and actively joining collaborative research projects for 5G communications, including Ericsson, Huawei, T-Mobile and others. It is also promising to see that with ...
5G: Towards Energy-Efficient, Low-Latency and High-Reliable Communications Networks Shunqing Zhang, Xiuqiang Xu, Yiqun Wu and Lei Lu Huawei Technologies, Co. Ltd., Shanghai, China Email: {zhangshunqing, xuxiuqiang, wuyiqun, kevin.lu}@huawei.com (Invited Paper)
Abstract—In the past few decades, wireless communications have been evolved from GSM system (2G) to LTE-A networks (4G) with the major interest focusing on the throughput related criteria. 5G communication networks, however, extend to a 3-D performance metric cube based on throughput, number of links and delay simultaneously. Moreover, 5G networks confronts a wider range of future applications, including person-to-person, person-to-machine, or even machine-to-machine types. To deal with all the above cases and challenges, we start from the investigation of most popular 5G scenarios and identify the requirements on the energy efficiency, reliability and delay. Detailed technologies are surveyed and discussed thereafter, to achieve critical requirements and facilitate the fundamental tradeoffs among 3-D performance criteria. Through this study, we hope to shed some lights on the novel 5G communication system design, and further pave the way towards energy efficient, low latency and high reliable communication networks. Index Terms—5G, energy efficient, low latency, high reliable, non-orthogonal multiple access
I. I NTRODUCTION In the past few decades, wireless communications have been evolved from GSM system (2G) to LTE-A networks (4G) with the major interest focusing on the throughput related criteria. While in the pioneering work of the next generation (5G) communications, researchers start to consider a 3-D performance metric cube based on throughput, number of links and delay simultaneously. With the vision to have 1000 times throughput enhancement, 100 billions connections and closeto-zero delay, 5G wireless communication networks attracted global research attention in the recent years. For example, the European Union initiated an integrated project in the seventh framework program (FP7) named “Mobile and wireless communications Enablers for the Twenty-twenty Information Society (METIS) ” [1], which aims to lay the foundation of 5G mobile and wireless communications system, and the Chinese government formed an official promotion group named “International Mobile Telecommunications Twenty-Twenty” (IMT 2020) for joint 5G research and standard promotion. In addition, industrial partners from vendors and operations are highly motivated to investigate their internal research programs and actively joining collaborative research projects for 5G communications, including Ericsson, Huawei, T-Mobile and others. It is also promising to see that with the booming of mobile internet, 5G wireless communications will confront a wider
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range of future communication applications, including personto-person (P2P), person-to-machine (P2M), or even machineto-machine (M2M) types, which raises additional challenges for the innovative network design. To name a few, vehicle-tovehicle (V2V) communications requires high reliable guarantee for emergency cases, and real-time industrial control or virtual reality gaming demands for extremely low latency. Typical applications like sporting events and video conferencing may strike a balance between the reliability and delay requirements. Moreover, with the massive deployment of small cells and access points, energy efficiency problem [2] will be another important aspect to investigate, which is also shown to be the key performance metric of 5G systems [3]. Nevertheless, the above three metrics may tradeoff with each other, and thus there will be no unified solution to solve all the issues and satisfy all the requirements from all the scenarios. To tackle the above issues, naturally we need to figure out a series of technologies, including the physical layer coding and modulation up to all the network topology and routing schemes. In this article, we start from the investigation of most popular 5G scenarios and identify the requirements on the energy efficiency, reliability and delay. Detailed technologies are surveyed and discussed thereafter to achieve critical requirements and facilitate the tradeoff among 3-D performance criteria, which further paves the way for energy efficient, low latency and high reliable communication networks. The rest of the paper is organized as follows. We first summarize the potential important scenarios according to the underlying communication behaviors and propose framework solutions for 5G communication systems in Section II. Typical technology families from the network architecture, the medium access control (MAC) mechanism and the physical layer schemes are elaborated in Section III to Section V. We give some concluding remarks in Section VI. II. S CENARIOS , TARGETS AND F RAMEWORK S OLUTIONS Apart from the traditional 4G personal communications, 5G aims to exploit more communication opportunities in the social activities and penetrates into nearly everyone’s daily lives. However, it makes more challenging to predict the complicated 5G communication behaviors, either for P2P, P2M or M2M communications. In this article, we investigate the evolutions of human activities and generate the implicit rules based on the current observations.
