Abstract
Relaying is supposed to be a low energy consumption technique since the long distance transmission is divided into several short distance transmissions. When the power consumptions (PCs) other than that consumed by transmitting information bits is taken into account, however, relaying may not be energy efficient. In this article, we study the energy efficiencies (EEs) of one-way relay transmission (OWRT) and two-way relay transmission (TWRT) by comparing with direct transmission (DT). We consider a system where two source nodes transmit to each other with the assistance of a half-duplex amplify-and-forward relay node. We first find the maximum EEs of DT, OWRT, and TWRT by optimizing the transmission time and the transmit powers at each node. Then we compare the maximum EEs of the three strategies, and analyze the impact of circuit PCs and data amount. Analytical and simulation results show that relaying is not always more energy efficient than DT. Moreover, TWRT is not always more energy efficient than OWRT, despite that it is more spectral efficient. The EE of TWRT is higher than those of DT and OWRT in symmetric systems where the circuit PCs at each node are identical and the numbers of bits to be transmitted in two directions are equal. In asymmetric systems, however, OWRT may provide higher EE than TWRT when the numbers of bits in two directions differ significantly.
Highlights
Since the explosive growth of wireless services is sharply increasing their contributions to the carbon footprint and the operating costs, energy efficiency (EE) has drawn more and more attention recently as a new design goal for various wireless communication systems [1,2,3], compared with spectral efficiency (SE) that has been the design focus for decades.A widely used performance metric for EE is the number of transmitted bits per unit of energy
When only transmit power is taken into account, the EE monotonically decreases with the increase of the SE [4] at least for point-to-point transmission in additive white Gaussian noise (AWGN) channel
Since we assume perfect channel knowledge at each node, the nodes can estimate the transmit power and the transmission time required for each block, which depend on the channel distribution and packet sizes Bab and Bba
Summary
Since the explosive growth of wireless services is sharply increasing their contributions to the carbon footprint and the operating costs, energy efficiency (EE) has drawn more and more attention recently as a new design goal for various wireless communication systems [1,2,3], compared with spectral efficiency (SE) that has been the design focus for decades. Two-way relay transmission (TWRT) can recover the SE loss when properly designed [8,9,10] It is not well-understood whether these relay strategies are energy efficient, when various energy costs in addition to transmit power are considered. In [17], TWRT was shown to be more energy efficient than OWRT via simulations, where only transmit power was considered in the EC model. We first maximize the EEs of TWRT, OWRT, and DT by optimizing transmission time and transmit powers, respectively, for the three strategies. TWRT is not always more energy efficient than OWRT if the numbers of bits to be transmitted in two directions are unequal, or the circuit PCs at each node are different. We will use the Shannon capacity formula to represent the relationship between data rates and transmit powers in OWRT and TWRT cases later
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