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把高频载波信号变成调制波的方法有很多。对于xDSL技术中最多提到的ADSL来说,现有两种互相竞争的方案:无载波幅相调制(CAP)以及离散多音频调制(DMT)。CAP和DMT都基于正交调幅(QAM),但应用的方法却不同。 QAM是一种宽带会话过程,一般用于Modem,允许两路数字载波信号占用同一传输带宽。利用QAM,两路独立的消息信号用来调制两路具有相同频率但波幅和相位不同的载波信号。QAM接收器能够辨别究竟是利用较高的还是较低的幅度和相位才能克服电线对上出现的噪声和干扰。 CAP 产生一种能反映幅度和相
There are many ways to make high-frequency carrier signals into modulated waves. There are two competing solutions for ADSL most mentioned in xDSL technology: carrierless phase modulation (CAP) and discrete multi-tone modulation (DMT). Both CAP and DMT are based on Quadrature Amplitude Modulation (QAM), but the method of application is different. QAM is a broadband session process, generally used in Modem, allowing two digital carrier signals to occupy the same transmission bandwidth. With QAM, two independent message signals are used to modulate two carrier signals with the same frequency but different amplitude and phase. QAM receivers can recognize whether the use of higher or lower amplitude and phase can overcome the noise and interference present on the pair. CAP produces a can reflect the magnitude and phase