Communications and Network, 2013, 5, 654-660 http://dx.doi.org/10.4236/cn.2013.53B2117 Published Online September 2013 (http://www.scirp.org/journal/cn) A Survey on Energy Efficiency in Cellular Networks Xiaochen Su1, Enchang Sun1, Meng Li1, F. Richard Yu1,2, Yanhua Zhang1 1College of Electronic Information and Control Engineering, Beijing University of Technology, Beijing, China 2Department of Systems and Computer Eng., Carleton University, Ottawa, ON, Canada Email: hinata05269@sina.com, ecsun@bjut.edu.cn Received July 2013 ABSTRACT With the continuous increase of user requirement, the Information and Communication Technology (ICT) has devel- oped rapidly. As a result, both capacity and coverage performances have b een improved, and at the same time, the communication networks have been more energy-intensive. In order to improve the energy efficiency of cellular net- works, the green communication must be realized in the future networks. In this article, we present a short survey on energy-efficient technology of cellular networks. Moreover, we classify them into three categories based on their appli- cation scenarios: energ y-efficient architectures, energy-efficient resource management and energy-efficient radio tech- nologies. For the first scenario, the applications of relay, Coordinated Multiple Points (CoMP) and heterogeneous net- work are discussed in detail. For the second scenario, the switching off scheme will be introduced as an emphasis of this part. And for the third scenario, the Multiple-Input Multiple-Output (MIMO) and Orthogonal Frequency Division Mul- tiplexing (OFDM) will be introduced as the representatives of the Energy-Efficient Radio Technologies. Finally, based on the technologies introduced above, a prospect forecast of the energy-efficient wireless cellular network is pr esented . Keywords: Green Wireless Communication; Energy-Efficient; CoMP; Relay; Switching Off Scheme 1. Introduction In the last several decades, due to the continuous increase of communication requirement, the Information and Com- munication Technology (ICT) developed rapidly focused on the higher capacity and larger coverage [1]. And cor- respondingly, the energy consumption is also growing at an amazing speed: the ICT is responsible for about 3% of energy consumption and 2% - 4% of CO2 emission all over the world [2,3], and the energy consumption will increase at a rate of 15% - 20% per year and will double after 5 years [4]. Within the wireless cellular networks, the Base Station (BS) occupies a significant portion of energy about 60% - 80%, which means a breakthrough point to decrease the energy consumption [5]. In order to reduce the energy consumption, several organizations or projects, such as the Energy Aware Ra- dio and Network Technologies (EARTH), have been founded, and many workshops have been organized at international conference [6,7]. Due to the endeavor and attempt for several years, various significant methods have been proposed. In this paper, we present a survey on energy-efficien t technology within the cellular network. The technologies can be classified into thr ee categories according to their application scenarios. And for each aspect, the r epr esent- ative technologies are introduced and discussed further. After that, a prospect forecast of the energy-efficient wireless cellular netwo rk is presented. The rest of this paper is organized as follows. In Sec- tion 2, the energy-efficient ar chitecture is introduced. The energy-efficient resource management is discussed in Section 3. In Section 4, some Radio Technologies are discussed. Within Section 5, the prospect forecast is pre- sented. Finally, Section VI concludes this paper. 2. Energy-Efficient Architectures A rational architecture is very important to reduce the energy consumption since it has a comprehensive influ- ence on networks. In order to optimize the network ar- chitecture, a mass of methods based on some technolo- gies, such as the heterogeneous network, CoMP and re- lay, have been proposed. With deploying some special cells, such as microcells, picocells and femtocells, to assist the conventional macro cells, the heterogeneous network will increase coverage of the cellular networks as well as EE [8 ]. In [9], the au- thors analyze the energy consumption of heterogeneous network, and obtain a higher EE based on a balance point they found between coverage range and outage probabil- ity. In [10], the trade-off between SE and EE within the heterogeneous networks is studied, and in [11], the trade- off between capacity and EE of heterogeneous network is Copyright © 2013 SciRes. CN
X. C. SU ET AL. studied deeply and the authors obtain a result that the small cell has a good potential to improve the EE and network capacity at the same time. In [12,13], the fore- most trade-off between the deployment efficiency (DE) and EE within heterogeneous network is investigated. With their analysis of deployment density, the proper cell size and number of BSs have been presented as a refer- ence for future design. With the cooperation of multiple nodes, CoMP can in- crease the coverage and throughput of cellular networks, and the mobile users can be served with a relatively sta- ble performance and quality even located at other cells [14]. In [15], the capacity and EE of an idealized CoMP system are analyzed by assuming the perfect backhauling and cooperative processing. In [16], the authors study the trade-off between EE and SE within the uplink of CoMP system and drive their power consumption models for both idealized and realistic scenario. And