高速铁路LTE系统可靠传输链路研究
摘要
现有的铁路通信系统GSM-R在高速度、高密度的高速铁路列车运行中,存在诸多挑战与需求,如高清视频监控行车安全,高度冗余与纠错的通信系统,时刻保障通信可靠性等。LTE系统以其显著的高数据传输速率和较低的系统时延,已经被国际铁路联盟确定为下一代铁路通信系统的演进技术。过渡阶段,可以将LTE作为GSM-R的冗余热备份网络,在异常情况下,由LTE进行语音和部分数据业务的传输,从而保障铁路通信正常进行。
本文对基于LTE技术的高速铁路通信系统进行分析介绍,描述其网络架构及关键技术,探讨了从GSM-R向LTE-R演进的可行性,并给出了可能的演进模式。
为了克服无线信道时变衰减给信号带来的影响,LTE系统采用HARQ技术来提高系统吞吐量和可靠性。然而,在LTE无线通信系统应用到高速铁路环境时,较高的移动速度、复杂的沿线环境将给系统HARQ性能带来较大的不利影响。本文首先对LTEHARQ在高速铁路场景下的性能进行了分析,结果表明,相对于不采用HARQ技术,带有HARQ机制的系统具有较低的误块率,较高的吞吐量；然而,相比于公网静止用户场景,高铁移动用户的性能有所下降；之后提出了一种增强小区重叠区HARQ传输性能的机制,即在重叠区某一位置开始降低最大重传次数,同时保存之后传输错误的和切换前未来得及确认的数据包,待接入目标小区后再由目标eNodeB重发这些数据包,进而在车载用户设备端合并解码。性能分析表明,保存错误包待接入目标小区后合并解码可使这些数据包在目标小区重发时的错误率降低55%,提升了吞吐量。相比于不降低最大传输次数,此方案在切换触发前给了新数据包更多的传输机会,使得接入目标eNodeB重传前转数据包时,吞吐量提升了12%左右。
同时,高铁下快时变信道对LTE话音业务QoS保障提出了严峻的考验,本文针对这一问题进行了研究。整理分析了目前一些针对VoIP的降低延时、提高可靠性的研究方向,然后重点从调度方面入手,先就高斯白噪声信道,分析了调制编码方式对VoIP可靠性的影响,进而扩展到高速场景,结合HARQ技术,通过建立高铁信道下VoIP传输分析模型,对其性能进行分析,得出调度时为VoIP业务选择低阶MCS,可以降低中断率,减少传输次数,进而降低时延；最后针对话音业务,给出了一种新的MCS选择策略,可以在提高可靠性的同时不消耗过多的资源。
The existing railway communication system, GSM-R (GSM for Railway), is facing with lots of challenges and requirements from the high-density of high speed trains, such as high-definition video surveillancing trains' safety, high redundant communication system with error correction, always ensuring communication reliability. LTE (Long Term Evolution) system, with its remarkable high data rate and low delay, has been adopted as the next generation of railway communication system by International Union of railway. In a period of transition, LTE can be used as the redundant hot backup, when GSM-R is abnormal, the transmission of voice and part of data service can be replaced through LTE network. Thus the railway communication can be operated correctly.
First of all, this thesis introduces the high-speed railway communication system based on LTE, describes its network architecture, and analyzes the key technologies. It also discusses feasibility of the GSM-R system's evolution to LTE-R and proposes the possible evolution model.
In order to overcome the effect of time-varying wireless channel attenuation to the signal, LTE adopts HARQ (Hybrid Automatic Retransmission reQuest) to improve the system throughput and reliability. However, in high-speed railway environment, the HARQ performance of LTE wireless communication system is deteriorated due to the high speed and the complicated railway environment. In this thesis, the HARQ performance in high-speed railway environment is analyzed. It shows that the system with HARQ technology has lower block error rate and higher throughput, compared to the case without it. However, the HARQ performance in high-speed environment is not as good as that in the scenario of public network, where most of the users are with low-speed. And then, a novel HARQ scheme is proposed, which could enhance the HARQ performance in the overlapping region. In this scheme, the number of maximum retransmission is reduced when the train arrives at a predefined location, and the packets, which suffer transmission error or can not be acknowledged before handover triggering, will be saved by the system, and retransmitted by the target eNodeB (after the train accesses it), and then combined and decoded by the onboard UE (User Equipment). Performance analysis shows that the error rate decreases by55%when these packets are retransmitted in the target cell. Compared to the situation with fixed number of maximum retransmission, the proposed scheme provides more opportunities for new packets to be transmitted before handover, thus the throughput increases by about 12%when target eNodeB retransmits those forward packets.
