Understanding Cell Phone Technology: What’s RF?

The Electromagnetic (EM) spectrum refers to the complete range of electromagnetic radiation, typically described by frequency or wavelength. The frequencies of EM waves range from 3 to 300 * 10^18 Hertz (Hz), or wavelengths of 1 * 10^5 to 1 * 10^-12 meters (m).

In a vacuum, all EM waves travel at the speed of light c, where c is 3 * 10^8 meters per second. There is a basic relationship between c, and the EM wave’s frequency lambda and its wavelength nu:

c = lamba * nu

There are many different portions of the EM spectrum. They are broadly labelled as radio, infrared, visible light, ultraviolet, x-rays, and gamma rays. The radio portion of the EM spectrum, called the RF (Radio Frequency),  spans 3 Hz to 3 * 10^12 Hz. The diagram attached below shows the bands into which RF is partitioned, and some typical applications for each band. If you’re curious about what each acronym stands for, do a little internet research. If you’re curious about how we use all these bands in the US, see https://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf.

Many of us are familiar with radio in our cars. AM (amplitude modulation) radio lives in the radio band spanning frequencies 525 – 1606.5 * 10^3 Hz or 525 – 1606.5 kHz. AM is good for speech, but not so good for signals with higher information content, like music. FM (frequency modulation) radio, at 87.5 – 108 * 10^6 Hz or 87.5 – 108 MHz, has greater bandwidth, so it can broadcast a higher quality signal. That’s one reason that music sounds so much better on FM than AM. While AM and FM radio originally broadcast analog signals, new technology is allowing them to migrate to digitally enhanced signals.

The newest addition we have to car radio is satellite radio, which in the U.S. broadcasts a digital signal nationwide in the 2.3 GHz band.

Of course, the RF is used for far more than broadcast radio. Radar systems can use frequencies in bands from 50 MHz to 40 GHz, and above. Satellite communications systems rely on bands from 3 GHz to 40 GHz. Bluetooth wireless technology uses 2.45 GHz. And U.S. cell phones use a wide variety of frequencies, including 800 MHz, 850 MHz, 1700 MHz, 1900 MHz, and 2100 MHz. The newest generation of cell phone technology, called 5G, will add frequencies at 600 MHz, 2.5 GHz, 28 GHz, and 39 GHz.

In point of fact, your cell phone has several radios in it, and an antenna for each radio.  These include a cellular radio, a Bluetooth radio, a Wi-Fi radio, and a GPS (Global Positioning System) radio. What does a cell phone need these all for?

The cellular radio allows you to connect to the Internet via the cellular system, relying on the phone’s cellular antenna to transmit and receive. The Bluetooth radio allows you to communicate (transmit and receive) a short distance between your phone and your headset, earbuds, or car stereo system.

The Wi-Fi (IEEE 802.11)  radio allows you to connect to the Internet via a wireless (computer network) router to transmit and receive. Wi-Fi uses various frequencies, including 900 MHz, 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz, 5.9 GHz, and 60 GHz.

The GPS radio allows your phone to receive position (and time) data from the constellation of GPS satellites. GPS is the US version of what is known as a Global Navigation Satellite System (GNSS). Many cell phones can also receive GNSS signals from competing GNSS systems, like  Galileo (European), GLONASS (Russian), and perhaps even Beidou (Chinese).

GPS itself does not track your location, nor does your GPS radio transmit any information about your location. Instead, other radios in your cell phone transmit information about your location with the cellular and/or Wi-Fi networks. We’ll talk more about GPS another day.

All in all, your cell phone is a remarkable little device which relies on multiple radios and antennas for its functionality. Its messages are carried from transmitter to receiver on an analog wave traveling over the RF. How the original message gets prepared to be transmitted over the EM spectrum will be the subject of our blogs for the next couple of weeks.