OFDM COMMUNICATION SYSTEMS

1. DEPARTMENT OF ELECTRONICS ENGINEERING
SCHOOL OF ENGINEERING AND TECHNOLOGY
PONDICHERRY UNIVERSITY
KALAPET,PUDUCHERRY-605014.

2. By
Ashok kumar
Pavan Kumar
Under the esteemed guidance of
smt.ANSUDHA Phd.

3.  For video communication,
Very high bit rate and
High speed communication required.
 To satisfy this, we must have the modulation
scheme that can read more number of bits at a time
and send it with considerably low bit errors

4.  Introduction
 Block diagram
 Effect of fading on OFDM
 Parameter choice and implementation issuses in
OFDM
 references

5.  OFDM
 ORTHOGONALITY
 MULTI CARRIER MODULATION
 CYCLIC PREFIX

6.  Modulation
Changing the characteristics of carrier wave w.r.t. to
message signal.
 Multiplexing
It is a method of sharing a bandwidth with other
independent data channels.
OFDM is a Combination of both modulation and
multiplexing.

7. Orthogonality
In order to assure a high spectral efficiency the sub
channel waveforms must have overlapping transmit
spectra.
To enable simple separation of these overlapping
sub channels at the receiver they need to be
orthogonal.

8. Definition of Orthogonality
Orthogonality is a property that allows the signals
to be perfectly transmitted over a common channel
and detected without interference.
   
C i j
  

  i 
j
S t S t dt j
T
i 0 0

9. Multicarrier Modulation
In OFDM, we can use MC modulation
.
each one of these sub carriers has a
width of f W / Nc
Adv :Intersymbol interference can
be reduced

10. Cyclic Prefix
In an OFDM, loss of orthogality appears due to
isi resulting in ici

 


 
  
T
LOSS T

g
sym
SNR 10 log 1 10

13.  Initially it was assumed that there was no time
variations imposed by the channel on signal envelope.
 It was also assumed that errors produced by IFFT/FFT
processing were negligible.
 But looking at these effects carefully we will encounter
with inter carrier interference or inter channel
interference or simply ICI.
 ICI can be made zero by making Doppler spread as
zero

14.  The use of OFDM in fading/multipath channels
appears to reduce to a few issues. These are:
 OFDM Symbol rate for combating delay spread
 Realizing diversity in OFDM
 Implementation issues such as Envelope variation
and synchronization

15.  High spectral efficiency
 Simple implementation by FFT
 Low receiver complexity
 Suitability for high-data-rate transmission
 High flexibility in terms of link adaptation

16.  Xu.fang,xu, Ru and sun, Haixin, ”implementation of
OFDM by using pulse shaping technique in freq
domain”2007 IEEE pp 319-323.
 Anwar.k and yamamoto.H,” a new design of OFDM
with FFT as spreading codes” 2006IEEE,pp:543-546.