城市軌道交通車地通信綜合承載系統(LTE-M)性能測試與分析

朱東飛 洪 婷

(武漢地鐵集團有限公司,430030,武漢∥第一作者,副總工程師)

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城市軌道交通車地通信綜合承載系統(LTE-M)性能測試與分析

朱東飛 洪 婷

(武漢地鐵集團有限公司,430030,武漢∥第一作者,副總工程師)

在城市軌道交通系統中,各子系統車地通信模塊是完全獨立的。獨立建網造成城市軌道交通沿線纜線密集,結構復雜,對珍貴的頻率資源也造成了極大的浪費。目前車地無線普遍采用基于IEEE 802.11的WLAN(無線局域網)技術,該技術存在頻點干擾嚴重、高速性能瓶頸等重要缺陷。為了解決這些問題,將LTE(長期演進)應用到城市軌道交通的車地無線通信系統是很有必要的。設計了基于LTE的城市軌道交通車地通信綜合承載系統 (LTE-M),并在武漢地鐵進行了LTE-M系統的性能測試。測試結果表明:LTE-M系統具有系統穩定、綜合承載能力強的特點,不僅能滿足CBTC(基于通信的列車自動控制)系統傳輸要求,同時還具有為PIS(乘客信息系統)和CCTV(閉路電視監控)系統提供良好通道的能力。

城市軌道交通; 車地通信; 綜合承載

Author′s address Wuhan Rail Transit Group Co.,Ltd.,430030,Wuhan,China

0 引言

車地通信是城市軌道交通安全運營的關鍵。而目前城市軌道交通CBTC(基于通信的列車運行控制)、CCTV(閉路電視監控)和PIS(乘客信息系統)等系統的車地通信,大部分使用的是在公共開放頻段工作的局域網技術。目前,實踐證明,基于WLAN(無線局域網)的車地通信網絡是滿足城市軌道交通高速、高密度和高安全性等需求的最佳技術之一[2]。但這種技術不是完美的,也存在一些局限性。比如,WLAN使用的是免費開放頻段,不需要無線電委員會的準許。也正因如此,很多民用設備也工作在這一開放頻段,如便攜式WiFi(Wireless Fidelity)設備、電磁設備等,這些設備都可能干擾車地無線傳輸。還有就是,WLAN不是為高速移動設計的[2]。2012年深圳地鐵2、5號線就曾因上述原因數次出現無線干擾,影響列車運行[3-4]。此外,WLAN不能設置優先級,不能確保給優先級較高的業務更大真實帶寬,因此不適合車地通信的綜合承載。

LTE(長期演進)是基于OFDMA(正交頻分復用多址接入)技術、由3GPP(第三代合作伙伴計劃)制定的全球通用技術標準。LTE技術在設計之初,就考慮了滿足高吞吐率的需求。在20 MHz帶寬組網情況下,上下行峰值速率分別可達100 Mbit/s和50 Mbit/s。并且采用扁平化架構以減少控制平面和用戶平面時延。LTE采用了OFDM(正交頻分復用)、HARQ(混合反饋重發) 、MIMO(多輸入多輸出)等先進技術,這些技術能夠大幅提升頻譜效率、傳輸速率和抗干擾能力,同時能夠支持綜合業務承載(滿足不同優先級合理分配調度和高移動速度特性),并采用安全機制和抗干擾的辦法確保無線數據傳輸的安全性和可靠性。

TD-LTE是TDD(時分復用)的LTE技術,它是一種專門為移動高寬帶應用而設計的無線通信標準,是中國擁有核心自主知識產權的4G(第4代移動通信技術)國際通信標技術標準。本文提出的基于LTE的城市軌道交通車地無線通信綜合承載系統(LTE for Metro,LTE-M) 正是利用了TD-LTE技術。

