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Communications and Network, 2013, 5, 344-347 http://dx.doi.org/10.4236/cn.2013.53B2063 Published Online September 2013 (http://www.scirp.org/journal/cn) Analysis of Relay Deployment Based on Handover Outage Probability in High Speed Scenarios Liang Jing, Xiaojie Xu, Y afen g Wang Beijing University of Posts and Telecommunications, Beijing, China Email: liangjing@bupt.edu.cn Received July, 2013 ABSTRACT The LTE and LTE-Advanced systems are supposed to be the most popular cellular network in 4G networks. Relay technology is one of the most preferred technologies proposed in LTE-Advanced standardization to improve the capac- ity and coverage. This paper analyses the handover outage probability through relay deployment, and proposes some proofs and suggestions for relay deployment to minimize the h andover outage probability in high speed scenarios. Keywords: Outage Probability; Relay Deploy ment; Handover; High Speed Scenarios; LTE-advanced 1. Introduction LTE-Advanced network, with one or more relays in one cellular, may be used to cover some high speed scenarios such as railway, highway and so on. As people are mov- ing faster and faster via variou s vehicles, which brings up some challenges to security and reliability of the network , handover is becoming an important issue in those high speed scenarios. Handover is usually based on sign als transmitted from base stations (BS) or relay stations (RS) to mobile sta- tions (MS). If MS finds out that the quality of signals from neighbor BS or RS is better than that from home BS or RS while other conditions are met, then MS releases channels in home cell and begins to use channels in the neighboring cell after a handover process. In high speed scenarios, signals transmitted in LTE-Advanced network not only experience shadowing and multi-path, but also suffer Doppler shift due to high speed. As a result, some solutions may be adopted to solve the problem. In previ- ous work, handover algorithms are proposed to set dif- ferent handover threshold [1-3] depending on different speeds. As we know, handover often occurs in over- lapped areas, which is far away from BS, an accurate threshold can hardly be set to meet different speeds. Relay technology can improve the coverage, enlarge the network capacity and reduce the energy cost. Of course, relay technology can also be used to assist hand- over in LTE-Advanced network. In LTE-Advanced net- work cells with relays, when MS decides to make hand- over decisions, it not only measures signals from neighboring BS and home BS, but also from RS. As a result, the deployment of RS is very important for hand- over. The position of RS may affect the outage probabil- ity and the stability after h andover. In this paper, a novel analysis of handover outage probability based on signal strength in different relay deployments is made. Based on the analysis results, conclusions about relay deployment in high speed scenarios are made. 2. System Model In cellular networks, there are two main deployments, which are regional coverage and linear coverage respec- tively. Regional coverage is usually applied in urban area, where people in wireless communication needs are dis- tributed in a large area. For those with wireless commu- nication service demands in high speed such as railway, linear coverage is preferred and should be applied. As showed in Figure 1, the center of round cells is in the same line with MS traveling along. Th e network showed in Figure 1 has a frequency reuse factor of 3, and they are 1 f , 2 f and 3 f respectively. 