A Power-Distance Based Handover Triggering ... - IEEE Xplore

The 22nd Asia-Pacific Conference on Communications (APCC2016)

A Power-Distance Based Handover Triggering Algorithm for LTE-R Using WINNERII-D2a Channel Model Ehab Ahmed Ibrahim Computer Networks and Data Center Arab Academy for science and Technology Alexandria, Egypt [email protected]

Ehab F. Badran Electronics and Engineering Department Arab Academy for Science and Technology Alexandria, Egypt [email protected]

capacity and capability. These shortcomings become a major issue for railways because they cannot support advanced data services.

Abstract— Railway systems cannot be apart from the rapid evolution of wireless communication systems. The Long Term Evolution communication system for Railway (LTE-R) is believed to be the normal evolution for current Global System for Mobile Communications Railways (GSM-R). One of the essential targets of LTE is to provide seamless and fast handover from one cell to another to achieve a strict delay requirement while, at the same time, keeping network management as simple as possible. Hence, the decision to trigger a handover is very important in the design of handover process. For handover to be successful, it requires to choose handover parameters correctly and optimize their setting. The LTE-R system uses two parameters to decide handover triggering: Hysteresis and Timeto-Trigger (TTT). The handover triggering is highly depended on the train speed, which means that as the train speed changed, the handover triggering time is correspondingly varied. To determine a more accurate handover triggering point is a critical issue. Too late handover triggering caused by train speed change will lead to handover failure. In this paper, we propose a new handover triggering algorithm called Power-Distance Algorithm that assets to avoid this problem. It eliminates the dependency on the train speed, also, reduces the system processing power. This Algorithm depends on the distance between the two base stations and power received from them. At a certain (predefined) point, the handover triggering must occur. This triggering point is the same for all values of train speed .i.e., it doesn’t change as the train speed changes. The change occurs only when the Hysteresis value changes.

Therefore, alternative technologies such as LTE [3, 4], is considered as a future railway communication technology. LTE, also called Evolved Packet System (EPS) is the fourth generation of mobile network standard proposed by 3rd Generation Partnership Project (3GPP) [5], the 3GPP structures their standards as Releases. LTE is specified in Release 8 which was published in December 2008. LTE is designed to increase the capacity, coverage, and the speed of mobile wireless networks over earlier wireless systems. Long Term Evolution for Railway (LTE-R) [6] defines the specifications of LTE applied for railway transportation system. It has been proposed to fulfill the requirement of broadband mobile communication systems in high speed railway environment. Handover performance is considered as one of the critical issues in radio resource management especially for real-time service because the handover failure rate increases with the higher moving speed. Only hard handover is supported in LTE. This causes an interruption time in the user plane. The performance of handover expressed in terms of success rate is highly important. The handover decision must be taken properly to avoid call drops and critical data loss such as control signals especially in case of trains. To determine a more accurate handover triggering point is a critical issue greatly reduce the failure probability of handover. Too early handover triggering will cause ping-pong effect problems because the UE tries to switch to the source cell again shortly after a successful handover to the target cell. Too early handover triggering occurs because of a failure in the target cell radio link after a handover has been finished. The UE tries to re-establish its radio link with the source cell. Another reason for too early handover is radio link failure in the target cell during the handover process. The UE tries to re-establish its radio link in the source cell.

Keywords ‫ ـــ‬LTE–R, 3GPP, EPS, A3 event, Hysteresis, TTT, WINNER II, D2a.

I.

