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Wireless Networks 4 (1998) 109–124 109

Issues in satellite personal communication systems

Erich LutzInstitute for Communications Technology, Deutsche Forschungsanstalt fur Luft- und Raumfahrt, DLR, D-82234 Wessling, Germany

In the paper various issues in personal satellite communications are addressed. Basic geostationary and non-geostationary satelliteconstellations are considered. The narrowband and wideband characterization of the mobile satellite channel and related system impli-cations are discussed. Satellite diversity is presented as a measure to overcome signal shadowing. The capacity of TDMA and CDMAmultiple access is estimated, taking into account co-channel interference. Various network issues, such as mobility management, radioresource management, call control, routing, and network integration are addressed. Finally, some regulatory and political issues arementioned which may be most relevant for market development and financial success of satellite personal communication systems.

1. Introduction

The central idea of personal communications is the abil-ity of a (mobile) subscriber to set-up a call and to receive acall at any place and time. The handheld terminals shouldbe small and light-weight, with small, omnidirectional an-tennas and low transmit power. They should provide dif-ferent services (voice, fax, data, etc.) with high quality andshould be usable in various mobile networks.

As an alternative and complement for terrestrial personalcommunication networks (PCN), global satellite systems(S-PCN) are being developed, such as Iridium [16], Glob-alstar [49], and ICO [37]. Starting in 1998/1999, thesesatellite systems will provide worldwide mobile commu-nications services. S-PCNs will usually be based on non-geostationary satellites in low earth orbits (LEO) or mediumearth orbits (MEO). In parallel to S-PCNs, satellite systemsfor portable and mobile multimedia communications are be-ing developed (Galaxy/Spaceway [13], Teledesic [43], etc.),which will start service around the year 2000.

With S-PCN, long-distance (business) travellers will beable to overcome the problem of incompatible terrestrialPCN standards (GSM, AMPS, IS-95, etc.). Further ap-plications are the geographic extension of the coverage ofterrestrial mobile networks or the provision of basic com-munication means in less developed countries. Recent mar-ket analyses expect roughly 10 million S-PCN users in theyear 2002 [35].

Figure 1 shows the basic S-PCN system architecture.Each satellite covers a circular area on the earth’s surface,which increases with increasing orbit height and decreasingminimum elevation angle εmin. The choice of orbit planesand the satellite phasing within the orbits must guaranteecontinuous coverage of the service area (being the full earthsurface for global systems). The number of required satel-lites is determined by orbit height and minimum satelliteelevation [46].

Direct communication via satellite using a handheld ter-minal with low transmit power and omnidirectional antennarequires a high antenna gain on board the satellite, which

can be achieved with spotbeam antennas. Accordingly, thecoverage area of the satellite is composed of a large num-ber of spotbeams. This allows the reuse of frequency bandsf1, f2, . . . in separated cells, increasing the bandwidth effi-ciency of the system.

The gateway stations comprising a fixed earth stationand a mobile switching center (MSC) are connected to theterrestrial fixed network typically via international switch-ing centers (ISC). The use of data bases (home locationregister, HLR and visitor location register, VLR) allowsto maintain contact with globally mobile users (mobilitymanagement). The network control center (NCC) amongother tasks allocates spotbeam frequencies and distributesrouting tables to the satellites. The satellite control cen-ter via telemetry and command links keeps the satellitesin their correct orbit positions. Some systems (Iridium,Teledesic) will use intersatellite links (ISLs) to provide forlong-distance transmission within the satellite network.

S-PCN frequency bands have been allocated at theWARC-92 and the WRC-95. Frequencies around 140 and400 MHz can be used for data systems (“little LEOs”). Forvoice systems (“big LEOs”, such as Iridium and Global-star), the bands 1.610–1.6265 GHz and 2.4835–2.500 GHzcan be used for the mobile up- and downlink, respectively.The band 1.6138–1.6265 GHz is allocated for both direc-tions and is used by Iridium in time division duplex (TDD).After the year 2000, additional frequency bands at 1.980–2.025 GHz (uplink) und 2.160–2.200 GHz (downlink) maybe used (as intended by the ICO system). Higher frequen-cies at 5/7 GHz, 15 GHz, and 20/30 GHz are foreseenfor feeder links. Intersatellite links may work at 23 GHz,60 GHz, or at optical frequencies.

2. Satellite constellations and system concepts

Different basic system options are followed for S-PCNs,characterized by the underlying satellite orbits.

J.C. Baltzer AG, Science Publishers

110 E. Lutz / Issues in satellite personal communication systems

Figure 1. Basic S-PCN system architecture.

2.1. Geostationary (GEO) system concept

Global coverage (excluding the polar regions) can beprovided by only 3 geostationary satellites. However, thelarge orbit height of 36 000 km requires a very high gainsatellite antenna, which for frequencies below 2.5 GHz re-sults in extremely large antenna dimensions. Due to thenarrow beam, a very large number of spotbeams is neces-sary to fill up the footprint area, further increasing antennacomplexity. Another drawback of the GEO orbit is thelarge propagation delay (0.5 s round-trip between mobileuser and fixed earth station).

Up to now, GEO systems are used for less demand-ing applications such as data communications (Inmarsat-C), telephony with briefcase/laptop terminals (Inmarsat-M/Inmarsat-3) or truck fleet management (Omnitracs, Eu-teltracs).

New GEO system concepts aim at broadband and mul-timedia services (typically up to 1.54 Mb/s), mainly forportable terminals with small (< 1 m) antenna dishes. Theglobal Spaceway/Galaxy system and the Asian CellularSatellite (ACeS) system belong to this category. The SEC-OMS/ABATE project in the frame of the ACTS programmeof the European Community develops a GEO system formobile multimedia services [29], which may lead to a com-mercial system called EuroSkyWay.

2.2. Low Earth Orbit (LEO) system concept

Low earth orbits at 700–1500 km avoid the large signalattenuation and delay of the geostationary orbit. However, alarge number of LEO satellites are needed to continuouslycover the earth’s surface. The Globalstar system uses a

Walker constellation of 48 satellites in 8 inclined orbits at1414 km, and Iridium is based on 66 satellites in 6 polarorbits at 780 km height. The large number of LEO satel-lites is partly compensated for by their lower weight andcomplexity, compared to GEO satellites. Due to the non-geostationary characteristic of LEO satellites, spotbeam andsatellite handovers are necessary during calls.

Corresponding to the small footprint of LEO satellites,a large number of gateway stations is required in systemswithout intersatellite links (ISL). Globalstar will use some150 gateways all over the world. If ISLs are used, the num-ber of gateways can be reduced, and their positions can bechosen freely. The Iridium system which will use ISLs,initially foresees 11 gateway stations. Moreover, ISLs al-low to route long-distance calls within the satellite network,saving costs for terrestrial lines. As a consequence of theuse of ISLs, signal processing and switching is necessaryon board the satellites, to route the calls within the ISLnetwork.

