- Open Access
A CoMP soft handover scheme for LTE systems in high speed railway
© Luo et al; licensee Springer. 2012
- Received: 6 February 2012
- Accepted: 13 June 2012
- Published: 13 June 2012
With the development of high-speed railway and public growing demand on data traffic, people pay much more attention to provide high data rate and high reliable services under high mobility circumstance. Due to the higher data rate and lower system latency, long-term evolution (LTE) has been chosen as the next generation's evolution of railway mobile communication system by the International Union of Railways. However, there are still many problems to be solved in the high mobility applications of LTE, especially the higher handover failure probability, which seriously degrades the reliability of railway communication. This article proposes an optimized handover scheme, in which the coordinated multiple point transmission technology and dual vehicle station coordination mechanism are applied to improve the traditional hard handover performance of LTE. The scheme enables the high speed train to receive signals from both adjacent base stations and obtain diversity gain when it moves through the overlapping areas, so it improves the quality of the received signal and provides reliable communication between train and ground eNodeBs. Numerical analysis and simulation results show that the proposed scheme can decrease the outage probability remarkably during handover and guarantee the reliability of train to ground communication.
- soft handover
- coordinate multiple point transmission (CoMP)
- high mobility
- outage probability
Due to the lower energy consumption, less environmental pollution, larger transport capacity and more safety, railway transportation plays an important role for the development of country. Japanese Shinkansen, French TGV, German ICE and China Railway have achieved remarkable successes. Nowadays, the development of high-speed railway makes people's lives more and more convenient. Meanwhile, it puts forward higher requirements on high-2 speed railway communication services. The existing GSM for Railway (GSM-R) network is mainly based on the second-generation Global System for Mobile Communications (GSM), and its data rate is too low to meet the broadband mobile communication access and other value-added service demands of passengers. In order to provide broadband services and applications for users not only at home but also on trip, long-term evolution (LTE) has been chosen as the next generation's evolution of railway mobile communication system by International Union of Railways (UIC), which supports significant higher data rates and lower system latency. This article focuses on the applications of LTE technology in railway broadband wireless communication network.
Normally, the main problems caused by user's high-speed movement in cellular wireless communication system are over-frequent handover, Doppler shift and large penetration loss, among which over-frequent handover needs to be paid special attention as it seriously affects the communication quality of service (QoS) and traffic reliability. Currently only traditional hard handover scheme is supported in LTE, which encounters two challenges under high speed movement circumstance. On the one hand, the handover delay caused by hard handover is relatively large. The high-speed train passes through the overlapping areas so fast that the handover procedure can not be accomplished timely. On the other hand, the speed of MRS is so fast that it would miss the optimal handover position, which degrades the handover success probability. In order to overcome the challenges mentioned above, the existing handover scheme of LTE should be optimized to improve the handover success probability in high-speed movement circumstance.
Currently, more and more researches focus on the broadband communication access issues in railway communication. In , a fast handover algorithm suitable for dedicated passenger line is devised by setting a new neighboring list. However, the optimization of system parameters has limited improvement on the overall performance of system. A novel moving extended cell (MEC) concept is introduced in [2, 3], which is based on user centric virtual groups of adjacent cells that transmit the same data to the user. The MEC concept utilizes a mechanism for restructuring the virtual multi-cell area according to the user's mobility pattern. This handover scheme is supposed to support high end-user mobility in a 60GHz broadband pico-cellular radio-over- fiber network. Jeon and Sanghoon , Pabst et al.  have introduced relays to the process of handover in cellular networks in order to improve handover performance. In [6, 7], multiple radios in devices are exploited to eliminate handover latency. In the proposed approach, multi-scan nodes rely on using their (potentially idle) second wireless interface to opportunistically scan and pre-associate with alternative Access Points (APs) and eventually seamlessly handover ongoing connections. A novel handover scheme based on on-vehicle antennas is introduced in , and the numerical analysis results show that the proposed scheme can pre-trigger handover appropriately, which ensures a higher handover success rate and has an improvement on system throughput.
