Positioning and Relay Assisted Robust Handover Scheme for High Speed Railway

Transcription

1 Positioning and Relay Assisted Robust Handover Scheme for High Speed Railway Linghui Lu, Xuming Fang, Meng Cheng, Chongzhe Yang, Wantuan Luo, Cheng Di Provincial Key Lab of Information Coding & Transmission Southwest Jiaotong University Chengdu 63, China Abstract- With the epansion of the railway industry and the upgrade of railway speed, the reliability and efficiency of wireless communication systems in high speed railway attract the public attention. The high speed brings about more severe Doppler Effect, the postponement of handover triggering location and more frequent handover. Therefore the influence caused by the high speed decreases the reliability of wireless communication network in high speed railway. In this paper, a scheme based on the train position information and relay power control has been proposed to optimize the system handover performance. When the train is at the handover point, by power control the relay generates an enhanced signal to meet the minimal communication requirement. The results show that the proposed scheme increases the handover success probability effectively, thus improves the reliability and efficiency of wireless communication systems in high speed railway. Keywords- Handover, Positioning, Relay, Power Control, Outage Probability I. INTRODUCTION With the continuous construction of high-speed railway and the development of Communication- Based Train Control (CBTC) technology [], the reliability and efficiency of wireless communication systems in high-speed railway have attracted public attention. The key techniques of CBTC include bidirectional wireless communications and train positioning technique. Using the CBTC s information (e.g. position information) to enhance the wireless communication system's performance has become a kind of research trend. The main problems eisting in wireless communications of high-speed railway include frequent handover, postponement of the handover triggering position and Doppler Effect. The handover is the focus of attention because the wireless communication systems of high-speed railway (e.g. GSM-R) are mainly in charge of train dispatch which transmits information having close relations with safety. The handover triggering mainly depends on the detection of received signal strength. When the base station () signal strength of target cell is larger than that of source for a certain threshold, the train will be handed over to the neighboring cell. The handover happens at the boundary area of two cells. The quality of This work is supported by NSFC under Grants 63, 678, 67785, Key Research Plan of MoR of China under Grant 9X9-E and the State Key Laboratory of Rail Traffic Control and Safety under Grant RCS8Κ of Beiing Jiaotong University. received signal is low because of shadow and multi-path. Additionally, the Doppler Effect which results from high-speed movement of trains brings about high outage probability to traditional handover schemes. The eisting references [] and [3] have suggested decreasing handover failure probability by optimizing the threshold of handover udgment. It is actually the optimization of system parameters which cannot improve system performance significantly. References [4] and [5] have introduced relays to the process of handover in cellular networks and managed to improve the handover performance. We noticed that the combination of precise train positioning information which can be obtained easily from CBTC and relay power control technology is of great benefit to system capacity and handover. In this paper, a scheme based on the train positioning information and relay power control technology is proposed to improve the system performance. The rest of this paper is arranged as follows: Part II introduces concepts and usage scenarios. Part III discusses the optimal handover scheme based on location information and relay power control in detail. Part IV analyzes the system performance. Part V illustrates the simulation results and the improvement brought by the proposal. Finally we conclude our work. II. A. System Topology CONCEPT AND USAGE SCENARIOS Wireless communication systems in high-speed railway have the linear coverage topology shown in Fig.. The is with the coverage radius R. The width of overlapping region is α. The vertical distance between a and track is d min. The carrier frequency is denoted as f m, mi,, k,. In Fig., the train moves toward cell along the track. The train measures i fi f k fk Fig.. Network topology diagram. l fl //$6. IEEE

