Wimax

BY : SAYED QAISAR SHAH

What is WiMAX?
WiMAX has the potential to replace a number of existing telecommunications infrastructures. In
a fixed wireless configuration it can replace the telephone company's copper wire networks, the
cable TV's coaxial cable infrastructure while offering Internet Service Provider (ISP) services. In
its mobile variant, WiMAX has the potential to replace cellular networks. How do we get there?

Figure 1 WiMAX has the potential to impact all forms of telecommunications

What is WiMAX or Worldwide Interoperability for Microwave Access? WiMAX is an Institute
ofElectrical and Electronics Engineers (IEEE) standard designated 802.16-2004 (fixed wireless
applications) and 802.16e-2005 (mobile wire-less). The industry trade group WiMAX Forum has
defined WiMAX as a "last mile" broadband wireless access (BWA) alternative to cable modem
service, telephone company Digital Subscriber Line (DSL) or T1/E1 service.

Fixed WiMAX

Figure 2 Fixed WiMAX offers cost effective point to point and point to multi-point solutions

What makes WiMAX so exciting is the broad range of applications it makes possible but not
limited to broadband internet access, T1/E1 substitute for businesses, voice over Internet
protocol (VoIP) as telephone company substitute, Internet Protocol Television (IPTV) as cable
TV substitute, backhaul for Wi-Fi hotspots and cell phone towers, mobile telephone service,
mobile data TV, mobile emergency response services, wireless backhaul as substitute for fiber
optic cable.

WiMAX provides fixed, portable or mobile non-line-of sight service from a base station to a
subscriber station, also known as customer premise equipment (CPE). Some goals for WiMAX
include a radius of service coverage of 6 miles from a WiMAX base station for point-to-
multipoint, non-line-of-sight (see following pages for illustrations and definitions) service. This
service should deliver approximately 40 megabits per second (Mbps) for fixed and portable
access applications. That WiMAX cell site should offer enough bandwidth to support hundreds
of businesses with T1 speeds and thousands of residential customers with the equivalent of
DSL services from one base station.

Mobile WiMAX

Figure 3 Mobile WiMAX allows any telecommunications to go mobile

Mobile WiMAX takes the fixed wireless application a step further and enables cell phone-like
applications on a much larger scale. For example, mobile WiMAX enables streaming video to
be broadcast from a speeding police or other emergency vehicle at over 70 MPH. It potentially
replaces cell phones and mobile data offerings from cell phone operators such as EvDo, EvDv
and HSDPA. In addition to being the final leg in a quadruple play, it offers superior building
penetration and improved security measures over fixed WiMAX. Mobile WiMAX will be very
valuable for emerging services such as mobile TV and gaming.

WiMAX is not Wi-Fi

Figure 4 Where Wi-Fi covers an office or coffee shop, WiMAX covers a city

One of the most often heard descriptions of WiMAX in the press is that it is "Wi-Fi on steroids".
In truth, it is considerably more than that. Not only does WiMAX offer exponentially greater
range and throughput than Wi-Fi (technically speaking 802.11b, although new variants of
802.11 offer substantial improvements over the "b" variant of 802.11), it also offers carrier grade
quality of service (QoS) and security. Wi-Fi has been notorious for its lack of security. The "b"
variant of 802.11 offered no prioritization of traffic making it less than ideal for voice or video.
The limited range and throughput of Wi-Fi means that a Wi-Fi service provider must deploy
multiple access points in order to cover the same area and service the same number of
customers as one WiMAX base station (note the differences in nomenclature). The IEEE
802.11 Working group has since approved upgrades for 802.11 security and QoS.

Converged voice and data easy as FM radio?

Figure 5 With WiMAX, converged voice and data can be as easy as FM radio

Visualize turning on an FM radio in your office. You receive information (news, weather, sports) from that service (the
FM radio station) and hardware (the FM radio with attached antenna). WiMAX can be described as being somewhat
similar. In place of a radio station there is a base station (radio and antenna that transmits information (internet
access, VoIP, IPTV) and the subscriber has a WiMAX CPE that receives the services. The major difference is that
with WiMAX the service is two-way or interactive.

