Wi max tutorial

Mr. Sandeep Saxena (For Academic purpose only)

WiMAX Tutorial

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

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

http://www.ieee.org/

http://www.wimaxforum.org/


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

http://www.wimax.com/pdf/wimax-tutorial/wimax-antennas.pdf


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.

Interference

Wireless services have been around for a century. There is always the potential for interference and the service provider must engineer accordingly.


Figure 21: Interference occurs in a number of forms and interference mitigation is a matter of good engineering

Source: IEEE

Countering interference is a matter of understanding it and engineering accordingly. Interference occurs naturally in the electromagnetic spectrum.

Figure 21 above shows out-of-channel interference, which comes from other transmitters that are not on the same frequency as the primary radio. A co-

channel interfering transmission occurs on the same frequency as the desired signal. Step One in interference mitigation is to avoid co-channel

interference through thorough frequency planning, use of licensed spectrum, and dynamic frequency selection. Step Two is to pay close attention to the

link budget on the wireless network and plan power and spectrum to overcome interfering signals. Much of the im-pact of out-of-channel interference

can be avoided using such technologies as OFDM, OFDMA, and a host of antenna technologies.

Solutions to Interference - OFDM

Figure 22: Multipath and intersymbol interference occur with all wireless transmissions

OFDM mitigates interference by breaking the signal into subcarriers. The loss of the data on a small percentage of the subdivided signal does not


degrade the reception of the received signal.

Figure 23: OFDM and OFDMA mitigate interference by breaking the signal into mul-tiple subcarriers

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

Unsolicited Grant Service

(UGS) VoIP

-Jitter tolerance

-Maximum latency tolerance

-Maximum sustained rate

Real-time Packet Services

(rtPS) Streaming Audio/Video

-Traffic priority


-Maximum reserved rate


Extended real time Packet

Services (ertPS)

VoIP (VoIP with

Activity Detection

-Traffic priority

-Jitter tolerance




Non-real time Packet

Services (nrtPS) FTP

-Traffic priority



Best Effort (BE) Data transfer, web

browsing

-Traffic priority


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.

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