New Rules for Unlicensed Digital Transmission Systems

Thomas N. Cokenias

New FCC rules could create significant opportunities for digital services and applications in unlicensed bands. Processing-gain requirements would be eliminated.

Since 1986, FCC rules have provided for unlicensed spread-spectrum operation in the 915 MHz (902–928 MHz), 2.4 GHz (2400–2483.5 MHz), and 5.7 GHz (5725–5850 MHz) bands. Spread-spectrum devices, like all Part 15 devices, may not cause harmful interference to authorized (i.e., licensed) services, and they must accept any interference received from other devices. Operation under rules in Section 15.247 is limited to frequency-hopping and direct-sequence spread-spectrum systems.

When spread-spectrum rules were originally developed, the emphasis at the time was to encourage designers to develop unlicensed industrial, scientific, and medical (ISM) microwave-band radio devices. The rules emphasized the relatively high power output (up to 1 W) available and the interference immunity of spread-spectrum systems. High-data-rate applications were not anticipated, but with the development of corporate wide local-area networks (WLANs) and the Internet, high-capacity, license-free radio links became feasible. Designers worldwide began to develop products.

FCC has modified the Part 15 spread-spectrum rules a number of times over the years to support new technologies and evolving industry standards, such as HomeRF, Bluetooth, and IEEE 802.11. In May 2001, FCC issued a Further Notice of Proposed Rulemaking and Order (FNPRM) to change the current unlicensed spread-spectrum radio requirements in Part 15.¹ This notice is further to one requested by 13 petitioners for revising parts of the rules governing frequency-hopping spread-spectrum devices. The second notice was released primarily as a result of actions taken on a certification application from Wi-Lan Inc. (Calgary, AB, Canada).

The notice goes beyond changes to frequency-hopping systems by proposing the elimination of processing-gain requirements for direct-sequence systems. It also creates a category for so-called digital transmission systems that are neither frequency hopping nor direct sequence in nature. If adopted as written, the rule changes will create significant opportunities for valuable services and applications not currently permitted in the unlicensed bands. As with any FNPRM, FCC may elect to adopt requirements that are substantially different from those originally proposed, or it could elect to withdraw the FNPRM entirely and not act upon it. As of this writing (April 2002), however, it is the consensus of industry and FCC representatives that FCC will adopt the rules as proposed.

In this most recent notice, FCC seeks to relax or eliminate requirements that would hinder new technologies or the efficient use of the radio-frequency (RF) spectrum. The proposed changes are expected to remove unnecessary restrictions to the successful deployment of systems with high data rates (20 Mb/sec or greater).

Frequency Hopping

The petitioners for the original NPRM cited studies that show how adaptive-hopping techniques in the 2.4 GHz band would reduce the potential for interference with direct-sequence systems by allowing frequency-hopping systems to avoid transmitting on frequencies used by collocated direct-sequence systems.

The current version of Part 15 is restrictive in the sense that, for narrow-bandwidth hopping channels (bandwidth less than or equal to 1 MHz), at least 75 hopping frequencies are required. For systems with hopping channels with a bandwidth greater than 1 MHz, the rules require a minimum of 15 nonoverlapping channels covering a total span of at least 75 MHz.

FCC will likely amend the rules to incorporate the changes as proposed by the petitioners. Section 15.247(a)(1)(iii) would be amended as follows:

Eliminating the Processing-Gain Requirement

For direct-sequence systems, the current rules require a minimum processing gain of 10 dB. Processing gain is most commonly determined using the continuous-wave (CW) jamming margin test. A signal generator is stepped in 50-kHz increments across the system passband. At each point, both the system output power and the CW level required to produce the recommended bit error rate (BER) is recorded. Processing gain is calculated as:

G_p = (S/N)_o + M_j + L_sys

where G_p = processing gain, (S/N)_o = signal-to-noise ratio required for the chosen BER, M_j = jammer-to-signal ratio, and L_sys = system losses (not more than 2 dB).

The processing-gain requirement was originally adopted to keep manufacturers from designing narrowband systems that would take advantage of the 1-W power output allowed under FCC rules. Now, 15 years later, this requirement appears to be no longer necessary. It benefits the manufacturer to design in processing gain to minimize interference problems when operating near other RF devices. Consequently, the proposed changes recommend eliminating the processing-gain requirement.

Digital Transmission Systems

Recently, a number of digital-modulation techniques, such as orthogonal frequency-division multiplexing (OFDM), have been developed for use in the 5 GHz unlicensed national information structure (U-NII) band governed by Section 15.401 of the rules. These new digital technologies are very attractive for use in the 2.4 GHz unlicensed band.

Wi-Lan submitted a certification application for a 2.4 GHz device using wideband OFDM. In its petition, Wi-Lan argued that its device met the technical requirements for a spread-spectrum device. FCC did not agree, but saw merit in making provisions in the rules for alternative digital-modulation technologies. Rather than create a separate rule section, FCC elected to include provisions for digital transmission systems by amending the existing spread-spectrum rules.