Figure 6 WiMAX indoor CPE, courtesy Motorola

Wireless Architectures
The following section will provide a simple overview of wireless concepts and nomenclature to
help the reader understand how WiMAX works and will assist the reader in com-municating with
the WiMAX industry.

Wireless architecture: point-to-point and point-to-multipoint

There are two scenarios for a wireless deployment: point-to-point and point-to-multipoint.

Figure 7: Point-to point and point-to-multipoint configurations

Point-to-point (P2P)

Point to point is used where there are two points of interest: one sender and one receiver. This
is also a scenario for backhaul or the transport from the data source (data center, co-lo facility,
fiber POP, Central Office, etc) to the subscriber or for a point for distribution using point to
multipoint architecture. Backhaul radios comprise an industry of their own within the wireless
industry. As the architecture calls for a highly focused beam between two points range and
throughput of point-to point radios will be higher than that of point-to-multipoint products.

Point-to-Multipoint (PMP)

As seen in the figure above, point-to-multipoint is synonymous with distribution. One base
station can service hundreds of dissimilar subscribers in terms of bandwidth and services
offered.

Line of sight (LOS) or Non-line of sight (NLOS)?

Figure 8: The difference between line of sight and non-line of sight

Earlier wireless technologies (LMDS, MMDS for example) were unsuccessful in the mass
market as they could not deliver services in non-line-of-sight scenarios. This limited the number
of subscribers they could reach and, given the high cost of base stations and CPE, those
business plans failed. WiMAX functions best in line of sight situations and, unlike those earlier
technologies, offers acceptable range and throughput to subscribers who are not line of sight to
the base station. Buildings between the base station and the subscriber diminish the range and
throughput, but in an urban environment, the signal will still be strong enough to deliver
adequate service. Given WiMAX's ability to deliver services non-line-of-sight, the WiMAX
service provider can reach many customers in high-rise office buildings to achieve a low cost
per subscriber because so many subscribers can be reached from one base station.

WiMAX Radios

At the core of WiMAX is the WiMAX radio. A radio contains both a transmitter (sends) and a
receiver (receives). It generates electrical oscillations at a frequency known as the carrier
frequency (in WiMAX that is usually between 2 and 11 GHz). A radio might be thought of as a
networking device similar to a router or a bridge in that it is managed by software and is
composed of circuit boards containing very complex chip sets.

WiMAX architecture, very simply put, is built upon two components: radios and antennas. Most
WiMAX products offer a base station radio separate from the antenna. Conversely, many CPE
devices are also two piece solutions with an antenna on the outside of the building and
subscriber station indoors as illustrated in the figure below.

Figure 9: Most WiMAX solutions use radios separate from antennas

The chief advantage of this is that the radio is protected from extremes of heat cold and
humidity all of which detract from the radio's performance and durability. In addition, having the
antenna outdoors optimizes the link budget (performance of the wireless connection) between
transmitter and receiver especially in line of sight scenarios. The antenna is connected to
WiMAX radio via a cable known as a "pigtail". One simple rule for wireless installations: keep
the pigtail as short as possible. Why? The longer the pigtail the more signal is lost between the
antenna and the radio. The popular LMR-400 cable, for example will lose about 1 dB
(pronounced "dee-bee" for decibel, a measure of signal strength) for every 10 feet of cable.
Very simply put, if an antenna is placed at the top of a 20-story building and the radio in the
wiring closet on the ground floor, one may lose all signal in the cable.

Radios and Enclosures

Figure 10: WiMAX performance can be optimized by placing the radio in a weather resistant or
weatherproof enclosure near the antenna

Radio placement

The photo above shows the WiMAX radio deployed in an enclosure. Note from left to right: a)
copper grounding cable on the inside of the enclosure b) Ethernet connection to the data source
c) Heliax "pigtail" to the antenna (Heliax is a heavy duty, lightning resistant cable) d) 110v power
via an APC UPS (note black box in top right hand corner of enclosure.

