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Study on the Integration B etween theGSM Cellular Network and a Satellite SystemFrancesco Delli Priscoli (*), Flavio Muratore (**)

(*) Univ. di Roma "La apienza", Dip. di Inform. e Sistem., Via Eudossiana. 18 - 00184 Roma, Italy(**) CSELT - Centro Studi e Laboratori Telecomunicazioni S.p.A. - Via G. Reiss Romoli, 274 - 10148 Torino, Italy

ARSTRACT This paper deals with the full integration of theGlobal System for Mobi le communica t ion (GSM) pan-European cellular network with a multi-spot geostationarysatellite. The basic requirement of full integration is that thepre-existing GSM network must not be modified because of theinsertion of the satellite.A fully integrated scenario is presented in the paper; inparticular, the satellite counterpart of the G SM cell is id entified.In a fully integrated scenario "single-mode'' MSs only equippedwith the GSM terminal coexist with " dual-mod e'' MSs equipped

with both the GS M and the satelli te terminal. Two basiccategories of dual-mode MSs are considered: the transportableMSs and the vehicular MSs. For both categories the satellitelink feasibility is studied by exploiting an ad hoc simulationtool; moreover, the system capacity is determined taking intoaccount the presence of the voice activation mechanism.The presen ted paper i s part ly based upon the workperformed by the authors in the framework of an EuropeanSpace Agency (ESA) study on the compatibility betweensatellite s ystems and the GSM cellular network. Telespazio(Rome) is the prime contractor of this study. The op inionsherewith reported are not necessarily those of ESA.1. IntroductionThe fu ll integration between cellular networks and satellitesystems is a very prom ising issue since it permits to immediately

provide areas lacking in terrestrial facilities with the radioservices offered by the cellular networks. In particular, thispaper deals with the integration of the GSM cellular networkwith a multi-spot geostationary satellite operating at L band.In general, th e satellite systems can provide a limitedcapacity with respect to terresmal networks, nevertheless theyare particularly suited in order to cover large terrestrial areasoffering a scarce amount of traffic, since in these areas it is notconvenient to implement the cellular network equipment.Finally, the satellite systems can be profitably used inorder to cope with co ntingent situations of unavailability of theterrestrial carriers (e.g. due to traffic congestion in some cells[11) and/or in order to shorten the terrestrial tails (a mobile anda fixed user involved in a call can communicate, via satellite,through the base station closest to th e fixed user).The basic requirement of the ful l integration is that theinsertion of the satellite system does not entail any m odificationof the hardware, the software and th e functionalities of thealready existing GSM entities.In section 2, the integrated system architecture is outlined;as it is shown in [2], such system architecture permits a naturalex t ens ion of the GSM proce dures (e .g . ca l l se t -up, ca l lselectionheselection, handover, initial acquisition, tracking) to0-7803-0917-0/93$03.000 1993 IEEE

the integrated system. In section 3, the required performance ofthe satellite system entities, i.e. the reference p ayload, the FixedEarth Station (FES) and the Mobile Station (MS) are analyzed;moreover, th e statistical characteristics of the propagationchannel are explained and the relevant performance (obtainedby means of the simulation tool already presented in [3]) aredisclosed; finally, the satellite system capacity is determinedtaking into account the voice activity factor.2. System Architectu re

