Communication Networks of Visible Light Emitting Diodes with Intra-Frame Bidirectional Transmission
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Unlike traditional radio frequency communication of consumer devices, the "optical antenna" direction of Visible Light Communication (VLC), i.e., the Field-Of-View (FOV), varies greatly from device to device. This encompasses wide FOVs of ambient infrastructure and directional FOVs of light emitted by low-end embedded devices. This variety of light wave propagation can severely affect the transmission reliability, despite "pointing" devices to each other may seem enough for a reliable link. In particular, the fact that FOVs are unknown makes traditional access protocols in VLC unreliable in presence of interference among nodes of different FOVs and 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 channel and it makes the communication robust to different types of FOVs. We build a software-defined embedded platform running on Linux operating system, implement the CSMA/CD-HA protocol, and evaluate its performance through experiments. Results show that collisions caused by hidden nodes can be reduced and our protocol can increase the saturation throughput by nearly up to 50% and 100% under the two-node and four-node scenarios, respectively.