While fibers are mostly being deployed in the backhaul networks, a new approach of building flexible mobile networks is being pushed forward where fiber is also used from the base station to the antenna, which is called fronthaul.

1. D-5143_Rev. A 1
Backhaul and Fronthaul
Technologies
Evolving Technologies for
Seamless Mobile Front and Backhauling
White paper
Edition 2015

2. Ring/ Daisy Chain
Point-to-Point
Small Cells
Macro Cells
Central Office
RRH
OADM/ MUX
BBU
C-BBU
RNC

3. D-5143_Rev. A 3
Backhaul and Fronthaul
Technologies
Introduction
A recent Cisco VNI Global Mobile Data Traffic Forecast reported that in the next five years
there will be 4 billion more mobile-ready devices and connections and the average mobile
connection speed will increase 2.4 fold [1].
The Citrix mobile consumer survey in 2014 revealed that 54% of mobile subscribers would
abandon a slow-loading page in less than 10 seconds. Furthermore, 63% of millennials
were more likely to blame the mobile network for mobile video stalling [2].
With the advent of smart devices, cloud services, newer technologies for fixed and wireless
connectivity and very impatient consumers, there is tremendous pressure to strengthen
the access and mobile backhaul segment of the network. The copper and microwave
connections to the towers can no longer endure the exploding capacity requirements. Fiber
is fast becoming the de-facto solution to meet these demands [3].
This paper provides a primer addressing the current technological developments, for an
economic passive fiber-based backhauling and fronthauling architecture. The various
network elements namely; transceivers, multiplexers, monitoring and protection systems are
discussed in detail clarifying how these technologies overcome the challenges depicted in
Fig. 1.
The paper introduces the possible fronthaul and backhaul layout when looking at centralized
and/or distributed architectures. It delves deeper into fully passive solutions for the remote
sites that reduce latency, space, power and finally cost requirements.
Since fiber convergence to mobile networks is a demanding application with quickly
changing requirements, it is a much debated subject matter for all mobile service operators.
Figure 1
Mobile Front-/Backhaul
Challenges
Fig1
C apac ity Demand
Network R eliability
R es ilience
- Lowest latency
- Improved performance
with high QoS
- Fiber line protection and
monitoring
- Multiple RRU per radio
site
- Multiple sites connected
to same CO/C-BBU
- Fronthaul with backhaul
- Maximize MTBF
- Environmentally hardened
- Passive component at
remote radio sites

4. Centralized Architecture
While fibers are mostly being deployed in the backhaul networks, which interconnects the
baseband unit (BBU) to the core network, a novel approach of building flexible mobile
networks has been pushed forward since a couple of years where fiber is also used from
the base station to the antenna, which is called fronthaul [4].
Traditionally, as depicted in Fig. 2(a), the BBU and the remote radio head (RRH) are
collocated inside a cabinet close to the antenna and a coax cable is used to connect
the RRH to the antenna located at the top of the cell site. With migration to fiber based
connection, the RRH is placed close to the antenna at the top of the cell site and connected
to the BBU as shown in Fig. 2(b). Fiber overcomes any limitation imposed by Coax, such as
distance, weight and energy. Due to the possibility of longer distances, one can design the
fronthaul network with centrally located base station at a central office location as depicted
in Fig. 2(c) equipped with the number of baseband units for several base stations.
(a)
(b)
(c)
Fig 2
Central Office
Macro Cell
Central Office
Macro Cell
Central Office
Macro Cell
Backhaul
Backhaul
Fronthaul
Fig 2
Central Office
Macro Cell
Central Office
Macro Cell
Central Office
Macro Cell
Backhaul
Backhaul
Fronthaul
Fig 2
Central Office
Fronthaul
Macro Cell
Figure 2
Distributed and Centralized
Architectures for
(a) Backhauling
(b) Fronthauling-Backhauling Mix
(c) Fronthauling