The proposed changes would amend a number of rule sections in Section 15.247 to include the terms digitally modulated and digital modulation techniques in addition to the direct sequence and frequency hopping terms already in use. Digital transmission systems must meet the same requirements as direct-sequence spread-spectrum radios, including:

Interim Waivers

As part of the notice, FCC is waiving, on an interim basis, the restriction of Section 15.247(a) that limits operation in Section 15.247 to frequency-hopping and direct-sequence spread-spectrum systems. Prior to adoption of the new rules, FCC is accepting applications and issuing equipment authorization grants for products that meet the provisions of the proposed new rules. Products must comply with the maximum peak output power limit of 100 mW. In addition, any conditionally authorized devices will have to comply with whatever rules are ultimately adopted for digital-modulation systems in the 2.4 GHz band.

U-NII Rules

In 1998, U-NII rules were adopted to regulate devices operating in the 5.15–5.35 and 5.725–5.825 GHz bands. Modulation types were not specified. However, modulation was required to be digital in nature. Power output, up to 1 W in the 5.7 GHz band, is based in part on channel-occupied bandwidth. In addition to a frequency stability requirement, the U-NII band has a maximum PSD requirement and a maximum EIRP requirement at the band-edges (expressed in dBm/MHz). As with the spread-spectrum rules, U-NII rules in Section 15.407 limit transmitter EIRP for point-to-point and point-to-multipoint systems.

For unlicensed operation, the newer modulations such as OFDM could be practically employed only under U-NII band rules. Operation is possible at the 915 MHz, 2.4 GHz, and 5.7 GHz bands under 15.249 of the rules, but power levels are limited to 1 mW or less, too low to be practical in WLAN applications. Under the proposed spread-spectrum regulations, 2.4 GHz devices will be able to use the more advanced modulations, and 5.7 GHz devices will be afforded advantages not available under current U-NII regulations.

Under U-NII rules, the maximum output power for 5.7 GHz devices is 17 dBm + 10 log (26-dB bandwidth). This translates to 30 dBm (1 W) for emission bandwidths greater than 20 MHz. The proposed rules do not impose power-output penalties for transmitters with emission bandwidths narrower than 20 MHz because the full 1 W would be allowed. This change is significant because a number of commercially available high-capacity systems employ channels that are only 6 MHz wide.

The digitally modulated systems proposed for use limit peak power spectral density to 8 dBm in any 3-kHz bandwidth. For a signal with uniform spectral distribution across the emission bandwidth, this is equivalent to 8 dBm + 10 log (1 MHz/3 kHz) = 33 dBm/MHz (compared with the 17 dBm/MHz allowed under U-NII rules).

Radiated emissions outside the 5.725–5.825 GHz U-NII band must be less than –17 dBm/MHz EIRP within 10 MHz of the band-edge and –27 dBm/MHz EIRP beyond that. For a 1-W output into a 0-dBi antenna, this represents suppression of –47 to –57 dBc at the band-edges, with a greater attenuation requirement for devices using high-gain directional antennas. For devices operating under the proposed new rules, maximum attenuation is only –20 dBc.

For point-to-point systems, the regulations in 15.247 do not limit EIRP for the 5.7 GHz band. A 5.7 GHz point-to-point transmitter is permitted to transmit a full watt into directional antennas with unlimited gain, whereas the U-NII rules limit maximum antenna gain to 23 dBi. Table I summarizes the new requirements and compares the 5.7 GHz limits for U-NII with the proposed requirements.

Conclusion

The proposed changes to the existing spread-spectrum rules will undoubtedly be very attractive to WLAN and wireless Internet service providers. For instance, the IEEE 802.11a specification, which supports up to 54 Mb/sec at 5 GHz, could theoretically be employed at 900 MHz and 2.4 GHz with the advantages of less-costly components and significant decreases in path loss when compared with 5 GHz. The proposed changes would also relax requirements for 5.7 GHz devices currently regulated under the U-NII requirements in 15.401.

Manufacturers can now certify products to the new requirements before they are adopted as long as maximum output power is 100 mW or less. Devices must be certified to meet whatever requirements are eventually adopted. FCC believes the proposed rules will not increase interference to other users in the spectrum, but has invited comment as to whether the flexibility it is allowing for digitally modulated systems warrants a reduction in permitted power levels to reduce the likelihood of any adverse impact on other systems operating in this spectrum, similar to the reduced power levels adopted for wideband frequency-hopping systems.

Reference

1. Federal Register, 66 FR: 31585, June 12, 2001.

Acknowledgment

This article is based on a presentation at Wireless Systems 2002, February 25–28, 2002, in San Jose, CA.

Thomas N. Cokenias worked for eight years as an engineer at the FCC Laboratory in Columbia, MD. Since 1983, he has held senior engineering positions at a number of EMC laboratories and has been involved with spread-spectrum radio testing since the rules were first adopted. He currently serves as director of engineering for Compliance Certification Services (CCS) and is founder and principal consultant at T.N. Cokenias Consulting in El Granada, CA. He can be reached at tom@tncokenias.org.