Teledesic is a pioneer LEO concept, planning 288 satel-lites in 12 polar orbits at a height of 1600 km. It shouldprovide global mobile telephony with 16 kb/s to small Kaband (20/30 GHz) terminals with 8 cm antenna diameter.Larger, portable terminals will allow video conferencing at64 kb/s, multimedia, and high rate data up to 1.2 Gb/s.

2.3. Medium Earth Orbit (MEO) system concept

Systems with satellites in medium earth orbits around10 000 km avoid the large signal attenuation and delay ofthe geostationary orbit and still allow a global coveragewith few (10 to 15) satellites. Since the required satel-lite antennas are state of the art, and no ISLs are neces-

E. Lutz / Issues in satellite personal communication systems 111

(a)

(b)

Figure 2. Received power level for narrowband measurements in urban environment. (a) Driving van with roof-mounted antenna, 20 . . . 30 degelevation. (b) Randomly moving user with handheld, 30 . . . 40 deg elevation.

sary, the technical risk of MEO systems is acceptable. Asan example, the ICO (Intermediate Circular Orbits) sys-tem is based on 10 MEO satellites at 10 354 km orbitheight.

3. Signal propagation and channel model

The availability and quality of the service S-PCN canoffer is crucially influenced by the particular characteristicsof signal propagation in the link between the mobile orpersonal user and the satellite. In order to investigate thissubject, a number of propagation measurements have beenperformed, and several channel models have been derived,

describing the transmission path between a mobile/personaluser and a GEO or non-GEO satellite [9,23–25,28,33].

In the mobile satellite link, multipath fading occurs be-cause the received signal does not only contain the directsignal but also echo components being reflected from ob-jects in the surroundings. The received total power of theechoes mainly depends on the type of user environment(urban, suburban, rural, etc.) and on the antenna char-acteristic of the user terminal. Antennas with wide-anglepatterns tend to gather more echo power than directive an-tennas. Opposite to antennas mounted on top of a vehicle,handheld terminal antennas may pick-up strong specular re-flections from the ground [25]. Variation of the receivedpower with time is caused by movement of the user, of the(non-geostationary) satellite, or of reflecting objects.


Shadowing of the satellite signal is caused by obstaclesin the propagation path, such as buildings, bridges, andtrees. The percentage of shadowed areas on the ground, aswell as their geometrical structure strongly depends on thetype of environment. For low satellite elevation the shad-owed areas are larger than for high elevation. Especiallyfor streets in urban and suburban areas, the percentage ofsignal shadowing also depends on the azimuth angle of thesatellite.

Due to the movement of non-geostationary satellites, thegeometrical pattern of shadowed areas is changing withtime. Similarly, the movement of a mobile/personal usertranslates the geometrical pattern of shadowed areas intoa time-series of good and bad channel states. The meandurations in the good and bad state, respectively, dependon the type of environment, satellite elevation, and mobileuser speed.

3.1. Narrowband channel characterization

In order to investigate the personal communicationssatellite channel, a measurement system has been set upwith a transmitter part carried by an aircraft and a receiverpart in a van. For narrowband measurements the transmitterradiated a CW signal at 1.82 GHz. On ground, two chan-nel sounders were available. A handheld mock-up withan RHCP drooping dipole antenna and a car roof-mountedRHCP antenna have been used. The measurements are de-scribed in more detail in [25].

Figure 2 shows typical results for the received powerlevel. Figure 2(a) refers to a mobile user with a car roof-mounted antenna. Here, signal shadowing is the major im-pairment, causing attenuation of 15 . . . 30 dB. In figure 2(b)the received power level is given for a quasi-fixed (per-sonal) user with a handheld terminal. The handheld ter-minal mainly suffers from head shadowing and two-pathfading caused by specular reflections from the ground.

Earlier narrowband measurements have been carried outat 1.54 GHz, using the geostationary MARECS satellite anda car roof-mounted receive antenna. A two-state channelmodel was derived and its parameters were fitted to mea-surement results for different environments and elevationangles [33], figure 3.

The fading process is switched between Rician fading,representing unshadowed areas with high received signalpower (good channel state) and Rayleigh/lognormal fad-ing, representing areas with low received signal power (badchannel state). The Rician fading is characterized by thedirect-to-multipath signal power ratio (Rice-factor) c. TheRayleigh/lognormal fading is determined by mean powerlevel decrease µ (in dB) and standard deviation σ (in dB)of the power level.

The switching process between Rician and Rayleigh/log-normal fading is modeled by a two-state Markov chain andis characterized by the mean durations Dg and Db the chan-nel stays in the good or bad state, respectively. For per-sonal communication of pedestrians the durations Dg and

Figure 3. Narrowband model for a land mobile satellite channel.

Db may be given in meters; for mobile applications witha certain speed they may be translated into bit durations.The time-share of shadowing, A, representing the percent-age of time when the channel is in the bad state is givenby A = Db/(Dg +Db).

Values for the channel parameters are published in[4,33,34] for different environments, satellite elevations,and terminal types. Typical parameter values are com-piled in table 1. Compared to mobile terminals, handheldterminals tend to exhibit a somewhat larger time-share ofshadowing and a lower Rice factor. In general, the resultsof the propagation measurements indicate that the chan-nel behaviour is dominated by the effect of signal shadow-ing.

3.2. Wideband channel characterization

For the wideband measurements the aircraft transmitteda spread spectrum signal centred at 1.82 GHz. The signalwas spread by a pseudo noise bit sequence to a bandwidthof 30 MHz, corresponding to a spatial resolution of 10 m.16 power delay profiles could be measured per second [25].

Figure 4 shows a typical example of power delay pro-files p(t, τ ) for a mobile user on a highway [23]. The timeaxis refers to measurement time t, the delay axis shows theecho delay τ . In general, near echoes with 10 . . .30 dBattenuation appear with small delays below 600 ns (cor-responding to detours below 200 m). In addition to nearechoes, usually some far echoes occur with delays rangingto a few µs. Typical values of the delay spread ∆ for hand-held terminals range from 100 to 500 ns [2] if, accordingto CCIR, echoes below −25 dB are neglected.

Assuming an exponential power delay profile and deter-mining the coherence bandwidth as the bandwidth in whichtwo fading signal envelopes have a correlation > 0.5, the


Table 1Typical parameter values for the narrowband characterization of the personal and land mobile satellite channel.

