Technology finally makes millimeter waves practical to use, enabling the continued growth of wireless communications before we run out of spectrum.
Millimeter waves occupy the frequency spectrum from 30 GHz to 300 GHz. They’re found in the spectrum between microwaves (1 GHz to 30 GHz) and infrared (IR) waves, which is sometimes known as extremely high frequency (EHF). The wavelength (λ) is in the 1-mm to 10-mm range. At one time this part of the spectrum was essentially unused simply because few if any electronic components could generate or receive millimeter waves.
All that has changed in the past decade or so. Millimeter waves are now practical and affordable, and they’re finding all sorts of new uses. Best of all, they take the pressure off the lower frequencies and truly expand wireless communications into the outer limits of radio technology (see the table). If we go any higher in frequency, we will be using light.
| Table 1. | Microwave and millimeter-wave bands. | ||||||||||||||||||||||||||||||||
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The Pros And Cons
Millimeter waves open up more spectrum. Today, the spectrum from dc through microwave (30 GHz) is just about used up. Government agencies worldwide have allocated all of the “good” spectrum. There are spectrum shortages and conflicts. The expansion of cellular services with 4G technologies like LTE depends on the availability of the right sort of spectrum. The problem is that there isn’t enough of it to go around.
As a result, spectrum is like prime real estate—it’s expensive. And the expression “location, location, location” is apt for spectrum. Millimeter waves partially solve the problem by providing more room for expansion. You can take all of the useful spectrum we now use from dc to 30 GHz and drop it into the lower end of the millimeter-wave region and still have 240 GHz left over.
Millimeter waves also permit high digital data rates. Wireless data rates in microwave frequencies and below are now limited to about 1 Gbit/s. In the millimeter-wave range, data rates can reach 10 Gbits/s and more.
The bad news is that while this spectrum gives us some expansion room, it isn’t useful for all types of wireless applications. It has its limitations. Overcoming those shortcomings has been the challenge of making millimeter waves practical and affordable. That time has come.
One of the key limitations of millimeter waves is the limited range. The laws of physics say that the shorter the wavelength, the shorter the transmission range for a given power. At reasonable power levels, this limitation restrains the range to less than 10 meters in many cases.
The free space loss in dB is calculated with:
L = 92.4 + 20log(f) + 20log(R)
R is the line-of-sight (LOS) distance between transmit and receive antennas in kilometers, and f is the frequency in gigahertz. For example, the loss at 10 meters at 60 GHz is:
L = 92.4 + 35.6 – 40 = 88 dB
Designers can overcome this loss with good receiver sensitivity, high transmit power, and high antenna gains.
Also, the atmosphere absorbs millimeter waves, restricting their range. Rain, fog, and any moisture in the air makes signal attenuation very high, reducing transmission distances. Oxygen (O2) absorption is especially high at 60 GHz (Fig. 1). Water (H2O) absorption is responsible for the other peaks. Selecting frequencies within the curve valleys minimizes the loss. Additionally, high-gain antenna arrays can boost the effective radiated power (ERP), significantly increasing range.
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| Figure 1. | The plot of signal attenuation at sea level and 20°C versus log frequency shows how oxygen (at 60 GHz) and water at the other peaks in the atmosphere significantly increase signal attenuation. |
In fact, the short range can be a benefit. For example, it cuts down on interference from other nearby radios. The high-gain antennas, which are highly directional, also mitigate interference. Such narrow beam antennas increase power and range as well. And, they provide security that prevents signals from being intercepted.
Small size is another major advantage of millimeter-wave equipment. While ICs keep the circuitry small, the high frequency makes very small antennas necessary and possible. A typical half-wave dipole at a cellular frequency like 900 MHz is six inches long, but at 60 GHz one half-wave is only about 2.5 mm in free space and even less when it’s made on a dielectric substrate. This means the entire structure of a radio including the antenna can be very small. It’s easy to make multiple-element phased arrays on a substrate chip that can steer and focus the energy for greater gain, power, and range.
Another challenge is making circuitry that works at millimeter-wave frequencies. With semiconductor materials like silicon germanium (SiGe), gallium arsenide (GaAs), indium phosphide (InP), and gallium nitride (GaN) and new processes, though, transistors built at submicron sizes like 40 nm or less that work at these frequencies are possible.
Applications
Video signals demand the greatest bandwidth and, accordingly, a higher data rate. Speeds of many gigabits per second are needed to transmit 1080p high definition (HD) video. That data rate can be reduced if video compression techniques are used prior to transmission. Then, data rates of several hundred meagbits per second can get the job done, but usually at the expense of the video quality.
