DRDO SCIENCE SPECTRUM 2009
DRDO Science Spctrum, March 2009, pp. 130-139
© 2008, DESIDOC
Satellite Mobile Communications Technology-Trends
Jagdish Prasad
Scientific Analysis Group, Metcalfe House, Delhi-110 054
ABSTRACT
This oration paper describes in brief the basics of satellite communications and technological trends in the area
of Mobile satellite communications. Starting with the history, types of orbits, launching of satellites, propagation
characteristics of electro magnetic signal when the user device is stationary v/s moving, carrier modulation and multiple
access techniques, the principle of mobile satellite communication and finally the brief mention of presently operational
satellite systems is given. design to development.
Keywords: TDMA, FDMA, TDM, FEC, Modulation
1.
HISTORY
The first satellite equipped with on-board radio-transmitter
that worked on two frequencies, 20.005 and 40.002 MHz
was the Soviet Sputnik 1, launched in 1957. The first American
satellite to relay communications was Project SCORE in
1958, which used a tape recorder to store and forward
voice messages. Telstar was the first active, direct relay
communications satellite, launched by NASA from Cape
Canaveral on July 10, 1962, the first privately sponsored
space launch. Telstar was placed in an elliptical orbit (completed
one rotation once every 2 hours and 37 minutes), rotating
at a 45° angle above the equator. An immediate antecedent
of the geostationary satellites was Hughes’ Syncom 2,
launched on July 26, 1963 revolved around the earth once
per day.
2. INTRODUCTION
A communications satellite is an artificial satellite stationed
in space for the purposes of telecommunications. Modern
communications satellites use a variety of orbits including
geostationary earth orbit (GEO), medium earth orbit (MEO),
low (polar and non-polar) earth orbit (LEO) and elliptical
earth orbits (EEO). A satellite in a geostationary orbit appears
to be in a fixed position to an earth-based observer. A
geostationary satellite revolves in a circular orbit around
the earth at a constant speed once per day over the equator
at a distance of 35786 km. A minimum of three satellites
are needed for global coverage except at the Polar Regions.
A Medium Earth Orbit (MEO) typically is a circular orbit
about 10000 -20000 kilometers above the earth’s surface
and, correspondingly, a period (time to revolve around the
earth) of about several (5-10) hours depending upon the
radius of the orbit. They may be revolving in circular orbit
in the equatorial plane and/ or in inclined planes. There
130
may be 10-15 MEO satellites to provide global communications.
A Low Earth Orbit (LEO) typically is a circular orbit about
400-2000 kilometers above the earth’s surface and,
correspondingly, a period (time to revolve around the earth)
of about few (1-2) hours. In addition, satellites in low earth
orbit change their position quickly relative to the ground
position. So even for local applications, a large number
(hundreds) of satellites are needed if the mission requires
uninterrupted connectivity. A satellite in an Elliptical orbit
follows an elliptical path. Since a geostationary satellite
operates above the equator, is not suitable for providing
services at high latitudes. The first such satellite (popularly
known as Molniya satellites) was launched on April 23,
1965 and was used for experimental transmission of TV
signal from Moscow uplink station to downlink stations,
located in Siberia and Russian Far East. The Molniya orbit
is highly inclined, guaranteeing good elevation over selected
positions during the northern portion of the orbit. Furthermore,
the Molniya orbit is so designed that the satellite spends
the great majority of its time (one third day) over the far
northern latitudes. In this way a constellation of three
Molniya satellites (plus in-orbit spares) can provide
uninterrupted coverage.
3.
SATELLITE LAUNCHING
In the early 1960s, converted Inter Continental Ballistic
Missiles (ICBM) and Intermediate Range Ballistic Missiles
(IRBM) were used as launch vehicles. These all had a
common problem: they were designed to deliver an object
to the earth's surface, not to place an object in orbit. Upper
stages had to be designed to provide a delta-Vee (velocity
change) at apogee to circularize the orbit. The DELTA
launch vehicles, which placed all of the early communications
PRASAD:SATELLITE MOBILE COMMUNICATIONS TECHNOLOGY-TRENDS
satellites in orbit, used the VANGUARD upper stage to
provide this delta-Vee. Later on velocity change technology
was improved and 1968 afterward, satellite launch became
quite reliable.
4.
PROPAGATION CHARACTERISTICS
The frequency range of interest covers L/S/C/K/Kabands and the EHF-band.
