Implementation and performance of an adaptive QAM modulation-level-controlled system for land mobile communications

An adaptive modulation system for land mobile communications that can select one of the quadrature amplitude modulation levels as a suitable modulation for the propagation conditions is described. The main characteristics of the system are a mode in which information cannot be transmitted under adverse propagation conditions and a buffer memory for maintaining the data transmission rate. We confirmed the basic performance of the adaptive modulation system using the equipment we developed and we found the measured performance was consistent with computer simulation results. We also found that the adaptive modulation system provided a noticeable improvement in the spectral efficiency and transmission quality.