8/8/2019 10.1.1.11.6291[1]

1/5

Design of Mobile Cellular Coverage in Tunnel Environments

Ramn M Ruiz Tarrs*, Florentino Jimnez Muoz**; Rafael Herradn Dez**,Jos M Hernando Rbanos***.

*TELEFNICA MVILES ESPAA, S.A. E-mail:ruiz_r@tsm.es**DIAC-E.U.I.T. de Telecomunicacin(UPM)

Ctra de Valencia, Km. 7, 28031 MADRID. Tel:91 3367785, E-mail:fjimenez@diac.upm.es***SSR-E.T.S.I.Telecomunicacin(UPM)

ABSTRACT

When planning cellular coverage in special areas, astunnels or indoors, the continuity of connections inprogress in entrances and exits is a critical point. Awider overlapping concept is introduced as a method toinclude environment relevant elements, like existingnetwork or the mobile channel, necessary in order toachieve a correct design. As practical example theanalysis in tunnels of a high-speed railway line ispresented, where a dedicated dual GSM/UMTScoverage system is designed. Also a prediction model isproposed and utilisation of radiating cable instead of antennas is discussed

I. INTRODUCTION

When a cell to cell handover should be done at theentrance or the exit, coverage overlapping between thecells involved must be ensured in order the handoverprocess is correctly achieved. This necessaryoverlapping is usually defined like a distance in bothdirections, in which the signal levels from de cell A andB are high enough to make possible the handover.

A

B

A B

Fig. 1. Handover process

This overlap concept does not take into account all thecharacteristics of the environment and the involvedcellular mobile system. This can lead to designs that,once done, do not achieve the desired service qualityrequirements. Modifying the installed systems is usuallyvery expensive.

As we will see, its much more reasonable definingoverlapping as the handover overlapping which means

the necessary overlapping for making possible that allhandover processes can be done. We will also see thatthe necessary level its not fixed only by the cells signallevel involved in the handover and that it does not exista symmetry between both overlap directions.

II. HANDOVER TIMMING

We will suppose that in instant t1 the access takes placeto cell A. Several timers begins at this time that avoidthat the mobile station leaves the cell A during a definedperiod of time, depending on the conditions of the radiolink.

time

t 1 A c

c e s s t o c

e l l A

. . .HO Temporizations

t 2 S t a

r t r e p o r t s

t 3 C e

l l B i n r e p

o r t s

t 4 M o

b i l e s y n c h

r o n i z e

d t o c

e l l B

?

( 3 - 5 s )

( 1 - 2 s )( 1 - 5 s )

Fig. 2. Time in HO process Detection

Independently of these timers, in time t2 the mobile hasreceived enough information from the network to beginto send information about neighbouring cells. In GSMthe neighbouring information is send to the BTS each480 ms using the associated channel SACCH. In UMTSthis associated channel doesn't exist. The transmissionof neighbour measurements is carried out when theyoccur certain "events" defined in specifications [1].

UMTS supports handover between different carriers andsystems that can be classified by the way of operation of the system and for the execution mechanism [2].According to the way of operation we can classify themin the following way:

- Intra-mode Handover: Between two FDD or TDDcarriers with the same or different frequency.- Inter-mode Handover: Between FDD and TDD

mode.

8/8/2019 10.1.1.11.6291[1]

2/5

- Inter-system Handover: Between different systems3G or 3G-2G.

Attending to the way of execution it can be classified as:

- Hard Handover: Between different frequencycarriers. The commutation to the new channel

means liberation of the old one. There is notemporary overlapping among both connections.- Soft Handover: The mobile station establishes

simultaneous connections through several stations.For this reason the communication is notinterrupted during the transfer, contrary to theprevious case.

- Softer Handover: They are soft handovers betweensectors of the same base station. The differencewith soft handover is in the combinationmechanism used in the uplink connection.

In this document we refer to the intra-mode handover inFDD between different sites, at the same frequency(soft) or among different frequencies (hard). Softhandover provides an additional gain in front of fastfading, reducing the Eb /No necessary to assure thedesired quality.

In t3 the mobile detects cell B. Time between instant t3and t2 is very variable depending on the followingfactors:

Cell B level and mobile channel conditions. Cells in service with same or better level that B.

Candidate set of neighbouring cells in UMTS orACTIVE cell list in GSM. Multi Band Cells Reported parameter (MBCR) in

dual mobiles GSM/DCS.

Mobile stations report information about neighbouringcells. In GSM mobile reports include information on thereceived level and the BSIC of up to 6 neighbouringcells. As much as higher is the number of neighboursmore cells they will be excluded of the report. Newneighbours in service can retard the entrance in thereport of cell B and, consequently, a handover to atunnel or interior fails where it was perfectly adjusted

before. In the same way, the MBCR parameterdetermines the number of neighbours reported by eachband in dual mobiles. Their modification affects at themoment that our cell B enters in the reports and,therefore, affects to the handover adjustment.