2 the strength of the signals transmitted from the source i and neighboring. If the strength of signal from is larger than that from i for Γ db, the handover triggering condition is satisfied. B. Positioning Information In CBTC [], some of the positioning devices that are employed by vehicles and railroad to determine the train s location on the railroad include tachometers, accelerometers, gyroscopes, global positioning system (GPS), transponders (or tags), radar, lasers, loop transpositions and digital track maps. Different manufacturers of train-control systems employ different combinations of these devices for train location. Therefore, accurate speed and position information can be obtained to aid wireless communications. C. Relay Power Control Relay station () can be used to increase system capacity, epand network coverage, improve the performance of weakfield-strength zone and decrease the consumption of system resource. In this paper, is deployed in the middle of the overlapping region. Assume that the only attaches to the source, and the is the amplify-and-forward (AF) relay, and the delay of replay can be neglected. III. (a) Phase I: train in [d, d) (b) Phase II: train in [d, d3] Fig.. Positioning and Relay Assisted Robust Handover Scheme. POSITIONING AND RELAY ASSISTED ROBUST HANDOVER SCHEME The basic mechanism of positioning and relay assisted robust handover scheme is described in Fig.. Assume that the i is located at the middle of overlapping region of i and and it only attaches to source i. Particularly, in this paper, the signal of the only refers to the signal from and the signal of the cell refers to the compound signal from both and in the cell. i works from location d to location d3. The whole process can be described in two phases: Phase I: [ d, d) When the train arrives at d, i starts to forward the received signal from i with full power. The train combines the signals both from i and i to obtain diversity. The circle of real line in Fig. (a) shows the coverage of i. When determining the location d, it is necessary to make sure that the signal received from the must be useful signals which can obtain diversity gains rather than interference signal. Phase II: [ d, d3] When the train arrives at d, the strength of signal from is larger than that from i for Γ db, that is, the handover triggering condition is satisfied. Then i starts the power control to make the signal of the cell i satisfy the minimal communication requirement. It is of benefit to the triggering and operation of handover in the condition that the outage with source cell does not happen. The circle of dashed line in Fig. (b) shows the coverage of, which changes continuously. When determining the location d, only the circumstance that signal strength of is larger than that of i for Γ db needs to be considered. When the train arrives at d3, the is stopped working because the handover is finished. In this paper, if the locations of d and d are determined appropriately, the optimal scheme can be operated successfully. IV. PERFORMANCE ANALYSIS As we know, Doppler Effect deteriorates the BER (Bit Error Rate) while has little influence on received signal strength so that it is unnecessary to consider Doppler Effect when computing handover triggering probability which depends on signal strength. In addition, because Doppler Effect has the same influence over traditional scheme and our proposal on outage probability, it is not considered in this paper. Moreover, wireless communications in high-speed railway use a dedicated band in which the interference is small enough to be ignored. Finally two neighboring s are within a relatively short distance (the typical distance is 5km in China) and railroads are usually built in wide suburban environment, in which multipath effect is small, therefore, only the main path signal is considered. A. Traditional Handover Strategy In order to compare the differences between the traditional strategy and our proposal, let s consider the traditional handover strategy first. Without loss of generality, let the train be located at. The power of received signal from m is: ε( m / m m () R(, ) P PL(, ), m i, where, i is source ; is neighboring ; P is the transmit power; is the measurement location, the values of which assemble a number sequence { n } and in this number sequence, the neighboring difference can be epressed as measurement periodic Δτ multiplied by speed ν; PL is the path loss ( m, ) between m and location ; ε denotes shadow fading with