Compression techniques invariably diminish the quality to allow available wireless standards like Wi-Fi 802.11n to be used. Standards like 802.11ac that use greater bandwidth in the 5-GHz band are now available to achieve gigabit data rates. Millimeter-wave technologies make gigabit rates commonplace and relatively easy to achieve, making uncompressed video a reality.
Common applications include video transmission from a set-top box (STB) to an HDTV set or transmission between a DVD player and the TV set or from a game player to the TV set. Video also can be sent wirelessly from a PC or laptop to a video monitor or docking station. Transmitting signals from a laptop or tablet directly to the HDTV screen is popular as well. Other applications include wireless HD projectors and wireless video cameras. Millimeter-wave technologies allow the wireless transmission of popular video interfaces such as HDMI 1.3 or DisplayPort 1.2. A wireless version of PCI Express is also available.
There is now considerable interest in implementing a wireless version of USB 3.0. It is becoming the interface of choice not only on PCs and tablets but also TV sets and other consumer gear. USB 3.0 specifies a maximum rate of 5 Gbits/s with about 80% of that rate being achieved in a real application. A 10-Gbit/s USB version could be in the works as well. Wouldn’t it be nice to have a millimeter-wave dongle that could achieve those rates?
Other applications for millimeter-wave equipment include backhaul for wireless basestations, short-range radar, and airport body scanners. One interesting potential use is PCB-to-PCB (printed-circuit board) or chip-to-chip wireless links. At millimeter-wave frequencies, cables, connectors, and even short PCB runs add attenuation. A short (inches or less) wireless link eliminates the problem.
The 60-GHz unlicensed industrial-scientific-medical (ISM) band from 57 to 64 GHz is getting lots of attention. It is already being used for wireless backhaul, and greater use is expected. Two short-range wireless technologies are also addressing this band’s potential: IEEE 802.11ad and WirelessHD.
IEEE 802.11ad WiGig
The designation 802.11ad is an extension of the IEEE’s popular 802.11 family of wireless local-area network (LAN) standards generally known as Wi-Fi. The 11ad version is designed for the 60-GHz band. It is backward compatible with all previous versions including 11a/b/g/n/ac, as the media access control (MAC) layers of the protocol are similar. The 11ad version is also known by its trade name WiGig. The Wireless Gigabit (WiGig) Alliance supports and promotes 11ad, and it recently announced plans to consolidate with the Wi-Fi Alliance under the Wi-Fi Alliance banner.
WiGig uses the unlicensed ISM 60-GHz band from 57 to 64 GHz, divided into four 2.16-GHz bands. The primary modulation scheme, orthogonal frequency division multiplexing (OFDM), can support a data rate up to 7 Gbits/s, making it one of the fastest wireless technologies available. The standard also defines a single carrier mode that uses less power and is a better fit for some portable handheld devices. The single carrier mode can deliver a data rate up to 4.6 Gbits/s. Both speeds permit the transmission of uncompressed video. The WiGig specification also provides security in the form of the Advanced Encryption Standard (AES).
Because of the small antenna size at 60 GHz, gain antennas are normally used to boost signal power and range. The maximum typical range is 10 meters. WiGig products use antenna arrays that can provide beamforming. This adaptive beamforming permits beam tracking between the transmitter and receiver to avoid obstacles and maximize speed even under changing environmental conditions.
One clever feature of the standard is its use of a protocol adaption layer (PAL). This software structure talks to the MAC layer and allows simplified wireless implementation of other fast standard interfaces like USB, HDMI, DisplayPort, and PCI Express.
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| Figure 2. | Incorporating the 802.11n chip from Qualcomm Atheros and the 60-GHz chip from Wilocity, this three-band module suits hot spots, routers, and other WLAN products. |
Wilocity, which is the primary source of WiGig radios, makes a single-chip 60-GHz transceiver. Its most common use is in conjunction with a standard 802.11n implementation. Qualcomm Atheros packages its AR9642 802.11n transceiver with the Wilocity 60-GHz chip, forming a module that covers the three main Wi-Fi bands of 2.4, 5, and 60 GHz (Fig. 2). Wilocity also has an arrangement to package its device with Marvell’s Wi-Fi transceivers. Look for more combinations as wireless LANs (WLANs), routers, and hot spots begin adopting a three-band strategy that may include 11ac.
