Multicarrier/QAM-level-controlled adaptive modulation for land mobile communication systems

A multicarrier and an adaptive modulation combined system, where a suitable level of quadrature amplitude modulation (QAM) is selected for each carrier and time-slot using a QAM-level selection scheme, is proposed for digital land mobile communications. The advantages of the system are a mode in which information cannot be transmitted under adverse propagation conditions and a buffer memory to limit a transmission delay time. The QAM-level of each carrier is also controlled collectively according to the amount of the stored data bits in the memory. We confirmed the bit error rate (BER) performances of the system using computer simulation and we found that the system provided a noticeable BER improvement over frequency-selective fading channels and that the selection scheme was effective for a low maximum Doppler frequency and a small size of the memory.