Unit 4 Multiple Access.ppt presentation for Satellite communication

Unit 4
Satellite Multiple Access
Mrs B.Prathyusha
Asst.Professor
Department of ECE

A radio wave – called a carrier wave – contains little information of its own. The
carrier has a frequency, a magnitude, and a phase angle; one or more of these
parameters must be varied with time to convey information.
An unmodulated carrier is called a Cwsignal, for continuous wave. The earliest
radio transmissions, pioneered by Marconi and others around 1900, were made
by turning the carrier on and off to send short and long bursts of the carrier in
the form of Morse code.
Information is transmitted on radio waves by modulating the carrier in
proportion to the signal that is to be transmitted.

Text Book:
Satellite Communications, Third Edition. Timothy Pratt and Jeremy
Allnutt.© 2020 JohnWiley & Sons Ltd. Published 2020 by JohnWiley &
Sons Ltd.

UNIT IV
• MULTIPLE ACCESS [1] [2] :
• Frequency division multiple access (FDMA) Intermodulation,
• Calculation of C/N.
• Time division Multiple Access (TDMA) Frame structure,
• Examples. Satellite Switched TDMA Onboard processing,
• DAMA,
• Code Division Multiple access (CDMA),
• Spread spectrum transmission and reception.

Analog Modulation
In analog modulation, the modulating signal (the baseband signal carrying the information
to be sent on the radio wave) is proportional to a physical quantity, for example a voltage
derived from a microphone that is proportional to the sound pressure on the microphone’s
transducer.
The RF carrier has only three parameters – amplitude, frequency, and phase – so an analog
modulation system must vary one of those three parameters (or possibly two at the same
time). The main analog modulation methods are:
amplitude modulation (AM),
frequency modulation (FM),
and phase modulation.

Analog modulation is usually AM or FM, and both are in widespread use
for broadcasting.
Phase modulation is rarely used directly in analog form. AM is the
oldest form of modulation, and its use today is largely restricted to
broadcasting in the radio frequency bands below 30MHz.
It is also the easiest to produce in a radio transmitter and to
demodulate in a radio receiver, so it was employed by the first
broadcast radio systems in the 1920s.
AM is not the best modulation technique for sound broadcasting, as it
has poor noise performance, and its use is confined to the lower RF
frequencies where interference from manmade noise is greatest.

Multiple Access
• The ability of a satellite to carry many signals at the same time is known
as multiple access.
• Multiple access allows the communication capacity of the satellite to be
shared among a large number of earth stations, and to accommodate the
different mixes of communication traffic that are transmitted through a
satellite transponder.

• The basic form of multiple access employed by most communications
satellites is the use of many transponders, as discussed earlier .
• A large GEO satellite can have a communication bandwidth many times
the allocated RF bandwidth; for example, 4000MHz of capacity can be
used within an allocated RF bandwidth of 1000MHz.
• Through frequency re-use with multiple antenna beams and orthogonal
polarizations, the spectrum can be re-used many times over – as many as
18 times in the case of some large GEO satellites.

• The frequency spectrum used by the satellite is divided into smaller
bandwidths, which are allocated to transponders, allowing separate
communication links to be established via the satellite on the basis of
transmit frequency.
• Transponder bandwidths from 20 to 200MHz have been employed on
GEO communications satellites, with a trend toward larger bandwidths
over time.
• The individual transponders may carry one signal – a high speed digital
stream made up of a number of television programs, for example, or
hundreds of signals, as with mobile satellite telephone systems.

• Smaller low earth orbit (LEO) satellites may have only one transponder
used for a specific service, or multiple transponders connected to
multiple beams. When the satellite has a particular application, such as
earth surveillance, the information it collects is transmitted on a
downlink that is usually sized to match the rate at which data is collected.
• If that is not possible, the LEO satellites are designed to collect and store
data as they orbit the earth and then download the contents of the
memory when in range of a receiving earth station.

• There are no transponders as in a communications satellite, but there is
always an uplink to the satellite for control purposes.
• The use of multiple transponders to divide up a frequency band is no
generally considered as multiple access, although the reason for their use
is to make it easier for earth stations to share the available frequency
spectrum efficiently.