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Smart Living People in 2020 shall enjoy a more convinient life than the current days. For example, the medical cares may happen at home through remote connections and most of the buildings may equipped with smart devices or sensors. Virtual reality (VR) gaming, real-time VR conferences and other communication aided applications will eventually penetrate to every corner of our daily lives. Smart Transportation Transportation systems will be upgraded to facilitate the intelligent communications among different cars. Typical applications may include the emergency reporting, automatic cruise control as well as augmented reality (AR) navigation, which eventually provides more comfortable driving experiences in 2020. Smart Control Another important aspect shall happen in the industrial control systems for emergency cases. Timely and accurate reactions need to be performed when the environment induced turbulence occurs. Meanwhile, major sporting events or festival ceremony may require the similar control techniques.
Fig. 1 summarizes the most popular scenarios based on the current survey results, where we categorize them into three typical classes based on the underlying communication principles. The first class is called “Massive Reporting”, which corresponds to the massive uplink reporting and the large scale downlink transmission for smart control capabilities. The typical behaviors include massive uplink transmission requests and wide range downlink broadcasting phenomena. “Reliable M2M” is the second one, which targets to provide high reliable machine communications for bidirectional information exchange. The third class named “Real-time Virtualization” is focusing on the VR applications for driving, gaming or conferencing, which basically relies on the point-to-point information streaming. Table I provides an overview of different classes and describes the corresponding design targets measured by link densities, delay and reliability requirements. In order to achieve the aforementioned goal and balance the tradeoffs among different criteria for 5G communication
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systems, framework solutions with flexible and reconfigurable properties shall be considered as top priority and the associated challenging problems may include: 1) How to balance the control signal overhead and the massive access capability? 2) How to reduce the multiple access delay and meet the “closeto-zero” end-to-end delay requirement? 3) How to deliver information reliably before the transmission deadline? 4) How to operate the network efficiently when massive cells are deployed? In what follows, we focus on layered structures to organize the potential research directions, which are dedicated to solve the associated issues in network, medium access and physical layers. III. N ETWORK A RCHITECTURE Mobile internet has changed our daily lives significantly and triggered large scale deployment of wireless communication networks. Compared with traditional mobile networks, today’s network architecture is much more complicated, either in terms of network functionalities or inter-connection relations. To facilitate the flexible network control and reduce the network capital and operational expenditures, two complimentary concepts, “Software Defined Networks (SDN)” [4] and “Network Functionality Virtualization (NFV)” [5] have been promoted by both the IT and the telecom industries, and the standardization process has also been initiated in the various organizations, such as European Telecommunications Standards Institute (ETSI) [5] or Internet Engineering Task Force (IETF) [4]. Nevertheless, in the wireless base stations or access points, upper layer information with different network functionalities will be jointly transmitted using a uniform physical layer protocol and the quality of services is guaranteed through control plane signaling. This approach requires the control plane signaling overhead to proportionally increase with the total amount of transmitted data. Moreover, in the massively deployed communication networks, roaming-induced handover signaling problems become more serious and incur the so called “signaling storm” in the radio network controllers.
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Fig. 2. A candidate network architecture for 5G communication systems, where the SDN, NFV and control-data separation technique are adopted. To be more specific, SDN and NFV focus on the functionality virtualization and hardware normalization in the core network side, while the control plane and data plane separation are mainly implemented in the radio access network side. Through the proper cooperation between newly developed core network and radio access network, 5G user equipments can enjoy a much better quality of experience, e.g. with lower access delay and higher reliability.
