Lancom 1821n Wireless Manual page 17

LANCOM 1811n Wireless – LANCOM 1821n Wireless
Chapter 1: Introduction
above, it is virtually impossible to transmit different signals on the same chan-
nel simultaneously as the receiver cannot distinguish between them. MIMO
uses the reflection of electromagnetic waves and the associated spatial aspect
to obtain a third criterion for identifying the signals.
A signal sent by transmitter A and received by receiver 1 follows a different
path than a signal from transmitter B to receiver 2. Due to the different reflec-
tions and changes in polarization that both signals experience along their
paths, each of these paths takes on its own characteristics. When data trans-
mission starts, a training phases records the characteristics of the path by
transmitting standardized data. Subsequently, the data received here is used
to calculate which data stream the signals belong to. The receiver decides for
itself which of the incoming signals is to be processed, thus avoiding loss from
interference.
MIMO thus allows the simultaneous transmission of several signals over one
shared medium, such as the air. Individual transmitters and receivers must be
positioned a minimum distance apart from one another, although this is just
a few centimeters. This separation results in differing reflections and signal
paths that can be used to separate the signals.
Generally speaking, MIMO can provide up to four parallel data streams, which
are also called "spatial streams". However, the current generation of chips can
only implement two parallel data streams as the separation of data streams
based on characteristic path information demands high levels of computing
power, which consumes both time and electricity. The latter tends to be unde-
sirable particularly for WLAN systems, where attempts are often made to
achieve independence from power sockets at the WLAN client or when using
PoE as the electricity supply for the Access Point.
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A
MIMO AP 802.11n
B
1
MIMO Client 802.11n
2