Carrierless Amplitude Phase Modulation
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Carrierless Amplitude Phase Modulation

Carrierless amplitude phase modulation (CAP) is a variant of quadrature amplitude modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates a QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert 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 the carrier 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 in HDSL and in early proprietary ADSL variants.[2][3] For HDSL, the American ANSI standard specifies 2B1Q rather than CAP, while the European ETSI ETR 152 and the international ITU-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[] of ADSL installs. The standardized variants of ADSL, ANSI T1.413 Issue 2 and G.dmt, as well as the successors ADSL2, ADSL2+, VDSL2, and, do not specify CAP, but rather discrete multi-tone (DMT) modulation.

CAP used for ADSL divides the available frequency spectrum 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, in a frequency-division duplexing (FDD) scheme.


  1. ^ a b c Sjöberg, Frank (April 2000). "Section 5.1: Carrierless Amplitude/Phase Modulation". A VDSL Tutorial (PDF). Luleå University of Technology. ISSN 1402-1528. Retrieved .
  2. ^ a b Starr, Thomas (ed.). DSL Advances. Uppser Saddle River, NJ: Prentice Hall. ISBN 0-13-093810-6.
  3. ^ Conlan, Patrick J. (2009-04-20). "WAN and Teleworker Connections". Cisco Network Professional's Advanced Internetworking Guide (CCNP Series). Indianapolis: John Wiley & Sons.
  4. ^ "G.991.1: High bit rate digital subscriber line (HDSL) transceivers". International Recommendation. ITU-T. 26 August 1998. Retrieved 2013.
  5. ^ "ETR 152: Transmission and Multiplexing (TM); High bitrate Digital Subscriber Line (HDSL) transmission system on metallic local lines" (PDF). ETSI. February 1995. Retrieved .

Further reading

  • Im, G. -H.; Werner, J. (1993). "Bandwidth-efficient digital transmission up to 155 Mb/s over unshielded twisted pair wiring". Proceedings of ICC '93 - IEEE International Conference on Communications. 3. p. 1797. doi:10.1109/ICC.1993.397590. ISBN 0-7803-0950-2.
  • Gi-Hong Im; Werner, J. -J. (1995). "Bandwidth-efficient digital transmission over unshielded twisted-pair wiring". IEEE Journal on Selected Areas in Communications. 13 (9): 1643. doi:10.1109/49.475537.

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