Math lesson needed dBm

Being no mathematician, can anyone give a simple rule of thumb on how to compute various dBm receive sensitivity specs at various speeds?

For example, if receive sensitivity is -87dBm at 11 Mbps, what will it be at 1 Mbps?

Also, what is the practical range difference between a sensitivity rated at -92dBm and one rated at -95dBm for say most Muni APs?

Finally, if a few small twig like branches obscure 90 degrees right of the line of sight between my sighted AP can I guesstimate the degree this will interfere with a good connection, for a small wubber duckey antenna? and for a panel antenna? Guess I ask alot of (dumb) questions, eh?

Reply to
JDavidson
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JDavidson hath wroth:

No. They're measured, not computed.

That can't be done. It's not linear, changes with modulation method, and is not very consistent between implimentations. There is also considerable creativity in measurement setup. Some vendors include the loss of the pigtail between the entenna connector and the board. Some do not. Some just use the chipset vendors sensitivity figures, without consideration for the circuitry between the antenna connector and the chipset (i.e. diversity switch, board traces, bandpass filter, matching network, etc).

Receiver sensitivity is the signal level at which one gets a 10E-5 BER (bit error rate). Sometimes, the PER (packet error rate) at 10% is used as it's easier to measure. The BER/PER is really a measure of SNR (signal to noise ratio).

At such low signal levels, digital noise from the board, card, or computer have a large effect on sensitivity. The same design, but on a different board layout, will result in different receive sensitivities.

Doing it backwards, the minimum signal to noise ratio (Eb/No) can be calculated for different speeds at a BER = 10E-5.

Speed SNR(dB) 11 6.99 5.5 5.98 2 1.59 1 -2.92

54 24.6 48 24.1 36 18.8 24 17.0 18 10.8 12 9.0 9 7.8 6 6.0

There are quite a few articles on communications design under:

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That's easy. A 6dB improvement is good for doubling your range. 12dB would be 4 times the range. 3dB is about 1.4 times the range.

improvment = 10^(dB/20) = 10^(3/20) = 1.4

For example, if your access point has the stock 2dBi vertical omni rubber ducky antenna on it, and you replace it with a 7dBi aftermarket omni rubber ducky antenna (in violation of FCC 15.204), the 5dB increase in gain should yield a 1.8 time improvement in horizontal range.

The antenna doesn't matter. What's important is the type of tree, thickness, and whether the tree clears the Fresnel Zone. 2.4GHz wireless requires MORE than line of sight to operate. It requires that at least 0.8 times the Fresnel Zone clearance at midpoint is unobstructed. Anything inside the Fresnel Zone will create diffraction effects and losses. See: |

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guess(tm) is that you have more than just a few branches in the way. If the range is considerable, you may need to elevate your antenna as the Fresnel Zone may hit the ground.

Nope.

Reply to
Jeff Liebermann

Jeff Liebermann hath wroth:

Duh. I forgot the punch line.

You can use this chart to guess how far you can go at different speeds. For example, if you can get a tolerable connection at

36Mbits/sec, slowing down the speed to 12Mbits/sec will result in a: 18.8 - 9.0 = 9.8dB improvment in S/N ratio. That's good for an: 10E(9.8/20) = 3.1 times increase in range.

Similarly, if you can get a solid but slow connection and you want more speed, you can add an antenna and guess how fast you'll be able to go. For example, an 18Mbit/sec connection, with perhaps a 12dB increase in antenna gain, will yield a SNR of: 10.8 + 12 = 22.8dB which is approximately the SNR of a 36Mbit/sec connection.

Reply to
Jeff Liebermann

Thanks again for a good answer. One day I'll go to enjuneering skul and lurn all about this, :-)

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

Reply to
JDavidson

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