INTRODUCTION

All national railways had their own incompatible analog systems for railway communications up to the end of the last century. At this time, a group of manufactures came together to define a universal standard for railway communication which resulted in the GSM-R standard [1, 2]. The GSM-R is a digital standard based on 2G GSM mobile technology. It is achieved the interoperability between various railway companies. GSM-R is used for voice communication and data communication between the train and the Radio Block Center (RBC). GSM-R is considered now as an obsolete mobile technology with a number of shortcomings in terms of

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M.R.M. Rizk Electrical Engineering Department Faculty of Engineering Alexandria, Egypt [email protected]

Similarly, too late handover triggering will also lead to handover failure. In this situation, the handover procedure in the source cell is initialized too late since the UE is moving faster than the handover (HO) parameter settings. Hence,

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functions such as Radio Admission Control and eNodeB Measurement Configuration Control. The MME manages mobility, provides user authentication and user profile download EPS Mobility Management and EPS Session management. The S-GW and P-GW act as termination points [12]. The S-GW terminates the interface towards an EUTRAN. It serves as local point for data connection for inter – eNodeB. The P-GW provides a User Equipment (UE) with access to PDN by assigning an IP address from the address space of the PDN. Also it is responsible for handover between 3GPP and non 3GPP systems.

when the HO command from the serving cell is transmitted, the signal strength is too weak to reach the UE which is located now in the target cell. Hence, connection is lost. How to find an exact handover triggering point to increase the success probability of handover and to decrease handover overheads is a critical issue and many researches were targeted this point. A. Preliminaries In the following, related works on handover procedures is introduced. Then, summery of the proposed algorithm is given. Finally, the paper organization is clarified.

III.

In [7], X2-handover Performance is evaluated based on Reference Signal Received Power (RSRP) Measurement with Free Space Path Loss, using Network Simulator Version 3 (NS-3). In [8], the X2-handover triggering is studied based on A3 Triggering Algorithm using MATLAB. In [9], a mobile relay based fast handover triggering scheme is proposed targeting high-speed mobile environment. Two reference points are introduced to ensure handover in time. Prepreparation and packet bi-casting is introduced to reduce communication interruption time and realize seamless handover.

A. Overview The handover procedure provides transferring a connected user’s session from a base station to another without disconnecting the session. Handover is an important concept in mobile networks. The system must provide mobility to the users reliably and without dropping any of their calls / losing their data. In mobile networks, there are two types of handover, hard (Break before Connect) and soft (Connect before Break) [13]. Hard handover requires disconnecting from source eNodeB before establishing connection to a target cell. In soft handover, it is possible for a UE to simultaneously connect to two cells during an ongoing session. The user connects to the target cell before disconnecting from the source cell. In LTE – R, only hard handover is supported.

In [10] the direction and speed information provided by GPS is used to facilitate the selection of eNodeB, enhancing handover triggering and shorten TTT. In [11] a fast handover scheme for high speed rail network is proposed. Railway is divided into segments. When a train enters a predefined handover zone between two segments, the handover triggering starts automatically.

B. handover Procedure The HO procedure in LTE-R [14] can be generalized in three phases: handover preparation, handover execution and handover completion. In handover preparation phase, the UE sends a measurement report to its serving cell. The UE performs radio channel measurements periodically. It measures the reference signal received power (RSRP) and the reference symbols received quality (RSRQ) [15]. The UE sends a measurement report to its serving cell which decides if the user needs to do handover and identifies the target cell then it sends a handover Request message to the target cell. If the target cell accepts the request, it allocates the required resources for the UE and sends a handover Request Acknowledge message to the source cell.

In this paper, a Power-Distance based handover Triggering Algorithm is proposed, in which the classical A3 handover Triggering Algorithm is adapted by taking into consideration the distance between the train and the eNodeBs. This Algorithm reduces the number of measurement procedures which improves the system performance by decreasing the total processing power. In this simulation, WINNER II Channel Model is chosen to simulate the LTE-R network. The rest of the paper is organized as follows: In Sec. II, LTE-R network architecture is discussed in details. In Sec. III, the handover procedure in LTE-R is presented. Then, in Sec. IV, the WINNER channel model is explained. The proposed handover Algorithm is demonstrated in Sec V. Simulation results are analyzed in Sec. VI, followed by the conclusion in Sec. VII. II.

