| 摘要 |
Closed loop frequency control allows for much closer packing of adjacent transmission channels or sub-channels in that cross channel interference can be adaptively minimized by detecting frequency mismatch between the received signal frequency and the nominal signal frequency. For systems employing quadrature amplitude modulation (QAM), the frequency mismatch can be detected by detecting rotation of the received constellation using a phasor rotation filter. Once phasor rotation is detected, a frequency error value can be derived and a correction value can be transmitted from the receiving unit to the transmitting unit with instruction for the transmitting unit to adjust its transmission frequency accordingly. Frequency mismatch arising from Doppler frequency shift to the transmitted signal can be compensated for, as well as inadvertent oscillator mismatch. <IMAGE> |
