Design and Construction of a DQPSK Modern for Mobile Communication

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Justin Ladu Akidas, Lamoro
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The aim of this work is to design and construct a non-coherent QPSK digital modern to be used for mobile communications. It looked into the very often used digital modulation methods i.e. FSK, PSK, QPSK and DPSK and those modulation types suited for mobile use like MSK, GMSK and GTEM; each has merits as regards to baud rate and the bandwidth. Mobile radio communications, for which the design is intended, is previewed. It is found that particular types of modulation methods that offer higher spectral efficiency are better suited for wireless communications. QPSK is one of the modulation methods that have high spectral efficiency and it is simple to implement. When the phase angles of QPSK symbols are differentially encoded, the resulting modulation is DQPSK. It is done by either differentially encoding the source bits and mapping then onto absolute phase angles of QPSK signal constellation or by directly mapping the pairs of input bits on the relative phases. The DQPSK modulation provides the spectral efficiency of QPSK and reduces the effects of amplitude fluctuations of the mobile radio channel. This modulation has the advantage that it can be implemented with differential detection or discriminator detection. It is a four-phase system thought of as two DQPSK systems in Parallel in which the carrier are in phase quadrature. The encoding of the information at the transmitter removes the difficulty of establishing and maintaining the correct reference phase at the receiver. The information is encoded in terms of phase changes at the transmitter and the signal is detected by comparing phases of adjacent symbols. The design and construction of the DQPSK MODEM in this work went through two stages. The stages are the modulator and the demodulator. The modulator stage gave very good modulation results. The demodulator stage, due to the problem of the delay equalizers for time delayed signal to the correlators, could not give exact replica of the transmitted information of the transmitter at the demodulation output. This however could be resolved by the use of appropriate time delay I.C instead of the many cascaded OP AMPS to give the delay which was the cause of the problem
Design , Construction , DQPSK Modern , Mobile Communication