Monday, July 25, 2011

WiMAX Network Survey:Guideline

In a series on how to plan a city-wide wireless WiMAX network.

Companies or cities that have decided to or are considering building and operating wireless broadband networks have several important issues to consider in terms of how to proceed. Once demographic, competitive, financial, commercial and/or residential market analysis is complete, the operator must conduct a thorough engineering site survey of the area to be served. Once that is completed, the data learned and gathered during the survey will aid in the development of a preliminary network design.

The initial spectrum for WiMAX in the US is unlicensed spectrum in the 5GHz range. Given that this spectrum is 'open to the public', it has inherent interference issues and risks which need to be studied carefully. A quality site survey can provide an invaluable insight into current or potential interference issues. As will be discussed in the WiMAX network design document, there are many effective ways to minimize interference in unlicensed WiMAX networks both from a site selection and equipment selection perspective.

The In-field Site Survey
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 WiMAX network blueprint, or 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 & bolt.

There are many tools, online databases and mapping resources available that were developed specifically for wireless broadband technologies and applications. Much of the pre-survey and post-survey planning can be done with reasonable accuracy 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 a wireless network with which trained and experienced network engineers and administrators are quite often unfamiliar.
These wireless network variables include:
  • The impact of the fresnel zone on signal propagation
  • The difference between passive gain and powered amplification, and when use of each is appropriate
  • The orchestration of sector, space, frequency and polar diversification(s)
  • The use of a Front to Back ratio
  • How many channels are available for use and where are potential interferers located
  • The +/- 3dB rule and how it influences the link budget
  • How and why "application" must drive the hardware
  • Lastly, what to do when the light just won't come on
All these engineering variables apply to site-specific situations, scenarios and topology. Most represent only a fraction of the "issues" that will be encountered, and must be addressed during the deployment of a wireless network.
Quality wireless network engineers will have years of in-the-field experience recognizing and solving these issues. As wireless network planning is concerned, more information equals higher engineering and performance accuracy, which minimizes the total cost of network deployment. The operator's goal is to design a quality network with no surprises at the end.
Some elements of a standard RF site survey:
  • A quality site survey starts several days before the site visit by documenting tower and other attachment sites available. Once these sites have been researched and documented, they should be geo-coded on a topological map to assist in selection of best candidates once on the ground. This prep work and mapping work will also reduce the time on the ground, thus reducing the overall site survey costs.
  • Geo-code and photo-document the target locations (mounting points, cable runs, electrical closets, etc.) This allows the engineering firm to recommend what components are needed in the network, not what is generally or historically needed.

  • Perform and document multi-spectrum (900MHz, 2.4GHz, 4.9 GHz, 5.3 and 5.8GHz) activity by utilizing a spectrum analysis, as appropriate to the application. This allows engineers to work around potential frequency issues before the fact, not onsite during installation with tower rigger crews charging by the hour as they wait for the solution.
  • Spectrum analysis can be accomplished with varying degrees of precision. A scan of a frequency can be done in 10 minutes (with minimal collection of data) or in 10 hours (with considerably more data collection). It's even recommended to sweep both horizontal and vertical polarities, something most firms do not do.
  • Detect and document existing wireless networks including SSID and frequency info.
  • Verify available mounting locations/altitudes which aids in an accurate site parts list.
  • Initial verification and planning for available A/C power helping eliminate onsite power issues.
  • Verify visual LOS to target feeder tower's (potential tower bearings having already been determined pre-survey). This is a redundancy check against engineering software for potential manmade obstructions not represented in the software calculations.
  • Verify available space on recommended towers (expedites the lease process by getting it right the first time) and the type of tower utilized. This information is provided to the tower owners during the lease process, and to the tower riggers and/or equipment vendor to insure that the right kind of mounting brackets are ready before installation.
  • Deploy and evaluate a temporary test link to verify RF functionality for less than certain solutions. For all the recent science and technology, nothing works as dependably as installing and testing a system first.
  • A detailed view of the operator's current or planned backhaul location and documentation of potential hand-off requirements.
  • Identify many more location-specific issues that must be addressed and resolved in the final deployment plan.
  • Identify the general tree foliage environment and document the estimated losses needed to apply to the coverage propagation software. Inputting reasonable losses for foliage will provide a more accurate picture of the overall coverage capability.
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