Flip OFDM for Optical Wireless Communication

PREPARED BY

DARSHAN BHATT

(150320705001)

M.E (EC – SEM 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

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]

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)

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.

FLIP OFDM IN UNIPOLAR COMMUNICATION SYSTEM

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]

Model of unipolar comm. system

Fig.1 – Equivalent model for Unipolar communication System [1]

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 –

Classification of Unipolar OFDMUnipolar OFDM

DCO - OFDMACO - OFDM

Asymmetrically Clipped Optical OFDM

DC-Offset OFDM

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.

Fig 2 – Flip OFDM Transmitter Section

(cont)

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

(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.

Fig 3 – Flip OFDM Unipolar frame [2]

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.

Fig 4 – Flip OFDM Receiver Section

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.

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.

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.

Flip OFDM for optical wireless communication

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).

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.

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

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]

Transmitter Block[2]

Receiver Block[2]

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.

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

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