In the meantime, the QoS guarantee for LTE Voice faces severe challenge in the time-varying channel of high-speed railway. Some possible solutions of reducing delay and improving reliability are proposed and analysed. This thesis fucoses on the scheduling, discusses the impact of modulation and coding scheme on the reliability of VoIP in AWGN first, and then extends to high-speed scenario. The VoIP transmission combined with HARQ is modeled. The performance results show that a low-level modulation and coding scheme can reduce the outage probability and decrease the number of transmissions, thereby the delay is decreased. Finally, the thesis presents a novel MCS selection criterion for LTE VoIP, which can improve reliability without consuming more resource.
本文对基于LTE技术的高速铁路通信系统进行分析介绍,描述其网络架构及关键技术,探讨了从GSM-R向LTE-R演进的可行性,并给出了可能的演进模式。
为了克服无线信道时变衰减给信号带来的影响,LTE系统采用HARQ技术来提高系统吞吐量和可靠性。然而,在LTE无线通信系统应用到高速铁路环境时,较高的移动速度、复杂的沿线环境将给系统HARQ性能带来较大的不利影响。本文首先对LTEHARQ在高速铁路场景下的性能进行了分析,结果表明,相对于不采用HARQ技术,带有HARQ机制的系统具有较低的误块率,较高的吞吐量；然而,相比于公网静止用户场景,高铁移动用户的性能有所下降；之后提出了一种增强小区重叠区HARQ传输性能的机制,即在重叠区某一位置开始降低最大重传次数,同时保存之后传输错误的和切换前未来得及确认的数据包,待接入目标小区后再由目标eNodeB重发这些数据包,进而在车载用户设备端合并解码。性能分析表明,保存错误包待接入目标小区后合并解码可使这些数据包在目标小区重发时的错误率降低55%,提升了吞吐量。相比于不降低最大传输次数,此方案在切换触发前给了新数据包更多的传输机会,使得接入目标eNodeB重传前转数据包时,吞吐量提升了12%左右。
同时,高铁下快时变信道对LTE话音业务QoS保障提出了严峻的考验,本文针对这一问题进行了研究。整理分析了目前一些针对VoIP的降低延时、提高可靠性的研究方向,然后重点从调度方面入手,先就高斯白噪声信道,分析了调制编码方式对VoIP可靠性的影响,进而扩展到高速场景,结合HARQ技术,通过建立高铁信道下VoIP传输分析模型,对其性能进行分析,得出调度时为VoIP业务选择低阶MCS,可以降低中断率,减少传输次数,进而降低时延；最后针对话音业务,给出了一种新的MCS选择策略,可以在提高可靠性的同时不消耗过多的资源。
The existing railway communication system, GSM-R (GSM for Railway), is facing with lots of challenges and requirements from the high-density of high speed trains, such as high-definition video surveillancing trains' safety, high redundant communication system with error correction, always ensuring communication reliability. LTE (Long Term Evolution) system, with its remarkable high data rate and low delay, has been adopted as the next generation of railway communication system by International Union of railway. In a period of transition, LTE can be used as the redundant hot backup, when GSM-R is abnormal, the transmission of voice and part of data service can be replaced through LTE network. Thus the railway communication can be operated correctly.
First of all, this thesis introduces the high-speed railway communication system based on LTE, describes its network architecture, and analyzes the key technologies. It also discusses feasibility of the GSM-R system's evolution to LTE-R and proposes the possible evolution model.
