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Table of Content

    25 December 2021, Volume 19 Issue 4
    Special Topic
    Editorial: Special Topic on OTFS Modulation for 6G and Future High Mobility Communications
    YUAN Jinhong, FAN Pingzhi, BAI Baoming, AI Bo
    2021, 19(4):  1-2.  doi:10.12142/ZTECOM.202104001
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    A Survey on Low Complexity Detectors for OTFS Systems
    ZHANG Zhengquan, LIU Heng, WANG Qianli, FAN Pingzhi
    2021, 19(4):  3-15.  doi:10.12142/ZTECOM.202104002
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    The newly emerging orthogonal time frequency space (OTFS) modulation can obtain delay-Doppler diversity gain to significantly improve the system performance in high mobility wireless communication scenarios such as vehicle-to-everything (V2X), high-speed railway and unmanned aerial vehicles (UAV), by employing inverse symplectic finite Fourier transform (ISFFT) and symplectic finite Fourier transform (SFFT). However, OTFS modulation will dramatically increase system complexity, especially at the receiver side. Thus, designing low complexity OTFS receiver is a key issue for OTFS modulation to be adopted by new-generation wireless communication systems. In this paper, we review low complexity OTFS detectors and provide some insights on future researches. We firstly present the OTFS system model and basic principles, followed by an overview of OTFS detector structures, classifications and comparative discussion. We also survey the principles of OTFS detection algorithms. Furthermore, we discuss the design of hybrid OTFS and orthogonal frequency division multiplexing (OFDM) detectors in single user and multi-user multi-waveform communication systems. Finally, we address the main challenges in designing low complexity OTFS detectors and identify some future research directions.

    Signal Detection and Channel Estimation in OTFS
    NAIKOTI Ashwitha, CHOCKALINGAM Ananthanarayanan
    2021, 19(4):  16-33.  doi:10.12142/ZTECOM.202104003
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    Orthogonal time frequency space (OTFS) modulation is a recently proposed modulation scheme that exhibits robust performance in high-Doppler environments. It is a two-dimensional modulation scheme where information symbols are multiplexed in the delay-Doppler (DD) domain. Also, the channel is viewed in the DD domain where the channel response is sparse and time-invariant for a long time. This simplifies channel estimation in the DD domain. This paper presents an overview of the state-of-the-art approaches in OTFS signal detection and DD channel estimation. We classify the signal detection approaches into three categories, namely, low-complexity linear detection, approximate maximum a posteriori (MAP) detection, and deep neural network (DNN) based detection. Similarly, we classify the DD channel estimation approaches into three categories, namely, separate pilot approach, embedded pilot approach, and superimposed pilot approach. We compile and present an overview of some of the key algorithms under these categories and illustrate their performance and complexity attributes.

    Message Passing Based Detection for Orthogonal Time Frequency Space Modulation
    YUAN Zhengdao, LIU Fei, GUO Qinghua, WANG Zhongyong
    2021, 19(4):  34-44.  doi:10.12142/ZTECOM.202104004
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    The orthogonal time frequency space (OTFS) modulation has emerged as a promising modulation scheme for wireless communications in high-mobility scenarios. An efficient detector is of paramount importance to harvesting the time and frequency diversities promised by OTFS. Recently, some message passing based detectors have been developed by exploiting the features of the OTFS channel matrices. In this paper, we provide an overview of some recent message passing based OTFS detectors, compare their performance, and shed some light on potential research on the design of message passing based OTFS receivers.