4.1 Signal Characteristics at L-Band
An RHCP drooping dipole antenna for the handheld
and a car-roof mounted RHCP antenna have been used for
determining the signal characteristics. For narrowband signal
the results are shown in Fig. 1 and 2. The upper graph
shows the power level for narrowband signal while the
user is standing keeping the set in hand. Lower graph
shows the power level for narrowband signal while the
user is inside the standing car and antenna is mounted
on the car roof. It is obvious that signal is well defined
for user in the car with the antenna mounted on the car
roof. Similarly the signal power levels are shown for the
user in the driving car with the antenna on car roof and
the other for the set held in hand in the running car. Here
also the signal level is well defined for the user in the
driving car with the antenna mounted on the car roof.
350
350
Figure 2. Narrow band power level, upper graph for car roof
mounted driving, lower graph for handheld in car
driving for L band.
4.1.1 Signal Characteristics at C- and Above Bands
Figure 1. Narrow band power level, upper graph for handheld
standing, lower graph for car roof mounted standing
for L band.
Since these frequencies are not used by mobile terminals
and mainly used by feeder link and inter satellite links, the
effect of rain, atmosphere etc are as follows
a) Tropospheric (gaseous atmosphere) effects
•
Absorption by air and water vapor (no condensed):
This is nearly constant for higher elevation angles,
adding only a few tenths of decibels to the path loss.
It generally can be ignored at frequencies below
15 GHz.
•
Refractive bending and scintillation (rapid fluctuations
of carrier power) at low elevation angles: Earth stations
that point within 10° of the horizon to view the satellite
are subject to wider variations in received or transmitted
signal. Troposphere scintillation is time varying signal
attenuation (and enhancement) caused by combining
of the direct path with the refracted path signal in the
receiving antenna.
•
Rain attenuation: This important factor increases with
frequency and rain rate. Rain also introduces scintillation
due to scattering of electromagnetic waves by raindrops.
b) Ionospheric effects
•
Faraday rotation of linear polarization: This is most
pronounced at L- and S-bands, with significant impact
at C-band during the peak of sunspot activity. It
131
DRDO SCIENCE SPECTRUM 2009
•
5.
is not a significant factor at Ku- and Ka- bands.
Ionosphere scintillation: This is most pronounced
in the equatorial regions of the world (particularly
along the geomagnetic equator). Like Faraday rotation,
this source of fading decreases with increasing frequency,
making it a factor for L-, S-, and C-band links.
CARRIER MODULATION
Modulation is the process of translating the information
signal either in analog or digital form onto the suitable
carrier (analog) signal. Analog modulations are (i) Amplitude
Modulation, (ii) Frequency Modulation, and (iii) Phase
Modulation. Nowadays, analog modulation is not used in
satellite communications. There are three major classes of
digital modulations used for transmission of digitally
represented data:
•
Amplitude-shift keying (ASK)
•
Frequency-shift keying (FSK)
•
Phase-shift keying (PSK)
All convey data by changing some aspect of a carrier
wave, (usually a sinusoid) in response to a data signal.
However, nowadays, digital phase modulation is mostly
used in satellite communications.
A constellation diagram of PSK shows the points in
the Argand plane where, the real and imaginary axis are
termed the in-phase and quadrature axis respectively due
to their 90° separation. The amplitude of each point along
the in-phase axis is used to modulate a cosine (or sine)
wave and the amplitude along the quadrature axis to modulate
a sine (or cosine) wave where the constellation points are
positioned with uniform angular spacing around a circle.
This gives maximum phase-separation between adjacent
points and thus the best noise immunity. They are positioned
on a circle so that they can all be transmitted with the same
energy.
5.2 Quadrature phase-shift keying QPSK)
Fig. 4 below shows its Constellation diagram Sometimes
known as quaternary or quadriphase PSK, 4-PSK, or 4QAM, QPSK uses four points on the constellation diagram,
equispaced around a circle. With four phases, QPSK can
encode two bits per symbol, shown in the diagram with
Gray coding to minimize the BER and cater for twice the
rate of BPSK. QPSK can be viewed as two independently
modulated quadrature carriers.
Figure 4. Constellation diagram for QPSK with gray coding.
With this interpretation, the even (or odd) bits are
used to modulate the in-phase component of the carrier,
while the odd (or even) bits are used to modulate the
quadrature-phase component of the carrier. BPSK is used
on both carriers and they can be independently demodulated.
