Energy-Efficient Cooperative Beamforming Using Timer Based Relay Subset Selection
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I. I NTRODUCTION It has been shown that cooperative communications can significantly enhance system capacity and reliability [1]-[3]. Cooperative communication is also considered as a promising technique to improve the energy efficiency in wireless networks [4]. Choosing the optimal number of cooperative nodes among the available nodes is one of the main challenges while designing energy efficient cooperative communication systems. Most existing cooperative schemes select either the best relay [5][6] or all available relays [7][8] to cooperatively transmit data to the destination. However, using all relays for cooperative transmission is not always beneficial. For cooperative systems with limited feedback, recruiting a subset of available relays leads to significant performance improvements compared to best relay selection or deploying all relays [9]. Nevertheless, cooperative communications may increase signalling overhead in comparison to point to point communications [10]. Most existing work on energy efficient cooperative relaying ignores the overhead needed for acquisition of channel state information (CSI), relay selection, coordination among recruited relays etc. [8][11]. Quantitative analysis of the overhead for cooperative relaying schemes was provided in [10], where, however, only the impact of overhead on spectral efficiency was considered. Energy consumption for cooperative beamforming considering the overhead to obtain and feed back CSI to the selected relays, is investigated in [12], which shows that the required
overhead restricts the number of relays used for energyefficient cooperative beamforming. Though, it overlooked the maximum transmission power constraint and omitted direct communication. In [13], energy efficiency of cooperative beamforming is analysed, including the energy consumption for the overhead needed to form a virtual Multiple-Input Single-Output (MISO) system. However, it recruits all decoding relays. The optimal number of relays used for cooperative beamforming to maximize energy efficiency is obtained in [14], under the assumption of very large data packets. This assumption has two implications. First, it requires very long channel coherence time. Second, it may not show the real impact of overhead on energy efficiency, as the overhead could become negligible. Furthermore, it is assumed in [14] that cooபைடு நூலகம்erating relays can hear each other and hence the hidden node problem is ignored [6]. Both [13] and [14] focus only on the case that cooperating relays are distributed near the source, while the impact of different locations of cooperating relays on energy consumption has not been investigated. In this work we propose an energy efficient cooperative communication scheme that uses a timer based method to select the best subset of available relays for cooperative beamforming. We analyse energy efficiency under the maximum transmission power constraint and using practical data packet length. In our analysis, we take into account the overhead needed to obtain CSI, to select cooperating relay(s) and to perform cooperative beamforming. We study also the impact of different locations of cooperating relays on energy efficiency and provide performance comparison to the cooperative scheme in [12], direct communication, best relay selection and all relay selection schemes. In addition, the impact of the data packet length on the energy efficiency is investigated. The rest of the paper is organised as follows. In Section II we describe the system model of the proposed cooperative communication system. Section III analyses the associated energy efficiency. We present simulation results in Section IV. In Section V conclusions are given. II. S YSTEM M ODEL We consider one source-destination pair and a set N = {1, . . . , N } of Decode-and-Forward (DF) relays as depicted in Fig. 1. The independent block Rayleigh fading channel model is assumed. The channel power gains for
Abstract—Energy efficiency has become a crucial performance metric in wireless communications due to the increasing energy consumption resulting from increasingly widely deployed wireless communication systems and devices. We propose a timer based relay selection scheme that selects a subset of available relays to perform cooperative beamforming for data transmission in an energy-efficient way. We analyse the resulting energy efficiency taking into account the additional energy consumed by the overhead required to obtain channel state information, to select relay subset and to perform cooperative beamforming. We also study the impact of the number of cooperating relays, their location and different data packet length on energy efficiency. The simulation results show that for relays located close to the source the proposed scheme is more energy efficient than direct communication, conventional best relay selection, all relay selection and an existing cooperative relaying scheme. Moreover, through simulation it is revealed that increasing data packet length leads to significant energy savings.
