WiMAX:
Considerations from Planning to Deployment

WiFi has set the bar that WiMAX must maintain. In the enterprise, businesses have gone wireless. WiFi networks are almost taken for granted, as gains in productivity, the desire for mobility and the continuing deployment of municipal WiFi networks drives more and more wireless adoption. And, as workers become accustomed to highly available “anytime, anywhere” broadband access in the workplace, they’ve begun to demand the same level of connectivity outside of the office. This demand represents both the market opportunity and challenge for WiMAX. Complicating matters, the adoption curve of WiMAX is likely to be steeper than the adoption curve of WiFi, with applications like voice, video and data quickly driving the need for high performance, low latency and highly secure networks. To support these requirements, WiMAX must be able to deal with two major issues: interference and security.

The Impact of Interference

Fortunately, WiFi has blazed the trail for WiMAX regarding interference management. In WiFi, interference can come from WiFi devices, and also from non-WiFi devices, including popular devices such as cordless phones, Bluetooth devices, wireless video cameras and microwave ovens. When operating in a licensed band, WiMAX enjoys some level of freedom from non-WiMAX interference. However, the issues of shared channel interference, harmonics and unauthorized use mean WiMAX must to deal with interference challenges. While operating in the unlicensed bands, the interference concern with WiMAX is similar to that of WiFi. While a microwave oven won’t transmit at 5GHz, many other devices listed above do. And these devices can have a dramatic affect on performance and range.

So, what are the potential effects of interference and wireless “noise”? In WiFi, interference can cripple the network and paralyze mission-critical applications. A recent study by Farpoint Group indicated that as little as one interferer can drop a connection’s throughput by as much as 20% to 62%, and as that device moves closer to the access point, it throughput reduction up to 100%.

WiMAX has similar issues. Interference ultimately raises the noise floor, decreases signal-to-noise ratio and reduces range and performance. The lessons of large WiFi deployments dictate that interference, both transient and persistent, must be understood and built around for a large-scale network deployment to be successful.

Spectrum Options

The initial spectrum for WiMAX in the United States is either licensed 2.5GHz or unlicensed spectrum in the 5GHz range. Given that the unlicensed spectrum is “open to the public,” it has inherent interference issues and risks that must be considered carefully. A quality site survey can provide invaluable insight into current or potential interference issues. There are many effective ways to minimize interference in unlicensed WiMAX networks, from both site selection and equipment selection perspectives.

A site survey is required, in short, because the operator wants to minimize the "unknowns" regarding the network prior to construction and the operator wants to develop a vision of "how" the network will work once it is built. A site survey provides the necessary information for a complete wireless network design, which is needed in order to minimize hurdles and costly mistakes as the network is being built-out. A quality site survey will be precise to the last frequency, spectrum analysis, connector, nut and bolt.

There are many RF planning tools, online databases and mapping resources available that were developed specifically for wireless network design. With the correct tools and data resources, much of the pre-survey and post-survey planning can be done with reasonable accuracy from hundreds of miles away. There are "predictable" formulas that can be applied to the variables on any network rollout. But unlike copper or fiber networks, there are variables in both an enterprise wireless LAN and a municipal or metro-scale wireless network that even trained and experienced network engineers and administrators can often be unfamiliar with.

Interference is an example. In many network deployments, unexpected interference can adjust accuracy of predictive models significantly. As a result of this, a search that finds persistent hot spots of interference should be undertaken, and the results of these surveys incorporated into planning models.

Preparing For Wireless Networks

So what can be done, and where does a city, service provider or a business with an enterprise environment begin? Planning! Many mistakes and challenges of wireless service delivery can be avoided with solid planning, optimization and project management. Whenever possible, stakeholders should talk to constituents and understand what the network will be used for. Some important questions to ask are:

• Is mobility required? If so, at what speeds?
• What levels of data encryption and encapsulation are necessary?
• What applications will be employed?
• What is the estimated traffic load on the network?
• Will different users, groups and/or applications get separate QoS priority?
• What is the total network capacity?
• How are we going to provision and manage individual users or groups of users?
• Do we need dedicated spectrum, and how are the RF channels managed?

From a network perspective, setting spectrum policies on channel assignment rules, minimum signal-to-noise ratios and the amount of acceptable transient interference all play a vital role in ensuring that networks perform. In addition, spectrum policies make troubleshooting escalation more straightforward as the “required minimums” are plainly documented.

