FOA Guide to Fiber Optics

Topic: Fiber Optics For Wireless

Table of Contents: The FOA Reference Guide

Fiber Optics For Wireless

    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.

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 and the future 5G) 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

To provide the bandwidth needed to the tower or cell site, fiber is being used to connect the towers to the phone systems. The connection to the tower is generally called "backhaul."

Wireless backhaul

   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. 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. This is called "fiber to the antenna" or FTTA. 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.
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.

The New FTTA Architecture - "Fronthaul"
The large amount of activity on wireless means that technology is being developed to make it faster, more available and cheaper. One of the interesting developments is traceable directly to the capabilities of fiber for long links and high bandwidth. It's called "fronthaul" by one of the developers, Nokia (new parent of Lucent.) Fronthaul copies the cabling architecture of small cells and DAS (see below) into the outdoor cell site. It replaces backhaul by moving the electronics (base band unit - BBU) at the cell tower back to a telco office or head end. at the tower, you basically leave an active antenna or passive antenna with remote radio head (RRH) connected with passive fiber. This move of the electronics to a head end saves costs, power, maintenance, etc. 

Wireless fronthaul

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. They only need a single fiber and power to operate. The electronics to control them is in a CO or head end.

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.

Smart Street  Light

Small cells are ideal for metro areas where residents don't like unsightly cell towers. The small cells can be installed in many locations, but this may become one of the most popular. This is a "smart street light" in Los Angeles. It's a product from Phillips which created the LED street light and Ericsson which added the small cells which are inside the larger diameter of the light pole toward the top. Each of these lights require power, of course, and fiber for the small cells. The cable architecture is "fronthaul" as described above.

Distributed Antenna Systems
Wireless traffic is growing at a phenomenal rate. But the majority of calls originate inside buildings so wireless coverage inside buildings has become more important. Cellular wireless often cannot penetrate walls and even windows in large buildings, so low-power cellular antennas are placed inside buildings to provide reliable service. There are several other reasons for cellular systems inside buildings also. Sometimes the number of users inside a building like a convention center or sports facility exceeds the bandwidth of a single cellular system and local laws specify the need for public safety signal coverage.

passive DAS

Passive DASs  are simply repeaters for signals to/form an outside antenna. The electronics amplified the signals and connected remote antennas over large (12-25mm/0.5-1inch) coax cables. Besides the obvious noise problems with all analog systems, the coax cable runs were limited to about 100m (330ft) making this a difficult system to use in large buildings or structures. Passive systems have been used mainly in small to medium sized buildings - ~10,000-200,000 sq ft size but are being replaced by digital systems.

Today’s solution distributes the signal inside the building or facility over fiber as digital signals. (Although there have been some systems that use RF over fiber inside the building, it’s not common anymore.) 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 antennas on the building for smaller facilities. Fiber backhaul enhances service and simplifies installation since coax cabling to the carrier antenna is not necessary.

DAS digital

The block diagram here is generic – practically every manufacturer of DAS systems has different names for the various operational blocks and even some 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.
Active DAS systems are used in larger buildings, up to 1milion sq ft.

More on DAS.

 Wireless In Premises Networks
   Wireless in the corporate premises network is WiFi (IEEE 802.11), the common network built into most laptop or netbook computers, tablets, VoIP phones, many cellular phones and other portable devices. The wireless “antenna” in the network, called an "access point," is a lot more than that. It’s a radio transceiver and network adapter that connects to your laptop to allow access to the network, with some logic that implements part of the network protocols allowing access to the network. The transceiver in the antenna has limited power as does the transceiver in the portable device, so the distance from the antenna to your laptop is limited. Connection between devices and antennas can be affected by metal in a building that reflects or attenuates signals. Signals can even be absorbed by people in the building. A typical office building may need 4-8 antennas per floor to get consistent connections throughout the area. 

