We now have FCC SAR test procedures for 5-GHz WLAN
David A. Case, Cisco Systems, Inc.
Richland, OH
The engineers involved with certifying 802.11-based products, specifically portable devices, are aware of the headaches that Specific Absorption Rate (SAR) testing can cause. Until recently there was not a clear test procedure for qualifying such devices for Federal Communications Commission (FCC) approval. FCC Bulletin 65C revision 01-01 did address some of the issues, as did the TCB (Telecommunications Certified Body) guidance note issued in April 2004. Still, to some extent, the actual test procedures and requirements were left up to the test engineer. Further, if the system exceeded a certain power or operated above 3 GHz, one was required to send the submission to the FCC, instead of a TCB, for review and approval, thus adding further delay to the approval cycle time.
Though the IEEE and IEC standards groups are addressing SAR test procedures for body-worn and handheld devices, including those involving measurements above 3 GHz, the FCC, with prompting from the industry and the TCBs, has developed an interim solution; and, as of October 10, 2006, TCBs that sent personnel for training on these procedures can review 5-GHz SAR test reports. These interim solutions can be accessed from the FCC Knowledge Data Base documents KDB 248227 and KDB 865664.
Look at the Process
Unfortunately, there were several “bumps in the road” before the final procedures were approved for release. The FCC drafted and presented the first versions of the test procedures for >3 GHz and 802.11 in May 2005 at a TCB training session. The TCBs themselves were happy with the procedures; however, the 802.11 industry was not. Though the procedures would allow the TCBs to review WLANs above 3 GHz and did provide guidance on how to test 802.11 products, manufacturers of these systems expressed concern about the impact of the test standards on the future and widespread acceptance of these products.
First, none of the SAR testing labs or the manufacturers had been given a real chance to review the procedures or to determine the appropriate test setups and operating parameters required by these new procedures. Second, there were legitimate concerns as to how these proposed test procedures would accord with those being proposed by the IEC and IEEE standards committees addressing these issues. Finally, some observers noted that the proposed procedures touched, in part, on some of the ongoing discussion involving the still unresolved NPRM 03-137 on RF exposure.1 Might some parts of these proposals be regarded as de facto rulemaking on the part of the FCC?
A series of meetings between representatives of the industry, including the Wi-Fi Alliance Health and Safety Work Group and the FCC lab, were arranged after the industry weighed in with their concerns. Industry representatives submitted a number of suggested changes, proposals, and modifications to the proposed procedures to the FCC lab. The result of these discussions was the release of a test procedure for 802.11 systems, as well as for testing above 3 GHz, that was more acceptable to the industry and that did reflect some of same issues being studied by professional and international standards bodies.
The 802.11 Test Procedures
The 802.11 SAR test procedures are not only applicable to the 2.4-GHz and 5-GHz WLANs that operate under Part 15 of the rules, but are also applicable to 4.9-GHz Public Safety systems that operate under Part 90. It can be assumed that the basics of the procedures would also be applicable to 802.11 and 802.16 systems that could operate in the 3650-MHz band under Part 90 rules—as well as Bluetooth and ZigBee products operating under the Part 15 rules.
As part of the procedure, the document calls out the required channels that must be evaluated as part of the compliance testing (Table 1). If the maximum power output of the 802.11g channel is less than ¼ dB higher then the 802.11b channel, there is no requirement to repeat the tests for the 802.11g portion. When testing a 5-GHz product for compliance with Part 15.247 in the 5.7-GHz band, the tester defaults to the highest channel in the Part 15.247 band for testing purposes, instead of the highest channel for systems actually operating under Part 15.407.
Those who have followed the issues set out in the FCC NPRM 03-137 are familiar with the questions regarding the appropriate duty cycle and data rate for testing. For data rates, the test procedure requires the use of the lowest data rate for each modulation since comparison studies have shown that, in most cases, this option achieves the highest SAR value. Even though the worst case EMC profile may be obtained using the highest data rates, the SAR test software must be configurable for the slowest data rate. Also, testing with turbo mode, or the fastest data rates, may not be required if the total power output in these modes amounts to less than ¼ dB difference from the non-turbo modes using the slowest data rates.
As for the actual duty cycle of the transmitter, it should be configured to operate at 100% duty cycle, or as close as possible, if it cannot be configured to operate at a true 100% duty cycle.2 The test standard also addresses the issue of testing with diverse antennas. Though the standard is written mainly for SiSo (Single input, Single output), the standard does address some of the issues concerned with testing MiMo antennas (Multiple input, Multiple output) as well. These requirements must be taken into account when performing the testing. As with the power output of different modulations, there are exemptions from performing multiple tests with some antenna combinations based on transmitter power-out levels.
SAR Procedures for 3-GHZ to 6-GHZ Band Testing
Even with the 802.11 test procedure, TCBs would still be limited to doing SAR evaluations on systems operating below 3 GHz. A TCB cannot evaluate a product when there are not clear procedures, and the technical standards do not address SAR testing above 3 GHz. To remedy this situation, the FCC lab did some research and has developed a test procedure for testing products for SAR compliance above 3 GHz. While the 802.11 procedure addressed the various issues including appropriate testing modes, test channels, antennas, etc., the test methodology for above 3 GHz actually addresses the test equipment requirements, calibration of the probes, and the dielectric material to use for the frequency bands between 3 and 6 GHz. This procedure also specifically addresses, at least in part, some of the anomalies that need to be resolved for SAR testing of 802.11 devices. On the issue of test probes, the procedure requires a probe tip of
Conclusion
If testing is done in accordance with these procedures, a TCB can do the review and can issue the FCC grant. If other methodology is used, then the TCB must submit the product to the FCC for review and approval. Like most FCC test procedures, these will evolve as more information and data become available. Also, I suspect that as the IEEE and IEC standards evolve to include both 802.11 products and a test methodology above 3 GHz, additional changes will occur, or perhaps these procedures will be regarded simply as guidelines.In either case, we now have FCC SAR test procedures for 5-GHz WLAN. Besides these two procedures, there was one other result of these meetings between 802.11 industry members and the FCC. Insightful input from industry professionals most likely played a part in bringing about the FCC decision to hold a round table discussion with the industry on the development of test procedures for 3G wireless devices3 that took place in October 2005.
FOOTNOTES
- As of the date of this writing, the NPRM (Notice of Proposed Rulemaking) 03-137 was still in limbo.
- This duty cycle issue is not resolved as per the ongoing discussion in NPRM 03-137, and the final decision in this consultation could bring about a change in this procedure. However, until such time, testing must be done with 100% duty cycle as stated in the test procedure
- Based on input from this round table discussion, the FCC lab released a test procedure for 3G equipment in June 2006. See FCC document KDB 941225 in their Knowledge Data Base.
David A. Case, NCE, NCT, is the Regulatory Technical Leader for Cisco Systems Corporate Compliance EMC Standards and Operations Group. He is responsible for providing higher end technical support on wireless for various Cisco technology groups. He is also responsible for addressing regulatory issues with various agencies worldwide. He currently serves on many technical and standards committees, including chairing the Information Technology Industry Council TC8 Committee on Telecom and serving as Vice Chair of the WFA (WiFi Alliance) Health and Science Work Group.He has written numerous articles and has lectured at a number of different forums on wireless regulations and testing.
