Understanding ibeacon distancing

Question

Trying to grasp a basic concept of how distancing with ibeacon  beacon  Bluetooth-lowenergy BLE  can work  Is there any true documentation on how far exactly an ibeacon can measure  Lets say I am 300 feet away   is it possible for an ibeacon to detect this   Specifically for v4  amp   v5 and with iOS but generally any BLE device   How does Bluetooth frequency  amp  throughput affect this  Can beacon devices enhance or restrict the distance   improve upon underlying BLE    ie                   Range         Freq         T sec        Topo                                                                                                                                                                                       Bluetooth v2 1   Up to 100 m    lt  2 481ghz    lt  2 1mbit    scatternet                   ------------- ------------ ------------ ------------  Bluetooth v4                    lt  2 481ghz    lt  305kbit    mesh                         ------------- ------------ ------------ ------------  Bluetooth v5                    lt  2 481ghz    lt  1306kbit   mesh

User · Accepted Answer

The distance estimate provided by iOS is based on the ratio of the beacon signal strength (rssi) over the calibrated transmitter power (txPower). The txPower is the known measured signal strength in rssi at 1 meter away. Each beacon must be calibrated with this txPower value to allow accurate distance estimates.

While the distance estimates are useful, they are not perfect, and require that you control for other variables. Be sure you read up on the complexities and limitations before misusing this.

When we were building the Android iBeacon library, we had to come up with our own independent algorithm because the iOS CoreLocation source code is not available. We measured a bunch of rssi measurements at known distances, then did a best fit curve to match our data points. The algorithm we came up with is shown below as Java code.

Note that the term "accuracy" here is iOS speak for distance in meters. This formula isn't perfect, but it roughly approximates what iOS does.

protected static double calculateAccuracy(int txPower, double rssi) {
  if (rssi == 0) {
    return -1.0; // if we cannot determine accuracy, return -1.
  }

  double ratio = rssi*1.0/txPower;
  if (ratio < 1.0) {
    return Math.pow(ratio,10);
  }
  else {
    double accuracy =  (0.89976)*Math.pow(ratio,7.7095) + 0.111;    
    return accuracy;
  }
}

Note: The values 0.89976, 7.7095 and 0.111 are the three constants calculated when solving for a best fit curve to our measured data points. YMMV

User · Answer

Distances to the source of iBeacon-formatted advertisement packets are estimated from the signal path attenuation calculated by comparing the measured received signal strength to the claimed transmit power which the transmitter is supposed to encode in the advertising data     A path loss based scheme like this is only approximate and is subject to variation with things like antenna angles  intervening objects  and presumably a noisy RF environment   In comparison  systems really designed for distance measurement  GPS  Radar  etc  rely on precise measurements of propagation time  in same cases even examining the phase of the signal   As Jiaru points out  160 ft is probably beyond the intended range  but that doesn t necessarily mean that a packet will never get through  only that one shouldn t expect it to work at that distance

