multi antenna system in wireless communication

8/4/2019 Multi Antenna System in Wireless Communication

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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



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.



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]



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.

multi antenna system in wireless communication

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