IMPLEMENTING A PI/4 SHIFT D-QPSK BASEBAND MODEM USING THE TMS320C50

Pi/4 D-QPSK (Pi/4 shift Differential-Quadrature Phase Shift Keying) is a four level modulation scheme first proposed by Baker in 1962 and is currently the focus of intensive research. The scheme is currently implemented in the American Digital Cellular, Japanese Handy Phone and the European TETRA systems.

The aim of this undergraduate project was to implement a Pi/4 Shift D-QPSK baseband modulator on the Texas Instruments (TM(TM)) TMS320C50 digital signal processor (DSP). The functions included Pi/4 D-QPSK signal mapping using a look-up table and raised cosine filtering to reduce Inter-Symbol Interference (ISI), and spectral spreading. The report also describes the differential detection and data recovery implementations in the demodulator; and the theory behind the symbol timing recovery module is introduced.

Raised cosine filtering of the I and Q channel streams at the modulator output was implemented digitally using a twenty tap FIR filter. A technique called interpolation was used in the filtering algorithm to increase the sampling rate and relax the reconstruction filter design. The filter coefficients for use in the DSP program were derived using a Matlab program.

This document was part of the first European DSP Education and Research Conference that took place September 26 and 27, 1996 in Paris. For information on how TI encourages students from around the world to find innovative ways to use DSPs, see TI's World Wide Web site at www.ti.com.

*DISCLAIMER: This document was part of the DSP Solution Challenge 1995 European Team Papers. It may have been written by someone whose native language is not English. TI assumes no liability for the quality of writing and/or the accuracy of the information contained herein.

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