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Cellular Interference with Digi Modems

Q. Has the possibility of RF Interference from cell phones operating in the 2.4GHz and 900 MHz band been investigated?

A. All Radios are designed to transmit or receive electromagnetic waves at a particular frequency. Ideally, they would only receive their own signals and completely reject all others. However, we all have been in situations where the radio station we have been listening to or the cell phone call we are on goes fuzzy or gets lost due to either interference from other electronic devices or due to a weak signal at the receiver side.

Anytime you are doing anything with a wireless system, interference that will affect performance is always a possibility. Within the 900 MHz and 2.4 GHz bands there exist technologies such as 802.11 (WiFi), Bluetooth, cordless phones and microwave ovens among others. Cell phones typically operate in the 850 MHz or 1900 MHz bands for the US and 900 MHz to 1800 MHz in Europe and other areas. For transmissions directly outside the 900 MHz and 2.4 GHz bands, Digi’s radios have over 70 dB of interference rejection, meaning that these out-of-band interfering signals get attenuated by ten million times. Unless your antenna is located right next to a cell tower, the transmissions will usually work as planned. As a general rule, unless the out-of-band interferers are really strong, the in-band interference generally will cause more loss of data packets than outside interferers like cell phones.

So can you use radios in critical environments where in-band or out-of-band interferers may be present? The answer in most cases is yes, as long as you have a good data protocol in place. All radios in the 900 MHz and 2.4 GHz bands must use a form of spread spectrum if transmitting over a certain output power. If interference does occur, a good protocol can be used to retransmit or recover the data later on. Using a good protocol has allowed Digi radios to be used where reliable data transmissions becomes critical, such as medical, control and safety monitoring applications.

 

 

 

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