• The signals that earth stations transmit to a satellite may differ widely in
their character – video, data, voice – but they can be sent through the
same satellite using multiple access and multiplexing techniques.
• Multiplexing is the process of combining a number of signals into a single
signal, so that it can be processed by a single amplifier or transmitted
over a single radio channel. Multiplexing can be done at baseband or at
an IF. The corresponding technique that recovers the individual signal is
called demultiplexing.
• Multiplexing is a key feature of all commercial long distance
communication systems, and is part of the multiple access capability of all
satellite communications systems.

• The designer of a satellite communication system must make decisions
about the form of multiple access to be used. The multiple access
technique will influence the capacity and flexibility of the satellite
communication system, its cost, and its ability to earn revenue.
• The basic problem in any multiple access system is how to permit a
changing group of earth stations to share a satellite in such a way that
satellite communication capacity is maximized, bandwidth is used
efficiently, flexibility is maintained, and cost to the user is minimized
while revenue to the operator is maximized.

• In frequency division multiple access
• (FDMA) all users share the satellite at the same time, but each uplink
earth station transmits at a unique allocated frequency. This approach to
sharing the frequency spectrum is familiar to us all, as it is the way that
radio broadcasting has always shared the air waves.
• Each radio station is allocated a frequency and a bandwidth, and
transmits its signals within that part of the frequency spectrum. FDMA
can be used with analog or digital signals.

• In time division multiple access (TDMA)
• each user is allocated a unique time slot at the satellite so that signals
pass through the transponder sequentially.
• Because TDMA causes delays in transmission, it is used only with digital
signals.
• The signals in below Figure have equal bandwidth or occupy equal time
periods; in practice, different bandwidth signals can share a transponder
in FDMA and signals with different durations can share a TDMA frame.

• In code division multiple access (CDMA) all users transmit to the satellite
on the same frequency and at the same time, so the signals are overlaid
on one another as illustrated in Figure 6.10. The earth stations transmit
coded spread spectrum (SS) signals that can be separated at the
receiving earth station by correlation with the transmitted code.
• For example, in the global positioning system (GPS) each individual GPS
satellite transmits a different coded spread spectrum signal. The signals
are nearly orthogonal, allowing a GPS receiver to extract the spread
spectrum signal for one satellite in the presence of similar spread
spectrum signals from other visible GPS satellites. CDMA is inherently a
digital technique.

• In all three of the classical multiple access techniques, some resource is
shared.
• If the proportion allocated to each earth station is fixed in advance, the
system is called fixed access (FA) or preassigned access (PA).
• If the resource is allocated as needed depending on changing traffic
conditions, the multiple access technique is called demand assignment
multiple access (DAMA).

• Systems that combine both FDMA and TDMA techniques are sometimes
called hybrid multiple access schemes or multi-frequency time division
multiple access (MFTDMA).

FDMA
• The main advantage of FDMA is that filters can be used to separate
signals. Filter technology was well understood when satellite
communications began, and microwave filters were used in earth
stations to select the signal from a given transponder.
• In a fixed assignment system, each transmitting earth station was
allocated a frequency and bandwidth for each group of signals it wished
to send.

Implementing FDMA
• Figure 6.11 shows a transponder operating with FDMA.
• Three transmitting earth stations send signals at different uplink
frequencies to a single transponder on a GEO satellite.
• The transponder amplifies the received signals and retransmits them on
the downlink at frequencies f1, f2, and f3.
• All earth stations within the satellite’s coverage zone receive all three
signals.
• The three receivers shown in Figure 6.11 could be at one earth station or
at three separate earth stations; in either case, BPFs centered at the
frequencies f1, f2, and f3 are used to select the wanted transmission from
within the bandwidth of the transponder.