5. D-5143_Rev. A 5
Backhaul and Fronthaul
Technologies
The architecture with the stacked BBUs at the central office or hostel to form a centralized
BBU (C-BBU) is referred to as centralized architecture. This architecture aids in easy
maintenance at the single location and provides improved security (no cabinets to break
into) and reduces energy utilization. Furthermore, in LTE networks, the collocation of BBUs
simplifies the X2 interface and also makes the associated latency and synchronization
issues insignificant. The X2 interface provisions information exchange between BBUs for
smooth handover and coordination.
While the connection between each RRH and BBU can be deployed with a dedicated
fiber, the most efficient way would be via the deployment of wavelength multiplexing over a
single fiber. An active wavelength division multiplexing (WDM) solution in fronthaul would
have stringent signal synchronization requirements. Additionally, space and power limitation
would dominate the design of the active system based network.
Passive WDM on the other hand provides low latency solution where colored transceivers
are directly deployed in the RRH to provide the necessary WDM wavelength signal.
Network Offloading via Small-Cells and DAS
Once the mobile service provider (MSP) wants to deepen coverage and improve capacity
in a targeted area, network offloading via distributed antennas systems (DAS) and small
cells is recently gaining popularity as the alternative architecture to macro site expansion.
DAS solution is also important where macro coverage is inferior.
A DAS network has a spatially separated antennas connected individually via fibers to
the basestation or central office where the baseband processing occurs. The advantage
of a DAS system is that it can be shared by multiple operators. Each of the operators
then connect their own basestations to the shared distribution system [5]. A DAS can be
implemented both indoors (iDAS) and outdoors (oDAS).
Small cells as depicted in Fig.3 are fully equipped to performs the processing at the same
location. Most of the small cells today are designed as single frequency band for a single
operator. Similar to macro site back/fronthauling, fiber can be optimized to DAS and small
cells backhauling.
As depicted in Fig. 3, due to low loss and multiplexing facility of fiber, multiple small cells
and macro cells can be linked over several km to the central office while still performing the
processing at the central location.
.
Figure 3
Small Cells and Macro
Cell Backhauling
Fig 3
Central Office
Macro Cell
Small Cells

6. The fiber-based access technology has matured over years with passive optical network
(PON) fiber-to-the-home applications. Though the penetration of fiber to wireless backhaul
or fronthaul network is very recent, the technological maturity is well proven. One must
however note that the optical elements as discussed in the following sections need to fulfill
certain conditions specific to wireless environment to make the fiber-based wireless network
resilient and future-proof.
Different Flavors of Transceivers
Transceivers are the key components of any network. Today grey optics at either 850nm
or 1310nm with data rates up to 10Gb/s are mostly deployed on a simple point-to-point
fronthaul fiber network and the MSP or carriers are increasing capacity per cell site by
stacking more antennas [6]. Each of the antennas with its RRH then requires a dedicated
fiber for fronthaul connection. Additionally, multiple of these sites with numerous RRH are
connected to the C-BBU at the central office. Thus bandwidth demand is currently satisfied
by addition of more fiber strands.
As mobile operators upgrade their networks to 4G LTE with supplementary small cells and
DAS, fronthaul will be ubiquitous and installing new fiber will be deemed too expensive.
Once the fiber reaches a cell site, the capacity can be simply increased by implementing
colored transceivers and multiplexing technology. For passive front/ backhauling, a colored
transceiver is directly connected to the RRH with fitting transceiver at the C-BBU using the
common public radio interface (CPRI) or open base station architecture initiative (OBSAI)
protocol.
The collaboration of Ericsson, Huawei, NEC, Nortel and Siemens in 2003 set CPRI as the
standard for the digitized base station interface between the controller and the RRH [7].
The radio equipment controller (REC) at the central office and RRH need to support at least
one of the CPRI options as depicted in Fig. 4 with the respective line bit rates.
The CPRI line rate includes the information data with signaling and control data. Currently,
CPRI rates are defined up to 10.1376Gb/s. A 9.8Gb/s CPRI line rate can sustain a LTE
carrier bandwidth of 20MHz implemented with an 8x8 MIMO antenna.
Figure 4
(a) SFP+ and XFP Transceivers
(b) CPRI Options
Fig 4
CPRI Options CPRI Line Bit Rate
1 614.4 Mb/s
2 1.2288 Gb/s
(2 x 614.4 Mb/s)
3 2.4576 Gb/s
(4 x 614.4 Mb/s)
4 3.0720 Gb/s
(5 x 614.4 Mb/s)
5 4.9152 Gb/s
(8 x 614.4 Mb/s)
6 6.1440 Gb/s
(10 x 614.4 Mb/s)
7 9.8304 Gb/s
(16 x 614.4 Mb/s)
8 10.1376Gb/s
(20 x 491.52 x 66/64 Mbit/s)

