Adaptive modulation

Adaptive Modulation is a transmission scheme in Digital Communications where the transmitter adapts its transmission mode in accordance with the channel. Depending on the severity of the channel, the transmitter could be adapting one or more of the following: constellation size, code rate, and power. Adaptive modulation systems invariably require some channel information at the transmitter. This could be acquired in time division duplex systems where the channel from the transmitter to the receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel knowledge can also be estimated at the receiver, and fed back to the transmitter. Adaptive modulation systems improve rate of transmission, and/or bit error rates by exploiting the channel information that is present at the transmitter. Especially over fading channels which model wireless propagation enviroments, adaptive modulation systems exhibit great performance enhancements (as measured in terms of rate or bit error rate).