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 by assuming 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 directly measured 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 environments, adaptive modulation systems exhibit great performance enhancements compared to systems that do not exploit channel knowledge at the transmitter.

Cellular phones adaptively adjust their power level. Cell phones close to a cell tower transmit using very little power. As they get further away, the cell tower tells them to transmit using more power. This adaptive transmission has 2 benefits:

• A fixed transmit power level would either (close to the tower) use more energy than necessary, reducing the battery life of the cell phone, or (far from the tower) transmit too quietly to be heard, reducing the range of the cell phone.
• The cell tower adjusts the power level of each cell phone so that -- at the tower -- they all have close to equal power levels. This makes it much easier to seperate all the signals from each other -- if one phone were much "louder" than the others, it would be more likely to bleed through into the other signals.