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.23919@JCC.2019.08.

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.23919@JCC.2019.08.
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  China Communications • August 2019  1 Keywords: 6G visions; THz communications; symbiotic radio; satellite-assisted communica- tions; articial intelligence; machine learning .    NTRODUCTON It is foreseen that the global IP trafc in 2022 will be threefold of that in 2017 and will reach about 400 exabytes (EB) per month [1]. Notably, the traffic from wireless and mobile devices accounts for 71 percent of the total IP trafc, which is largely driven by the expansion of Internet of Things (IoT), the in - crease of powerful mobile devices like smart  phones and tablets, as well as the popularity of content-based applications like YouTube and Netix [2]-[5]. To accommodate the enor  - mous trafc, in recent years, the engineers and researchers from both industry and academia are developing innovative technologies, con - ducting testbed experiments, and creating new international telecommunication standards for the fth-generation (5G) mobile communica - tion. Abstract:  With a ten-year horizon from con - cept to reality, it is time now to start thinking about what will the sixth-generation (6G) mobile communications be on the eve of the fth-generation (5G) deployment. To pave the way for the development of 6G and beyond, we provide 6G visions in this paper. We rst introduce the state-of-the-art technologies in 5G and indicate the necessity to study 6G. By taking the current and emerging development of wireless communications into consider- ation, we envision 6G to include three major aspects, namely, mobile ultra-broadband, super Internet-of-Things (IoT), and artificial intelligence (AI). Then, we review key tech - nologies to realize each aspect. In particular, teraherz (THz) communications can be used to support mobile ultra-broadband, symbiotic radio and satellite-assisted communications can be used to achieve super IoT, and machine learning techniques are promising candidates for AI. For each technology, we provide the  basic principle, key challenges, and state-of-the-art approaches and solutions. Received: Mar. 10, 2019Revised: May 10, 2019Editor: Honggang Zhang I NVITED P APER  Ying-Chang Liang IEEE Fellow, Professor, University of Electronic Sci-ence and Technolo-gy of China 6G Visions: Mobile Ultra-Broadband, Super Internet- of-Things, and Articial Intelligence Lin Zhang 1 , Ying-Chang Liang 1, *, Dusit Niyato 2 1 University of Electronic Science and Technology of China, Chengdu 611731, China 2  Nanyang Technological University, 639798, Singapore* The corresponding author, email: liangyc@ieee.org  China Communications • August 2019 2 access to a variety of platforms. Typical evolutions for the 5G core network include mobile edge computing (MEC), software defined networking (SDN), network func - tion visualization (NFV), and network slic - ing.The rst 5G standard is the 3rd generation  partnership project (3GPP) Release 15 (R15), which has been frozen in June 2018 and will  be deployed in around 2020. This standard mainly focuses on the eMBB scenario, in which the data-rate is the key metric. Howev - er, the requirements for mMTC and URLLC are more challenging. Meanwhile, both the ap -  plication demands and business models for the mMTC and URLLC usage scenarios are not fully developed by now. Thus, 3GPP R15 just reuses the long-term evolution (LTE) based narrow-band IoT (NB-IoT) and enhanced MTC (eMTC) in 3GPP R14 for the mMTC, which only guarantees the same reliability and latency for the URLLC as those in the eMBB. Enhanced technologies for the mMTC and URLLC usage scenarios may be discussed in 3GPP R16 or R17.With a ten-year horizon from concept to reality, it is time now to start thinking about what the sixth-generation (6G) mobile com - munications will be on the eve of the fth-gen - eration (5G) deployment. Recently, different organizations are initializing the study of 6G. For instance, the International Telecommuni - cation Union (ITU) has established a new ITU Focus Group for Network 2030, which aims to guide the global information and communica - tions technology (ICT) community in studying the network capability for the year 2030 and  beyond. This means that the extensive re - search on 6G is on the way. To pave the way for 6G and beyond, we provide visions and key technologies of 6G in this paper.By taking the current and emerging de - velopment of wireless communications into consideration, 5G is far from meeting the re - quirements in the future due to the following three shortages. Firstly, new applications in the future such as holography may require a data rate up to Terabits per second, which is Generally, 5G has three typical usage scenarios [6]: enhanced mobile broadband (eMBB), massive machine-type communica - tions (mMTC), and ultra-reliable low-latency communications (URLLC). In particular, eMBB is to provide high data rates for mo -  bile users (up to 1G bits per second), mMTC focuses on the number of connected ma - chine-type devices in IoT (up to 1 million wireless connections per kilometer square), and URLLC puts an emphasis on the reliabil - ity and latency for real-time applications such as vehicular network and industry automation (the reliability and latency are respectively in the orders of 99 . 