Upon successful HO preparation, the HO decision is made and consequently the HO Command will be sent to the UE. The connection between UE and the serving cell will be released. In handover execution phase, the source cell forwards user data packets to the target cell with using X2 or S1 interface according to the interface used in the handover. The UE releases the resources of the source cell, synchronizes with the downlink of the target cell and tries to access the target cell with using the random access procedure.

LTE-R NETWORK ARCHITECTURE

The network architecture of LTE-R is similar to that of LTE. To simplify the architecture of the system, LTE is designed to be a fully packet switched network based on IP. The LTE system architecture consists of three elements: evolved-NodeB (eNodeB), Mobile Management Entity (MME), and Serving Gateway (S-GW)/ Packet Data Network Gateway (P-GW). The eNodeB provides the user with radio interfaces and performs Radio Resource Management (RRM)

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HANDOVER PERFORMANCE WITHIN LTE-R

In the completion phase of handover, the UE responds with a handover Confirm message. The target cell sends Path Switch Request message to the MME. After receiving the request, the MME informs the S-GW about the change in the data path of the UE. Finally, the source UE receives UE

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the serving cell by a number equals to HO hysteresis (in dBs); this condition has to be satisfied for a period equal to the TTT. The values that can be assigned to the Hysteresis and TTT are defined in the LTE specification. These values are: Hysteresis (0 – 30 dB), TTT (0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560 and 5120 ms). Fig.1 illustrates the A3 handover triggering within 3GPP LTE.

Context Release message, upon receiving these messages the source cell can release the resources for the UE. C. Measurement Report Metrics In LTE, the serving cell is the responsible for the handover decision. To assist these decisions, UEs send measurement reports on specific conditions. As mentioned before, there are two measurement metrics defined in LTE: RSRP and RSRQ. RSRP is defined as the average received power of all resource elements that carry cell specific reference signal. It is calculated from the source eNodeB transmit power (Ps) and the path-loss values (Lue) observed by UE.

RSRSP (dBm)

Target eNodeB

RSRP values which UE receives are as follows: RSRPs,ue = Ps – Lue

TTT (1) Hysteresis

and RSRQ = N × (RSRP/RSSI)

(2) Serving eNodeB

where N is the number of Resource Block and RSSI is Received Signal Strength Indicator. RSSI is the wideband received power by the UE including thermal noise and interference in the target eNodeB. The RSRQ parameter provides additional information when RSRP is not sufficient to make a reliable handover decision. For handover procedure, the LTE specification provides the availability of using RSRP, RSRQ or both.

HO Triggering Event Fig. 1. Triggering of A3 event handover

IV.

D. handover Triggering Events The reporting criterion can be event triggered or periodic. For event based triggers, there are some events from which one can choose the desired condition of HO Triggering. These events are defined in the specification [16] and listed in Table 2.

WINNER (stands for Wireless World Initiative New Radio) is an organization of 41 partners coordinated by Nokia Siemens Networks working towards enhancing the performance of mobile communication systems [17]. WINNER I work package 5 (WP5) focused on multipleinput multiple-output (MIMO) channel with frequency range of 5 GHz.

TABLE 2 HANDOVER TRIGGERING EVENTS Event

Triggering Condition

A1

Serving eNodeB power is greater than threshold

A2

Serving eNodeB power is lower than threshold Target eNodeB power is greater than serving one by certain threshold Target eNodeB power is greater than threshold Serving eNodeB power is less than threshold 1 and target eNodeB power is greater than threshold 2

A3 A4 A5

WINNER II continued the work of the WINNER I project by developing and optimizing it towards a detailed system definition. The WINNER II project work package 1 (WP1) continued WINNER I channel modelling work and extended the frequency range to 6 GHz. It also extend also the number of scenarios. WINNER I models were updated and a development of new set of multidimensional channel models was presented. The channel models cover various propagation scenarios including rural area moving networks where both the eNodeB and the UE are moving possibly at very high speed. An example of this scenario is high-speed trains where wireless coverage is provided by moving relay stations (MRS) mounted, on the roof, this propagation scenario is called D2. The connection from the eNodeB to the MRS is called D2a while the connection from the MRS to UE is modelled with the model (D2b). This is the suitable

