1.
OFDM Transmitter & Receiver
Guide: Dr. Vijaya Prakash A.M.
Professor, ECE Department
Group members: Shriram Shridhar Shanbhag(1BI12EC101)
Shubham Dhingra (1BI12EC103)
Supreem H(1BI12EC111)
Mohammed Ibrahim Adilshah(1BI13EC407)
BANGALORE INSTITUTE OF TECHNOLOGY
ELECTRONICS & COMMUNICATIONS ENGINEERING
Project Seminar on

2.
Agenda / Topics
• Introduction to OFDM
• Single carrier modulation v/s Multi-carrier modulation
• Block Diagram
• Constellation Mapper
• Role of IFFT block
• Cyclic Prefix
• Design of Constellation Mapper
• Design of IFFT block
• Simulation Results
• Conclusion
• References

3.
Introduction to OFDM
 Multi Carrier modulation technique
 Subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other
 Two signals are said to be orthogonal to each other if the integral of the product of
two signals is zero over a time period
 Orthogonality is defined by:
0
𝑇
cos(2𝜋𝑛𝑓0 𝑡)cos(2𝜋𝑚𝑓0 𝑡)𝑑𝑡 = 0 (𝑛 ≠ 𝑚)
 For OFDM, T is one symbol period and f0 set to 1/T for optimal effectiveness
 Orthogonality prevents interference between overlapping carriers

4.
Basis for OFDM – Multi Carrier Modulation(MCM)
 Operates by dividing the data stream to be transmitted into a number of lower data rate
data streams
 Each of the lower data rate streams is then used to modulate an individual carrier
 MCM is being used as a modulation format for high data rate transmissions
Single Carrier Modulation v/s MCM – An example
 Usually, symbol period must be greater than delay time to avoid ISI
 Generally, delay time Td = 2-3 μs
Say, bandwidth B = 10Mhz
Case (i) Single Carrier Modulation
Symbol time, T = 1/B = 1/10M = 0.1μs
T < Td which implies there will be ISI
Case (ii)Multi-Carrier Modulation
Suppose , there are 1000 sub-carriers, i.e. N=1000
Symbol time, T = 1/(B/N) = 1/(10M/1000) = 100μs
T>>Td which implies there will be no ISI B
. . .. . .
Transmission in MCM systems
0 B/N 2B/N-B/N-2B/N

5.
 OFDM signal consists of a number of closely spaced modulated carriers (until they are
actually overlapping)
Spectrum overlap in OFDM
 Because of orthogonal carriers, inter carrier
guard bands are not required and hence provides
high bandwidth efficiency compared to FDM
 Doesn’t require filter at the receiver side as in
the case of FDM
Fig.(A)Spectrum of FDM showing guard
bands
Fig.(B)Spectrum of OFDM showing
overlapping carriers

6.
Block Diagram of OFDM
Serial to
Parallel
Constellation
Mapper
(QPSK/QAM)
IFFT
Parallel
to Serial
Channel
Serial
to
Parallel
Delete
Cyclic
Prefix
FFT
Constellation
demapper
(QPSK/QAM)
Parallel
to Serial
d0,d1,d2,…
OFDM signal
Cyclic Prefix
(xN-Ncp ,xN-Ncp-1,….,xN-1)
s0
s1
sn-1
X0
X1
Xn-1
x0
x1
xn-1
r0
r1
rn-1
R0
R1
Rn-1
s0
s1
sn-1
d0,d1,…

7.
Constellation mapper
Constellation mapper is nothing but the modulator that takes word as an
input and maps it to a point on the constellation diagram
The size of the word depends on the type of modulation used
In case of QPSK the size is 2 bits/word, in 16-QAM it’s 4 bits/word and in
case of 64-QAM it’s 6 bits/word
The use of phase shift keying produces a constant amplitude signal while
QAM produces signal with variable amplitude
Hence PSK modulators are easier in terms of implementation

8.
The constellation diagram for QPSK, 16-QAM and 64-QAM are shown below

9.
Role of IFFT block
• In OFDM we divide the available bandwidth into N orthogonal
carriers
• If Xi is the symbol to be transmitted on ith subcarrier, transmitted
signal equation can be written as
Si = Xi*ej2*pi*i*(B/N)*t where B is the total bandwidth
• The set of symbols transmitted can be written as
S = Σ Xi*ej2*pi*i*(B/N)*t
• The above expression is nothing but IFFT(X)