2.1. Traditional Handover Scenario (without Relay) In th is sc enar io, whe n MS in cell travels along the line in Figure 1. LTE-Advanced network deployment with reuse factor of 3. C opyright © 2013 SciRes. CN L. JING ET AL. 345 LTE-Advanced network, it constantly measures the sig- nals from BS in cell (home BS) as well as from BS in cell and cell (neighboring BS). As long as conditions for handover are met, MS handovers are from cell to cell. Without loss of generality, signals transmitted from source to destination can be described as follows [4]: ()/1 410 xx xx SSr 0 x=i, j, k (1) where is the ID number of home cell, and , k are the ID numbers of neighboring cell. ij x S is the power received from BS in relative cell, S is the transmitted power of the source. x r is the distance between the source and the destination. x , which represents the shadowing of signals received from the source, is a Gaussian distributed random variable with a zero mean and a standard deviation x . x is a constant that rep- resent the Doppler shift effect of signals received from BS or RS in corresponding cells. When MS moves faster, x is getting smaller. x G, representing path gain can be defined as follows: ()/1 410 xx xx Gr 0 G x=i, j, k (2) The condition for handover in network without relay can be described as follows[5]: when signals received from home BS is at least smaller than that received from neighboring BS, then the MS handover to the neighboring BS. The MS handover probability is: Pr , handoverj ijk PGGG (3) Let 0 , since is a Gaussian distributed ran- dom variable with zero mean and a standard deviation x , Equation (3) become s[6 ] : 2 02 40 2 400 Pr , (10log(/)()/) 1 *(10log( / )()/)*2 j handoverj ijk iji ji kjk jk j PGGGG rr rre d (4) After MS handovers to the target cell, it may be outag e because of the co-channel interference and other inter- ference. With the assumption that interfering signals re- ceived only from BS in cell l that has the same frequency as cell j, the co-channel interference ratio can be repre- sented by: ()/10 ()/1 44 ()/10 ()/10 44 *10 10 *10 10 jj jj ll ll jj ll Sr r C ISr r 0 (5) where j r, l represent the distance between MS and BS j or BS l, l r is a Gaussian distributed variable with zero mean and a standard deviation l . j , l repre- sent Doppler shift effect of signals received from BS in cell j and in cell l, and j <0, l <0. The outage prob- ability after handover is given by: __target cell outage P target_cell_outage C< I P=Pr (6) where is a predefined threshold. Let 0l , then Equation (6) becomes 2 02 l target_cell_outage 1 4j40jl j l - -20 l P C =Pr<=Pr I*> + C I r =F(10log() +(-)/) r 1 **ed 2 (7) 2.2. LTE-Advanced Handover Scenario (with relay in cells) In this scenario, with relay in cells[7], when MS travels along the line, it not only constantly measures signals from home BS and neighboring BS, but also measures signals from the RS that is deployed in cell j. In this situation, the handover probability can be divided into two parts: handover to th e BS in target cell and handover to the RS in the target cell, which can be given by fol- lows: _ Pr, , Pr , *Pr handover BS jijkj *Pr RS j ij RS handoverRS j P GG GGS GGGGSS PaG k S G j , * k k (8) _ Pr* , Pr* ,Pr Pr, *Pr handover RS RS iRSRSj RS iRSRSj R SiRSRS j P SSSS SS SSSS SS aG GaGaGG k S S G __RS (9) Then the total handover probability in the scenario with relay in cells is defined as follows: _handover sumhandoverhandover BS PPP (10) In LTE-Advanced system, frequencies are orthogonal and we assume that the two relay links are distinguished in time division multiplex way. Thus the co-channel in- terference ratio with BS can be represented as follows: Pr target_BS_outage target_cell_outage C PP I (11) Copyright © 2013 SciRes. CN L. JING ET AL. 346 Considering that the link between BS and RS in LTE-Advanced system is wireless, so the outage prob- ability of the relay access should also involve the BS-RS link, whic h is given by: arg __ __ __ 1(Pr )*(Pr) tet RS outage BS to RS RS RS RSBS to RS P C C II (12) where R S is the a predefined threshold for RS, and ()/ 4 ()/10 4 ()/10 4 ()/10 4 10 10 10 10 RS RS ll RS RS ll RSRS RS RS l RS l CSr ISr r ar 10 (13) The in Equation (13) represents the transmit p ower ratio of RS and BS, that is: a R S S aS (14) ()/1 4 __ __ ()/10 4 __ __ 10 10 jj ll l BS to RSBS to RS BS to RSBSto RS Cr Ir 0 (15) where __ l B StoRS represents the distance between the BS l and the RS and r R S, and represent the distance be- tween MS and RS or BS l. rl r 3. Numerical Result In order to show the impact of RS on handover in high- speed scenarios, we run some simulations using Mathe- matica and Matlab tools. First we need to take a look