Radio Modems Information
Radio modems encode, transmit, receive and decode serial data using radio waves. They connect to serial ports on devices such as video cameras and data acquisition systems, and send signals to and receive signals from other radio modems. Although these modems do not require wires, their uses may be limited by uneven terrain, inadequate antenna heights, and antenna feeder cable loss. In these situations, a radio modem repeater may be required.
Radio modems transmit serial data as radio waves in a specific frequency according to a defined radio technique. This digital data is encoded prior to transmission, and then decoded prior to receipt. Once decoded, the serial data is provided to the connected device.
Frequency Bands and Radio Techniques
When selecting radio modems, buyers need to specify a frequency band.
- 900 MHz and 2.4 GHZ are unlicensed industrial, scientific and medical (ISM) bands that do not require FCC registration. For both, the maximum transmission power is 1 W.
- 5 GHz is part of the unlicensed national information infrastructure (UNII).
- 23 GHz is for microwave radio systems.
- UHF and VHF radio modems are also available.
To reduce the effect of localized frequency interference and noise, these modems use one of two spread spectrum
techniques. Both methods use more bandwidth than the system requires.
Direct sequence spreads the signal on a larger band by multiplexing it with a code or signature, a fast repetitive pattern. In the receiving radio modem, the original signal is recovered by receiving the whole spread channel and demodulating it with the same code. Direct sequence modems generally require faster circuits and a digital signal processor (DSP).
Frequency hopping moves the signal through a set of narrow channel in sequence. The transmission frequency band is divided into a certain number of channels. Following a predetermined cyclic hopping pattern, the system periodically moves to a new channel. Frequency hopping avoids interference and noise by never staying in the channel for a long period of time.
Frequency Hopping
Operating Modes
There are three operating modes for radio modems.
- Point-to-point devices can transmit to only one radio modem at a time.
- Point-to-multipoint devices can transmit to multiple radio modems at a time.
- Repeater mode devices are designed to receive a radio signal and then retransmit it a higher level and or power.
When selecting radio modems, buyers should also consider range reducers and enhancers, advanced networking and security, low-cost and low-poer options, frequency usage, and certifications and approvals.
Resources
The Radio Modem - Physical Layer
Image Credits:
RF Industries Inc. | B&B Electronics | HP / Jean Tourrilhes