Once the terminal has detected cell B, it needs to obtainthe synchronisation with the cell.

In GSM, the mobile must read the channel SCH of theneighbouring cell. Theoretically, due to the GSMtemporary scheme, a mobile terminal connected to aTCH/FR channel needs between 0.6 and 2.28 seconds

(average value=1.38s) for synchronising to a neighbourand send the first report to the system containing thisneighbour. In practice, tests carried out with severalmobiles and real traffic analysis shown that this time is

usually between 1 and 5 seconds. However, it can belonger, depending mainly of the terminal manufacturerand the conditions of the radio link.

Also the UMTS mobile should be synchronised to theneighbour before performing the handover. UMTS is anasynchronous CDMA system, so the cell searchprocedure o synchronisation procedure differs greatlyfrom the procedure in a synchronous one like IS-95. InUMTS cells use different scrambling codes instead of different code phase shifts, nowadays terminaltechnology cannot search for 512 code of 10ms withoutany prior knowledge [3].

UMTS cell synchronisation procedure has basicallythree steps. Specification defines the process in ageneral way [1], but there are no requirements as towhich steps to perform and when. The practicalimplementation is open to the manufacturer.

Synchronisation properties scheme need to be taken intoaccount when setting network parameters. For handoveroptimal performance, target cell search in connectionmust be optimised. For example, correct planning of scrambling codes can improve it.

time

t 4 M

o b i l e s y

n c h r o n

i z e d t o c

e l l B

t 5 E n

o u g h r e p

o r t s

t 6 H O D

e c i s i o n

t 7 M o

b i l e s e r v e

d b y c

e l l B

?( 1 - 3 s ) ( 0.6 s GSM )

Fig. 3. Time in HO process Execution

At the moment network receives the first report of aneighbouring cell (instant t4) it is already possible to tryto carry out a transfer toward it. However, the systemgenerally evaluates several reports to assure that thehandover is performed at the right moment. Thenetwork operator determines the number of reports.

In t5, radio network controller has enough number of reports to decide. Handover algorithms will determinethe moment when handover should be performed (t6).Handover algorithms are also opened to the supplier.

Once the handover decision has been taken, the systemand the mobile begin the process that will take themobile station to be served by cell B (t7). This processtakes about 0,6 seconds in GSM under normal quality

conditions.Summarising, to evaluate the handover overlapping firstof all it is necessary to determine the geographical point

8/8/2019 10.1.1.11.6291[1]

3/5

where wanted neighbouring cell B is detected as apossible candidate. Starting from this time (t3) it ispossible to evaluate the necessary overlapping timeusing equation (1) and (2).

ii t t ti = + 1 (1)

= =6

33 i tihot (2)In GSM system their minimum value is:

st mhoi

it 04.76.0044.156 33 =+++== = (3)

III. HANDOVER IN TUNNELS

We are going to examine the specific problem withhandover in tunnels. We will suppose a tunnel with onlyone indoor cell (I) that assures the continuity of the calls

inside.

Clulas de entrada E

Clula interior IClulas de salida S

Fig.4. In HO (concentration effect) and Out HO(expansion effect)

A. In Handover

The entry cell or E cells are the set of possible serversbefore entering in the tunnel. It is usually the mostproblematic handover for the following reasons:

It is necessary to assure overlapping handover withcell I for each one of the possible entrance cells. The mobile is in a multiple neighbouring cells

environment that will retard the entrance of cell I inthe reported list.

The real problem is that cell I enters in the neighbouringreport (instant t3) when cell E still has tmho3 seconds inthose the cell E radio link has enough level and qualityto assure the whole handover process.

B. Out Handover

The cell or exit cells S are the possible candidates toreceive the call from the indoor cell I. In this case, theout handover benefits of the following:

There is only one handover origin cell (cell I) andseveral exit cells candidates. It is enough to assurethe overlapping time with anyone of them(expansion effect).

Before doing the handover, the only existing cell isthe server cell (cell I). Therefore, the exit cell entersin the neighbouring report when he is strong

enough to be detected.The average level necessary for the exit cell to bedetected will be:

Lmint3 = S + AtV + AtC + Mg (4)

Where:S: Mobile Sensibility

AtV: Vehicular lossAtC: Body lossMg : Fading margins

C. Tunnel inside handover

In tunnel inside handover case, we can suppose thatthere are only two cells involved (I1 and I2). Theminimum overlapping time will be tmho3 seconds sinceI2 cell average level is better than Lmint3. As in theprevious case, cell I1 should maintain a better level thanLmint3 during this whole time. Therefore, the existingoverlapping time is the time when the signal levels of both cells are better than Lmint3.

