Les mécanismes de la localisation WiFi : Trilateration et RSSI expliqués

THE MECHANICS OF WIFI WAYFINDING: TRILATERATION AND RSSI EXPLAINED A Purple Technical Briefing Podcast — Approximately 10 Minutes --- SEGMENT 1: INTRODUCTION AND CONTEXT (approx. 1 minute) Welcome to the Purple Technical Briefing series. I'm your host, and today we're getting into the mechanics of WiFi wayfinding — specifically how trilateration and RSSI work together to tell you where someone is inside a building, and what that means for your deployment strategy. If you're a network architect, IT manager, or venue operations director, this is the episode for you. We're not going to spend time on the basics of WiFi — you know what an access point is. What we're going to cover is the positioning layer that sits on top of your existing infrastructure, how it actually works under the hood, and the practical decisions you need to make to get it right. The question "what is wayfinding?" comes up constantly in enterprise WiFi conversations, and the honest answer is: it's a lot more nuanced than most vendors let on. So let's get into it. --- SEGMENT 2: TECHNICAL DEEP-DIVE (approx. 5 minutes) Let's start with the fundamentals. WiFi wayfinding is the use of your existing wireless infrastructure to determine the physical location of a device — and by extension, the person carrying it — inside a venue. No GPS, no additional hardware in most cases, just the access points you already have. The core mechanism is trilateration. Not triangulation — that's a common misconception worth clearing up immediately. Triangulation uses angles. Trilateration uses distances. Your access points measure signal strength from a device, convert that signal strength into an estimated distance, and then the system calculates where those distance circles intersect. That intersection is your device's estimated position. The signal strength measurement is called RSSI — Received Signal Strength Indicator. It's expressed in decibels relative to a milliwatt, or dBm. The scale runs from zero, which would be an impossibly strong signal, down to around minus 100 dBm, which is effectively noise. For practical wayfinding deployments, you want your access points seeing client devices at minus 67 dBm or better. Below minus 75, you're in unreliable territory. Below minus 85, forget it — you won't get consistent positioning. Now, here's where it gets technically interesting. The relationship between RSSI and distance is not linear. It follows a logarithmic path-loss model. The standard formula is: RSSI equals minus 10 times n times the log base 10 of distance, plus a constant A. Where n is the path-loss exponent — typically between 2 and 4 depending on your environment — and A is the RSSI at one metre from the access point, your calibration reference. In an open office with line of sight, n might be 2.0. In a dense hotel corridor with concrete walls, steel doors, and lift shafts, n could be 3.5 or higher. This is why a deployment that works brilliantly in one venue can give you 10-metre errors in another with the same AP density. The environment is a variable, and it has to be measured, not assumed. This brings us to calibration. There are two approaches. The first is radio frequency fingerprinting — you physically walk the space with a device, recording RSSI values at known coordinates, and build a lookup table. Accurate, but labour-intensive, and it needs to be redone whenever the physical environment changes significantly. The second is model-based positioning, where you apply the path-loss formula with measured or estimated environmental parameters. Faster to deploy, less accurate, but sufficient for zone-level wayfinding in most venue types. For precision wayfinding — think hospital ward-level accuracy, or retail shelf-level product guidance — you typically need a hybrid approach, combining WiFi RSSI with additional signals. Bluetooth Low Energy beacons are the most common complement. BLE operates at shorter range and lower power, which means tighter signal circles and better intersection accuracy. The IEEE 802.11mc standard, also known as WiFi Round-Trip Time or RTT, is another option — it measures the actual time of flight of the signal rather than just its strength, giving you distance estimates that are far less susceptible to environmental interference. But RTT requires compatible hardware on both the AP and the client device, so check your estate before specifying it. Now let's talk about the positioning stack architecture. At the bottom, you have your physical layer — the access points, their placement, and their antenna characteristics. Above that, you have the RSSI collection layer, which is typically handled either by your wireless controller or by a dedicated location engine. Then you have the positioning engine itself, which runs the trilateration calculations and applies any calibration data or machine learning corrections. Above that sits the application layer — the wayfinding interface that the end user actually sees, whether that's a map on their phone, a digital signage display, or an analytics dashboard showing dwell time and footfall patterns. Purple's platform operates at the application and analytics layer, consuming positioning data from your existing infrastructure — whether that's Cisco, Aruba, Ruckus, or any other vendor — and translating it into actionable intelligence. That hardware-agnostic approach is significant because it means you're not locked into a single vendor's location engine, and you can evolve your underlying infrastructure without rebuilding your wayfinding application. One more technical point worth covering: the impact of the 2.4 GHz versus 5 GHz band on positioning accuracy. The 2.4 GHz band propagates further and penetrates walls better, which sounds like an advantage for coverage. But for positioning, that propagation characteristic actually works against you — the signal circles are larger, which means the intersection area is larger, which means lower precision. The 5 GHz band attenuates faster, giving you tighter circles and better positional resolution. For wayfinding deployments, you generally want your positioning engine consuming 5 GHz RSSI data where available, with 2.4 GHz as a fallback. --- SEGMENT 3: IMPLEMENTATION RECOMMENDATIONS AND PITFALLS (approx. 