| Software radio is an important research field, whose aim is to construct a common hardware platform with the A/D and D/A converter close to antenna as possible as it can and realize all the functions of communication by corresponding signal processing software. The software radio products have the merits of simplified circuit, high reliability, convenient produce and maintenance. Also system function can be updated by renewing software. There are two keys to its practical application on a large scale: one is capability improving and price falling for DSP, the other, upswing and perfect for signal processing algorithm.Modulation Demodulation algorithm is the core of software radio and affects the system performance directly. After expatiating and discussing the character of digitalized demodulation, research actuality and correlative theories, the author offers a DFT-based digitalized demodulation and bit synchronization scheme for QDPSK signal. The basic idea is: since the code wave of the signal received can be divided into transitional section and stable section actually, the author implements DFT on the sampling data in the stable section and gets the skip-phase of fore-and-aft code, then accomplishes demodulation using the skip-phase information. Meanwhile, the author gets the carrier wave amplitude by doing DFT on sampling data of signal in every carrier wave and establishes the bit synchronization by analyzing the amplitude wave. As to the validity and noise-resistance of demodulation, simulation in software is also implemented in addition to theoretical analysis. The result indicates that this scheme not only simplifies the demodulation process, reduces the calculation, but also improves the noise-resistance. This demodulation method can also be applied to other DPSK signal.In the end, the paper discusses the whole design scheme of software radio experiment system based on DSP for the above demodulation method and some pivotal technology included. |
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