Environment City Highway

Satellite elevation 10◦ . . . 30◦ . . . 50◦ 10◦ . . . 30◦ . . . 50◦

Mean duration of good states, Dg 7 m . . . 25 m . . . 50 m 102 m . . . 103 m . . . 104 mMean duration of bad states, Db 70 m . . . 50 m . . . 35 m 50 m . . . 30 m . . . 20 mRice factor for line-of-sight, c 5 dB . . . 9 dB . . . 10 dB 10 dB . . . 14 dB . . . 18 dBTime-share of shadowing, A 0.9 . . . 0.7 . . . 0.4 0.3 . . . 0.2 . . . 0Mean attenuation due to shadowing, −µ 12 dB . . . 12 dB . . . 15 dB 9 dB . . . 10 dB . . . 14 dBStandard deviation of power level for shadowing, σ 4 dB 4 dB

Figure 4. Measured power delay profiles. Car-roof mounted receive antenna, highway, 65 deg elevation.

Figure 5. Wideband model of the land mobile satellite channel.

coherence bandwidth of the channel (with regard to thesignal amplitude) can be derived as [27]

BC =1

2π∆. (1)

The above values for the delay spread result in a coherencebandwidth ranging from 0.3 to 1.6 MHz.

Following the approach of Bello [1] and assuming thatechoes arriving from different distances are uncorrelated,the channel is wide-sense stationary with uncorrelated scat-tering (WSSUS). Accordingly, the complex valued chan-nel impulse response h(t, τ ) can be expressed as a sum ofa direct path E1(t) · δ(τ − τ1(t)) with delay τ1(t) and a

number of echoes Ek(t) · δ(τ − τk(t)), k > 2, with delaysτk(t) = τ1(t) + ∆τk(t):

h(t, τ ) =∑k>1

Ek(t) · δ(τ − τk(t)

). (2)

With this approach, the wideband satellite channel can bemodeled as tapped delay line [24], figure 5. The powerdelay profile is related to the impulse response by

p(t, τ ) =∣∣h(t, τ )

∣∣2. (3)

The parameters of the wideband channel model have beenfitted to measurement results. The fit equations as well asnumerical values are given in [21].

3.3. System implications

3.3.1. Signal shadowingThe most detrimental propagation impairment is signal

shadowing. Light shadowing, e.g., caused by a single tree,or head shadowing may be compensated by power control.In the presence of heavy shadowing (blockage) the link isinterrupted.


For realtime services, such as speech, the influence ofsignal shadowing can be reduced by satellite diversity, seesection 4. The gain in increased link availability has to bepaid for by increased (e.g., doubled) transmit power andspectrum usage.

For data services and signalling, the application of ARQ(automatic repeat request) and related transmission proto-cols is suitable, also without satellite diversity. In essence,data packets are transmitted repeatedly until a positive ac-knowledgement is returned from the recipient.

3.3.2. Frequency non-selective multipath fadingMultipath fading appears to be frequency non-selective

(flat) if the coherence bandwidth of the channel is largerthan the signal bandwidth. Here, any kind of diversitycan improve signal transmission. The most common typeof diversity is receive antenna diversity, which improvesthe link quality without increasing the transmitted poweror bandwidth. For personal communications terminals, an-tenna diversity is not favourable, however. Of course, also(microscopic or macroscopic) transmit antenna diversity de-creases the influence of fading.

Frequency diversity reduces frequency non-selectivefading by periodically changing the transmit frequency withsteps larger than the coherence bandwidth of the channel(frequency hopping). The hopping sequence may be slowor fast compared to a bit duration. Packet retransmissionor channel coding can cope with the transmission errorsoccuring during a hop within a fade.

A similar effect is achieved by time diversity. Inter-leaving the transmitted bits and deinterleaving the receivedbits time-spreads the error bursts caused by fades. Channelcoding can much better cope with such randomized errorsthan with burst errors [15].

For continuous transmission with a given net rate, chan-nel coding increases bandwidth (except if trellis coded mod-ulation is used) but achieves a net gain in transmit power,compared to uncoded transmission. For data transmissionwith a given transmission rate, the redundancy bits of chan-nel coding lower the information throughput, however, thereliability of the transmitted data is increased.

3.3.3. Packet error rate in non-interleaved fading channelsIf messages have to be transmitted in single packets,

it may not be possible to interleave the data exceedingthe packet duration. This is especially true for all kindsof slotted multiple access schemes where the user has totransmit his packet within a given time slot, according tothe multiple access protocol. The transmission of user data,short messages, and system control messages will be orga-nized in short blocks without or with minimal interleav-ing.

The performance of a Reed–Solomon (43,21) code overGF(64), a binary BCH (255,131) code and a rate-1/2 con-volutional code (soft decision, zero tail) is compared infigure 6 for the non-interleaved flat Rayleigh channel. Allcodes have approximately the same block length and code

Figure 6. Packet error rate versus normalized fading bandwidth BT forthree rate-1/2 codes [3]. T = symbol duration.

rate. For the convolutional code the bits were interleavedwithin the packets.

For small fading bandwidths, the block codes have ap-proximately equal performance, whereas for large fadingbandwidths the difference is very strong. The lowest PERwas obtained with the convolutional code, but the com-parison with the block codes is not quite fair, because weassumed only hard decision for the block codes but softdecision decoding for the convolutional code. The convo-lutional code with intra-packet interleaving is least sensitiveto slow fading. For large fading bandwidths the BCH codeis much better than the symbol error correcting RS code,because the bit errors are randomly distributed over thewhole packet.

3.3.4. Frequency selective multipath fadingIf the signal bandwidth exceeds the coherence bandwidth

of the channel, the duration of the impulse response is not-icable compared to the bit duration. The transmitted pulsesare spread in time, and intersymbol interference (ISI) arises.This filtering effect of the channel can be compensated forby an inverse filter at the receiver, known as equalizer. Usu-ally, the channel is unknown and time-varying. This meansthat the equalizer must be adaptive. With an equalizer, thelink quality is improved without increasing the transmit-ted power or bandwidth. Since TDMA systems exhibit ahigh burst rate compared to the user bit rate (see section 5),such systems often require adaptive equalization adding tothe complexity of the receiver.

In CDMA systems, the chip rate is usually larger thanthe coherence bandwidth of the channel. However, dueto the pseudo noise feature of signature sequences there isvery low correlation between successive chips. Moreover,the interchip interference is averaged out by the correla-


tor within one bit duration. Therefore, no equalization isnecessary.

If the delay of a multipath echo is larger than a chipduration (but shorter than a bit duration), and if anothercorrelator is tuned to the according delay, then the echosignal can be separately detected. Finally, it can be com-bined with the time delayed version of the original signal.A rake receiver collects the time-shifted versions of theoriginal signal by providing a separate correlation receiverfor each of the multipath signals [39]. In this way, thesignal energy contained in the echo signals is exploited forthe bit decision, in addition to the energy of the directlyreceived signal.