7. D-5143_Rev. A 7
Backhaul and Fronthaul
Technologies
OBSAI came out of the collaboration between Hyundai, LGE, Nokia, Siemens, and ZTE in
2002. Similar to CPRI, OBSAI rates range from 728 Mbps to 6.8 Gb/s. Currently, CPRI has
a higher market penetration than OBSAI. The link length between BBU and RRU with CPRI
can be several tens of km. In densely populated areas or locations where wireless internet
services are inferior, offloading of small cells and DAS can be designed by positioning
additional colored transceiver with Ethernet protocol at the cell and colocation site. Ethernet
speeds range from GbE up to 100Gb/s.
The transceivers at the remote sites however need to be robust and resilient to withstand
extreme environmental conditions, as these locations will not all be environmentally
hardened. Currently, all 2.45Gb/s coarse WDM (CWDM) wavelengths and eight
9.83Gb/s CWDM red lambda (1470nm-1610nm) CPRI transceivers are available
for harsh environments with operating temperature ranging from -40 to +85°C. While
designing a front/backhauling network, it is important to investigate the available form-
factors, wavelengths, power budgets and most importantly if designing a passive network,
the temperature range where the transceivers functions properly.
WDM for Higher Capacity
Multiplexing can be achieved in different dimensions: time, wavelength, modulation/phase,
polarization and also space. A wavelength division multiplexing (WDM) as the name
reflects is the multiplexing of different channel wavelengths into a single fiber and is one of
the best ways to easily expand the network capacity. Depending on the fiber availability,
services and locations of connecting base stations and central office, a fronthaul/backhaul
network can have variety of topologies i.e. ring, bus, daisy chain or point-to-point, use of
single fiber or fiber pairs; all reinforced with multiplexing. An example is depicted in Fig. 5.
Depending on the wavelength or frequency spacing between the neighboring channels,
WDM is subdivided into two major types, dense WDM (DWDM) and coarse WDM
(CWDM). Because of the tightly spaced channels in DWDM and lower linewidth
requirements of DWDM system, more stable lasers like DFB or ECL are essential.
A CWDM system on the other hand uses FP laser, which costs lower than DWDM
Figure 5
WDM Enabled Fronthauling
Fig5
Central Office
Macro Cell
Macro Cell

8. counterparts. The transmit laser might often be used with a temperature controller, to assure
that the central wavelength of the laser does not drift far from the operational bandwidth.
The choice of CWDM or DWDM during the network design depends mainly on distance,
number of channels and data rate required. Additionally, add/drop multiplexers make them
flexible with respect to locations. SK Telecom in South Korean was one of the first operators
to use WDM in fronthaul networks.
CWDM provides up to 16 channels while DWDM can carry 80 wavelengths (in C
band), that can be extended to 80 more channels when considering the L band. With
WDM technology, the capacity increase is limitless. However as cost plays a vital role
in deployments, low cost DWDM technology with outdoor-hardened specification is still
in research. CWDM based fronthauling is much more economically beneficial and as
CWDM transceivers withstand the extreme conditions, are therefore today used in most of
the new fronthaul installations.
Passive WDM is also gaining interest as it requires no power and management at the
remote sites and thus does not generate extra OPEX. WDM can be built in extremely
compact and robust housings as shown in Fig. 5 and is outside plant (OSP) compliant.
Protecting Front/Backhaul Network
Since the revenue per bit for mobile data service is very low, it is imperative to deploy a
solution that is cost effective in both CAPEX and OPEX and that also represents a future
proof technology for seamless communication. While cost can be an important deciding
factor, optical fiber line protection for small cell to macro cell deployments also needs to be
resilient to support harsh environmental conditions and any electrical power outages [8].
Fig. 6 depicts the application of semi-passive OPS in mobile fronthauling application.
This example assumes the use of 16 bi-directional services (CPRI or GbE) for transmission
between the central office (CO) and the RRH node over two fibers. A passive coupler is
deployed at RRH node and an active switch is implemented at the central office. A RRH
node typically has three or more RRHs. In this example three RRHs are considered which
are independently operating in three different wavelengths.
Fig6
Central Office
Figure 6
Optical Link Protection
for Extended Single Fiber
Bi-Di Fronthauling

9. D-5143_Rev. A 9
Backhaul and Fronthaul
Technologies
The appropriate wavelength is achieved by inserting the corresponding transceiver at
the RRH. With the aid of a multiplexer the three different wavelengths in three fibers are
combined into a single fiber. The 3-dB coupler after the multiplexer splits the WDM signal
into two working and backup line. At the central office, the active switch detects the signal
in the working like and switches to the backup line when fault occurs.
Because of the use of purely passive coupler at the RRH end, with such a semi-passive
OPS architecture, no power supply is required at the remote node thus minimizing the
operational cost and making the design resilient to electrical power outages. There is also
a possibility of passive remote monitoring and management via the implementation of a
reflector at the RRH node when desired. This design is appropriate for harsh environment
and extended temperature ranges and can support WDM or Optical Add/Drop (OADM)
integration for point-to-point and ring topologies. The BBU unit at the central office has
network management access to the switch to monitor and user defined threshold values.
Passive Monitoring for Front-/Backhauling
The novel reflector-based real-time fiber monitoring for fronthauling application is
implemented as shown in Fig. 7. This method not only detects the fault but also identifies the
underlying cause aiding the operator to quickly diagnose the problem and reduce truck
rolls. A WDM multiplexer is required at the CO to couple the CPRI/ GbE services with
1625/1650 nm port for OTDR monitoring.
At the remote edge, a reflector with form-factor that of an in-line attenuator is inserted. A filter
integrated in the adapter blocks the 1650 nm wavelength and transmits the wavelengths
from 1260-1618 nm. The reflected 1650 nm OTDR signal traverses back to the OTDR
receiver which then displays the distance where the reflectors are positioned. The reflector
can also be manufactured as a patch cable or a pigtail. The choice of the form factor
depends on the desired specification and the installation requirements.
In practice, the reflectors at different fronthauling branches are not located at the same
distance from the central office [9]. Once installed properly, the reflectors can be used to
diagnose live network without interrupting the services.
The fault is allocated by comparing the reference trace taken during installation from every
branch with the live trace. Because of the high reflectivity (95%) of the reflector, the spatial-
resolution of the OTDR is improved for better performance outlook. The reflector introduces
an insertion loss of maximum 1dB which needs to be calculated during network planning.
Figure 7
Passive Monitoring
for Fronthauling
Fig7
Macro Cell
Small Cells
Central Office

