ABSTRACT
In this paper, we propose and validate an artificial neural network-based equalizer for the constant power 4-level pulse amplitude modulation in an optical camera communications system. We introduce new terminology to measure the quality of the communications link in terms of the number of row pixels per symbol Npps, which allows a fair comparison considering the progress made in the development of the current image sensors in terms of the frame rates and the resolutions of each frame. Using the proposed equalizer, we experimentally demonstrate a non-flickering system using a single light-emitting diode (LED) with Npps of 20 and 30 pixels/symbol for the unequalized and equalized systems, respectively. Potential transmission rates of up to 18.6 and 24.4 kbps are achieved with and without the equalization, respectively. The quality of the received signal is assessed using the eye-diagram opening and its linearity and the bit error rate performance. An acceptable bit error rate (below the forward error correction limit) and an improvement of ~66% in the eye linearity are achieved using a single LED and a typical commercial camera with equalization.
ABSTRACT
The ever-increasing number of vehicles on a global level signifies the need for communications between vehicles and the surrounding environment. Visible light communications (VLC) is a viable complementary technology to congested radio-frequency-based wireless systems. In order to increase the reliability of the VLC link, two novel algorithms based on (i) channel inversion (CI) and (ii) frame subtraction and CI (FSCI) schemes are proposed to successfully extract the data in a non-line-of-sight multiple-input-multiple-output spatial-division multiplexing optical camera communications system. We have adopted differential modulation and frame subtraction schemes and proposed a unique packet structure to mark the packet and the position of the footprint of transmitters (Txs) in the image frame. We show that the FSCI scheme with much simpler receiver structures can offer almost the same bit error rate performance compared with the hybrid selection/equal gain combining (HS/EGC) technique at lower transmit power (illumination) levels of <13dBm for a single Tx and improved performance at higher illumination levels of >20dBm for multiple Txs. Compared with HS/EGC, CI schemes have a higher tolerance to the spacing between Txs, where the payload threshold level can be set to a fixed value of 0.5.
ABSTRACT
We propose, for the first time to the best of our knowledge, a novel non-line-of-sight 2×N indoor optical camera communication system, where N is the number of pixels in the camera with a unique packet structure and a detection methodology for extracting the data from the recorded video streams. A comprehensive theoretical model for the proposed system is presented. The proposed system is experimentally investigated, and the measured results show that higher International Standards Organization (ISO) levels and exposure times lead to a reduced transmit power level by 3 dB for every doubling of the exposure time and ISO at a bit error rate of 10-3. It is also shown that when the overlapping area of two interfering transmitters is larger than approximately 30% of the footprints, the data cannot be recovered.