• The BPFs are usually in the intermediate (IF) section of the receiver to
simplify their design.
• Figure 6.12 shows a typical fixed assignment FDMA plan for two C-band
transponders.
• The triangles represent RF carriers with the transmitting earth station
country and RF bandwidth shown inside the triangle.
• The signals could be video, data, or voice. Frequencies shown are for the
downlink from the satellite; the triangles are not spectral diagrams and
may also be shown as rectangles.

• The triangles represent the location of each signal within an allocated
bandwidth such as that of a transponder. Transponder #1 in Figure 6.12
receives three signals from different uplink earth stations; in this
example, two are in the United Sates and one is in Chile. Each of the
signals has a bandwidth of 10MHz.
• The uplink signals from the two earth stations in the United States are
transmitted on carrier frequencies of 5939 and 5951MHz, and the uplink
signal from the earth station in Chile is transmitted with a carrier
frequency of 5963MHz.

• The transponder down converts each received signal by 2225MHz giving
the downlink carrier frequencies of 3714, 3726, and 3738MHz. All earth
stations within the antenna beam connected to transponder #1 can
receive all of the signals transmitted by the transponder, and each
receiving earth station can extract any signals that are destined for that
particular earth station.

• Transponder #2 in Figure 6.12 carries two signals with different
bandwidths. The 20MHz wide signal originates from an earth station in
the United States at a carrier frequency of 5984 and the 10MHz
bandwidth signal originates from an earth station in Chile at a carrier
frequency of 5996MHz. Transponder #2 down converts these signals by
2225MHz and transmits them at carrier frequencies of 3759 and
3771MHz.

• Both of these signals can be received by the same earth stations that
receive signals from transponder #1.
• Typically, large C-band earth station receivers have front ends with a
bandwidth of 500 or 1000MHz to allow reception of all C-band carriers.
• Down conversion to an IF of 140MHz, for example, allows IF filters with a
bandwidth of 36MHz to separate the signals from the two transponders.
• Further filtering and down conversion is needed to extract the individual
carriers from each transponder, as illustrated in Figure 6.11.

• The use of microwave filters to separate transponders makes the fixed
assignment approach to FDMA very inflexible.
• Changing the frequency assignment or bandwidth of any one
transmitting earth station requires retuning of the filters at several
receiving earth stations.
• The fixed assignment FDM-FM-FDMA scheme illustrated in Figure 6.12
also makes inefficient use of transponder bandwidth and satellite
capacity.

• As an example, consider an earth station in the west of the United States
using a Pacific Ocean GEO satellite to send telephone channels to earth
stations in Korea, Japan, and Chile.
• The time difference between North America and the Pacific Rim
countries means that the channels will be busy for only a few hours per
day, and at a different time of day than the United States–Chile links.
• With fixed assignment, the frequencies and satellite capacity cannot be
reallocated between routes, so much of the satellite capacity remains
idle.

• Estimates of average loading of Intelsat satellites using fixed assignment
were typically around 15%. It is not possible to achieve 100% loading of
satellites used for international traffic, or even for domestic traffic in
many cases.
• Demand assignment and single channel per carrier techniques allow
higher loadings and therefore give satellite operators increased revenue.
Fixed assignment systems are rarely used now with new satellite
systems; demand assignment is preferred.
• The development of agile frequency synthesizers was a key factor in the
introduction of demand assignment FDMA.

FDMA Receiver
• Every earth station that operates in a FDMA network must have a
separate IF receiver for each of the carriers that it wishes to receive.
• SCPC systems can have a very large number of carriers in one
transponder; as a result, FDMA earth stations tend to have a very large
number of IF receivers and demultiplexers which select individual carriers
using narrowband IF filters.
• Figure 6.13 shows how the IF bandwidth of a receiving earth station
could be configured to receive 25 digital data channels, each with an
occupied bandwidth of 1.94MHz from a 54MHz wide Ku-band
transponder.

• The IF band is centered at 70MHz requiring the BPFs that extract the
individual signals to have a Q factor of 36. The 200 kHz frequency spaces
between the channels are called guard bands.
• Guard bands are essential in FDMA systems to allow the filters in the
receiver to select individual channels without excessive interference from
adjacent channels.
• All filters have a roll off characteristic, which describes how rapidly a filter
can change from near zero attenuation in its pass band to high
attenuation in the stop band.

