PERFORMANCE OF MULTICARRIER QAM-LEVEL-CONTROLLED ADAPTIVE MODULATION FOR LAND MOBILE COMMUNICATION-SYSTEMS/

A system combining multicarrier modulation and adaptive modulation in which a suitable level of quadrature amplitude modulation (QAM) is sel ected for each subcarrier and time-slot, is proposed for high-bit-rate and high-quality digital land mobile communications. The advantages o f the system are a mode in which information cannot be transmitted und er adverse propagation conditions and a buffer memory to limit a trans mission delay time. If the allowable delay time is small, such as in v oice and video transmissions, the system tends to have a poor bit erro r rate (BER) because of the forcible QAM-level selection. Our new sele ction scheme improves the BER for small transmission delay time. Suita ble distribution of the delay time among subcarriers is obtained by us ing the scheme where the QAM-level of each subcarrier is chosen collec tively using the number of data bits stored in memory. Computer simula tion of the system's NER performance showed that the system could prov ide a noticeable BER improvement over frequency-selective fading chann els as well as flat Rayleigh fading channels. The QAM-level selection scheme was also effective for a low maximum Doppler frequency and a sm all memory size. The system could thus attain about 25-fold improvemen t in BER at E-s/N-0 greater than or equal to 30 dB compared to the mul ticarrier/16QAM system. It also attained about 60-fold and 3.5-fold im provement in BER at f(d)=10 Hz compared with the system with multicarr ier/16QAM and without the QAM-level selection schemed respectively.