2.1 Basic characteristics of the integ rated systemIn a fully integrated scenario "single-mode'' MSs onlyequipped with the GSM terminal coexist with "dual-mode'' MSsequipped with both the GSM and the satellite terminal. Twobasic categories of dual-mode MSs are considered in this paper:- the "transportable" dual-mode MSs equipped with a GSMterminal such that the Base Band uni t can be extracted, placedin a suitable position and linked to a proper satellite RadioFrequency section (including, for instance, a folding antenna).The transportable MS users, when roaming inside the GSMcoverage , can communica te via the GSM terminal ;conversely, when roaming outside the G S M coverage, inorder to communicate via satellite, they stop and use themobile terminals after a proper antenna positioning (the sameterminals can be useful also inside the GSM coverage in caseof unavailability of the GSM link);- the "vehicular" MSs equipped with a completely integratedGSM-satellite terminal including a single Base Band unit andtwo different Radio Frequency sections. These M S s accessthe GSM network or the satellite system in a way completelytransparent to the users (see [ l ] and [ 2 ] ) ; n pmicular, sincethe satellite resources are the most valuable, they access thesatellite system only if the GSM link is unavailable (e.g.because the MS is outside the GSiM co verage).The FESs must play the same role as the G S M BaseStation Systems (BSSs). This means that the FESs must rcusethe BSS hardware up to the Intermediate Frequency equipmentand most of the BSS software.The satellite link between FESs and dual-mode MSs mustreuse the channels, the access technique, the modulation, thecoding and most of the protocols of the GSM l ink betweenBSSs and single-mode MSs.From now o n, only dua l-mode MSs will be considered: forbrevity, they will be referred to as MSs.

2.2 Reference scena rioIn order to extend to the integrated system th e GS Mprocedures (see [2]), it is important introducing the "satellitecell". i.e. the satellite counterpart of the GSM cell.588

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A multi-FES multi-spot scenario is considered. Let usassume that the satellite system in cl ud es N E S S and M spots(in the mobile link) and let us define a parameter Cij (i=l, ...,N ) , ( j = l , ...,M ) e q u a l to 1 or 0 ccor d ing to whe the r aconnection, via satellite, between th e i-th FES and the j-th spotis or is not present. Clearly,N MN j = C C i j , M i = C C i j

i= 1 j= 1represent the number of FESs connected with the j-th spot andthe number of spots conne cted with the i-th FES, respectively.Since the j-th sp ot iscovered by Nj FESs with Nj differentlinks via satellite, the above spot can be considered as theoverlap of N j "satellite cells". These Nj satellite cells have thesame coverage area (i.e. . t h e area covered by the spot inquestion); however, each of these satellite cells is served by adifferent FES (one of the Nj FESs coverin g the spot).On the other hand, the i-th FE S serves a number ofsatellite cells equ al to the number of spots it is connected with,i.e. equa l to Mi. Such arran geme nt facilitates frequency reuse innon-adjacent spots.From the above discussion, it can be deduced that. ingeneral, a MS roaming in the integrated system, is covered bymore than on e satellite cell. Proper criteria can be defined (see[2]) for selecting the most convenient satellite cell (e.g. eachMS can compile a satellite cell preference list on the basis ofthe a r e a (s ) i t m o r e f r e q u e n tl y c o m m u n i c a t e s w i t h ).Nevertheless,as already m entioned in section 2.1, a MS selectsa satellite cell only in case no GS M cell is ava ilable.

As fa r as the FES-to-spot connection is concemed, thepossible solutions range from the "s imple connection" (each FESis allowed to com mu nica te only with a spot, i.e., in general, thespot where the FES s placed), to the "ful l connectio n" (each FESis allowed to com municate with all the spots). Clearly, in thesimple connection case, the satellite system includes N satellitecells; in the full connection case, it includes N -M satellite cells.Obviously, the full connection solution is more flexible.However, it could be inefficient since, in the GSM system, adedicated carrier (the BCCH carrier) must be transmitted in everycell. Moreover, the full connection solution imposes morestringent requirements on the on-board processor (e.g.. p a t e rnumber of on-board filters) and/or more comp lex frequency plans.

3. Satellite Link Feasibility3.1 Payload characteristicsThe reference payload in this paper is the L-band LandMobile (LLM) payload. LLM is a transparent payload coveringWestern Europe with a global beam at the Ku-band feeder link(from the FES to the satellite and vice versa) and with threespots at the L-band mo bile link (from the satellite to the MS an dvice versa). The p ayload allows on-board switching (controlledvia telecommand) of the carriers from the global beam to anyspot and vice versa. The most recent spe cifications indicate thefollowing EIRP and G / r performance:- at Ku-band: EIR P = 33.5 dBW, G/r = -1.4 dB/K;- at L-band: EIRP = 45 dBW. G/T = 0 dB/K.