• The transmitter is basically the complement of the receiver in Figure 6.14
and operates in demand assignment.
• The RF frequencies of the transmitted signals are assigned by a controller
that tells the uplink earth station which three of the 25 RF channels to
use.
• If a change to a different transponder is required, both the transmitting
and the receiving earth stations must change their RF local oscillators in
Figures 6.14 and 6.15 to new RF frequencies.

• When demand assignment is provided in the transmitter and receiver, 25
IF local oscillator frequencies and 25 SRRC filters centered at the IF
frequencies are required.
• A frequency synthesizer is needed to generate the LO frequencies. This
requires a great deal of hardware, so many FDMA-DAMA links use digital
signal processing (DSP) to generate the SRRC waveforms at the required IF
frequencies under software control.
• In the transmitter in Figure 6.15, only three channels are present, so only
three SRRC waveforms need to be generated. This can be done in a single
(ASIC) or field programmable gate array (FPGA) instead of providing the 25
sets of local oscillators and SRRC filters required by a hardware transmitter.

• FDMA is widely used as a method of sharing the bandwidth of satellite
transponders. In large GEO satellites with multiple downlink beam
antennas, a transponder is connected to each beam and can carry a
single RF carrier.
• If the satellite has 72 downlink beams, there may be 72 transponders
with single polarization and 144 transponders if each beam has two
polarizations.
• Alternatively, one transponder may be connected to several beams via RF
filters that select the frequency band to be transmitted by each beam.

• The builders and operators of satellites have historically shown a strong
preference for wideband transponders that can carry any type of traffic –
the bent pipe transponder that can carry video, data, or voice as the
marketplace demands.
• Bent pipe refers to a transponder that amplifies a signal received from
the uplink and retransmits it on the downlink at a different frequency
and at a higher power.
• By contrast, an onboard processing satellite has transponders that
demodulate signals received from the uplink, process the signals at
baseband, and then remodulate the signal onto a downlink RF carrier.

• Bent pipe transponders on commercial GEO satellites usually have wide
bandwidths, with bandwidths of 24, 36, 54, 72, and up to
200MHzcommonly employed.
• When an earth station has a carrier that occupies less than the
transponder bandwidth, FDMA can be used to allow that carrier to share
the transponder with other carriers.
• When an earth station sends one signal on a carrier, the FDMA access
technique is called single channel per carrier (SCPC). Thus a system in
which a large number of small earth stations, such as mobile telephones,
that access a single transponder using FDMA is called a single channel per
carrier frequency division multiple access (SCPC-FDMA) system.

• Hybrid multiple access schemes can use TDM of baseband channels,
which are then modulated onto a single carrier. A number of earth
stations can share a transponder using FDMA, giving a system known as
TDM-SCPC-FDMA. Note that the sequence of abbreviations is baseband
multiplexing technique first, then multiple access technique next. TDM-
SCPC-FDMA is often used by VSAT networks in which the earth stations
transmit many digital signals.

• FDMA has a disadvantage in satellite communications systems when the
satellite transponder has a non-linear characteristic. Most satellite
transponders use HPAs, which are driven close to saturation, causing
non-linear operation.
• A transponder using a traveling wave tube amplifier (TWTA) is more
prone to non-linearity than one with a solid state high power amplifier
(SSHPA). Equalization at the transmitting station, in the form of
predistortion of the transmitted signal can be employed to linearize the
transponder when fixed assignment is used.

• Linearization of solid state and traveling wave tube high power amplifiers
(TWT HPAs) in the satellite transponder is also possible.
• Non-linearity of the transponder HPA causes a reduction in the overall
(CNR)o at the receiving earth station when FDMA is used because
intermodulation (IM) products are generated in the transponder.
• Some of the IM products will be within the transponder bandwidth and
will cause interference. The IM products are treated as though they were
thermal noise, adding to the total noise in the receiver of the receiving
earth station.