User · Answer

I m very thoroughly investigating the matter of accuracy rssi proximity with iBeacons and I really really think that all the resources in the Internet  blogs  posts in StackOverflow  get it wrong   davidgyoung  accepted answer    100 upvotes  says      Note that the term  accuracy  here is iOS speak for distance in meters    Actually  most people say this but I have no idea why  Documentation makes it very very clear that CLBeacon proximity      Indicates the one sigma horizontal accuracy in meters  Use this property to differentiate between beacons with the same proximity value  Do not use it to identify a precise location for the beacon  Accuracy values may fluctuate due to RF interference    Let me repeat  one sigma accuracy in meters  All 10 top pages in google on the subject has term  one sigma  only in quotation from docs  but none of them analyses the term  which is core to understand this   Very important is to explain what is actually one sigma accuracy  Following URLs to start with  http   en wikipedia org wiki Standard error  http   en wikipedia org wiki Uncertainty  In physical world  when you make some measurement  you always get different results  because of noise  distortion  etc  and very often results form Gaussian distribution  There are two main parameters describing Gaussian curve    mean  which is easy to understand  it s value for which peak of the curve occurs   standard deviation  which says how wide or narrow the curve is  The narrower curve  the better accuracy  because all results are close to each other  If curve is wide and not steep  then it means that measurements of the same phenomenon differ very much from each other  so measurement has a bad quality     one sigma is another way to describe how narrow wide is gaussian curve  It simply says that if mean of measurement is X  and one sigma is s  then 68  of all measurements will be between X - s and X   s   Example  We measure distance and get a gaussian distribution as a result  The mean is 10m  If s is 4m  then it means that 68  of measurements were between 6m and 14m    When we measure distance with beacons  we get RSSI and 1-meter calibration value  which allow us to measure distance in meters  But every measurement gives different values  which form gaussian curve  And one sigma  and accuracy  is accuracy of the measurement  not distance   It may be misleading  because when we move beacon further away  one sigma actually increases because signal is worse  But with different beacon power-levels we can get totally different accuracy values without actually changing distance  The higher power  the less error   There is a blog post which thoroughly analyses the matter  http   blog shinetech com 2014 02 17 the-beacon-experiments-low-energy-bluetooth-devices-in-action   Author has a hypothesis that accuracy is actually distance  He claims that beacons from Kontakt io are faulty beacuse when he increased power to the max value  accuracy value was very small for 1  5 and even 15 meters  Before increasing power  accuracy was quite close to the distance values  I personally think that it s correct  because the higher power level  the less impact of interference  And it s strange why Estimote beacons don t behave this way   I m not saying I m 100  right  but apart from being iOS developer I have degree in wireless electronics and I think that we shouldn t ignore  one sigma  term from docs and I would like to start discussion about it   It may be possible that Apple s algorithm for accuracy just collects recent measurements and analyses the gaussian distribution of them  And that s how it sets accuracy  I wouldn t exclude possibility that they use info form accelerometer to detect whether user is moving  and how fast  in order to reset the previous distribution distance values because they have certainly changed

User · Answer

It s possible  but it depends on the power output of the beacon you re receiving  other rf sources nearby  obstacles and other environmental factors   Best thing to do is try it out in the environment you re interested in

User · Answer

The iBeacon output power is measured  calibrated  at a distance of 1 meter   Let s suppose that this is -59 dBm  just an example    The iBeacon will include this number as part of its LE advertisment     The listening device  iPhone  etc   will measure the RSSI of the device   Let s suppose  for example  that this is  say  -72 dBm     Since these numbers are in dBm  the ratio of the power is actually the difference in dB   So   ratio dB   txCalibratedPower - RSSI   To convert that into a linear ratio  we use the standard formula for dB   ratio linear   10    ratio dB   10    If we assume conservation of energy  then the signal strength must fall off as 1 r 2   So   power   power at 1 meter   r 2   Solving for r  we get   r   sqrt ratio linear    In Javascript  the code would look like this   function getRange txCalibratedPower  rssi        var ratio db   txCalibratedPower - rssi      var ratio linear   Math pow 10  ratio db   10        var r   Math sqrt ratio linear       return r      Note  that  if you re inside a steel building  then perhaps there will be internal reflections that make the signal decay slower than 1 r 2   If the signal passes through a human body  water  then the signal will be attenuated   It s very likely that the antenna doesn t have equal gain in all directions   Metal objects in the room may create strange interference patterns   Etc  etc     YMMV

User · Answer

iBeacon uses Bluetooth Low Energy LE  to keep aware of locations  and the distance range of Bluetooth LE is 160ft  http   en wikipedia org wiki Bluetooth low energy

User · Answer

With multiple phones and beacons at the same location  it s going to be difficult to measure proximity with any high degree of accuracy  Try using the Android  b and l bluetooth le scanner  app  to visualize the signal strengths  distance  variations  for multiple beacons  and you ll quickly discover that complex  adaptive algorithms may be required to provide any form of consistent proximity measurement   You re going to see lots of solutions simply instructing the user to  please hold your phone here   to reduce customer frustration

[ios] Understanding ibeacon distancing

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