3.2 Fixed Earth Station characteristicsIt is assumed that the FES is a Ku-band VSAT having anantenna diameter of 1.8 m, a gain of 46 dBi and a figure ofmerit (G/T)of 20 dB/K.

The required FES power per carrier results from theforward link budget shown in table 1.This budget shows that,by assuming mobile power losses equal to 1dBW, FES powersper canie r equal to 1.4 dBW (1.4 W) and to 15.4 dBW (34.7W) are required in the transponable and in the vehicular cases,respectively.3 3 Mobile Station characteristics33.1 Radio Frequency equipmentThe frequency band (L band) i n which the MS operatesforces the use of a specific front-end in addition to the GSMone. Radio Frequency filters. power and low noise amplifiers,up and down converters and frequency synthesizers need aduplicationor a new matched design.

As far as the antenna is concemed, in the transportablecase, it is considered a parabolic antenna of gain equal to 16dBi (diameter 0.6 m) and figure of merit (G/T) of -8 d B K . I nthe vehicular case, it is adopted a very directive antenna of gainequal to 12 dBi, figure of merit (Gmof -12 dB/K.The required mobile power per carrier results from thereturn link budget shown in table 1. This budget sho ws that, byassuming m obile power losses equal to 1 dB, peak mobilepowers per carrier equal to 14.4 dBW (27.5 W) and to 15.6dBW (36.3 W) are required i n the transportable and in thevehicular cases, respectively.I t is impor tant noting that such peak powers can besupplied by solid-state amplifiers because of the MS burstyemission; as a matter of fact, in the full-rate channel case, theMS only transmits during 1/8 of the frame period (U16 in thehalf-rate channe l case).3.3.2 Base Band and Intermediate Frequency equipmentThe Inte rmedia te Frequency equip ment is comp atible bothfor the terrestrial and the satellite links.As to th e Base Band equipment, i n the performanceevaluation carried ou t in the next section, the considered BaseBand filter is a Butterworth filter (ideally equalized) with 5poles and 3-dl3 bandwidth equal to half the bit rate. In [3] it is

shown that this f ilter is particularly suita ble to a satelliteenvironment.The Base Band GSM functions ( speech and channelc o d i n g a n d d e c o d i n g , d e m o d u l a t i o n , f r a m e a n d c l o c kgener a t ion) a r e co l lec ted i n some Appl ica t ion Spec i f icIntegrated Circuits (ASICs). The complexity of these circuitscan be estimated close to 600000 transistors. Moreover, thecontrol procedures of the terminal require micro programmeddevices with at leas t 150 kbytes of code. Since the framestructure and its synchronization can remain the sam e for theterrestrial and the satellite links, the AS ICs developed for GSMare compatible also for the satellite environ men t; minor change sare only necessary in the control firmware, for instance, in orderto handle the satel l i te ini t ia l acquis i t ion synchronizat ionprocedure (as it is shown in [2], this proced ure is different fromthe GSM one due to the very higher t ime delays of the satellitelink).

3.4 Th e propagation channel and the simulation resultsA typical model for the propagation channel in a landmob ile satellite system con siders that the total field received bya MS is the vector sum r of three major components: the directl ine-of -s ight component rd between satel l i te and mobile589

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antenna, the specular (or reflected) multipath component rs dueto the ground reflection in the mobile antenna direction and thediffuse multipath component rm, which represents the powerscattered into the antenna main beam and side lobes fromsurrounding objects (terrain irregularities, buildings, roadsidetrees, etc.). Moreover, the propag ation channel model assumesthe presence of additive white Gaussian noise.In [4] it is shown that, in rural areas, the reflected energy isprimarily contained in the diffuse multipath compon ent, with anegligible amount of energy in the specularly reflected one.Then, the diffuse multipath, with its well known Rayleighdistributed amplitude and uniformly distributed phase, representsthe only component to be added to the line-of-sigh t one.Therefo re, the envelope of th e vector sum r can becharacterized by the Rice-Nakagami probability densityfunction:(3.1)

where 42s is the root mean square envelope of the multipath, Iothe modified Bessel function of order zero, while r and rd arethe r ece ived enve lope and t h e l ine - of - s igh t componentamplitude, respectively. The parameter characterizing the Ricestatistics is the ratio cr between the line-of-sight signal powerand the diffuse multipath power:

