In the previous posts, the implementation demo and the implementation guidance of the Screen-OFDM system were shown. The measured performance comparison between Screen-OFDM and a traditional A-QL in IEEE 802.15.7m is reported in this post.
Overview of 2D-OFDM vs A-QL
A-QL is the single-carrier modulation while 2D-OFDM is the multi-carrier modulation approach applied for the screen-camera system.
- A-QL turns the color cells ON or OFF at the locking time. This is why A-QL is a single-carrier modulation.
- 2D-OFDM generates orthogonal subcarriers by using 2D-IDFT at Tx and 2D-DFT at Rx.
Detailed parameters used for A-QL system and Screen OFDM system are given in Tables 1 and 2.
For performance comparison, a 16×16 cells A-QL code (the A-QL code of the IEEE802.15.7m ongoing standard) is at the size of 8cm×8cm, in which each Tx cell has a resolution of 5×5 pixel. Accordingly, for a fair BER comparison to A-QL code, the size of each element (i.e., Tx cell) of the Screen OFDM Tx must be set equal to the previous size of each cell of the A-QL system used in the measurement. Thus 5×5 pixel resolution (width × height) is set for the cell size of each cell element within Tx for both A-QL and Screen OFDM.
The error measured results at three layers, layer one without error correction, layer two with a Convolutional Code, and layer three(optional) with a Reed-Solomon code. The accumulated error bits is counted for many situations to practice the impacts of distance and the perspective viewing angle to BER. We checked performance at distances of 1.5m, 3m, and 4.5m; and viewing angles of 0-degree, 30-degree, and 60-degree.//
Below Figure compares the BER performance of A-QL and Screen-OFDM measured at distances of 1.5m, 3m, and 4.5m; and viewing angles of 0-degree, 30-degree, and 60-degree.
In particular, the following figures draw the BER performance recorded at different FEC layers, by A-QL system and Screen-OFDM system under varying conditions of distance and viewing angle.
The Orthogonal Frequency Division Multiplexing (OFDM) waveform is a well-known implementation for Light Fidelity (LiFi) but quite a novel implementation for Optical Camera Communication (OCC). Our Screen-OFDM implements a single screen luminaire composed of multiple cells so that a data rate of up to 50 kbps can be achieved with the 2D-OFDM waveform.
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.