The need for more bandwidth and capacity in wireless systems currently is the main culprit for the great interest in the development of wireless communications systems operating at millimeter wave frequencies and higher. The future needs of broad-band interactive services (1Gb/s) demand the application of optical fiber feed networks for distribution of the radio signals to and from the antennas at the various base stations. Fiber-optic technologies have reached the stage where insertions into various commercial RF systems are being considered. Today, there are three main steps in the evolution of RF/Photonics systems for wireless communications. The first step has been in the direction of using photonics to slowly replace conventional RF components, such as, the coax that is used to interconnect the antenna to the electronics. Optical fibers, in contrast to coaxial cable, provide a more ideal medium for broadband RF communication systems. The light weight property of fibers, and its immunity from other signal interference make them very critical in the development of future RF distribution systems. The second, and more challenging step, is in the seamless integration of photonics and RF wireless circuits. The challenge in this step is to use photonics and RF circuits as complementary systems and blend them together. Finally, the third step is towards the development of optically coupled antennas. In this step the aim is to eliminate the need of local oscillators, mixers, amplifiers and a host of other parts by directly feeding an antenna through a fiber at millimeter wave frequencies. Here, it is proposed that an array of RF modulator/photodetectors be integrated directly to an array of antennas. This new RF/photonic antenna array system, with the appropriate space-time processing and coding, will form a iosmart antennaln that can enhance network coverage, capacity, and quality. It is envisioned that a large number of such RF/Photonic antenna elements could be networked together into a star configuration, feeding in and out of a radio hub. As a transmitter, the proposed optoelectronic device operates as a photodiode, while as a receiver the device operates as an optical modulator. It has already been demonstrated that this dual function of a semiconductor electroabsorption modulator and photodiode in the same device for duplex operation, can occur, using bias control as a transmit/receive mode control. For full duplex operation, two modulator/photodiode devices need to be incorporated in the each transceiver element. We propose to directly drive a coplanar waveguide (CPW)-fed slot antenna by converting optical power into microwave power and vice versa using these RF modulator/photodetectors. As a transmitter, the CPW line is connected to the active surface of the photodetector, from which the microwave power propagates to feed the radiating slot. The photodetector is fed via an optical fiber from beneath. When the device functions as an optical modulator, the receive function can also be achieved. Preliminary results for a single antenna show that a very good bandwidth and radiation patterns can be achieved. It should be noted that these elements can be interconnected via the fiber to achieve summation, mixing and other signal processing functions, at the antenna site or at a remote site. Some preliminary results have been achieved in the area of multiple functionality for the optoelectronic components, such as modulation, photodetection, self-biasing and RF frequency mixing. They have shown properties, such as high bandwidth and high power, that are desirable for the antenna applications. A main emphasis here is to further investigate the material and device designs for the optoelectronic component that can incorporate into the smart antenna architecture. The proposed approach will have significant impacts on wireless communication systems by providing higher system bandwidth capacity and enhancing their reliability. It may lead to a new type of long distance, broadband network infrastructure that supports transparent transport of optical signals. Our team is formed to provide the expertise in the four key elements for this proposed research. Our project will provide a good opportunity to train graduate and undergraduate students in one of the most exciting interdisciplinary areas in science (RF, photonics, signal processing and communications). The interactions between the researchers at the different institutions will be aided by the close collaboration that exists between the members of the group.
