This document provides information about the differences in RF power
between the Cisco Access Point (AP) 3500 and the AP 3600.
This document provides insight on FCC rules and use of Power Spectral
Density (PSD) requirements that causes a slight reduction in RF power as the
new AP 3600 complies with the new FCC rules.
A. Traditionally, the UNII-1 band has always had lower RF power
restrictions placed on it because these frequencies are set aside for indoor
use only in the United States. During the development of the AP 3600 some new
FCC guidelines on RF emissions took effect, which changed the RF power
Note: The AP 3600 is the first commercial Access Point to be certified
under these new guidelines. Review this Wi-Fi spectrum for a better
Figure 1 - This chart shows the Wi-Fi spectrum and those services
that are primary (licensed users). Source:
When looking at the UNII-1 band 5150-5240 MHz (channels 36-48), you can
see that the primary or dedicated (licensed use) for this spectrum is for the
Aeronautical Radio Navigation service and fixed satellite use, applications
such as Microwave Landing systems and outdoor communications.
When the UNII-1 frequencies are used indoors with limited RF power, the
FCC permits these frequencies to be used for unlicensed Wi-Fi because such
devices can co-exist with limited interference provided the Peak RF power and
the Power Spectral Density (PSD) are kept within acceptable levels.
On October 25th 2011, the FCC Office of Engineering and Technology
Laboratory Division released a paper about the testing of transmitters with
Multiple Outputs in the same band. These new guidelines help to reduce
potential interference and apply to Smart Antenna systems and all Wireless LAN
products utilizing Multiple Input Multiple Output (MIMO) technology regardless
This FCC bulletin was released to address how manufacturers such as
Cisco must comply with the new guidelines. These guidelines are available at
these FCC URLs:
The “take-away” is that the FCC has introduced additional clarification
about the method manufacturers must comply with PSD limits.
From the FCC paper, directional gain calculations can be done using
Directional Gain Calculations—In the re-occurring case
of N transmit antennas, each with the same directional gain GANT dBi driven by
N transmitter outputs of equal power, directional gain is to be computed as
If any transmit signals are correlated with each
other, Directional gain = GANT + 10 log(N) dBi
If all transmit signals are completely uncorrelated with
each other, Directional gain = GANT
From the FCC paper—Correlation between signals
transmitted from different antennas can lead to array gain, which increases the
directional gain of the device and leads to higher radiated levels in some
directions. The contribution of array gain to the directional gain of the
transmitter must be considered in rule parts where conducted in-band emission
limits vary with directional gain, or in situations where conducted
measurements are combined with directional antenna gain to determine compliance
with in-band radiated limits.
Because the Cisco AP 3600 is the first to market Access Point to comply
with the new FCC guidelines, Cisco (and other manufacturers) is now required to
reduce RF power in the UNII-1 band and slightly reduce RF power in the UNII-2
and UNII-2 extended bands when releasing new products that take advantage of
MIMO or Smart Antenna technology. Or, other methods such as reducing the
overall antenna gain permitted by their products.
Again, this is done to reduce potential interference with Aeronautical
Radio Navigation services, Radar and fixed satellite communications services.
The key take-away here is that in order to comply with In-Band PSD
requirements under the new rules, you must consider the number of transmitter
paths and how it effects the total RF power and PSD emissions.
This includes beam-forming whether the beam-forming was intentional or
not and given that under certain conditions, RF power can add up driving up the
overall PSD value. In order to keep that under the FCC limits, in some cases
you have to reduce RF power based on the MCS rate and frequency being used to
The reduced power is more pronounced in the UNII-1 band where PSD
limits are lower especially when multiple transmitter paths exist. For example,
in the next screenshot you can see under the new FCC rules that the more
transmitter paths (physical transmitters) the higher the PSD rises. In some
cases, RF power is reduced by 6 dB with four transmitters present.
Figure 2 – When more transmitters are enabled the PSD lowers. This
forces a reduction in RF power.
Figure 3 - When more transmitters are enabled the PSD lowers. This
forces a reduction in RF power. Fortunately this reduction in power is much
less in the UNII-2 and UNII-2 extended bands.
When considering the new FCC rules, RF output power is rolled back a
bit when all four transmitters are used concurrently to comply with the lowered
PSD limitations. A worst case power reduction, for example, a 6 Mbps packet can
have up to 6 dB less power with an AP 3600 than the AP 3500 (8 dB versus 14 dB)
because the AP 3600 now complies with the new FCC rules about PSD emissions and
the two additional transmitters.
In most cases, especially with clients using higher data rates, the
reduced TX power is compensated by having a better downlink performance due to
the gains from beam-forming now enabled with multiple transmitters and using
Cisco Client Link 2.0.
Note: Client Link 2.0 is only available on the AP 3600. Therefore, it has
an advantage over the AP 3500 allowing 802.11n and 802.11ac clients to maintain
a better downstream link.
Also, while there has been a noticeable reduction in RF power primarily
in the UNII-1 band, and while it is possible to perhaps characterize better
performance with an AP 3500 running at maximum RF power in the UNII-1 band,
testing has indicated the AP 3600 when installed in an over-all network
employing multiple APs across channels in the UNII-1, 2 and 3 bands the
performance of the AP 3600 exceeds AP 3500 performance by far.
Again, the AP 3500 only has the ability to beam-form to legacy
802.11a/g clients, whereas the newer AP 3600 using Client Link 2.0 can
beam-form to 802.11a, g, and n clients as well as 802.11ac clients in
In summary, the AP 3600 is the first industry Access Point that fully
complies with all the new FCC rules and regulations. Also, the AP 3600 offers
many features above and beyond those in the AP 3500.
Additionally, any new multi-transmitter MIMO devices, including those
by other vendors, now need to comply with these new FCC rules.
Support for four transmitter chains, which enables 3-ss client
support up to 450 Mbps.
Client Link 2.0 (Beam-forming) for all 802.11n clients for a better
overall Bring Your Own Device (BYOD) experience.
Note: The 4th transmitter allows Client Link to work with 3-ss
Module support for expandability and investment
Cisco APs have full support of UNII-2 extended channels, a key
feature for the newer clients because more clients are emerging with UNII-2
extended support including .11ac clients in enterprise (802.11n) compatibility
For more information on the AP 3600, refer to:
Aironet 3600 Series.