On the Optimum Energy Efficiency for Flat-Fading Channels with Rate-dependent Circuit Power

This paper investigates the optimum energy efficiency (EE) and the corresponding spectral efficiency (SE) for a communication link operating over a flat-fading channel. The EE is evaluated by the total energy consumption for transmitting per message bit. Three channel cases are considered, namely static channel with channel state information available at transmitter (CSIT), fast-varying (FV) channel with channel distribution information available at transmitter (CDIT), and FV channel with CSIT. The link's circuit power is modeled as ρ+κφ(R) Watt, where ρ>0 and κ≥0 are two constants and φ(R) is a general increasing and convex function of the transmission rate R≥0. For all the three channel cases, the tradeoff between the EE and SE is studied. It is shown that the EE improves strictly as the SE increases from 0 to the optimum SE, and then strictly degrades as the SE increases beyond the optimum SE. The impact of κ, ρ and other system parameters on the optimum EE and corresponding SE is investigated to obtain insight. Some of the important and interesting results for all the channel cases include: (1) when κ increases the SE corresponding to the optimum EE should keep unchanged if φ(R)=R, but reduced if φ(R) is strictly convex of R; (2) when the rate-independent circuit power ρ increases, the SE corresponding to the optimum EE has to be increased. A polynomial-complexity algorithm is developed with the bisection method to find the optimum SE. The insight is corroborated and the optimum EE for the three cases are compared by simulation results.