What are some strategies to ensure the antenna can be as high as possible to take advan-tage
of line-of-sight topologies where ever possible while keeping the pigtail as short as possible?
One approach is to co-locate the radio on or near the roof with the antenna in an enclosure.
Considerations for enclosures include: a) security and b) weather resistance-how hot or cold
can your radio gets and still function?

Sheet metal or fiberglass enclosures with a lock provide security. Next, it is necessary to
determine how well suited the radio is for local atmospherics (hot or cold). Most Wi-MAX radios
are rated as operating between -20 degrees Fahrenheit to 120 degrees F at the upper end. If
you will be operating in locations that will exceed those parameters you need an enclosure that
will shield your radio form those extremes. As the radio will generate its own heat, surrounding
it with insulation will ensure the temperature of the radio will not suffer from sub-zero
temperatures.

WiMAX Antennas

Figure 11: Different antenna types are designed for different applications

WiMAX antennas, just like the antennas for car radio, cell phone, FM radio, or TV, are designed
to optimize performance for a given application. The figure above illustrates the three main
types of antennas used in WiMAX deployments. From top to bottom are an omni directional,
sector and panel antenna each has a specific function.

Omni directional antenna

Figure 12: An omni-directional antenna broadcasts 360 degrees from the base station

Omni directional antennas are used for point-to-multipoint configurations. The main drawback
to an omni directional antenna is that its energy is greatly diffused in broad-casting 360
degrees. This limits its range and ultimately signal strength. Omni directional antennas are
good for situations where there are a lot of subscribers located very close to the base station.
An example of omni directional application is a WiFi hotspot where the range is less than 100
meters and subscribers are concentrated in a small area.

Sector antennas

Figure 13: Sector antennas are focused on smaller sectors

A sector antenna, by focusing the beam in a more focused area, offers greater range and
throughput with less energy. Many operators will use sector antennas to cover a 360-degree

service area rather than use an omni directional antenna due to the superior per-formance of
sector antennas over an omni directional antenna.

Panel antennas

Figure 14: Panel antennas are most often used for point-to-point applications

Panel antennas are usually a flat panel of about one foot square. They can also be a
configuration where potentially the WiMAX radio is contained in the square antenna enclosure.
Such configurations are powered via the Ethernet cable that connects the ra-dio/antenna
combination to the wider network. That power source is known as Power over Ethernet (PoE).
This streamlines deployments as there is no need to house the radio in a separate,
weatherproof enclosure if outdoors or in a wiring closet if indoors. This configuration can also
be very handy for relays.

Subscriber Stations

The technical term for customer premise equipment (CPE) is subscriber station. The generally accepted
marketing terms now focus on either "indoor CPE" or "outdoor CPE". There are advantages and
disadvantages to both deployment schemes as described below.

Outdoor CPE

Figure 15: An outdoor CPE device

Outdoor CPE, very simply put, offers somewhat better performance over indoor CPE given that WiMAX
reception is not impeded by walls of concrete or brick, RF blocking glass or steel in the building's walls.
In many cases the subscriber may wish to utilize an outdoor CPE in order to maximize reception via a
line of sight connection to the base sta-tion not possible with indoor CPE. Outdoor CPE will cost more
than indoor CPE due to a number of factors including extra measures necessary to make outdoor CPE
weather re-sistant.

Indoor CPE

Figure 16: Indoor WiMAX CPE, courtesy Motorola

The most significant advantage of indoor over outdoor CPE is that it is installed by the subscriber. This
frees the service provider from the expense of "truck roll" or installation. In addition, it can be sold online
or in a retail facility thus sparing the service provider a trip to the customer site. Indoor CPE also allows a
certain instant gratification for the subscriber in that there is no wait time for installation by the service
provider. Currently, many telephone companies require a one month wait between placement of order
and in-stallation of T1 or E1 services. In addition, an instant delivery of service is very appeal-ing to the
business subscriber in the event of a network outage by the incumbent service provider.

Site Survey
Before any equipment is deployed, there must be a site survey to determine what is needed in order to
have a successful wireless operation. By understanding the dynamics of the market where the
deployment will take place and planning accordingly, the service provider can ensure success on Day
One of operations.

