Intra-Frame Bidirectional Transmission in Networks of Visible LEDs
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The optical antenna's directionality of nodes forming a Visible Light Communication (VLC) networks, i.e., their Field-Of-View (FOV), varies greatly from device to device. This encompasses wide FOVs of ambient light infrastructure and directional FOVs of light from low-end embedded devices. This variety of light propagation can severely affect the transmission reliability, despite "pointing" the devices to each other may seem enough for a reliable communication. The presence of interference among nodes with different FOVs makes traditional access protocols in VLC unreliable and it also exacerbates the hidden-node problem. In this paper, we propose a Carrier Sensing Multiple Access/Collision Detection&Hidden Avoidance (CSMA/CD-HA) medium access control protocol for a network where each node solely uses one Light Emitting Diode (LED) to transmit and receive data. The CSMA/CD-HA can enable in-band intra-frame bidirectional transmission with just one optical antenna. The key idea is to exploit the intra-frame data symbols without emission of light to introduce an embedded communication channel. This approach enables the transmission of additional data while receiving in the same optical frequency band and it makes the communication robust to different types of FOVs. We implement the CSMA/CD-HA protocol in a software-defined embedded platform running Linux, and evaluate its performance through analysis and experiments. Results show that collisions caused by hidden nodes can be largely reduced and our protocol can increase the saturation throughput by nearly up to 50% and 100% under the two- and four-node scenarios, respectively.