Lancom 1821n Wireless Manual page 18

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Chapter 1: Introduction
Even if the aim of four spatial streams has not yet been achieved, the use of
two separate data connections results in a doubling of data throughput,
which represents a true technological leap in the area of WLAN systems. Com-
bined with the improvements in OFDM modulation, the data throughput that
can be attained increases to 130 Mbps.
The short description "transmitter x receiver" expresses the actual number of
transmitting and receiving antennas. 2x2 MIMO describes two transmitting
and two receiving antennas.
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 POLARIZATION
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-
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