1 Energy Efficiency and Spectrum Efficiency Trade-off Over Optimal Relay Location In Bidirectional Relay Networks M I Khalil 1 , S M. Berber, and K W. Sowerby Department of Electrical and Electronic Engineering, The University of Auckland Auckland, New Zealand e-mail 1 : mkha232@aucklanduni.ac.nz Abstract—This paper presents a new method combines Optimal Relay Location (ORL), Energy Efficiency (EE) and Spectrum Efficiency (SE) in a bidirectional relay networks. The proposal aims to minimize power consumption with lower data rate loss. For this work, a flat fading channel is considered for exchanging signals between two wireless users separated by a specified distance. This distance is splitted by a set of bidirectional Amplify-and-Forward (AF) relay nodes that allow to amplify the input signal without any further processing. The locations of these relays are optimized first and the trade-off expression between EE and SE is then derived. All analytical expressions (i.e., optimal relay location and trade-off EE SE ) are corroborated by the simulation. Index Terms—Amplify-and-Forward relay,Amplify-and-Forward relay, Energy Efficiency, Relay location, Spectral Efficiency. I. I NTRODUCTION Recently, the world has seen dramatically growth in the number of of wireless portable devices such as cell phones and laptops. With this number of devices, data rate demand and the energy required to deliver such data have become a serious concern for the wireless community. Data rate, defined as bits per second unit (bits/sec), is commonly evaluated by metric named as Spectrum Efficiency (SE) [1]. While the energy required to serve the data is evaluated by the Energy Efficiency (EE) metric, which is defined as the number of transmitted data bits per unit of transmitted power[2]. Increasing EE value (i.e., decreasing the energy consumption) can be achieved by maximizing bit rate or minimizing power per unit. Several researches have been proposed to develop EE for wireless networks. One of the most promising evolution is wireless relay networks [3]. The main concept of the relay network is that the relay nodes intermediate a single-hop communication link between a transmitter and its receiver. Thus, wireless networks use two or more wireless hops to transmit information from a source to a destination. The relay element is commonly classified into two categories: Amplify and forward (AF) and Decode and Forward (DF). The AF relay, amplifies the source signal and forwards it to the destination without any further processing. In contrast, DF relay decodes signal before delivering it to its receiver. Both AF and DF can be operated in one-way (unidirectional) or two-way (bidirectional). For the bidirectional AF relay network which was proposed by [4], optimizing EE under the constraints of both the SE and the transmission power, considering affect of self-interference cancellation on EE, is investigated by [5]. It showed that the bidirectional relay networks can achieve higher SE than unidirectional relay networks, at the price of lower optimal EE. Similar result is obtained by [6], when the circuit energy (CE) consumption is also considered. The study demonstrated that the optimizing both EE and SE with considering CE is more complicated issue in practical relay networks However, the low EE presented by [6] and [5] appeared because their analysis is limited increase SE at the cost of EE. Other studies [7–9] have investigated combining EE and SE in order to decrease network energy consumption with ensuring a desirable SE. Reference [7] investigated a transmission scheme to reduce the power consumed per transmitted bit for both unidirectional and bidirectional relay networks by combining power allocation and relay selection. Similar work is achieved [8] with considering CB to combine EE and SE of OFDM system. In some research, such as [9], average channel status is adopted to calculate average EE and SE. But, such studies have not considered relay location in their works. Motivated by the above problems related with EE and SE in TAF-relay networks, we propose a new method to optimize EE and SE in a balance scheme considering CB and the best relay allocation. This allows to increase EE associated with less loss in SE. Furthermore, integration optimal relay location beside BEeS has not been presented previously for TAF-relay networks. While [9] recognized that the relay location is an important determinant of both energy and spectral efficiency The rest of the paper is organized as follows; Section 2 outlines the system model; Section 3 discusses the proposed method for combining EE and SE with relay location; Section 4 shows the simulation results; Finally, conclusions and remarks are discussed in Section 5. 978-1-5090-0470-6/16/$31.00 ©2016 IEEE 298