1 城市軌道交通車地生產業務以及服務質量(QoS)需求分析

城市軌道交通中的車地通信承載了CBTC、CCTV、PIS和TOSM(列車運行狀態監測)系統等4項基本業務。

CBTC技術主要包括無線電通信技術和自動化控制技術。CBTC車地通信系統的功能是完成車站設備、中心設備、車載設備和軌旁設備之間的信息傳輸,支持點對點、點對多點之間數據通信,支持對一列車、一組列車或者所有列車有選擇地進行通信。CBTC系統要具有控制列車安全行駛的功能,車地通信業務應滿足以下要求:① 車地通信覆蓋區域應包括正線(折返線、聯絡線)、段(場)咽喉區、段(場)車庫、出入段線、出入場線、試車線;② 采用獨立的雙網冗余信道;③ 為了保證無線網絡的安全性,基站與車載無線單元要進行授權認證,認證通過后才可以關聯,并加密傳輸數據,加密密鑰至少為128位;④ 車頭和車尾能夠與A網、B網雙網通信,信號系統無線網絡部分與其它部分應隔離。還有,單網傳輸速率上下行分別至少達到100 kbit/s;單網信息丟包率要低于1%,單網信息誤碼率小于或等于10;單網跨區切換時間為100 ms以內,信息經有線和無線網絡的時延應在150 ms之內;應實現200 km/h行駛速度范圍內實時雙向通信。

在全自動無人駕駛系統中,CCTV系統起著至關重要的作用。通過車地之間的視頻傳輸,地面控制中心可以實現對駕駛室、設備間、車廂等重點區域的監控,滿足列車安全管理的需求,是實現列車安全運行的重要輔助手段。在正常情況下,如果控制中心需要查看車廂視頻信息,列車需向控制中心上傳不少于2路客室監控畫面,每路上行車載CCTV業務帶寬需求為1～2 Mbit/s,同時要求監控視頻圖像清晰、無卡頓。

PIS系統,在正常情況下播放控制中心下發的節目,在異常情況下播放乘客通知和發布運營服務信息。如在車載PIS顯示屏上實時顯示緊急信息、行車信息、換乘信息、旅行指南、新聞、廣告等信息。車地無線綜合寬帶傳輸平臺要適配PIS的不間斷、低時延、高帶寬需求。每路圖像帶寬需求為下行4～6 Mbit/s。緊急文本為上行信息,帶寬需求為10 kbit/s。

TOSM系統主要有信息采集、信息傳輸、信息顯示、信息處理分析和信息發布5個工作流程,通過車輛狀態信息做到安全投入的效益優化。 TOSM系統需要采集1 500個開關量和500個模擬量。因此要求傳輸帶寬應保證在100 kbit/s,系統車地無線通信的丟包率應低于1%。

總結城市軌道交通車地通信業務的QoS需求如表1所示。

目前,城市軌道交通車地通信業務都采用各自獨立的車地通信系統。這種獨立的建網方案存在的問題是:設備數量大、故障隱患多;頻譜利用率低;投資建設時間長;維護難度大,成本高;系統缺乏可擴展能力。

另一方面,城市軌道交通車地通信系統需要承載綜合業務,所承載業務中CBTC系統信息、緊急文本信息、CCTV系統信息等,直接涉及安全問題,對傳輸的可靠性和實時性都有很高的要求。其中CCTV和PIS還要求滿足一定傳輸帶寬。要進行綜合承載業務的高可靠、高實時、高帶寬傳輸,接收信號的SINR(信號與干擾加噪聲功率比)必須滿足一定的要求。除了進行合理的網絡規劃保證接收信號的功率超過一定的接收門限,還要求接收信號的干擾功率足夠小。這就要求要有一個相對干凈的無線傳輸環境保證傳輸的正常進行。現有的ISM(工業、科學和醫療)頻段干擾嚴重,難以滿足綜合承載業務傳輸的要求。因此,有必要設立專用的頻段進行城市軌道交通綜合業務的傳輸,以減少無線信號干擾,提高車地通信的可靠性,保證綜合業務的傳輸,滿足承載綜合業務的要求。

鑒于建立綜合承載系統以及設立專用的頻段進行綜合業務的傳輸的必要性,提出業界基于LTE的車地通信綜合承載系統LTE-M,下文進行具體說明。

2 城市軌道交通車地通信綜合承載系統LTE-M

2.1 系統結構設計

目前,LTE-M主要有兩種組網方案。

方案一:同頻交織組網(見圖1)。該方案中,軌旁設備在一個無線接入網下連接成二層網絡,二層網絡的基站依照站址交織放置,并且采用同樣的載波頻率來配置。

圖1 同頻交織組網示意圖

相鄰的RRU(無線射頻單元)與不同的無線基站BBU(射頻拉遠單元)連接,單個BBU有問題時,相鄰RRU增大發射功率從而覆蓋故障BBU下RRU覆蓋區域。未出現設備問題時,減低發射功率防止相鄰基站互相干擾；當有基站問題時,相鄰基站增大發射功率覆蓋故障基站覆蓋區域。這種組網方式的突出特點是頻率資源利用率高,因為全網只用一個頻段;缺點是增加了不穩定性,因為軌旁設備沒有設備備份,若兩個相鄰基站失效,則系統將無法正常工作。因此本次沒有采用這種方案。

