Wireless sensor network at 868MHz

Why does it have to be 868MHz? In many countries there are lower frequency bands which will give you longer range for the same transmitter power with a simple antenna.

Does it need to be a transceiver, or would a simple transmitter be enough?

Why does it all need to be on one chip?

You haven't really given us enough information to be able to help.

You will get better answers if you tell us what range and power consumption and physical size you are looking for, as well as the data rate required. How much are you willing to spend per unit? Even better would be for you to explain what you are trying to achieve, because there is a good chance that somebody here has done it already (or tried to).

John

Thanks for the help...

I want to realize a wireless sensor network for environmental parameters (temperature,humidity.ecc..). Data rate is not important for me (each node can transmit every minute or more); instead power consumption is very important. Range shoud be 1Km or more...

Because i wanted to use an open standard: so i thought about ZigBee at

868MHz. But i haven't found so much, so i think i should use proprietary protocol or change the frequency (433MHz?)....Any suggestion?

Not very usable requirements in order to get a usable answer ...

Anyway, I assume that each station will generate 60 bits and the measurements should be repeated every 60 seconds, thus the effective bandwidth would be about 1 Hz.

Unfortunately 1 Hz bandwidth is not practical at UHF frequencies, so you would have to use some spread sequence sequences such as 16383 bit sequences on a radio with 25 kHz channel spacing.

With a centralized mains powered high power master, you only have to worry about the slave power consumption during receive, since the short burst (1W for 1 km) would not be an issue.

However, for true peer-to-peer networks, the transfer rate and hence power requirement will increase quite rapidly for nodes that are close to the network hub (take off) point.

On a sunny day (Sun, 23 May 2010 15:41:14 +0300) it happened Paul Keinanen wrote in :

Well, that is purely theoretical, you would use some packet like system, and transmit a short bursts of data every 60 seconds. The main problem here is the distance, 1km may need special antennas, or even licenses. Perhaps the simplest solution is to use existing GSM modules, and send for example short data burst in the form of SMS (text messages). You would have to pay for each SMS, but those come cheap in quantity, and using existing licensed modules will save you development cost. Old cellphones are cheap too, many have a modem connection. An that solution provides a return path too.

Thanks for your answer...

your assumption are correct

I want to use free licensed frequencies, so i have: 433MHz, 868MHz,

2.4GHz. I wanted to use Zigbee for 868MHz, but isn't so simple... I think 1Km as range at 868MHz isn't impossible (but i may be mistaken...)

example

I don't want to use GSM modules due to the costs.

Transferring a few bits per second between two sites 1 km apart is no problem, as long there is a direct line of sight path between the stations _and_ there is no other signals on the same frequency. The communication can be done with a few milliwats of DC power.

As soon as we are talking about non-line-of-sights paths, the power requirement can grow to something of an order of 1 W.

If you want to use unlicensed frequencies, you might in some cases achieve 1 km contact distance between your stations.

The next day I (or anyone else) could legally bring my own unlicensed transmitter 1 m from your receiver, completely blocking the communication with your slaves. You can not legally complain about this, unless you own several hundred square kilometers around your receiver.

Well, in America we then round up some men with horses and double-barrel rifles and then ... oh wait, that doesn't work anymore :-)

The only way to achieve this in a license-free band is with frequency hopping. That's how most systems I dealt with do it, including the one I have here right now. That, and tons of correlation if needed.

--
Regards, Joerg

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