| 单词 | QAM (Quadrature Amplitude Modulation) |
| 释义 | QAM (Quadrature Amplitude Modulation) A signal modulation technique that splits the carrier into two waveforms that are 90° out of phase, and specifies two possible amplitude values for each of four phase shifts separated by 90° (0°, 90°, 180°, and 270°). This yields eight distinct signal states, as illustrated in the signal constellation graph in Figure Q-1.Thereby, each signal impulse, or symbol, carries one of eight possible signal combinations and represents three bits (23 = 8). As a result, the transmission rate is thrice the signaling rate, or baud rate.At a signaling rate of 2400 baud, for example, this tribit modulation scheme yields a transmission rate of 7200 bps.The ITU-T V.29 recommendation is for 16-QAM. More complex schemes include 64-QAM, 128-QAM, 256-QAM, and 512-QAM.Trellis-coded modulation (TCM) uses the same modulation scheme as QAM, but adds forward error correction (FEC) to overcome the increased susceptibility to signal impairments which make it harder for the receiver to judge correctly which state is signaled with each baud. QAM applications include asymmetric digital subscriber line (ADSL). See also 16-QAM, 64-QAM, 128-QAM, 256-QAM, 512-QAM, carrier, FEC, modulation, phase, signal, signaling rate, symbol, TCM, transmission rate, and tribit. Binary Values 0° 000 001 010 011 100 270° 90° 101 110 111 180° |
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