10.
Pictorially, it can be represented as follows :

11.
Cyclic Prefix
• Usually to minimize the ISI we increase the symbol time
• But even after increasing symbol time, some effects are not removed

12.
• To eliminate this problem, we have to find a way to recover the lost part
• One way to do this is duplicating the initial part of the symbol and append
it to the end of the symbol
• This increases the overall length of the symbol but the gains outweigh the
increase in length

13.
• From the figure below, it can be seen that with cyclic prefix the problem no
longer exists
• The length of the cyclic prefix should be large enough to eliminate the
effects of multipath components but short enough to keep the symbol time
low

14.
Design of constellation mapper
QPSK
rst
in Out_real
Out_im
Binary Mapping points Hexadecimal
00 0.707 + j0.707 3fe6 3fe6
01 -0.707 + j0.707 bfe6 3fe6
10 -0.707 –j0.707 bfe6 bfe6
11 0.707 – j0.707 3fe6 bfe6
16
16
clk

15.
ROM MODULE 4x32
4x32
ROM
rst
address
Out
2
clk
32
Address Value
00 3fe63fe6
01 bfe63fe6
10 bfe6bfe6
11 3fe6bfe6

16.
SIPO
4x32
ROM
ROM Clk
rst
address
Out_real
clk
2 16
16
32
/2
in
din
rst
Out_img

17.
Design of IFFT block
X(0) = x(0)+ x(4)+ x(2)+ x(6)+ x(1)+ x(5)+ x(3)+ x(7)
X(4) = x(0)+ x(4)+ x(2)+ x(6)- x(1)- x(5)- x(3)- x(7)
X(2) = x(0)+ x(4)- x(2)- x(6)+ jx(1)+ jx(5)- jx(3)- jx(7)
X(6) = x(0)+ x(4)- x(2)- x(6)- jx(1)- jx(5)+ jx(3)+ jx(7)
X(1) = x(0) - x(4) + jx(2) - jx(6) + 0.7071x(1) + j0.7071x(1) - 0.7071x(5) - j0.7071x(5) -
0.7071x(3) - j0.7071x(3) + 0.7071x(7) + j0.7071x(7)
X(5) = x(0) - x(4) + jx(2) - jx(6) - 0.7071x(1) - j0.7071x(1) + 0.7071x(5) + j0.7071x(5) +
0.7071x(3) + j0.7071x (3) - 0.7071x(7) - j0.7071x(7)
X(3) = x(0) - x(4) - jx(2) - jx(6) - 0.7071x(1) + j0.7071x(1) + 0.7071x(5)- j0.7071x(5) +
0.7071x(3) - j0.7071x(3) - 0.7071x(7) + j0.7071x(7)
X(7) =x(0)- x(4) - jx(2) - jx(6) + 0.7071x(1) - j0.7071x(1) - 0.7071x(5) + j0.7071x(5) -
0.7071x(3)+ j0.7071x(3)+ 0.7071x(7)- j0.7071x(7)

18.
Design of IFFT Block
Pass
Path0
Path1
Path2
Path5
Path3
Path4
Path6
Path7
X0
X1
X2
X3
X4
X5
X6
X6
x(0)
x(1)
x(2)
x(3)
x(4)
x(5)
x(6)
x(7)

