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MULTI ANTENNA SYSTEM IN WIRELESS COMMUNICATION
1. When we send something from transmitter to receiver we experience
something unexpected known as fading.1.large scale fading2.small scale fading
2. The small scale fading is directly related to the reasons like
Multipath propagation.
Speed of the mobile.
Speed of surrounding objects.
The transmission bandwidth of the signal.[when signal bandwidth is less
then channel bandwidth then small scale fading is more significant. Here
the amplitude is changed rapidly but the signal will not be distorted in
time]
3. There are different types of small scale fading are there like…..
Depending on the relation between the signal parameters [bandwidth,
symbol period] and the channel parameters [rms delay spread and Doppler
spread] different signals undergoes in different types of fading as given
bellow………..
Multipath delay spread: [time dispersion comes in to picture]
Flat fading:
BW of signal<BW of channel.
Delay spread <symbol period.
N.B: Since here the delay spread is less than the symbol period so hb(t,τ) can be
approximated as having no excess delay ,means expressed as single delta
function with delay(τ) =0. Also known as narrowband channels. We can say
that the time delay of the arriving waves is approximately equal, so amplitude
does not depends on the carrier frequency. The total received signal can be
shown as .
Frequency selective fading:
BW of signal > BW of channel.
Delay spread>Symbol period.
N.B: Since here the delay spread is greater than the symbol period so the
receiver experience the multiple versions of the transmitted waveform which
are faded and delayed in time and hence ISI is come in to picture. Frequency
fading models are must more difficult to model than flat fading channels. since
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here each multipath signal must be modeled. Models like two ray Rayleigh
fading models are used here .Also known as wideband channels. Here the
received signal is composed of waves from all scatterers, whose phase and
amplitude depends on the reflection co-efficient and scattering characteristics
of the scattered and whose time delay is τ. here Y is given by……………
Doppler spread: [frequency dispersion come in to picture]
Fast fading:
High Doppler spread
Coherence time<Symbol period
Channel variation faster than the
base band signal variation.
Slow fading:
Low Doppler spread
Coherence time>Symbol period
Channel variation slower than
baseband signal variation.
N.B: A channel may be flat fast fading channel[here no time delay will be
there in multipath propagation] only a single delta function can characterize
the impulse response. Therefore only the amplitude of the delta function will
vary faster than the rate of change of transmitted base band signal. Otherwise
if the channel is a frequency selective fast fading channel[here time delay is
there] then the amplitude, the phase, the time delays of any one of the
multipath component changes very faster than the rate of change of the
transmitted signal. Velocity of the mobile and the base band signal determines
whether the signal goes under fast or slow fading.
4.Comparision between AWGN and Narrow band fading channel is
given bellow:
AWGN channel:
Signal may experience only some noise and some fixed
multipath loss[including shadowing].
It is applied in a mobile system where the mobile and
surrounding object are not in motion.
Here the noise is composed of real and imaginary
component which have zero mean, components are
independent and with a real Gaussian process.
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Noise is known as white Gaussian because [here white
means has a constant power spectral density and
Gaussian means has a normal distribution.
Given byy(t)=Au(t)+n(t)
NARROW BAND FADING CHANNEL:
It is a time variant process.
The received signal at time t is now given by
Y(t)=Aα(t)u(t)+n(t)here α(t) is a fading parameter.
5. Due to this fading we may experience different phenomena like----1.Inter
symbol interference [ISI]generally occurs when modulated signal bandwidth
exceeds the coherence bandwidth of the radio channel.[coherence bandwidth
is the maximum frequency difference over which the signals are still strongly
correlated in amplitude].
2.Rapid changes in signal strength over a small travel distance or time interval.
3.Random frequency modulation due to Doppler shifts on different multipath
signals.
4.time dispersion [echoes 0caused by multipath propagation delays.
To reduce these effects and to improve the system performance there are
three basic techniques which can be used independently or in a tandem
manner….These are
1.Equalization 2.Diversity and 3.Channel coding.
Multipath fading is due to the constructive and destructive combination of
randomly delayed, reflected, scattered, and diffracted signal components. This
type of fading is relatively fast and is therefore responsible for the short-termsignal variations. Depending on the nature of the radio propagation
environment, there are different models describing the statistical behavior of
the multipath fading envelope……………..
1 .Rayleigh fading 2.Rician fading 3.Nakagami –q (hoyt) model 4.Nakagami-
q(rice)model 5.Nakagami –m model 6.Two ray Rayleigh fading model.
According to each model we get different distribution function and as well as
different PDF(Probability Density Function),moment generation
function(MGF)and amount of fading(AF) etc. Depending on these we can
calculate the particular situation of a particular environment.[already we
have]
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DIVERSITY: It is one of the various techniques use to improve the performance
of a communication system. There are main 4 types of diversity are there………
1.time diversity 2.frequency diversity 3. Space diversity 4.polarized diversity
Space diversity only can be achieve by use of multiple antennas which helps to
get a higher data rate without increase the bandwidth of the channel. So it
increase the channel capacity. To improve the performance of the diversity
there are two main requirement………………..
1.The channels should be un-correlated. Means should have a low cross-
correlation.
2.The mean power available for each branch should be almost equal.
The multiple antenna may be have in the receiver side or in the transmitter
side or both side. Depends on that we get three model……….
SIMO[Single input multiple output].
MISO[Multiple input single output].
MIMO[Multiple input multiple output].
Diversity can be implemented two ways in the system.
1.Using receive diversity 2. Using transmitter diversity
Received diversity can be gain by three combing method------
1.Selection combining.2.Gain combining------------
Equal gain combining.
Maximum ratio combining.
Maximum mean square error combining.
3.Hybrid selection/gain combining.[description already with us]
Transmitter diversity can be achieve by two main method……….
When the transmitter has a perfect channel knowledge, beamforming
can be performed.
When the transmitter has no channel knowledge, pre-processing known
as space –time coding[generally used Alamouti scheme ]is used to
achieve a diversity gain, but no array gain.
MIMO:
With perfect channel knowledge-------
1.dominant eigenmode transmission.2.Dominant eigenmode transmissionwith antenna selection.3. Multiple eigenmode transmission.
Without channel knowledge--------
1.Space –time block coding.2.Space –time trellis coding.3.Space-frequency
coding.