As a result, the radio access network architecture for 5G communication shall align with the development of SDN and NFV, and decouple the control plane signaling from the traditional transmission approaches to avoid the mentioned issues. Fig. 2 provides a candidate solution for 5G communication networks. In order to extend the control and data separation concept in the radio access networks, [6] suggested a unified framework to jointly deliver the wireless signals based on the proposed functionality separation concept. Through this approach, the traditional admission control and system information maintenance will be covered by the control base stations and the data base stations can focus on pure data transmission, which results in lower transmission delay and higher reliability. Meanwhile, by adopting this framework, the data base stations can highly adapt to the traffic variations via efficient switch on/off schemes and offer much more energy efficient networks. Within the separation framework, several interesting research problems are still open in the literature. For example, the efficient network deployment problems, the optimized
Fig. 3. Software-defined MAC protocols for 5G communication systems, where different base stations can equipped with different MAC protocols to deliver different types of data information. For example, as shown in the figure, the streaming packet with large payload (pico cell) may need reliable MAC header and additional redundancy check bits while the voice or instant message applications with tiny packet size (macro cell) may get rid of those unnecessary overheads.
routing and coordination protocols and the smart backhaul mechanisms, which are shown to be the key enabling technologies for for the separated base stations. We refer interested readers to [6] and references therein for more detailed discussions. IV. MAC M ECHANISMS One of the most critical tasks for 5G MAC layer design is to provide diversified delay and reliability requirements. In order to balance this tradeoff, traditional ways to provide uniform package and equal redundancy check is no longer sustainable and the most promising solution is intended to introduce a flexible MAC protocol design with dynamic scheduling and variable packet length characteristics. However, to achieve such a goal is not as simple as performing some marginal modifications due to the coordination nature among different MAC functions and the interaction property with the lower physical layer transmissions. In this section, we introduce two important research directions for the flexible MAC layer design in the following part. The straight forward way to design the MAC mechanisms for 5G communication systems is to incorporate the software defined MAC concept. Different data transmission requirements will be allocated to different types of base stations with local caching capabilities as shown in Fig. 3. Each base station employs a reconfigurable MAC protocol according to the data applications, e.g., the streaming packet with large payload may need reliable MAC header and additional redundancy check bits while the voice or instant message applications
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with tiny packet size may get rid of those unnecessary overheads. Through the software defined MAC approach, specific traffic quality requirements can be supported by different MAC protocols with different MAC headers, payload sizes or redundancy check bits, which balances the tradeoffs among delay, reliability and energy efficiency. Another potential direction for designing the novel MAC mechanisms is to consider the “user-centric” scheduling, where the radio resource management and the transmission mode adaptation are mainly moved from the base station to the user terminal side. In the “user-centric” scheduling, the user terminal exploits available resources, make the scheduling decision according to the current traffic requirements and notifies the final result to the base station. Compared with the current “network-centric” solution, “user-centric” approach avoids the traditional resource request/grant procedures and reduces the random access as well as the queueing delays. However, to facilitate this type of scheduling mechanism, we need several pre-requested conditions. The first one is to have sufficient physical layer resources with smart distributive resource management schemes to avoid the collision problem in the random access procedures. Smooth handover mechanism is selected as the second one to handle the user roaming related issues, including the discontinuous transmission and the cooperative reception. Other conditions shall also include the forward error detection and notification process, the packet retransmission and the efficient exponential backoff protocol. The above discussions brings two novel directions, which requires significant changes in the current MAC layer procedures. Other potential MAC layer evolution possibilities may include the ultra fast HARQ protocol or the low overhead or even zero overhead handover procedures. In addition, since the physical layer for 5G systems has not been finalized yet, it is also possible to introduce additional MAC control parameters for advanced signal transmission or processing. V. P HYSICAL L AYER S CHEMES To have a better matching with the flexible MAC mechanisms, the physical layer design in 5G systems shall provide a tunable air interface design with adaptable frame structures, waveforms and other transmission technologies. In this section, instead of proposing a fully integrated software defined air interface, which is so far open in the literature, we elaborate several key technology components as follows. One of the key enabling technologies in 5G physical layer is to adapt the frame structure according to the MAC payload. Conventional approach using fixed frame structure relies on the coding and modulation schemes to adapt the transmission rate with constant control overhead1 . While in the massive reporting scenarios with huge small packet delivering, new frame structure with limited control overhead and shorter transmission durations needs to be identified. Meanwhile, short block length causes significant performance degradation 1 The control overhead in the physical layer frame include the cyclic prefix, the pilot symbols, and the transmission mode indicators in frame control areas.
