Design And Implementation Of QPSK Modulation And Demodulation Based On FPGA

Software radio is a pioneering technology in the wireless communication field. Its basic idea is starting from the angle of modular, standardization and generalization, and to pursuit the operation mode of full-band, multi-mode and reconfigurability. In the communication systems which adopt software radio, a variety of basic functions can be defined according to the needs of the users. So, the design of hardware circuit is as convenient as software, driving the software radio to be widely used in all aspects of the field of communication.Because of its high band width efficiency, anti-interference ability, fast transfer rates, etc., QPSK is widely used in modern digital communication systems as a digital modulation scheme. Field-programmable gate array(Field Programmable Array, FPGA) uses hardware description language for digital circuit design, so its circuit design has features of small investments, short cycle and intelligence in its development process. So, applying FPGA to implement QPSK modulation and demodulation is of great significance.This paper introduces the background and significance of FPGA-based QPSK modulation and demodulation, namely in the context of the idea of software radio, using FPGA to design QPSK modulation and demodulation system. Then, the research status of QPSK modulation and demodulation techniques, development of FPGA and the necessary of using FPGA to implement QPSK modulation and demodulation are analyzed. Principles and system design diagram are described in detail, providing a solid theoretical foundation for the design. The modulator is composed of the up mixer, shaping filter, a numerically controlled oscillator and summator. This article focuses on the oscillator and the filter, and uses raised cosine shaping filter to mold baseband signal. By using raised cosine drop filter to solve the baseband signal band width and inter symbol crosstalk problem; through the look-up table method to produce two orthogonal sinusoidal signal, solve problems caused due to bad carrier orthogonal QPSK constellation discrimination variable. Demodulator composed by a down mixer, a matched filter, carrier synchronization module, bit synchronization module and the sampling decision modules and other components. This article focuses on the carrier synchronization module and bit synchronization module, wherein the carrier synchronization module using a modified model Costas loop, bit synchronization module using Gardner phase lock loop. Costas loop in the phase detector, this paper on the traditional multiplier as the phase detector do some improvements, use differential phase detector to achieve instead, to a certain extent, improve the sensitivity of the PFD. By using the output of the loop filter controls the numerically controlled oscillator frequency control word to achieve loop stability and accurate output. So Costas loop realizes the carrier synchronization and enhance the understanding of the accuracy of adjusting input carrier recovery. In the Gardner phase locked loop interpolation filter, this paper improves the data processing speed by improving the structure of the traditional filter and adopting the cubic interpolation filter. The Gardner phase lock loop itself is independent of the characteristics of the carrier synchronization, so the performance of the bit synchronization is improved. On the correctness of the MATLAB modeling and simulation to validate each design based, with Quartus II platform, hardware circuit description Language Verilog to achieve the QPSK modulation and demodulation system design of each module. The generated download file is downloaded to the Altera CycloneII series EP2C35F672C6 NX chip to complete the FPGA modulation and demodulation of the QPSK implementation. In this process, the Verilog file is simulated by Modelsim, and the simulation waveforms of each module are analyzed, and the correctness of Verilog program is verified.. In the end, the signal of the signal and modulation end is transmitted, and the comparison is made in the oscilloscope, which verifies the correctness of the design and implementation of the QPSK modulation and demodulation circuit in the paper.Based on the analysis of the QPSK modulation and demodulation principle, each module of the modulation and demodulation of the terminal including shaping filter, numerical controlled oscillator, carrier synchronization, bit synchronization module was implemented with MATLAB modeling and simulation analysis and FPGA, we have achieved the desired results of correct sending data by sending the signal though QPSK modulation after launch and received. With the help of company internship before, this paper could be completed, and due to length and workload, this issue involves only the modulation and demodulation part.