Energy and Spectral Efficiencies Trade-off in Multiple Access Interference-Aware Networks

This work analyzes the power allocation problem in DS/CDMA systems under the energy efficiency (EE) metric, maximizing the transmitted information per energy unit. As the spectral efficiency (SE) is one of the most important performanc e metrics and is maximized with infinite power, the trade-off between these two metrics (EE-SE trade-off) are investigated a nd characterized in relation to multiple-access interference (MAI) power level, resulting that the optimum operating point is the max-EE point and the EE limitation imposed by MAI. To corroborate these results, we develop two algorithms for power allocation based on distributed instantaneous SINR level. Keywords— Energy efficiency, spectral efficiency, DS/CDMA, distributed power control. I. I NTRODUCTION Due to the explosive demand of mobile services worldwide and to battery lifetime limitations, resource allocat ion techniques in wireless networks had become a great concern and a challenge to many researchers in the last decades. In general, the problems aims to maximize system throughput given the maximum power or minimize power consumption given a minimum rate criterium. Besides this scenario, the increasing importance of rational use of scarce resources (specially energy), core of Green Communications approach , brings the metric of Energy Efficiency (EE), which aim to maximize the number of transmitted bits per energy unit, measured in bits per Joule. The problem of maximizing EE is mainly investigated in wireless systems for code-division multiple access (CDMA) [1], multi-carrier direct sequence CDMA (MC-DS-CDMA) [2] and orthogonal frequency-division multiple access (OFDMA ) [3], [4]. The strategy presented in those papers is that all users transmit at the maximum achievable EE, constrained by the maximum power available. As a consequence, some users can allocate the maximum power without transmitting at the optimum EE, increasing the generated interference. As CDMA systems are limited by interference, specially when using single-user detections techniques, putting some of these n onoptimum users in outage can result in interference reductio n and greater EE. On the other hand, since rate and spectral efficiency (SE) remain as fundamental metrics for modern wireless systems, it becomes necessary to investigate the gap between energy and spectral efficiencies-based allocat ion processes, determining the trade-off among these two metri cs and the optimum operating point. This issue is an important opened topic in Green Communications area [5]. Previously, Álvaro R. C. Souza and Taufik Abr ão, Dept. of Electrical Engineering and Computer Science. State University of Londrina, Londrin a-PR, Brazil. Fábio R. Durand, Dept. of Electrical Engineering, UTFPR Camp o Mour̃aoPR. Lucas H. Sampaio and Paul Jean E. Jeszensky, Dept. of Telec omm. and Control Engineering, Escola Polit écnica of the University of S̃ ao Paulo. S̃ao Paulo-SP, Brazil. E-mails: {alvarorcsouza, lucas.dias. sampaio }@gmail.com, taufik@uel.br, fabiodurand@utfpr.edu. br, pjj@lcs.poli. usp.br the EE-SE trade-off has been investigated in [6] considerin g OFDMA systems and some simplifications in the system model, given the difficulty in the analytical characterizat on. This work proposes to analyze the existing EE-SE trade-off for DS-CDMA systems under matched-filter (MF) detection, determining the optimum EE-SE trade-off, as well as to the impact of multiple-access interference (MAI) on this trade off. In order to corroborate the conclusions of this analysi s, we develop two algorithms for power allocation using game theory (specially non-cooperative games), since the overa ll network EE depends on the behavior of all users. II. N ETWORK SYSTEM MODEL For analysis simplicity, we have assumed an uplink direct sequence code division multiple access (DS/CDMA) network. However, the extension for multi-cell multi-carrier multi ple access systems is straightforward. The received signal by t he base station can be described as