3 dimension of db which obeys Gaussian distribution with mean and standard deviation σ. The interference received from co-frequency while the train stays in cell m is: N ( m, ) ( n n ε ( n / I P PL, m i, where, N is the number of co-channel s. The SINR of received signal in cell m is: ( m ( m ) NBW / I( m, ) + () R SINR,, m i, (3) where, N is thermal noise density (db/hz); BW is signal bandwidth (Hz). The handover triggering mechanism which depends on signal strength can be epressed as: When the signal strength of target cell is larger than that of source one for Γ db, the train will be handed over to target cell. That is, the handover probability can be epressed as: P( i, ) Pr log R( log R ( i Γ (4) With the upgrading of train speed, the neighboring difference of number sequence { n } becomes larger and the handover trigger location is deferred which will leads to more handover failure. While Pr[ ε( ε ] N(, σ( ) and σ (,) is the standard deviation of ε (,), equation (4) can be epressed as equation (5). P( i, ) Pr log R( log R( i ε( ε Γ Pr ε(, ) ε dε log ( R(, ) ) Γ ε σ (, ) log e dε σ (, i) P PL ( i, ) πσ(, ) Φ (5) Another important concept is the outage probability. Outage probability refers to the probability that received signal quality is too weak to be properly decoded and leads to link interruption. It is assumed that when the SINR of received signal is smaller than a threshold γ th (db), the outage happens: outage P Pr[ SINR < γ ] (6) th where, SINR is the SINR while the train stays at location. In the traditional handover schemes, there are two main reasons for handover failure: one is that outage happens before handover triggering because of weak signal quality of source cell; and the other is that outage happens after the handover because of weak signal quality of target cell. Therefore, the handover failure probability should be considered in two parts, can be epressed as (7): Pfail P (, i ) Pr SINR(, iy) < γ thpr[ y z] z di, + Pr SINR( y, ) < γ th Pr[ y z] z where, z is the udgment location and z { n }; d i, is the distance between two neighboring s. B. Our Proposed Scheme First of all, the locations of d and d should be determined. When trying to determine the location d, it is necessary to make sure that the signal received from the must be useful signals which can obtain diversity gain rather than interference signal. And the diversity gain can be obtained only while the signal forwarded by is one of the multi-path signals. The arrival time interval between i and i should be smaller than the maimal delay spread: [ d + ( d d)] d < τ ma (8) c where, d is the distance between i and i; c is the spread velocity of electromagnetic wave; τ ma is the maimal delay spread of multi-path. Net, the maimal transmit power restriction of should be considered. If the signal from i can be ignored compared to that from i, the relay forwarding is unnecessary. In this paper, it is assumed that starts to forward information only when the signal strength from is larger than half of the signal strength from attached. That is: ε(, ) / P (, ) / (, ) i d ε (, ) i P d PL i d PLi d (9) where, P is the maimal transmit power of ; PL is ( i, d) the path loss between i and the location d. And then, when trying to determine the location d, only the scenario that signal strength of neighboring is larger than that of source for Γ db needs to be considered: ( R d ) ( Ri d ) (, ) (, ) (7) log log Γ () After determining the location d and d, the effect of relays in the process of handover could be analyzed. The useful signal received by i from the i in source cell is: ε(, ) / R(, ) (, ) i d i d P PL i d () The useful signal received by the train from the i is: ε(, ) / R(, ) AR(, ) PL(, ) i () i i d i where, PL is the path loss between i and location ; A is ( i the amplify factor of the and can be epressed as:

4 P, d < d NBW / R( i, ) d + Rth A, d < d3 N (, ) / BW / ε i ( R( i, ) ) (, ) d + PL i, otherwise (3) where, R th is the receive power which meets the minimal communication requirement. The noise signal which has been amplified by i while the train located in source cell is: N A PL (4) NBW / ε ( i / ( i ( i The interference received from co-frequency relay is: N ( n / ( m, ) (, ) n,, (5) n I P PL m i where, N is the number of co-channel. Therefore, the useful signal received by the on-board transceiver from source cell is: R(, i ) R( i + R( i ε( i / ε( i / P PL + AR PL (6) ( i ( i, d ) ( i The total interference signal received by the on-board transceiver is: I,, ( m, ) I( m, ) + I( m, ) m i (7) Then the SINR can be epressed as: SINR (, i) R I R I(, i) + (, i) NBW / (, i) + + N( i ( i NBW/,, d< < d3 otherwise After the handover, the SINR of the received signal is: SINR R ( (, ) NBW / I(, ) + (8) (9) In the proposed scheme, handover cannot be triggered while d because of the relay. It can only be triggered while >d, so when Pr[ ε(, ) ε ] N(, σ(, )) and σ is the standard (, ) deviation of ε (, equation (4) can be epressed as (). Similarly, the outage cannot happen while d because of the relay. The handover failure probability can be epressed as equation (). V. SIMULATION AND ANALYSIS In order to verify the effect of the proposal upon the handover operation, we present the MATLAB simulation results and analysis. Detailed simulation parameters are shown in Table I. [6, 7] TABLE I. SIMULATION PARAMETER Parameter Value Channel bandwidth (BW) MHz Antenna pattern Omni Carrier Frequency 9MHz transmit power (P ) 43dBm transmit power (P ) 33dBm antenna height (h b) 3m Train antenna height (h m).5m d min 3m Lognormal Shadowing Std. Dev (σ) 4dB Correlation Distance