FOA Guide to Fiber Optics


Topic: Fiber To The Antenna For Wireless

Table of Contents: The FOA Reference Guide



FTTA - Fiber To The Antenna

Today's users of mobile devices depend on wireless connections for their voice, data and even video communications. Even homes and businesses may depend on wireless, especially those who are not in urban or suburban areas served by FTTH (fiber to the home) or FTTC (fiber to the curb.) Some of us in the business now use the term FTTW for fiber to wireless, since wireless depends on fiber for the communications backbone and increasingly the connection to the wireless antennas, no matter what kinds of wireless we use.

Wireless is not entirely wireless. The easiest way to understand wireless is to think of it as a link that replaces the cable that connects your cellular or wireless phone to the phone system or the patchcord that connects your computer or other portable Internet device to the network. To understand wireless, it is necessary to look at several different and unique types of wireless systems, including cellular wireless phones, wireless in premises cabling, municipal or private wireless links and even some of the short distance links used for computer peripheral connections.

This FOA page focuses on fiber to the antenna, primarily looking at cell towers, but also antennas mounted on rooftops, small cells and distributed antenna systems (DAS.) Because of its variety, DAS will be covered in a separate page in more detail.

Why fiber to the antenna?

The reason fiber is being used to connect towers and then go up the tower to connect the antennas is consumers insatiable desire for bandwidth. To accommodate more bandwidth in the cellular systems, new cellular protocols being are used (4G, LTE, and whatever comes next) but also more antennas are needed to support more frequencies. Thus cell towers that once had 3 antennas for coverage may have two dozen antennas.

The increased demand for cellular bandwidth to support fast growing data usage from smartphones and tablets requires upgrading towers – more bandwidth means more antennas. More antennas means more cables up the towers. If those cables are coax, it means more weight and wind resistance, perhaps more than the tower was designed for. And RF (radio frequency) signals require lots of power to transmit up the tower since the coax cable attenuates the signals at high frequencies.


Wireless tower connections
Today’s cell towers are being modified to replace older copper coax cables with fiber optic cables to reduce weight and cost. Like other applications of fiber, the small size and light weight allows one fiber cable (which often includes power conductors also) to replace many coax cables. This diagram shows what a current cell tower looks like. The diagram is way too complicated for a quick view so we’ll focus on various areas of the tower to show how fiber is used, then we’ll go into issues of installation and testing.



Cellular Wireless


Cellular phone systems have grown to dominate the telecommunications marketplace. Countries that have had extensive landline phone systems for a century now already have more cell phones than land lines. Countries that had not developed landline-based phone networks skipped them entirely and went directly to cellular wireless where the adoption rates have been extremely high.

While cellular wireless started out as a voice network, text messaging became very popular, eclipsing voice for most users. Smart phones brought the Internet to the phone, and soon data became the largest traffic generator for cellular networks. In the first 3-1/2 years of the iPhone, AT&T claimed their data traffic grew 8000% - 80 times! Now video is coming to these same devices, creating an even faster growth rate for cellular network traffic.

fiber to the wireless  tower


To accommodate this traffic level, wireless needs new systems with more radio frequency spectrum. Current systems (CDMA for some systems, in the US, GSM for the rest of the US and the world) are evolving into new generations of systems (4G, LTE) that have more data bandwidth. Almost from the beginning, cellular towers were connected to the telco networks over fiber optics, just like any other  connection. Wireless towers have small huts at the base that connect to fiber backbones that connect towers to the various phone companies. As traffic grows, towers need more antennas. Instead of 3-4 antennas on a tower, now one sees dozens, so towers and buildings now look like this:

Cellular antennas connected over fiber


or on buildings.

Cellular antennas connected over fiber


All these antennas on a tower or the side of a building have created another problem. In the past, each antenna has been connected by a large (~2", 50mm) coax cable that carries both signal and power to the antenna. But with all these antennas, the size, weight and even wind resistance of these cables has become a big problem, as has the cost. These towers which have been upgraded to add many antennas show the problem with these large coax cables.

Cellular towers connected by fiber optics


This is another application where copper cable is being replaced by optical fiber. One small fiber cable can replace all those coax cables and a separate power cable is used for the drivers on the antennas. These applications use mostly prefab cable assemblies since making terminations on top of the tower is difficult to say the least. Some applications use prefab at the top of the tower and conventional termination at the base. Many of these systems use multimode fiber because the distances are so short and the transceivers are much less expensive for MM fiber.

