Understanding Solutions For The Crowded Electromagnetic Frequency Spectrum

The combination of electric and magnetic fields at right angles produces radio waves. These waves occur at different frequencies, and the range of useful frequencies is called the electromagnetic spectrum. Here is a look at the state of the wireless spectrum and the issues related to it.

Table Of Contents

1. Spectrum Regulation
2. The Spectrum Up Close
3. The Spectrum Crisis And Its Causes
4. Mitigating the Spectrum Problem
5. References

Spectrum Regulation

All electronic communications requires some transmission medium to carry the information to be communicated, like coax and fiber-optic cables. But for radio, the medium is free space—or the ether, as they used to call it.

In radio communications, electromagnetic waves travel through the air at the speed of light (300 million meters per second) from transmitter to receiver. That means they’re all around us. The air is jammed with every conceivable wireless signal you can think of and many you can’t.

So how can any radio communication be successful with all those seemingly interfering signals? What makes it possible for any wireless connection to be successful? The answer lies simply in the fact that each communication is on its own assigned frequency.

If there’s one thing that good receivers and transmitters do best, it’s operate on their own frequency. Signals are separated and selected by frequency. The electromagnetic frequency spectrum is the full range of frequencies that wireless signals use.

Since the frequency spectrum is after all “free” space, you would think that anyone could use it. Wrong! Imagine the chaos of thousands or even millions of devices trying to use the same frequencies at the same time. The result would be massive non-communications because of the interference.

That’s why governments control and manage virtually all frequency spectrum. Most of the more than 150 countries of the world have frequency management organizations. These agencies and other interested parties meet every three or four years at the World Radiocommunications Conference (WRC), sponsored by the International Telecommunications Union (ITU). They discuss spectrum issues, argue the merits of different spectrum proposals, and resolve spectrum conflicts.

The ITU also facilitates satellite management. The geostationary satellite orbit that’s 22,300 miles above the world around the equator is getting full, and satellites are literally in sight of one another over some parts of the world. It gets trickier each year to place new satellites in this orbit. The ITU and WRC help manage the placement issues.

In the United States, the Federal Communications Commission (FCC) manages all commercial and domestic frequencies, and the National Telecommunications and Information Administration (NTIA) handles all government and military frequency assignments. These frequencies involve radio and TV broadcasting, cellular, land mobile radio (LMR), marine and aircraft radio, radiolocation, amateur radio, satellites, radar, and anything else you can think of.

The NTIA offers a chart of all U.S. frequency allocations, available at www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf. The Code of Federal Regulations (CFR) Title 47 Parts 0 through 100 from the Government Printing Office includes a more complete listing of spectrum assignments.

The CFR was updated in October 2011. Part 2 lists the frequency assignments, but changes in spectrum allocations are posted in the Federal Register daily. If you work with wireless in any way, this is a good reference to all the relevant regulations.

The Spectrum Up Close

Figure 1 gives you the big picture of the total spectrum from dc to light and beyond. Note both the frequency and wavelength measures. In the wireless world we mainly use frequency. But in the upper reaches of the spectrum, we use wavelength in meters as spectrum bands.

The radio spectrum, those frequencies used for wireless, roughly extends from 30 kHz to 300 GHz. Frequencies above 1 GHz are generally known as microwaves (Table 1). Frequencies in the 30- to 300-GHz range are also called millimeter waves. The radio range is also subdivided into sections for identification (Table 2).

Right above 300 GHz is the terahertz (THz) region, right before the optical spectrum begins. This has been a dead zone up until recently. With no hardware available to generate or receive signals in this range, there were no applications. Today, with several key breakthroughs, this region has become promising. Considerable research and development is in progress, so the future may bring equipment and useful applications.

Then there’s the optical region. Yes, optical radiation is electromagnetic. We switch from a frequency designation to a wavelength designation. At the lower-frequency end of the range is far infrared (IR) beginning at 300 GHz (1000 nm) followed by mid-infrared and near infrared.

IR is widely used in electronics for thermal (temperature measurement) and electronic imaging (night vision). It also is the light of choice for fiber-optical communications. Typical operating channels are 800 to 1600 nm.