    Performance of LDPC Coded OTFS Systems over High Mobility Channels
    ZHANG Chong, XING Wang, YUAN Jinhong, ZHOU Yiqing
    2021, 19(4):  45-53.  doi:10.12142/ZTECOM.202104005
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    The upcoming 6G wireless networks have to provide reliable communications in high-mobility scenarios at high carrier frequencies. However, high-mobility or high carrier frequencies will bring severe inter-carrier interference (ICI) to conventional orthogonal frequency-division multiplexing (OFDM) modulation. Orthogonal time frequency space (OTFS) modulation is a recently developing multi-carrier transmission scheme for wireless communications in high-mobility environments. This paper evaluates the performance of coded OTFS systems. In particular, we consider 5G low density parity check (LDPC) codes for OTFS systems based on 5G OFDM frame structures over high mobility channels. We show the performance of the OTFS systems with 5G LDPC codes when sum-product detection algorithm and iterative detection and decoding are employed. We also illustrate the effect of channel estimation error on the performance of the LDPC coded OTFS systems.

    Coded Orthogonal Time Frequency Space Modulation
    LIU Mengmeng, LI Shuangyang, ZHANG Chunqiong, WANG Boyu, BAI Baoming
    2021, 19(4):  54-62.  doi:10.12142/ZTECOM.202104006
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    Orthogonal time frequency space (OTFS) modulation is a novel two-dimensional modulation scheme for high-Doppler fading scenarios, which is implemented in the delay-Doppler (DD) domain. In time and frequency selective channels, OTFS modulation is more robust than the popular orthogonal frequency division multiplexing (OFDM) modulation technique. To further improve transmission reliability, some channel coding schemes are used in the OTFS modulation system. In this paper, the coded OTFS modulation system is considered and introduced in detail. Furthermore, the performance of the uncoded/coded OTFS system and OFDM system is analyzed with different relative speeds, modulation schemes, and iterations. Simulation results show that the OTFS system has the potential of full diversity gain and better robustness under high mobility scenarios.

    OTFS Enabled NOMA for MMTC Systems over LEO Satellite
    MA Yiyan, MA Guoyu, WANG Ning, ZHONG Zhangdui, AI Bo
    2021, 19(4):  63-70.  doi:10.12142/ZTECOM.202104007
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    As a complement of terrestrial networks, non-terrestrial networks (NTN) have advantages of wide-area coverage and service continuity. The NTN is potential to play an important role in the 5G new radio (NR) and beyond. To enable the massive machine type communications (mMTC), the low earth orbit (LEO) satellite is preferred due to its lower transmission delay and path loss. However, the LEO satellite may generate notable Doppler shifts to degrade the system performance. Recently, orthogonal time frequency space (OTFS) modulation has been proposed. It provides the opportunity to allocate delay Doppler (DD) domain resources, which is promising for mitigating the effect of high mobility. Besides, as the LEO satellite constellation systems such as Starlink are thriving, the space spectrum resources have become increasingly scarce. Therefore, non-orthogonal multiple access (NOMA) is considered as a candidate technology to realize mMTC with limited spectrum resources. In this paper, the application of OTFS enabled NOMA for mMTC over the LEO satellite is investigated. The LEO satellite based mMTC system and the OTFS-NOMA schemes are described. Subsequently, the challenges of applying OTFS and NOMA into LEO satellite mMTC systems are discussed. Finally, the potential technologies for the systems are investigated.

    Orthogonal Time Frequency Space Modulation in Multiple-Antenna Systems
    WANG Dong, WANG Fanggang, LI Xiran, YUAN Pu, JIANG Dajie
    2021, 19(4):  71-78.  doi:10.12142/ZTECOM.202104008
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    The application of the orthogonal time frequency space (OTFS) modulation in multiple-antenna systems is investigated. The diversity and/or the multiplexing gain can be achieved by deploying various multiple-antenna techniques, and thus the reliability and/or the spectral efficiency are improved correspondingly. We provide two classes of OTFS-based multiple-antenna approaches for both the open-loop and the closed-loop systems. Specifically, in the open-loop system, a transmitting diversity approach, which resembles the space-time coding technique, is proposed by allocating the information symbols appropriately in the delay-Doppler domain. In the closed-loop system, we adopt the Tomlinson-Harashima precoding in our derived delay-Doppler equivalent transmission model. Numerical evaluations demonstrate the advantages of applying the multiple-antenna techniques to the OTFS. At last, several challenges and opportunities are presented.