In this work, A3 Triggering event is selected. In this type, handover is triggered based on two triggers controlled the handover process, first trigger is the Hysteresis, or "HO hysteresis", and the second is "Time to Trigger" (TTT). The UE makes periodic measurements of RSRP and RSRQ using the RS received from serving cell and the strongest adjacent cells. If handover Algorithm is based on RSRP measurements, handover is triggered when the RSRP value from an adjacent cell (target cell) is greater than the one from

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WINNER II CHANNEL MODELS

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time


WINNER propagation scenario for moving trains and is selected in this work to simulate LTE-R network, its specification is listed in Table 3 [17].

event then adapting it to be suitable for the modified Algorithm. In the Power-Distance Algorithm, the received power from target cell must exceed the received from the serving one by a certain threshold called Triggering Margin (same as HO Hysteresis in A3 Algorithm). The TTT metric is discarded because LTE-R network is piecewise linear network which implies that as the received power from the target cell exceeds a certain value and as the train moves toward this cell, it is obvious that the power received will increase continuously and there is no need to continue measuring the received power for a certain time to ensure that choosing this cell as a target is the right decision. So, as soon as the HO Margin condition is achieved, the HO Triggering will be initiated immediately.

TABLE 3 D2A PROPAGATION SCENARIO SPECS Scenario D2a D2b

Defination Rural Moving networks Scenario: eNodeB – MRS, Rural Moving networks Scenario: MRS – UE,

LOS / NLOS

Mobility (Km/h)

Frequency (GHz)

LOS

0 – 350

2–6

LOS / NLOS

0–5

2–6

Moving networks scenario D2a targets mainly fast trains with maximum speed of 350 km/h. The connection to the train is established by using a moving relay station (MRS) which is mounted on the roof of the train carriage. The D2a scenario is specified as follows:    

The benefits of Power-Distance Algorithm over A3 Triggering Algorithm are: 1. The HO Triggering is carried out at a certain predefined distance based on the system parameters. 2. The HO Triggering is independent on the train speed. 3. The total processing power in UE is reduced due to reducing the number of power measurement procedure before HO decision is taken. Only one measurement procedure is needed to trigger HO. This feature will be discussed in more details latter in this section.

Base Stations are arranged in a track with intervals of 1000 - 2000 m. The distance between base stations and the tracks is 50 m with antenna heights of 30 – 32 m, or 2 m with antenna heights of 5 m. Height of the train (with MRS mounted on the roof) is 1.5 m. Train speed is up to 350 km/h.

Fig.2 demonstrates the network architecture of the proposed Power-Distance Algorithm. The distance between the two eNodeBs, named (y) in Fig, is ranged from 1000 to 2000 m. These values are stated in the WINNER Channel specification. Train will handover when it reach a point that is x m far from the serving cell.

The D2a model is based on the TUI fast-train measurements. The carrier frequency of 5.2 GHz and 120 MHz bandwidth are used in the measurement. Antenna is as vertically polarized with max power of 27 dBm. Finally, the path loss models have been developed for various WINNER channel models based on measurements results carried out within WINNER project. These path loss models formula is given by [17]: PL = A log10(d) + B + C log10(fc / 5.0)

(3)

where d is the distance between the transmitter and the receiver in m, fc is the system frequency in GHz, the propagation parameters A, B and C are defined for each scenario. For D2a Propagation scenario, the path loss parameters are: A =21.5, B = 44.2, C =20 and the path loss equation is given by: PL = 21.5 log10(d) +20 log10 (fc / 5.0)+ 44.2 V.

Fig. 2. Network Architecture for Power – Distance Algorithm.

