Carrierless Amplitude Phase Modulation

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Carrierless amplitude phase modulation (CAP) is a non-standard variation of quadrature amplitude modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair.

CAP used for ADSL divides the available space into three bands. The range from 0 to 4 kHz is allocated for POTS transmissions. The range of 25 kHz to 160 kHz is allocated for upstream data traffic and the range of 240 kHz to 1.5 MHz is allocated for downstream data traffic.

History

CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs. Now it is deprecated in favour of Discrete MultiTone Modulation (DMT), but it is still used for some variants of HDSL.

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