A HIGHLY EFFICIENT FPGA-BASED QAM MODULATOR IMPLEMENTATION
Date
2002
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Degree Level
Masters
Abstract
The purpose of this thesis is to show that Field Programmable Gate Arrays (FPGAs) are a viable option for low volume manufacturers to compete in the digital cable marketplace. The major advantage of this technology is that it allows them to design
and build low cost solutions tailored for specific customers.
The present paper describes a Quadrature Amplitude Modulator (QAM) archi-tecture that is optimized for implementation in an FPGA. The traditional QAM modulator implementation consists of parallel in-phase (I) and quadrature-phase (Q) square root raised cosine baseband pulse shaping filters implemented as a con-volution using multipliers and adders. The proposed implementation does not use physical multipliers to implement these filters, but uses distributed lookup tables as an alternative. A single time-shared square root raised cosine filter designed in this fashion is capable of implementing parallel I and Q signal chains. The square root raised cosine baseband filters are followed by an upsampling and interpolation stage to prepare for modulation on an intermediate frequency carrier. A fractional delay filter is used in order to provide an output sampling rate that is independent of the input data rate. The final intermediate frequency modulator stage uses a numerically controlled oscillator to provide a variable output frequency. The proposed square root raised cosine filter implementation provides an order of magnitude reduction in FPGA area requirements when compared to more traditional implementations. The example modulator implemented as part of the research adheres to the specifications for the International Telecommunications Union (ITU) Standard J.83 Annex A [1], and supports 16, 32, 64, 128, or 256 QAM modes with a square root raised cosine rolloff factor of 0.15. In this thesis, theoretical results are compared to simulation results for the specific parameters of 256 QAM and a rolloff factor of 0.15.
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Degree
Master of Science (M.Sc.)
Department
Electrical Engineering