Techsplanations: Part 6, Mobile Connectivity – an Incomplete Explanation of the Radio Spectrum

Written by Stan Adams

Previously in this series, we talked about what the internet is and how it works, what net neutrality means, and what a virtual private network, or VPN, is. In this post (and in a few that follow), we will cover some basics of mobile communications with a focus on modern mobile telecommunications networks. As before, please refer to this glossary for quick reference to some of the key terms and concepts (in bold).

If this is your first Techsplanation, welcome!

This post will start with a basic discussion of the radio spectrum, some physics, and how mobile networks use spectrum. The next post will break down some of the ways that mobile carriers get the most out of the spectrum they use, followed by a third post discussing the wired parts of mobile networks. After that (and some along the way), we will specifically dissect 5G networks and discuss what sets them apart from current generations of mobile networks (3G, 4G, LTE).

If you haven’t already read the first posts in this series, you may want to start at the beginning (What Is the Internet?) because most modern mobile networks use the same core infrastructure and protocols as the internet. As a disclaimer, these posts will not attempt to explain every aspect of mobile communications in detail (it gets really complex), but they will try to provide enough information to understand the fundamental building blocks.

If you are already impatient let me give you the TL:DR for this post: your mobile communications use radio waves. Some waves travel shorter distances and can carry more information, other waves can travel further but carry less.

If you are game for more, let’s start with the basic elements of a mobile communications network: the Radio Access Network and the Core Network (CN). The RAN is the wireless part of the network, all that invisible mystery between your phone and the nearest tower(s). The CN is all the wires and boxes connecting the towers to each other and to other networks – that’s right, your mobile carrier’s core network is part of the internet. When you use your wireless device to communicate over a mobile network, information from your device travels through the air to a receiving antenna (such as a wifi router or a cell tower) where it is then sent over wires to its destination, which may be wired into the internet or may require wireless transmission to another mobile device.

Today, the most common types of wireless connections include RFID, NFC, Bluetooth, WiFi, and radio access networks (RANs). Although this series will focus on RANs, the physics of radio communications are the same for all of these technologies. RANs are the networks your mobile phones use to make calls and connect to the internet. These are often called cellular networks because their geographical coverage areas are divided into multiple “cells,” each with its own antenna to transmit and receive communications. All of these kinds of connections use radio signals to convey information through the air.

Radio signals? Do you mean radio like the one I used to listen to in the car? Or like a walkie-talkie?

Yes, the radio signals your phone uses are essentially the same as those used to broadcast audio or television signals. Put simply, radio signals are sent using parts of the electromagnetic spectrum, called the Radio Frequency (RF) spectrum. Different parts of the RF spectrum have different qualities.

Longer wavelength, lower frequencies travel farther and are better at going over, around, and through more obstacles without losing their integrity. That’s why AM and Ham radio stations (operating at the lower end of the spectrum) are capable of transmitting signals over great distances. The part of the spectrum used for FM radio can carry clean signals for shorter distances (depending on broadcast power) and those waves are more likely to be broken up by physical obstacles (mountains, trees, buildings, etc).

Shorter wavelengths/higher frequencies are even more susceptible to being absorbed, bounced, or broken up by anything in the way, including water vapor and atmospheric gasses. One way to think about it is that waves don’t travel well through things that are physically bigger than their wavelength, which is why “millimeter” waves have trouble passing through raindrops and why your cell phone (which probably uses wavelengths in the 10-50 cm range) loses its connection when your train goes into a tunnel or you step into an elevator.

In short, longer wavelengths travel further but carry less information; shorter wavelengths have shorter ranges but can carry more information.

An aside about frequencies and wavelengths.

To unpack all of this a little more, wavelength and frequency are related. Longer wavelengths have lower frequencies; shorter wavelengths have higher frequencies. (For reference, Extremely Low Frequency (ELF) wavelengths are upwards of 100,000 km long and complete only a few, or even less than one, cycles per second, hence the “extremely low” frequency. Gamma rays are about one trillionth of a meter long (picometer) and complete upwards of 300 quintillion cycles per second. It’s a broad spectrum.)

A cycle is the time it takes for one complete oscillation of the wave (think sine wave) and is expressed in terms of Hertz. One Hertz equals one cycle per second. In between the two extreme ends of the spectrum are things like VHF (very high frequency), UHF (ultra high frequency), “millimeter waves” (which have wavelengths between 1-10 mm), infrared, visible light, ultraviolet, and X-rays. How these waves are produced gets a bit more complicated, but it basically involves charged particles giving off energy – electromagnetic radiation. All of these waves propagate or travel at (or near) the speed of light.