Intermodulation
• Intermodulation (IM) or intermodulation distortion (IMD) is the
amplitude modulation of signals containing two or more different
frequencies, caused by nonlinearities or time variance in a system.
Intermodulation (IM) products are generated whenever more than one
signal is carried by a non-linear device.
• Sometimes filtering can be used to remove the IM products, but if they are
within the bandwidth of the transponder they cannot be filtered out.
• The saturation characteristic of a transponder can be modeled by a cubic
curve to illustrate the generation of third order intermodulation.
• Third order IM is important because third order IM products often have
frequencies close to the signals that generate the intermodulation, and are
therefore likely to be within the transponder bandwidth.

Time Division Multiple Access (TDMA)
• In TDMA a number of earth stations take turns transmitting bursts of RF
signals through a transponder.
• The bit rate of a burst is determined by the bandwidth of the RF signals
and the modulation.
• The RF bandwidth can be equal to the full transponder bandwidth that
typically will create a high bit rate, or in a MF-TDMA system can be a
fraction of the transponder bandwidth with a lower bit rate.

• Since all practical TDMA systems are digital, TDMA has all the advantages
over FDMA that digital signals have over analog.
• TDMA systems, because the signals are digital and can be divided by
time, are easily reconfigured for changing traffic demands, are resistant
to noise and interference, and can readily handle mixed video, data, and
voice traffic.

• One major advantage of TDMA when using the entire bandwidth of a
transponder is that only one signal is present in the transponder at one
time, thus overcoming some of the problems caused by non-linear
transponders operating with FDMA.
• However, using all of the transponder bandwidth requires every earth
station to transmit at a high bit rate, which requires high transmitter
power, making the basic form of TDMA not well suited to narrowband
signals from small earth stations.

• TDMA can be used to assemble multiple bit streams into a single higher
speed digital signal that has an RF bandwidth much less than the
transponder bandwidth.
• Several such MF-TDMA signals can then share a transponder using
FDMA.
• MF-TDMA is well suited to internet access systems using GEO and LEO
satellites, and systems with satellite telephones and mobile video links.

• It is important to distinguish between TDM and TDMA. The difference
between TDM and TDMA is that TDM is a baseband technique used at
one location (e.g., a transmitting earth station) to multiplex several
digital bit streams into a single higher speed digital signal.
• Groups of bits are taken from each of the bit streams and formed into
baseband packets or frames that also contain synchronization and
identification bits.

TDMA Frame Structure
• A TDMA frame contains the signals transmitted by all of the earth
stations in a TDMA network, or all of the earth stations in one MF-TDMA
group.
• A frame typically has a fixed length, and is built up from the burst
transmissions of each earth station, with guard times between each
burst.
• The frame exists only in the satellite transponder and on the downlinks
from the satellite to the receiving earth stations.

• The frame structure can differ greatly between different satellite
communication systems depending on whether the satellites are GEO or
LEO, whether the data has a high bit rate or a low bit rate, and whether
the system has fixed or mobile earth stations.
• The following discussion relates primarily to earth stations that have
fixed locations, communicating via satellites in GEO, and data rates that
are relatively high.

• A simplified diagram of a generic TDMA frame for four transmitting earth
stations.
• Each frame contains synchronization and other data essential to the
operation of the network, as well as data.
• Each earth station’s transmission is followed by a guard time to avoid
possible overlap of the following transmission.

• In GEO satellite systems, frame lengths of 125 μs up to 20ms have been
used, although 2ms has been widely used by stations using Intelsat
satellites.
• Earth stations must be able to join the network, add their bursts to the
TDMA frame in the correct time sequence, and leave the network
without disrupting its operation.

• They must also be able to track changes in the timing of the frame
caused by motion of the satellite toward or away from the earth station.
GEO satellites are never in a perfectly circular orbit above the earth’s
equator.
• The orbit always has some ellipticity and inclination, resulting in variation
of the distance from an earth station to the satellite. Each earth station
must also be able to extract the data bits and other information from
burst transmissions of other earth stations in the TDMA network.
• The transmitted bursts must contain synchronization and identification
information that help receiving earth stations to extract the traffic
portions of the frame without error.