In the mobile satellite channel, only relatively weakechoes are to be expected, cf. figure 4. Therefore, not muchgain can be achieved by a rake receiver. A rake receivercan, however, be used in CDMA systems to implementsatellite diversity and seamless satellite handover. In thesecases, the signal is simultaneously transmitted over morethan one satellite. Thus, artificial multipath is produced,which can efficiently be exploited by the rake receiver.

None the less, CDMA with rake reception is not the onlyway to implement satellite diversity or seamless handover.Diversity can as well be achieved with TDMA systems, ifthe signal is received from different satellites at differentfrequencies or within different time slots. The combiningor selection can be done at the bit or packet level.

4. Satellite diversity

4.1. Concept

Systems such as Globalstar or ICO essentially providedouble coverage of the earth. This feature enables the ap-plication of satellite diversity, i.e., the simultaneous com-munication with a user via two or more satellites. If one ofthese satellites is shadowed, there is some chance for an-other satellite being still in view to the user and maintainingthe service. In this way, satellite diversity can substantiallyimprove service availability (the percentage of time whenthe service is available).

Of course, gain in service availability can only beachieved if the considered satellite channels behave dif-ferently. Therefore, any dependency between the channelsinfluences the benefit of satellite diversity. In [31] a con-cept for modelling two statistically dependent satellite chan-nels was developed. The two-state Markov channel modelcharacterizing the shadowing process (i.e., the switchingbetween good and bad channel states, cf. figure 3) was ex-tended to two separate channels. In order to take into ac-count the correlation between the shadowing processes ofthe channels, the transition probabilities of the combinedMarkov model were modified in such a way that the ini-tial channel models are maintained but a certain correlationcoefficient ρ is produced.

An example for the azimuth correlation of shadowing inan urban area is given in figure 7. The correlation decreases

Figure 7. Azimuth correlation of shadowing in urban environment [4].

with increasing azimuth separation of the satellites, and issmaller if the satellites have different elevation angles.

4.2. Numerical results

Extensive simulations have been performed [4] for agateway at 100◦ W/40◦ N and 16 user positions equallydistributed in a circle around the gateway, where it wasassumed that the gateway always selects the two satelliteswith the highest elevation angles. Figures 8(a) and 8(b)show the service availability for the LEO system Globalstarand the MEO system ICO, respectively. For both systemswe assumed a link margin of 7 dB. The elevation-dependentchannel parameters were taken from measurements.

For both systems, satellite diversity results in a signifi-cant improvement of service availability. The improvementis more distinct in the Globalstar system than in the ICOsystem, whereas the absolute availability figures in the di-versity case are nearly the same in both systems. A reasonfor this are the better visibility statistics for Globalstar. Itturned out from simulations that a user has two (mutually)visible satellites for 80% of time in the Globalstar systemand for 50% of time in the ICO system.

Comparing the highway environment with the city envi-ronment it can be concluded that S-PCN can provide highservice quality in highway or rural areas, whereas in urbanareas the satellite system will be not more than a comple-ment to terrestrial systems.

Assuming on–off channel behaviour for the mobile userlink, the concept of satellite diversity is equivalent to a sys-tem initiating a seamless satellite handover each time whenthe currently used satellite gets shadowed. A requested han-dover is only successful, however, if an alternative satelliteis available to the user. These handovers would occur in ad-


(a)

(b)

Figure 8. Service availability versus user distance from gateway. (a) Glob-alstar system. (b) ICO system.

dition to the handovers required due to time-limited satellitevisibility.

5. Multiple access

The active mobile users within a satellite spotbeam (cell)are simultaneously using the satellite transponder. Two as-pects arise:

• Multiplexing of the signals can be achieved by fre-quency separation (leading to frequency division mul-tiple access, FDMA), time separation (leading to time

division multiple access, TDMA), or separation throughsignal signatures (leading to code division multiple ac-cess, CDMA). Moreover, combined schemes are possi-ble, such as FDMA/TDMA or FDMA/CDMA (multi-frequency schemes).

• The rules according to which the users may use thefrequency channels, time slots, or signature codes aredefined by a multiple access protocol. This protocolmust take into account the services to be provided.

5.1. Multiple access protocols

Random access protocols are suitable for the transmis-sion of single data packets (connection requests, signallingmessages, short message transmission). A frequently usedrandom access protocol is slotted Aloha. Here, the time isdivided into slots, and the users transmit their packets inthe next time slot after the transmission has been initiated.If a time slot is used by more than one user, a collisionoccurs, and the packets can not be received by the satelliteand have to be retransmitted after a random delay. Pack-ets can also get lost, if they are transmitted during a badchannel state. In [30], data transmission with slotted Alohais analyzed, taking into account forward error correctionand packet retransmission. The slotted Aloha scheme canbe improved if the users transmit their packets only duringgood channel states [22].

For speech communication and for the transmission oflarger data files it is advantageous to request (e.g., via slot-ted Aloha) the assignment of a traffic channel before trans-mission (demand assignment multiple access, DAMA). Ac-cording to the multiplexing scheme, the assigned channelcan be a frequency channel, a time slot, or a code.

5.2. TDMA

With TDMA the time is divided into frames, which areagain divided into time slots. A traffic channel is formed bya certain time slot within each frame. In order to account forsynchronisation errors, the user bursts transmitted duringdifferent time slots are separated by a guard time.

A frequency band must not be simultaneously used inneighbouring cells, rather, in order to limit co-channel in-terference (CCI), cell clusters are formed using differentfrequencies. Typical cluster sizes are C = 4 or 7.

For system bandwidth Bs, user bit rate Rb, and M -levelmodulation, the possible number of active users within acell is approximately given by

Nu =1C· Bs

Rb· ldM

1 + H+GI+P ldM

. (4)

The burst header H (for carrier and clock synchronization)and the guard time G in transmitted symbols are taken intoaccount, relative to the user information (I information bitsplus P parity bits) contained in a burst. As an example,for C = 4, I + P = 1000 bits, H = 100 symbols, G = 30


symbols, and M = 2, the number of users within a cellbecomes

Nu = 0.22Bs

Rb. (5)

5.3. CDMA

In CDMA the user signals are modulated with (usuallyapproximately) orthogonal signature sequences. In directsequence CDMA the modulation is binary phase-shift key-ing (BPSK) or quaternary phase-shift keying (QPSK). Sincethe signature signal usually is of much higher rate than theuser signal, the transmission bandwidth is much larger thanthe user bit rate. Usually, all spread user signals are simul-taneously transmitted in the same frequency band. In thereceiver, the incoming signal is correlated with the signa-ture sequences, thus reconstructing the information signalsof the users. The residual correlation of the signatures pro-duces interference.

We assume the following features:

• There are Nu active users in each satellite spotbeam.

• The same frequency band is used in each spotbeam.

