The Role of Bluetooth RSSI in Indoor Positioning

Table of Contents

Outdoor positioning technologies like GPS, Beidou, GNSS are all the rage today and are in wide application. The prominent integration can be seen in apps like Google Maps. Nonetheless, we spend almost 90% of our time indoors, where those GPS signals don’t work well. This is why RSSI positioning is popular for indoor locations. For Bluetooth communication, RSSI values show the signal strength and connection quality. Here, we’ll explain more about Bluetooth RSSI, including its normal range and impact on Bluetooth communication.

Understanding Bluetooth RSSI

RSSI, which is short for Received Signal Strength Indication, refers to the measurement of evaluating the signal strength received by a wireless device. In Bluetooth communication, it’s easy to understand the Bluetooth RSSI values, it represents the strength of the signal the Bluetooth device receives and affects the communication quality and distance directly. The Bluetooth RSSI values are typically shown as negative dBm values. Higher values (closer to 0) indicate better signal strength. Theoretically, the RSSI range is from 0 to -100dBm.

The commonly used formula is

RSSI value = Received Signal Power – Noise Floor + Calibration Factor.

It’s impossible to overstate the benefits of Bluetooth RSSI, as we can see from its wide range of indoor applications. With more iterations of Bluetooth versions, especially with the development of Bluetooth low energy technology, Bluetooth RSSI positioning will become even more readily available.

Types of Bluetooth RSSI positioning methods

Bluetooth RSSI localization methods can be mainly categorized into network-side localization and terminal-side localization.

Network-side positioning

Network-side positioning mainly includes cell phones, Bluetooth beacons, Bluetooth gateways, and servers. We need to deploy Bluetooth beacons and gateways in the target area. When the terminal device enters the area, it will receive RSSI data from the beacon. Then the gateway will send the cpllected data to the cloud server, and then the server will use the positioning algorithm to calculate the location of the device. If the distance between the transmitter and receiver is closer, the signal strength will be better.

This method has the advantage that the cloud server centralizes the calculation, and the mobile device only needs to scan and report the data. The disadvantage, of course, is that you need to pre-deploy the relevant infrastructure for positioning.

Terminal-Side Positioning

The architecture of terminal-side localization is much simpler and mainly involves the device itself and Bluetooth beacons in the target area. An end device such as a smartphone receives RSSI data from different beacons and then uses its built-in positioning algorithm to estimate its location.

Undoubtedly, both methods require us to deploy a certain number of Bluetooth beacons in the target area. In terms of application, network-side positioning is mainly used to track and locate the position of people and assets, while terminal-side positioning is more suitable for indoor navigation.

Bluetooth RSSI positioning algorithms

It goes without saying that the key to Bluetooth RSSI localization is how to accurately convert RSSI data into position coordinates. Here, we classify Bluetooth RSSI positioning algorithms into two main categories based on distance measurement or not: algorithms that require distance measurement and algorithms that do not require distance measurement.

Distance Measurement Required

Let’s first look at localization algorithms that require distance measurements, this type of algorithm needs to first estimate the distance between the target device and a reference node (i.e., a beacon) using RSSI values. Then, based on the distance information obtained, the final location coordinates of the target device are then computed.

(1) Trilateration

Trilateration is a relatively simple localization algorithm based on the geometric principles of triangles. Let’s assume that there are three non-collinear Bluetooth beacons AP1, AP2, AP3, and their distances to the target M point are d1, d2, d3, respectively. we can draw three circles with the position of these three beacons as the center, and d1, d2, d3 as the radius, respectively. The intersection point of these three circles is also the coordinates of the target M point.

(2) The Least Squares Method

In reality, we usually deploy more than 3 Bluetooth beacons in the target area. At this point, we need to use the least squares method to estimate the location of the target node. Assuming that we measure the distance from each beacon to the target node M, we can obtain an equation based on each pair of known distance and beacon location. By combining all the equations, we can get a system of overdetermined equations. Finally, solving this system of equations using the least squares method gives us an estimate of the optimal location coordinates of the unknown node M.

Distance Measurement No Required

These algorithms directly utilize RSSI fingerprint data for localization. We don’t need to calculate the distance first, so we can effectively avoid the error of distance estimation.

(1) Centroid Determination

The center of mass is an imaginary point where the mass of a system is considered to be concentrated. The centroid positioning method estimates the location of the centroid of its geometric shape using the positions of the surrounding beacons it receives. The algorithm of this method is easy to understand, the computational effort is small, and positioning accuracy depends on the density of beacons deployed.

(2) RSSI Fingerprinting  

RSSI fingerprinting is a nonparametric localization technique based on scene analysis. It locates by matching real-time signals against an RSSI fingerprint database. Fingerprint positioning consists of two stages: 1) An offline phase builds the fingerprint library from indoor RSSI data. 2) An online positioning phase finds the best fingerprint match to the received signal, using its location as the target’s position estimate. Its advantages are high accuracy with few reference points, but disadvantages include the arduous fingerprint database creation and difficulty adapting to environmental changes.

RSSI measurement and Bluetooth communication

In the previous section, we have discussed various methods and algorithms for measuring Bluetooth RSSI values. In practice, it’s difficult to shape the optimal environment, so the Bluetooth signal strength will decay as its communication distance increases. The RSSI signal ranges from 0 ~ 100, with 0 being the ideal case, which does not exist in practice. When the RSSI value is close to -50, the signal signals of Bluetooth connections are good.