Link Budget

Figure 17: The link budget determines the success or failure of a wireless operation

The figure above illustrates a link budget. It is the equation of the power of a signal transmitted minus
detractions between the transmitter and receiver (rain, interference from other broadcasters, vegetation,
gain at the antennas ate either end) and what signal is received at the receiver.

Frequency Plan

Part of the site survey process is to determine a viable frequency plan. The wireless op-erator must make
maximum use of limited spectrum assets. How does one do that?

Figure 18: By reusing frequencies at different base stations, a WiMAX operator can avoid interference
from their own network

The diagram above illustrates how a wireless operator (cellular, WiMAX, etc) uses their limited spectrum
allocation to deliver the best service possible while avoiding interfer-ence between their base stations.
Note there are nine different base stations with three different frequencies but no similarly shaded circle
touches another. If they did touch, there would be interference between base stations because they
would be operating on the same frequency.

Its about windows, not roof tops

Traditional wireless thinking dictated that a radio and its associated antenna should be at the highest
point possible with a line of sight to a majority of the service area (note mountain tops and the Empire
State Building). This is not necessarily so with WiMAX. As indoor subscriber units mature, the value of
antenna placement is not necessarily in height above subscribers, but in achieving as short and direct a
line of sight possible be-tween base station and subscriber's CPE.

Figure 19: Imagine each window or floor paying $500 per month in WiMAX services

Objections to WiMAX

A discussion of WiMAX is not complete without taking on objections to the technology. Before any one
can sell a high technology product, they must first sell the customer on the technology.

Figure 20: Objections to WiMAX are best understood via the provisions built into the WiMAX Physical and
MAC layers
Source: IEEE

Technology sales people invariably encounter objections to the technology they are selling. The primary
objections to WiMAX are:

1. Interference: Won't interference from other broadcasters degrade the quality of the WiMAX service?

2. Quality of Service (QoS): Wireless is inherently unstable so how can it offer voice and video services?

3. Security: Is WiMAX secure? Can anything wireless be secure?

4. Reliability: Nothing can be as reliable as the telephone company's service (rumored to offer "five 9s"
of reliability or 5 minutes of downtime per year).

The answers to those objections are best understood via the Physical (known as the PHY, pronounced
"fi") and Medium Access Control (MAC pronounced "mac") Layers. The WiMAX Working Group no
doubt were aware of these objections based on experiences with earlier wireless technologies (Wi-Fi,
LMDS, MMDS, CDMA, GSM) and have engineered WiMAX to fix failures of past wireless technologies.

Antenna Technologies & Interference

Adaptive Antenna System (AAS)

Figure 24: By utilizing AAS and beam steering technologies, WiMAX overcomes interference while
boosting range and throughput

Adaptive Antenna Systems (AAS) use beam-forming technologies to focus the wireless beam between
the base station and the subscriber. This reduces the possibility of interference from other broadcasters
as the beam runs straight between the two points.

Dynamic Frequency Selection, MIMO, and Software Defined Radios

Figure 25: Dynamic Frequency Selection enables a radio to shift frequencies when interference is present

One of the simplest remedies to interference is to simply change frequencies to avoid the frequency
where interference occurs. Dynamic frequency selection (DFS) does just that. A DFS radio sniffs the

airwaves to determine where interference does not occur and selects the open frequency to avoid the
frequencies where interference occurs.

Multiple in and multiple out (MIMO) antenna systems work on the same principle. With multiple
transmitters and receivers built into the antenna, the transmitter and receiver can coordinate to move to
an open frequency if/when interference occurs.

Software defined radios (SDR) use the same strategy to avoid interference. As they are software and not
hardware defined, they have the flexibility to dynamically shift frequencies to move away from a
congested frequency to an open channel.

Quality of Service

Quality of Service (QoS) is what determines if a wireless technology can successfully deliver high value
services such as voice and video. The chief detractors from good QoS are latency, jitter and packet loss.
Solve these issue and you have a carrier-grade service. Very simply put, WiMAX offers a very low
latency across the wireless span. Most ven-dors have products where latency is less than 10
milliseconds from base station to CPE (and vice versa). To put this in perspective, latency must be
measured end-to-end. VoIP, for example, is highly susceptible to latency. If latency exceeds 150
milliseconds for ex-ample, the quality of the conversation begins to drag. At or above 200 milliseconds
many listeners may find a conversation unintelligible.