Phase ambiguity problems at the receiver are resolved by
differentially encoded QPSK. Major components of the
transmitter and receiver are shown below in Fig. 5.
The binary data stream is split into the in-phase and
quadrature-phase components. These are then separately
5.1 Binary phase-shift keying (BPSK)
Fig. 3 shows the Constellation diagram for BPSK BPSK
(also sometimes called PRK, Phase Reversal Keying) is the
simplest form of PSK. It uses two phases which are separated
by 180° and so can also be termed 2-PSK. It does not
particularly matter exactly where the constellation points
are positioned, and in this figure they are shown on the
real axis, at 0° and 180°. This modulation is the most robust
of all the PSKs since it takes serious distortion to make
the demodulator reach an incorrect decision. It is, however,
only able to modulate at 1 bit/symbol and so is unsuitable
for high data-rate applications when bandwidth is limited.
Figure 5. Conceptual transmitter structure for QPSK.
modulated onto two orthogonal basis functions. In this
implementation, two sinusoids are used. Afterwards, the
two signals are superimposed, and the resulting signal is
the QPSK signal. A QPSK receiver is shown below Fig.6.
In the receiver, the matched filters can be replaced
with correlators. Each detection device uses a reference
0
0
1
Figure 3. Constellation of BPSK.
132
Figure 6. Receiver structure for QPSK.
PRASAD:SATELLITE MOBILE COMMUNICATIONS TECHNOLOGY-TRENDS
threshold value to determine whether a 1 or 0 is detected.
5.3 Offset QPSK (OQPSK)
Constellation diagram for OQPSK is shown in Fig.7.
As is visible from the constellation diagram above,
it is seen that Signal doesn't pass through origin because
only one bit of the symbol is changed at a time Offset
Quadrature Phase-Shift Keying (OQPSK) is a variant of
phase-shift keying modulation using 4 different values of
the phase to transmit. It is sometimes called staggered
quadrature phase-shift keying (SQPSK).
Figure 7. Constellation diagram for OQPSK.
5.4 Difference of the phase between QPSK and
OQPSK
Taking four values of the phase (two bits) at a time
to construct a QPSK symbol can allow the phase of the
signal to jump by as much as 180° at a time. When the
signal is low-pass filtered (as is typical in a transmitter),
these phase-shifts result in large amplitude fluctuations,
an undesirable quality in communication systems. By offsetting
the timing of the odd and even bits by one bit-period, or
half a symbol-period, the in-phase and quadrature components
will never change at the same time. In the constellation
diagram, it can be seen that this will limit the phase-shift
to no more than 90° at a time. This yields much lower
amplitude fluctuations than non-offset QPSK and is sometimes
preferred in practice.
ð /4–QPSK Modulation: Recently ð /4–QPSK & ð /4
shift QPSK have become very poplar for mobile satellite
communications as well terrestrial mobile communications
because it has a compact spectrum with small spectrum
restoration due to non linear amplification and differential
detection can be performed.
This final variant of QPSK uses two identical constellations
Fig. 8a. which are rotated by 45° ( ð /4 radians, hence the
name) with respect to one another. The maximum phaseshifts are reduces from a maximum of 180°, but only to a
maximum of 135° and so the amplitude fluctuations of ð /
4–QPSK are between OQPSK and non-offset QPSK. An
implementation of transmitter is shown in Fig. 8b. On the
other hand, ð /4–QPSK lends itself to easy demodulation,
as shown in Fig.8c., and has been adopted for use in, for
example, Thuraya Mobile System & TDMA cellular telephone
systems.
5.5 Higher Order M-PSK Modulation
Any number of phases may be used to construct a
PSK constellation but 8-PSK is usually the highest order
PSK constellation deployed. Without coding transitions
may be from any point to any point in the constellation.
With gray coding, there will be transition only in the
neighborhood on left or right. With more than 8 phases,
the error-rate becomes too high and there are better, though
more complex, modulations available such as quadrature
amplitude modulation (QAM).
Bit error rate for high order M and its E b / N 0 can be
approximated by:
The bit-error probability for M-PSK can only be determined
exactly once the bit-mapping is known. However, when
Gray coding is used, the most probable error from one
symbol to the next produces only a single bit-error and
Figure 8. Constellation diagram & Trans for
ð /4–QPSK.