Below are photos from Corning showing a remote antenna head end and antenna and the fiber terminal serving the antennas. Note the use of a prefab cable system at the top of the tower, making installation much easier. Some installations use a composite cable that includes both fiber and power conductors so only one cable need be installed up the tower.
FTTA
FTTA
Photos courtesy of Corning.


Many cell towers are independently owned and space for antennas is rented to the service providers. Installation of fiber to the towers and fiber up to the antennas is generally done by independent contractors who specialize in this kind of work.


Another Option - Small Cells
 

Small cells go by many names including micro-cells. They are small integrated radios and antennas intended for small geographic areas. They can cover the range of 700MHz to 2.6GHz with power outputs from 1-5W, much less than regular cellular antennas. They are intended to be be mounted on typical urban fixtures – walls, street lights, traffic lights, bus stops, whatever gets them slightly up off the ground.

LightCube
Alcatel-Lucent LightCube Radio Small Cell

Because they cover smaller areas than regular cellular antennas, they will have fewer users connecting, spreading out available bandwidth to increase bandwidth available per user. Most will require only a single SM fiber and DC power making installation easy where municipal cable plants are available. Fiber technology for installations is standard OSP and premises – nothing new required. We understand they can even use PON technology to reduce the electronics near the antenna. You can place several of these small cells in one dome providing extended coverage over many frequencies.


Distributed Antenna Systems  (DAS)

Distributed antenna systems  (DAS) have many uses, from providing coverage in dead zones like inside large office buildings or campuses. They are also used in facilities that have large crowds expecting cellular (and wireless) services where the density of antennas is high but the power requirements are low.

DAS
DAS diagram - typical application

DAS antenna
Indoor DAS Antenna and Remote Radio Unit (KLA Labs)


A DAS has to connect to service providers which is done with fiber backhaul to all the service providers for large facilities and may be done by wireless for smaller facilities. Connections inside the facility will generally be done with singlemode fiber following standard premises cabling installation practices to the remote antenna units (RAU) which can drive several antennas even through coax splitters.

The block diagram here is generic – practically every manufacturer of DAS systems has different names for the various operational blocks and some include unique architectures, even using PONs (passive optical networks) like OLANs and FTTH. But the idea is to get wireless signals to numerous remote antennas over fiber, convert to coax at remote antenna units (RAU) and then distribute to numerous low-power antennas, often multiples through coax splitters, covering small areas.


Fiber To The Tower

In a cell tower, the cellular antennas connect to users' mobile devices, but all those communications devices must connect into the public telephone networks. The older towers connected on copper phone lines just like many landline subscribers still have for their phone connections. However older copper wires do not have adequate bandwidth for modern mobile devices like smartphones or tablets that consume vast amounts of digital data.

Thus most towers are being connected on fiber that offers virtually unlimited bandwidth. In remote areas, where installing fiber optic cables can be expensive, microwave links are often used. The microwave links are more cost effective for remote towers and their limited bandwidth compared to fiber optics may not be as big a problem since remote towers generally have fewer users than those in urban or suburban areas.

tower connections
Connections to the cell tower

The installation of an OSP fiber optic cable is conventional, underground, direct buried or aerial to the tower and terminated at the base using the hardware for the BBU. The crew doing the OSP cable install may be different than the one doing the tower work because the OSP crew may use different equipment and procedures than the FTTA installation.


Connecting To Antennas

Traditional cell towers (below) use copper coax cables. The network feed goes into a base transceiver which drives analog signals up the tower to a masthead amplifier which is connected by a short coax cable to a passive antenna. Coax has high attenuation so the final antenna drive amp is needed to provide adequate signal to drive the antenna. The base transceiver station has interfaces for either a digital telephone network over cable, usually fiber, or a microwave antenna feed.

Coax
Traditional coax link to antennas

Today’s towers are moving to a digital system based on fiber optic cable to a remote radio unit (RRU, sometimes called RRH for remote radio head) that converts the digital signal to analog and drives the passive antenna over copper coax cable. Cables up the tower have fiber and electrical conductors, usually inside an armored jacket. The base band unit (BBU) connects to the telecom network, either by a fiber optic cable or sometimes a microwave antenna.

RRU
Today’s tower diagram-This is the most common system in use now so we will focus on it.

