection. By using the FTB-720G V2 OTDR test,
the missing interconnection was identified at exactly 102 meters from the BBU hotel site,
as shown in the figure below.
Figure 11. First OTDR test result—fiber span only 102 meters
With any fiber
optic-based network
installation, a critical
first step is to ensure
proper cleanliness of
fiber optic end-face
connectors.
This ensures
minimal optical loss
along the fiber path
and optimal system
performance.

9. case study
© 2017 EXFO Inc. All rights reserved. 9
After further investigation, the MNO RAN team was able to determine that a fiber mislabeling
was the root cause of the missing fiber interconnection. Using EXFO’s FTB-720G V2 with
the visual fault locator (VFL), which is a highly visible laser light source, the team was able
to identify the correct fiber missing in the interconnection.
Figure 12. Fiber interconnection room
Based on the fiber installation plans, the MNO RAN team was able to determine the
geographic location of the far-end missing fiber interconnection (at 2 km). Operating
EXFO’s test solution in real-time OTDR mode, the team was able to provide live feedback to
the technician at the far-end interconnection site (at 2 km). In real-time OTDR mode, fiber
characterization is continuously performed and allows the user to see instant changes in
the fiber span. In this situation, once the correct fiber interconnection was completed by
the technician, the real-time OTDR display instantly went from 2 to 8 km, displaying the full
fiber span.
Figure 13. EXFO real-time OTDR

10. case study
© 2017 EXFO Inc. All rights reserved. 10
Once the second missing fiber interconnection was found and resolved, the fiber span in
each direction was fully characterized. Furthermore, the MNO RAN team reduced their testing
time by 50% by using EXFO’s iLoop feature. Using iLoop and placing a loopback fiber at the
far-end RRH site between the two fibers, the team was able to test the two fibers (transmit
and receive) with one single OTDR test.
Technician 2
Loop fiber
Technician 1
20 m loop
56 m
Tx Rx
Figure 15. FTTA testing with the FTB-1 Pro platform and iOLM application
BA
Pos. –0.1580 0.0000 0.0562 0.0763 0.1325 0.2876 km
km0.1580 0.0562 0.0201 0.0562 0.1551Len.
Original measurement
Automatically separates
the two fibers for individual results
LAUNCH RECEIVE
FIBER 1 FIBER 2LOOP
BA
Pos. –0.1580 0.0000 0.0562 0.0763 0.1325 0.2876 km
km0.1580 0.0562 0.0201 0.0562 0.1551Len.
LAUNCH RECEIVE
FIBER 1 FIBER 2LOOP LOOP
Split
iLoop
Figure 16. iLoop application splits the iOLM results into two individual links—one for each fiber
The iLoop software application splits up the total fiber span (2 x 8 km: 16 km) and
displays each fiber separately, thus speeding up the testing process and simplifying test
result interpretation.

11. case study
© 2017 EXFO Inc. All rights reserved. 11
The importance of OTDR testing in fronthaul networks
In working with the MNO RAN team during this field trial, the consensus was that C-RAN
architectures provide many advantages. However, they also add complexities to the fronthaul
optical network. The reality today is that the mobile architectures are evolving to C-RAN.
As such, the fiber spans are reaching longer distances (10-15 km) with multiple fiber
interconnections, as seen in this field trial.
Throughout the years, EXFO has seen that OTDR testing is very valuable during the
installation and commissioning of D-RAN cell sites (distributed RAN—where BBUs and RRHs
are colocated). Although the fiber spans are much shorter (20-100 m), quickly finding fiber
issues in D-RAN cell site deployments increases delivery speed and quality. In addition,
C-RAN, with their added complexities, significantly increase the importance of using OTDR
test and measurement tools for RAN installation and commissioning.
OTDRs have been used for many years to characterize and troubleshoot fiber networks in
order to quickly and efficiently find impairments along the fiber span. During this C-RAN
field trial, OTDR testing proved to be instrumental in troubleshooting and validating the fiber
network between the BBU hotel and the remote RRH site.
Nevertheless, OTDRs can be complex to configure even for optical experts. Various settings such
as pulse widths, pulse durations and fiber span settings can be difficult to properly configure.
Additionally, OTDR trace results can be very complex to read and interpret even for OTDR experts.
Figure 17. Traditional OTDR test results
Many technicians in the RAN industry have extensive RF expertise but are new to the optical
field. It is therefore important to consider test and measurement tools that are easy to use
for engineers and technicians at all skill levels.
While on site, the MNO RAN team used EXFO’s intelligent Optical Link Mapper (iOLM) to
characterize the fiber spans. EXFO’s iOLM is an innovative OTDR-based application that uses
multipulse acquisitions and advanced algorithms to deliver detailed information on every
element on the fiber link. By using the iOLM, it removed the complexity of OTDR configuration
and eliminated OTDR trace interpretation guesswork. The team easily understood the fiber
impairments detected and the steps they needed to take in order to quickly resolve these
issues. The figure below shows an iOLM test result with various optical events along the
fiber span. Users can easily see issues such as high optical loss with fiber connectors,
macrobends or even splices.
Figure 18. EXFO’s iOLM intuitive test result
EXFO’s iOLM is
an innovative
OTDR-based
application that
uses multipulse
acquisitions
and advanced
algorithms to deliver
detailed information
on every element on
the fiber link.