2'd (3.2)2 2o=-

Note that for cr =- B(i.e, if the line-of-sight componentis absent) , the relation (3.1) becomes the usual Rayleighprobability density function. In rural areas, in case of verydirective antenna at the MS the cr ratio ranges from 18 to 20 dBThe described propagation channel has been sim ulated bymeans of a sof tware package developed at CSELT. Thisprogram is capable of simulating the overall radio chainspecified by GSM, including co-decoding, mo-demodulation

and propagation effects.The simulation results display the BER (Bit Error Rate)performance versus the mean value of Ec/No (Energy pertransmitted bit/Noise spectral density) by varying the cr ratio(3.2) between -00 and +-.The performance has been evaluated for the GSM speechchannels at both full-rate ( T C W S ) and half-rate (TCHn-IS)(*) and for the GSM full-rate data channel (T C W . 4 ) .In the speech channel case, for the different classes of bitprotection, i.e. class 1 and 2 (these classes refer to coded anduncoded case, respectively, in the sense specified in the GSMRec. 05.03). the BER has been evaluated in terms of- "Gross BER" (ratio of the number of errors over all th etransmitted frames to the total number of transmitted bits),- "Residual BER" (ratio of the number of errors in the "good"frames to the number of transmitted bits in the "good"

frames),- "Bad Frame Indication (BFI) ratio" (ratio of the number offrames defined as "bad" to the total number of transmittedframes).

(see W.

(*) As TCH/HS has not yet been specified by ETSI, for the voice andchannel coders the CSELT proposal, that passed the first ETSIselection [5].has been adopted.

In Figs. 1, 2 and 3 the curves relevant (0 the Gross BER,the Residual BER and the BFI ratio of a (full-rate and half-rate) speech channel are shown. In Fig. 4 the curve relevant tothe BER of a full-rate data channel is show n. These figures arerelevant to a MS speed equal to 20 0 kmh; additional results(not shown in this paper for space reasons) have been obtainedfor a MS speed equal to 3 km h and 100km/h.By using the above-mentioned results, it is possible todeduce that, in order to satisfy the BER constraints specifiedby the GSM Rec. 05.05, an EC/Noequal to 3 dB is required.In the vehicular case, the attenuations caused by obstaclesin the direct path between the MS antenna and the satelliteantenna must be taken into account. A shadowing margin isconsidered just for compensating these attenuations. In thetransportable case, by placing the MS in a position such thatthe direct path between the antenna and the satellite is freefrom any obstacle, the shadowing margin is of 0 dB .In the vehicular case, var ious tables and empir icalexpressions are available in the literature giving the shadowingmargin versus the environment, the satellite elevation angle,the terrestrial coverage probability. From these results, i tcomes that, in the vehicular case, the integrated system understudy can work only in a rural environment, since differentenvironments (i.e. sub-urban, urban, ...) would require veryhigh figures (greater than 10 dB) of shadowing margin and,consequently, a too high power level of the mobile unit. In arural environment i t is possible to calculate (see 151) that, at anelevation angle of 40, wi th in a 60% time probability, theattenuation experienced is less than 3 dB (75% and 90% if theattenuations experienced are less than 4.7 dB and 7.2 d B ,respectively). A 3 dB shadowing m argin is reponed i n the l inkbudgets shown in table 1.It is important noting that, in the vehicular case, thein tegr a ted sys tem becomes ab le to wor k also i n otherenvironments in case a m ore advanced payload than LL M (i.e.a pay load wi th h igher EI RP and G/T and / or a non-geostationary payload allowing higher elevation angles) isconsidered.