Many systems use a digital communications standard called CPRI - The Common Public Radio Interface –to connect the BBU and RRU. CPRI is an industry cooperation aimed at defining a publicly available specification for the key internal interface of radio base stations between the Radio Equipment Control (REC) and the Radio Equipment (RE). The parties cooperating to define the specification are Ericsson AB, Huawei Technologies Co. Ltd, NEC Corporation, Alcatel-Lucent and Nokia Siemens Networks.

active antenna
Active antenna system

Active antennas are the newest technology
and are beginning to be installed in new systems. The antenna is an active device with the RRU integrated into the antenna. Some towers are already being fitted with antennas like this (2014). With an active antenna one only needs a fiber/copper cable to bring digital signals directly from the BBU and power the antenna.




Typical Tower Diagram

Cables up the tower have fiber and electrical conductors, usually inside an armored jacket. The top of the tower may have a fiber/copper distribution box that connects the RRUs. There are usually 3 RRUs for the 3 antenna systems aimed for full wireless coverage. Some systems use one cable to feed all 3 antennas so a distribution box is used to break out the main cable to 3 cables for each RRU. If 3 cables are used, one for each RRU, the distribution box may not be used. Smaller fiber-only distribution boxes are also available. The RRUs have fiber and copper power inputs from the BBU below and coax going out to the antennas, either one or two cables depending on the antenna. Various suppliers may partition the equipment at the top of the tower differently but the functions generally follow this diagram.

Wireless tower connections
FTTA tower diagram



Cables are often prefab assemblies, terminated in a factory to the proper length and shipped on large spools for safe transport. Typical FTTA cable has both fiber and copper power conductors in one cable, often with armor under the jacket for protection. The fibers may be either singlemode or multimode fiber depending on the electronics. Connectors are usually duplex LC types chosen for their small size. The main issues for installers are to make certain the fiber optic cables are protected and not damaged during installation.


cable
Cable on spool

Cable
This cable has 12 fibers for 6 links plus three power conductors


cable

Here you can see the individual fiber optic cables and copper conductors exiting the cable’s armored jacket and the rugged sealed fitting on the cable for the distribution box. The individual cable breakouts and their duplex fiber optic connectors should be handled carefully to prevent damage or contamination from dirt. Some cables that plug directly into equipment use LCs in a ruggedized sealed housing.


sealed LC


Another alternative for cabling is to use these cables (below) with rugged outlet boxes adapted from aerial cables used for FTTH – fiber to the home. The connections are sealed with o-rings for reliability and mate to duplex patchcords. This alternative is easy to install, lightweight and rugged, but may not include the power which must be handled separately.

drop cables

FTTA cables may have either multimode (MM) or singlemode (SM) fiber depending on the fiber requirements of the BBU/RRU units specified. MM fiber has a larger core than SM so is somewhat more forgiving for cleanliness but both must be handled carefully and and cleaned thoroughly before use.

Installing Cables

As you can see from the cable on the spool above, these cables are not lightweight, especially since they must be pulled up to the top of cell towers which may be as high as 300 feet (100m.) A crane or winch is needed to raise the cable to the top and hold it while it is secured. In this photo (provided by Corning) the cable is being winched up along with a bucket holding the accessories needed to secure and connect it.


pull cable up tower
Pulling cable up the tower (Corning photo)

Installers must use a winch or crane to get the cable up the tower. If the cable is armored, a kellum’s grip can be used to hold it, or a pulling loop may be provided. The cable is secured to the tower with appropriate fasteners.


Tower Hardware

Below is an example of the equipment for a tower as used in the training lab of Wireless Workforce (http://thewirelessworkforce.com). The top distribution box has a patch panel for the fiber to break out to three RRUs. It also includes a copper breakout with lightning surge protection. The lower distribution box also handles both fiber and copper connections but is bigger to allow for storage for excess cable. If the cable is too long, the corrugated jacket can be stripped and the excess fiber stored in the box. Copper conductors can be cut to length. The distribution boxes also have provision for grounding and bonding the corrugated metal armor of the cable.

tower top
Equipment at the top of the tower

top
The top distribution box has a small fiber patch panel to connect patchcords from the RRU to the cable coming up the tower. It also has power connections for the equipment and lighting surge protectors.

bottom
At the bottom of the tower, another distribution box handles both fiber and copper power connections and provides storage for excess fiber optic cable. Patchcords connect from here to the BBU.