for regular channel coding schemes, which motivates extra research efforts for the next generation channel coding technologies, including polar code [7] and multi-dimensional lowdensity parity check code [8]. New waveforms are considered as the second key enabling technology in the physical layer. In 4G communication systems, out-of-band (OOB) power emission is often regarded as one of the practical threats for the OFDM-based waveform and the traditional approach to deal with this issue is to introduce the guard band intervals, which requires up to 10% margin of the total bandwidth. Recently, the filtering based waveform is proposed to tackle the OOB leakage problems by introducing the non-orthogonality among neighboring subcarriers. For example, Generalized Frequency Division Multiplexing (GFDM) [9], as a generalization of traditional OFDM waveform, is invented to achieve higher spectrum efficiency by OOB elimination in a broadband system and Filter-Bank Multi-Carrier (FBMC) [10] forces zero energy at the integer multiples of the symbol period by jointly designing two half-Nyquist filters at the transmitter and the receiver, which provides an exciting approach to completely remove CP while still maintaining the reasonable OOB performance in the frequency selective environment. Other transmission technologies in the 5G research are dedicated to create more transmission resources to improve the delay and reliability performance. The brute-force ways to increase the radio resources are focused in the frequency domain (e.g., the utilization of millimeter wave [11]) or the spatial domain (e.g., the massive MIMO technology [12] or the ultra dense network deployment [13]). Another way to create virtual resources is to realize the non-orthogonality property in the user domain, where the spreading sequence/matrix technique is applied to enhance the number of simultaneous serving links. For example, [14] proposed an enhanced CDMA technology in OFDM systems and [15] proposes a possible solution to use Low Density Signature (LDS) based spreading technique for CDMA systems with reasonable multi-user interference performance loss and affordable computational complexity, and [16] extends this idea to OFDM systems. However, in LDS-OFDM systems, entries of the signature matrix in LDS-OFDM systems are restricted to zero or one only. If we consider the downlink transmission direction, modulated symbols from different users will be naturally coupled together at the transmitter side, which may cause significant performance degradation. To solve this issue, sparse code multiple access (SCMA) [17] was recently proposed to allow complex entries in the signature matrix and each user enjoys a different codebook to avoid the coupling issues as shown in Fig. 4. The previous discussions are mainly point-wise technologies with limited interactions. In order to provide a unified physical layer transmission framework, the coexistence issues among the above solutions and the backward compatibility shall be carefully investigated. Besides, the additional influences on the MAC and higher layer protocols, the feasibility for multipoint/multi-channel coordinations, and the hardware imple-
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Fig. 4. An illustrative example of the sparse code multiple access scheme. In this example, we have six independent users with six SCMA codebooks to access four radio resource elements simultaneously. Each user select one codeword from its own codebook based on the information bits and the receiver can jointly decodes all the information bits from the coupled observations.
mentation complexity issues are important research directions in the near future. VI. C ONCLUSION In this paper, we survey various application scenarios for future 5G communication networks. To design an energy efficient, low latency and high reliable networks, we adopt topdown approach to consider the potential framework solutions from network architecture, MAC mechanisms as well as physical layer schemes. In particular, the control and data plane separated network with software defined MAC, the “usercentric” scheduling as well as the adaptive air interface are investigated. By integrating all the valuable solutions together, we hope to make 5G as an energy efficient, low latency and high reliable communication networks.
