Comment by Dwolb | Hacker News Reader

Comment by Dwolb | Hacker News Reader

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You're right we're speaking a bit tangentially. Hadn't heard of the Friis argument that frequency doesn't matter as long as antennas on RX and TX are the same. I'm not sure I understand it yet.

Nevertheless, let's get some numbers on the page to see if I'm misunderstanding the subject.

Here's a rough link budget with some estimates

TX/RX antenna gain is around 0dBi (seems reasonable [1]) TX power = 16mW = 12dBm [2] RX sensitivity = -90dBm (personally, better than -100dBm seems aggressive)

Roughly, our link budget formula can be... Path loss = TX power + TX gain + RX gain - Rx power [3] Path loss = 102dB

Ideal free space loss over 1km at 900MHz is around 91dB [4]. Therefore, we have about 10dB margin in our link budget to transmit 1km. We also have a few dB margin in the antenna and receive sensitivity estimate.

[1] http://store.rfdesign.com.au/antenna-900mhz-2dbi-monopole-rp... [2] http://www.cpcstech.com/dbm-to-watt-conversion-information.h... [3] http://skypilot.trilliantinc.com/pdf/link_budget.pdf [4] http://en.wikipedia.org/wiki/Path_loss

Take this as the kind of half-cooked thought it is at this point (long day, about to go to bed and don't want to crack open the textbook), but the fact that your path loss leaves 10dBm of power above your Rx sensitivity doesn't mean you have 10dBm of link margin. I'm not completely sure, but I'd guess it means you have 10dB of SNR (theres a units from dBm to dB thing I haven't completely resolved in my head). Whether that makes your link work or not depends on the bitrate, modulation you choose and BER you can accept (and etc). If your modulation scheme needs 9dB of SNR (proportional to Eb/N0) to operate at an acceptable BER, your system may only have 1dB of actual link margin.

We're circling around some good stuff. Night though, thanks for the information to think about.

They cancel perfectly when the antennas are the same. There's a scale factor when they're different, but the frequency dependence still goes away.

So I'm adding about 10dB for noise, and getting a path loss that's about 15 dB higher, giving me an expected range closer to 100 m. What are you using for the antenna's effective area?

Do you agree with lpmay's explanation of Friis? Path loss increases as frequency increases. However, if aperture is constant while frequency decreases, which increases path loss, gain increases, which offsets the increase in path loss, and there is no change in link budget.

My receive sensitivity has noise included. It's why I think -100dBm seems for a 900MHz TI chip (could be wrong? didn't design this system...). Factor in noise/packet loss/PER or however you want to quantify, and I'd guess you'll get to a measured RX sensitivity of ~-90dBm.

Mentally, I equated RX sensitivity at data rate with RX power, which shouldn't be done for explanation purposes, but my original formula should still hold.

What are you using for path loss [1]?

[1]: http://en.wikipedia.org/wiki/Path_loss

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