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I'd like to track a ball during a game using wristbands on players and a chip in the ball. I'd like the type of ranging given by Bluetooth low energy (immediate, near, mid, far) so that I can tell post-game who had the ball at a given time.

If BLE is the right solution I'm trying to figure out how chips would exchange the ranging distance. Would the chip in the ball need to be smart-ready (dual mode)? The phone would be on the sideline, so it can't be the in-game tracker of the ranges.

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I've done some experimentation with BLE range sensing and I've found that it is not very accurate. It is based on RSSI - a received signal strength indication - and the received signal strength is affected by a lot of things apart from distance. For example, putting a BLE device into your pocket so that some of the RF energy is absorbed by your body causes a similar decrease in RSSI as moving it 10 meters away in free space. Also the radiation pattern from a device is not even in all directions, so 3 meters in one direction may give the same rssi as 8 meters in another, etc.

One way that this is got around in beacon type applications is in taking several readings over time and taking an average. This is not ideal in your case as you need fairly instant results.

One of the ways in which low-energy is achieved in BLE is by not communicating very often. I.e. a peripheral device may communicate with a central device only once a second or so. This is what gives keyring type devices that can be on for a year. You get to choose these intervals when you design a system so in your case you can probably have a shorter interval and turn the devices off when no one is playing.

BLE works as a hub-and-spoke type network where devices are either a central or a peripheral. A peripheral can establish a link to only one central (but broadcast packets from a peripheral may be received and ranged by any central). Ranging is from each peripheral to the central.

You don't need a dual mode chip in a BLE system, you can have an entirely BLE implementation. Common embedded BLE chipsets (Nordic Semiconductor, TI, CSR) can all operate in central or peripheral mode.

I'd be tempted to use a simpler ranging system between your wristbands and your ball. Maybe iR, ultrasonic or other radio techonolgy would do the job. You could interface this to a BLE device and use that to communicate scores, who has the ball to a smart phone

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  • \$\begingroup\$ Thank you, great to hear that single mode devices can operate in central mode. That was not clear to me elsewhere. The issue with other ranging solutions is that they don't identify the player, which I should have said is something I'd like to do. If I were simply ranging the distance I would probably use a hall-effect or reed sensor. Do you have any suggestions for that? \$\endgroup\$ – Tinkerer26 Sep 29 '14 at 16:43
  • \$\begingroup\$ It the game needs to be co-ordinated by a smartphone then I think BLE wristbands are a good solution, just not for the ranging part of the problem. If you have a lower-tech solution in the wrist band and connected to the gpio of the ble device then you can tell the smartphone (the central) who has the ball (because you know from the wrist band). If you want the ball to be the BLE part then you need a system of identifying which wrist band you are near. Maybe have a coil in the wrist band and modulate it at a different frequency for each player? Or post as a separate question? \$\endgroup\$ – Will Sep 29 '14 at 16:58
  • \$\begingroup\$ Thanks again! I would prefer the wristbands to be cheap and if I'm not getting range from BLE on them, I'd want to just make it a magnet for a hall effect or reed sensor and find a way to get unique identification to the chip in the ball. I will be researching... \$\endgroup\$ – Tinkerer26 Sep 29 '14 at 18:05
  • \$\begingroup\$ "A peripheral can talk to only one central" - untrue - ranging as commonly used with BLE is one-way broadcast traffic from a peripheral to any central(s) which care to pay attention. \$\endgroup\$ – Chris Stratton Sep 29 '14 at 19:05
  • \$\begingroup\$ Fair enough, edited. I'm not sure that BLE ranging of that kind would help much in this application though, unless you have a cunning plan? \$\endgroup\$ – Will Sep 29 '14 at 19:12
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I personally spent a bit of time trying to accurately track position and distance using BLE. I was unable to every get accurate/repeatable results with a changing environment using just BLE. I would suggest some serious sensor fusion using gyro and accelerometer to get anything decent.

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  • \$\begingroup\$ Thanks. I actually plan on having those sensors in the ball anyway. How would they help with ranging from the wristbands? \$\endgroup\$ – Tinkerer26 Sep 29 '14 at 16:09
  • \$\begingroup\$ Because the BLE often will give terrible data that would yield less then usable data. With this much movement your going to be facing some serious issues regardless. There is a lot of noise in the Bluetooth spectrum...I wouldn't count on BLE as an accurate source of info pertaining to this data ...I am talking accuracy to the tune of a few meters(2-5 depending on environment)... If used other sensors and weighted your filter appropriately the bluetooth could help. \$\endgroup\$ – tman Sep 29 '14 at 16:19

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