Black Box LWN602A User Manual page 36

Chapter 3: The SmartPath AP (LWN602HA) Overview
In previous 802.11 standards, access points and clients each used a single set of components, or RF chain, for transmitting or
receiving. Although two antennas are often used for diversity, only the one with the best signal-to-noise ratio is used at any given
moment, and that antenna makes use of the single RF chain while the other antenna remains inactive. A significant improvement
that MIMO introduces is to permit each antenna to have its own RF chain and for all antennas to function simultaneously. For the
SmartPath AP, you can connect up to three antennas per radio and configure the radio to use two or three transmit chains and
two or three receive chains.* Using two or three transmit and receive chains simultaneously increases the amount of data that can
flow across the WLAN and accelerates the processing of that data at each end of the wireless link.
*The convention for presenting the configuration of transmitting and receiving MIMO RF chains is TxR. For example, a SmartPath
AP radio functioning in access mode might be configured to use two RF chains for transmitting and three for receiving. In that
case, its configuration can be presented as "2x3." In general, the number of receive antennas is equal to or greater than the
number of transmit antennas.
Another major aspect of MIMO is how it turns multipath signals from a curse to a boon. As a radio signal moves through space,
some objects reflect it, others interfere with it, and still others absorb it. The receiver can end up receiving multiple copies of the
original signal, all kind of muddled together. However, the digital signal processors in the multiple receive chains are able to
combine their processing efforts to sort through all the received data and reconstruct the original message. Furthermore, because
the transmitter makes use of multiple RF chains, there is an even richer supply of signals for the receive chains to use in their
processing. To set the transmit and receive RF chains for a radio profile, enter the following commands:
radio profile <name> transmit-chain { 2 | 3 }
radio profile <name> receive-chain { 2 | 3 }
There are two sets of antennas—three antennas per set—that operate concurrently in two different frequency ranges: 2.4 GHz
(IEEE 802.11b/g/n) and 5 GHz (IEEE 802.11a/n). Using two different frequency ranges reduces the probability of interference that
can occur when numerous channels operate within the same range. Conceptually, the relationship of antennas and radios is
shown in Figure 3-11.
RP-SMA
Connectors
Cut-away view of the SmartPath AP to show the relationship of the antennas and the two internal radios
The wifi0 interface links to radio 1 (frequency range = 2.4 GHz for IEEE 802.11b/g), and the wifi1 interface links to radio 2
(frequency range = 5 GHz for IEEE 802.11a). These interface-to-radio relationships are permanent.
When deciding how many antennas to use, consider the types of wireless clients—802.11n only, 802.11g/n, 802.11b/g/n, or
802.11a/n—the area needing coverage, and the RF environment.
Page 36
2.4 GHz (A)
Radio 1
RF 802.11b/g/n
2.4 GHz (B)
2.4 GHz
2.4 GHz (C)
Figure 3-11. Antennas and radios.
724-746-5500 | blackbox.com
5 GHz (A)
Radio 2
RF 802.11a/n
5 GHz (B)
5 GHz
5 GHz (C)
RP-SMA
Connectors