Lancom L-310agn Wireless Manual page 18

LANCOM L-300 Access Point series
Chapter 1: Introduction
signals cannot be uniquely identified, their computation can still be performed
with the aid of the third signal. The use of additional antennas does not con-
tribute to an increase in data throughput, but it does result in a more even,
stronger coverage for clients.
MIMO in outdoor use
Outdoor 802.11n applications cannot use natural reflections since signal
transmission usually takes place over the direct path between directional
antennas. In order to transmit two data streams in parallel, special antennas
are employed that use polarization channels turned through 90° to each
other. These so-called "dual-slant" antennas are really two antennas in one
housing. Since a third signal does not offer additional reliability, outdoor
applications generally use as many antennas (or polarization channels) as
there are data streams for transmission.
BUILDING BUILDING
POLARIZATION POLARISATION
MIMO AP 802.11n MIMO AP 802.11n
DIVERSITY DIVERSITY
40 MHz channels
As the above explanation of OFDM modulation states, data throughput rises
with an increasing number of carrier signals because this allows several sig-
nals to be transmitted simultaneously. If a channel with a bandwidth of
20 MHz supports no more than 48 (802.11a/g) or 52 (802.11n) carrier signals,
the obvious choice would be to use a second channel with additional carrier
signals.
This method was used in the past by a number of manufacturers (including
LANCOM Systems) and was referred to as "turbo mode", allowing data rates
of up to 108 Mbps. Turbo mode does not form part of the official IEEE stan-
dard but is frequently employed on point-to-point connections, for example,
because compatibility to other manufacturers tends to play a secondary role.
However, the success of the underlying technology has lead to its incorpora-
tion into 802.11n. IEEE 802.11n uses the second transmission channel in a
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