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In many wireless networks energy management is an important issue for reducing the size and cost of communication devices and/or extending a device's usable life-time. Often, the required transmission power is one of the main energy consumers in a wireless device; consequently, there has been much interest in efficiently utilizing this resource. A basic technique is transmission power control, i.e. adapting the transmission power over time in an attempt to not use any more energy than needed to communicate reliably. With data traffic, in addition to adjusting the transmission power, energy efficiency can be further improved by adjusting the transmission rate or equivalently the transmission time per packet, for example, by using adaptive modulation and coding. Such approaches exploit the well-known fact that the required energy per bit needed for reliable communication is decreasing in the number of degrees of freedom used to send each bit. In a fading channel, another benefit of adapting the transmission rate and power is that it enables the transmitter to be "opportunistic" and send more data during good channel conditions, which again reduces the required average energy per bit.