From ‘Good Enough’ to ‘Great’: The Evolution of Indoor Positioning
Indoor location technology has come a long way. Many of us started with RSSI-based positioning, measuring signal strength from our access points (APs) using either Wi-Fi or Bluetooth radios to triangulate device locations. While RSSI gets the job done for basic ‘which zone am I in?’ applications, its accuracy limitations become apparent when you need precise sub-meter asset tracking or navigation.
The Wi-Fi industry’s first answer was time-based ranging. Wi-Fi Fine Timing Measurement (FTM) brought Time-of-Flight measurements to the table, improving from RSSI-based techniques. By measuring the round-trip time of signals between devices and access points, FTM delivers the precision that RSSI simply can’t match.
But FTM has a scalability problem for dense deployments. In dense enterprise environments with hundreds of Wi-Fi devices, those ranging exchanges create airtime overhead. Each client needs multiple frame exchanges with several APs to get a position fix. When everyone’s trying to range, the result is increased overhead. It’s like trying to have precise conversations in a crowded room. Technically possible, but not practical at scale. This is where Ultra-Wideband (UWB) changes the game. With its nanosecond pulses and Time Difference of Arrival (TDOA) techniques, UWB delivers both the centimeter-level accuracy we need, and the scalability to handle enterprise deployments. Whether using uplink TDOA (tags transmitting to synchronized anchors) or downlink TDOA (anchors broadcasting to devices), UWB sidesteps the collision issues that plague Wi-Fi ranging.
The Missing Piece: Getting UWB Deployment Right
But there’s a critical aspect that’s often overlooked. Having UWB-capable access points doesn’t guarantee a working, precision location system. At Cisco Wireless, we’ve conducted extensive measurements that revealed deployment requirement which hasn’t been systematically addressed in the industry.
Here’s what we discovered when the AP is transmitting at 23 dBm on 5 GHz (RRM off, max power):
When Wi-Fi signal strength drops below approximately -60 dBm RSSI, as measured by site survey equipment, UWB reception becomes consistently unreliable. Conversely, when Wi-Fi signal strength remains above -60 dBm RSSI, UWB measurement maintains reliable performance.
This discovery fundamentally changes how we need to think about AP deployment for location services. We can’t just deploy for Wi-Fi coverage anymore. We need to ensure adequate signal strength specifically for UWB performance.
Additionally, unlike BLE RSSI (which requires 3 access points for trilateration) the nature of TDOA positioning means we need at least four anchors (access points) with good (above X dBm) signal strength to any point on our floor map. Three anchors give us ambiguous results, while four anchors provide precision. This requirement stems from the fundamental geometry of hyperbolic multilateration.
A New Deployment Methodology
Based on these findings, we’ve developed a systematic approach to deploying APs for optimal UWB performance:
Step 1: Predictive Modeling with New Parameters
Just like traditional Wi-Fi deployments, start with deployment software and import floor plan CAD files. But here’s the difference: instead of planning for typical Wi-Fi coverage (-67 to -75 dBm) for one AP, model for -60 dBm RSSI coverage from 4 APs (assuming APs are at 23 dBm power in 5GHz, i.e. RRM is off). This ensures UWB will function reliably wherever precision tracking is needed.
Step 2: Deploy & Verify
Once deployment is complete based on the model, validation becomes critical. Tools like Hamina, Oscium Nomad, or Ekahau Sidekick can verify coverage, but here’s where Cisco has been leading the industry by working with partners to implement necessary updates. For example, Wi-Fi site planning and site survey tools typically use the strongest RSSI value at each location. For UWB deployment validation, we need them to use the four strongest 5G RSSI readings, confirming that RSSI readings from at least 4 APs exceed our ~-60 dBm threshold.
This -60 dBm threshold applies specifically to site survey measurements of Wi-Fi RSSI on 5 GHz channels for APs that are transmitting at 23 dBm TX power (i.e., RRM off). If RRM is on, the number needs to be adjusted accordingly.
Step 3: Fill the Gaps
Any location showing fewer than four APs above -60 dBm needs additional coverage. This might mean adding APs in areas where Wi-Fi-only deployments would consider coverage ‘good enough’.
When deploying APs solely for connectivity and data, a general guideline is to install one AP for every 1,500 square feet, and there is no strict requirement to place APs along the perimeter. However, when designing for real-time location (RTLS), a similar AP density can be used, but it becomes important to position APs near the perimeter so that the entire floor plan lies within the ‘convex hull’ of the APs, ensuring better location accuracy across the entire coverage area. For advanced location services, the deployment density increases to approximately one AP per 1,000 square feet in the middle of the building, again with APs placed along or as close to the perimeter as possible, ensuring that any point on the floor has a strong connection to at least four APs for optimal location precision.
An example of AP deployment in a 200ft by 200 ft area is shown deploying 25 APs at roughly 40 ft intervals to meet basic connectivity needs. Increasing perimeter coverage to ensure the entire space lies within the convex hull of access points raises the count to 36 APs for RTLS, and spacing APs at about 33 ft intervals further increases density to 49 APs for advanced location accuracy.
When considering perimeters, the overall AP count varies depending on how the building is shaped. For example, in the Network Design for Advanced Location outcomes below, each AP in the middle of the building covers 1,000 square feet. The overall AP count is 49 for 40,000 square feet, not 40.
The Payoff: Location Services That Actually Work
This deployment methodology might seem excessive from a pure Wi-Fi connectivity perspective. But when assets need to be tracked within inches, when automated vehicles navigate through narrow aisles, or when high-value equipment requires real-time monitoring, the difference between ‘mostly works’ and ‘always works’ is everything. Moreover, there are additional Wi-Fi benefits of increased AP density – increased Wi-Fi capacity with smaller cells, enhanced Wi-Fi performance at short range with 4k QAM, improved support for high-speed, low-latency applications like industrial robotics, medial robots, 8K streaming, AR/VR, and increased Wi-Fi redundancy in case of AP failure.
By understanding the relationship between Wi-Fi signal strength and UWB performance, and planning deployments accordingly, we can finally deliver on the promise of enterprise-grade, precision location services. No more dead zones. No more ‘it worked in the lab’ excuses. Just consistent, reliable, centimeter-accurate positioning wherever the business needs it.
The future of indoor location isn’t about choosing between technologies. It’s about deploying them correctly, so they work together seamlessly. With modern Cisco Wi-Fi 7 access points integrating UWB alongside traditional radios, we have the hardware. Now, with proper deployment methodology and with the Cisco Spaces platform for smart spaces, we have a complete blueprint for success.
It’s time to move beyond ‘good enough’ and build location services that deliver precision at scale.
For more information on deployment, indoor location services and/or smart spaces, reach us here.