IV. HIGH SPEED RAILWAY TUNNELS

A. Tunnel Propagation Model

Tunnels in high-speed railway lines are straight-linetunnels or their curvatures have a very wide radius.Their transversal section is large due to tunnels shouldallow both circulation lines. In addition, a large sectiondecreases the pressure wave when trains enter in thetunnel. The pressure wave effect is also the reason toreduce the number and size of structures inside thetunnel, as the on-fly power supply system based onaerial wires.

In these environments, a coverage tunnel system basedon antennas show a guide-wave effect and no blockingeffect when the LOS area is left. Besides, the trainblocking losses is low because the train has a smallertransversal section than the tunnel.

Figure 5 and 6 shows the average signal strength curvesand the model curves for systems working at 910MHzand 2GHz in a typical high-speed railway tunnel,obtained in a measurement campaign in a Spanish high-speed railway line.

8/8/2019 10.1.1.11.6291[1]

4/5

Fig. 5. Averaged received signal and model at 910 MHz

The average signal strength curve is the result of averaging all the measures taken inside the tunnel,changing the transmission antenna position and thereceiving paths. Near the antenna, the real antennadiagram pattern causes the difference between theaverage signal strength curve and the model curvebecause the last uses the main features of the diagrampattern, but not the complete diagram.

Fig. 6. Averaged received signal and model at 2 GHz

The model curve responds to:

Line-Of- Sight (LOS) section

In this area the hybrid model [4] is used. It assumes tworegions in the LOS section.

[ ] LOSrup10rup0

ddd ) / log(10)log(20dd )log(20

8/8/2019 10.1.1.11.6291[1]

5/5

Doppler effect. For all these reasons we should take intoaccount the radiating cable as a good alternative tocover high-speed railway tunnels, even for highfrequencies as UMTS and in spite of the high cost of thecable and their installation.

Figure 8 shows a compensating attenuation comparativetable for GSM and UMTS, using antennas or radiatingcable. The Rayleigh margin used for this environment is4 dB [6].

Fig. 8. GSM and UMTS Power Budget in a tunnel

Figure 9 shows the maximum length that can be reachedfor two types of Andrews radiating cable [7].

Fig. 9. Maximum length for GSM and UMTS

Figure 10 shows the predicted propagation attenuationfor GSM, using the proposed model, in a tunnel of 1510m, with two antennas in the tunnel ends. There isnot direct vision among both ends.

It can be observed that, with the proposed solution, onlyan overlapping of approximately 400m is obtained. Itcan seem enough. However, the handover overlappingfor this case, when a handover inside the tunnel must beperformed, it should be of about 685 m for a maximumspeed (350 Km/h).

A possible solution to get the handover overlapping canbe to modify the position of the antennas or to useradiating cable instead of antennas.

Fig. 10. Attenuation in a high-speed railway tunnel

V. CONCLUSIONS

A wider coverage-overlapping concept has beenpresented. The effect of existing cells, mobile channelfeatures, network parameters or cell synchronisationprocedures over the radio network performance andhow the handover overlapping concept allows includethese factor in the whole design has been shown. GSMand UMTS handover timing keys are explained. Thenecessary handover overlapping coverage in severalsituations is analysed and calculated.

Also a model for high-speed tunnel propagationprediction and utilisation of radiating cable instead of antennas is evaluated. Finally, an example of application of the concepts introduced in this paper isshown.

REFERENCES

[1] 3GPPTechnical Specification 25.304 and 25.922,www.3gpp.org

[2] Jos M Hernando Rbanos, Cayetano LluchMesquida, Comunicaciones Mviles de TerceraGeneracin, Telefnica Mviles Espaa S.A.,2000, Tomo-I, Parte 2.

[3] Harri Holma and Antti Toskala,WCDMA for

UMTS , Ch. 6 Physical Layer, 2000.[4] Y.P.Zhang, A Hybrid Model for Propagation LossPrediction in Tunnels , Proc-MillenniumConference on Antennas and Propagation,Switzerland, 9-14 April 2000.

[5] Universidad Politcnica de Madrid - Grupo deRadiocomunicaciones Mviles, "Caracterizacinde instalaciones para comunicaciones mviles ",Oct-1999, Informe 3.

[6] Farrokh Abrishamkar, James Irvine,Comparisonof Current Solutions for the Provision of VoiceServices to Passengers on High Speed Trains ,IEEE - VTC 2000, pp. 2068-2075

[7] ANDREW Series RADIAX - Radiating Cable,www.andrew.com