    Review
    Study on Security of 5G and Satellite Converged Communication Network
    YAN Xincheng, TENG Huiyun, PING Li, JIANG Zhihong, ZHOU Na
    2021, 19(4):  79-89.  doi:10.12142/ZTECOM.202104009
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    The 5G and satellite converged communication network (5G SCCN) is an important component of the integration of satellite-terrestrial networks, the national science, and technology major projects towards 2030. Security is the key to ensuring its operation, but at present, the research in this area has just started in our country. Based on the network characteristics and security risks, we propose the security architecture of the 5G SCCN and systematically sort out the key protection technologies and improvement directions. In particular, unique thinking on the security of lightweight data communication and design reference for the 5G SCCN network architecture is presented. It is expected to provide a piece of reference for the follow-up 5G SCCN security technology research, standard evolution, and industrialization.

    Research Paper
    Payload Encoding Representation from Transformer for Encrypted Traffic Classification
    HE Hongye, YANG Zhiguo, CHEN Xiangning
    2021, 19(4):  90-97.  doi:10.12142/ZTECOM.202104010
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    Traffic identification becomes more important, yet more challenging as related encryption techniques are rapidly developing nowadays. Unlike recent deep learning methods that apply image processing to solve such encrypted traffic problems, in this paper, we propose a method named Payload Encoding Representation from Transformer (PERT) to perform automatic traffic feature extraction using a state-of-the-art dynamic word embedding technique. By implementing traffic classification experiments on a public encrypted traffic data set and our captured Android HTTPS traffic, we prove the proposed method can achieve an obvious better effectiveness than other compared baselines. To the best of our knowledge, this is the first time the encrypted traffic classification with the dynamic word embedding has been addressed.

    AI-Based Optimization of Handover Strategy in Non-Terrestrial Networks
    ZHANG Chenchen, ZHANG Nan, CAO Wei, TIAN Kaibo, YANG Zhen
    2021, 19(4):  98-104.  doi:10.12142/ZTECOM.202104011
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    Complicated radio resource management, e.g., handover condition, will trouble the user in non-terrestrial networks due to the impact of high mobility and hierarchical layouts which co-exist with terrestrial networks or various platforms at different altitudes. It is necessary to optimize the handover strategy to reduce the signaling overhead and improve the service continuity. In this paper, a new handover strategy is proposed based on the convolutional neural network. Firstly, the handover process is modeled as a directed graph. Suppose a user knows its future signal strength, then he/she can search for the best handover strategy based on the graph. Secondly, a convolutional neural network is used to extract the underlying regularity of the best handover strategies of different users, based on which any user can make near-optimal handover decisions according to its historical signal strength. Numerical simulation shows that the proposed handover strategy can efficiently reduce the handover number while ensuring the signal strength.

    Truly Grant-Free Technologies and Protocols for 6G
    MA Yihua, YUAN Zhifeng, LI Weimin, LI Zhigang
    2021, 19(4):  105-110.  doi:10.12142/ZTECOM.202104012
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    The further integration of telecommunications and industry has been considerable and is expected to bring significant benefits to society and economics in 6G. It also forms some evolution trends for next-generation communication systems, including further rises in machine-type communications (MTC), uplink-dominated systems, and decentralized structures. However, the existing access protocols are not friendly to these trends. This paper analyzes the problems of existing access protocols and provides novel access technologies to solve them. These technologies include contention-based non-orthogonal multiple access (NOMA), data features, enhanced pilot design and successive interference cancellation (SIC) of diversity. With these key enablers, truly grant-free access can be realized, and some potential modifications of protocols are then analyzed. Finally, this paper uses massive and critical scenarios in digital transformations to show the great necessity of introducing novel access technologies into future communication protocols.

    Table of Contents, Volume 19, 2021
    Table of Contents, Volume 19, 2021
    2021, 19(4):  111-114.  doi:10.12142/ZTECOM.202104013
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