2D-OFDM for Optical Camera Communication

OFDM is well-known for LiFi; however, it is still fresh advanced waveform for OCC. This post shares an implementation of 2D-OFDM for Screen-to-Camera communication system in which a massive number of display cells brings a great advantage for communication.

  • A full journal version of this topic is available online at IEEE Access here.

System Architecture

Figure 1 shows the reference architecture for implementation of Screen-OFDM. In this system, Hermitian mapping and IDFT are two-dimensional. In Rx side, DFT and channel equalizer are also two-dimensional.

What’s new in this Screen-OFDM system? 

  • MIMO-OFDM screen-camera 
    • MIMO channel with sub-channels consisting of subcarriers for reliable communication.
    • Configurable size of Tx, with OFDM symbols based on experimentally verifying the channel in conjunction with the channel analysis 
  • The new design of Tx that jointly supports critical functionalities 
    • 360-degree rotation support.
    • Asynchronous communication with frame rate variation support.
    • Correction of perspective distortion.
  • Complete system architecture and detailed implemental guidance.
    • OFDM symbol creation and Tx post-processing (2D-cyclic prefix insertion and clipping processes).
    • Scaling and DC-bias control.
    • 2D pilots design and channel equalization.
    • PHY frame format PPDU co-existent to 802.15 standards.
    • Effective FEC.
  • Comprehensive Rx processing and decoding guidance 
    • Specific decoding processes are given based on implemental results, significantly including
    • Spatial downsampling and the correction of perspective distortion,
    • Temporal downsampling to deal with frame rate variation, and
    • Rotation correction

Figure 1 – Reference Architecture for the Screen-Camera DFT-OFDM system

Code Design

The design of Tx (see FIGURE 2) consists of the following features:  

  • The bolder area: to support the detection and extraction ofthe code.
  • The rotation tracking corners: to determine the rotation.
  • Clock cells at the surrounding of the code: to support the Rxdetermining the proper sampling ratio under the presence of perspectivedistortion.
Figure 2 – Allocation of multiple-OFDM symbols into Tx

Implementation Demo

Video demo for Screen-OFDM is available here! 

In this demo, the measurement of 2D-carriers profile (96×96 spatial carriers) and the data mapping into these spatial carriers for text transmission are demonstrated.

The reliability of the proposed system was verified through numerical performance measurements, as recorded that the overall BER of 10-5 is achievable within the communication distance of 4.5m and the viewing angle of 300. The overall performance of the screen OFDM is also compared to a single-carrier modulation approach,asynchronous-quick-link (A-QL) code within the ongoing IEEE 802.15.7m standard, to demonstrate the reliable performance of the proposing system.

These are some of the implementation figures:

Conclusion:

A mature screen-camera system employing 2D-OFDM has been proposed in this first paper. The results of the theoretical analysis and implementation have been addressed in detail. Numerical results demonstrated that the implementation of the Screen OFDM system was more reliable compared to the implementation of the A-QL code even though Screen OFDM carries over ten times more data than A-QL, under the sample testing conditions.

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