In the case of WiMAX, the large majority of latency will not occur on the air link be-tween subscriber and
base station but rather on the wired portion of the connection between the subscriber and what ever the
"other end" might be (web site server, IPTV server or VoIP called party). The figure below illustrates how
any latency on the wireless portion of a network is minimal relative to that on the wired portion of a
network.

Figure 26: Over-the-air latency in a WiMAX network is minimal relative to the latency on the IP backbone
or the rest of the network

Prioritizing Traffic

The chief solution in offering good QoS is to prioritize time sensitive traffic such as VoIP and video. Fixed
WiMAX offers 4 categories for the prioritization of traffic and mobile WiMAX has 5 categories:

Service Class Applications QOS Specifications
-Jitter tolerance
Unsolicited Grant Service (UGS) VoIP -Maximum latency tolerance
-Maximum sustained rate
-Traffic priority
-Maximum latency tolerance
Real-time Packet Services (rtPS) Streaming Audio/Video
-Maximum reserved rate
-Traffic priority
-Jitter tolerance
Extended real time Packet VoIP (VoIP with Activity
-Maximum latency tolerance
Services (ertPS) Detection
-Maximum reserved rate
-Traffic priority
Non-real time Packet Services
FTP -Maximum reserved rate
(nrtPS)
-Traffic priority
Best Effort (BE) Data transfer, web browsing

Early Wi-Fi offered no prioritization of traffic and the technology has not gone beyond the wireless local
area network (WLAN) stage. WiMAX is different in that, in the case of fixed WiMAX, there are four
categories of traffic prioritized per their needs in delivery with VoIP and video at the top and web surfing
at the bottom. Mobile WiMAX offers 5 such prioritized categories with VoIP being top priority.

OFDM & Dynamic Bandwidth Allocation

Good QoS

Figure 27: WiMAX coding and modulation schemes ensure steady signal strength over distance by

decreasing throughput over range to deliver the best QoS possible

An old wisdom in the networking world goes "Bandwidth is the answer, now what was the question?"
WiMAX offers a pair of mechanisms that ensure good QoS. First, the coding and modulation schemes
(64-QAM/16-QAM/QPSK) ensure a steady signal strength over increasing distance. Secondly, Dynamic
Bandwidth Allocation (DBA) is a mechanism that monitors the network and, when interference or other
detractions to sig-nal strength occur, the base station allocates more bandwidth and power for the
afflicted stream.

Spectral Efficiency

Figure 28: Beam width is a measure of a product's spectral efficiency

Spectral efficiency is the measure of the width of the signal's beam through the air. It is also the measure
of the WiMAX radio's scalability. In mobile WiMAX, for example, commonly used beam widths range from
1.25 MHz to 20 MHz. Efficiency of the product is determined by how much bandwidth (measured in
megabits per second in this case) can be transported over how little beam width (MHz in this case).
Spectral efficiency is especially important in cases where a service provider is paying a high price for
spectrum (example: 40 MHz at 2.5 GHz). With high spectral efficiency, the service provider can service
more customers at a lower cost per subscriber for the spectrum in use.

WiMAX Security

Figure 29: WiMAX offers state of the art security via authentication and strong encryption

Security in WiMAX is set in the Privacy Sublayer in the MAC Layer. Per their respective specifications,
fixed WiMAX (802.16-2004) uses X.509 certificates for authentication and 56-bit Digital Encryption
System (DES) for encryption of the data stream. Mobile WiMAX (802.16e-2005) uses EAP for
authentication and Advanced Encryption System (AES, also used by the US government) for encryption.
Vendors may use variants of these. Some vendors offer 152-bit AES, which is rumored to take millions of
years to crack with a consumer grade PC. Both variants use Privacy Key Management (PKM) for
authentication between base station and subscriber station. While Wi-Fi may have suf-fered a bad
reputation for security given early problems in the industry, WiMAX offers strong security measures to
thwart a wide variety of security threats.