Additionally, IR is the main communications band for TV and other consumer remote controls, which operate in the 800- to 1000-nm region. Then there’s the visible optical spectrum from 700 nm (red) to 400 nm (violet). Beyond optical are ultraviolet, x-rays, and gamma rays, which are beyond our scope of discussion here.

The Spectrum Crisis

Mark Twain’s advice—buy land, they’re not making it any more—applies to spectrum. Once we use it up, where will we go for wireless? We haven’t used it all yet, but we’re getting close. Some segments of spectrum are more useful than others, and those segments are essentially full.

These full segments include VHF, UHF, and the low microwave frequencies from roughly 100 MHz to 4 GHz. That’s where cell phones, broadcast TV, wireless local-area networks (LANs), and lots of popular short-range technologies like Bluetooth and Wi-Fi operate. Without more spectrum, we face a crisis of wireless expansion.

Today, wireless is the hottest and fastest growing segment of electronics. Without a place to expand, will our wireless fortunes come to an end? The cellular carriers think so. So does the U.S. government, which is looking to wireless as a solution to rolling out its National Broadband Plan for universal high-speed Internet service across the U.S. What to do?

Let’s be more specific about this so-called spectrum crisis. The crisis is the lack of spectrum to expand cell-phone and broadband wireless services. Several key trends are making the problem worse:

• Growth in mobile Web access as a result of the smart phone’s exceptional adoption and success
• Growth in the “Internet of Things” connectivity phenomenon, with wireless machine-to-machine (M2M) communications
• The pressure to implement the National Broadband Plan to provide high-speed Internet connectivity to most of the U.S.

More to the point, the crisis is a shortage of the specific frequency ranges where wireless works best in those applications. Spectrum is available but just not where cellular operators and others want it to be. Everyone wants to be in the 100-MHz to about 4-GHz range, VHF, UHF, and low-frequency microwave as indicated earlier. What’s so great about this region?

First, it’s ideal for short-range communications, roughly less than 100 miles. Much of it is line-of-sight (LOS) spectrum where the antennas have to “see” one another. However, the lower frequencies in the range work in some non-LOS environments. This characteristic keeps the signals from traveling too far and interfering with others on the same frequencies.

Also, the signals aren’t refracted off the ionosphere like many lower-frequency signals are. Shortwave signals called sky waves in the 3- to 30-MHz range can “bounce” off the ionosphere, causing the signal to travel huge distances, in some cases around the world with multiple bounces from earth to ionosphere and back. VHF, UHF, and microwave signals penetrate the ionosphere, so they can be used with satellites and other spacecraft. This is desirable in some applications.

Frequencies higher than 6 GHz do not travel as far because of physics unless very high-power and highly sensitive receivers are used. The well-known Friis formula says that for a given power level, the range of a signal is proportional to the wavelength (see “Friis Formula For A Radio Signal”). The higher the frequency, the shorter the wavelength, and the more limited the range of the signal for a given power level and antenna gains.

So there’s really only a shortage of the most desirable frequencies as well as a shortage of contiguous frequencies. Most new cell-phone and other technologies use orthogonal frequency-division modulation (OFDM) methods and require wide bandwidths. You can’t break it up into narrower channels.

Furthermore, frequency division duplex (FDD) cellular systems need one contiguous segment for uplink and one for the downlink. These segments must be properly spaced to avoid interference between transmitters and receivers. Finding these segments within the cellular operator’s spectrum holdings has been difficult and in some cases impossible. Some spectrum may be available, but it just isn’t wide enough or spaced as needed. Spectrum fragments are a problem and actually waste spectrum.

An interesting part of this issue is that there are still many unused chunks of spectrum in the desirable range. These frequencies are assigned by the FCC or NTIA and owned by a company or government organization, but they aren’t being used. They’re tied up with specific licensees who are holding on to them for future use.

For example, TV channels were assigned years ago, but the broadcasters didn’t have to pay for them, as most other companies have to pay for spectrum today. Many organizations covet these unused bands because of their potential for so many other useful services.

Lower frequencies are more plentiful, and there are numerous gaps. The bad news is that these lower frequencies cannot support the high data rates so common in today’s digital technologies. Data has to be modulated onto a carrier to transmit. If the modulating frequency or data rate is greater than the carrier frequency, modulation won’t work. In some cases it could work, but the resulting modulation bandwidth would occupy too much of the surrounding spectrum. For high-speed data, we need the higher frequencies.