• Each information bit is modulated by G chips of a sig-nature sequence, with G usually representing a periodof the sequence.

• The spread user signals are transmitted asynchronouslyand are uncorrelated.

• Gaussian channels are assumed (no signal shadowing,no multipath).

• The transmission power of the user terminals is perfectlycontrolled, i.e., the differences in distance and antennagain are compensated for. Thus, the signals are receivedwith equal power at the satellite.

• During speech pauses, the signal transmission is inter-rupted.

Under these premises, the total noise power in the correla-tion receiver results in [45]

Ntot = N0 + α(1 + k)(Nu − 1)Eb

G. (6)

N0 represents thermal noise, and the second term describesthe interference caused by other users, the signals of whichare received with bit energy Eb. This interference is re-duced by processing gain G with typical values rangingfrom 100 to 1000. α is the speech activity of the users.

The factor (1+k) accounts for the additional interferenceproduced by users in neighbouring cells, with k being de-fined as other-cell interference power divided by own-cellinterference power. For terrestrial CDMA, usually a fourthpower law with distance is assumed. Here, a typical valuefor the other-cell interference factor is k = 0.44 [44].

In satellite systems with spotbeams, the interferencepower is determined by the characteristic of the spotbeamantenna (the terminal antenna may be assumed to be hemi-spherical), which typically is a tapered-aperture antenna [7].

Table 2Other-cell interference factor k for different S-PCN system scenarios.

h = orbit height.

Scenario Spotbeam contour k

Lower bound −3 dB 1.20for small nadir angles −4.3 dB 0.78

MEO-system (h = 10 354 km, −3 dB 1.22169 spotbeams, εmin = 10 deg) −4.3 dB 0.79

LEO-system (h = 1414 km, −3 dB 1.6319 spotbeams, εmin = 10 deg) −4.3 dB 0.97

Another parameter determining other-cell interference is thedefinition of the spotbeam contour. Usually, the spotbeamcontour is defined by a 3 dB decrease of antenna gain.Higher beam isolation leading to less interference can beachieved by choosing a larger gain decrease at spotbeamcontour. A convenient choice is a value of 4.3 dB.

For typical LEO and MEO system scenarios and uni-formly distributed users, the other-cell interference fac-tor k was evaluated by numerically integrating the own-and other-cell interference power over a satellite footprint,assuming that an interferer belongs to that spotbeam fromwhose center the angular deviation is smallest. Tapered-aperture spotbeam antennas, perfect power control of allusers with regard to their respective spotbeam, and idealchannels are assumed. The resulting values for k, table 2,indicate that other-cell interference is much more critical forsatellite systems than for terrestrial cellular systems [32].

Usually, thermal noise can be neglected, compared tothe interference produced by other users. The number ofusers within a cell can therefore be approximated by

Nu ≈G

α(1 + k)· 1

(Eb/N )req(7)

with (Eb/N )req designating the signal to noise ratio requiredfor signal transmission. For (Eb/N )req = 5 dB (assum-ing a Gaussian channel, a rate-1/2 convolutional code withconstraint length 7, and a required bit error rate of 10−6),α = 0.5, and k = 1.2 we get

Nu ≈ 0.29G = 0.29Bs

Rb. (8)

A comparison with equation (5) shows that CDMA maybe more bandwidth efficient than TDMA. The differenceis not crucial, however. In practice, the above CDMA ef-ficiency may not be fully achieved because of imperfectpower control and statistical variations of speech activity.Additionally, the interference from other users increases therequired transmission power compared to transmission inthermal noise only.

In the mobile link of CDMA satellite networks, bothcircular polarisations can be used, because the user signalsin neighbouring cells are additionally separated by differentsignature sequences [14]. This requires additional antennaeffort on board the satellite, however. Also, for mobile


terminals the cross polarisation attenuation is often verysmall.

In chip synchronous CDMA, the other-user interfer-ence from the own cell can be drastically reduced by us-ing orthogonal sequences. Other-cell interference is stillpresent, however, because the corresponding signals remainnon-synchronous. In forward direction, chip synchronousCDMA is easily possible, because the signals are transmit-ted from a common entity (satellite or gateway). Synchro-nous CDMA is also proposed for the return link [10]. Fornon-geostationary satellites and mobile users, chip synchro-nisation may be a formidable task, however. An extensiveoverview of satellite CDMA is given in [11].

5.4. Comparison

Besides bandwidth efficiency, there are further criteriafor the choice of the multiple access scheme.

TDMA has the following advantages:

• TDMA is a mature technology.

• TDMA can easily be combined with dynamic channelallocation, in this way avoiding interferences and adapt-ing the system to unbalanced and time-varying trafficdistribution [36].

• Because the user signals are transmitted one at a time,the satellite transponder can be operated with smallback-off.

Disadvantages of TDMA are:

• High burst transmission rates combined with high ter-minal peak power. This drawback is alleviated by usingFDMA/TDMA.

CDMA is characterized by the following features:

• In each cell the same frequency band can be used. Thisincreases bandwidth efficiency and eliminates the needfor frequency allocation. The latter advantage may beeliminated, however, if allocation of different code fam-ilies is required.

• CDMA systems can share a part of the allocated S-PCNfrequency band (1610–1621.35 MHz in the US), allow-ing dynamic allocation of system capacity, while TDMAsystems need separate bands (e.g., 1621.35–1626.5 MHzfor Iridium in the US).

• Different and varying data rates can easily be im-plemented; different services can easily be combined;reducing transmission rate and power during speechpauses directly increases system capacity.

• Satellite diversity and soft handover can easily be im-plemented by using a rake receiver.

CDMA requires a remarkable complexity, however:

• In order to compensate for different receive levels of theuser signals, fast and exact power control is necessary.Due to the large signal delay in satellite networks, thisis especially difficult to achieve.

• Due to high speed signal processing, signal and codeacquisition, and rake demodulators, CDMA terminalsexhibit increased complexity.

Planned S-PCNs do not show a clear preference of TDMAor CDMA, respectively. Iridium and ICO will use TDMA,Globalstar and Ellipso will use CDMA.

6. System and network aspects

Cellular mobile networks are characterized by specificfeatures and requirements:

• The service area is divided into radio cells, and the usersare mobile. Therefore, mobility management (MM) isrequired to keep in contact with the users.

• Radio cells use different frequencies (TDMA), frequen-cies are reused in separated cells, radio channels arenon-ideal and are allocated only when required. Thisrequires radio resource management (RR).

• Establishment, maintenance, and release of connectionsis performed by connection/call control (CC). Due touser mobility and satellite movement, the need for han-dover (HO) may arise during a call.

The concepts for the call processing related functionsMM, RR and CC are basically adopted from the GSM sys-tem [38]. The functions of MM, RR, and CC mainly residein the ISO/OSI network layer (layer 3) and require the in-volvement of various network entities, such as terminals,satellites, gateways, and the network control center.

