Direct Digital Synthesizers (DDS) - Theory Applications

Abstract

 

Traditional designs of high bandwidth frequency synthesizers employ the use of a phase-locked-loop (PLL). A direct digital synthesizer (DDS) provides many significant advantages over the PLL approaches. Fast settling time, sub-Hertz frequency resolution, continuous-phase switching response and low phase noise are features easily obtainable in the DDS systems. Although the principle of the DDS has been known for many years, the DDS did not play a dominant role in wideband frequency generation until recent years. Earlier DDSs were limited to produce narrow bands of closely spaced frequencies, due to limitations of digital logic and D/A-converter technologies. Recent advantages in integrated circuit (IC) technologies have brought about remarkable progress in this area. By programming the DDS, adaptive channel bandwidths, modulation formats, frequency hopping and data rates are easily achieved. This is an important step towards a "software-radio" which can be used in various systems. The DDS could be applied in the modulator or demodulator in the communication systems. The applications of DDS are restricted to the modulator in the base station. The aim of this research was to find an optimal front-end for a transmitter by focusing on the circuit implementations of the DDS, but the research also includes the interface to baseband circuitry and system level design aspects of digital communication systems.
The theoretical analysis gives an overview of the functioning of DDS, especially with respect to noise and spurs. Different spur reduction techniques are studied in detail. Four ICs, which were the circuit implementations of the DDS, were designed. One programmable logic device implementation of the CORDIC based quadrature amplitude modulation (QAM) modulator was designed with a separate D/A converter IC. For the realization of these designs some new building blocks, e.g. a new tunable error feedback structure and a novel and more cost-effective digital power ramp generator, were developed.

Design and Implementation of an FPGA-based Soft-Radio Receiver utilizing adaptive tracking

Abstract :


The wireless market of the future will demand inexpensive hardware, expandability, interoperability, and the implementation of advanced signal processing functions-i.e. a software radio. Configurable computing machines are often ideal software radio platforms. In particular, the Stallion reconfigurable processors efficient hardware reuse and scalability fulfill these radios demands. The advantages of Stallion-based design inspired an FPGA-based software radiothe proto-Stallion receiver. This thesis introduces the proto-Stallion architecture and details its implementation on the SLAAC-1V FPGA platform. Although this thesis presents a specific radio implementation, this architecture is flexible; it can support a variety of applications within its fixed framework. This implemented single-user DS-CDMA receiver utilizes an LMS adaptive filter that can combat MAI and constructively combine multipath; most notably, this receiver employs an adaptive tracking algorithm that harnesses the LMS algorithm to maintain symbol synchronization. The proto-Stallion receiver demonstrates the dependence of adaptive tracking on channel noise; the algorithm requires significant noise levels to maintain synchronization.

Implementation of HMMer on a Reconfigurable Processor

Intelligent and Reconfigurable Architecture for remote image application

Maximizing Performance in Long Distance Wireless Networks

ABSTRACT :


Today we are witnessing a large disparity between the levels of network connectivity in industrialized countries, and the ones in the developing world.This digital divide is caused by the uneven distribution of wealth around the world, and has the effect of reinforcing this polarization, by providing increased economic opportunities to people that already afford access to information technology. This divide is partially addressed by the growth of wireless and cellular technologies, but these technologies remain financially inviable in rural regions, with sparse and financially-constrained users.
To address rural and remote network coverage, we propose the use of multi-hop wireless networks relying on long-distance point-to-point links. By using inexpensive, off-the-shelf Wi-Fi radios, and connecting them to high-gain directional antennas, we can build inexpensive, high-throughput links exceeding tens or even hundreds of kilometers in length. In theory, these wireless long-distance (WiLD) networks have the potential to deliver low-cost connectivity to remote areas. Unfortunately, the performance achieved using the standard 802.11 MAC in long links is very poor, with high and asymmetric packet loss rates, and with low throughput over wireless paths spanning multiple hops.
In this dissertation, we understand the causes for low performance in these scenarios, and build MAC- and PHY-layer mechanisms that address these problems and maximize end-to-end network performance. Using extensive measurements we identify the primary sources of performance degradation. To deal with these problems we design and build WiLDNet, a system that includes a spatial-reuse TDMA MAC and a combination of FEC and ARQ-based link-layer loss recovery mechanisms. We deploy WiLDNet in real-world networks, and show that it eliminates most packet losses and increases link utilization, delivering good end-to-end UDP and TCP throughput. We incorporate the lessons learned from our deployments in the design of JazzyMAC, a MAC that maximizes network-wide throughput and minimizes packet delay by using variable-length transmission slots which change dynamically according to traffic.
We demonstrate the appropriateness of our solutions by deploying them in several networks in developing countries, including the Aravind Eye Hospital network in India that uses our technology to provide telemedicine services to many thousands of patients.