SparkFun Forums

[Wastora]

Hello World,

I've got kind of a vague question. I need to transmit data in a building, with a BER < 10^4, within a range of 10-50 meters. Would that be possible with an affordable transmitter (lets say, < 100 euro)? I can only use a small antenna at the receiver end. Does anybody have any examples or literature on this topic? I have very little experience in wireless communication, so any info would be great! I don't even know if a BER < 10^4 is achievable or very easy to achieve...

If somebody would have some examples of Bit Error Rates in a building or mall, with an affordable transmitter, that would be great too!

[60amp_relay]

It's all about how you design your protocol, and what kind of data rate you need. I'm assuming you'll have a laptop/Arduino/PIC/Atmel/or similar at each end. You can use something like these:

http://www.sparkfun.com/commerce/produc ... ts_id=8946
http://www.sparkfun.com/commerce/produc ... ts_id=8950

Then you simply trade off speed for message integrity as needed. Let's say you use a 1200 baud rate, and find that your signal is 80% reliable at 50m. (which is probably about what you'd get with these modules in an open field) If you send each data packet twice, you'd be up to 96% reliable, but at only 600 effective bits per second. Send each packet four times, and you'll get over 98% reliability, but at 300 effective bits per second. Tune as needed, and have fun!

[Wastora]

It's all about how you design your protocol, and what kind of data rate you need. I'm assuming you'll have a laptop/Arduino/PIC/Atmel/or similar at each end. You can use something like these:

http://www.sparkfun.com/commerce/produc ... ts_id=8946
http://www.sparkfun.com/commerce/produc ... ts_id=8950

Then you simply trade off speed for message integrity as needed. Let's say you use a 1200 baud rate, and find that your signal is 80% reliable at 50m. (which is probably about what you'd get with these modules in an open field) If you send each data packet twice, you'd be up to 96% reliable, but at only 600 effective bits per second. Send each packet four times, and you'll get over 98% reliability, but at 300 effective bits per second. Tune as needed, and have fun!

Thanks for your reply! But actually, this wasn't exactly what I meant. With the BER, I meant the number of bits that was transmitted wrong, not the number of packets. Adding redundancy doesn't change the BER, it only makes the transmission more reliable. So if the BER is to high, I will have to add more redundancy to achieve the same effect as with a lower BER.

[theridane]

I'm assuming you ment a BER of 10^-4 and not 10^4 otherwise a two pound slab of granite would work great for you.

XBee PROs ($38 or 31 € a piece) should be able to punch through a decent amount of interior walls. They like having about 5 dB SNR to achive a BER of 10^-5 (10x better than what you wanted). Their so-called datasheet specifies, if somewhat vaguely, an indoor range of 100 m. I believe that in the 50 meters you want they would do a great job.

The 802.15.4 protocol (xbee) is much more capable at penetrating walls with low power than 802.11 (wifi), so if you can get decent connection with a low power (10 dBm) desktop wifi router, then you'll be swinging with this xbee (18 dBm).

If you're American or Canadian you might be interested in the 900 MHz xbees. The lower frequency reduces the attenuation to about a third, effectively allowing you to punch through three times as much masonry as with 2.4 GHz. If you're European, there's the 868 MHz version. It is however very limited in data rates (~2 kbit/s), difficult to obtain, and a general pain to work with.

What data rates would you like to see? The lower the data rate, the lower the bit error rate, as receiver sensitivities increase with reduced data rates.