Coordinated multiple point transmission (CoMP) transmission and reception allows geographically separated base stations to joint sending data to one terminal and joint receiving data from one terminal, by which the inter-cell interference could be reduced and the system frequency spectral efficiency would be improved. In , CoMP is introduced to solve inter-cell interference issues, and the simulation results show that the CoMP schemes achieve different gains in average sector throughput and 5% edge-user throughput gain as compared to that of conventional pre-coding scheme. Zhou and Wan  proposed an approach to improve the throughput of systems applied CoMP by adjust Time advance (TA). A new handover scheme was designed for CoMP scenarios in , and the handover model of CoMP system was analyzed and the signaling transmission procedure of CoMP cooperating sets handover was designed. Unfortunately the system performance was not discussed.
From the analysis above we can see that, there are few researches on the application of base stations interaction and multiple vehicle stations cooperation. In order to take full advantage of the multiple base stations cooperation feature of CoMP systems under highspeed scenarios, this article proposes a seamless soft handover scheme based on CoMP, which allows the train to receive signals from both adjacent base stations when the train travels through the overlapping areas. Thus, the handover failure rate is degraded and the reliability of train to ground communication is guaranteed.
The rest parts of the article are arranged as follow: Section 2 introduces the current handover scheme in LTE systems. Section 3 describes the proposed handover scheme in detail. Section 4 analyzes the system performance. Section 5 illustrates the simulation results and the improvement achieved by the proposal. And finally Section 6 concludes the whole article.
As the train moves into the overlapping area from cell i, eNodeB i decides whether to handover or not according to the reported received signal strength indication (RSSI), reference signal received power (RSRP) or reference signal received quality (RSRQ) measurement information by the train and the radio resource management (RRM) information, as shown in Figure 1a. Once the handover is triggered, the train disconnects with eNodeB i and tries to synchronize with the target eNodeB j. Then the Mobility Management Entity (MME) switches communication route and the previous station eNodeB i releases both user plane resources and control plane resources when the handover procedure is completed, as shown in Figure 1b.
As analyzed above, the current handover scheme in LTE systems is a Break-Before-Make approach. The scheme allows the train to receive signals from only one base station at one time. It will cause a larger outage probability and interrupt latency, which severely affect the reliability of train to ground communication.
3.1 A co-channel deployment approach for railway communication
In order to achieve CoMP joint processing and transmission between the two adjacent eN- odeBs along the railway track, an interference-avoid co-channel deployment approach is proposed in this article.
3.2 Dual on-vehicle stations collaborative scheme
This scheme can solve the 'processing capacity bottle-neck' problem caused by the conventional single on-vehicle station scheme. Moreover, a good diversity gain would be obtained since the distance between Antennas 1 and 2 is far away enough.
3.3 CoMP based soft handover scheme
This article proposes a seamless soft handover scheme utilizing CoMP joint processing and transmission technology, which can significantly improve the handover performance when the train moves through the overlapping areas.
Summarily, the optimized handover scheme based on CoMP can significantly degrade the outage probability and improve the handover performance, by which the train can communicate with the two adjacent eNodeBs in the overlapping area. The scheme achieves a seamless handover performance as soft handover.
In high-speed environments, the Doppler Effects would lead to irreducible bit error rate (BER) which is called error floor [12, 13]. However, according to technical specifications (TS) of LTE [14, 15], the procedure of triggering handover contains three phases: the user equipments (UEs) measure the RSSI, RSRP or RSRQ, sent the measurement reports to source eNodeB, and then the radio resource control (RRC) of source eNodeB decides whether handover is triggered or not . The 3GPP evaluation documents  also point out that the handover measurement and radio link failure (RLF) only depend on the RSSI, RSRP or RSRQ. Though the BER performance would degrade the QoS, if the RSRP remains above a certain threshold for a fixed duration, the wireless link will be re-established and assured to complete the handover. At most of time, the high-speed train travels through the wide plain and viaduct, the line-of-sight (LOS) path experienced free-space loss only between MRS and BSs is available and there are few reflectors or scatterers. The major influence on wireless channel caused by relative motion between transmitter and receiver is Doppler shift instead of Doppler spread. Therefore in high-speed railway scenario, instead of considering Doppler Effects which degrades BER, we only need to consider Doppler shift which would impair handover performance. In , Doppler shift in the overlapping region of two neighboring eNodeBs (handover region) is almost unchanged, and can be compensated . In this article we suppose that Train Control Information, such as train's velocity and location, can be shared by the ground eNodeBs, and Doppler shift has been compensated [20, 21].
