Abstract-This paper proposes a new power allocation technique to jointly optimize link-layer energy efficiency (EE) and effective capacity (EC) of a Rayleigh flat-fading channel with delay-outage probability constraints. Specifically, EE is formulated as the ratio of EC to the sum of transmission power and rate-independent circuit power consumption. A multi-objective optimization problem (MOP) to jointly maximize EE and EC is then formulated. By introducing importance weight into the MOP, we can flexibly change the priority level of EE and EC, and convert the MOP into a single-objective optimization problem (SOP) which can be solved using fractional programming. At first, for a given importance weight and a target delay-outage probability, the optimum average transmission power level to maximize the SOP is found. Then, the optimal power allocation strategy is derived based on the obtained average input power level. Simulation results confirm the analytical derivations and further show the effects of circuit power, importance weight, and transmission power constraint limit on the achievable tradeoff performance.
I. INTRODUCTIONDuring the last decade, climate change has emerged as a global challenge and many governments, academics and industries are now increasingly unified in a call to action [1]. It is reported that information and communications technology (ICT) industry is estimated to contribute between 2% to 3% of global greenhouse gas emissions [2], a share which is quickly raising. Besides, although silicon technology is exponentially progressing, the power consumption of the processor is also increasing by 150% every two years [3]. In contrast, the improvement in battery technology is much more sluggish, about 10% increase every two years [3], which leads to a rapidly increasing gap between the demand for energy and the battery capacity offered. Therefore, to meet the challenges raised by the high demands of wireless traffic and energy consumption, green communication has become an urgent need. Energy efficiency (EE), in b/J/Hz, and spectral efficiency (SE), in b/s/Hz, are considered as two key performance indicators for green wireless communication systems. Unfortunately, it is known that EE and SE are inconsistent and conflict with each other.To tackle this problem, many studies on the EE-SE tradeoff have been carried out [4]- [10]. In particular, the EE-SE tradeoff problem was formulated as a constrained optimization model, for interference-limited wireless networks in [4], downlink orthogonal frequency division multiple access (OFDMA) networks in [5], and cooperative cognitive radio networks in [6]. In the aforementioned studies, EE was fixed as the objective function and a constraint on achievable rate