Carrierless Amplitude Phase (CAP) Modulation

CAP is closely aligned to quadrature amplitude modulation (QAM). QAM as a technique is widely understood in the industry and well deployed in older modems. Both CAP and QAM are single-carrier signal techniques.

The data rate is divided in two and modulated onto two different orthogonal carriers before being combined and transmitted. The main difference between CAP and QAM is in the way they are implemented. QAM generates two signals with a sine/cosine mixer and combines them onto the analog domain.

CAP was one of the original proposals for use with ADSL technology. Unfortunately, this was a proprietary solution offered by a single vendor, which turned heads away from acceptance. Most industry vendors agree that CAP has some benefits over DMT, but also that DMT has more benefits over CAP. The point is that two differing technologies were initially rolled out for ADSL (and the other family members), which contradict each other in their implementation.

CAP uses the entire loop bandwidth (excluding the 4-kHz baseband analog voice channel) to send the bits all at once. The DMT technique has no subchannels. The lack of subchannels removes the concern and problems with individual channel transmission. Most of the RBOCs started using CAP but have since moved on to DMT.