19.
Path 0 & 4
adder
adder
adder
adder
adder
adder
adder
x0
x4
x2
x6
x1
x5
x3
x7 x(0)
8bits

20.
Path 2 & 6
adder
adder
adder
x4
x2
x6
x0
x(2) R
adder
adder
adder
x5
x3
x7 x(2) I
x1

21.
Path 1, 3, 5 & 7
X(1) = x(0) - x(4) + jx(2) - jx(6) + 0.7071x(1) + j0.7071x(1) - 0.7071x(5) - j0.7071x(5) - 0.7071x(3) -j0.7071x(3) +
0.7071x(7) + j0.7071x(7)
X(5) = x(0) - x(4) + jx(2) - jx(6) - 0.7071x(1) - j0.7071x(1) + 0.7071x(5) + j0.7071x(5) + 0.7071x(3) + j0.7071x (3) -
0.7071x(7) - j0.7071x(7)
X(3) = x(0) - x(4) - jx(2) - jx(6) - 0.7071x(1) + j0.7071x(1) + 0.7071x(5)- j0.7071x(5) + 0.7071x(3) - j0.7071x(3) -
0.7071x(7) + j0.7071x(7)
X(7) =x(0)- x(4) - jx(2) - jx(6) + 0.7071x(1) - j0.7071x(1) - 0.7071x(5) + j0.7071x(5) - 0.7071x(3)+ j0.7071x(3)+
0.7071x(7)- j0.7071x(7)
• Each expression contains 12 products and sum of those product terms.
• So the path can be split into 2 modules one to find the products, and one to add
them up.

22.
Path 1, 3, 5 & 7
Path 5
x0
x3
x1
x2
x4
x6
x7
XR
XI
16bits
8bits
x5

23.
Product
Generator
clk
start
x0
x3
x4
x1
x2
x5
x6
x7
Cascade
of adders
p1
p2
p3
p4
p5
p10
p11
p12
p6
p7
p8
p9
XR
XI
8bits
16bits
busy enable

24.
M1
a
b
p
M2
M11
M3
M12
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.start
clk
busy
multiplier
clk
inputs
outputs
busy

25.
Tools Used
• MATLAB R2014a
• Xilinx 9.2i
• ModelSim-Altera 10.1d (quartus 13.1)

26.
Results
Simulation of constellation mapper

27.
RTL schematics of
QPSK constellation
mapper

28.
Simulation of path 0

29.
Simulation of path 2

30.
Simulation of path 5

31.
Comparison of Transmitted signal with the received signal
using OFDM system simulated in MATLAB with SNR = 10 dB

32.
BER v/s SNR for different modulation schemes

33.
Fig.(A) SNR = 30 dB
Fig.(B) SNR = 20 dB
Fig.(C) SNR = 10 dB

34.
Conclusion
Because of orthogonal carriers, inter carrier guard bands are not required and hence
OFDM provides higher bandwidth efficiency compared to FDM
OFDM overcomes even severe intersymbol interference through the use of the IFFT and a
cyclic prefix
BER v/s SNR curve shows BER reduces at low SNR values for QPSK while it reduces to the
same levels at relatively higher SNR in case of QAM
Data rates of 16-QAM is twice of QPSK and that of 64-QAM, it’s thrice of QPSK
Choice of modulation is a trade-off between accuracy and speed

35.
References
• “HDL Programming (VHDL and Verilog)” Nazeih Botros- 2008 edition
• Foisal Ahmed,1,* Md. Liakot Ali,2 and Mohammad Imam Hasan Bin Asad2 “Design of high speed
ofdm transmitter and receiver” 8th International Conference on Electrical and Computer
Engineering 20-22 December, 2014, Dhaka, Bangladesh
• “Hardware implementation of ofdm trasnmitter and receiver using fpga” Shabaz Abbasi and
Shazer Baig National University of Computer and Emerging Sciences Fast June 2008
• “OFDM SIMULATION in MATLAB” by Paul Gaunming Lin California Polytechnic State University
• “Implementation the Technique of Orthogonal Frequency Division Multiplexing Using 16-Point
Fast Fourier Transform and Inverse Fast Fourier Transform” Muhammad Waqas – Sharad
institute of science and information technology, Peshawar, Pakistan
• “Design and Implementation of Orthogonal Frequency Division Multiplexing (OFDM) Signaling”
Alan C. Brooks and Stephen J hoelzer.
• Mathuranathan Viswanathan, “Introduction to OFDM”, June 2015;
https://www.gaussianwaves.com

36.
References
• https://www.rf-wireless.com
• Charan Langton, “Orthogonal Frequency Division Multiplex (OFDM, DMT)”, July 2015;
https://www.complextoreal.com
• Krishna Sankar M, “Cyclic Prefix in Orthogonal Frequency Division Multiplexing”, July 2015;
https://www.dsplog.com
• Matlab help (https://www.mathworks.com)

37.
THANK YOU