It is often said that microwave ovens operate at 2.45 GHz because it corresponds to a particular excitation mode of water molecules. This is sometimes even
offered as a reason why 802.11 cannot be used over long distances. Atmospheric water vapor would severely attenuate any microwave signals in rain or in humid
climates.
The existence of a water excitation mode in the microwave range is a myth. If there was an excitation mode, water would absorb a significant amount of the microwave energy. And if that energy was absorbed effectively by water, microwave ovens would be unable to heat anything other than the water near the surface of food, which would absorb all the energy, leaving the center cold and raw. An absorption peak would also mean that atmospheric water vapor would disrupt satellite communications, which is not an observed phenomenon. NASA Reference Publication 1108(02), Propagation Effects on Satellite Systems at Frequencies Below 10 GHz, discusses the expected signal loss due to
atmospheric effects, and the loss is much more pronounced at frequencies above 10 GHz. The microwave absorption peak for water, for example, is at 22.2 GHz.
Microwave ovens do not work by moving water molecules into an excited state.
Instead, they exploit the unusually strong dipole moment of water. Although electrically neutral, the dipole moment allows a water molecule to behave as if it were composed of small positive and negative charges at either end of a rod. In the cavity of a microwave oven, the changing electric and magnetic fields twist the water molecules back and forth. Twisting excites the water molecules by adding kinetic energy to the entire molecule but does not change the excitation state of the molecule or any of its components.
Use of equipment in the ISM bands is generally license-free, provided that devices operating in them do not emit significant amounts of radiation. Microwave ovens are high-powered devices, but they have extensive shielding to restrict radio emissions.
Unlicensed bands have seen a great deal of activity in the past three years as new
communications technologies have been developed to exploit the unlicensed band. Users can deploy new devices that operate in the ISM bands without going through any
licensing procedure, and manufacturers do not need to be familiar with the licensing procedures and requirements. At the time this book was written, a number of new communications systems were being developed for the 2.4-GHz ISM band:
• The variants of 802.11 that operate in the band (the frequency-hopping layer and both spread spectrum layers)
• Bluetooth, a short-range wireless communications protocol developed by an industry consortium led by Ericsson
• Spread-spectrum cordless phones introduced by several cordless phone manufacturers
• X10, a protocol used in home automation equipment that can use the ISM band for video transmission
Unfortunately, "unlicensed" does not necessarily mean "plays well with others." All that unlicensed devices must do is obey limitations on transmitted power. No regulations specify coding or modulation, so it is not difficult for different vendors to use the
spectrum in incompatible ways. As a user, the only way to resolve this problem is to stop
using one of the devices; because the devices are unlicensed, regulatory authorities will not step in.
9.2.1.2 Other unlicensed bands
Additional spectrum is available in the 5-GHz range. In the United States, the following three bands are called the Unlicensed National Information Infrastructure (UNII) bands:[2]
[2] The UNII bands are defined by FCC part 15.407.
• 5.15-5.25 GHz
• 5.25-5.35 GHz
• 5.725-5.825 GHz
Devices operating in the UNII bands must obey limitations on radiated power, but there are no further constraints imposed on them. European regulatory authorities have set aside the same frequency bands, but the first two bands are dedicated to HiperLAN technology; the third band is the only one potentially available for 802.11 networks.
9.2.2 Spread Spectrum
Spread-spectrum technology is the foundation used to reclaim the ISM bands for data use. Traditional radio communications focus on cramming as much signal as possible into as narrow a band as possible. Spread spectrum works by using mathematical functions to diffuse signal power over a large range of frequencies. When the receiver performs the inverse operation, the smeared-out signal is reconstituted as a narrow-band signal, and, more importantly, any narrow-band noise is smeared out so the signal shines through clearly.
Use of spread-spectrum technologies is a requirement for unlicensed devices. In some cases, it is a requirement imposed by the regulatory authorities; in other cases, it is the only practical way to meet regulatory requirements. As an example, the FCC requires that devices in the ISM band use spread-spectrum transmission and impose acceptable ranges on several parameters.
Spreading the transmission over a wide band makes transmissions look like noise to a traditional narrowband receiver. Some vendors of spread-spectrum devices claim that the spreading adds security because narrowband receivers cannot be used to pick up the full signal. Any standardized spread-spectrum receiver can easily be used, though, so additional security measures are mandatory in nearly all environments.
This does not mean that spread spectrum is a "magic bullet" that eliminates interference problems. Spread-spectrum devices can interfere with other communications systems, as well as with each other; and traditional narrow-spectrum RF devices can interfere with spread spectrum. Although spread spectrum does a better job of dealing with interference within other modulation techniques, it doesn't make the problem go away. As more RF
devices (spread spectrum or otherwise) occupy the area that your wireless network covers, you'll see the noise level go up, the signal-to-noise ratio decrease, and the range over which you can reliably communicate drop.
To minimize interference between unlicenced devices, the FCC imposes limitations on the power of spread-spectrum transmissions. The legal limits are one watt of transmitter output power and four watts of effective radiated power (ERP). Four watts of ERP are equivalent to 1 watt with an antenna system that has 6-dB gain, or 500 milliwatts with an antenna of 9-dB gain, etc.[3] The transmitters and antennas in PC Cards are obviously well within those limits— and you're not getting close even if you use a commercial antenna.
But it is possible to cover larger areas by using an external amplifier and a higher-gain antenna. There's no fundamental problem with doing this, but you must make sure that you stay within the FCC's power regulations.
[3] Remember that the transmission line is part of the antenna system, and the system gain includes transmission line losses. So an antenna with 7.5-dB gain and a transmission line with 1.5-dB loss has an overal system gain of 6 dB. It's worth noting that transmission line losses at UHF freqencies are often very high; as a result, you should keep your amplifier as close to the antenna as possible.