256 QAM Modem for High Capacity Digital Radio System

This paper presents topics related to the 256 QAM modem for high capacity digital microwave radio. Specifications for modem circuitry are derived from the allowable carrier-to-noise ratio degradation point of view. In this process, various degradation factors are categorized so that the same evaluation is possible. Two representative circuits, the automatic threshold control (ATC) and carrier recovery circuits, of particular importance for developing a 256 QAM modem, are presented. For the ATC circuit, the operation principle as well as the presence of the false-lock phenomenon and its evasion method are described. The prototype 256 QAM modem performance indicates the prospect of a 400 Mbit/s 256 QAM modem.