Cognitive Radio Unified Spectral Efficiency and Energy Efficiency Trade-off Analysis

Spectral efficiency and energy efficiency are fundamental trade-off in wireless communications. Spectral efficiency (SE), defined as the average data rate per unit bandwidth, quantifies how efficiently the available spectrum is utilized. Energy efficiency (EE), defined as the successful transmitted information bits per unit energy from transmitter to receiver, quantifies how efficiently the energy is utilized. Basically, with higher average energy per bit to noise power spectral density ratio at the receiver, the packet can be more successfully detected, thus utilizing the spectrum more efficiently, giving higher SE; however, in this case, it requires more energy, lowering EE, and vice versa. In this paper, we study the trade-off between SE and EE, specifically for the cognitive radio considering its configurability. We propose a general metric SEE (Spectral/Energy Efficiency) to facilitate the analysis which quantifies the preference of SE or EE. Closed-form solutions for symbol transmission energy and the length of information bits per frame are obtained for various combined modulation and channel coding schemes. The closed-form solutions further facilitate the adaptivity of cognitive radio considering both SE and EE in various scenarios. Using the proposed metric shows that our scheme is capable to perform balanced trade-off between SE and EE. Considering only maximizing SE, our scheme gains much larger EE while only sacrificing little SE; and comparing with maximizing EE, larger SE can be obtained while sacrificing a small amount EE.