3.5 System capacityThe required EIRPs per carrier (at Ku band and at Lband) result f rom t h e l ink budgets shown i n t ab le l . B yconsidering the overall LLM EIRPs specified in section 3.1, itcan be easily deduced that the satellite system is able tosupport 1 7 and 10 simultaneously active GSM carriers in thetransportable and in the vehicular cases, respectively.In a power limited system, the voice activity factor lessthan 1 can be used to improve the number of simultaneousactive GSM carriers, by properly increasing the occupied band.Such mechanism is character ized by the fol lowingparameters:- Pb: tolerated block probability (it is assumed equal to 0.05);- V,: average voice activity of a single voice channel (it isassumed equal to 0.4);- M: n u m b e r o f s i m u l t a n e o u s l y a c t i v e G S M c a r r ie r s

supportable by t h e system (see above);- N: number of available carriers in the system (N is greaterthan M thanks to the voice activation mechanism);- Pv: f ract ion of M utilized by the voice channels ( i t isassumed equal to 0.75); 0.25 is the fraction of M utilized bydata channels;- Nv: number of available voice carriers.590

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The above parameters satisfy the Bernoulli relations:

RETURN LINK BUDGETUPLINK BAND)

N =Nv +M - [M Pv]

Transportable Vehicular

where the symbol [....I indicates the closer integer number.Taking into account that each GSM carriers supportseither 8 full-rate channels or 16 half-rate channels, by applyingthe previous relations, the available channels listed in table 1can be determined.

Available full-rate channels (**)Available half-rate channe ls (**)

4. ConclusionsThe presented satell i te cell concept allows a naturalextension of the GSM network protocols to a multi-spot multi-FES satellite system.It is shown that, both for the transportable and for thevehicular case, the satell i te l ink is feasible. Howev er, thetransportable cas e is less demanding than the vehicular one interms of FE S and MES performance: in particular, in thevehicular case, a MES peak power per carrier equal to 36.3 Wis required (the average power is equal to about 4.6 W and 2.3W in the ful l - ra te and i n th e ha l f - r a t e channe l c a se ,respectively).In the transportable case, the satellite system can supporte i t he r 224 fu l l - r a t e channe l s o r 448 ha l f - r at e channe l s ,guaranteeing the GSM quality for the 95% of the time. In thevehicular case, the satellite system can support either 128 full-rate channels or 256 half-rate channels, guaranteeing the GSMquality for the 57% of the time (the shadowing and the voiceactivity can be considered as two statistically independentphenomena): by increasing the shadowing m argin (see section3.4). it is possible to increase this percentage of t ime at theexpense s of the satellite system capacity.In general, more advanced payloads could permit toincrease th e coverage probabili ty and the satell i te systemcapacity as well as to relax the mobile terminal specifications.

224 128448 256

AcknowledgementsThe author wish to thank Dr. Xavier Benedicto and Dr.Martin Langhorn (ESA), Dr. Laura Bonzano, Dr. RaffaeleMenolascino, Dr. L o r i s Stola and Dr. Franco Settimo (CSELT),Dr. Fulvio Ananasso. Dr. Marco Carosi and Dr. Antonio Puccio(TELESPAZIO), F'rof. Enrico Del Re (Universid di Firenze)for their helpful suggestions in the framework of thework.References[ l ] F. Delli Priscoli, E. Del Re, P. Iannucci ,R. Menolascino, F.Settimo: "Architectures and Protocols for an IntegratedSate ll i te-Terrest r ia l Mobi le System" , 3rd Int . Mob i leSatellite Conference and Exhibition. Pasadena (USA). June1993[2] F. Del l i P r i sco l i , "Ne twork a spec t s r e l evan t to th eintegration between the GSM network and a satell i tesystem", 2nd Int. Conf. on Universal Personal Comm.(ICUPC). Ottawa (Canada), Oct. 1993[3] F. Delli Priscoli, F. Muratore, "Link Feasibility Analysis fora S atellite System Operating at L-band Integrated with the