方案二:同站址雙網覆蓋(見圖2)。此方案系統為 A、B雙網(兩張網完全獨立,并行且互不干涉)。每張網包括了核心網(EPC)、軌旁無線接入網(eNodeB)、車載無線終端(TAU)。A網絡只承載CBTC業務,B網則備份CBTC業務和承載PIS業務。

2.2 專用頻段選擇

根據我國有關頻率資源劃分情況,目前可申請的城市軌道交通TD-LTE頻段有:1 447～1 467 MHz(固定移動用戶頻段),1 785～1 805 MHz(行業專網頻段),5 850～5 920 MHz (TD-LTE可用頻段)。

圖2 同站址雙網覆蓋示意圖

因5.9 GHz頻段的空間傳輸損耗過大,且硬件設備仍不成熟,所以1.4 GHz和1.8 GHz更適用于城市軌道交通。已經有城市申請和使用1.8 GHz頻段。工信部在最近發布的文件中提出,城市軌道交通可以申請1.8 GHz頻段。

為進一步驗證LTE技術應用于城市軌道交通綜合業務承載的可行性,在武漢地鐵開展了現場實地測試。采用實際工程的組網結構,測試LTE-M系統在真實環境中的性能。然后結合目前城市軌道交通車地通信綜合承載業務的需求進行分析,最后判斷其應用的可行性。

2.3 測試方案和工具

測試搭建的LTE-M系統采用A、B雙網冗余的組網方式,一同承載測試業務數據。A網用15 MHz帶寬承載CBTC業務、車載CCTV和PIS等業務;B網用5 MHz帶寬承載CBTC業務、緊急文本。每個網絡都包含EPC、BBU、RRU以及車載無線終端(TAU)。BBU通過以太網交換機直接與兩套LTE核心網設備連接,使用光纜連接至軌旁RRU。

測試工具采用業界著名的Ixchariot,服務器端設置在地面,分別在地面和車載布置測試節點。

2.4 測試內容

本次現場實地測試是在真實城市軌道交通環境下,測試LTE-M系統的覆蓋能力、系統的車地通信傳輸性能以及穩定性。測試的內容包括LTE-M的無線覆蓋測試、傳輸性能測試和穩定性測試。

3 測試結果與分析

3.1 LTE-M無線覆蓋測試

LTE-M無線覆蓋測試,使用1臺場強接收儀(羅德斯瓦茨)和1臺PC機實現,并將1臺GPS(全球定位系統)接收機連接到PC機以獲取當前位置。場強接收儀帶寬設置為5 MHz,取樣間隔設置為:10 ms的情況下,列車從1個RRU運動到相鄰RRU時的接收功率變化。由圖3可知,在5 MHz帶寬下,接收功率的變化范圍為-95～-33 dBm(藍色區域),圖中白色曲線表示平均功率在-58～-38 dBm范圍內。

圖3 5 MHz帶寬下接收功率覆蓋范圍

3.2 LTE-M傳輸性能測試

傳輸性能測試項為丟包率、傳輸時延和吞吐量等。

3.2.1 丟包率測試

測試步驟為:① 按照基本配置完成系統配置;② 記錄設備型號、軟件版本、RSRP(參考信號接收功率)、SINR、系統帶寬、AMC(自適應調制編碼)參數、ICIC(小區間干擾協調)是否關閉;③ 測試軟件與網絡側進行時間同步;④ UE(用戶設備)發起業務;⑤ 使用Ixchariot測試丟包率,包長400 B,丟包率測試應使用100 kbit/s速率發包;⑥ 在試驗段多次運行列車,過程中進行丟包測試,結束后,查看測試結果;⑦ 列車以最高限速進行。

測試結果表明:CBTC系統車地通信的上下行丟包率都小于0.01%,滿足系統要求。

3.2.2 傳輸延遲測試

測試步驟為:① 按照基本配置完成系統配置;② 記錄設備型號、軟件版本、RSRP、SINR、系統帶寬、AMC自適應參數、ICIC是否關閉;③ 測試軟件與網絡側進行時間同步;④ UE發起業務;⑤ 使用測試軟件測試RTT(往返傳輸時間)時延,包間隔為100 ms大小為400 B。