With digital data rates continuously increasing, the bandwidth also must increase—the faster the data, the greater the bandwidth needed to transmit it. The Shannon-Hartley formula relates speed and bandwidth and noise (see “Shannon Hartley Theorem: Data Rate Versus Bandwidth”). Reducing the data rate or using other measures like data compression or higher-level modulation schemes minimizes the bandwidth needs and reduces spectrum requirements.

The FCC’s Spectrum Dashboard provides a public means for reviewing how spectrum bands are allocated and what they’re used for, as well as who holds licenses in what areas. It covers the most useful frequency ranges from 225 MHz to 3.7 GHz. It’s available at http://reboot.fcc.gov/reform/systems/spectrum-dashboard.

Mitigating The Spectrum Problem

This problem is not new. It has been going on for years but has gotten worse as technology changes have brought us smart phones, high-speed data, HDTV, and fast Internet access. This unending dilemma has brought forth a mix of constant new methods, policies, and technologies. Yet more needs to be done.

One of the oldest methods of preserving spectrum is to allow multiple users to use the same frequency in situations where one signal won’t interfere with another, such as AM and FM broadcasting.

There are dozens of AM radio stations at 780 kHz, but they’re widely spaced throughout the country. There is little chance that one will interfere with another, especially during the day when range is limited to a hundred miles or so. At night with sky-wave bounce conditions, some interference may occur. But typically, frequency and geographical locations are selected to minimize such problems.

The same goes with FM. Many stations use 98.7 MHz. But since most FM signals only travel a maximum of 70 to 100 miles, many stations can be assigned to the same frequency.

Cellular phone systems reuse frequencies by carefully spacing adjacent cell sites and adjusting cell antenna height and directivity as well as power level. Since most cell site ranges are good for only a few miles or so, many calls can be added on the same frequencies over a coverage area.

The movement to femto cells and smaller pico and metro cells by the cellular operators will help extend the frequency reuse method further by limiting cell site range to less than a mile and in some cases only several hundred feet. LMR stations operate the same way, using VHF and UHF for handhelds and mobile radios over short distances.

If you run out of space at the lower frequencies, seek spectrum at higher frequencies where there’s more space. That’s what we’ve been doing the last few decades. As ICs and other solid-state devices have been developed, improved higher-frequency operation has been enabled, making it possible to move some services to higher frequency ranges. Cellular radios started out in the 800- to 900-MHz range, which is still used. But today, there are many newer bands at frequencies in the 1800- to 2700-MHz range.

Wi-Fi LANs initially used 2.4 GHz and still do, though newer frequency assignments in the 5-GHz range are now available. Most radio services gradually move up in frequency if the spectrum is available and electronic equipment is available to support it. Look for Wi-Fi to move to 60 GHz as the new wireless standards are developed and approved, such as 802.11ad.  

Meanwhile, the industry has been striving to develop more spectrally efficient electronics for years. Modulation and access methods have gone a long way in boosting the number of signals per given bandwidth.

The original cellular phones using FM were quickly replaced by digital modulation methods using time division multiplexing (TDM). Next came spread spectrum such as direct sequence and frequency-hopping methods that squeezed many users into a common spectrum assignment.

OFDM is even more spectrally efficient. The Long Term Evolution (LTE) cellular standard uses OFDM and higher-level modulation schemes to get high data rates into narrower channels. An OFDM version called discrete multitone (DMT) is used to get high speeds on limited bandwidth telephone twisted pair cable in DSL high-speed Internet systems (see “Understanding Modern Digital Modulation Techniques”).  

Multiple-input multiple-output (MIMO) is another technique for boosting data rate in limited spectrum. By using multiple transceivers and antennas, high-speed data is divided into different streams and transmitted simultaneously in the same channel. Using encoding and the inherently different signal characteristics of each transmitted signal, the data can be extracted at the receiver and reconstituted into the original fast data stream. Wi-Fi uses this technique, as does LTE.

Time division duplexing (TDD) also saves space by putting both send and receive digital signals on the same frequency but in different time slots. TDD is a spectrum-efficient way to implement full duplex (simultaneous two-way conversations). Most cellular systems still use FDD, which uses two segments of spectrum spaced to avoid interference between transmitted and received signals. One segment is the uplink and the other is the downlink. But using TDD, only one segment is needed.

