WiMAX is an IP based, wireless broadband access technology that provides performance similar to 802.11/Wi-Fi networks with the coverage and QOS (quality of service) of cellular networks. WiMAX is also an acronym meaning "Worldwide Interoperability for Microwave Access (WiMAX).

WiMAX is a wireless digital communications system, also known as IEEE 802.16, that is intended for wireless "metropolitan area networks". WiMAX can provide broadband wireless access (BWA) up to 30 miles (50 km) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations. In contrast, the WiFi/802.11 wireless local area network standard is limited in most cases to only 100 - 300 feet (30 - 100m).

With WiMAX, WiFi-like data rates are easily supported, but the issue of interference is lessened. WiMAX operates on both licensed and non-licensed frequencies, providing a regulated environment and viable economic model for wireless carriers.

At its heart, however, WiMAX is a standards initiative. Its purpose is to ensure that the broadband wireless radios manufactured for customer use interoperate from vendor to vendor. The primary advantages of the WiMAX standard are to enable the adoption of advanced radio features in a uniform fashion and reduce costs for all of the radios made by companies, who are part of the WiMAX Forum - a standards body formed to ensure interoperability via testing. The more recent Long Term Evolution (LTE) standard is a similar term describing a parallel technology to WiMAX that is being developed by vendors and carriers as a counterpoint to WiMAX.

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 of Electrical 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.

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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

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.

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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.

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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

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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 layersSource: 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.

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