GSM Network", 2 nd Int . Conf . on U niversal PersonalComm. (ICUCP), O ttawa (Canada), Oct. 1993[41 V.J. Vogel and E.K. Smith, "Propagation Consideration inLand Mobile Satellite Transmission", Microwave Journal,Oct. 1985[5] CSEL T, OTE, ELESPAZIO: "Ass essment of a PublicMobile Satellite System Compatible with the GSM CellularNetwork", Final Report, 1993

[FORWARD LINK BUDGETIlUPLINK N uBAND)FES power per carrier (dBW)FES antenna gain (dBi)FES power loss (dBW)Uplink path loss (dB)Miscellaneouslosses (dB) (*)Boltzman constant (dBW /K)Satellite G/T (dB/K)Uplink C/Nn (dBH z)DOWNLINK (LBAND)Satellite EIRP per carrier (d BWDownlink path loss (dB)Miscellaneous osses (dB) (*)Shadowing margin (dB)Boltzman constant (dBW/K)Mobile G f l (dB/K)Downlink C/No (dBH z)

Transportable1.446.0-1.0-207.4-4.5228.6-1.461.732.1-188.2-1.00.0228.6-9.063.1

Vehicular15.446.0-1.0-207.4-4.5228.6-1.475.735.0-188.2-1.0-3.0228.6-12.059.4

Mobile power per carrier (dBW)Mobile antenna gain (dBi)Mobile power loss (dBW)Uplink path loss (dB )Miscellanoeus losses (dB) (*)Shadowing margin (dB)Boltzman constant (dBW /K)SatelliteG/T dB/K)Uplink C/Nn (dBHz)

14.416.0-1.0-188.8-1.00.0228.60.068.2

15.612.0-1.0-188.8-1.0-3O228.60.062.4DOWNLINK (Ku BAND)SatelliteEIRPper carrier (dBW)Miscellaneous losses (dB) (*>FES G f l (dB/K) 20.0 20.0Downlink path loss (dB) -206.4 -206.4Boltzman constant (dBW /K)Downlink C/Nn (dBHz)

IOVERALL ITransportable Vehicular IRequired &/No (dB)Implementation margin (dB)Carrier bit rate (dB)(270.833 Kbps)Overall C/No (dBHz)

3 O2.054.359.3 54.359.3 I(*) Rain loss,Pointing loss,Polarization loss(**) The se are alternative solution s (e.g.. in the transportable case, either224 full-rate channels, or 448 half-rate chann els are available)

Tab. 1 - Satellite link budgets591

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Mean EC/No (dB) -I

Fig. 1 - Gross BER (classes 1 an d 2) vs mean EC/No (Energy pert ransmit ted bi t /noise spect ral densi ty) in a t ime varying fadingchanne l p lus add i t i ve whi t e Gauss i an no i se fo r a speed of t hemobile receiver equal to 20 0 km/h. Parameter of the curves is thep o w e r r a t i o b e t w e e n t h e c o n s t a n t p r o p a ga t io n p a t h a n d t h emultipath component. The continuous curves refer to the TCH/FSvoice channel and the dashed one to the TCH/HS voice channel

-:0 -5 0 5 :0 15Mean EdNo (dB)

Fig. 3 - As Fig. 1, but for BFI

UW1Q:30v)WU

m

-

1

I! 3

Ii'

1:3

10-1 0 -5 0 ! 5Mean EdNo (dB)

1

Uw1Q:n

m3 1i2

i ' 1-!O - 5 0 1 15Mean E ~ N odB )

!O

Fig. 2 - As Fig. 1, but for Residual BE R

16'

10 -U

J10

10

- 5 0 5 1010 -10Mean EdNo (dE)

Fig. 4 - B E R v s m e a n EJ" ( E n er g y p e r t r an s m it te d bi t/ no is espect ral densi ty) in a t ime varying fa ding chann el plus addi t ivewhite Gaussian noise for a speed of the mobile receiver equal to200 !un/h. Parameter of the curves is the power ratio between thec o n s t a n t p r o p a g a t io n p a t h a n d t h e m u l t i p a t h c o m p o n e n t. T h econt i nuous curve s re fe r t o t he TCH/FS voice channe l and t hed as he d o ne to t he T C W S voice channel

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