It has been observed that higher-order modulations exhibit
higher error-rates; in exchange however they deliver a
higher raw data-rate. The error rate performances of BPSK
& QPSK or OQPSK are same hence QPSK or OQPSK are
invariably used in practical satellite systems.
Orthogonal Frequency Division Multiplexing (OFDM)
— essentially identical to Coded OFDM (COFDM) and
Discrete multi-tone modulation (DMT) — is a frequencydivision multiplexing (FDM) scheme utilized as a digital
133
DRDO SCIENCE SPECTRUM 2009
multi-carrier modulation method. A large number of closelyspaced orthogonal sub-carriers are used to carry data. The
data are divided into several parallel data streams or channels,
one for each sub-carrier. Each sub-carrier is modulated
with a conventional modulation scheme (such as quadrature
amplitude modulation or phase shift keying) at a low symbol
rate, maintaining total data rates similar to conventional
single-carrier modulation schemes in the same bandwidth.
The primary advantage of OFDM over single-carrier
schemes is its ability to cope with severe channel conditions
— for example, attenuation of high frequencies in a long
copper wire, narrowband interference and frequency-selective
fading due to multipath — without complex equalsation
filters. Channel equalization is simplified because OFDM
may be viewed as using many slowly-modulated narrowband
signals rather than one rapidly-modulated wideband signal.
The low symbol rate makes the use of a guard interval
between symbols affordable, making it possible to handle
time-spreading and eliminate intersymbol interference (ISI).
6.
MULTIPLE ACCESS TECHNIQUES FOR
SATELLITE COMMUNICATION
Multiple Access is a technique of sharing a common
facility (satellite transponder) by many geographically separated
users for establishing telecommunication services. It is
Figure 10. Pictorial concept of FDMA, TDMA & CDMA.
Figure 9. Constellation diagram for 8-PSK with Gray coding.
the technique used to exploit the satellite geometric advantage
and is at the core of satellite networking. Although there
are many specific implementations of multiple access systems,
however three basic multiple access techniques are used
primarily in satellite communication systems.
•
Frequency Division Multiple Access
(FDMA)
•
Time Division Multiple Access
(TDMA)
•
Code Division Multiple Access (CDMA)
The pictorial concept of basic multiple access technique
is shown in Fig.10.
6.1 Frequency Division Multiple Access (FDMA)
The simplest and most widely used multiple access
technique of satellite communication is frequency division
multiple access, where each earth station in satellite network
134
transmits one or more carriers at different center frequencies
to the satellite transponder. Each carrier is assigned a
frequency band with a small guard band to avoid overlapping
between adjacent carriers. Satellite transponder receives
all the carriers in its bandwidth, amplifies them and retransmits
them back to the earth. The earth station in the satellite
beam can select the carrier that contains message intended
for it. In this type of system each carrier employ analog
modulation such as frequency modulation or digital modulation
that is phase shift keying. An excellent example of FDMA
is the 4-6 /11-14GHz satellite communication system.
In C-band (4-6 GHz) system, the overall bandwidth
is 500 MHz and each transponder bandwidth is 36
MHz or 72 MHz in Ku -Band. 36MHz/72MHz bandwidth
has many carriers accommodating different number of
channels. Each carrier provides access to the satellite
from the ground station. Access scheme is shown in
following Fig.11 with various multiplexing cum carrier modulation
cum multiple access technique.
Depending upon the type of base band and the
type of modulation used, FDMA can take several forms
as shown above. Some popular FDMA systems are:
FDM/FM/FDMA is one form of FDMA which is
Frequency Division Multiplexed multiple channel base
band and FM for modulating a carrier.
PRASAD:SATELLITE MOBILE COMMUNICATIONS TECHNOLOGY-TRENDS
Figure 11. FDMA scheme.
TDM/PSK/FDMA indicates time division multiplexed
multiple channel digital base band and PSK modulating
the carrier.
SCPC/FM or PSK/FDMA indicates the single
channel per carrier FDMA scheme where each analog
and digital channel FM and PSK modulates a carrier
respectively.