– Above -50dBm: Extremely strong signal for near-field communication

– -50dBm to -80dBm: Ideal signal strength for normal short-range use

– Below -80dBm: Very weak signal with risk of packet loss and intermittent disconnects

In Bluetooth Low Energy, RSSI is one of the parameters that must be reported. RSSI of periodic advertising channels is important for localization because mobile devices are scanning them without connection. For BLE devices, due to their lower power, their RSSI values will be lower than classic Bluetooth at the same distance. In order to achieve low power communication, the general value of BLE RSSI is maintained at -50 ~ -80dBm.

How to check Bluetooth RSSI on smartphones

Due to the popularity of Bluetooth technology, we often use our smartphone Bluetooth for indoor navigation in our daily lives. Nowadays almost all smartphones provide the function to view the Bluetooth RSSI value of the connected device. Here we take the Android system as an example, the steps are as follows:

  1. Open “Settings” and go to “Connected Devices” or “Connection Preferences”.
  2. Enable Bluetooth.
  3. The phone will scan and display nearby connectable Bluetooth devices.
  4. In the device list, an RSSI value between -100 and 0 is shown for each device, representing the signal strength between the phone and that device.

If you want to check and monitor the comprehensive Bluetooth RSSI value records, we can use professional tools such as BLE Scanner and BLE Tool. By observing the RSSI value changes of the devices, we can have a very intuitive understanding of the approximate distance and signal strength between the phone and each device. If we move the phone, the RSSI value will also change. Interestingly, RSSI readings may be different between devices at the same distance.

Applications of Bluetooth RSSI in real-world scenarios

Despite some challenges that need to be overcome, Bluetooth positioning technology based on RSSI still has a promising application. As is the case with any advancement it is rather promising to come across Bluetooth RSSI positioning in more areas. Currently, some typical application scenarios include but are not limited to:

Indoor Positioning Navigation

In large indoor venues such as shopping malls, exhibition halls, museums, etc., Bluetooth RSSI positioning can provide customers with navigation routes and location information services. Besides, it can guide the order in which visitors tour in large exhibitions, avoiding crowding issues.

Asset and Personnel Management

In factories, parks, and hospitals, we can install Bluetooth beacons and tags on important assets and personnel. Combined with the Bluetooth RSSI positioning system, we can achieve real-time monitoring and management.

Smart Retail and Business Analytics

In retail, using Bluetooth RSSI positioning can open new opportunities to bring business value to retailers. Through tracking the trajectories of movement of the customers and their dwelling time, retailers can analyze customer shopping behaviors. Consequently, it is effective for merchants to identify popular products and high-traffic areas, and set up their promotional plan and arrange the placement of their employees.

Smart Home and Indoor Positioning

By deploying a small number of Bluetooth beacons in homes, homeowners can achieve indoor location tracking of personnel and pets, such as knowing the current location of the elderly or children to prevent them from having accidents. Additionally, homeowners can automatically adjust home appliances such as lights, temperature controls, etc., according to the user’s location, achieving a truly smart home experience.

Comparison of Bluetooth RSSI and Bluetooth AoA

Apart from the RSSI-based positioning, there is another type of positioning technique that has recently emerged in Bluetooth positioning, known as Bluetooth AoA. It determines the location of the mobile device by measuring the angle at which the radio signal arrives at antennae array with triangulation. The potential AoA positioning can achieve one meter or even higher in theory, which greatly surpasses the positioning accuracy of the traditional RSSI technology.

Therefore, the major benefit of the RSSI positioning is no complicated infrastructure as well as the rather low cost. However, I can note that, the primary disadvantage is the poor sensitivity to the conditions. Known for its high accuracy, AoA positioning is more accurate as compared to RSSI positioning. Of course, it also has disadvantages such as high-cost infrastructure.

Moreover, we can apply both technologies together. For example, after estimating the specific area where the target is located through RSSI, we can employ the AoA method for precise positioning inside a certain area. Soon, we are able to incorporate many discussed technologies like RSSI, AoA, etc., alongside other technologies such as UWB to utilize the best of each and develop a much more accurate and reliable positioning solution.

Future development of Bluetooth RSSI positioning

The Bluetooth 5.2 version, released in 2020, significantly enhanced its positioning capabilities, including support for AoA/AoD positioning and RSSI calibration. Meanwhile, these improvements will further enhance the accuracy and reliability of Bluetooth positioning. There’s no doubt that future versions of Bluetooth will continue to strengthen positioning capabilities.

Using a single wireless technology for positioning has its limitations. Therefore, integrating the advantages of multiple technologies is the future trend. Such as combining Bluetooth RSSI with WiFi fingerprinting positioning or using it in conjunction with more accurate technologies like UWB (Ultra-Wideband).

If you are wondering about adopting any Bluetooth RSSI positioning solutions, just talk to our Bluetooth expert!

Written by ——
Nick He
Nick He
Nick, a seasoned project manager in our R&D department, brings a wealth of experience to MOKOSMART, having previously served as a project engineer at BYD. His expertise in R&D brings a well-rounded skill to his IoT project management. With a solid background spanning 6 years in project management and get certifications like PMP and CSPM-2, Nick excels in coordinating efforts across sales, engineering, testing, and marketing teams. The IoT device projects he has participated in include Beacons, LoRa devices, gateways, and smart plugs.
Nick He
Nick He
Nick, a seasoned project manager in our R&D department, brings a wealth of experience to MOKOSMART, having previously served as a project engineer at BYD. His expertise in R&D brings a well-rounded skill to his IoT project management. With a solid background spanning 6 years in project management and get certifications like PMP and CSPM-2, Nick excels in coordinating efforts across sales, engineering, testing, and marketing teams. The IoT device projects he has participated in include Beacons, LoRa devices, gateways, and smart plugs.
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