Square-QAM adaptive modulation/TDMA/TDD systems using modulation level estimation with Walsh function

The authors propose a square-QAM adaptive modulation/time division multiple access/time division duplex (TDMA/TDD) system for land mobile communications to achieve high quality and high bit rate transmission in both flat and frequency selective fading environments. To further improve performance, a new modulation level estimation scheme using the Walsh function and maximum likelihood (MS) estimation is also proposed. Computer simulation confirms that the proposed system gives 8 dB better performance and four times larger delay spread immunity than that of conventional QPSK systems at BER=10^-3