Carrierless Amplitude Phase Modulation (CAP) Explained

This page explains the basics of Carrierless Amplitude Phase Modulation (CAP), including its advantages and disadvantages. CAP modulation is primarily used in xDSL transmissions where the carrier frequency is suppressed at the Central Office (CO).

Introduction to CAP Modulation

CAP is essentially a variation of Quadrature Amplitude Modulation (QAM). The key difference is that CAP suppresses the carrier frequency, requiring it to be regenerated at the DSL device to decode the modulated signal. Unlike QAM, which uses the amplitude of two carrier frequency waves, CAP generates a QAM modulated signal by combining two filtered PAM (Pulse Amplitude Modulation) signals. These signals have an impulse response that forms a “Hilbert pair.”

The primary distinction between QAM and CAP is that in CAP, the phase of the carrier is reset at the beginning of each symbol. The figure above illustrates the transmitter and receiver modules in a CAP system.

Advantages of CAP Modulation

Here’s a rundown of the benefits of CAP modulation:

• Lower Latency: CAP offers better latency characteristics, meaning it doesn’t significantly delay bit throughput.
• Suitable for Single-Carrier Designs: Vendors implementing non-DMT (Discrete Multi-Tone) designs in their xDSL products often use CAP modulation.
• Simpler Implementation: Compared to QAM modulation at the same carrier frequency and symbol rates, CAP is simpler to implement.
• Historical Use: CAP has been used in various applications such as HDSL and ADSL (up to 1996).

Disadvantages of CAP Modulation

Here are the drawbacks associated with CAP modulation:

• More Complex Electronics at User End: CAP requires more electronic circuits at the end-user’s premises.
• Poor Noise Adaptation: It doesn’t adapt well to noisy conditions.
• Obsolete in Modern Standards: CAP is not used in modern standards like ADSL2, VDSL2, and G.Fast, which utilize DMT modulation.