Also, these functions require the establishment of var-ious (logical) signalling channels for the exchange of re-lated information. Broadcast control channels are used todistribute general network information from gateways andsatellites and to allow mobile users to get in contact withthe network. For ongoing connections, dedicated controlchannels are set up, to allow specific signalling (for powercontrol, e.g.).

6.1. Mobility management (MM)

Mobility management should allow both, terminal mo-bility and user mobility (usage of a subscriber identity mod-ule in different terminals and networks).

6.1.1. Location updateAfter switch-on of a terminal, it searches for a broadcast

control channel at known frequencies. According to the re-ceived information, the terminal may choose the network(terrestrial or satellite) and identifies the current radio cell.Also, the broadcast control channel indicates the randomaccess channel which is subsequently used to inform thegateway currently responsible for the mobile user about thelocation area the user belongs to. If the user is situatedwithin the service area of his/her home gateway the loca-tion area is registered in the corresponding home location


register, HLR, cf. figure 1. If the user position belongs toanother gateway, the location area is stored in the corre-sponding visitor location register, VLR, and the address ofthe currently visited gateway is reported to the home gate-way HLR. If due to user mobility the location area changes,this is reported to the responsible gateway/register (locationupdate).

Due to the movement of non-GEO satellites, the cellbased location area concept used in the GSM system isnot applicable for S-PCNs. Two alternative location updat-ing methods have been developed in [42]: the guaranteedcoverage area approach and the terminal position based ap-proach.

The guaranteed coverage area is defined as the fixed ge-ographical area around an S-MSC, where the coverage isguaranteed by the connected earth stations [17]. The loca-tion area identity is broadcast over the guaranteed coveragearea via the broadcast control channels of the spotbeams(possibly from more than one satellite) which overlap theguaranteed coverage area. The mobiles will initiate a loca-tion update when a new location area identity is receivedthrough the selected satellite. In this way, the guaranteedcoverage area approach excludes the effect of satellite dy-namics in the location update procedure.

The terminal position based approach requires the mo-bile terminals to have positioning capability (e.g., GPS). Ifa mobile’s traveling distance exceeds a certain threshold, itinitiates the location update procedure.

6.1.2. PagingWhen a mobile terminated call arrives, the mobile user

is informed via the paging channel of the currently respon-sible gateway. The paging signal is broadcast within thecurrent location area of the user. Therefore, a trade-offarises between the signalling effort for paging and for loca-tion updating, respectively. Large location areas (contain-ing many radio cells) require much signalling for pagingbut few location updates, and vice versa.

Using a one-step paging scheme, the whole location areais paged in each paging step. More than one paging stepsmay be necessary because of signal shadowing. This effectcan be reduced if the mobile terminal continuously choosesthe best visible satellite by means of a spotbeam re-selectionprocedure.

Normally, a location area consists of a number of spot-beams. With the two-step paging scheme [17], in the firststep the mobile is only paged in the spotbeam with the high-est probability of user presence, thus minimizing the pagingsignalling overhead. If the first paging step fails, the pag-ing signal is retransmitted in the whole location area. Ifsatellite diversity is applied, the paging signal is broadcastvia two satellites.

6.2. Radio resource management (RR)

Since the frequency bands allocated to S-PCN are strictlylimited, the radio resource is precious and has to be used

as efficiently as possible. This requires bandwidth efficientmultiple access and transmission schemes, as well as effi-cient allocation of frequency bands and channels to radiocells.

6.2.1. Fixed channel allocationIf a new call is to be established, a channel must be

allocated to the respective cell/spotbeam. With fixed chan-nel allocation, the set of available channels is divided intoC (equally big) groups of channels. Regular groups ofC cells (clusters) are formed, with C = 3, 4, 7, etc., suchthat the required distance between co-channels (frequencyreuse distance) is maximized for given cluster size C. IfNch = Bs/Bch channels of bandwidth Bch are available inthe system, Nch/C channels can be allocated to each cell. Ifthe system contains Ncell cells, each channel can be reusedNcell/C times. Thus, decreasing cluster size C increasesfrequency reuse and enhances bandwidth efficiency. How-ever, C must be chosen large enough to provide sufficientfrequency reuse distance guaranteeing the required signalto co-channel interference ratio.

A new call can only be accomodated if a free channelis available in the spotbeam. If all channels are used, thenew call will be blocked.

6.2.2. Channel borrowingFor high network load, fixed channel allocation is effi-

cient, if the traffic is equally distributed among the spot-beams. For unequal or time-varying traffic load, more flex-ible allocation schemes are preferable. Channel borrowingis a simple step into this direction: Initially, channels areallocated according to fixed channel allocation. If all chan-nels within a spotbeam are occupied, and a new call arrives,a channel is borrowed from a neighbouring spotbeam, ifthis channel provides the necessary reuse distance. Afterany call within the own spotbeam is finished, the channelallocation may be rearranged, and the borrowed channel isreturned.

6.2.3. Dynamic channel allocationWith dynamic channel allocation (DCA), all channels

are kept in a common pool, from which any channel canbe allocated to any cell, as long as the reuse distance isguaranteed (or a certain signal quality can be maintained).The main idea of all DCA schemes is to evaluate the cost ofusing each candidate channel, and to select the one with theminimum cost, provided that certain interference constraintsare satisfied [26]. In distributed DCA schemes (especiallysuitable for microcellular systems) base stations and userswithin a radio cell are responsible for the channel allocationin that cell.

In DCA schemes for S-PCNs the total satellite capac-ity can be variably shared by all beams. In centralizedschemes, a channel from the central pool is assigned to acall by a centralized controller, which may reside in thesatellites. In the “first available” scheme, the first availablechannel satisfying the reuse distance encountered during a


channel search is assigned to the call. With “locally opti-mized dynamic assignment”, the selected cost function isbased on the blocking probability in the vicinity of the cellin which a call is initiated. Channel reuse optimizationschemes try to maximize the utilization of every channel inthe system, e.g., by defining a cost function for the alloca-tion of a channel, which is low, if the channel resembles anFCA scheme. The 1-clique scheme uses a global channelreuse optimization approach based on graph theory. In-terference based DCA [5] relies on downlink interferencemeasurements of a mobile requesting a channel, and allo-cates the channel with lowest co-channel interference Idown,provided Idown is smaller than a given maximum Idown,max,and Iup (measured by the satellite) is smaller than Iup,max.A DCA scheme based on carrier to interference ratio (CIR)uses estimated values of CIRup and CIRdown to maximizethe minimum CIRup among the set of mobiles that wouldbe affected by the new channel allocation, while guaran-teeing that CIRdown > CIRdown,min. Schemes with channelrearrangement improve the performance at high traffic load:If a connection is released, the channels used in the respec-tive cell are rearranged, and the most “expensive” channelis returned to the common pool.