12. case study
© 2017 EXFO Inc. All rights reserved. 12
Type of fault Diagnostic Solving the issue
Bad
connector
The connector or
bulkhead is damaged,
dirty or not well
connected
Inspect and clean as needed
Macrobend Excessive fiber bend Inspect the fiber in this area for excessive
bending. Use of a visual fault locator (VFL)
could help identify the exact location of the
macrobend.
Bad splice Excessive loss of a
non-reflective fault
Inspect the splice at this location, and
resplice if needed. Use of a VFL could help
identify the exact location of a bad splice.
Table 1. iOLM diagnostic examples
RAN optical interface validation
Once the issues of the common optical link were resolved and validated, the next step was
to validate the optical interfaces at the RAN equipment (BBUs and RRHs). Through various
other customer field trials, EXFO has documented a complete list of issues found when
installing and commissioning RAN network equipment. The table below describes the most
common issues observed:
Issues Description
BBU/RRH SFP mismatch Mismatch in terms of SFP wavelength or SFP type
(multimode single mode, high/low rate)
Improperly seated SFP in
the RRH
SFP cage inside the RRH is often recessed, which can make the
SFP insertion difficult
Broken fiber jumper cable
at the RRH
Damage or cracked LC connectors at the RRH commonly seen in
cell tower installations
LC fiber connectors not fully
inserted in the RRH’s SFP
SFP inside the RRH is often recessed for weather proofing
purposes, which increases difficulties in LC fiber connector insertion
SFP installed in the wrong
optical port of the RRH
RRHs have a primary and secondary optical interface port.
The primary port must be used to interconnect with the BBU.
The secondary port is used for daisy chaining to another RRH and
cannot be used to interconnect directly to a BBU.
Table 2. Most common BBU and RRH optical interface issues
These issues, if not discovered during the installation phase, will require a service call that
may cost anywhere between $2,000 to $5,000, excluding delays in the commissioning
process. It is therefore critical for the MNOs to ensure that these issues are avoided or
resolved quickly during the installation phase.
For this trial, the MNO team validated the optical interfaces at the BBUs and the RRHs to
ensure proper RAN equipment configuration and operation.

13. case study
© 2017 EXFO Inc. All rights reserved. 13
Solution 3—BBU optical interface validation
During this field trial, the team wanted to validate the optical interfaces of the base stations.
This meant validating that the optical interfaces of the BBUs, and the SFPs in the BBUs,
were operational at the expected CPRI rate. By configuring EXFO’s CPRI Test application in
RRH emulation mode, the MNO RAN team was able to validate these optical interfaces on
the BBUs.
The EXFO CPRI test application performed a CPRI validation test and within seconds,
provided valuable information to the team regarding the actual CPRI configured rate on
the 3G BBU. CPRI test results indicated that the configured rate of the optical ports was
1.2 Gbit/s (CPRI rate option 2) as opposed to the expected rate of 2.4 Gbit/s (CPRI rate
option 3). A lower configured BBU CPRI rate would result in lower mobile bandwidth to
subscribers. The benefit of knowing this information allowed the team to potentially save a
considerable amount of time from troubleshooting other root causes.
Next, testing the 4G BBU validated that the CPRI optical interfaces were operational and
configured at 2.4 Gbit/s (CPRI rate option 3) as expected.
CPRI test application
indicated that 3G BBU
configured CPRI rate
was 1.2 Gbit/s, not
2.4 Gbit/s
CPRI test
to validate
BBU optical
interface
operation and
configured rate
Figure 19. BBU optical interface validation
Solution 4—RRH optical interface validation at the RRH site
At the RRH site, the next step was to validate the RRH optical interfaces. The team wanted
to ensure that the RRHs and the SFP (inside the RRH) were fully operational.
By configuring EXFO’s CPRI test application in BBU emulation mode, the team proceeded
to validate the 3G RRH. A CPRI link up could not be achieved regardless of the CPRI rate
selected. The team noticed that the actual SFP optical transceiver was inserted into the
wrong SFP port of the RRH.
The EXFO CPRI
test application
performed a CPRI
validation test and
within seconds,
provided valuable
information to the
team regarding
the actual CPRI
configured rate on
the 3G BBU.