in [17], the authors investigate the realistic measurement method of EE for various downlink CoMP methods in LTE-A, and their results indicate that the coordinated beamforming and joint transmission could be the more suitable tech- niques to improve the EE. In [18,19], the ideas of switch- ing off sch eme and sleeping mode are combined with CoMP respectively to reduce the energy consumption fur- ther, and the switching off scheme and sleeping mode will be introduced primarily in the next section. In [20], the authors present a stochastic predictive control algo- rithm to optimize the BSs grouping for EE in CoMP. And in [21], a cooperative framework is proposed to dy- namically decide whether to perform CoMP transmission or not. Except the researches above, in [22], the authors present a transmission mode selection scheme to save energy within the CoMP system using Semi-Smart An- tenna. Within the transmission mode selection scheme, the conventional transmission mode, CoMP communica- tion, transmitting with SSA, and the transmission with both CoMP and SSA are involved. As same as CoMP, the relayed technology can also provides extra coverage and throughput to the conven- tional cellular networks and improve the communication quality within the cell edge. In [23], the energy-savin g performance of CoMP and relay, affected by the traffic intensity and network density, are compared based on the typical parameters setting. The comparing result is vary- ing with different intensity. In [24], the authors verify the Figure 1. Model of heterogeneous network and CoMP. beneficial effect on EE by deploying the fixed Relay Sta- tions (RS). The result within this paper shows that the energy minimization will not decrease the Signal to In- terference Noise Ratio (SINR) of cellular relay network. In [25], the EE of cellular network with Realistic system characteristics and Amplify and Forward (AF) r elay is investigated, and by using the convolution coding before transmission, the EE will be increased at high Signal to Noise Ratio (SNR) scenario. In [26], the scenario of De- code and Forward (DF) relay and Direct Transmission (DT) are also investigated. For a given outage probability, the network coding is beneficial for the situation with higher cell radius. In [27-32], the relay selection chara c- terized on EE, such as taking the optimal locations of nodes to minimum transmitting power, and the instanta- neous channel state obtained by channel estimation into account respectively in [27,30], has been investigated to reduce the energy consumption further. Moreover, the research result within [32] shows that the dynamic time allocation an d increasing the cooperative relays within the transmitting procedure properly are also beneficial to EE. In addition, based on the relay selection, the selec- tion of transmission modes can improve the EE further. In [33], a self-adaptive energy-efficient transmission scheme is presented to choose the most su itable trans- mission mode based on the EE calculated before. Fur- thermore, the number of relays deployed in cells is also important to the EE. In [34], the authors investigate the trade-off between EE and deployment density deeply. The analysis shows that a proper deployment density of relays will satisfy the requirement both of EE and DE. Too many relays will lower the EE and increase the dep- loying cost as well. In the next generation cellular networks, CoMP will play a crucial role in improving the communication qual- ity and EE at the same time. Therefore, the joint consid- eration of CoMP and other technologies needs to be put into more attention. And the optimized deployment of small cells and relays for energy efficiency is still an open issue needed to be studied further. 3. Energy-Efficient Resource Management Although the improved architecture can bring some ben- efits, the corresponding resource management is also in- dispensable to realize the green communication. For this reason, several technologies have been taken into account the energy efficient resource management, such as the switching off scheme, cell zooming, the using of renew- able energy and so on. In traditional cellular networks, the operators deployed many BSs to cover the communication blind district and improve the communication quality. Although it really works sometimes, plenty energy has been wasted due to Copyright © 2013 SciRes. CN
X. C. SU ET AL. the low utilization of BSs. For this reason, many switch- ing off schemes based on the variation of traffic load have been proposed in [35-42]. Except the common cri- terion presented above, the distance between the User Equipments (UEs) and the BSs mentioned in [35], the additional load increments transferred to the adjacent BSs considered in [37], the maximized coverage provided by active BSs and the coverage overlap of BSs adopted in [39,42] respectively are also the important criterions that should be taken into account. Bes ides the operation in BS, the switching off scheme can also be applied in relay networks. Alt hough the t ransmitti ng power of RSs is much lower than BSs, RSs w ith lower utilizatio n will also cause the energy wastage. In [43], the relays are dynamically switched off according to the variation of traffic loads. And in [44], the throughput and energy consumption are treated with the criterions simultaneously to decide which RSs should be switched off to improve EE of the net- work. As a power control technology, the cell zooming is used to satisfy the demand of traffic load in general, but can also increase EE of cellular networks. And