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wireless communication systems
INTRODUCTION
Traditional wireless communication systems use a single antenna for
transmission and a single antenna for reception. Such systems are
known as single input single output (SISO) systems (Fig.1). In
recent years, significant progress has been made in developing
systems that use multiple antennas at the transmitter and the receiver
to achieve better performance[3]. Such systems are known as
multiple input multiple output (MIMO) systems (Fig.2).
There are two types of benefits of using multiple antennas: link
budget / spatial diversity improvement and throughput improvement
from spatial multiplexing. Both are intrinsic to wireless channels,
where rich spatial variations or spatial dimensionality exist [3].
Spatial diversity refers to the fact that the probability of having
all antennas at bad locations is significantly lower as the number of
antennas increases. Link budget improvement refers to the fact that
the signals from the various antennas can be combined to form a
signal stronger than any of the individual signals. For receive
spatial diversity, signals received on multiple antennas are weighted
and combined, e.g. maximal ratio combining (MRC)[3]. There are
two types of transmit spatial diversity, open-loop and closed-loop.
Open-loop transmit diversity involves transmitting signals from
multiple antennas in some deterministic pattern, that does not
depend on the channel. Open-loop techniques include cyclic delay
diversity (CDD) and space-time block codes (STBC)[3]. Closedloop
transmit diversity techniques, in contrast, require channel
information to guide transmissions. An example is transmit
beamforming (TxBF), where proper magnitude and phase weights
computed from the channel estimation are re-applied across
antennas to aim the signal in a given desired direction[3]. MIMO
systems with spatial diversity achieve better performance, i.e. longer
range for a given data rate, or higher data rate than SISO systems at
a given same location.