999% and milliseconds). To satisfy the requirements in these usage scenar  - ios, the 5G evolution is mainly from three as -  pects: spectrum bands, new radio, and 5G core network.• 5G spectrum bands can be divided into sub-GHz, 1-6GHz and above 6GHz. Sub-GHz bands are suitable to provide a wide coverage for machine-type devices in IoT due to good attenuation properties of the signals propagated at these frequencies. The 1-6GHz bands offer a reasonable balance  between the coverage and data-rate for 5G services. Spectrum bands above 6GHz have large bandwidths, and thus can provide high data-rates to support eMBB applications within a limited coverage.• 5G new radio targets new technologies to enhance the transmission efficiency, e.g., high end-to-end data-rate, low latency, and low energy consumption. Related technol - ogies include ultra-dense heterogeneous network, millimeter wave (mmWave) com - munication, massive multiple-input-mul - tiple-output (MIMO), scalable orthogonal frequency division multiplexing (OFDM) waveform, and non-orthogonal multiple ac - cess (NOMA).• 5G is designed to satisfy diverse demands from various applications within a single 5G core network. As such, the 5G core net - work needs to provide multiple new func - tionalities, such as agile resource allocation, exible network reconguration, and open  China Communications • August 2019  3  band is still far from meeting the bandwidth requirements in 6G. To further increase the  bandwidth and boost the data rate, 6G is envi - sioned to utilize THz band, in which the avail - able bandwidth is theoretically three orders of magnitude higher than that in the mmWave  band [8], [9], [10], [11]. In fact, IEEE has formed a dedicated study group under IEEE 802.15 for the THz spectral allocations and standardizations. Likewise, multiple compa - nies like Huawei and Intel are conducting in - door/outdoor experiments in these bands. 2.1 Challenges and state-of-the-arts In the past decades, it is challenging to gen - erate THz signals and develop a full-function THz communication system due to the lack of THz transceivers, making the THz band one of the least-studied electromagnetic spectrum [8], [9]. Nevertheless, with the technological advances, THz communication is envisioned to become reality in the next few years and will become mature in 6G era [10], [11]. 1) THz sources: Since THz band is between the microwave frequency band and the opti - cal frequency band, the THz is too high for electronics-based devices which is used to generate microwave signals, and is too low for photonics-based devices which is used to generate optical signals [9]. As such, existing terahertz signals are generated either from electronics-based devices via frequency multi -  plication or from the photonics-based devices via photomixing. Recently, graphene-based technology emerges as a promising candidate to generate THz signals due to the extraordi - nary electro-optical properties of graphene. The generated THz signals can be divided into two categories: pulsed signal and continuous signal. In particular, the pulsed signal is wide - ly studied in the existing literature on THz communications since it can be generated with reasonable size and complexity of the trans - mitter/antenna [12]. In contrast, the generation of the continuous signal has a more stringent requirement on the size and complexity of the transmitter/antenna and thus needs more investigations to facilitate the THz communi - around three orders higher than the data rate of 5G. Secondly, as the exponentially increase and expansion of IoT devices in the future, it is urgent to further enhance both the con - nection capability and coverage of 5G IoT. Thirdly, current network configurations/opti - mizations are typically achieved in a manual manner. The manual network configurations/optimizations are no longer suitable to the wireless network in the future, which is surely ultra-large-scale, and has complicated/mul - tidimensional/dynamic profiles in terms of, for instance, user demands, radio resource, trafc load, and network topology. As a result, 6G is expected to provide proper solutions to overcome these shortages. In particular, 6G is defined to include three major aspects, namely, mobile ultra-broadband, super Inter  - net-of-Things (IoT), and articial intelligence (AI) as shown in gure 1, in which typical use cases for each aspect in 6G are envisioned. The mobile ultra-broadband can provide Tera -  bits per second wireless transmissions. Super IoT can enhance the connection capability and the coverage of the current IoT. AI can cong - ure/optimize the wireless network in the future in an intelligent manner. More specifically, teraherz/THz communications can be used to support mobile ultra-broadband, symbiotic radio and satellite-assisted communications can be used to achieve super IoT, and machine learning techniques are promising candidates for AI. For each technology, we provide the  basic principle, key challenges, and state-of-the-art approaches and solutions in the rest of the paper. .   