IEEE 802.11 Counters Proposed FCC's Rulemaking
Let me start this month's column by reporting on progress made
at the IEEE 802.11 plenary meeting held July 5 to 9 in Montreal. This session convened the various wireless standards working groups. In fact, it was the first official plenary session for the 802.15 Wireless Personal Area Network (WPAN) working group—a spin-off the 802.11 and the 802.16 broadband wireless access working group.The 802.11 working group closed out the week drafting responses to comments received from those who voted in the last round of the re-circulation sponsor letter ballot, as preparation for final approval of the draft text by the standards boards later this year. In addition, the 802.11 group wrote a letter to the Federal Communications Commission (FCC) in response to their Notice of Proposed Rule Making (NPRM) on wideband frequency hopped (WBFH) spread spectrum, which was posted by the FCC on June 24, 1999. The NPRM was a major topic of discussion for the 802.11 committee, so I'll devote the rest of this month's column to that topic.
What prompted the NPRM?
The FCC wrote the NPRM (ET Docket 99-231, FCC 99-149) in response to a request championed by the HomeRF Working Group and some of its members. The NPRM defines proposed changes to the rules found in CFR Part 15.247, which govern radio operation in the 2.4 to 2.483 GHz unlicensed spectrum. The changes, if adopted, would widen operational bandwidth for frequency hopping, spread spectrum (FHSS) systems and re-define the method used to measure processing gain in direct sequence, spread spectrum (DSSS) systems.
We in the industry have 75 days (from July 20) to respond to the FCC's notice. Following that, the FCC is expected to respond to those comments in the form of a report; an order would be the next step if the rules actually do change.
More on the proposed changes
Current Part 15.247 rules govern operation of either FHSS or DSSS radio operation. FHSS radios are required under these rules to limit RF transmit power (measured at the antenna) to 1 W. Further, the radio must hop over a minimum of 75 channels, with a maximum dwell time of 400 ms and cannot occupy more than 1 MHz of channel bandwidth.
The proposed rule change for FHSS operation would widen the occupied channel bandwidth from 1 MHz to 3 and 5 MHz. At an occupied channel bandwidth of 3 MHz, the RF transmit power would be limited to +25 dBm (320 mW) and the channel dwell time would max at 50 ms. If the occupied channel bandwidth used is 5 MHz, then the RF transmit power limit would be +23 dBm (200 mW), with a maximum 20 ms channel dwell time.
Is this the best use of the spectrum?
In my opinion, widening the occupied bandwidth will result in inefficient use of the 2.4 GHz spectrum. I say this because widening the occupied bandwidth to 3 and 5 MHz forces the channels to overlap between hops, resulting in self-interference of such WBFH systems and increasing the level of interference with existing 1 MHz wireless local-area network (WLAN) radios—whether FHSS or DSSS.
I'm also concerned that since current FHSS systems use frequency shift keying (FSK) for modulation of voice and data packets, it's very likely that multi-level FSK will be used under the proposed rules changes for these wider channels. Frankly, multi-level FSK will not deliver the quality of service necessary. I believe higher order, more robust modulations like quadrature phase-shift keying (QPSK) and/or complementary code keying (CCK), are necessary for quality transmission of video streams in the home market.
Streaming video is not as forgiving as voice and data, especially when the consumer in the home expects the same quality that's transmitted over existing coax cable in the home. If WBFH systems are developed in the WLAN market, it is very likely such systems will use the maximum allowable RF power of 320 mW in order to provide the same spectral efficiency that QPSK or CCK delivers at 100 mW. The installed base for existing 2.4 GHz 1 Mb/s FHSS and DSSS WLAN systems today transmit at 100 mW. WBFH systems in practice will result in higher levels of interference, resulting in higher packet errors and in some instances even loss of network connection. This is the fear shared by many I've spoken to about this rules change proposal.
IEEE 802.11 position
The IEEE 802.11 committee reviewed a number analytical papers and presentations on the effects caused by FHSS systems using 1 MHz versus 3 and 5 MHz overlapping channels. The committee concluded that in practice, systems employing WBFH would increase the amount of interference on existing 1 MHz FHSS and DSSS systems. As a result, the IEEE 802.11 plans to forward a letter to the FCC opposing widening of the occupied channel bandwidth to 3 and 5 MHz.
Now, a short farewell!
Well, this concludes my final report in the WLAN Beat column on Wireless Design Online. I am turning the column over to Bruce Kraemer, my colleague of many years. Bruce plans to cover the processing gain issue of the FCC's NPRM in next month's column. Though I won't meet you here anymore, I'm sure I'll see you at committee meetings, conferences and the like. Stay wireless and have fun!
About the author:
Al Petrick was Senior Manager of Strategic Marketing for Wireless Products at Harris Semiconductor at the time this article was written. He is also the vice chairman of the IEEE 802.11 standards committee. He holds a BSET degree from the Rochester Institute of Technology (Rochester, NY) and an EMBA from the Crummer School of Graduate Business at Rollins College (Winter Park, FL). Al left Harris in July and is now employed by ParkerVision (Jacksonville, FL).
Edited by Robert Keenan