14. case study
© 2017 EXFO Inc. All rights reserved. 14
RRHs normally have two optical ports, one primary and one secondary. The primary port
is used for optical communication with the BBU and the secondary port is used for optical
communication to a second RRH for use in a daisy chain configuration. In this situation, the
SFP was inserted in the secondary port. This is a very common issue seen during all basic
RRH equipment installations. The SFP ports on the RRHs are quite often not clearly labeled,
which makes it difficult for RAN field technicians to know which port to use.
Once the SFP optical transceiver was inserted in the correct port, the primary port, the CPRI
validation test was performed successfully on the 3G RRH at 1.2 Gbit/s. The 4G RRH CPRI
validation test was performed successfully without any issues (at 2.4 Gbit/s).
3G RRH CPRI ports
are located at the
bottom of the RRH
Wrong CPRI port
(daisy chain port)
3G RRH
Wrong CPRI port—no CPRI link
with EXFO CPRI test app
Good CPRI port
3G RRH
Good CPRI port—validation
test operational at 1.2 Gbit/s
Figure 20. Field trial images—3G RRH optical interface validation test
“Without a CPRI validation test at the RRH site, it would have been difficult to quickly identify
the incorrectly used RRH SFP port, causing our RAN team to potentially spend many hours
troubleshooting the root cause of the issue,” explained a manager from the Tier 1 MNO.
“Also, other very common issues seen at the RRH are SFP mismatch and inversion of the
two fiber cables connected in the transmitter (TX) and receiver (RX) of the SFP,” explained a
senior manager from the Tier 1 MNO RAN team.
These issues, if not found at the RRH site with a CPRI protocol tester, can cause increased
deployment delays and cost for the mobile network operators.
Solution 5—RRH optical interface validation from the BBU
hotel location
In a C-RAN architecture, performing a CPRI validation test from the BBU hotel site to the RRH
location is the final validation step in the installation process. This test will confirm proper
CPRI link connectivity and operation to the far-end RRH, which may be 10-15 km away. This
test will ensure that the complete fiber span, the fronthaul transport equipment and the
RRH are fully operational. Additionally, a CPRI round-trip delay (RTD) measurement can be
performed to confirm that the complete link delay is within 200 µsec (round-trip) to avoid
any potential communication issues.
The SFP ports on the
RRHs are quite often
not clearly labeled,
which makes it
difficult for RAN field
technicians to know
which port to use.

15. case study
© 2017 EXFO Inc. All rights reserved. 15
As highlighted in the previous section, comprehensive testing of the RRH and antenna
system can also be included in these tests to simulate operational conditions.
By emulating the BBU using the EXFO CPRI test application, final tests were performed and
confirmed that the 3G and 4G CPRI communication links were fully operational from the
BBU hotel site all the way to the far-end RRHs. Additionally, CPRI RTD measurements were
performed, which indicated that the complete link delay from the BBU hotel to the far-end
RRHs was 79 µsec—well below the upper threshold of 200 µsec.
RRH
CPRI transport system
(CWDM/DWDM/OTN)
Up to 15 kmUp to 15 km
FTB-700G V2 Series
Figure 21. CPRI round-trip delay (RTD) measurement

16. case study
© 2017 EXFO Inc. All rights reserved. 16
Solution 6—RRH and antenna RF tests using BBU emulation
Testing the optical link to the RRH, as performed in the previous step, ensures that the CPRI
interface of the RRH is working correctly. However, additional tests, such as BBU emulation,
are required to validate that the RF components of the RRH, as well as the co-axial cable and
antenna system, have been installed and are functioning as desired. RRH and antenna tests
are used to bring the RRH up to an operational state so that RF signals can be generated and
received. By configuring the test to run at the CPRI link rate and RF frequencies to be used
during normal use, the test simulates real-life operation of the RRH. As part of the test, the
received frequency band can be checked to ensure the absence of any abnormal signal, such as
RF interference, which, if present, can be cleared before the site is integrated into the network.
Normally, line sweep tests would be performed on the co-axial cable and antenna system
connected to the RRH. Unfortunately, it is no longer possible to perform these functions
without climbing the tower, where the co-axial cable connects to the RRH. However, EXFO’s
RRH antenna RF test solutions use the RRH to perform these tests over the CPRI link enabling
voltage standing wave ratio (VSWR), return loss and PIM detection measurements to be
performed at the bottom of the tower.
Performing a test during cell site commissioning allows archived results to be compared
with future tests performed when abnormal operation of the site is detected, highlighting
certain differences that are potential areas of concern.
Tests are initially conducted at the RRH location so that issues can be fixed locally. However,
these tests can also be performed from the BBU hotel location to produce a comprehensive
test of the entire fronthaul network, from BBU to antenna site, located many kilometers away.
Optical transceiver
EXFO's
BBU emulation
application
Optical transceiver
application
Antenna
Coax jumper
Base
junction box
Remote radio unit
Gamma sector
Alpha sector
Beta sector
Fiber jumper
Junction box
Fiber cable
Optical
transceiver
BBU emulation
Figure 22. RRH validation