• These goals are achieved by dividing TDMA burst transmissions into two
parts:
• A preamble or header that contains a synchronization waveform,
identification bits, and control bits, and a traffic portion containing data
bits.
• Synchronization of a TDMA receiver is achieved with the portion of the
frame that contains carrier and bit clock synchronization waveforms.

• Traffic bits are the revenue producing portion of each frame, and the
preamble and reference bursts represent overhead.
• The smaller the overhead, the more efficient the TDMA system, but the
greater the difficulty of acquiring and maintaining network synchronization.
• The preamble of each station’s burst transmission requires a fixed
transmission time.

• A longer frame contains proportionally less preamble time than a short
frame, so more revenue producing data bits can be carried in a long
frame.
• Early TDMA systems were designed around 125 μs frames, to match the
sample rate of digital speech in telephone systems, in exactly the same
way that T1 24 channel systems operate.
• A digital telephone channel generates one 8-bit digital word every 125 μs
(8 kHz sampling rate), so a 125 μs frame transmits one word from each
speech channel.

• However, it is more efficient to lengthen the frame to 2 ms or longer so
that the proportion of overhead to message transmission time is reduced.
• It must be remembered that a longer frame requires multiple 8-bitwords
when transmitting digital speech.
• For example, in a time period of 2 ms, a digital terrestrial channel will
deliver sixteen 8-bit words to a transmitting earth station, so a 2 ms TDMA
frame requires sixteen 8-bit words (128 bits) from each terrestrial channel
to be sent in each transmitted burst.
• GEO satellites are not widely used for telephone traffic now, with use
restricted to places that are not served by optical fiber cables such as
small islands in a large ocean.

CBTR stands for carrier and bit timing recovery, often 176 symbols in
duration, formed of a period of unmodulated carrier to synchronize the
locally generated carrier that drives the demodulator in the receiver, and
a sequence of modulated symbols that are used to synchronize the
receiver bit clock. Once the demodulator is synchronized, the
demodulator can output bits and the bits are used to synchronize the
receiver bit clock.

• The next symbols in the burst are a unique word (UW), typically 16–64
bits that are used to identify the transmitting earth station and to
determine whether the demodulator locked up correctly.
• A transmitting station identifier (address) may be added if all
transmitting stations use the same unique word.
• The next block in the burst is for control, marked CNTL in Figure 6.22, and
can take many forms.
• Information in the control block includes instructions for the receiver
such as the modulation and FEC applied to the preamble and traffic
segments, the length of the traffic burst, and warnings of any changes
that will occur in the next frame.

• There may be a forward error correction (FEC) segment at the end of the
preamble that can be used by both the transmitting and receiving
stations to ascertain whether the preamble was received correctly.
• Errors in the preamble can result in the traffic section of the burst being
corrupted, requiring a retransmission of the entire frame.
• For example, in the DBS-S2 standard for satellite television very powerful
forward error correction coding is applied to header information and a
different FEC rate can be selected for traffic bits (ETSI 2009)

• A known bit sequence is required in the received signal for ambiguity
resolution, called a unique word.
• The pattern of ones and zeroes in the unique word allows the receiver to
check for phase ambiguity and to invert the appropriate bit stream (I, Q,
or both) if ambiguity is found.
• The unique word correlator functions in exactly the same way as a
baseband correlator in a direct sequence spread spectrum (DSSS)
receiver.

• The UW and the correlator circuits must therefore be designed to ensure
that the UW is detected correctly in every burst with a very low
probability of a timing error.
• An incorrectly detected UW is known as a miss, and the probability that a
miss occurs can be calculated from the bit error probability (BER) of the
recovered bit stream and the length of the UW.

• A false alarm can occur if a unique word sequence happens to occur
within the traffic data when an earth station is trying to achieve
synchronization of a TDMA burst.
• Once the time position of a UW within the TDMA frame is determined, a
window can be placed over the UW so that the correlator is operated
only during a period slightly longer than the UW duration.
• This will greatly reduce the chances of a false alarm. Use of a long unique
word reduces the likelihood of false alarms

Calculate the voice channel capacity for the above INTELSAT frame given the voice channel bit
rate is 64 kb/s and that QPSK modulation is used. The frame period is 2ms?

