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线性调制方式,虽然能够实现高频谱效应,但人们却很少将它用于移动无线电系统。这主要是因为使用线性调制方式,传输器功率放大器的非线性往往会将频谱扩展,因而会削弱所有频谱的获益。本文所提出的线性调制系统,解决了上述困难,并能获得较传统的数字式调频(FM)更高的频谱效应。调制/解调是一种相位为π/4 的四相移键控(QPSK)和带有限幅鉴相器的相移检波以及积分门(Integrate—and—Dump)滤波器。通过引入一直角坐标的负反馈控制,利用“AB”级放大器,依靠29dB的反馈增益,能同时实现10W输出功率时的35%功率效应和-60dB的相关带外发射。通过使用带有限幅鉴相器的非相干检波,接收机结构能很容易地实现抗扰性,并能避免快衰落。通过使用新型的检波方法,在非衰落和衰落两种情况下的比特误差率与数字式调频所得到的比特误差率不相上下。由于这些原因,在移动无线电系统中,线性调制方式更有可能得到较传统的数字式调频方式更高的频谱效应。
Linear modulation, although able to achieve high spectral effects, but very few people use it for mobile radio systems. This is mainly due to the fact that using linear modulation, the transmitter power amplifier’s nonlinearity tends to spread the spectrum, thus undermining the benefit of all the spectrum. The proposed linear modulation system solves the above difficulties and achieves higher spectral effects than the traditional FM. Modulation / demodulation is a quadrature phase shift keying (QPSK) with a phase of π / 4 and phase-shift detection with amplitude-limited phase detector and an Integrate-and-Dump filter. By introducing a negative feedback control in a rectangular coordinate system, using the “AB” amplifier, with a feedback gain of 29dB, the 35% power effect at 10W output power and the associated out-of-band emission at -60dB can be achieved simultaneously. By using incoherent detection with a slicer phase detector, the receiver structure can easily achieve immunity and avoid fast fading. By using the new detection method, the bit error rate in both non-fading and fading cases is comparable to that obtained with digital FM. For these reasons, in the mobile radio system, the linear modulation method is more likely to obtain a higher spectral effect than the conventional digital FM method.