in most instances, cell zooming is adopted as an assistant tech- nology to improve the EE with other technologies. In [45,46], the authors put the low utilization BS into sleep- ing mode (operating with a low energy level) and cover the uncovered area by using cell zooming. And in [47,48], the cell zooming is combined with the switching off scheme to minimize the energy consumption. Except the technologies introduced above, the usage of renewable energy can also reduce the electric energy con- sumption more dir e ctly. In [49], a handover method is presented to guide the users to access the BSs powered by green energy. In [50], the authors propose a scheme to enable more users to be served by the green energy through cell zooming. In [51], the authors investigate a system model with BSs powered by the combination of electronic and renewable energy. And In [52], the au- thors combine the BS switching off scheme with hybrid power to further increase the EE. Although the ren ewable energy can reduce the electric energy consumption directly, the study of this aspect is beyond our research. For this reason, to our best know- ledge, the switching off scheme is considered as the most effective method to manage the resource within future Figure 2. The switching off scheme and cell zooming. networks. Moreover, the switching off scheme can also be combined with CoMP, cell zooming and/or other tech- nologies to maintain a better coverage and improve the EE at the same time. Therefore, the joint switching off scheme needs further investigation. 4. Energy-Efficient Radio Technologies The green radi o technology can effectively reduce the energy consumption within the transmitting procedure, and then improve the EE of total network. In this section, MIMO and OFDM will be introduced as representatives. Due to the noteworthy improvement on average data rate and SE, MIMO has drawn many attentions within cellular networks. And pushed by the trend of green com- munication, the EE of MIMO is also becoming a re- search hotspot. In [53], the trade-off between energy and bandwidth efficiency is studied within a MIMO multi- hop wireless networks. And the analysis results show that the number of transmit antennas, receive antennas and communication hops are responsible to improve EE. In [54], the EE of different MIMO transmission schemes, such as the Open Loop Spatial Mu ltiplexing (OLSM) and Close Loop Spatial Multiplexing (CLSM), have been in- vestigated and compared with the traditional SISO mode. And in [55], the authors prove that the feedback informa- tion is very important to improve the throughput and EE especially in the CLSM which utilizes all available feed- back information. In addition, for the case of the UE has only one antenna in general, the energy-efficient MU- MIMO has been investigated in [56,57]. In [56], the au- thors devote themselves to balance the cell-edge EE and the average EE within cell. And in [57], the emphasis is put on the balance between EE and capacity by improv- ing the power allocation. As same as MIMO, OFDM also has a good potentiali- ty to reduce the energy consumption. In [58,59], some important trade-offs are studied in the OFDMA downlink. In [58], the authors prove that the relationship between EE and SE is quasi-concave and they find the upper bound and lower bound on the EE-SE curve for general scena- rios. In [59], the authors propose an algorithm to increase the EE and reduce the computation overhead properly at the sa me time. Except these fundamental res ea rches, Figure 3. The scenarios of SISO, MIMO and Mu-MIMO. Copyright © 2013 SciRes. CN
X. C. SU ET AL. OFDM is often combined with the power and resource allocation to improve EE further. In [60], the authors improve EE by optimizing the number of sub-carriers. And in [61], the overhead of CSI feedback is reduced to achieve higher EE. Although the research a imed at im proving EE of MIMO and OFDM systems have been continued for years, there are many aspects, such as the MU-MIMO and multi-cell OFDM, need to be investigated more deeply. 5. Prospect Forecast Based on the introduction and discussion of the energy - Efficient Technologies above, to our best knowledge, an optimized energy-efficient wireless cellular net work can be conceived. According to the fact that the BSs occupy a significant portion of energy consumption, the hetero- geneous network can be used to reduce the energy con- sumption while providing a better communication per- formance. Besides, the combination of heterogeneous net- work and switching off scheme will further increase the EE. In order to avoid the frequent switching, the low utilization BSs or RSs can be turned into a sleeping mode first. If the low utilization lasts for a long time, the sleep- ing nodes will be switched off completely. And as the important supplement, the cell zooming and CoMP can be adopted to cover the communication blind district. For a more optimized consideration, the OFDM and MIMO should be introduced into the signal transmission due to their better performance of balancing the EE and other criterions such as SE and capacity. 6. Conclusion In this article, we present a short survey on energy-effi- cient technologies of cellular networks. Many hotspot technologies such as the CoMP, MIMO and switching off scheme are introduced as the repr es entatives, which are divided into three categories according to their appli- cation scenarios. For each part, the current research situ- ations of technologies are introduced and discussed in detail. 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