1) OFDM is used to combat channel impairments like fading and intersymbol interference that occur due to high transmission rates in wireless systems. 2) In OFDM, high rate data streams are converted to low rate parallel streams and modulated using techniques like BPSK before transmission. 3) The use of a cyclic prefix preserves orthogonality in OFDM and prevents intersymbol interference. 4) The document describes the key components of an OFDM system including the transmitter, channel model, and receiver section. It focuses on modeling the wireless channel as a Rayleigh fading channel using the Jakes model.

1. Motivation
• The high demand for a large volume of
multimedia services in wireless
communication systems require high
transmission rates. But high Transmission
rates may result in severe fading and ISI.
These channel impairments can be combated
by OFDM.

2. OFDM Transmitter Section

3. OFDM Symbol generation
• Symbol Mapping: The high rate data streams
are first converted into parallel data of M
subchannels. The data is modulated using BPSK
before S/P conversion.
OFDM symbol is given by-
bm[k] (mapped data)
• Where m= mth data stream
• k= kth data symbol

4. OFDM signal

5. Concept of orthogonality

6. ISI
• If a sinusoid has no longer an integer number
of cycles within an FFT interval than
orthogonality is lost and Inter Symbol
Interference (ISI) occurs.

7. OFDM spectra

8. Guard interval and Cyclic prefix…..
The orthogonality in OFDM system may be
preserved by use of Guard interval and Cyclic Prefix.
• The most effective way of inserting a guard
period is to extract a portion of an OFDM symbol
at the end and append it to the beginning of the
OFDM symbol.
Cyclic prefix is a copy of the last part of OFDM
symbol.

9. ….cont.

10. Channel Model
The channel used for the project is a Rayleigh-
Jakes-WSSUS process model

11. Channel equation

12. Tap delay line
• The channel is modelled as a multipath
frequency selective fading channel using a
tapped delay line with time varying
coefficients and fixed tap spacing.

13. Frequency selective and Flat fading
• In many cases the reciever recieves more than
one copy of the transmitted signal due to
multipath between the transmitter and the
reciever ie., a multipath channel. In such cases
the channel is said to be frequency selective
ie; the channel varies in frequency.
– The other case is when reciever only recieves one
copy of the transmitted signal which esults in a
Flat Frequency Fading spectrum.

14. Rayleigh channel model
Rayleigh Fading Distribution in wireless channel is commonly
used to describe the statistical time varying nature of the
received envelope of a flat fading signal or the envelope of an
individual multipath component.
where
is the rms value of the received voltage signal before envelope
detection,
is the time-average power of the received signal before envelope
detection.
The channel is taken to be Rayleigh when there is more number of multipath.

15. Jakes Model
• A real time channel is both time and
frequency variant. The Jakes channel model
with an exponential PDP is used to generate
time-frequency-variant channel.
Jakes model is closely related to
Doppler shift and Doppler shift is obtaind when
iether or both the transmitter and reciever is in
motion.

16. • Maximum Doppler shift is given by-
• fd = vd fc / c
Where fd = Doppler shift
vd = vehicle velocity
fc = carrier frequency
c = speed of light
Doppler shift is connected with fast and slow fading and hence channel
is modeled using Jakes model whose impulse response is

17. • The autocorrelation of different transmission
paths is a zero order Bessel function that depends
on the time difference T and Doppler shift ,fd.
Autocorreletion in time domain is given by:
Φt[m]= Jo( 2πfdT)
fdT is the factor that controls how much the
channel varies betwwen two successive symbols.
Not:the above equation also indicates that the
power spectral density of the channel satisfies Jakes
model.

18. Power delay profile.

19. WSSUS process
• In practice the channel is changing
stochastically, so we must use statistical
characterisation in order to cope with the real
channels. In the real life it can be assumed
that the channels are wide sense stationary
both in time and in frequency - these channels
are called WSSUS channels

20. Reciever section

21. At the reciever end….
• The received signal r(t) at kth symbol duration
by the OFDM reciever is given by:-
• from the received signal first the CP is
removed
• After CP removal the signal is demodulated
using FFT, which converts the time domain
signal to frequency domain signal.

22. ….cont.
• The output of the demodulator block is called the
measured signal, ‘Sk,q ‘.
• Where k= kth symbol interval, k=1.2.3….
• q = number of sub-carriers generated by IFFT.
• This signal is actually the input to the estimator block.
• The delay blocks are inserted to synchronise the output
of the demodulator and the estimator block.
• The estimator block esrimates the channel states of
each sub-carrier.

23. Channel Estimation
• There are mainly two problems with the
design of channel estimators
• ------how the pilot information should be
transmitted
• -------design of an interpolater with low
complexity and good performance.

24. Pilot Arrangement
• Channel estimates are often achieved by
multiplexing pilot symbols into data sequence
and this technic is called Pilot Symbol Assisted
Modulation(PSAM)
• Block Type
– All sub-carriers reserved for pilots wit a specific
period
• Comb Type
– Some sub-carriers are
reserved for pilots for each symbol