OFDM is the advanced FDM technique used for wireless communication. But for Uni-polar communication and Optical Wireless communication (OWC) its modified version called Flip OFDM is quiet efficient technique.

1. Flip OFDM for Optical
Wireless Communication
PREPARED BY
DARSHAN BHATT
(150320705001)
M.E (EC – SEM 2)

2. Content
• Introduction
• Major application of Flip OFDM
• Concept of unipolar Flip OFDM
• Tx/Rx sections of Flip OFDM for unipolar comm.
• Comparitive study of Flip OFDM & ACO-OFDM
• Flip OFDM for OWC
• Model and Tx/Rx sections of Flip OFDM for OWC
• Conclusion
• Reference

3. Introduction
• Orthogonal frequency division multiplexing (OFDM) is the
frequency division multiplexing (FDM) scheme used as a
digital multi-carrier modulation method.[3][4]
• A large number of closely spaced orthogonal subcarriers are
used to carry data and that data is divided into several parallel
data stream or channel, one for each carrier.[3]
• The concept of flip OFDM is quiet similar to ACO-OFDM
except polarity of negative part of signal is inverted and then
transmitted with IFFT operation.[1]

4. Two Major Applications of Flip OFDM
• Flip OFDM technique is used mainly in these two fields of
communication engineering –
1. Unipolar Communication Systems
2. Optical Wireless Communication Systems (OWC)

5. Concept of Flip OFDM
• In Flip OFDM, positive & negative parts is extracted from the
real bipolar OFDM signal.
• Then the polarity of negative parts are inverted before
transmission of both positive and negative parts of OFDM
signal.
• So, transmitted signal is always positive.
• Flip OFDM is used in unipolar communication.

6. FLIP OFDM IN UNIPOLAR
COMMUNICATION SYSTEM

7. What is Unipolar Communication ?
• Unipolar communications systems can transmit information using only
real and positive signals.[1]
What is the need of Unipolar OFDM ?
• Channel dispersion or multipath fading may cause the inter-symbol-
interference and degrade the performance of such unipolar
communication systems. To compensate for these effects, unipolar
orthogonal frequency division multiplexing (OFDM) can be used.[1]

8. Model of unipolar comm. system
Fig.1 – Equivalent model for Unipolar communication System [1]

9. Unipolar communication model
• Let x(t), z(t) and h(t) represent transmitted signal, noise
component and channel impulse response.
• Then the communication is said to be unipolar if –
1. x(t) is real and positive.
2. If the equivalent received signal y(t) can be modeled as –

10. Classification of Unipolar OFDM
Unipolar OFDM
DCO - OFDMACO - OFDM
Asymmetrically Clipped
Optical OFDM
DC-Offset OFDM

11. Transmitter Section
• Let Xn be the transmitted QAM symbol in the nth OFDM
subcarrier. The output of the Inverse Fast Fourier Transform
(IFFT) operation at the k-th time instant is given by –
• The signal x(k) is complex signal so a real signal can be obtain
by Hermitian symmetry property.

12. Fig 2 – Flip OFDM Transmitter Section

13. (cont)
• O/p of IFFT operation is a real signal but not unipolar. So it can
be decomposed as -

14. (cont)
• These two components are separately transmitted over two
successive OFDM symbols.
• Positive signal x+(k) is transmitted in first subframe, while
flipped or inverted polarity signal -x-(k) is transmitted in second
subframe.
• CP addition is used to eliminate ISI between data blocks and
also used for error detection and correction purpose.
• The negative subframe is delayed by N+Δ and transmitted after
positive subframe.

15. Fig 3 – Flip OFDM Unipolar frame [2]

16. Receiver Section
• Reconstruction and detection process at receiver is given in fig.
• Cyclic prefixes associated with each OFDM subframes are
removed.
• Original signal can be reconstructed as –
• Here y1(k) and y2(k) represent time domain samples received in
+ve and –ve subframes.
• To detect transmitted QAM signal, FFT is used at receiver.

17. Fig 4 – Flip OFDM Receiver Section

18. Comparison of Flip OFDM & ACO-OFDM
• Noise Power:
In Flip-OFDM, the noise power of the flip OFDM is doubled
during the recombination of the +ve and –ve signals. But in
ACO-OFDM, there is no recombination so that noise power is
half of the amount in flip OFDM.
• Complexity:
Complexity is defined as the number of FFT/IFFT operations at
Tx or the Rx.

19. Complexity Table
Table : Complexity comparison of flip OFDM and ACO-OFDM [1][2]
NOTE : In the flip OFDM at the receiver side 50% complexity saving
compared to ACO-OFDM.

20. Flip OFDM ACO - OFDM
It is also known as inverted OFDM. It is known as Asymmetrically clipped
optical OFDM.
Flipping process is used in Flip OFDM. Clipping process is used in ACO-
OFDM.
It has polarity separator block in its
system.
It has not polarity separator block in its
system.
It has not negative clipper circuitary. It has negative clipper circuitary.
There is a recombination process of +ve
and –ve signals.
Recombination doesn’t occure.
Noise power is doubled during
recombination process.
Noise power is half of the amount of flip
OFDM.
In flip OFDM, 50% complexity saving
at receiver side.
More complex system at receiver side.

21. Flip OFDM for optical wireless communication

22. What is optical wireless communication ?
Optical wireless communications
(OWC) is a form of optical
communication in which
unguided visible, infrared (IR),
or ultraviolet (UV) light is used
to carry a signal.
Also known as Visible Light
Communication (VLC).

23. Concept of Flip OFDM in OWC
• OFDM is widely used to compensate dispersion effects in
optical wireless communication.
• In OWC, intensity modulation with direct detection (IM/DD)
technique is commonly used for data transmission.
• OWC is also used a concept of unipolar communication that is
it works on only real and positive signal.

24. Fig 5: Equivalent model of Optical Wireless Channel [2]

25. Optical wireless channel
• In most OWC systems, an infrared emitter is used as optical
transmitter to generate optical signal x(t).
• At the receiver, a photodetector collects optical signal and
converts it into electrical current y(t).
• Optical wireless link can be operated in two modes: directed
and non-directed. [2]

26. Transmitter Block[2]

27. Receiver Block[2]

28. Conclusion
• Unipolar communication system (Flip OFDM) is equivalent to
well known ACO-OFDM in terms of spectral efficiency and
error performance. We can save nearly 50% of receiver
complexity over ACO-OFDM.
• The same concept of Flip OFDM is also used for optical
wireless communication (OWC) using intensity modulation
technique.

29. References
[1]. N Fernando, Yi Hong, E Viterbo, “Flip OFDM for Unipolar
communication system”, IEEE Transactions on communication, vol 60,
No.12, December 2012.
[2]. N Fernando, Yi Hong, E Viterbo, “Flip OFDM for Optical Wireless
Communication”, IEEE Information theory workshop, 2011.
[3]. www.wikipedia.org
[4]. Book - Wireless communication by Andrea Goldsmith

30. THANK YOU