David A. Case NCE, NCT
It seems that no matter how we plan it, the latest version of the standard at times never seems to completely address the latest technology completely. Not to say this is bad in fact it is just the opposite since there is limited experience on how to test some of the newer technologies. Therefore the problem to solve in the short term is how to test the product properly.
There are several avenues to address this, for example a few years ago, this issue existed for WiMAX and the issue was partly resolved when several manufacturers got together to develop an ad-hoc procedure for testing client devices. This was vetted thru the FCC and the procedure was allowed to be used for testing some specific configurations.
The Federal Communications Commission Authorization Branch addresses these issues by developing Knowledge Data Base guidance notes on testing various technologies.
These guidance notes are used as supplements along with the recommended test standards to provide guidance to industry on how to test the devices as well as guidance for TCB’s to review the reports for these new technologies.
Shortly after the C 63 work group sent the C 63.10 rev 2 to ballot, the FCC issued new draft guidance on proposed test methods for 802.11ac radios for comments. Given that the 802.11ac is still being sorted out and the technology being prepared for market is based on the draft standard, adding it to the standard would be premature at the best.
Therefore the KDB issued on this will allow the industry players from manufacturers to test labs as well as TCB reviewers to gain experience in testing these products and therefore be able to improve the KDB itself for future inclusion in a later version of the standard.
Before testing the systems, one needs to understand exactly what 802.11ac is exactly. For the most part as far as the general testing requirements the current procedures we use for 802.11 b,g,n or 802.11a,n will suffice in general.
However 802.11 ac allows the use of channel bandwidths of 80 and 160 MHz, in comparison with the current maximum bandwidth of 40 MHz as allowed by 802.11n. As such , with these increased bandwidths several issues arise.
First is the issue of measuring these wide band signals which will exceed some of the bandwidths of our test equipment.
The second is that the wider bandwidths would require that the channels exceed the allowed spectrum use in specific bands and as such, the channel will operate in frequencies that are covered by different technical regulatory requirements.
As such the challenges faced by 802.11ac will include the need in some case’s to modify the various regulatory regimes requirements or adapt guidance on how to allow operation under these conditions.
The FCC addressed this by issuing guidance on this under KDB’s #.644545. This guidance should be used to supplement testing per C 63.10 standard as well as several other KDB’s the FCC has drafted on testing DTS, U-NII, smart antennas, as well as DFS.
The 802.11ac as drafted not only addresses 2.4 and 5GHz but the 4.9 GHz public safety band as well.
Approvals for 802.11ac currently require either filing with the FCC or thru the TCB PBA process.
802.11ac issues to consider
The testing of 802.11ac will offer some challenges for the test engineer. These challenges include
A) Channel bandwidths up to 160 MHz
B) Non contiguous 80 plus 80 MHz channels
C) Allowance for up to 8 MiMo antenna outputs
D) A higher order of modulation 256K QAM
E) As well operation of new channels between frequency bands.
There are a number of issues of regulatory to consider, first if the channel bandwidth falls into two different frequency bands, the requirements for both bands need to be addressed. If the channel falls between what is referred to as U-NII and U-NII 2 band, the operation will be restricted to indoor use only.
The requirement for Dynamic Frequency Selection operation is applicable to any part of a signal from a channel that falls within the DFS frequency band range.
As per KDB 443999, operation in the 5600-5650 Terminal Doppler Weather Radar band is prohibited and as such a channel cannot operate across that frequency band nor can any of the channel fall into this band. All the requirements of the referenced KDB must apply.
If operating in the 5725 – 5850 MHz band under Part 15.247, one cannot also operate simultaneously in the 5725 -5825 MHz band under Part 15.407 rules. Note the FCC also issued a KDB of alternate test procedures for operation in the 5725 – 5850 MHz band .which addresses specific issues such as allowing operation in the whole 5725- 5850 MHz band for Part 15.407 for the wider channels (note one cannot operate a 20 MHz 15.407 channel in the 5725- 5850 MHz band).
As far as emissions, one needs to study the KDB’s quite closely , especially in regards to Out of Band Emissions, for example as quoted from the KDB
For devices operating in the 5.15-5.25 GHz band, the -27 dBm/MHz peak EIRP limit applies
outside of the lower pair of U-NII bands, i.e., 5.15-5.35 GHz. However, any transmission that
does not intentionally extend into the 5.25-5.35 GHz band must be down 20 dB above 5.25 GHz
per section 15.215(c) of the rules. If the emission does intentionally extend into the 5.25-
5.35 GHz band, DFS and TPC must be implemented per section 15.407(h) of the rules.
(ii) For devices that operate in the 5.25-5.35 GHz band and are restricted to indoor operation, the
27 dBm/MHz peak EIRP limit applies outside of the lower pair of U-NII bands, i.e., 5.15-
(iii) For devices that operate in the 5.25-5.35 GHz band and are not restricted to indoor operation,
the -27 dBm/MHz peak EIRP limit applies outside of the 5.25-5.35 GHz band (i.e., the out-of band and spurious limit applies below 5.25 GHz).”
Therefore in approaching the testing, depending on the channels BW and Occupied BW, the OOBE tests will not be as straight forward as previous for 802.11n devices.
Transmitter power output will also provide a challenge as one needs to address testing a wide band channel. Further given it is likely that the channel power will between to contingent bands with different maximum power level, one will need to verify that the total conducted power does not exceed the maximum Power Spectral Density. Since 802.11ac allows up to 8 antenna ports, one will need to carefully follow the guidance not only of this DB but the various KDB’s on smart antenna / MiMo antennas in order to insure compliance.[i]
As with 802.11n and other technologies, one needs to test all the legacy modes as well, which will make for a rather large report. Given this product must be approved under the PBA process, one will need to run the test program by the TCB and FCC to get approval before one tests.
Further the KDB’s do not address SAR issues at this time, there is no information available when this will be addressed in the future.
[i] Note: The latest draft version of ANSI C 63.10 Rev 2 out for balloting addresses some of the issues on testing MiMo antennas as well as provides some background on measuring wideband signals.