Information transmission through the use of video imagining systems has the potential to build upon the success of imaging communication systems, such as QR Codes and 2D Barcodes. Video communication systems add a temporal dimension to image-based information transmission systems, greatly expanding the amount of data that can be transferred. Such a system allows for secure transfer of data through the use of a line-of-sight (LOS) visual channel, making it attractive for several applications, including in-store purchases, banking, and stealth military missions. The nature of the LOS visual channel also reduces interference from other applications, making it a very attractive option for implementing short range communications systems from a cell phone, computer, tablet, or other device. This paper investigates how a video imaging system can be used for information transmission. A prototype system has been designed and implemented as a proof of concept, and the performance of the prototype is evaluated.

Underlay communications has the potential to become a valuable tool for increasing the capacity of spectrum provided that the quality of communications is upheld. In this paper, we propose a method for underlay nodes to detect, and coordinate with, other underlay nodes to form a network on preoccupied spectrum. Specifically, we introduce a novel optimization algorithm, called the Coordinated Optimization of EXposure Introduced by a Secondary Transmitter (COEXIST), which selects the best communications channel for all nodes within a network while minimizing interference with the primary user. This algorithm does not require knowledge of spatial location of the primary users or underlay nodes. With the proposed solution, underlay nodes can detect other nodes at power levels far below the power level of the primary user and, once a network is formed, nodes can continue to communicate at very low power levels without causing interference to primary users transmitting at the same time.

This correspondence examines the number of users that can be supported at a given signal-to-interference ratio in asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems. It is shown that for many chip waveforms the user capacity for asynchronous systems is larger than for optimally designed synchronous systems. The degree to which the user capacity improves by exploiting asynchrony is characterized by defining the effective dimension of the chip waveform. Finally, simulation results are shown to verify the analysis.

User capacity in synchronous CDMA systems has been well characterized and is fixed by the processing gain and target SIR required by each user. Asynchronous systems have not been as well studied. This paper examines the user capacity of asynchronous CDMA systems. It is shown that the user capacity can be increased over synchronous systems by allowing the users to be chip asynchronous and adapting their spreading codes to decrease interference. The degree to which the system experiences a gain by asynchrony is characterized by the effective dimension of the chip waveform. Simulation results are provided to verify that the user capacity is improved by allowing asynchrony when interference avoidance algorithms are employed.

A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs well in severe near-far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations.

Full-rate, full-diversity space-time codes are consid- ered in the framework of asynchronous direct-sequence code- division multiple access with aperiodic random spreading se- quences. The system is described in a multiuser uplink channel with multiple transmit and receive antennas. Chip-level oversam- pling is used to exploit the cyclostationarity of multiple-access interference. A sphere decoder is employed to jointly decode the space-time codes and suppress the interference. Simulation is performed to show the validity of applying the sphere decoder to multiple-access interference suppression. The performance im- provement of the sphere decoder over the conventional minimum- mean-square-error detector is also studied.

A simple transmit diversity method for asynchronous direct-sequence CDMA systems is proposed. In addition, a receiver structure that can effectively suppress multiple-access interference and exploit transmit diversity is examined. The proposed system is shown to perform well even when there are severe impairments to the desired user from high-power interferers. Finally, an adaptive implementation of the receiver is proposed, and its performance is evaluated with simulations.