WiMAX Reliability

Figure 30: Telephone wires and cable TV cables represent a single point of fail-ure in their networks.
Hurricanes and high winds can cause serious outages.

Some supporters of the telephone network say it offers 99.999% reliability or that it is down 5 minutes per
year. That may be true of the switches in the Central Office, but is not true of the telephone network as a
whole. The copper wires coming to the home or office, for example, represent a single point of failure
(that is, there is no back-up if the wire or fiber optic cable breaks or is cut). Businesses using the
telephone company should ask themselves two questions:

1. What does it cost us per hour to be down?
2. What back up, if any, do we have if the telephone line is cut or broken?

WiMAX service providers have no wires or cables that can be cut and can offer 99.999% of reliability by
using redundant radios to cover a given market. Use of licensed spectrum ensures that only one service
provider is broadcasting on a given frequency. Finally, radios with high quality chips have a mean time
between failure (MTBF) of 40 or more years. If nothing else, businesses should consider WiMAX as a
cost effective disaster recovery solution. Note: a backhoe operator cannot cut a WiMAX wireless
connection to the home or office.

WiMAX - Reference Network Model

The IEEE 802.16e-2005 standard provides the air interface for WiMAX but does not define
the full end-to-end WiMAX network. The WiMAX Forum's Network Working Group (NWG), is
responsible for developing the end-to-end network requirements, architecture, and
protocols for WiMAX, using IEEE 802.16e-2005 as the air interface.

The WiMAX NWG has developed a network reference model to serve as an architecture
framework for WiMAX deployments and to ensure interoperability among various WiMAX
equipment and operators.

The network reference model envisions a unified network architecture for supporting fixed,
nomadic, and mobile deployments and is based on an IP service model. Below is simplified
illustration of an IP-based WiMAX network architecture. The overall network may be logically
divided into three parts:

1. Mobile Stations (MS) used by the end user to access the network.
2. The access service network (ASN), which comprises one or more base stations and
one or more ASN gateways that form the radio access network at the edge.
3. Connectivity service network (CSN), which provides IP connectivity and all the IP
core network functions.

The network reference model developed by the WiMAX Forum NWG defines a number of
functional entities and interfaces between those entities. Fig below shows some of the more
important functional entities.

Base station (BS): The BS is responsible for providing the air interface to the MS.
Additional functions that may be part of the BS are micromobility management
functions, such as handoff triggering and tunnel establishment, radio resource
management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host
Control Protocol) proxy, key management, session management, and multicast
group management.
Access service network gateway (ASN-GW): The ASN gateway typically acts as
a layer 2 traffic aggregation point within an ASN. Additional functions that may be
part of the ASN gateway include intra-ASN location management and paging, radio
resource management and admission control, caching of subscriber profiles and
encryption keys, AAA client functionality, establishment and management of mobility
tunnel with base stations, QoS and policy enforcement, foreign agent functionality
for mobile IP, and routing to the selected CSN.
Connectivity service network (CSN): The CSN provides connectivity to the
Internet, ASP, other public networks, and corporate networks. The CSN is owned by
the NSP and includes AAA servers that support authentication for the devices, users,
and specific services. The CSN also provides per user policy management of QoS and
security. The CSN is also responsible for IP address management, support for
roaming between different NSPs, location management between ASNs, and mobility
and roaming between ASNs.

The WiMAX architecture framework allows for the flexible decomposition and/or combination
of functional entities when building the physical entities. For example, the ASN may be
decomposed into base station transceivers (BST), base station controllers (BSC), and an
ASNGW analogous to the GSM model of BTS, BSC, and Serving GPRS Support Node
(SGSN).

Overview
I. What is WiMAX?
- Fixed WiMAX
- Mobile WiMAX
- WiMAX is not Wi-Fi
- Converged voice and data easy as FM radio?

II. Wireless 101
- Simple Wireless Architecture
- Radios and Antennas
- Subscriber Stations
- Site Survey

III. Objections to WiMAX
- Interference
- Antenna Technologies and Interference
- Good Quality of Service
- WiMAX Security
- WiMAX Reliability

Wimax

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