17. case study
© 2017 EXFO Inc. All rights reserved. 17
Solution 7—Interference hunting and CPRI link monitoring
Once the mobile network has been commissioned and is operational, attention turns to
supporting and maintaining the cell sites. Key performance indicators (KPIs) are used to
monitor how the mobile network is performing. When KPIs indicate a problem associated
with the fronthaul network, these are usually associated with RF interference or problems
on the CPRI link.
EXFO’s OpticalRF™ can be used to examine the uplink or downlink RF waveforms and to
detect issues on the CPRI link. These tests can be performed unobtrusively while the site
is fully operational and without disruption of the mobile network. In C-RAN installations,
these tests can be performed at the BBU hotel, avoiding the time and costs associated with
traveling to the far-end cell site. As well, the ability to access the test equipment remotely
allows maintenance personnel to view the results without being local to the test system.
CPRI link issues such as low optical power, bit errors or CPRI protocol flags can be isolated
allowing the course of corrective action to be determined. The solutions described in the
earlier sections can then be used to determine the exact location and component responsible
for the problem. Once repaired, the fix can be validated, as described previously.
If interference is the cause of the problem, then the EXFO test solution can be used to monitor
the RF waveform over the CPRI link. As well, using a USB-enabled RF spectrum analyzer
connected to the EXFO test solution allows for the monitoring of the over-the-air RF signal.
If interference is found to be external to the site, the USB-enabled module can be detached
and connected to a standard Windows-based device, which can in turn connect remotely to
the EXFO test system. This provides interference hunters with the mobility to track down the
source of the interference while simultaneously monitoring the digital RF waveform over the
CPRI link. Once found, the suspected source of interference can be minimized or eliminated
and immediately verified on the digital CPRI link using EXFO’s test solution.
Antenna
Coax jumper
Base
junction box
Remote radio unit
Fiber jumper
Junction box
Fiber cable
FTB-700G V2 Series
Up to 15 km away
Optical splitter
EXFO's
OpticalRF™
application
FTB-700G V2 Series
+ or
Up to 15 km away
Optical splitterOptical splitter
FTB-700G V2 Series
Figure 23. Interference hunting with USB-enabled over-the-air spectrum analyzer
Passive fronthaul
validation and
maintenance steps
1. Fiber connector inspection
(with FIP)
2. Common fiber link
characterization
(with iOLM)
3. BBU CPRI optical link
validation (at BBU site)
4. RRH CPRI optical link
validation (at RRH site)
5. Comprehensive CPRI, RRH
and antenna test (from
BBU hotel site)
6. RRH and antenna RF tests
(at RRH site)
7. Interference hunting and
CPRI link monitoring

18. case study
EXFO serves over 2000 customers in more than 100 countries.
To find your local office contact details, please go to www.EXFO.com/contact.
© 2017 EXFO Inc. All rights reserved. 18
Field trial summary
Fronthaul C-RAN architectures provide many benefits to MNOs. Along with adding more
bandwidth and coverage to their mobile subscribers, deploying a C-RAN architecture allows
MNOs to lower their operational expenses. Since the BBUs are pooled in one central location,
lower maintenance cost can be achieved as well as increased ease of access to the BBUs.
Additionally, remote sites hosting the RRHs can be located many kilometers away in small
non-ventilated locations, thus greatly reducing site rental cost and power consumption.
For MNOs that want to deliver bandwidth and coverage today and tomorrow with 5G on the
horizon, the move to C-RAN is a necessary step. However, as seen in this field trial, fronthaul
C-RAN comes with its share of challenges. It is imperative to note that the issues observed
during this field trial are commonly seen during optical and fronthaul network installations.
Without the right test and measurement solution, field technicians could spend hours, or
even days, troubleshooting many of the common optical fiber and RAN network installation
issues. Throughout this field trial, the MNO RAN team utilized all the functionalities of EXFO’s
FTB-720G V2 solution. Having the right tools and applications such as the automated fiber
inspection probe, iOLM, VFL, real-time OTDR, CPRI test application, OpticalRF™ and BBU
emulation, greatly improved efficiency when it came to troubleshooting and validating the
complete fronthaul network.
“Without EXFO’s fronthaul test solution, our RAN team would have spent hours, even days,
troubleshooting the issues we encountered during this field trial,” stated the senior manager
from the MNO RAN team.
“The wide range of test functionalities included in EXFO’s FTB-720G V2 is the perfect test
solution for all RAN contractors and cell technicians looking to quickly and efficiently deploy
C-RAN.”
Without the right test
and measurement
tools, field
technicians could
spend hours or even
days troubleshooting
many of the
common optical fiber
and RAN network
installation issues.
CSTUDY060.2EN 17/08