Highly Efficient Digital Mobile Communications with a Linear Modulation Method

Although linear modulation methods can achieve high spectrum efficiency, very little attention has been directed to their use in mobile radio systems. This is mainly due to the fact that the nonlinearity of the transmitter power amplifier tends to spread the spectrum and thus eliminate any spectrum efficiency advantage gained through the use of linear modulation methods. In this paper, a linear modulation system is proposed, which solves the above difficulty and which gives higher spectrum efficiency than conventional digital FM. The modulation/demodulation method is
\pi /4
shift QPSK and phase-shift detection with a limiter-discriminator and an integrate-and-dump filter. By introducing a cartesian coordinate negative feedback control, 35 percent power efficiency at 10 W output power and - 60 dB relative out-of-band radiation are simultaneously achieved with a class "AB" amplifier, owing to the 29 dB feedback gain. The receiver configuration is easy to realize and gives immunity against fast fading through the use of noncoherent detection with limiter-discriminator. By using a novel decision method, bit error rate performances under both nonfading and fading condition are comparable to those obtained by digital FM. These results make it possible for linear modulation methods to achieve higher spectrum efficiency than is possible with conventional digital FM methods in mobile radio communications.