THz   C OMMUNCATONS In general, low-frequency spectrum band has a preferable propagation property to support wide coverage, but achieves a low transmis - sion rate due to the relatively narrow band - width. With the explosive increase of high data rate demands, 5G is suggested to make use of the mmWave band, which is able to  provide new bandwidths in the order of sev - eral gigahertz [7]. Nevertheless, the mmWave  China Communications • August 2019 4 cations in the future. 2) Path loss: THz signals suffer from a high free-space path loss, which arises from both the spreading loss and molecular absorption loss [13]. In particular, the spreading loss is caused by the expansion of the electromag - netic wave in the space and increases quadrat - ically with the operating frequency and the distance between two communication nodes  based on Friis’ law. The molecular absorption loss is caused by the fact that partial energy of THz signals is converted into the internal kinetic energy of the molecular in the air, and is dominated by the percentage of water vapor molecules. If we denote  L  s   as the spreading loss and denote  L a   as the molecular absorption Fig. 1.  Typical use cases in 6G visions which include three aspects: mobile ultra-broadband, super IoT, and  AI. Fig. 2.  Path loss in THz band [14].  China Communications • August 2019  5 the molecular absorption noise is colored, since different types of molecules poses dif  - ferent resonant frequencies and absorption  properties. By taking the peculiar behaviors of both the path loss and the noise into con - sideration, [16] derived the network capacity of a THz communication system with pulsed signals. Besides, experiments are conducted to measure and analyze the THz channel capacity in [17], [18], [19]. 2.2 Potential applications 1) Ultra-broadband wireless communica-tions: THz band can meet ultra-high data-rate requirements. Typical scenarios include ultra-high data-rate small cells, wireless per  - sonal area networks. In small cells, a THz access point can support bandwidth-intensive applications like virtual/augmented reality and holographic remote for both static and mobile users within a small coverage (in the order of meters). Meanwhile, THz band is an attractive spectrum resource to meet the fronthaul/back  - haul capacity requirements of access points in - stead of a wired solution, especially when the wired solution is infeasible or its deployment cost is unacceptably high. In wireless personal loss, we have  L  s  =     4 π  c fd  2  and  Le a  =  Kfd  () , where c is the speed of light,  f is the operat - ing frequency, d is the distance between two communication nodes, and  K  (  f ) is the overall absorption coefficient of the medium [14]. Meanwhile, we have the free-space path loss as  L  p   =  L  s  L a , which is typically high and limits the communication distance d  between two communication nodes. As shown in figure 2, we provide the path loss of THz communica -tions for different distances and at different frequencies. From the figure, the free-space  path loss can easily exceeds 80 dB when the communication distance d is as small as 1 meter. In other words, THz band can only support short-distance (in the order of meters and below) rather than medium/long-distance wireless communications. To combat the dis - tance limitation, various techniques are inves - tigated [15], such as distance-aware physical layer design, ultra-massive MIMO communi - cation, reflectarrays, and intelligent surfaces. In addition, the molecular absorption causes  peaks on path loss curves, which forms trans - mission windows, i.e, bandwidth segments. For the distance d  below 1 meter, the effect of the molecular absorption is ignorable and the transmission window is almost 10 THz. For the distance over 1 meter, the effect of the molecular absorption becomes signicant and transmission windows become narrow. 3) Channel capacity: Different from the wireless receiver operating on the microwave  band, which suffers from only the additive Gaussian white noise (AWGN), there exists two kinds of noise at the wireless receiver operating on the THz band, i.e., the molecular absorption noise and the AWGN. In particular, the molecular absorption noise is produced  by the vibrating molecules that reradiate  part of their absorbed energy. Since different molecules may produce different amounts of absorption noise, the molecular absorption noise is determined by both the types and the amounts of the molecules experienced by the THz wave beam. It is worth pointing out that Fig. 3.  Typical network topologies of the symbiotic radio.

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