6.3. Call control

6.3.1. Call establishmentFor mobile originating calls, the terminal requests a con-

nection via the random access channel of the currently re-sponsible gateway. After authentication of the user a traf-fic channel is allocated and the mobile connection is fur-ther controlled by the responsible (home or visited) gate-way [20].

In S-PCNs without own country code, each mobile ter-minating call from the PSTN arrives at the home gatewayof the user. If the user is not in the service area of thehome gateway, the currently visited gateway is read fromthe HLR and the mobile user is paged by the visited gate-way within the current location area. After the user hasresponded to the paging (via the random access channel)and a suitable satellite has been selected, the mobile con-nection is established and a fixed connection is extendedfrom the home gateway to the visited gateway.

In S-PCNs with own country code, calls from the PSTNcan be accepted at any gateway (e.g., nearest to the callingsubscriber). For Iridium, this is called the PSTN connect-ing gateway [20]. For mobile originated calls, the visitedgateway will determine a PSTN connecting gateway basedon the digits dialed by the mobile subscriber.

6.3.2. HandoverIn terrestrial cellular systems, handovers are caused by

user mobility. In non-geostationary satellite systems satel-lite handovers arise because a satellite can be seen by auser only a limited time period. MEO satellites can be vis-ible for approximately 2 hours, a period much longer thana typical telephone call. LEO satellites may be visible for

less than 10 minutes, therefore, handover of a call to an-other satellite will be necessary with high probability. Innon-geostationary satellite networks the user movement canbe neglected compared to the satellite velocity.

Under ideal line-of sight conditions, the criteria for ini-tiating a satellite handover could be based on the deter-ministic geometric properties. Since the satellite footprintsoverlap, different handover strategies exist:

(a) always the satellite with maximum elevation is used;

(b) a satellite is used until its elevation falls below a min-imum.

Strategy (a) yields better channel characteristics butneeds more handovers, compared to strategy (b) [6].

In the mobile/personal environment, the handover proc-ess should also consider the impact of the satellite channel.Optimizing the quality of service implies that always thebest mobile link should be chosen, which would producea large number of handovers, however, corresponding toa high signalling overhead in the network. Applying apower hysteresis threshold or a time-out period to handoverinitiation reduces the frequency of handovers and leads toa trade-off between service quality and signalling load [8].In [50] a handover algorithm is proposed, which is basedon user terminal position and signal strength measurements.

In “satellite-fixed cell systems”, the cells move over theearth, according to the satellite motion, and handovers be-tween spotbeams of a satellite arise, occuring more fre-quently than satellite handovers. At beam handover, a chan-nel has to be assigned to the call in the new beam. In orderto minimize the probability of forced call termination dueto the lack of a new channel, some channels may be ex-clusively reserved for handover requests (guard channels).Also handover requests may be queued to wait for a channelreleased by a terminating call. With DCA, a given channelcan migrate from beam to beam and remain associated witha given call during the whole satellite passage [41]. In [12]a handover strategy based on the queueing of handoversand dynamic channel allocation is investigated.

In “earth-fixed cell systems”, cells are geographical re-gions attached to the earth [40]. Satellite beams consec-utively serve a cell. During the time interval a beam isassigned to a cell, the beam is continuously steered into theopposite direction of the satellite motion. Within a satellitefootprint, no beam handovers occur. If the satellite foot-print leaves a cell, the respective beam is redirected to anew incoming cell. This eliminates uncertainty as to whena handover occurs, resulting in a near-to-zero handover fail-ure probability. However, earth-fixed cell systems requirea larger number of satellites compared to satellite-fixed cellsystems [40]. Moreover, the satellites must be capable ofperforming beam steering and cell switching. An examplefor an earth-fixed cell systems is the Teledesic system.

If the satellite network is integrated with a terrestrialmobile network, inter-network handovers may occur, e.g., ifthe service area of the terrestrial system is left. In the Amer-ican GEO-system MSAT this kind of inter-system handover


is already implemented. For inter-network handovers in theother direction the terrestrial network standard has to be en-hanced.

6.4. ISL routing

For every connection between two users, a route throughthe communication network must be chosen. Routing is acentral task in large networks and influences network per-formance as well as quality of service (e.g., message delay).

In LEO/MEO systems, two aspects must be consid-ered [47]:

• For both, the mobile user and the gateway serving satel-lites have to be chosen (up/downlink routing).

• Long-distance traffic can be routed via terrestrial links orvia intersatellite links, if available. In the latter case, ISLrouting chooses the way between start and end satellites.

Because of the time-varying network topology, ISL routingis especially complex:

• A route between certain start and end satellites may notbe optimum or even possible during the whole connec-tion. Rather, route changes will be necessary, resultingin problems connected with, e.g., flow control and delayvariations.

• Due to time-limited satellite visibility, the start and endsatellites may change during a connection (satellite han-dover). This requires additional route changes withinthe ISL network.

New developments of intelligent routing schemes for S-PCN are based on ATM principles [47].

6.5. Integration of terrestrial and satellite mobile networks

If S-PCNs should complement terrestrial PCNs, the net-works have to be integrated to a certain degree.

Terminal integration is the simplest way of providingcombined usage of the networks. Basically, a dual modeterminal contains a terrestrial and a satellite terminal. Themobile user has a separate number in both networks. Both,mobile terminal originating and terminating calls use thenetwork chosen by the calling subscriber. No network han-dover is possible.

The combination of terrestrial and satellite networks ismore efficient for network integration. In figure 9, the mo-bile switching centers (MSC, S-MSC) can mutually accessthe registers of both networks, and the S-PCN appears asan international network with its own country code. Now,the mobile user has one unique number regardless of thenetwork to which it is attached, and a network handover ispossible. For this purpose, an active dual mode terminalhas to listen also to control channels of the other network.Also, some kind of interworking unit (IWU) between ter-restrial MSC and S-MSC is necessary. If the networks usecommon registers, the S-PCN appears as an extension of

Figure 9. Network integration with dual mode terminals and mutual accessto registers.

a terrestrial mobile network, and a subscriber will get acountry code depending on his home base [19].

The highest degree of integration is reached if the satel-lite segment is treated as a homogeneous part of total cov-erage (system integration). Both networks may use a com-mon radio interface, which can be adapted to the differ-ent propagation conditions. Also, the protocols for bothnetwork segments have to be compatible. This approach,which considers a satellite cell as an umbrella cell of theintegrated network, will be followed for the future UMTS.

In [18] a single MSC integration scenario with GSM atsystem level is compared to a split MSC integration sce-nario at network level. It is shown that split MSC integra-tion is advantageous with regard to network signalling loadand required modifications of GSM.

