Performance of modulation-level-controlled adaptive-modulation under limited transmission delay time for land mobile communications

The performance of an adaptive-modulation/time division duplex (TDD) scheme to support constant-bit-rate services under transmission delay time constraint conditions is evaluated in this paper. According to the received signal conditions, the modulation levels or transmission modes are selected from transmission-off, QPSK, 16QAM, or 64QAM by using the reciprocity of the propagation path characteristics in the TDD systems. Moreover, the transmission delay time is limited to keep data transmission rate constant by the memory buffer. The received signal condition is estimated by using extrapolation of pilot symbol or estimated received signal distortion at the decision stage. The performance is evaluated by computer simulation in case of 32 ksymbol/s transmission and is compared with the performance of conventional trellis-coded (TCM) 32QAM. The results show that the proposed method improves the BER performance and delay spread immunity of a system transmitting data at a constant rate under multipath fading conditions and the performance is better than that of TCM-32QAM in Rayleigh fading conditions with small Doppler frequency.