2 minutes) Right, let's get practical. The three most common failure modes I see in wayfinding deployments are: insufficient AP density, poor calibration, and ignoring multipath interference. On AP density: the rule of thumb is that for reliable trilateration you need a minimum of three access points with overlapping coverage at any given point in the venue. In practice, for a 2 to 3 metre accuracy target, you're looking at one AP per 15 to 20 square metres in a typical indoor environment. That's denser than you'd deploy purely for connectivity, which means wayfinding requirements should feed into your RF design from day one, not be bolted on afterwards. On calibration: do not skip the site survey. Even if you're using a model-based approach, you need measured path-loss exponents for your specific environment. A 30-minute walk-through with a spectrum analyser will save you weeks of troubleshooting inaccurate positioning post-deployment. On multipath: this is the big one that catches people out. In environments with lots of reflective surfaces — think glass-fronted retail, airport terminals, sports halls — signals bounce off walls and floors and arrive at the receiver via multiple paths. The RSSI reading becomes an average of all those paths, not a clean line-of-sight measurement. The mitigation is a combination of denser AP deployment, fingerprinting calibration, and — where budget allows — the move to RTT-based positioning which is inherently more resistant to multipath because it's measuring time, not amplitude. From a compliance perspective: if you're collecting location data on individuals, you're in scope for GDPR in the UK and EU. The key principle is that passive RSSI collection from probe requests — where the device is broadcasting its MAC address — is generally considered personal data processing. You need a lawful basis, typically legitimate interests for aggregate analytics, or explicit consent for individual-level tracking. MAC address randomisation, which is now default on iOS 14 and above and Android 10 and above, significantly complicates individual tracking but doesn't affect aggregate footfall analytics. --- SEGMENT 4: RAPID-FIRE Q&A (approx. 1 minute) A few questions that come up regularly: "Do I need to upgrade my access points for wayfinding?" — In most cases, no. If your APs are less than five years old and running a current firmware, they'll support RSSI reporting. RTT-based positioning is the exception — that requires 802.11mc-compatible hardware. "What accuracy can I realistically expect?" — For a well-calibrated WiFi-only deployment, 3 to 5 metres is a realistic target. Add BLE beacons and you can get to 1 to 2 metres. RTT can get you under 1 metre in favourable conditions. "How does this work with Wi-Fi 6?" — Wi-Fi 6 and Wi-Fi 6E improve throughput and reduce latency, but they don't fundamentally change the RSSI-based positioning model. The higher channel density in 6 GHz does offer some positioning benefits in terms of signal resolution. We've covered the Wi-Fi 6 versus Wi-Fi 5 comparison in detail in our guides section if you want to go deeper on that. "What about privacy?" — Aggregate zone analytics don't require individual identification. If you're doing individual wayfinding — turn-by-turn navigation — you need explicit opt-in. Purple's guest WiFi platform handles the consent capture at the point of network authentication. --- SEGMENT 5: SUMMARY AND NEXT STEPS (approx. 1 minute) To wrap up: WiFi wayfinding is a mature, deployable technology that works on your existing infrastructure. The core mechanic is trilateration using RSSI measurements — three or more access points, distance estimation via path-loss modelling, and intersection calculation to determine device position. The accuracy you achieve is directly proportional to your AP density, the quality of your calibration, and your ability to account for environmental variables like multipath and wall attenuation. For most venue operators — hotels, retail, stadiums, conference centres — a well-designed WiFi wayfinding deployment will deliver 3 to 5 metre accuracy, which is more than sufficient for turn-by-turn navigation, zone-level dwell analytics, and operational use cases like staff location and asset tracking. The next step is a site assessment. Map your current AP placement against the density requirements for your target accuracy, identify the calibration approach that fits your operational model, and make sure your data collection practices are GDPR-compliant from day one. Purple's platform integrates with your existing infrastructure to deliver the analytics and wayfinding application layer on top. If you want to explore what that looks like for your specific venue, the details are at purple.ai. Thanks for listening. We'll be back with the next technical briefing shortly. --- END OF SCRIPT