6.6. Network dimensioning

S-PCNs serve a globally distributed user group with arelatively high requirement for long-distance traffic. Thecapacity of the various links must be large enough to carrythe traffic generated by fixed and mobile users. Moreover,upper limits for blocking probability and message delayhave to be taken into account. The necessary capacity ofthe links depends on

• the number, distribution, and activity of users,

• the topology of the satellite constellation,

• the extent and spotbeam structure of satellite footprints,

• the number and positions of gateways,

• the routing strategy of long-distance traffic.

The global traffic requirement within and between suit-ably defined regions can be described by a traffic matrix.The traffic requirement can be matched to the topology (in-cluding the connection matrix) of the satellite network, andthe required link capacities can be determined [46]. Basedon the connection matrix, suitable routes are selected, whereterrestrial long-distance links or intersatellite links may bepreferred, or ISLs may be used for average traffic and ter-restrial links may be used for peak traffic. By considering a


series of times, it can be taken into account that the load ofa link varies due to satellite movement and traffic variationwith the time of day. Maximum link loads as well as meanvalues can be evaluated. These investigations result in

• the quantity of mobile user links per spotbeam and satel-lite,

• the quantity of gateway links per satellite,

• the quantity of intersatellite links (if considered),

• the quantity of terrestrial long-distance links and theirlength distribution.

7. Satellite technology

Although this is a very important issue in S-PCN, onlya few words will be mentioned here, in order to extent thepaper not too much.

Satellites with transparent transponders just frequency-shift and amplify the received uplink signal before it isretransmitted into the downlink (e.g., Globalstar).

In satellites with regenerative transponders, the receivedmultiplex signal is demodulated, decoded, and demulti-plexed. Various analog, digital, or hybrid techniques fordemultiplexing are possible, with digital solutions requir-ing complex and powerful signal processing on board thesatellite. Then, channels have to be routed to spotbeams,and if ISLs are used, a switching function must be imple-mented in the satellite. Before transmission, the signals aremultiplexed, coded, and modulated. Due to signal regener-ation, the transmission schemes can be separately adaptedto the fixed feeder link and the mobile user link, respec-tively, and better signal quality can be achieved.

The multiple spotbeams for the mobile link are formedeither by a reflector antenna with multiple feeds or witha phased-array antenna requiring a complex feed networkto generate multiple beams. GEO satellites tend to usemulti-feed reflector antennas, while LEO/MEO satellitesuse phased-array antennas. New GEO systems with a largenumber of spotbeams either have to use a large antennaaperture (ACeS system) or ressort to higher frequencies(SECOMS/EuroSkyWay). Earth-fixed cell systems withregional frequency assignment require agile digital beam-forming.

Inter-orbit ISLs require steerable antennas, which haveto track the neighbouring satellites, at least during a part ofthe orbit [46].

8. Regulatory, financing, and system operations

Apart from the technical aspects, there are a number ofother issues in S-PCN which have to be solved before asystem can successfully be operated:

• Frequency bands for the mobile and feeder links have tobe allocated. The ITU WARC-92 and WRC-95 provideda sound basis for S-PCN, but some questions remain tobe solved by the WRC-97.

• For each system, a licence must be granted for imple-mentation and operation of the space segment. This li-cence is filed for by the system proponent and is grantedby a regulatory authority of one country (e.g., the USFCC) in coordination with the ITU. Further, a licence forestablishing a gateway station is required by the gate-way operator from the respective country. Similarly, alicence for providing the service must be granted to theservice provider by the respective country. Connectionagreements with terrestrial fixed networks, and roamingagreements with terrestrial mobile networks must be setup.

• The allocated frequency bands will be used by more thanone system. While CDMA systems can share a band,TDMA systems must use separate band segments. Forthe 1610–1626.5 MHz band, the FCC proposed to usethe lower 11.35 MHz for CDMA systems (Globalstar,Odyssey) and to reserve the upper 5.15 MHz for theIridium TDMA system.

• Some political problems have been discussed at the re-cent World Telecommunications Policy Forum (WTPF)initiated by the ITU. Here, some conflicts between sys-tem proponents and developing countries came up [48]:System proponents are interested in low cost of licences,foreign ownership of gateways, type approval and freecirculation of terminals, and transparent regulation. Onthe other hand, developing countries are interested inlow cost of airtime, opportunity for domestic ownershipof gateways, and unrestricted service accessibility. Na-tional sovereignty is of prime importance for developingcountries. To prevent unauthorized traffic and bypassingof national telecommunications networks through satel-lite call-back services, they are interested in transpar-ent call records and reliable data on unauthorised traf-fic.

• Finally, a huge amount of costs must be financed duringsystem construction. Typically, about half of the costsis invested by strategic partners, and the rest is financedby credits. The service charging policy will be an im-portant instrument to penetrate the markets in developedand less developed countries.

9. Conclusions

In developing and assessing S-PCNs a great variety ofissues has to be considered. A fundamental choice is theselection of the satellite constellation, influencing the wholesystem design. Because of strictly limited satellite and ter-minal power, signal propagation characteristics are espe-cially important in the S-PCN environment, and variousmeasures have to be taken to guarantee reliable transmis-sion. Satellite diversity, error control, and other meansmay be used. Also, the choice of the multiple accessscheme has consequences for the further system design,hence, dividing S-PCNs into TDMA and CDMA systems.A number of network issues must be tackled in designing


an S-PCN. Here, the GSM standard provides a suitable plat-form, which can be adapted to and extended for S-PCNs.Satellite and terminal technology are further issues to beconsidered.

The above mentioned technical issues present taskswhich can be solved by engineering research and devel-opment. Regulatory and political issues might be more dif-ficult and time consuming to be solved, as was shown bythe recent WTPF. An especially important task for the Eu-ropean Union is the development of a common regulatorystrategy.

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Erich Lutz was born in Augsburg, Germany. Hereceived the Ing. grad. degree from the Polytech-nic Augsburg in 1972, the Dipl.-Ing. degree fromthe Technical University Munich in 1977, and theDr.-Ing. degree from the University of the ArmedForces, Munich, in 1983. From 1977 till 1982he was a research assistant at the Technical Uni-versity and the University of the Armed Forces,Munich, working in the field of digital transmis-sion over cables and optical fibers. Since 1982,

he has been with the Institute of Communications Technology of the Ger-man Aerospace Research Establishment (DLR) in Oberpfaffenhofen, Ger-many. Since 1986, he has been head of the Digital Network section ofthis institute. His current research interests include mobile radio channelcharacterization, error control techniques, multiple access techniques, andnetworking aspects, in particular for mobile satellite communication net-works. Since 1996, he lectures at the Technical University Munich onmobile satellite communication networks.

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