Rooftop Installations

Rooftop installations require permission from the building owner and the installation designer will work with the owner on the location of antennas (some do not allow external mounting like this) and negotiate on how the RRUs and BBUs will get installed, as well as how the fiber and power will be routed to the equipment on the roof. It’s not unusual to install cable trays or conduit for the cables on the roof to keep them organized and make them look more attractive.


antennas
Antennas on building

Note the different types of mountings on the antennas on this building, likely from different carriers on the same building.

roof

Rooftop installation (RFS Hybriflex)


It’s hard to generalize about rooftop installations as it depends on the building structure, location of the entrance facility in the building and number of service providers using the building. Most buildings will have the BBU somewhere downstairs in the building near the entrance facility for service providers and fiber running to the RRUs and antennas on the roof. Buildings must have space for entrance facilities, electrical services and cabling for the systems. Some building may mount the antennas on the building, some may have towers on the roof built for the antennas. Cables run in the building will probably have conduits or cable trays for managing all the necessary cables. Most issues will be negotiated with the building owners and managers.


Small Cells

Small cells go by many names including microcells. They are small integrated radios and antennas intended for small geographic areas. They can cover the range of 700MHz to 2.6GHz with power outputs from 1-5W, much less than regular cellular antennas. They are intended to be be mounted on typical urban fixtures – walls, street lights, traffic lights, bus stops, whatever gets them slightly up off the ground. Because they cover smaller areas than regular cellular antennas, they will have fewer users connecting, spreading out available bandwidth to increase bandwidth available per user.

cell
Two small cells in an outdoor housing

Most will require only a single SM fiber and DC power making installation easy where fiber optic cable plants are available. Fiber technology for installations is standard OSP and premises – nothing new required. We understand they can even use PON technology to reduce the electronics near the antenna. You can place several of these small cells in one dome providing extended coverage over many frequencies.


Cabling Options - Prefab Assemblies or Terminate Onsite?

Most FTTA cables will be prefabricated in a factory for the length needed for the tower, so field termination is not necessary. If you need to terminate onsite (hopefully at the ground level only), there are options with prepolished/splice connectors that produce good results – just check the manufacturer’s specifications to see if they are rated for the extremes of temperatures expected in the location of the tower. Fusion splice-on connectors or pigtails may be more reliable for extremes of temperature.

The FOA training curriculum and Guides have information on termination in general and every type of fiber optic termination. See the Table of Contents of the FOA Guide.


Connector Handling And Cleaning

You must never assume that factory-installed connectors are perfect or stay clean. Certainly they should have been perfect when made and tested at the factory, but the factory puts protective caps on the connectors to ship them. We call those caps “dust caps” and, as the joke goes, they are called  “dust caps” because they are usually full of dust. So after you receive the cables, you should first remove the dust caps and inspect the connector ferrule end face for dust and scratches. Then you clean them, inspect to assure yourslef the cleaning was done properly, then test them. Likewise before you insert them into the receptacles to mate with another connector, give them a quick dry cleaning before insertion.
Never touch the end of the connector because the oils on your finger will

Dirt is the #1 enemy of fiber optic connectors because it can cause loss and reflectance, even damage connectors. Inspect every connector before you make a connection with it. Check the connector and the receptacle it will be plugged into as either or both may be dirty.

Here is more information on cleaning and inspecting connectors.


Testing FTTA Cables

When dealing with prefab cables, testing involves careful cleaning and inspection with a microscope, insertion loss testing and in some cases, OTDR testing.

Like any fiber optic cable and especially any prefab cable, the tower cable should not be installed until it has been tested to confirm that the cable is OK. This also includes the patchcords used on the tower. Even short cables can cause major problems if they have been damaged or are not clean.

Testing includes cleaning and inspecting the connectors, checking continuity with a visual fault locator (VFL), then do a loss test with an optical loss test set to determine if all fibers are OK. Recording this data will help in the final test, after the cable has been installed, by comparing losses before and after installation to see if any damage was done during installation.

Remember to always keep protective caps on all the connectors except when cleaning, inspecting or testing.

After installation, the cable needs to be tested again to ensure no damage was done to the cable during installation. Insertion loss testing and perhaps OTDR testing will be required.

More on FTTA testing.






 


(C)2014, The Fiber Optic Association, Inc.