Best radio system for a distance of 3 km

I am trying to transfer just a few bits per minutes over a distance of 3 kilometers in open field,

the reliability of the data transfer is very important for me.

what do you suggest to keep the system cheap and reliable?

I suggest you use the most common CC1101 rf module with MAX2235 as a power amplifier. Search google about CC1101. You will get a lot of projects and DIY. But make sure you have cleared all the legal issues...

"the reliability of the data transfer is very important for me."

zigbee pro

May I ask which features and characteristics of RF modules defines its reliability?
I mean to make sure that less interference, less corruption on signal and data, continuous functioning and etc can be achieved?

If the link is going to transfer the important control bits, do you still propose the same modules?

thanks

The ZigBee Advantage

The ZigBee protocol is designed to communicate data through hostile RF environments that are common in commercial and industrial applications.

ZigBee protocol features include:

1, Support for multiple network topologies such as point-to-point, point-to-multipoint and mesh networks
2, Low duty cycle – provides long battery life
3, Low latency
4, Direct Sequence Spread Spectrum (DSSS)
5,Up to 65,000 nodes per network
6, 128-bit AES encryption for secure data connections
7, Collision avoidance, retries and acknowledgements


http://www.digi.com/products/wireles...odule#overview

Few bits per minute? The simplest CW ON/OFF code will do this job.

Or even simpler, is this method
http://en.wikipedia.org/wiki/Smoke_signal

But as I said, they are important condition monitoring and control data.
they have to be transferred reliably,

So far I am convinced that ZigBee is the best solution,
But can someone suggest any better solution?

thanks

If you want off the shelf, TI, nordic, etc chip work fine if you have a decent antenna and transmit with max power you are allowed to in your country. In some countries, like US, you can transmit enough power at 915 MHz to do 3 km if there is not too many obstructions.

If you are more technically adept, or have money for NRE effort, you could use a heavily coded or matched filtered FM signal (i.e. where you transmit 50 bits to send 1 bit of payload, or something like that) that is custom designed for your use.