To create a spectrum policy, typically one first conducts a survey to determine the baseline level of interference and general location of interfering devices that must coexist in the workspace or in the community; for example, certain pieces of medical equipment in a hospital or the number of hotspots in deployment grid for a city. This helps IT managers and municipal designers understand where interference hotspots are and/or where they could appear. As such, the policy enables IT managers to build wireless broadband networks that will accommodate applications requiring high reliability. Once this information is obtained, it can be used for comparisons over time to see if the area is out of specifications, and appropriate actions can be taken before service is affected.

Post-Deployment Safeguards

The appropriate architecture and spectrum management scheme will depend largely on the type of applications supported by the wireless broadband network being deployed. For instance, acceptable values of interferer power and channel duty cycles are quite different in a data-only WiFi network vs. a voice-enabled mobile WiFi network. While it might be acceptable for a data-only WiFi network to share its spectrum with cordless phones and Bluetooth devices, spectrum sharing might need to be dramatically reduced in a VoWiFi network.

Once the network is built, even with the most rigorous planning, problems are going to occur in the RF space. The most common will be rogue or interfering devices—whether WiFi, WiMAX or others—and understanding their impact on the RF spectrum is nearly impossible without the proper tools. For example, WiFi managers that have boosted their WiFi network’s bandwidth might be surprised to discover that VoWiFi calls are getting dropped seemingly out of the blue and according to no predictable pattern. When multiple devices are operating in the same environment, and in some cases sharing wireless spectrum, the device giving off the most powerful signal is likely to overwhelm the signal of nearby devices. Six people could be chatting away on VoWiFi calls, and a non-WiFi cordless phone device could pass by and knock all six calls off-line. But chances are, none of the callers, nor the user of the cordless phone device, will likely understand how these devices are interacting and the role interference is suddenly playing in their daily work. Unlicensed WiMAX can easily have the same problems, and even licensed WiMAX is susceptible to the same types of physical layer issues.

A performance policy based on supported services would dictate the threshold for critical spectral parameters. For example, the policy might not allow a channel to be utilized more than 40 percent to support G.729 calls. Another example of how performance requirements could impact spectrum policy could involve the allowable power output per AP or base station for a given network. If more network capacity or better coverage is required, then more base stations are needed. However, more base stations mean more opportunity for interference. It then becomes imperative for the network managers to set the appropriate power policy for base stations in their networks.

Security Concerns

Another important area of consideration is security. In general, questions that need to be asked include:

• How secure does the network need to be? How sensitive is the data?
• Is it important to see all devices coming onto, and off of, the network at all times?
• Are there certain devices (WiFi backdoors) that are absolutely not allowed?
• Is there concern about a Denial of Service attack from malicious parties?

While connectivity issues are more obvious, security is just as important. IT managers may believe that they are managing the security of their wireless networks diligently, but if an employee brings a Bluetooth or Ev-DO-equipped laptop to the office, trouble can ensue. If the laptop is misconfigured, it can provide hackers with a backdoor entrance to the enterprise WiFi or wired network. For instance, the laptop’s Bluetooth port could connect a computer in an office next door to the enterprise network. All wireless networks are subject to attack by jammer devices, so this must be considered for mission critical applications. The bottom line: service providers and enterprise IT managers should require specific methods of authentication and encryption.

Summary

In short, a large-scale wireless deployment is not an endeavor that network managers should handle lightly. With wireless traffic on the rise, not to mention the importance of the applications flowing across the air, broken or ineffective wireless implementations can be catastrophic for enterprises, and the service providers that serve them. All too often, interference can bring the whole thing down. Even more threatening is a wireless network breach when the applications contain sensitive data. Proper planning, including a site survey, is the first and arguably the most important safeguard against these threats. Establishing a spectrum policy and practicing continued interference monitoring is the best way to ensure ongoing success with a wireless infrastructure.

Neil Diener, CTO and Cofounder of Cognio (www.cognio.com), has 20 years of experience in the architecture and design of reliable communication systems and embedded software. At Cognio, he is responsible for leading the company’s technology and product strategy. Prior to becoming CTO, Neil was Director of Software Technology. Before joining Cognio, he held a series of director level positions at companies including Telogy/Motorola, Sun Microsystems, Xerox, and Ask Jeeves. Neil holds a BS in Electrical Engineering from MIT and an MS in Computer Engineering from USC.

Joseph Barnell is a strategy and technology consultant. His clients include wireless hardware vendors as well as service providers and municipalities in the United States and Asia. Previously, he was Director of Business Development for the municipal wireless division of mobilePro, and VP of Operations Planning for Evergreen Open Broadband, a national wholesale wireless broadband company.