PERFORMANCE OF MODULATION-LEVEL-CONTROLLED ADAPTIVE MODULATION SYSTEMS

The multivalued quadrature amplitude modulation (QAM), such as 16 QAM, is a very useful technique in land mobile communication since a high- speed transmission can be realized without enlarging the transmission band. The technique has a problem, however, in that it is not robust a gainst the decrease of the received signal level due to fading or the effect of the delayed waves, which easily deteriorates the transmissio n quality, To solve such a problem, this paper proposes an adaptive mo dulation system where the number of modulation levels is adjusted acco rding to the condition of the channel. In the proposed system, the ins tantaneous C/N-0 and the delay spread are estimated as the condition o f the channel, using a method based on time division multiple access/t ime division duplex (TDMA/TDD). When the channel is in a good situatio n, a high-speed transmission is executed using a larger number of modu lation levels such as 256 QAM. When the channel is in a bad condition, a modulation method with a small number of modulation levels, such as quadraphase-shift keying (QPSK), is used, which is robust against the noise or the delayed waves. By this scheme, it is possible to realize a high-speed transmission without deteriorating the transmission qual ity, compared to the conventional system with the fixed number of modu lation levels. The transmission performance is evaluated by a computer simulation, and it is shown that a high-speed transmission can be rea lized without deteriorating the transmission quality, compared to the QPSK system, in the uniform Rayleigh fading condition as well as in th e situation containing delayed waves.