[8] L. M. Zhang and F. R. Kschischang, “Multi-Edge-Type Low-Density Parity-Check Codes for Bandwidth-Efficient Modulation,” IEEE Trans. Commun., vol. 61, no. 1, pp. 43 – 52, Jan. 2012. [9] M. Kasparick, G. Wunder, Y. Chen, F. Schaich, T. Wild, and et. al., “5G Waveform Candidate Selection D3.1”, 5Gnow, Nov. 2013. [Online]. Available: http://www.5gnow.eu/system/files/5GNOW D3.1 v1.1 1.pdf [10] B. Farhang-Boroujeny, “OFDM Versus Filter Bank Multicarrier ,” IEEE Signal Process. Mag., vol. 28, no. 3, pp. 92 – 112, May 2011. [11] A. Ghosh, T. A. Thomas, and et. al., “Millimeter-Wave Enhanced Local Area Systems: A High-Data-Rate Approach for Future Wireless Networks,” vol. 32, no. 6, pp. 1152 – 1163, Jun. 2014. [12] E. Larsson, O. Edfors, F. Tufvesson, and T. Marzetta, “Massive MIMO for Next Generation Wireless Systems,” IEEE Commun. Mag., vol. 52, no. 2, pp. 186 – 195, Feb. 2014. [13] A. Osseiran, F. Boccardi, and et. al., “Scenarios for 5G Mobile and Wireless Communications: the Vision of the METIS Project,” IEEE Commun. Mag., vol. 52, no. 5, pp. 26 – 35, May 2014. [14] Y. Zhou, T. S. Ng, J. Wang, K. Higuchi, and M. Sawahashi, “OFCDM: A Promising Broadband Wireless Access Technique,” IEEE Commun. Mag., vol. 46, no. 3, pp. 38 – 49, Mar. 2008. [15] R. Hoshyar, F. Wathan, and R. Tafazolli, “Novel Low-Density Signature for Synchronous CDMA Systems over AWGN Channel,” IEEE Trans. Signal Process., vol. 56, no. 4, pp. 1616 – 1626, Apr. 2008. [16] R. Razavi, M. AL-Imari, M. A. Imran, R. Hoshyar, and D. Chen, “On Receiver Design for Uplink Low Density Signature OFDM (LDSOFDM),” IEEE Trans. Commun., vol. 60, no. 11, pp. 3499 – 3508, Nov. 2012. [17] H. Nikopour and H. Baligh, “Sparse Code Multiple Access,” in IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), London, Sep. 2013, pp. 332 – 336.
ACKNOWLEDGEMENT This paper is in part supported by the National High Technology Research and Development Program of China (863 Program No. 2012AA011400) and the National Basic Research Program of China (973 Program No. 2012CB316000). R EFERENCES [1] A. Osseiran and et. al., “The METIS 2020 Project – Laying the foundation of 5G”, METIS 2020, Nov. 2012. [Online]. Available: https://www.metis2020.com/ [2] Y. Chen, S. Zhang, S. Xu, and G. Y. Li, “Fundemantal Tradeoffs on Green Wireless Networks,” IEEE Commun. Mag., vol. 49, no. 6, pp. 30 – 37, Jun. 2011. [3] “5G Radio Access – Research and Vision”, Ericsson White Paper, June 2013. [Online]. Available: http://www.ericsson.com/news/1306255g-radio-access-research-and-vision 244129228 c [4] D. Jacobs, IETF SDN: I2RS uses traditional routing protocols in software networks, TechTarget, 2010. [Online]. Available: http://searchsdn.techtarget.com/tip/IETF-SDN-I2RSuses-traditional-routing-protocols-in-software-networks [5] ETSI, Network Functions Virtualisation, ETSI, 2014. [Online]. Available: http://www.etsi.org/technologies-clusters/technologies/nfv [6] X. Xu, G. He, S. Zhang, Y. Chen, and S. Xu, “On Functionality Separation for Green Mobile Networks: Concept Study over LTE,” IEEE Commun. Mag., vol. 51, no. 5, pp. 82 – 90, May 2013. [7] S. B. Korada, E. Sasoglu, and R. Urbanke, “Polar Codes: Characterization of Exponent, Bounds, and Constructions,” IEEE Trans. Inf. Theory, vol. 56, no. 12, pp. 6253 – 6264, Dec. 2010.
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