Then there’s data compression. In the beginning, the TV broadcasting system was given 6-MHz channels, which fit the original analog TV signals with truncated or vestigial sideband AM for the video. Today, broadcasters squeeze high-definition video into that 6-MHz band with MPEG2 video compression and multi-symbol modulation like eight-level vestigial sideband (8VSB). Voice compression is common in cell phones as well.

Two-way radio also uses spectrum-saving methods. One of the oldest, single sideband (SSB), is a form of AM radio that uses only one of the two sidebands generated by the AM process. This halves the needed spectrum. The military, marine radio, and amateur radio operators all use SSB.

LMR systems based on FM used in public safety and service now have to cut their bandwidths from 25 kHz to 12.5 kHz and eventually to 6.25 kHz to accommodate more users. This is now possible with voice compression and multi-symbol modulation like four-level frequency shift keying (4FSK).

One unique piece of new technology conserves spectrum indirectly via filtering. ISCO International’s Proteus PurePass signal processing unit is an adaptive digital filter designed to fit on the receive side of a cellular basestation (Fig. 2). It eliminates any interference to the small signals received on the uplink from handsets. Such interference can come from adjacent channel signals or other nearby sources. The filter maximizes existing spectrum utilization to support more calls in existing spectrum and enable higher data throughput.

Or, the industry can rob Peter to pay Paul. This happens sometimes when one service runs out of spectrum and cannot expand, so it petitions and pressures the FCC (or NTIA). The FCC finds some unused spectrum assigned to someone else, takes it, and reassigns it. It’s like eminent domain in real estate. With the pressure on, look for more of this in the future.

Auctions are one of the most useful of all solutions. Simply find and acquire unused spectrum, repurpose it, and sell it to the highest bidder. The FCC held auctions in 2008 and sold lots of spectrum to the cellular carriers, solving some of their problems and enriching the U.S. Treasury by $19 billion. Look for another auction in the near future where the reclaimed broadcast TV spectrum will be sold mainly for cellular expansion and the National Broadband effort.

Another solution for your spectrum problems is to find someone who has the spectrum you need and buy it or swap it for some other spectrum of your own. It’s often possible to piece together unrelated spectrum chunks into large segments by identifying the parties who have what you need. The FCC has to approve, though. Spectrum brokers help buyers and sellers find one another.

Or, try pressuring Congress, the FCC, and NTIA. The FCC and the NTIA hold the power to reassign spectrum. With most of it already assigned, finding new blocks of spectrum to reassign or auction is tough. But it can be done in some cases. These regulatory agencies need some good reason, political or financial, to get them moving. Outside influences like lobbyists help.

Of course, these agencies can improve their spectrum management. The FCC and NTIA have their hands full managing the spectrum assignments. It is very complex, and thousands of entities are involved. Hundreds or even thousands of spectrum transactions take place daily, and keeping track of them is a big challenge. Any improvement in management can result in found spectrum that can be reassigned. The goal is to optimize the use of the existing spectrum.

Then again, these agencies can assign more unlicensed spectrum. This doesn’t seem to be a solution until you look at what can be done. For example, anyone can use the unlicensed industrial, scientific, and medical (ISM) bands such as 902 to 928 MHz approved by Part 15 of the rules and regulations with approved equipment.

All sorts of people, services, and devices use this region without interference, like industrial telemetry and cordless phones. The widely spaced users and low-power limitations that minimize interference make its use possible.

Also, billions of unlicensed devices use the popular 2.4- to 2.483-GHz band. The biggest user is Bluetooth and all those billions of headset/cell-phone radios. Wi-Fi LANs are big users as well with millions of laptops, tablets, and smart phones accessing millions of access points and hotspots. So why isn’t there more interference?

Well, there is interference, but it’s short range and minimized by low power. Furthermore, frequency-agile methods can detect conflicts and avoid them. These unlicensed technologies have a way of solving interference problems and providing for co-existence in widely used spectrum. There should be more of these technologies. Still, there’s no guarantee that there won’t be any interference, as with exclusive licensed spectrum.