6.2 Time Division Multiple Access (TDMA)
Time division multiple access is a technique where
a number of earth stations share a common satellite
transponder by transmitting non-overlapping bursts of
single carrier, avoiding the generation of inter-modulation
products in the non-linear transponder and reducing
stringency of EIRP control. All the earth station
transmits at the same frequency using entire bandwidth
of transponder. Sharing is done in time division mode
by allotting time slots to various stations. The transmit
timing of the bursts is synchronized so that all the
burst arriving at the satellite transponder from an earth
station in the network are closely spaced in time
so that they do not overlap. The satellite transponder
receives one burst at a time, amplifies it and retransmit
it back to earth. Thus every earth station in the satellite
beam can receive the entire burst stream and extract
the bursts intended for it. TDMA scheme is shown in
Fig.12.
Figure 12. TDMA Scheme.
6.3 Code Division Multiple Access (CDMA)
This access scheme is based on the use of spreadspectrum modulation technique where all the users
transmit the signal simultaneously on the multiple
access channel. Message signal is spread over a wide
135
DRDO SCIENCE SPECTRUM 2009
band by multiplying it with noise-like or pseudo random
spreading signal.
6.4 Features of CDMA
•
CDMA is highly resistance to interference and therefore
satellite spacing can be reduced without causing
unacceptable degradation in the received signal
quality.
•
Spread Spectrum systems are resistance to multipath
noise.
•
This technique offers a highly secure form of
communication.
•
Small antenna can be employed without any
problem of interference from an adjacent satellite.
As shown in the following Fig.13. on the left top side
the input data is spread with the spreading pseudo random
sequence, resulting in very low power signal shown at
point C. This signal modulate a carrier signal, the spectrum
is translated to carrier. At this point, the thermal noise and
noise from other CDMA users and/ or intentional interference
is shown as peaked interference signal at point E. this
complex signal at point F is input to the demodulator and
despread by the spreading code at point H. the original
input data gets buildup and at the same time, the interference
& thermal noise get spread. The decision device is able
to decode correctly in spite of presence of heavy interference.
This is possible in CDMA scheme.
7.
BASE BAND MULTIPLEXING
In satellite communications only digital base band is
used for obvious reasons.
7.1 Time Division Multiplexing (TDM)
In circuit switched networks such as the Public Switched
Telephone Network (PSTN) there exists the need to transmit
multiple subscribers’ calls along the same transmission
medium. To accomplish this, network designers make use
of TDM. TDM allows switches to create channels, also
known as tributaries, within a transmission stream. A standard
DS0 voice signal has a data bit rate of 64 kbit/s, determined
by using Nyquist’s sampling criterion. TDM takes frames
of the voice signals and multiplexes them into a TDM
frame which runs at a higher bandwidth. So if the TDM
frame consists of n voice frames, the bandwidth will be
n*64 kbit/s. There are two widely used standards
a) ATT Standard
T1 : 24 PCM channels multiplexed to form 1.544 Mbit/
sec digital stream.
T2 : 4 Tl multiplexed to make 6.312 Mbit/sec bit stream.
T3 : 7 T2 multiplexed to form 44.736 Mbit/sec bit stream.
T4 : 6 T3 multiplexed to form 274.176 Mbit/sec bit stream.
b) CCIT Standard
E1: 30 PCM channels multiplexed to form 2.048 Mbit/sec
bit stream channel each PCM channel carry 64 Kbit/
sec digital speech with two channels for signaling.
E2: Four E1 stream multiplexed to form 8. 448 Mbit / see
bit stream.
E3: Four E2 stream multiplexed to form 34.368 Mbit/sec
bit stream.
E4: Four E3 stream multiplexed to form 139.264 Mbit/sec
bit stream.
Figure 13. CDMA Scheme.
136
PRASAD:SATELLITE MOBILE COMMUNICATIONS TECHNOLOGY-TRENDS
8.
FORWARD ERROR CORRECTION CODING
In satellite communications all types of coding is used
to reduce the errors introduced by the transmission channels.
Following coding techniques are used
•
Convolution codes as the inner code with the polynomial
and constrained length as dictated by the application.
•
Reed Soloman Code as the outer code from a large
set of n, k values
•
Interleaver to distribute the data in the matrix of column
and rows
•
Scrambling is used to randomize the data before
transmission to facilitate synchronization at the receiver
end.
•
Turbo codes are used in few recent systems
•
TCH (Tomlinson, Cercas, Hughes) Codes are also
used in recent systems.
