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Optical OFDM Waveform Construction by Combining Real and Imaginary Parts of IDFT

Gokce Hacioglu, Kadir Turk, Cenk Albayrak

TL;DR

Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation and does not effect the error performance.

Abstract

In optical communication systems, orthogonal frequency division multiplexing (OFDM) is widely used to combat inter-symbol interference (ISI) caused by multipath propagation. Optical systems which use intensity modulation and direct detection (IM/DD) can only transmit real valued symbols, but the inverse discrete Fourier transform (IDFT) or its computationally efficient form inverse-fast Fourier transform (IFFT) required for the OFDM waveform construction produces complex values. Hermitian symmetry is often used to obtain real valued symbols. For this purpose, some trigonometric transformations such as discrete cosine transform (DCT) are also used, however these transformations can eliminate the ISI only under certain conditions. In this paper, we propose a completely different method for the construction of OFDM waveform with IFFT to obtain real valued symbols by combining the real and imaginary parts (CRIP) of IFFT output electrically (E-CRIP) or optically (O-CRIP). Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation and does not effect the error performance. In addition, the O-CRIP method is less affected by clipping noise that may occur due to the imperfections of the transmitter front-ends.

Optical OFDM Waveform Construction by Combining Real and Imaginary Parts of IDFT

TL;DR

Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation and does not effect the error performance.

Abstract

In optical communication systems, orthogonal frequency division multiplexing (OFDM) is widely used to combat inter-symbol interference (ISI) caused by multipath propagation. Optical systems which use intensity modulation and direct detection (IM/DD) can only transmit real valued symbols, but the inverse discrete Fourier transform (IDFT) or its computationally efficient form inverse-fast Fourier transform (IFFT) required for the OFDM waveform construction produces complex values. Hermitian symmetry is often used to obtain real valued symbols. For this purpose, some trigonometric transformations such as discrete cosine transform (DCT) are also used, however these transformations can eliminate the ISI only under certain conditions. In this paper, we propose a completely different method for the construction of OFDM waveform with IFFT to obtain real valued symbols by combining the real and imaginary parts (CRIP) of IFFT output electrically (E-CRIP) or optically (O-CRIP). Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation and does not effect the error performance. In addition, the O-CRIP method is less affected by clipping noise that may occur due to the imperfections of the transmitter front-ends.

Paper Structure

This paper contains 12 sections, 59 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. System Model and Removing ISI
  3. Hermitian Symmetry Method
  4. Proposed CRIP Method
  5. Analysis of Clipping Noise
  6. Clipping Noise Power of a Gaussian Vector
  7. Clipping Noise Power of E-CRIP and Hermitian Symmetry Method
  8. Clipping Noise Power of O-CRIP Method
  9. Analytic and Simulation Results
  10. Complexity Analyzes
  11. Numerical Results for VLC System
  12. Conclusion

Figures (6)

  • Figure 1: Receiver and transmitter block diagram of a VLC system using the proposed CRIP method. Real and imaginary parts combined at the output of IFFT (E-CRIP).
  • Figure 2: Transmitter block diagram of a VLC system using the proposed CRIP method. Real and imaginary parts of IFFT are combined optically (O-CRIP).
  • Figure 3: Variation curves of clipping noise power versus input signal power for Hermitian symmetry and proposed CRIP methods.
  • Figure 4: BER performances of OFDM schema utilized Hermitian symmetry and proposed methods for various constellation sizes
  • Figure 5: BER performance reduction in Hermitian symmetry and CRIP schemas at $20$ dB due to shift in DC bias
  • ...and 1 more figures