The ultimate goal of wireless communication technology is to provide universal personal and multimedia communication irrespective of mobility and location with high data rates.
When the data is transmitted at high data/bit rates over mobile radio channels then the channels may cause :
Severe fading of transmitted signal when passed through channel
Inter symbol interference (ISI)
The focus of the future generation (4G) mobile system is on supporting higher data rates and providing seamless services across a multitude of wireless systems and networks.
Orthogonal Frequency Division Multiplexing (OFDM) is one of the promising technique for 4G to mitigate ISI and fading in multi-path environment.
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many sub-carriers, each one being modulated by a low data rate stream.
Fig:2 Single Carrier System Fig:1 Multi-Carrier System W=Bandwidth T= Sample time
5.
OFDM System Architecture Figure : 3 Transmitter Receiver
First the high serial data rate input with sampling time T s is modulated using any digital modulation technique (BPSK, QAM, QPSK etc), to give digital symbols b m [k].
Next the modulated serial data is converted to low rate parallel data streams (M) using S/P converter.
Due to this parallel conversion, the effective symbol duration is increased to T= MT s
Where index k=I,2,….. is the symbol interval and m=0,1,….M-1 are the number of sub-channels.
To mitigate the effect of ISI, signal y(t) in (1) is added with a cyclic prefix thus the mathematical expression of y(t) becomes
y(t)=∑ M-1 m=0 b m [k]exp(j2 π mt/T),
for - Ψ +kT ≤ t ≤ (k+1)T --------------(2)
where Ψ is guard interval.
11.
Channel The transmitted signal y(t) travels though wireless channel through multi-paths in various types of environments (indoor, outdoor, static and mobile) thus the signal y(t) undergo distortion, scattering, reflection and addition of noise. These phenomenon ultimately characterizes the channel mathematically in terms of (i) delay spread and (ii) fading coefficients of the channel, which are treated as random variables.
The channel is modelled as a multipath frequency selective fading channel using a tapped delay line with time varying coefficients and fixed tap spacing, which is mathematically expressed below,
Unit delay Unit delay Unit delay + + + h0 h1 h2 hx y(t) h(t,tau)
The channel considered in the present work is a Rayleigh with Jakes Doppler spectrum (in order to consider the mobility of either or both the transmitter and the receiver).
The channel impulse response h l (t) is considered as WSSUS (Wide sense stationary uncorrelated scaterring) process.
The WSSUS assumption of the channel means that the mean and the covariance (statistical properties of the channel) do not vary with respect to time.
The channel considered in the present work is a Rayleigh with Jakes Doppler spectrum (in order to consider the mobility of either or both the transmitter and the receiver). Also the channel impulse response is considered as WSSUS.
(a) Theoritical Rayleigh Jakes model:
Let the continuous time channel model of l th scatterer and m th
The Channel Fading parameter H k,q needs to be estimated accurately given the output s k,q .
For efficient Channel state estimation, a joint time-frequency domain channel estimation model will be developed in final stage of our work.
Channel estimates are often achieved by multiplexing pilot symbols into data sequence and this technique is called Pilot Symbol Assisted Modulation (PSAM).
Block Type
All sub-carriers reserved for pilots wit a specific period
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