With the increasing use of multimedia services and
personal communications, the design of communication
systems is being pushed to the limits of channel capacity
with the use of very ef?cient modulation and coding
schemes. Since most of these systems tend to be
portable and mobile, these terminals should be as small
as possible with low gain antennas. The power of these
terminals must also be kept as low as possible not only
for portability reasons but also for reducing the interference
on neighbor systems. However, these restrictions should
not compromise the overall performance in a real environment,
that is, when adverse propagation conditions are considered.
For example, we take into account propagation with
multi-path and other sources of fading that result in
low signal-to-noise ratios, namely a Ka or higher frequency
bands. This is even more critical in satellite receivers as
they operate under limited power of the satellite. The new
family of codes was devised for such applications,
namely for FEC (Forward Error Correction), and it was
shown that they can exhibit good performance and undertake
maximum likelihood soft-decision decoding with a very
simple decoder structure using DSP techniques known as
TCH receiver. A further analysis of the correlation properties
of TCH sequences, i.e. TCH codewords taken as sequences,
revealed that it is possible to identify some sets of sequences
that exhibit good auto and cross-correlation. These are
very important properties for this family of sequences since,
as we know, the performance of CDMA (Code Division
Multiple Access) systems depends not only on the cross
correlation properties of the sequences in order to minimize
inter-user interference, but also on their autocorrelation
because of the synchronization process. The details of
these techniques are not given to avoid the mathematics.
9.
provide flexible access to the space segment. This is preferred
because of its greater ability to penetrate foliage and nonmetallic
structures and bend around obstacles. The low end of
the usable spectrum is probably 100 MHz (50 MHz each
for uplink & downlink), which is able to penetrate the
ionosphere under all conditions.
The satellite bands and its typical applications are
shown in the following Table (1):
The overall architecture of MSS telephony is shown
in Fig. (14). There are four operating levels. Each of these
levels contributes to the functionality of the total system:
•
Satellite constellation, consisting of a quantity of
operational satellites that deliver the service over the
coverage area. These can employ any of the possible
orbit constellation arrangement. The inter satellite links
are optional. The inter satellite links normally operate
in Ka-band.
•
User terminals of various types: Vehicular, handheld,
transportable, ship & aircraft, and fixed terminals.
Table 1. Typical satellite bands
Frequency Band
L- Band (1-2 GHz)
S-Band (2-4 GHz)
C-Band (4-8 GHz)
X- Band (8-12.5 GHz)
Ku – Band (12.5-18
GHz)
K- Band (18-26.5 GHz)
Ka- Band (26.5-40
GHz)
•
Purpose
Mobile Satellite Service (MSS), UHF
TV, terrestrial microwave and studio
television links, cellular phone
MSS, Digital Audio radio Service
(DARS) NASA and deep space
research
Fixed Satellite Service(FSS), fixed
service terrestrial microwave
FSS military communication, DARS
feeder links, fixed service terrestrial,
Earth observation satellites
FSS, Broadcast Satellite service
(BSS), Fixed
Service terrestrial microwave
FSS, BSS, Fixed Service terrestrial
microwave. Local Multichannel
Distribution Service (LMDS)
FSS,Fixed Service terrestrial
microwave,LMDS, Intersatellite links,
Satellite Imaging
Gateway Earth Stations allow traffic to pass between
PRINCIPLE OF MOBILE SATELLITE
COMMUNICATION
Satellite communications is a natural facility for serving
users while they travel by various means. The mobile satellite
service (MSS) use frequencies 1-3 GHz where simple antennas
Figure 14. Overall architecture of an MSS system, showing the
four primary levels - the satellite constellation, the
user terminals, gateway earth station, and the
terrestrial networks.
137
DRDO SCIENCE SPECTRUM 2009
users and the public networks, and to manage the
service on a constant basis. There is Tracking, Telemetry
& Command (TT&C) facilities to control and monitor
the satellites. The feeder link between satellite and
GES may be in any one band i.e. C-, or Ku-, or Kaband.
•
Terrestrial networks to address the service needs of
the users,. These include the PSTN, the Internet, and
other networks, both public and private.