The Power-Distance Algorithm steps are summarized in the following points: 1. The UE is attached to the first eNodeB (with high RSRP), which acts as a serving cell until it reaches a certain point (at a distance “x” from serving cell). 2. At this point, UE measures the RSRP from both cells, generates a measurement report and sends it to the serving cell. 3. If the RSRP from target cell is greater than the serving one, the handover is triggered immediately, with no need for waiting any additional times. 4. Else, the UE remains attached to the serving cell and the UE returns to the classical A3 event based Triggering Algorithm. This happens if the target cell is out of

(4)

POWER-DISTANCE HANDOVER PROPOSED ALGORITHM

In this paper, a modified handover Triggering Algorithm is proposed for LTE-R network based on the distance between the MRS located in a train and both eNodeBs. This Algorithm is called Power-Distance Algorithm because it depends on the RSRP and intermediate distance between MRS and eNodeBs. Work in this Algorithm is started by applying the same conditions of the classical A3 Triggering

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service or has some problems which is rarely occurs. The UE may send a notification message to the control unit to declare this issue.

For y = 2000; Eq. (10) becomes: 21.5 log10(x) – 21.5 log10(2000 – x) ≥ 3

As one can see, the total number of measurement procedures is only one which reduces the processing power and consequently, the complexity of the UE. Based on the previous points, the very critical factor that controls the Triggering events is the distance (x). It must be defined preciously so as to get the correct handover decision. This distance is calculated as follows: first, the distance between the 2 cells is chosen to be 1000 m, i.e., y = 1000 (ignoring the distance between eNodeB and Railway Track).

(13)

Again x will be 1159.278 m which is also 57.96% of the distance between the two eNodeBs. This percent is changed only with changing handover Margin. It can be obviously shown that the train speed does not exist in our calculation. This makes our Power-Distance Algorithm independent on speed. In real world, the train speed is changed during its journey and it takes an amount of time in accelerating to reach its steady state speed. So, whenever the train travelled the predefined distance, the handover will be triggered regardless its speed variations.

The RSRP from the serving eNodeB is defined as: RSRP|s = P|s – PL|s

VI.

(5)

A. A3 Triggering Algorithm Simulation

where P|s is the serving eNodeB transmitted power and PL|s is the path loss value from MRS to the serving eNodeB. Substituting (4) in (5) one gets: RSRP|s = P|s – 21.5 log10(x) – 20 log10 (fc / 5.0) – 44.2

In this section, X2 handover is simulated based on A3 event Triggering Algorithm for a moving train. The simulation is carried out using MATLAB. In this simulation, only two eNodeBs are considered. One of them acts as a serving cell and the other is the target cell. A UE, located in the train, is attached to the serving cell at the beginning of the simulation. A train moves between both eNodeBs with constant speed 120 km/h. While the train is moving, UE measures RSRP and sends them back to the serving eNodeB. Based on the measurement report, the serving eNodeB will send handover request to the neighbor eNodeB when the measurement report of neighbor eNodeB is considered as better than serving eNodeB with the value of hysteresis, A3 event is triggered. After the event is triggered, the UE continues to measure the environment for a duration equals Time to Trigger (TTT). The effect of changing speed on the handover Triggering time is also simulated. The simulation parameters are summarized in Table 4.

(6)

Similarly, the RSRP from the target eNodeB is defined as: RSRP|t = P|t – PL|t

(7)

Substituting (4) in (7) one gets: RSRP|t = P|t – 21.5 log10(1000 – x) – 20 log10 (fc / 5.0) – 44.2

(8)

For handover to occur, RSRP|t must exceeds RSRP|s by the handover Margin, which is chosen to be 3 dB. The handover condition is: RSRP|t – RSRP|s ≥ 3

(9)

TABLE 4 SIMULATION PARAMETERS

Substitute (6) and (8) into (9) and take into consideration that P|s is equal to P|t one gets: 21.5 log10(x) – 21.5 log10 (1000 – x) ≥ 3

(10)

Solving this equation for x one gets: x ≥ 579.638 m

(11)

So, the minimum distance at which the handover is triggered is 579.638 m, which is equal to 57.96 % of the distance between the two eNodeBs. Now, repeat the same procedure for y = 1500 and 2000 m, respectively. For y = 1500, Eq. (10) is changed to: 21.5 log10(x) – 21.5 log10(1500 – x) ≥ 3