測試結果表明:CBTC系統車地通信的平均環回時延為32～43 ms,最大環回時延為295 ms。系統傳輸時延滿足CBTC系統小于150 ms的需求。

3.2.3 吞吐量測試

測試步驟為:① 按照網絡基本配置完成系統配置;② 記錄設備型號、軟件版本、RSRP、SINR、系統帶寬、AMC自適應參數、ICIC是否關閉;③ UE使用模擬軟件發起業務包長1 400 B,飽和流量;④ 采集各層吞吐量；⑤ 列車以最高限速運行,運行6圈;⑥ 雙網同時進行測試,A網測試5 M帶寬,B網測試15 MHz帶寬;⑦ 通過測試軟件觀察吞吐率是否正常。

測試結果表明:系統頻寬為5 MHz時,終端上行平均吞吐率約為5 Mbit/s;系統頻寬為15 MHz時,終端上行平均吞吐率為17 Mbit/s。統頻寬為5 MHz時,下行平均吞吐率為8～11 Mbit/s;系統頻寬為15 MHz時,終端下行平均吞吐率為19 Mbit/s。

由測試結果可知:LTE系統傳輸吞吐量滿足綜合承載需求。

3.3 LTE-M系統穩定性測試

LTE-M系統穩定性測試要求:當列車行駛時,系統中同時加載CBTC、PIS、CCTV和列車運行狀態監測業務。如表2所示,以能夠滿足小區和BBU之間發生200次切換的時間長度作為系統運行的最小持續時間,結果顯示整個測試過程無通信中斷發生。另一方面,通過分別模擬單EPC、單BBU和單RRU等故障情況,測試了冗余網絡的功能。測試結果表明:在單網故障情況下冗余網絡仍然可以正常工作,列車與控制中心之間的數據鏈路能夠保持連續不中斷。

表2 LTE-M系統穩定性測試

4 結語

本文設計了基于LTE的城市軌道交通車地通信綜合承載系統(LTE-M)，在武漢地鐵進行了實地測試。試驗結果滿足預期,驗證了本文設計的LTE-M系統的性能能夠滿足城市軌道交通業務需求。下一階段,將繼續加速推進LTE-M的技術規范,以規范和指導LTE-M系統的設計、研究工作。

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[2] ZHU L,YU F R,NING B,et al.Cross-layer handoff design in MIMO enabled WLANs for communication-based train control (CBTC) systems[J].IEEE J.Sel.Areas Commun.,2012,30(4):719-728.

[3] ZHU L,YU F R,NING B,et al.Cross-layer design for video transmissions in metro passenger information systems[J].IEEE Transactionson Vehicular Technology.2011,60(3):1171-1181 .

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Commentary

Advanced Informationization Network Architectures are the Foundation of “Smart Metro”

LI Zhonghao

(Deputy Director of Academic Committee of Experts of China Association of Metros)

The modernization construction of China′s urban rail transit began in 1994. After more than 20 years′ efforts, China has become a country with the longest total length of urban rail transit operating lines in the world. The urban rail transit network operation′s forming has become a symbol of first-tier and second-tier cities in China. By the end of “the 13thFive-Year Plan”, the total length of urban rail transit operating lines in Chongqing, Nanjing, Wuhan, Chengdu, Changsha, and Xi′an cities, etc. each will be more than 300 km. The total length of urban rail transit lines in medium- and long-term planning in Beijing, Shanghai, Guangzhou and Shenzhen cities, etc. each will be even more than 1 000 km. This trend will continue for 10～20 years. Especially under the guidance of national policies, such as “Smart Cities” and “Integration of Informationization and Industrialization” (referring to the deeply-combining of informationization and industrialization at the high level), etc., the field of urban rail transit is also pursuing realizing “Smart Metro”, and is pursuing “Intelligent Equipment”. However, the current operation mode of rail transit in most of the cities in China still stays in the single-line operation era. There is still a big gap in the informationization level and the construction speed. Without the foundation of information system and without the advanced network architectures, it is difficult to realize the potential value of large data. “Smart Metro” will only be a castle in the air.