Okay, so high frequencies are not as sturdy as low frequencies. What parts of the RF spectrum does my phone use?

Remember that the RF spectrum is a limited resource – there are only so many different frequencies available and, at least for sending communication signals, there needs to be space between them to avoid interference between neighboring frequencies. Regulating use of the RF spectrum is shared between the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA). In addition to granting licenses to use available frequencies, the FCC also controls the geographical ranges associated with those licenses. We’ll talk a bit more about this in the next post.

Most mobile phones now are capable of using a few different parts of the spectrum, depending on what protocols they are built for. These protocols are roughly associated with the different Gs (2G, 3G, etc) and have fancy sounding acronyms like GSM, UMTS, and LTE. Newer phones are usually capable of connecting to the most modern networks and any existing networks using older protocols, so your LTE phone could also work on GSM networks. This means that newer phones need more antenna capabilities than older phones, but are also more versatile. The currently operating networks use pieces of the spectrum between about 300 and 3500 MHz (3.5 GHz).

Cool, cool. But you still haven’t explained how are waves used to transmit information through the air!

The way information is encoded in radio waves gets pretty complicated, but you can think of it like this –

Imagine that you and a friend are each holding opposite ends of a long rope. You are each able to move the rope up and down (from your end) in a way that sends up-down waves along the rope to the other end, where the rope (and your friend’s hand) will go up and down according to the same wave pattern you shook. You have agreed on a system that establishes a specific sequence of ups and downs to have a certain meaning or value, such as a letter, word, or phrase. In this case (transmitting digital data) you only need two signals—1 and 0—this is called binary notation.

So you or your friend can shake the rope up and down to send messages composed of ones and zeros to the other end of the rope. The faster you shake the rope, the more wave signals you send, the more ones and zeros your friend receives. That’s why higher frequencies, which emit more cycles per second (more shakes of the rope) than lower frequencies, are capable of transmitting data at higher speeds.

Uhhhh, right. So my phone shakes an invisible rope up and down to send binary messages to the nearest tower, and/or to my WiFi router, but then what?

Your phone’s antennas (most 4G smartphones have a handful, 5G phones will have many more – we’ll talk about why in a future post) pick up these wave signals and translate them into ones and zeros that are sent to the phone’s processors in the form of electrical current – like flipping a lightswitch on and off. On the other end, the mobile carrier’s cellular antenna will be doing something similar, except it may transform the wave signals into either electrical pulses or tiny bursts of light (this is how fiber optic communication works). In some cases the antenna will merely relay the radio signals to another tower. But I digress… back to the spectrum.

As I was saying earlier, higher frequencies (more Hertz) in the spectrum are capable of carrying more information per second than lower frequencies, but are not as reliable as lower frequencies over longer distances. So while higher frequencies can serve more customers per cell, those cells are smaller than cells using lower frequencies. This means that, as carriers use higher and higher portions of the spectrum (this will be important when we get to 5G), the cell size will decrease, resulting in more cells and more antennas. Cell size also depends on the power of the transmitter, so there can be smaller or larger cells for any given frequency band. As a preview, 5G RANs may use a few different spectrum bands, including some upwards of 30 GHz, which gets into the “millimeter” wave bands. Cells using this band must be quite small, since millimeter waves work best in short-distance, line-of-sight environments.

Alright, so frequency and power determine cell sizes in mobile networks. What about Wifi and Bluetooth?

The same is true for all radio frequency communications. For these shorter-range wireless links, their maximum power is limited by the FCC, which helps prevent wireless systems from interfering with each other and with other systems using the same or similar parts of the spectrum. However, these shorter-range wireless technologies use unlicensed spectrum, so individual devices like your home router or your wireless headphones can operate anywhere without obtaining FCC permission first. Bluetooth uses a narrow band at 2.4GHz, while WiFi (the marketing name commonly used for a wireless local area network [WLAN], operating according to the IEEE 802.11 protocol) uses a few different bands. Near-field communications (NFC) are similar, but even shorter range (usually only a few centimeters), and support things like contactless payment systems.

So, that’s enough about spectrum for one post. Check out the next installment for a breakdown of the ways mobile carriers get the most out of their spectrum licenses.

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