The Use of 40 MHz Channels in the 2.4GHz Band is Not Reasonable for Commercial Networks

The 2.4 GHz band is often heavily congested, particularly in dormitories, apartments and condominiums, and multi-story office buildings. This alone precludes effective use of 40 Mhz channels. A home user, or a commercial user requiring only a single access point may obtain improved throughput (relative to 802.11g) using a single 40 MHz channel. Interior space larger than 3000 square feet will probably need more than one 802.11n access point to provide maximum bit-rate connectivity to 100% of the area. When adding a second access point to an 802.11n network is considered in the 2.4 GHz band the problem with 2.4GHz 40 MHz channels becomes evident.

Only One 40 Mhz 802.11n Channel Is Available in the 2.4 GHz ISM Band

Considering the channel map shown above, assume that a 40 MHz 802.11n channel were to be created with Channel 1 as the low side. That means that the new channel would span from 2400 MHz to 2440 MHz (the yellow channel rectangle on the left). If another 40 MHz channel were imagined in the band it would have to span from 2440 MHz to 2480 MHz (actually 2483 MHz if you include the required inter-channel gap between the two channels). 2483 MHz is outside the 2.4GHz ISM band. Two 40 MHz channels can not co-exist in the 2.4 GHz band - they would overlap! You can only configure a single 40 MHz channel when you're using 2.4 GHz 802.11n and that's not a reasonable or useable option for most commercial wireless LAN implementations. Only the 5.8 GHz band has enough channels to make 40 MHz commercial implementation feasible.

The 5.8 GHz Channel Space

Channel definitions in the 2.4 GHz band are part of the Industrial, Scientific and Medical (ISM) specifications. These definitions allow for the fact that the 2.4 GHz channels overlap. The channel space defined for unlicensed use in the 5 GHz range are defined under the Unlicensed National Information Infrastructure (U-NII, pronounced "you-nee") radio specifications. U-NII channels are exactly 20 MHz wide but they're spaced 20 MHz apart. There are 20 channels divided into four bands called "U-NII-1", "U-NII-2", "U-NII-2 Extended" and "U-NII-3".

The U-NII power restrictions in the United States are regulated under FCC Part 15 rules. Part 15 Subpart B is best known for its regulations regarding unintentional radiators (devices that produce electromagnetic noise) and Subpart C regulations for small transmitters (like FM microphones and "walkie-talkies"). Subpart E regulates unlicensed U-NII transmitters which include 802.11a and 5 GHz 802.11n access points. U-NII-1 is designated for indoor use and limits transmit power to 40 mW. U-NII-2 can be used for indoor or outdoor applications with a power limit of 200 mW. U-NII-3 is designated for outdoor use with an 800 mW power limit. Each channel shown in the diagram is 20 MHz wide.

802.11n 40 MHz Channels As They Relate to Dynamic Frequency Selection (DFS) and Radar Avoidance

Military radar and some weather radar can operate in the U-NII-2 band. To avoid conflict between unlicensed transmitters (like Wi-Fi access points) operating in the U-NII-2 band the FCC requires [Rule #15.407(h)(2)] that all unlicensed transmitters operating in U-NII-2 implement Dynamic Frequency Selection (DFS). DFS allows the radio to detect the presence of radar signals and to dynamically and automatically change to a different transmit frequency if radar is discovered. Because this requirement demands that a manufacturer of a Wi-Fi access point implement additional, specialized detection mechanisms some manufacturers have simply decided to not provide support for U-NII-2 operation. While almost all major manufacturers do support U-NII-2 it's important to check for U-NII-2 capability when selecting a particular brand of equipment.

Notice that without U-NII-2 support there are only four available channels for indoor use and four for outdoor use. That's a non-issue when 20 MHz channels are being used (i.e. for 802.11a). In mathematics, the "Four Color Theorem" states that if a plane (a floorplan or area map, for example) is divided into contiguous regions (analogous to access point coverage cell areas) then the regions can be colored using at most four colors so that no two adjacent regions have the same color. If we substitute "channel" for "color" then the Four Color Theorem tells us that if you have four channels to work with then a design can be created that avoids detrimental channel overlap (where two or more access points cover an area with the same channel resulting in system degradation).

When considering 40 MHz channels it can be seen that use of the U-NII-2 band is critical. You can't get four (or even three) 40 MHz channels for either indoor or outdoor use unless you take channel space out of the U-NII-2 band. The channel map shown below indicates how

Conclusions

The use of 40 MHz channels with 802.11n gives you just a little bit better than twice the throughput capacity of 20 MHz channel systems. Using a 40 MHz channel in the 2.4 GHz band is suitable only for residential use or single-access point designs and mixed-mode 802.11b/g with 802.11n in the 2.4 GHz band degrades 802.11n performance even more. Nonetheless, 802.11n will always offer greater maximum throughput than 802.11g.

802.11n should be implemented using 40 MHz channels in the 5 GHz U-NII band. You should be sure that your equipment implements support for the U-NII-2 band to be able to get four or more channels for use in your design.

Connect802 has the RF engineering expertise to help you design an optimal 802.11n wireless LAN system. We provide best-of-breed 802.11n equipment along with specialty 802.11n antenna systems when required. You're encouraged to call us to schedule a no-cost, no-obligation discussion with one of our experts to go over the requirements for your 802.11n project.