EXPERIMENT NO 7

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BE(EXTC) SEM VII (RCOE) Page 1

Name :

_________________________________________

Branch :

Electronics & Telecommunication (SEM VII)

Subject : Mobile Communcation

Experiment No: 07

Title : To Implement OFDM Receiver using MATLAB.

Date :

Subject In-Charge Sign

:

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7EXPERIMENT NO: 07

AIM

: To Implement OFDM Receiver using MATLAB.

APPARATUS

: MATLAB 7

THEORY:

BLOCK DIAGRAM OF OFDM

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL Internet access, wireless networks, powerline networks, and 4G mobile communications.

OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-multi-carrier signals are used to carry data on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.

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narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (on analogue TV these are visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.

ADVANTAGES

 High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc.

 Can easily adapt to severe channel conditions without complex time-domain equalization.

 Robust against narrow-band co-channel interference.

 Robust against intersymbol interference (ISI) and fading caused by multipath propagation.

 Efficient implementation using Fast Fourier Transform (FFT).

 Low sensitivity to time synchronization errors.

 Tuned sub-channel receiver filters are not required (unlike conventional FDM).

 Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.

DISADVANTAGES

 Sensitive to Doppler shift.

 Sensitive to frequency synchronization problems.

 High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.

 Loss of efficiency caused by cyclic prefix/guard interval.

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BE(EXTC) SEM VII (RCOE) Page 4 PROGRAM:

%...

channel = randn(1,2) + sqrt(-1)*randn(1,2); after_channel = filter(channel, 1, ofdm_signal); awgn_noise = awgn(zeros(1,length(after_channel)),0);

recvd_signal = awgn_noise+after_channel; % With AWGN noise

figure(7),plot(real(recvd_signal)),xlabel('Time'); ylabel('Amplitude');

title('OFDM Signal after passing through channel');grid on;

%...

recvd_signal_paralleled = reshape(recvd_signal,rows_Append_prefix, cols_Append_prefix);

%... %... % Remove cyclic Prefix

%... %... recvd_signal_paralleled(1:cp_len,:)=[]; R1=recvd_signal_paralleled(:,1); R2=recvd_signal_paralleled(:,2); R3=recvd_signal_paralleled(:,3); R4=recvd_signal_paralleled(:,4);

figure(8),plot((imag(R1)),'r'),subplot(4,1,1),plot(real(R1),'r'),

title('Cyclic prefix removed from the four sub-carriers')

subplot(4,1,2),plot(real(R2),'c')

subplot(4,1,3),plot(real(R3),'b')

subplot(4,1,4),plot(real(R4),'g')

%... %... % FFT Of recievied signal

for i=1:number_of_subcarriers,

% FFT

fft_data(:,i) = fft(recvd_signal_paralleled(:,i),16);

end

F1=fft_data(:,1); F2=fft_data(:,2); F3=fft_data(:,3); F4=fft_data(:,4);

figure(9), subplot(4,1,1),plot(real(F1),'r'),title('FFT of all the four

sub-carriers')

subplot(4,1,2),plot(real(F2),'c')

subplot(4,1,3),plot(real(F3),'b')

subplot(4,1,4),plot(real(F4),'g')

%... %... % Signal Reconstructed

%... %... % Conversion to serial and demodulationa

recvd_serial_data = reshape(fft_data, 1,(16*4));

qpsk_demodulated_data = pskdemod(recvd_serial_data,4); figure(10)

stem(data)

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stem(qpsk_demodulated_data,'rx');

grid on;xlabel('Data Points');ylabel('Amplitude');

title('Recieved Signal with error')

RESULT:

OFDM TRANSMITTER

0 10 20 30 40 50 60 70

0 0.5 1 1.5 2 2.5 3

Data Points

A

m

pl

itu

de

Original Data

0 10 20 30 40 50 60 70

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

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0 2 4 6 8 10 12 14 16

-1 0 1

Subcarrier1

0 2 4 6 8 10 12 14 16

-1 0 1

Subcarrier2

0 2 4 6 8 10 12 14 16

-1 0 1

Subcarrier3

0 2 4 6 8 10 12 14 16

-1 0 1

Subcarrier4

0 2 4 6 8 10 12 14 16

-0.5 0 0.5

IFFT on all the sub-carriers

0 2 4 6 8 10 12 14 16

-0.5 0 0.5

0 2 4 6 8 10 12 14 16

-0.5 0 0.5

0 2 4 6 8 10 12 14 16

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0 2 4 6 8 10 12 14 16 18

-0.5 0 0.5

Cyclic prefix added to all the sub-carriers

0 2 4 6 8 10 12 14 16 18

-0.5 0 0.5

0 2 4 6 8 10 12 14 16 18

-0.5 0 0.5

0 2 4 6 8 10 12 14 16 18

-0.5 0 0.5

0 10 20 30 40 50 60 70

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

Time

A

m

p

lit

u

d

e

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0 2 4 6 8 10 12 14 16 18

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

Orthogonality

0 10 20 30 40 50 60 70

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Time

A

m

p

lit

u

d

e

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OFDM RECIEVER:

0 2 4 6 8 10 12 14 16

-2 0 2

Cyclic prefix removed from the four sub-carriers

0 2 4 6 8 10 12 14 16

-5 0 5

0 2 4 6 8 10 12 14 16

-5 0 5

0 2 4 6 8 10 12 14 16

-5 0 5

0 2 4 6 8 10 12 14 16

-10 0 10

FFT of all the four sub-carriers

0 2 4 6 8 10 12 14 16

-10 0 10

0 2 4 6 8 10 12 14 16

-10 0 10

0 2 4 6 8 10 12 14 16

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0 10 20 30 40 50 60 70

0 0.5 1 1.5 2 2.5 3

Data Points

A

m

p

lit

u

d

e