The demand for high-speed Internet access for vehicles, such as high-speed trains (HSTs) and cars, is on the rise. Several Internet access technologies that use radio frequency are being considered for vehicular networking. Radio-frequency communications technologies cannot provide high data rates due to interference, bandwidth limitations, and the inherent limited data rates of radio technology. Free-space optical communications (FSOC) is an alternative approach and a line-of-sight (LOS) technology that uses modulated light to transfer data between two free-space optical (FSO) transceivers. FSOC systems for vehicular networks are expected to provide data rates in the range of Gbps for stationary and mobile stations. They also have additional benefits over radio frequency technologies including immunity to electromagnetic interference, high security owing it to the use of directed light, and the use of an unregulated range of the spectrum or license-free.
An introduction and mobility-specific challenges to FSOC are presented in the first chapter of this dissertation. A geometrical model for a ground-to-train FSOC system is presented and its performance is analyzed in the second chapter. Two beam modalities (i.e., narrow and wide) are compared using this geometrical model. A wide-beam modality that lowers the complexity of an FSOC system is proposed. In addition, a range of beam divergence angles, which are selected according to practical constraints, such as the maximum speed of a fast steering mirror to track an HST traveling at 300 km/h and the connection time between the train and a base station are proposed. All divergence angles in the proposed range mitigate the impairing effect of train-induced vibration without resorting to a feedback-control mechanism while guaranteeing high data rates (i.e., >= 1 Gbps).
An adaptive beam that adapts its divergence angle according to the receiver aperture diameter and the communication distance is proposed for a ground-to-train FSOC system in the third chapter of the dissertation. The proposed adaptive beam improves the received power and eases the alignment between the communicating parties in an FSOC system for HSTs. The received power, signal-to-noise ratio, bit error rate, and the maximum communication distance of the proposed adaptive beam technique are compared with those of the communications system that uses a beam with a fixed divergence angle of 1 mrad. The results indicate that the proposed adaptive beam technique yields a received power gain of 33 dB and extends the communication distance of an FSOC system for HSTs to about three times under different visibility conditions as compared to that of a fixed divergence beam. A new model on ground transceiver placement of ground transceivers of an FSOC system to increase connection efficiency is also proposed.
A novel diffused-light (DL) non-line-of-sight (NLOS) FSOC system for providing 1-Gbps Internet access to vehicles is proposed as the fourth chapter of this dissertation. This approach extends FSOC to locations that have no direct line-of-sight (LOS) between the transmitter and receiver. The amount of received power is shown for a receiving vehicle moving. Furthermore, the possible operation modes of the proposed diffused-light system is discussed to realize full-duplex communications.
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