Tips for Accurate Wi-Fi Predictive Site Surveys

Predictive Site Survey Software
What Are Predictive Site Surveys
Predictive site surveys use computer-based software programs to model the facility and RF environment. These programs allow you to outline the required coverage areas using facility blueprints; define facility structures to estimate RF signal attenuation; establish thresholds for minimum signal strength and application throughput that clients must achieve; predict the quantity, location, and type of access points that should be installed; and provide channel and power settings that maximize spectral capacity while minimizing co-channel and adjacent-channel interference (CCI/ACI).

The goal of a predictive site survey is to establish design criteria, such as AP quantity and placement with relative confidence. Doing so will then reduce the amount of effort required to perform the typically labor-intensive pre-deployment site survey (“AP-on-a-stick”). One of the major benefits of predictive modeling is the ability to quickly simulate various deployment scenarios and to narrow design alternatives. A predictive site survey will never be 100% accurate, and though it might not replace pre-deployment or post-deployment site surveys, it can help expedite them. When you perform adequate requirements gathering and planning, predictive site surveys can result in a Wi-Fi network design that meets requirements with a high level of confidence. It also provides validation of the forecasted AP capacity from the requirements gathering process and allows you to make adjustments by taking unique facility characteristics into consideration.

There are numerous software-based and web-based predictive site survey applications to choose from:
Predictive site surveys should follow proper requirements gathering and capacity forecasting to meet all design goals for coverage and capacity. You can read more about those topics in my High-Density Wi-Fi Design Series (including three videos: Forecasting AP Capacity, RF Design, and WLAN Configuration Best Practices).

Tips for Accurate Predictive Site Surveys
Follow these guidelines for a successful predictive site surveys:

Blueprint Calibration
Use blueprints that are accurately scaled representations of the facilities or coverage areas in which the WLAN will be installed. Calibrate (or “scale”) blueprints in the modeling software so they represent the correct physical dimensions of the coverage area. If necessary use a known distance between two reference points that are fairly far apart from each other to scale the blueprint as accurately as possible. A small error when defining a short distance between two points on the map results in a more serious error in scale when applied to the entire map than would a small error when defining a large distance. For example, scale the blueprint using the width of the building rather than the width of a cubicle, door, or ceiling tile. 

When importing electronic or scanned blueprints into predictive modeling site survey software, be sure to enter the drawing dimensions accurately. These programs often ask for the dimensions of the entire drawing, not the building or floor plan. The drawings dimensions must take into account any extra whitespace or outdoor areas surrounding a building.

Access Point Equipment Selection
Select the appropriate access point models, antennas, and accessories to use in various areas based on facility characteristics and desired coverage patterns. Document the access point models, mounting and external antenna orientation (if applicable) that have been selected to provide the proper coverage patterns and to minimize co-channel interference.

Transmit Power Levels
Determine appropriate access point power levels based on client device capabilities, which you learned during the requirements gathering process. Configure the access point power output similar to that of the client devices to provide successful bidirectional communication with minimal co-channel interference. Also consider differences in receive sensitivity between APs and client devices, if known, which might allow APs to transmit at slightly higher power than clients.

Access Point Capacity
Estimate the peak client device density in each coverage area to ensure that the planned AP capacity is sufficient to process the client and application throughput load on the WLAN. Use the forecasted AP capacity from the requirements gathering process as a starting point for the number of access points required in the environment or in individual coverage areas if requirements significantly differ between areas.

To learn about the requirements gathering process and how to forecast capacity, read my High-Density Wi-Fi Design Guide (PDF). You may also be interested in watching this short video on forecasting capacity that covers the same information.

Start with a foundation of dual-radio access points to provide a base layer of coverage and capacity for each area. If you need additional capacity, use the techniques described in the “Facility Characteristics” section of my high-density guide to supplement the base WLAN deployment. These techniques include deploying APs with directional antennas, deploying additional APs on 5 GHz, or installing additional APs in locations where RF obstructions will limit signal propagation to augment capacity in specific areas.

Access Point Placement
Determine the proper placement of access points to provide sufficient RF signal strength and coverage overlap between APs. The coverage overlap enables client devices to maintain high data rates throughout the environment and roam effectively. Base the minimum signal strength on client manufacturer recommendations or device specifications for receive sensitivity (minimum RSSI and SNR) needed to achieve the maximum supported data rate. If client receive sensitivity specifications are not published and cannot be acquired from the manufacturer, use a minimum RSSI of -67 dBm and SNR of 25 dB in all locations for planning purposes. 

Given the diversity of client devices in most networks, network designers typically use a minimum RSSI of -67 dBm for multimedia-grade network design as a baseline. 
You can best determine coverage overlap by ensuring that multiple APs provide coverage at the required minimum signal strength at the edge of each AP coverage area to facilitate client roaming. If clients will be moving at a higher speed than the typical pace of someone walking (about 5 MPH/8 KPH), then you might need to increase the amount of coverage overlap to provide sufficient time for AP discovery and fast roaming before the client connection with the initial AP becomes degraded.

Channel Plan
Validate that the dynamic or static channel plan assigns 2.4 GHz and 5 GHz channels optimally to minimize co-channel interference. Co-channel interference is minimized when AP placement provides adequate RF signal attenuation between different access points operating on the same channel. In high-density environments, co-channel interference is often the most significant limiting factor of performance and capacity. If you cannot eliminate co-channel interference, which is likely in the 2.4 GHz band, consider adjusting AP placement. It might be necessary to revise the number of dual-radio APs in the base coverage layer and supplement capacity with additional APs on 5 GHz. Finally, ensure that neighboring Wi-Fi APs are operating on nonadjacent 5 GHz channels to prevent adjacent-channel interference.

The ratio of 5 GHz to 2.4 GHz access point radios in the network design will determine the appropriate band steering ratio for client devices.

3-D Planning
The predictive site survey software should be capable of estimating signal propagation in 3-dimensions, that is, across multiple floors of a multi-story building. This will ensure that co-channel interference is minimized across floors through proper access point placement that is typically not directly above or below one another on adjacent floors and through proper channel planning so that APs that are near one another on adjacent floors are not assigned the same channels.

Compile Your Results
The result of a predictive site survey should include an initial Bill of Materials (BOM), which includes the following items:
  • All access point hardware and accessories, such as antennas, power injectors, and mounting hardware
  • Detailed equipment installation locations and instructions, including proper angling of external antennas
  • Configuration parameters, such as channel assignments and power levels
  • Graphical heat maps of anticipated coverage levels
  • Wired network requirements necessary to support the WLAN infrastructure, such as switch port capacity, speed, and PoE

In the next post, I'll cover on-site verification of the predictive model as well as how to perform pre-deployment site surveys in either active or passive modes. Stay tuned!