at the scenario without RS in the cells. As can be seen from Figure 2, the handover probabil- ity increases as the distance between the BS and MS de- creases, but the outage probability after the MS handover to the target BS is opposite, it decreases as the distance between the BS and MS decreases. Seen that handover requests start to increase from about 0.8~1 radius from BS, yet the outage probability need to be optimized dur- ing this range. We know that relay technology can improve the sys- tem capacity[8] and can also enhance the cell coverage, but whether wireless relay technology in LTE-Advanced system can make contribution to the handover in high speed scenarios is still a question to be studied. In the following, we will make a novel research on this ques- tion based on some numerical results that has been de- duced in previous analysis. Compared with the result of Figure 4, there are some differences in Figure 3. In this scenario, RS is deployed in the line that is 0.8 radius away from the BS j. When we calculate the outage probability after handover to RS, 0.20.4 0.6 0.811.2 1.4 1.6 1.82 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1Handover and outage probabi l i ty wi t ho ut RS Distance between M S and target B S P robabil i ty Handover probabili t y withou t RS Out age Probabilit y after handover Figure 1. Handover and outage probability without RS. 0.2 0.40.6 0.811.2 1.41.6 1.82 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1Handover and outa ge probabi l ity wit h RS Dis tance between M S and targe t B S P robabi l i t y Handover probabili ty wit h RS Out age P robabi l ity aft er handover Figure 2. Handover and outage probability with RS. considering that the BS-RS link is wireless, we calculate both the RS-MS link and the BS-RS link, which is giv en by Equation (12). The cells with RS in the boundary do make contribu- tions to the handover outage probability seen from Fig- ure 3, on one hand, it can enlarge the distance that MS is more possible to start a handover, on the other hand, the outage probability is significantly lower and gentler than that without RS. 4. Conclusions In LTE-Advanced systems, coverage in high speed sce- nario may be a question to be further studied. Based on the previous analysis and some numerical results, we can see that proper relay deployment in the target cell can improve the handover probability distance. For example, in the comparison of Figure 2 and Figure 3, relay de- ployed in 0.8 radiuses can enlarge about 0.4 radiuses at the same handover probability (0.5). Copyright © 2013 SciRes. CN L. JING ET AL. Copyright © 2013 SciRes. CN 347 At the same time, proper relay deployment can also make the outage probability after handover rise much more slowly as the distances increase, thus it can make the handover more reliable and stable for those in high speed who n e ed wirele ss commu n i c ation service s . 5. Acknowledgements This paper is supported by Key project (2012ZX- 03001030-004). REFERENCES [1] R. Pabst, B. Walke, et al., “Relay Based Deployment Concepts for Wireless and Mobile Broadband Radio,” IEEE Communications Magazine, pp. 80-89, New York, US, September 2004. [2] P. Liu, “Solution: How to Overcome the Coverage Issues for WCDMA in High-speed Scenarios,” Huawei Tech- nology, 2006, Vol. 11, pp. 53-55 [3] L. C. Huang and G. Zhu, “Analysis and Optimization of Handover in GSM-R Network,” Mobile Communications, 2007, Vol. 8, pp. 35-38. [4] A. Molisch, Wireless Communications, Wiley-IEEE Press, 2005. [5] Z. D. Zhong, X. Li and W. Y. Jiang, “Integrated Digital Mobile Communication System for Railway Transporta- tion (GSM-R) ,” Beijing: China Railway Press, 2003. [6] D. H. Lee, et al., “Fast Handover Algorithm for IEEE 802.16e Broadband Wireless Access System,” Wireless Pervasive Computing, 2006 1st International Symposium, Jan. 2006. [7] S. Cho, E. W. Jang and J. M. Cioffi, “Handover in Mul- tihop Cellular Networks,” IEEE Communications Maga- zine, Vol. 47, 2009, pp. 64-73. [8] H. Hu, et al., “Range Extension without Capacity Penalty in Cellular Networks with Digital Fixed Relays,” Proc. IEEE Globecom ’04, Vol. 5, 2004, pp. 3053-57. |