A state-of-the-art MSS system can provide other
capabilities besides connection of calls to the PSTN. The
most striking is direct mobile-to-mobile calling that allows
subscribers to talk to each other regardless of their location
and situation. The quality of the terrestrial telephone network
in different countries served by the system will also play
heavily into the attractiveness of direct mobile-to-mobile
calling Some systems will address this by connecting these
calls through a common gateway, introducing the delay
and degradation of a double hop. This is not a concern
in LEO and MEO systems, where the propagation delay
is relatively low. Details of the general operational characteristics
of LEO/ MEO global systems’ can be found in references
(2) & (3)
MSS communication requires a direct line-of-sight path
between the user and the satellite. The service quality in
this case is ideal because there is no outage due to the
mobile-to-satellite path. If the user or satellite is moving,
then the link will experience periods of blockage when the
user is “shadowed” from the satellite transmission. The
MSS network would either (1) allow the dropouts in the
data transfer, which would intermittently halt the conversation
or information flow, or (2) try to reduce the dropouts through
path diversity. The latter is very expensive because it requires
that the number of satellites be increased by at least a
factor of two. In most cases, the mobile user will want to
be connected with the PSTN, which is provided through
a GES that employs fixed satellite service (FSS) spectrum
at C- or Ku-band.
9.1 GEO MSS Systems Serving Handheld Terminals
Thuraya is one of the state of art system employing
latest technology. In June 1993, Hughes Communications,
Inc., introduced the concept of handheld service from a
GEO MSS system.
Most critical part of the design of this generation of
GEO MSS satellite is the technique for routing channels
and calls between the beams (no. in hundreds). This operation
is performed in digital form using an onboard digital processor.
The L-band spectrum in each beam is translated to IF as
in the analog approach and converted to a digital representation
in an analog-to-digital converter. From this point, the digitized
information can be filtered, routed, and applied to the downlink
with dynamic beam forming. This is shown in Fig. (15)
below
The concepts previously described were employed to
bring the first regional MSS system to the Middle East,
Africa, Europe, and Southern Asia, that is, by Thuraya of
the UAE. that covers literally every corner of the land with
138
more than 200 spot beams. Two satellites, Thuraya 1 and
2, operate at 44 EL and 28.5 EL, respectively.
The specific signal characteristics are summarized as
follows:
•
Channel bandwidth of 27.7 kHz, capable of supporting
a bit rate of 46.8 Kbps
Figure 15. GEO mobile payload with low-level digital beam
forming and mobile-to-mobile channel routing.
•
•
Modulation with ð/4-QPSK
TDMA within the individual FDMA channels for up
to eight multiplexed voice channels
•
L-band U/L: 1,626.5 to 1,660.5 MHz
•
L-band D/L: 1,525 to 1,559 MHz
•
U/L from the Gateway Earth station: 6,425 to 6,725
MHz
•
D/L Satellite to Gateway Earth station: 3,400 to 3,625
MHz
•
Antenna: 12.25-M
•
Spot Beams: 200 Appx. (Configurable)
A Provision of data transmission in increments of 4.3
Kbps up to the carrier maximum of 46.8 Kbps is available
to the controller. After the necessary operations are performed,
the selected information is routed to the appropriate transmission
channel where it can be converted back to analog form.
The resulting band of carriers is translated to either Cband (for mobile-to-gateway service) or again to L-band
(mobile-to-mobile service) and transmitted through the
appropriate amplifier and antenna feed. The Thuraya system
relies on a GPS receiver in the handsets to allow determination
of which beam the handset should be considered to be
in the network.
10. OPERATIONAL SATELLITE SYSTEMS
Following networks are presently in operation using
Ku-band & K-band transponder of GEO, MEO or LEO.
a) Astrolink Network: Astrolink satellite constellation
contains nine GEO with Ka-band transponder.
System is designed to support high speed multimedia
communication employing on board processing (OBP).
Data rate ranging from 16 Kbps to 9.6 Mbps
are supported by very small dishes which makes
it suitable for mobile platforms.
b) Cyberstar: Ka-band Cyberstar constellation consists
of three GEO satellites & designed to provide
IP multicasting services which is based on Frame
Relay & ATM technology. The capacity of Cyberstar
PRASAD:SATELLITE MOBILE COMMUNICATIONS TECHNOLOGY-TRENDS
c)
d)
e)
f)
g)
system is 9.6 Gbps.
Spaceway: Spaceway constellation consists of 3
GEO & 20 MEO satellites in Ka-band. It is
designed to support high speed data, internet
access, broad band multimedia information services.
It offers QoS (bit error rate, BER<exp-10) to users
with terminal as small as 0.66 Mtr. System is
compatible with ATM, ISDN & FR standards.