for Shadowing 5m Thermal noise density (N ) -74dBm/Hz Cell radius (R) 3km Site-to-Site Distance 4.8km Handover triggering threshold (Γ) 3dB Reuse factor 3 Path Loss Model HATA Minimum receiver power (R th) -86dBm SINR threshold (γ th) db d 45m d 65m Measurement periodic Δτ 5ms Mobility 36km/h The strength of received signal from resource changes when the train moves on, shown as the line with star in Fig. 3. When the train arrives at d (.45), it can receive two signals to obtain reliable and high-quality transmission due to the effect of in the middle of the overlapping region, though the signal quality from resource begins to degrade. When the train arrives at d (.65), uses power control to decrease the transmit power so that the signal from source cell meets the minimal communication requirement as well as the process of handover to target can be triggered and finished normally. Fig.4 shows the relationship between handover P(, i ) Pr log R( log R( i ε( ε Pr ε( ε dε Γ log ( R(, ) ) Γ ε σ (, ) Φ log e dε σ ( i, ) P PL( i, ) (, ) (, ) A R i d PL + i πσ(, ) () d, i Pfail P (, i ) Pr SINR(, i y) < γth Pr[ y z] + Pr SINR(, y) < γth Pr[ y z] z d z ()

5 success probability and the location of the train, where, the line with star denotes traditional handover scheme and the line with circle denotes the proposed scheme. In traditional scheme of which the ideal triggering position is the point.65km, the handover is triggered earlier and the overall success probability is lower. Due to space limitations of this paper, the analysis of outage probability is not described specifically. In the proposed scheme, when.65, the quality of received signal is kept at a good level due to the effect of. Therefore the handover will not be triggered. When.65<<3.5, the power control of leads to the postponement of triggering position. Thus in this range handover success probability of proposed scheme is a little lower than that of traditional one. However, the outage probability of proposed scheme is decreased because power control can guarantee the minimal communication requirement. When 3.5, the handover success probability of proposed scheme is higher than that of traditional one because the coverage of overlapping region is epanded equivalently by the power control of. Traditionally, premature handover triggering caused by fluctuations of signal results in high outage probability, while late handover triggering which is in the face of limited coverage of overlapping region and insufficient distance for the handover also leads to high outage probability. In the proposal, the effect of with power control can avoid handover failure caused by premature triggering and epand the coverage of overlapping region to decrease outage probability of late triggering so that the overall handover success probability is increased. The handoff success probability w/o relay relay with power control The distance between train and source (km) Fig. 4. Comparison of handover success probability of different schemes. VI. CONCLUSION Some wireless communication systems in high speed railway are needed to transmit railway control information, so its reliability and efficiency should be guaranteed. However, the high-speed brings about a series of communication problems: the more severe Doppler Effect, the postponement of handover triggering location and the more frequent handover. This paper proposed a positioning and relay assisted robust handover scheme to solve the issue. In this scheme, we make full use of the positioning information provided by the train control system and the installed with power control located in the middle of overlapping region. The results show that the proposed scheme can improve the handover success probability greatly. receiverd signal strength (dbm) X: Y: -.54 received signal strength from source cell w/o relay received signal strength from targer received signal strength from source cell with relay diversity X:.45 Y: The distance between train and source (km) X:.65 Y: X: 3 Y: -7 X: 4.8 Y: -.54 Fig. 3. Received signal strength according to train location. REFERENCES [] Pascoe, R.D. and Eichorn, T.N., What is communication-based train control? IEEE Vehicular Technology Magazine, Dec. 9, 4(4): 6- [] Licong Huang, Gang Zhu. Analysis and Optimization of Handover in GSM-R Network. Mobile Communications, 7, (8): [3] Jiying Huang, Jun Ma, Zhangdui Zhong. Research on Handover of GSM- R Network under High-Speed Scenarios, Railway Communications Signals, 6, 4(5): 5-53 [4] Sungho Jeon, Sanghoon Lee. A Relay-Assisted Handover Technique with Network Coding over Multi-hop Cellular Networks, IEEE Communications Letters, 7, (3): 5-54 [5] R. Rabst et. al. Relay-Based Deployment Concepts for Wireless and Mobile Broadband Radio, IEEE Communication Magazine, 4, 4(9): [6] IST WINNER II, D.. v.. WINNER II Channel Models, 7. [7] A. Molisch, Wireless Communications, Wiley-IEEE Press, 5