White space, which refers to the unused TV channels in the 50- to 700-MHz range, offer another solution. These spaces vary from city to city depending on TV channel assignments and how they are separated from one another. The FCC has authorized the use of these channels for data applications such as wireless high-speed Internet access. Some call it “Super Wi-Fi,” but it’s not 802.11 as such. It uses different radio technology to maximize the data rate in existing 6-MHz channels.

At the heart of the white space movement is the mandatory use of databases that the radios must access to ensure that they do not interfere with local TV stations or other wireless devices nearby, such as wireless microphones. The radio selects an operational channel and then accesses the database to see if there might be an interference problem. If none is detected, the transmission takes place. If interference is likely, another channel is selected to avoid it.

This frequency-agile characteristic is derived from a technology called cognitive radio, which is an offshoot of software-defined radio (SDR) that adds intelligence and decision-making capability to a radio, enabling it to find the best frequency for reliable communications while avoiding interference. FCC-approved databases are now available from Spectrum Bridge and Telcordia.

Carlson Wireless and U.K. partner Neul are among the first companies to offer white space radios. The Carlson Wireless RuralConnect IP Version II (RCIP VII) comprises a basestation and customer premise equipment (Fig. 3). It’s designed to create point-to-point and point-to-multipoint networks for voice, data, and video in the 470- to 786-MHz range. It also can achieve data rates of 4, 6, 8, 12, or 16 Mbits/s in a 6-MHz channel using quadrature phase-shift keying (QPSK) or 16-phase quadrature amplitude modulation (16QAM). Time division duplexing (TDD) is used as well.

A 6-MHz channel can accommodate from 40 to 60 concurrent users with a typical stream of 3 Mbits/s downstream and 1 Mbit/s upstream. Transmit power is +30 dBm, and receive sensitivity ranges from –86 to –89 dBm. Security is by AES-128 with a shared secret key. The units are designed to use the Telcordia and Spectrum Bridge databases and operate under the FCC’s Part 15 rules as well as the U.K. Ofcom regulations.

Some cellular operators are offloading high-speed data accesses like video streaming to nearby Wi-Fi networks. If a smart-phone user accesses a video and the carrier’s network is in overload, the data streaming can be rerouted through a nearby Wi-Fi hot spot, if one is available. With Wi-Fi access nearly ubiquitous thanks to millions of access points throughout the world, this is a workable solution. Some carriers are even expanding their existing Wi-Fi networks or building new ones to implement this workaround solution to the spectrum crisis.

Wi-Fi’s latest version, 802.11ac, will go a long way to help the offload situation (see “What’s The Difference Between 802.11n And 802.11ac”). It can achieve data rates greater than 1 Gbit/s in the 5-GHz ISM band. Broadcom’s BCM43460 system-on-a-chip (SoC) will help implement some of these new faster access points (Fig. 4).

Finally, if all else fails, use a cable. With a single coax cable, you can carry most of the lower part of the radio spectrum without interference with others. That’s what cable TV does. A normal RG-6/U coax cable carries TV signals and high-speed Internet service from 40 MHz to more than 1 GHz. Fiber-optic cables can carry data signals to 100 Gbits/s and beyond. Running cable is expensive, but it does mean you have the entire cable spectrum all to yourself.

References

1. Code of Federal Regulations 47, Chapter 1, Part 2
2. Federal Communications Commission
3. National Telecommunications and Information Administration

Friis Formula For A Radio Signal

The Friis formula illustrates the basic relationship between transmitted signal power (Pt), received signal power (Pr), transmitter antenna gain (Gt), receiver antenna gain (Gr), range or distance (d) in meters, and wavelength (λ) in meters for a free-space radio signal. The antenna gains are power ratios, not dB, and are referenced to an isotropic source:

P_r = (P_tG_tG_rλ²)/(16π²d²)

or:

d = √[(P_tG_tG_rλ²)/(16π²P_r)]

Range is proportional to wavelength. Path distance d is inversely proportional to frequency.

Remember, the relationship between frequency and wavelength in meters is:

λ = 300/f_MHz

Shannon Hartley Theorem: Data Rate Versus Bandwidth

The basic formula relating the channel capacity or data rate (C) and bandwidth (B) is:

C = Blog₂(1 + S/N)

C is in bits per second (bits/s), S is the signal power, and N is the noise power.

The logarithm to the base 2 for any number N can be found with standard common (base 10) logarithms with:

log₂N = 3.32 log₁₀N