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Making Sense of the 5-GHz Regulations for Unlicensed Transmitters
David A. Case
In 1997, the FCC, after reviewing petitions submitted by representatives of the computer and wireless industry—including Apple and WINForum—established a new unlicensed service in the 5 GHz band. A goal of the petitioners was to allow the nation's schools to connect to the Internet without the expense of hard wiring. This new service, called the Unlicensed National Information Infrastructure (U-NII), has been designated to provide high-speed wireless networking communication at low cost. The service allots a total of 300 MHz of bandwidth to help provide a low-cost alternative to wiring older schools as well as new ones.
This has gained more importance since connecting all schools in the United States to the Internet has been a high-profile goal of the Clinton administration. Because meeting this goal entirely with wired systems was both impractical and cost-prohibitive, the FCC opened up a wireless alternative.
The FCC is addressing the issue of cost in part through its Schools and Library Program, which helps provide funds for schools to connect to the Internet. To enable the technology, the commission established the U-NII, which opens up new frequency bands for unlicensed devices. U-NII offers the additional potential benefits of stimulating the growth of new industries and promoting the global competitiveness of U.S. manufacturers through the development of new unlicensed digital products.
The FCC will allow the use of 300 MHz of the 5 GHz band by these U-NII devices on a shared, noninterference basis. The frequency bands specified for these devices are 5.15 to 5.25 GHz, 5.25 to 5.35 GHz, and 5.725 to 5.825 GHz. The U-NII bands are envisioned as a tiered network. The lowest 100 MHz band is assigned to the lowest-power devices, the highest to the highest-power devices.
The 5.15-5.25 GHz band is intended for short-range devices, for functions such as connecting all the computers in the classroom to the teacher's computer. The 5.25-5.35 GHz band is intended for midrange devices. When added to the teacher's computer, it would connect to the school's local-area network. The 5.725-5.825 GHz band is designed for use with the wide-area network based on either point-to-point or multipoint operation. The device used with this band could connect the school's network to a district network across town.
Technical Requirements
The frequency range for this test was 50 MHz to 1 GHz. Given the size of the mode-stirred chamber, sufficient modal
density will not exist at frequencies below about 200 MHz. In the 50–200 MHz range, it is important for the tested device to be kept in
the same location and orientation to make a valid relative comparison. For all testing herein, this practice of maintaining cable
orientation between test configuration changes was strictly followed.
These U-NII devices are to be operated under the requirements of subpart E of FCC Part 15 of CFR 47. Per the requirements spelled out in Part 15.401, the radios operating in all three bands must meet the following basic specifications.
The U-NII bands are designated for wideband, high-data-rate digital communications. The bandwidth is 20 MHz and the data rates are to be 20 MB or better. The devices will share these bands on a noninterference basis. Both the modulation scheme and channel spacing will be determined by the technology. (Some of these requirements will probably be specified by the IEEE's 802.11 group, which, as noted below, is drafting a standard for U-NII devices.)
The devices must also meet some of the requirements of Part 47 subpart C, such as 15.203 (unique antenna connectors), 15.205 (restricted bands of operation), 15.207 (conducted emissions), and 15.209 (radiated spurious emissions).
The antenna gain is limited to 6 dBi with a 1-dB radio-power reduction for every 1 dB that the antenna exceeds 6 dBi. The devices must meet the requirements for RF safety outlined in FCC's OET Docket 65. The restricted band emissions are 54 dBµV/m (500 µV/m) at a distance of 3 m.
Table I. Specific device requirements for U-NII frequency bands.
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Frequency
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5.15–5.25 GHz
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5.25–5.35 GHz
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5.725–5.825 GHz
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Use
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Indoor use only; integral antenna
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Indoors or outdoors
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Indoors or outdoors only
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Power (Where B=26 dBm emission bandwidth)
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50 mW or 4 dBm + 10 logB
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250 mW or 11 dBm+10 logB
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1 W or 17 dBm + 10 logB
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Power spectral density (antenna gain up to 6 dBi)
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2.5 mW/MHz (4 dBm in a 1 MHz band)
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12.5 mW/MHz (11 dBm in a 1 MHz band)
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50 mW/MHz (17 dBm in a 1 MHz band)
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As shown in Table I, the FCC has also added specific requirements for each band, based on the projected uses of each. In addition, for point-to-point systems employing directional high-gain antennas in the 5.725-5.825 GHz band, the maximum antenna gain permitted for a 1-W radio is 23 dBi. For antennas exceeding 1 W, the power must be reduced by 1 dB for every 1 dB the antenna exceeds 23 dBi.
