Energy and spectral efficient transmissions of coded ARQ systems

Energy efficiency (EE) and spectral efficiency (SE) feature one of the most fundamental tradeoffs in communication systems. In this paper, we propose an optimum transmission scheme for coded Type-I automatic repeat request (ARQ) systems to balance the EE-SE tradeoff. The optimization is performed with respect to a new design metric, the normalized EE (NEE), which is defined as the energy per bit normalized by the SE. Minimizing NEE means either reducing the energy per bit or increasing the SE, it thus yields a balanced tradeoff between the two. The system design incorporates a wide range of practical system parameters, such as circuit power, modulation, coding, and detection errors in the physical layer, and frame length and protocol overhead in the media access control layer. Under the constraints of fixed modulation level and data rate, the optimum transmission energy and frame length are identified as closed-form expressions of the system parameters. Simulation results show that minimizing the NEE instead of energy per bit almost doubles the SE with less than 1 dB loss in EE.