Design and Analysis of Free Space Optical Sensor Networks for Short-Range Applications
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Free space optical communication (FSOC) systems using direct detection and line of sight (LOS) laser links can provide spatially efficient and physically secure connectivity for wireless sensor networks. The FSOC system can be developed with low power microcontrollers so that the entire sensor system can be implemented on a single printed circuit board. Available data rates can range from kb/s to hundreds of Mb/s with the complete system consuming power only in the tens of mW. These features are advantageous for low-power communication networks over short distances in environments where LOS is available, and where radio frequency (RF) connectivity must be avoided because of interference or security issues. In particular, the faster data acquisition rates of FSOC systems are extremely attractive in applications where the sensor systems, or "motes", remain in sleep mode most of the time and need to transmit large amounts of data in extremely short bursts when they wake up. However, in order for directional FSO sensor networks to become viable short-range solutions, the networks must provide signal coverage over a wide field of view without strict optical alignment requirements, operate with efficient media access protocols that can handle network traffic in an efficient manner, and minimize random access times for the independent transmitting motes within the network. These challenges are the focus of this dissertation.
In general, narrow optical beams used for FSOC require precise and complex pointing, acquisition, tracking and alignment methods. This dissertation addresses the challenge of alignment for FSO-based nodes by designing optical transceiver architectures with multiple narrow field of view (FOV) transmitters and a single, wide angle receiver. The architecture consists of rings of multiple transmitters surrounding a photodiode for light collection. Each ring is tilted at a different angle so that a wide transmission FOV can be obtained, thereby allowing point-multipoint communication. Depending on the number of transmitters and the transmitter's divergence angle, different FOVs can be tailored to fit the requirements of the target application. The developed transmitter design requires only a few milliwatts of transmission power from each transmitter to cover its respective FOV, which is sustainable with drive currents up to 10 milliamps using vertical cavity surface emitting lasers (VCSELs), making it a more practical strategy for a compact battery driven device.
The other major challenge is designing the proper media access control (MAC) protocol, which provides nodes with addresses and channel access capability so that directional links between multiple nodes can be formed. The challenge lies in the fact that most nodes are blind to other nodes' transmissions because of their relatively narrow directional links. Because of this blindness, packet collisions are inevitable. Therefore, an efficient multiple access protocol needs to be designed for the FSOC system to ensure successful directional communication between the motes and cluster heads for data collection and relaying. While there are many protocols that allow multiple access and provide collision avoidance for traditional RF systems, these protocols are not optimized for FSOC systems consisting of multiple narrow FOV transmitters. Instead, a directional MAC (DMAC) protocol is developed from existing RF protocols, but modified for FSOC technology. It overcomes the limitations in FSOC communication resulting from directionality by setting up a master-slave network architecture where communication takes place between a sensing system, "mote", and a central control station, or "cluster head", which is designed with a multiple VCSEL transmitters. In this way, the physical transmitter sources of the cluster head become an integral part of the FSOC DMAC protocol. In this type of architecture, the master node, or cluster head, has the dual functionality of coordinating network traffic and aggregating data from all the slave nodes, or motes, that are within its field of view (FOV). Multiple cluster heads can form a directional network backbone, and can relay signals collected from a mote through other cluster heads, until the signal is delivered to its destination.
In summary, this dissertation provides: 1) the design and implementation of small and inexpensive short-range FSOC systems that can be implemented using standard "off the shelf" components including a microcontroller and sensor device to form a complete standalone package; 2) development of a DMAC protocol that is optimized for the implemented FSOC system and target network applications; 3) network performance evaluation and optimization for the combined FSOC hardware, network architecture, and DMAC protocol. This is done through a series of hardware tests on an experimental prototype FSOC sensor network consisting of 10 motes and 1 cluster head and simulations of larger network sizes.