10. The Passive Advantage
With fast increasing mobile fronthaul/ backhaul deployments, it is important to select the
technology that promises a good return on investment. Spectrally efficient passive mobile
fronthauling and backhauling with the use of WDM wavelengths enjoys the following
advantages:
• Increase in capacity per cell site with simple plug-and-play modules.
• Economic advantage with lower CAPEX (< 50% compared to active) and lower OPEX
(support/maintenance, site rental and energy conservation).
• Lower latency improving the maximum allowable distance.
• Requires fewer resources w.r.t. space, energy, cooling with centrally located BBU.
• Robust and resilient for outside plant application.
• Complete transparency to carrier services, i.e., independent of transport, migration and
simple to long term changes.
Conclusion
In tomorrow’s online and interconnected world, we will see an unprecedented growth
in data traffic requiring deployment of novel front/backhaul technologies with improved
performance in harsh environments and with lower power consumption and lower cost.
Addressing these requirements, this paper sheds light on different temperature hardened
colored transceivers, multiplexers in different form factors together with protection planning
and monitoring that are an integral part of network design and planning while expanding
wireless capacity and coverage. There is no single easy solution to every upgrade, but
tackling the technical and business challenges step-by-step makes the back- and fronthaul
evolution more simple and efficient.
About the Author
Susmita Adhikari has a M.Sc. in Digital Communication with over six years of research
and product management experience from long-haul communication to metro. At
HUBER+SUHNER Cube Optics, she works as a Product Placement Manager and ensures
a good visibility of newly introduced products and network solutions via technical marketing
and customer interactions.

11. D-5143_Rev. A 11
Backhaul and Fronthaul
Technologies
Acronym
BBU base band unit
C-BBU centralized BBU
CAPEX capital expenditure
CPRI common public radio interface
CWDM coarse wavelength division multiplexing
DAS distributed antenna system
DWDM dense wavelength division multiplexing
LTE long term evolution
MSP mobile service provider
OBSAI open base station architecture initiative
PON passive optical network
OPEX operational expenditure
OSP outside plant
OTDR optical time domain reflectometry
RE radio equipment
REC radio equipment controller
RRH/U remote radio head/unit
References
[1] Cisco, “Cisco VNI Global Mobile Data Traffic Forecast, 2014 - 2019,” http://www.
cisco.com/c/en/us/solutions/service-provider/visual-networking-index-vni/index.
html~vniforecast 2015.
[2] Citrix, “Mobile Consumer Survey,” https://www.citrix.com/content/dam/citrix/en_
us/documents/oth/citrix-mobile-consumer-survey.pdf 2014.
[3] P. McClusky and J. Schroeder, “Fiber-to-the-antenna: Benefits and protection
requirements,” in Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th
International, pp.1-6, http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6
374502&isnumber=6374450, Sept. 30 2012-Oct. 4 2012.
[4] “Seamless Communiation with Passive Optical Mobile Front and Backhauling”,
HUBER+SUHNER Cube Optics, White Paper, D-5129, 2014.
[5] http://www.thinksmallcell.com/
[6] P. Rigby, “Mobile fronthaul: a new optical opportunity”, Fibre Systems. Issue 6, Winter
2015.
[7] http://www.cpri.info/
[8] “Resilient Semi-Passive Optical Link Failure Protection”, HUBER+SUHNER Cube Optics,
White Paper, D-5129, 2015.
[9] “Real-Time Monitoring of Passive Optical Networks”, HUBER+SUHNER Cube Optics,
White Paper, D-5134, 2015.

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WAIVER
It is exclusively in written agreements that we provide our customers with
warrants and representations as to the technical specifications and/or the
fitness for any particular purpose. The facts and figures contained herein are
carefully compiled to the best of our knowledge, but they are intended for
general informational purposes only.
D-5143 Rev. A