In order to overcome the effect of time-varying wireless channel attenuation to the signal, LTE adopts HARQ (Hybrid Automatic Retransmission reQuest) to improve the system throughput and reliability. However, in high-speed railway environment, the HARQ performance of LTE wireless communication system is deteriorated due to the high speed and the complicated railway environment. In this thesis, the HARQ performance in high-speed railway environment is analyzed. It shows that the system with HARQ technology has lower block error rate and higher throughput, compared to the case without it. However, the HARQ performance in high-speed environment is not as good as that in the scenario of public network, where most of the users are with low-speed. And then, a novel HARQ scheme is proposed, which could enhance the HARQ performance in the overlapping region. In this scheme, the number of maximum retransmission is reduced when the train arrives at a predefined location, and the packets, which suffer transmission error or can not be acknowledged before handover triggering, will be saved by the system, and retransmitted by the target eNodeB (after the train accesses it), and then combined and decoded by the onboard UE (User Equipment). Performance analysis shows that the error rate decreases by55%when these packets are retransmitted in the target cell. Compared to the situation with fixed number of maximum retransmission, the proposed scheme provides more opportunities for new packets to be transmitted before handover, thus the throughput increases by about 12%when target eNodeB retransmits those forward packets.
In the meantime, the QoS guarantee for LTE Voice faces severe challenge in the time-varying channel of high-speed railway. Some possible solutions of reducing delay and improving reliability are proposed and analysed. This thesis fucoses on the scheduling, discusses the impact of modulation and coding scheme on the reliability of VoIP in AWGN first, and then extends to high-speed scenario. The VoIP transmission combined with HARQ is modeled. The performance results show that a low-level modulation and coding scheme can reduce the outage probability and decrease the number of transmissions, thereby the delay is decreased. Finally, the thesis presents a novel MCS selection criterion for LTE VoIP, which can improve reliability without consuming more resource.
引文
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[2]钟章队,李旭,蒋文怡.铁路综合数字移动通信系统.中国铁道出版社,2003：159～172
[3]International Union of Railways (UIC), GSM-R procurement guide V. 5.0-01.02.2007:112-114. www.uic.asso.fr
[4]沈嘉,索士强,全海洋,赵训威,胡海静,姜怡华.3GPP长期演进(LTE)技术原理与系统设计.人民邮电出版社.2008：
[5]郑砍,汪玲,王文博.3GPP长期演进系统中的混合自动重传请求技术.中兴通讯技术.2007,13(5)：1
[6]马霓,邬钢,张晓博,张学军.LTE-UMTS长期演进理论与实践.人民邮电出版社.2009：63～65
[7]Tero Henttonen, Kennett Aschan, Jani Puttonen, Niko Kolehmainen. Performance of VoIP with mobility in UTRA Long Term Evolution. IEEE Vehicular Technology Conference (VTC-Spring 2008), May 2008:2492-2496
[8]赵飞.LTE混合自动请求重传技术.哈尔滨工程大学工程硕士论文.2010：39-49
[9]汤毅.LTE系统中HARQ技术及其相关技术研究.西南交通大学硕士论文.2010：37-42
[10]Xiangming Yu, Kaiying Sun, Dongfeng Yuan. Comparative Analysis on HARQ with Turbo Codes in Rician Fading Channel with Low Rician Factor.12th IEEE International Conference on Communication Technology (ICCT 2010), Nov. 2010:342-345
[11]Kian Chung Beh, Angela Doufexi, Simon Armour. Performance evolution of hybrid ARQ schemes of 3GPP LTE OFDMA system.18th annual IEEE international Symposium on personal, Indoor and Mobile Radio Communications (PIMRC 2007), 2007:1-5
[12]王玲,朱光喜,吴伟民,张毅杰.LTE系统跨层HARQ重传技术研究.小型微型计算机系统.2010,11(31)：2222-2226
[13]李垠韬,任皓,张忠皓,常永宇,杨大成.一种CO1辅助的HARQ改进方案.现代电信科技.2009(10)：54-58
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