For systems using multipoint or omnidirectional antennas, the power must be reduced by 1 dB for every 1 dB the antenna exceeds 6 dBi.
In the 5.725-5.825 GHz band, all emissions within 10 MHz of the spurious emission limit will be an equivalent isotropically radiated power (EIRP) of -17 dBm/MHz (40 dB) and by an EIRP level of -27 dBm/MHz (50 dB) outside these bands.
This new band should be a boon to the test instrument industry, since testing these types of devices will require some labs and companies to update their testing capabilities to include 5-GHz devices. For testing of U-NII devices, the FCC references the ANSI C 63.17 standard for settings such as bandwidth and other test requirements.
Possible Interference Problems
Devices operating in the U-NII bands face several potential interference problems. Both the lower edge of the 5.15-5.25 GHz band and the upper edge of the 5.25-5.35 GHz band are listed under Part 15.205 as restricted bands. This means that in these bands, the spurious emissions of the transmitters must meet the radiated limits specified in Part 15.209 of 500 µV/m at 3 m distance.
Devices operating at the lower frequency range of 5.15-5.25 GHz will be sharing this band with the digital broadcast satellite that has now been authorized to operate in the 5090-5250 MHz band.
In addition, devices operating in the 5.725-5.825 GHz band will be sharing it with Part 15.247 spread-spectrum devices and Part 15.245 perimeter sensor devices, as well as with ISM-type devices. Moreover, the next-generation microwave devices (microwave ovens) are starting to use the 5.8 GHz ISM band.
RF Safety
U-NII devices will also be required to meet the MPE\SAR limits per FCC's Bulletin 65 on RF safety.1 For those systems operating in a controlled environment, where the user is aware of potential exposure and can exercise some control over the exposure, the limit is 5 mW/cm2 over a time period of 6 minutes.
For those systems in an uncontrolled environment, where the user is unaware of potential exposure and cannot exercise some control over the exposure, the limit is 1 mW/cm2 over a time period of 30 minutes.
Conclusion
Although subpart E of Part 15 rules was officially published on January 31, 1997 (62 FR: 4649), and was officially revised on July 31, 1998 (63 FR: 40831), it will very likely be subject to further minor changes or clarifications. There are already standing requests to change some of the requirements, such as antenna restrictions and power outputs.
Industry Canada is currently working on a similar set of rules and regulations for the same 5 GHz bands of operation, based on Part 15.401. In addition, members of the IEEE 802.11 committee are currently working on a draft standard for these 5-GHz devices. The FCC currently only stipulates that the transmission must be a wideband type, which leaves the modulation scheme up to the various manufacturers. Since the idea behind IEEE 802 standards is interoperability, this group, made up of various wireless manufacturers, will need to come up with a standard and modulation schemes that are acceptable to all parties.
This group also faces a task of making it compatible with the requirements of the High-Performance European Radio Local-Area Network standard, or HIPERLAN, which covers the 5 GHz band in Europe. Japan has recently adopted the current HIPERLAN frequency range.
The HIPERLAN standard currently only allows operation in the 5.15 to 5.3 GHz band. The committee working on this standard has drafted a proposal to allow HIPERLAN to operate on additional higher-frequency bands in the 5 GHz frequency range. This proposal, however, is in the initial draft stages; no date for adoption or approval is known.
The opening up of 300 MHz of the 5 GHz band for high data rate digital communications should prove interesting for all those involved.
References
1. Robert Cleveland, Jr., David Sylvar, and Jerry Ulcek, Evaluating Compliance with the FCC Guidelines for Human Exposure to RF Electromagnetic Fields, OET Bulletin 65, Edition 97-01, (Washington, DC: Federal Communications Commission, Office of Engineering and Technology, 1997).
David A. Case, NCE, is the senior compliance and reliability engineer responsible for worldwide product approval at Aironet Wireless Communications Inc. (Akron, OH), a firm with expertise in spread-spectrum technology. His responsibilities include radio type approvals, as well as EMC design and safety approvals for spread-spectrum-based products. He also handles product reliability studies, EOS/ESD evaluation, and antenna and power supply evaluation for WLAN systems. Case is NARTE-certified in the fields of telecommunications and EMC and ESD control, and he currently chairs the IEEE EMC Society Representative Advisory Committee. He can be contacted by e-mail at dcase@aironet.com.
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