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2021-01-21 12:21
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2021年1月21日发(作者:游离)

The Structure and Design of MIMO

System

Abstract:


The next generation mobile communication, 4G called, will support very high data transmission rate. It’s necessary to
introduce
technology
with
high
frequency
utilization
in
limited
frequency
spectrum.
MIMO
is
a
technology
to
increase
channel
capacity
with
adding
more
antennas
and
has
very
high
spectrum
utilization.
It’s
one
of
the
most
competitive
technologies in future mobile communications. This thesis introduce basic theory about MIMO and make deep analyze and
deduct about the channel capacity and array gain. Also discussed the key techniques of MIMO and described its developing
trend in future.

Key Word


mobile communication

MIMO

LTE

channel capacity

array gain

space diversity, space multiplexing


1.

MIMO overview



Future wireless communication system to transmit the highest speed and capacity are put forward higher
requirements, how to use the limited unlimited spectrum to meet the increasing communication demand has become an
important subject. As early as 1948, Shannon has the SISO system is given in the literature of channel capacity limit: C =
Blog2 (1 + S/N). Maximum spectrum efficiency for C/B = log2 (1 + S/N). For this kind of single into SISO system, no matter
how advanced the way of modulation and coding, can only make the system channel capacity is close to the ceiling, and
impossible to surpass him. With mature Turbo code and LDPC code technology and practical application, the actual system
basically close to the Shannon channel capacity limit.


In a new generation of mobile communication systems, people put forward higher requirements on the rate of
transmission, it is presented under the premise of spectrum is a limited resource support the requirement of high rate. Studies
have shown that MIMO technology can meet the requirements. MIMO technology in indoor environment spectrum
efficiency can reach 20-40 bit/s/Hz, far more than the traditional wireless communication in 1-5 bit/s/Hz with microwave
transmission in 10-12 bit/s/Hz.
MIMO first be suggested in 1908, it uses multiple antenna suppress channel fading. MIMO technology refers to the
transmitter and the receiver using multiple transmit antennas and receive antennas. MIMO technology is the key to the
multipath effect factors exist in the traditional communication system become better factors for user communication. MIMO
technology immediately effective utilization of the decline and the possible existence of multipath propagation to multiply
business transmission rate. MIMO technology successes lies in it can without any increase in signal under the premise of
extra bandwidth wireless communication performance on several orders of magnitude. This contributed to the channel model,
information theory and coding, signal processing, antenna design and design of fixed/wireless antenna cellular.

Bell LABS e. Alater and ni independently in their own paper demonstrates the theory of the Shannon
capacity of MIMO channel. They point out that using the channel matrix to describe theN t
×
N rroot transmitting antenna
and N r receiving antenna system of wireless channel, and the matrix elements is ideal independent fading, system capacity
will be along with the transmitter and the receiver antenna number of smaller one min (N t, N r) the increase of linear
increase. It can multiply on the basis of SISO system capacity.


According to the definition of the MIMO system that based on transmit diversity and accept diversity into single out
(MISO) method and the single into more way (SIMO) is a part of MIMO.

2.

The limit capacity of MIMO system
i.

For SIMO system
By a single antenna to send, receive antenna structure, with a number of 2 N r = 2. At the receiving end of the
maximum ratio combining coherent reception technology, the received signal:




The limit capacity can be expressed as at this time



Among them, the P t is the total power to send, the delta n is the noise power. The ultimate capacity formula
can be seen that channel capacity will increase as the number of receiving antenna is a logarithmic.


ii.

For MISO systems

Sent by a single antenna, the structure of the antenna number is 2, namely the Nt = 2. In the sending end adopts
closed loop transmission diversity, the received signal



Among them, the P t is the total power to send, the δ n is the no
ise power. The ultimate capacity formula can be
seen that channel capacity will also be a logarithmic increase as the transmitting antenna number


iii.

For MIMO systems
Sent by a single antenna, the structure of the antenna number is 2, namely the Nt = 2. In the sending end adopts
closed loop transmission diversity, the received signal



The limit capacity can be expressed as at this time:

Among them, the Pt. is the total power to send, the δ n is the noise power. The ultimate capacity formula can be
seen that channel capacity will also be a logarithmic increase as the transmitting antenna number.
3.

Techniques in MIMO system
MIMO technology
is essentially for the system to provide the spatial
multiplexing
gain
and spatial diversity
gain. Spatial multiplexing technology can greatly improve the channel capacity, and spatial diversity can improve
the reliability of channel, reduce the channel bit error rate.
i.

Spatial multiplexing technology
Spatial
multiplexing
is
sender
and
using
multiple
antennas
at
the
receiving
end,
make
full
use
of
space
transmission
of
multipath
components,
using
multiple
data
channels
on
the
same
frequency
band
(MIMO)
sub-channels emission signal, thus making capacity increases linearly with the increase of the number of antennas.
This
signal
increase
don't
need
the
extra
bandwidth
capacity,
also
do
not
need
additional
transmission
power
consumption, and therefore is to improve the channel capacity of a very effective means.

Spatial reuse,
will first
need
to transfer the signal after a
string of translates into several parallel signal
flow,
more than in the same frequency band antenna to send at the same time. Data flow through the pulse forming, and
modulation.
Due
to
multipath
propagation,
each
pair
of
transmitting
antenna
of
the
receiver
to
create
a
different
spatial
signal,
the
receiver
using
signal
to
distinguish
their
different
data
streams.
Achieve
spatial
reuse
must
demand
spacing
is
greater
than
the
relative
distance
between
transmit
and
receive
antennas.
By
sending
and
receiving antenna spacing is greater than the relative distance (usually) more than 10 signal wavelength. In this way
can we guarantee to receive originated were not associated with each independent fading sub-channels channel.

For spatial reuse, BLAST is a kind of algorithm can realize the spatial multiplexing gain. In 1998 by Foschini
and G.G olden V - BLAST algorithm is put forward. V - BLAST algorithm is not for all signal decoding together,
but first of all, the strongest signal decoding, and then in the received signal minus the strongest signal, and then the
residual
signal
decoding,
the
strongest
signal
minus
this
signal,
so
once,
until
all
the
signals
are
rendered.
V
-
BLAST algorithm is considered algorithm complexity and decoding performance under a kind of optimal decoding
algorithm.
ii.

Transmission diversity and receive diversity
Space diversity technology can be divided into two categories, receive diversity and transmit diversity, usually
can be thought of SIMO system is receiving diversity, MISO systems transmit diversity. Wireless signal in complex
Rayleigh fading propagation in wireless channel, its decline characteristics different on different spatial location. If

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