Radio Frequency (RF) communications is based on laws of physics that describe the behavior of electromagnetic energy waves. For the purpose of providing a very cursory understanding of the technology this tutorial will use very informal terminology to describe what is happening.
General physics of radio signals
RF communication works by creating electromagnetic waves at a source and being able to pick up those electromagnetic waves at a particular destination. These electromagnetic waves travel through the air at near the speed of light. The wavelength of an electromagnetic signal is inversely proportional to the frequency; the higher the frequency, the shorter the wavelength.
Frequency is measured in Hertz (cycles per second) and radio frequencies are measured in kilohertz (KHz or thousands of cycles per second), megahertz (MHz or millions of cycles per second) and gigahertz (GHz or billions of cycles per second). Higher frequencies result in shorter wavelengths. The wavelength for a 900 MHz device is longer than that of a 2.4 GHz device.
In general, signals with longer wavelengths travel a greater distance and penetrate through, and around objects better than signals with shorter wavelengths.
How does an RF communication system work?
Imagine an RF transmitter wiggling an electron in one location. This wiggling electron causes a ripple effect, somewhat akin to dropping a pebble in a pond. The effect is an electromagnetic (EM) wave that travels out from the initial location resulting in electrons wiggling in remote locations. An RF receiver can detect this remote electron wiggling.
The RF communication system then utilizes this phenomenon by wiggling electrons in a specific pattern to represent information. The receiver can make this same information available at a remote location; communicating with no wires.
In most wireless systems, a designer has two overriding constraints: it must operate over a certain distance (range) and transfer a certain amount of information within a time frame (data rate). Then the economics of the system must work out (price) along with acquiring government agency approvals (regulations and licensing).
How is range determined?
In order to accurately compute range – it is essential to understand a few terms:
- dB - Decibels
Decibels are logarithmic units that are often used to represent RF power. To convert between milliWatts (mW) and decibels (dB), use the following conversion equations (P = power):
(mW to dBm) PdBm = 10 * Log10(PmW)
(dBm to mW) PmW = 10^(PdBm/10)
- Line-of-Sight (LOS)
Line-of-Sight when speaking of RF means more than just being able to see the receiving antenna from the transmitting antenna. In, order to have true line-of-sight no objects (including trees, houses or the ground) can be in the Fresnel zone. The Fresnel zone is the area around the visual line-of-sight that radio waves spread out into after they leave the antenna. This area must be clear or else signal strength will weaken.
There are essentially two parameters to look at when trying to determine range.
- Transmit Power
Transmit power refers to the amount of RF power that comes out of the antenna port of the radio. Transmit power is usually measured in Watts, milliwatts or dBm. (For conversion between watts and dBm see above.)
- Receiver sensitivity
Receiver sensitivity refers to the minimum level signal the radio can demodulate. It is convenient to use an example with sound waves; Transmit power is how loud someone is yelling and receive sensitivity would be how soft a voice someone can hear. Transmit power and receive sensitivity together constitute what is know as “link budget”. The link budget is the total amount of signal attenuation you can have between the transmitter and receiver and still have communication occur.
Digi 9XStream TX Power: 20dBm
Digi 9XStream RX Sensitivity: -110dBm
Total Link budget: 130dB.
For line-of-sight situations, a mathematical formula can be used to figure out the approximate range for a given link budget. For non line-of-sight applications range calculations are more complex because of the various ways the signal can be attenuated.
Regulations and licensing
The Federal Communications Commission (FCC) and other regulatory bodies around the world have set up a series of regulations defining the emission levels and usage for all the different frequencies. Digi radios operate within the Industrial, Scientific and Medical (ISM) bands that offer license free operation within certain frequencies. Within the United States, the most popular ISM band are at 902-928 MHz and 2.4 – 2.4835 GHz. Portions of the 902-928 MHz band are also available in Canada, Mexico, Australia and Israel. The 2.4 GHz band is generally more accepted worldwide.
At certain power levels some regulatory agencies require some form of spread spectrum. Spread spectrum can either be done by frequency hopping or by direct sequence. Frequency hopping consists of rapidly moving from one channel to the next while maintaining synchronization with the receiver. Direct Sequence is more complex, but works by slicing the carrier up with a code that can be decoded at the other end. Digi radios uses frequency hopping as its method of spread spectrum.
RF communications and data rate
Data rates are usually dictated by the system - how much data must be transferred and how often does the transfer need to take place. Lower data rates, allow the radio module to have better receive sensitivity and thus more range. In the XStream modules the 9600 baud module has 3dB more sensitivity than the 19200 baud module. This means about 30% more distance in line-of-sight conditions. Higher data rates allow the communication to take place in less time, potentially using more power to transmit.