Carrierless amplitude phase modulation (CAP) is a variant ofquadrature amplitude modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates a QAM signal by combining twoPAM signals filtered through two filters designed so that theirimpulse responses form aHilbert pair. If the impulse responses of the two filters are chosen as sine and a cosine, the only mathematical difference between QAM and CAP waveforms is that the phase of the carrier is reset at the beginning of each symbol.[1] If thecarrier frequency and symbol rates are similar, the main advantage of CAP over QAM is simpler implementation.[1] The modulation of the baseband signal with the quadrature carriers is not necessary with CAP, because it is part of the transmit pulse.[1]
CAP finds application inHDSL and in early proprietary ADSL variants.[2][3] For HDSL, the AmericanANSI standard specifies2B1Q rather than CAP, while the EuropeanETSI ETR 152 and the internationalITU-T G.991.2 standards specify both CAP and 2B1Q.[2][4][5] For ADSL deployments CAP was the de facto standard up until 1996, deployed in 90 percent[citation needed] of ADSL installs. The standardized variants of ADSL,ANSI T1.413 Issue 2 andG.dmt, as well as the successorsADSL2,ADSL2+,VDSL2, andG.fast, do not specify CAP, but ratherdiscrete multi-tone (DMT) modulation.
CAP used forADSL divides the available frequency spectrum into three bands.[citation needed] The range from 0 to 4 kHz is allocated forPOTS transmissions. The range of 25 kHz to 160 kHz is allocated forupstream data traffic and the range of 240 kHz to 1.5 MHz is allocated fordownstream data traffic, in afrequency-division duplexing (FDD) scheme.
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