Parameter

Value

Carrier Frequency

5200 MHz

Bandwidth

120 MHz

Number of Resource Blocks

25 RB

Thermal noise (kT)

-174 dBm / Hz

eNodeB Tx power

27 dBm

eNodeB noise figure

9 dB

Train speed Train travelled Distance

Hysteresis

120 Km/h 1.5 Km WINNER II D2a Path Loss Model Log Normal with Standard deviation = 4 dB 3 dB

Time To Trigger

1024 ms

Handover Triggering Event

A3

Path Loss Model Shadow Fading

(12)

So, x = 869.458 m which is also 57.96 % of the distance between the two eNodeBs.

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SIMULATION PROCEDURE AND RESULTS

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In Fig. 7, handover will occur at a distance 869.45 m from the serving cell (x = 57.96 % of y). Fig. 8 shows that the handover triggering point occurs at a distance x = 1159.27 which is also 57.96% of y.

In Fig. 3, RSRP in (dBm) is plotted versus time. As one can see, the handover occurs when RSRP for eNodeB2 (Target cell) is equal to – 120 dBm which is 3 dB greater than serving cell (the value of Hysteresis), at time 27.024 s (after 1024 ms (TTT value) from deciding to handover).

Fig. 4 Handover Triggering at speed 250 Km/h Fig. 3 Handover Triggering at speed 120 Km/h

Now, the simulation is repeated with the same parameters in Table 4 and change the speed of the train to be 250 km/h and 350 km/h, which represent the speeds of high speed and very high speed train respectively. Fig. 4 & Fig. 5 shows the simulation results for speed 250 and 350 km/h, respectively. As one can say, the HO triggering time is changed with train speed. It is obviously shown that as the speed increases, the HO Triggering time decreases. B. Power-Distance Algorithm Simulation Results The simulation parameters for Power-Distance Algorithm are previously summarized in Table 4. Simulation results are illustrated in the following figures. In Fig. 6, RSRP in (dBm) is plotted versus distance. As one can see, the handover occurs at a point far from the serving cell with a distance equal to 579.638 m. At this distance the difference between RSRP from target and serving cell is exactly 3dB (as verified by mathematical calculation). This handover Triggering point remains the same for all possible train speed. This means that, this triggering point is the same for the selected 3 values of train speed discussed in Figures 3 to 5 (120Km/h, 250Km/h and 350 Km/h). As the intermediate distance between the two cells changes, the handover Triggering point will also change. Again this point will be valid for all train speed values. Fig. 7 and 8 draw the power distance graph for intermediate distances 1500 and 2000 m, respectively.

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Fig. 5 Handover Triggering at speed 350 Km/h

Fig. 6 Handover triggering at intermediate distance 1000 m

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independent, so the handover decision does not change with the train speed. It is a good point because the train speed is changed many times during the journey. Also, the number of measurement procedures carried out by UE is reduced to only one which results in simplifying the UE design and reduces its total processing power and radio resources provided by eNodeB. REFERENCES [1] [2] [3] Fig. 7 Handover triggering at intermediate distance 1500 m

[4] [5] [6] [7]

[8]

[9] [10] Fig. 8 Handover triggering at intermediate distance 2000 m [11]

VII. CONCLUSION [12] [13]

In this paper, a modified handover Triggering Algorithm, called Power-Distance Algorithm, is proposed. This algorithm depends on the intermediate distance between the two base stations and the power received from them. The handover occurs at a certain predefined distance independent from train speed. This distance varies according to the handover Margin which has the same values as A3 Triggering event Algorithm. As results show, the distance at which handover occurs is a fixed ratio from the intermediate distance, 57.96 % in this work. This ratio varies by varying the handover Margin.

[14] [15] [16] [17]

This Algorithm surpasses the classical A3 Triggering Algorithm in some points: first, this Algorithm is speed

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