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A DIRECT PI/4 DQPSK MODULATOR WITH MEMORY BASED PREFILTERING AND PREDISTORTION

Date

1994-11

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Degree Level

Masters

Abstract

The cellular communication systems have experienced a very rapid growth. The demand for service is beyond the capacity of current analog systems. A time division multiple access (TDMA) digital cellular system with 7/4 DQPSK modulation has been specified as one of the next generation digital cellular systems for North America. An important consideration is that the mobile subscriber equipment be compact, power efficient and inexpensive. A directly modulated mobile transmitter has these desirable qualities. In this thesis, a direct 48.6 kb/s 7/4 DQPSK modulator is described. The modulator is implemented using microstrip circuits for RF and digital memory circuits for baseband prefiltering and predistortion. A square root raised cosine roll off prefilter with a roll off factor of 0.35 is implemented using digital memory circuits. Same circuits are used to incorporate predistortion for the nonlinearity of the biphase attenuators in the modulator. Although predistortion for the modulator only was implemented, predistortion for the power amplifier nonlinearities requires no additional circuits. Both hardware design and measured results are presented. These results show that the modulator vector error was within the 12.51,- specified for TDMA systems in standard IS-54. An 18dB improvement in the band edge spectrum spreading was realized using predistortion. The modulator is suitable for silicon monolithic microwave integrated circuit (MMIC) implementation using RF and digital circuits on the same chip.

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Degree

Master of Science (M.Sc.)

Department

Electrical Engineering

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