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After Searching on Web found out that the range/Distance of BLE device transmission can be controlled by varying its transmission power and frequency. But could not get how specific can we bet on the minimum distance covered by reducing the transmission power of a BLE capable device to the lowest.

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    \$\begingroup\$ You're assuming that there is a direct relation between transmit power and usable distance, which there is not. Other factors like sensitivity of the receiver, interference from other devices and obstacles attenuating and/or reflecting the signal play an important role. So at the lowest power you could still have a usable range between a couple of centimeters and several meters depending on the circumstances I mentioned. These circumstances are unpredictable to the range also is unpredictable. \$\endgroup\$ – Bimpelrekkie Dec 13 '16 at 7:16
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The goal of this rather long analysis is to ensure you look like the sharpest RF dude your company has ever hired :)

The free space path loss range of a an RF system can be calculated.

For a typical class 2 Bluetooth system, assuming +4dBm of transmit power, -70dBm receiver, & unity gain antennas, you will find that the range that gives you 74 dB of path loss (the maximum path loss that still works) is 50 meters. Below I did it in one of the many online path loss calculators:

enter image description here

From the path loss equation you will note that power is a function of the square of range, so, if you reduce the range by a factor of 2, the required transmit power (or receive sensitivity) falls by a factor of 4. (i.e. 6dB).

For example, if we reduce the transmit power from 4 dB to -2dB, the free space range will reduce to 25 meters. Lose another 6dB and you are around 12.5 meters.

Sounds easy huh? Nope...

Mobile links such as Bluetooth move around in a complex environment of objects that randomly reflect the signals. At the receiver, these multiple paths can either support or cancel each other, so as the mobile unit moves around, the signal power fluctuates wildly. How wildly? Well, it turns out that it's all statistics: The received signal strength turns out to be our old friend the normal distribution. The average signal strength is simply what is predicted by the path loss equation.

So first bad news is that if you depended on that 50m free space range, your link will only work half of the time. (top half of the distribution)

The second bad news is the standard deviation, which in this case turns out to be roughly 6dB. You'll see from this rule that 99.7% of signal strengths fall within -3 and +3 standard deviations.

That can be stated in a different way, and you are not going to like it: Your mobile Bluetooth signal will vary over a -18dB to +18db range around the mean value predicted by the path loss model!

Since range-squared and power is related through the first equation, we can also state the rule-of-thumb that your operating range will regularly vary between (roughly) 1/8x to 8x of the 50m estimated free space value!

In practice, this means that for around 0.15% of the time, you won't even be able to make the 50m link work at 50/8 = 6 meters, but on the other hand, for .15% of the time you will "get lucky" and the link will work as far out as 400 meters. If you are a smart product marketer, you will quote the range as 6 meters. Your product will meet your spec 99.85% of the time!

Why this long story? To provide you with sound reasoning regarding why you have to be very alert to the statistical nature of mobile links like Blueooth!

Your intended RF minimum distance is going going to vary over a 1:64 range! Is that OK?

If the range you are thinking of is less than, say, one meter, inductively coupled systems could be your answer, the key reason being that unlike RF, where received power is a function of 1/range-squared, in magnetically coupled systems power drops with 1/range-cubed. That extra order really takes care of "accidental" successes at further distances, but it's also the reason why the range is typically constrained to less than a meter.

Also, you might want to look at Alan Bensky's book on Wireless Positioning Technologies and Applications.

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  • \$\begingroup\$ Modern BT receiver sensitivity is around -90dBm, by the way! But the specification requires -70dBm. The best value to use for range estimation is the data sheet value of your IC. \$\endgroup\$ – neonzeon Dec 13 '16 at 9:43
  • \$\begingroup\$ Thanks alot neonzone for such a detailed answer... i'll make sure the distance advertised on my product remains closer to 5- 6 meters mark ... :-) \$\endgroup\$ – user2910111 Dec 13 '16 at 11:29
  • \$\begingroup\$ @user2910111 good luck with your project - if you are using a modern receiver, the receiver's -90dB is likely to result in more than 5m. \$\endgroup\$ – neonzeon Dec 13 '16 at 21:20

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