Concept of Flip OFDM and its applications
-
Upload
darshan-bhatt -
Category
Engineering
-
view
324 -
download
2
Transcript of Concept of Flip OFDM and its applications
![Page 1: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/1.jpg)
Flip OFDM for Optical Wireless Communication
PREPARED BY
DARSHAN BHATT
(150320705001)
M.E (EC – SEM 2)
![Page 2: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/2.jpg)
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
![Page 3: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/3.jpg)
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]
![Page 4: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/4.jpg)
Two Major Applications of Flip OFDM
• Flip OFDM technique is used mainly in these two fields of communication engineering –
1. Unipolar Communication Systems2. Optical Wireless Communication Systems (OWC)
![Page 5: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/5.jpg)
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.
![Page 6: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/6.jpg)
FLIP OFDM IN UNIPOLAR COMMUNICATION SYSTEM
![Page 7: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/7.jpg)
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]
![Page 8: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/8.jpg)
Model of unipolar comm. system
Fig.1 – Equivalent model for Unipolar communication System [1]
![Page 9: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/9.jpg)
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 –
![Page 10: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/10.jpg)
Classification of Unipolar OFDMUnipolar OFDM
DCO - OFDMACO - OFDM
Asymmetrically Clipped Optical OFDM
DC-Offset OFDM
![Page 11: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/11.jpg)
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.
![Page 12: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/12.jpg)
Fig 2 – Flip OFDM Transmitter Section
![Page 13: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/13.jpg)
(cont)
• O/p of IFFT operation is a real signal but not unipolar. So it can be decomposed as -
![Page 14: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/14.jpg)
(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.
![Page 15: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/15.jpg)
Fig 3 – Flip OFDM Unipolar frame [2]
![Page 16: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/16.jpg)
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.
![Page 17: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/17.jpg)
Fig 4 – Flip OFDM Receiver Section
![Page 18: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/18.jpg)
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.
![Page 19: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/19.jpg)
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.
![Page 20: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/20.jpg)
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.
![Page 21: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/21.jpg)
Flip OFDM for optical wireless communication
![Page 22: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/22.jpg)
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).
![Page 23: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/23.jpg)
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.
![Page 24: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/24.jpg)
Fig 5: Equivalent model of Optical Wireless Channel [2]
![Page 25: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/25.jpg)
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]
![Page 26: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/26.jpg)
Transmitter Block[2]
![Page 27: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/27.jpg)
Receiver Block[2]
![Page 28: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/28.jpg)
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.
![Page 29: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/29.jpg)
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
![Page 30: Concept of Flip OFDM and its applications](https://reader033.fdocuments.net/reader033/viewer/2022052514/5876d8e41a28ab1d238b6591/html5/thumbnails/30.jpg)
THANK YOU