Network supports data rate from 10 Kbps to 6
Mbps.
Sky Bridge: Sky Bridge constellation consists of
80 satellites in circular orbit LEO. This system
is networked to support advanced information
services at data rate from 16 Kbps to 60 Kbps.
System operates in Ku-band.
Switch in the Sky: In bent pipe satellite relay, the
satellite transponder performs signal amplification
and frequency translation. Signal detection, decoding
and protocol translation are not performed. Implementation
of "switch in the sky requires on board processing,
since they afford superior performance and more
sophisticated networking capabilities than the basic
transparent bent pipe relay. Examples of satellites
for OBP and OBR include US Milstar, the NASA
Advanced Communication Technology satellite &
Intelsat-2.
IP & ATM based satellite: IP, ATM and
combination of two is expected to form the backbone of efficient future multimedia information
networks. Various aspects of TCP and performance
of TCP/IP over satellite link is being implemented
to accommodate large bandwidth-delay product, to
overcome slow start algorithm, congestion control,
acknowledgement and error recovery mechanism.
The trend is toward flexible, packet oriented,
Quality of Service aware network & most of the
broad band satellite systems are expected to be
IP/ATM based.
IP security over Satcom: Potential for interception
and corruption is increased by wide area coverage
of satellite link. IP security mechanism is required
to ensure confidentiality, authentication, integrity,
access control and key management. Key management
is a key issue with respect to I P security over
multicast Satcom. Trends are also toward providing
service enabling platforms, resource allocation
together with adaptive beamforming technique &
development of VSAT and UltraSAT satcom systems
to provide voice, data, video, VOIP, VPN and other
IP based services for portable, mobile and fixed
satcom applications. BPSK, QPSK, 8PSK, 16QAM
modulation scheme with FEC and satellite channel
access with CDMA/TDMA/MF-TDMA/FDMA with
DAMA, PAMA as a channel assigned scheme
are employed with the present satcom system for
efficient use of bandwidth and EIRP of the satellite.
11. FUTURE SATELLITE COMMUNICATION
SYSTEMS
It is proposed to provide a longer-term activity dealing
with the exploitation of the satellite component to provide
services at higher rates (> 2 Mb/s) than presently assumed
in the UMTS to support wireless Internet access but also
simultaneous voice and data, multimedia, e-mail and broadband
integrated services It is recognized internationally that
satellite systems are necessary to provide the required
global coverage of future mobile and personal communications.
Features expected to be implemented by the new generation
systems include: personalized services with a capability
to respond to new services, facilities and applications (e.g.
multimedia and personal communications); mobile terminals
to offer the same services, facilities and applications as
a fixed terminal in a common-feel way; freedom to roam
on a worldwide basis; parity of quality, performance, privacy
and cost between fixed and mobile access. These systems
are intended to realize true personal mobile radio communications
from anywhere and to allow people to communicate freely
with each other from homes or offices, cities or rural areas,
fixed locations or moving vehicles (land, sea, air). The
satellite component of the future systems offers, in particular,
an effective means for providing services to areas where
terrestrial telecommunication infrastructures are not yet
well advanced.
12. CONCLUSION
In the tutorial paper an attempt has been made to
briefly describe the essentials of the satellite communications.
Many details could not be given place. The details of
many techniques could not be given due to length restrictions
and so also the mathematical formulations.
13. ACKNOWLEDGEMENT
This article will not be complete without extending my
sincere thanks to Dr. P.K. Saxena, Director SAG, for providing
me an opportunity to write on this topic and providing
dynamic guidance and requisite support
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
Bruce R. Elbert: Satellite Communications Applications
Handbook; Artech House Inc
M. Richharia: Mobile Satellite Communication; Addison
– Wesley
Krzysztof Wesolowski: Mobile Communication Systems;
John – Wiley & Sons, Ltd.
G. Maral, M. Bousquet: Satellite Communications systems;
Willey Publisher
Dr. Kamilo Feher: Advanced Digital Communication
systems and signal processing Techniques; Prentice
Hall Inc
Roger L. Freeman: Fundamentals of Telecommunications;
Wiley Interscience
Tri T Ha: Digital Satellite Communications; Howard
W Sams & Co.
Zhili Sun: Satellite Networking; John – Wiley & Sons,
Ltd.
Stephan C Pascall: Commercial Satellite Communications;
Focal Press
139