where P t is the transmit power in dBm, PL(i, x) is the path loss between eNodeB i and location x, and A(i,x,σ) is the shadow component at the location x, generally modeled as a Gaussian random variable with mean zero and standard deviation σ. Typically, σ is 6 or 8dBin .
where a2 + b2 = 1. Note that i is the link number. ξ0 is common to both A1 and A2, ξ i represents the independent part between the two adjacent eNodeBs. Furthermore, both ξ0 and ξ i are independent Gaussian random variables with mean zero and standard deviation σ. Meanwhile, because the tail of Gaussian distribution extends to infinity, a fade margin F dB is added to the transmit power.
4.1 Outage in the CoMP handover scheme
4.2 Outage in current hard handover scheme
In the current handover scheme, the MRS can connect to only one eNodeB at one time. For ideal case, the MRS is always switched to the eNodeB with the best signal quality. However, this may lead to the well-known 'ping-pong' effect around the cell boundary. In practical systems, handover will be triggered on the condition that the received power of the source eNodeB is lower than that of the target eNodeB by hysteresis level h.
- (1)If both signals from the two adjacent eNodeBs can't be received by the train, outage will happen. In such case, the outage probability Pboth_outage can be expressed as(8)
- (2)Before handover, if the signal strength of source eNodeB is unacceptable and that of target eNodeB is not large enough to trigger handover, this is obviously an outage. Let ε i be the event that the train is connecting to eNodeB i, and h be the hysteresis level. The outage probability Pbefore_ HO can be expressed as(9)
- (3)After handover, although the train has been successfully switched to the target eNodeB, outage will happen if the signal strength of the target eNodeB is too weak. It should be noted that the target eNodeB is determined by the direction to which the train moves towards, that is, the train can't be switched to the previous source eNodeB though the signal strength of the novel source eNodeB is terrible. Thus, the outage probability Pafter_ HO can be expressed as(12)
It should be noted that the value of second item in (9) is difficult to obtain analytically. Whether the train is connecting to eNodeB i or not can not be determined by a snapshot of the system. It is generally assumed to be 1/2 in the midpoint of the overlapping region. Since A j < A i in (9), that is, the attenuation loss to eNodeB j is smaller, and the train has a higher chance to connect to eNodeB j. Following , this probability is chosen as 0.6.
Simulation parameters 
Channel bandwidth (BW)
Height of antenna: hat
Height of eNodeB: heNodeB
Cell radius (R)
Path loss model
Additionally, from these figures, with the increasing of hysteresis level and standard deviation of shadow fading loss, both the current handover scheme in LTE systems and our proposal experience a higher outage probability. However, the proposals provide the better performance as the hysteresis increases.
As the evolution of railway mobile communication system is confirmed by UIC, LTE technology becomes a potential solution for future railway systems. The railway communication system has stringent requirements for wireless communication availability and latency. However the frequent handover caused by high-speed movement will seriously affect the performance. This article proposes a CoMP based soft handover scheme for LTE system in high speed railway, meanwhile, a co-channel deployment approach for railway communication and dual on-vehicle station coordination mechanism is proposed too. The proposal allows the train to receive signals of both adjacent eNodeBs, which significantly improves the handover performance and degrades the outage probability, as the simulation results show. It is worth mentioning that signaling interactions in control plane and performance analysis on MAC layer are the focuses in our following work.
The work of the authors' was supported partially by the 973 Program under the Grant 2012CB316100, NSFC under the Grant 61071108, 61032002, the Key Program of Technological R&D of the Ministry of Railway under the Grant 2011X011-A, and the ZTE R&D Foundation.