WIreless LAN architecture

    The antenna is connected to the network just like a PC, using UTP or fiber optic cable to a local switch which connects it into the network backbone. Not only does the wireless antenna require a network cable to connect to the network, but it needs power – uninterruptible power, just like any network hub or switch – to operate.
    So replacing a wired network with a wireless one doesn’t mean you don’t need cabling; you may in fact need more when you consider the power needs of the antennas.  Any advantage of a wireless network is not in the installation, it’s in the flexibility of users roaming but maintaining connections.
    Bluetooth  (IEEE 802.15) is a limited distance network mainly used for consumer devices. It has been used to connect a wireless printer or mouse to a PC, wireless headsets to cell phones and stereos, cell phones to cars for hands-free operation, digital cameras to printers, etc. In terms of installation, Bluetooth is built-in to many devices and adapters can be plugged into USB ports or added as cards to PCs, not installed as access points like WiFi, so it is not generally of interest for cabling installers.

DAS with structured cabling

The architecture of the cabling for a DAS is no different from an Ethernet LAN or standardized structured cabling. In some cases, DAS systems can be monitored by network management software used for LANs.
More on wireless in premises systems including wireless network standards

Metropolitan Wireless

    Metropolitan wireless systems have had a rocky road. Initially, they were proposed as an inexpensive way of offering broadband to everyone, but providing support and competitive issues with other broadband suppliers ended most of these early trials. Now cities often install WiFi for public service use and free Internet access in parks and plazas, like these examples. The distances these access points are from the network connections require fiber, usually SM fiber available in the metro fiber network used for other communications.

Metro WiFi in Santa Monica

Downtown Santa Monica California, where you can see two separate systems, one public, one private for city services, on one pole.

Istanbul Tureky WiFi

Istanbul, Turkey installed metro WiFi for visitors as the European City of the Year.

(San Diego Broadband)

Several services have offered metro WiFi wireless using line of sight WiFi connections instead of running fiber between buildings. This is possible using both licensed and unlicensed frequencies. The access point or antenna needs an Internet feed which is going to be fiber in most cases. Google bought Webpass for their work in this area. The ISP finds a fiber connected building and negotiates using their connections and rooftop to serve other buildings.

Rural WiFi

Rural wireless works the same way as line of sight metro WiFi. The wireless Internet service provider runs fiber to a location where antennas can service as many users as possible with line of sight connections. The ISP is often in a small town that has good Internet connections and a building tall enough for line of sight to local users. Rural WiFi is widely used in rural areas in California.

Rural WiFi

Rural WiFi antenna on home. (San Diego Broadband)

Proposed But Not Widely Accepted
Two wireless standards have been proposed but have not been widely adopted, WiMAX and Super WiFi.

    WiMAX is a further development of wireless network technology that expands the data capacity of wireless to ~ 100 Mb/s and it’s distance capability to several miles. Unlike WiFi which was primarily a short distance network aimed at private networks, WiMAX appears aimed at communications carriers who could use it in place of landline networks, substituting WiMAX, for example, for Fiber To The Home, in areas needing upgrades of their networks or using it to allow notebook PC or PDA roaming in a metropolitan area.

"Super WiFi" On Fiber

     Bringing broadband to the rural areas of the US or any large country with sparse population can be very expensive. The US has a plan, however, that may make it more affordable. "Super WiFi" is not your usual WiFi. It is using WiFi protocols but broadcasting on frequencies of unused TV channels, called "white space." The FCC is ready to open up new frequencies to broadband to allow delivering broadband Internet and phone to rural areas where cabling is too expensive.

Super WiFi
 An example of a super Wi-Fi antenna: Altai

    Super WiFi works at a lower frequency than either regular WiFi or cellular systems so it has more reach into areas that are too rugged for most wireless systems. Usage in more urban areas may be a problem however, if there are too many broadcast TV stations which can interfere with Super WiFi signals (and vice versa.) These antennas will also require fiber to connect into the network.
More in MIT Technology Review
 And the US Government Announcement.

More On Fiber For Wireless
FTTA- Fiber To The Antenna  
Testing FTTC Fiber  
DAS - Distributed Antenna Systems  
WiFi - Premises Wireless  


(C)2017, The Fiber Optic Association, Inc.