Quadrature Amplitude Modulation - QAM

QAM is a combination of amplitude modulation (changing the amplitude or voltage of a sine wave to convey information) together with phase modulation. There are several ways to build a QAM modulator. In one process, two modulating signals are derived by special pre-processing from the information bit stream. Two replicas of the carrier frequency sine wave are generated; one is a direct replica and the other is delayed by a quarter of a cycle (90 degrees). Each of the two different derived modulating signals are then used to amplitude modulate one of the two replica carrier sinewaves, respectively. The resultant two modulated signals can be added together. The result is a sine wave having a constant unchanging frequency, but having an amplitude and phase that both vary to convey the information. At the detector or decoder the original information bit stream can be reconstructed. QAM conveys a higher information bit rate (bits per second) than a BPSK or QPSK signal of the same bandwidth, but is also more affected by interference and noise as well.

This diagram shows that amplitude and phase modulation (QAM) can be combined to form an efficient modulation system. In this example, one digital signal changes the phase and another digital signal changes the amplitude. In some commercial systems, a single digital signal is used to change both the phase and the amplitude of the RF signal. This allows a much higher data transfer rate as compared to a single modulation type.

Quadrature Amplitude Modulation Operation Diagram

Related Cable Television - CATV Terms

Contribution Network
Headend
Off Air Receivers
Rate Shaper
Channel Signal Combiners
Metadata Management
Playout Scheduling
Hybrid Fiber Coax - HFC
Cable Ready Televisions
Cable Modems
Cable Telephone Adapters
Cable Telephony
Wireless Cable
Video on Demand – VOD
National Television Standards Committee – NTSC
Phase Alternating Line – PAL
Data Over Cable Service Interface Specifications – DOCSIS
Pay per View – PPV

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