- Jijing H, Jun M, Zhangdui ZH: Research on handover of GSM-R network under high-speed scenarios. Railway Commun Signals 2006, 42: 51-53.Google Scholar
- Tsagkaris K, Tselikas ND, Pleros N: A handover scheme based on moving extended cells for 60 GHz radio-over-fiber networks. In Proceedings of IEEE International Conference on Communications. Dresden, Germany; 2009:1-5.Google Scholar
- Pleros N, Vyrsokinos K, Tsagkaris K, Tselikas ND: A 60GHz radio-over-fiber network architecture for seamless communication with high mobility. Lightwave Technol 2009, 27: 1957-1967.View ArticleGoogle Scholar
- Jeon S, Lee S: A relay-assisted handover technique with network coding over multihop cellular networks. IEEE Commun Lett 2007, 11: 252-254.View ArticleGoogle Scholar
- Pabst R, Walke BH, Schultz DC: Relay-based deployment concepts for wireless and mobile broadband radio. IEEE Commun Mag 2004, 42: 80-89.View ArticleGoogle Scholar
- Bahl P, Adya A, Padhye J, Walman A: Reconsidering wireless systems with Multiple radios. Comput Commun Rev 2004, 34: 1-8.View ArticleGoogle Scholar
- Brik V, Mishra A, Banerjee S: Eliminating handoff latencies in 802.11 WLANs using multiple radios: applications, experience, and evaluation. In Proceedings of the 5th ACM SIGCOMM conference on Internet Measurement. New York, USA; 2005:22-23.Google Scholar
- Yang C, Lu L, Di C, Fang X: An on-vehicle dual-antenna handover scheme for high-speed railway distributed antenna system. In Proceedings of the 6th IEEE International Conference on Wireless Communicaitons Networking and Mobile Computing. Chengdu, China; 2010:1-5.Google Scholar
- Wang Z, Wang Y, Lin C, Wang Q: Enhanced downlink MU-CoMP schemes for TD-LTE-advanced. In Proceedings of IEEE Wireless Communications and Networking Conference. Sydney, Australia; 2010:1-6.Google Scholar
- Zhou M, Wan L: Anaysis into timing advance issue in CoMP systems. In Proceedings of the 70th IEEE International Conference on Vehicular Technology. Anchorage, USA; 2009:1-5.Google Scholar
- Xu X, Chen X, Li J: Handover scheme for coordinated multi-point transmission/reception system. ZTE Commun 2010, 1: 32-36.Google Scholar
- Fu H, Kam PY: Simple error probability derivation for binary DPSK over fast Rician channels with diversity. IEEE Electron Lett 2006, 42: 163-165. 10.1049/el:20063414View ArticleGoogle Scholar
- Fu H, Kam PY: Effect of Doppler shift on performance of binary DPSK over fast Rician fading channels with diversity reception. In Proceedings of IEEE International Symposium on Information Theory and Its Applications. Toronto, Canada; 2008:1-6.Google Scholar
- 3GPP TS 36.331 V9.4.0, Radio Resource Control (RRC)[http://www.3gpp.org/ftp/Specs/archive/36series/36.331/]
- 3GPP TS 36.300 V10.0.0, E-UTRA and E-UTRAN Overall Description[http://www.3gpp.org/ftp/Specs/archive/36series/36.300/]
- Sesia S: LTE - UMTS Long Term Evolution: From Theory to Practice. Wiley, Chichester; 2009.View ArticleGoogle Scholar
- 3GPP TSG-RAN WG1 #56bis, R1-091578, Evaluation model for Rel-8 mobility performance[http://www.3gpp.org/ftp/tsgran/WG1RL1/TSGR156b/Docs/]
- 3GPP TS 36.101 V10.0.0, User Equipment (UE) radio transmission and reception[http://www.3gpp.org/ftp/Specs/archive/36series/36.101/]
- Morelli M, Kuo C-CJ, Pun M-O: Synchronization techniques for orthogonal frequency division multiple access(OFDMA), a tutorial review. Proc IEEE 2007, 95: 1394-1427.View ArticleGoogle Scholar
- Piirainen Olli U.S. Patent 6,473,594 2002.Google Scholar
- Klotsche Ralf, Wunstel Klaus, Banniza Thomas-Rolf, U.S. Patent 7,653,347 (Jan 2010).Google Scholar
- Guidelines for evaluation of radio transmission technologies for IMT-2000[http://www.itu.int/rec/R-REC-M.1225-0-199702-I/en]
- Viterbi AJ, Viterbi AM, Gilhousen KS, Zehavi E: Soft handoff extends CDMA cell coverage and increases reverse link capacity. Sel Areas Commun 1994, 12: 1281-1288. 10.1109/49.329346View ArticleGoogle Scholar
- Fading Models[http://www.comlab.hut.fi/opetus/333/20042005slides/Fadingmodels.pdf]
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.