Adaptive modulation and coding (AMC) has been widely used to improve the spectral efficiency. In this paper, we take a different look at it from energy saving point of view. Specifically, we analyze the queuing behavior of AMC systems jointly with sleep mode where the wake-up process incurs time and energy cost. We formulate the optimization problem by jointly considering energy-efficiency, queuing delay and packet loss rate, and find the solution with cross-layer adjustment of the transmit power and the sleep threshold. Numerical results show that at low traffic range, when the power consumption of idle (no data transmission) mode is un-negligible, introducing sleep mode to the AMC system significantly improves the energy efficiency compared with non-sleep system. To achieve the energy-efficiency gain, the system tends to use higher-order modulation by increasing transmit power, which also reduces the number of dropped packets.
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