The information systems of urban rail transit are deployed respectively in the external service network, the internal service network and the safety production network. Because of the historical reasons, China′s urban rail transit is constructed in a way that a project′s approving, designing and constructing is separately implemented according to a single line, one line by one line. Therefore, their information systems are mainly deployed on the production networks according to their professional separation. In the 1990s, the signal systems of urban rail transit and AFC systems in China mainly purchased foreign equipment, so we have no ability to realize information systems′ being integrated and optimized. In recent years, with the continuous expansion of the urban rail transit construction scale, the construction speed′s continuous acceleration and the networking operation demand′s increasingly pressing, the seriousness of the urban rail transit informationization network architecture problem has gradually emerged. It has become the urgent task to accelerate the informationization construction of urban rail transit.

The core of the informationization network architectures′ integration of urban rail transit is on the safety production network. Integrating the safety production network inevitably brings about the adjustment of organizational relationship. For example, the wireless transmission between on-board and ground in the CBTC (communication-based train automatic control) system in urban rail transit, under the broadband LTE system, is needed to load comprehensively. This would break the original professional divisions of the signaling system. In the wired networks, it is necessary both to integrate the various professional IT network architectures of this line, and to integrate the network of other lines. The important systems of production and operation of a city′s rail transit will be integrated into one network. The integration of IT network architectures is the upgrading of informationization level, and is even the management revolution, involving the various professions of vehicles, machinery, engineering and electricity. It is a “Top Leadership” project.

Integrating many professional information systems together so as to realize the sharing of information inevitably involves the information security problem. The more information could be shared, the more attention should be paid to information security. For example, in the urban rail transit production system, it must be made clear that CBTC and SCADA (Supervisory Control and Data Acquisition) systems must conform to the 3rd level requirements in the national information security classification protection system. But if each line and each profession respectively realizes the 3rd level requirements of classification protection, then it is very possible to harass the people and waste money, and moreover to cause a lonely information island. It also needs to pay a greater price to realize the information sharing. If all the businesses of a city′s rail transit are integrated into one network, and that belongs respectively to the external service network, the internal service network and the safety production network. As a result, the classification protection requirements of the enterprise networks could be overall realized. Thus, yielding twice the result with half the effort will be achieved.

Some people would ask, “Could the existing networks independent to each other be integrated together under the condition of not affecting the urban rail transit operation if the newly-built urban rail transit lines are constructed according to the new informationization network architectures?” It depends on whether the newly-built networks and their information security guarantee measures are in place and whether they are more secure, reliable, smooth and easier to maintain and manage than the original systems. This would require that the maintenance service system and the management system of information security of the IT network should be established synchronously when the new-type IT networks of urban rail transit are constructed. In this respect, the national railways have much successful experience and that has been proved feasible.

Only if the network architectures of the production system have been integrated, the network architectures of all kinds of lines have been integrated into a production network, the safety production network, the internal service network and the external service network have been integrated into a internal network, and on the basis of such networks, the design and the deployment of information security architectures are implemented, could we stand in the forefront of information technology development, for example, using private clouds, realizing information sharing, and exploring large data applications, etc. Otherwise, each specialty′s implementing 3rd level of hierarchical protection, cloud computing and large data applications, etc. in a very small network scope will only be a kind of technology-following. It is difficult to produce a very good application effectiveness and efficiency, and the intelligent degree of “Smart Metro” will also be greatly reduced.

The next 5 years is 5 years of the unprecedented large-scale constructions of urban rail transit. Many cities each will open more than 200 km operating lines. We must seize this opportunity of a lifetime, with “Smart Metro” as the goal, start from Informationization network architectures, information security and IT services, stand on solid ground, and actively advance, so as not to fail to live up to the historical responsibility the times give us.

(Translated by SUN Zheng)

Test and Analysis of Integrated Service Capacity for Train-ground Communication Based on Metro LTE-M System

ZHU Dongfei,HONG Ting

Since the module of each communication subsystem is completely independent in urban rail transit system, the independent network building causes densely-laid cables along metro rail and complex structure, wasting a large amount of precious frequency resources. At present, the technology of WLAN based on IEEE802.11 is widely used in the field of train-ground wireless, which has some major defects like serious frequency interference, high performance bottleneck and so on. To solve these problems, the use of LTE (long term evolution) in urban rail transit wireless communication system is recommended, an integrated transport service capacity system (LTE-M) for urban rail transit LTE-based LTE is designed, and the performance of the LTE-M system is tested in Wuhan metro. The result shows that the LTE-M system is stable with good integrated service capacity, which can not only satisfy the transmission requirements of CBTC system, but also provide better channels for PIS (passenger information system) and CCTV.

urban rail transit; train-ground communication; integrated service capacity

U 231.7

10.16037/j.1007-869x.2017.05.037

2017-01-05)