QAM for terrestrial and cable transmission

We introduce quadrature amplitude modulation (QAM) and discuss QAM architectures used to overcome transmission impairments typically encountered on 6 MHz wide terrestrial and cable TV VHF/UHF channels. The input data to the QAM transmitter is mapped into N bit words called symbols; N determines the QAM level. The signal-to-noise ratio at the receiver determines the QAM level that can be used. Typical terrestrial and cable channels allow N=4 and N=8, leading to user data rates of about 20 and 40 Mbits/s, respectively. Transmission impairments include random noise, reflections, phase noise, and frequency offset. Adaptive equalizers are used by the cable and terrestrial QAM receivers to remove amplitude and phase distortion due to reflections and filter imperfections. The cable adaptive equalizer consists of feed-forward and feedback sections; the terrestrial equalizer uses a feed-forward section only. Both the cable and terrestrial receivers accomplish both start-up and steady-state equalization using only the user data. We also describe carrier recovery and symbol timing recovery for the terrestrial receiver.<
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