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Carrierless Amplitude Phase Modulation | CAP modulation

This page describes Carrierless Amplitude Phase Modulation i.e. CAP modulation basics. It mentions advantages of CAP modulation and disadvantages of CAP modulation.

What is Carrierless Amplitude Phase (CAP) Modulation?

CAP is used in xDSL transmissions which suppress carrier frequency at the CO (Central Office). This modulation scheme is a version of Quadrature Amplitude Modulation (QAM) scheme.

The modulation scheme suppresses carrier amnd hence it is regenerated at the end DSL device for decoding the modulated signal back.

It does not use amplitude of two carrier frequency waves as used by QAM. It generates QAM modulated signal by combining two filtered PAM signals. Their impulse reponse form "hilbert pair".

CAP modulation transmitter and receiver

The difference between QAM and CAP modulation is that, phase of carrier is reset at the beginning of each and every symbol. The figure depicts CAP modulation transmitter and receiver modules.

Benefits or advantages of CAP modulation

Following are the benefits or advantages of CAP modulation:
➨It offers better latency characteristics. Hence it does not delay bit throughput much.
➨Vendors which implement nonDMT (i.e. single carrier design) in their xDSL products often use CAP modulation.
➨It is simple in implementation compare to QAM modulation at same carrier frequency and symbol rates.
➨It has been used in various applications such as HDSL, ADSL (up to 1996).

Drawbacks or disadvantages of CAP modulation

Following are the drawbacks or disadvantages of CAP modulation:
➨It requires more electronic circuits at end user premises.
➨It does adjust to noise conditions.
➨It has not been used in latest standards such as ADSL2, VDSL2, G.Fast which uses DMT modulation (discrete multi-tone modulation).

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