Multiple Beam Antennas and Satellite Switched
TDMA
• One advantage that TDMA has when used with a satellite that has a
multiple beam downlink antenna and an onboard processing (OBP)
transponder is the option to employ satellite switched TDMA.
• Instead of using a single antenna beam to maintain continuous
communication with its entire coverage zone, the satellite has a number
of narrow antenna beams that can be used to cover the zone and to
concentrate transmitted power on those regions that have the greatest
volume of traffic.
• A narrow antenna beam has a higher gain than a broad beam, which
increases the satellite EIRP and therefore increases the capacity of the
downlink.

• Uplink signals received by the satellite are demodulated to recover the
bit streams, which are structured as a sequence of packets addressed to
different receiving earth stations.
• The satellite creates TDMA frames of data that contain packets
addressed to specific earth stations within each downlink beam, and
switches its transmit power and bandwidth to the direction of the
receiving earth station as the packets are transmitted.
• Note that control of the TDMA network timing could now be on board
the satellite, rather than at a master earth station.
• The satellite operates in much the same way as a data router in an
internet network.

Demand Assignment Multiple Access (DAMA)
• Demand assignment can be used in any satellite communication link
where traffic from an earth station is intermittent.
• An example is a LEO satellite system providing links to mobile
telephones.
• Telephone voice users typically communicate at random times, with call
duration ranging from less than one minute to several minutes.
• As a percentage of total time, the use of an individual telephone is likely
to be less than 1%. If each user were allocated a fixed channel, the
utilization of the entire system might be as low as 1%, especially at night
when demand for telephone channels is small.

• Demand assignment allows a satellite channel to be allocated to a user
on demand, rather than continuously, which greatly increases the
number of simultaneous users who can be served by the system.
• The two-way telephone channel may be a pair of frequency slots in a
SCPC-FDMADAMA system, a pair of time slots in a TDM or TDMA system,
or any combination or FDMA, TDM, and TDMA.
• Most SCPC-FDMA systems use demand assignment to ensure that the
available bandwidth in a transponder is used as fully as possible.
• VSAT networks also need to employ demand assignment because
individual terminals do not necessarily have sufficient data to transmit
continuously.

• In the early days of satellite communication, the equipment required to
allocate channels on demand, either in frequency or time, was large and
expensive.
• The growth of cellular telephone systems has led to the development of
low cost, highly integrated controllers and frequency synthesizers that
make demand assignment feasible.
• Cellular telephone systems use demand assignment and techniques
similar to those used by satellite systems in the allocation of channels to
users.
• The major difference between a cellular system and a satellite system is
that in a cellular system the controller is at a base station that is close to
the user and is connected by a single hop radio link.

• In a satellite communication system, there is always a two hop link via
the satellite to a controller at the gateway earth station and there are
much longer transmission delays in GEO links.
• In international satellite systems, the controllers are not placed on the
satellites largely because of the difficulties in determining which links are
in use, and who will be charged for the connection.
• As a result, all connections pass through a controlling earth station that
can determine whether to permit the requested connection to be made,
and who should be charged.

• In international satellite communication systems issues such as landing
rights require the owner of the system to ensure that communication can
take place only between users in preauthorized countries and zones.
• The presence of the signals from all destinations at a central earth
station in a particular country also allows security agencies the option of
monitoring any traffic deemed to be contrary to the national interest
(Everett 1992).

• Demand assignment systems require two different types of channel:
• a common signaling channel (CSC) and a communication channel.
• A user wishing to enter the communication network first calls the
controlling earth station using the CSC, and the controller then allocates a
pair of channels to that user. The CSC is usually operated in random access
mode because the demand for use of the CSC is relatively low, messages
are short, and the CSC is therefore lightly loaded, a requirement for any
random access link.
• Packet transmission techniques are used in demand assignment systems
because of the need for addresses to determine the source and
destination of signals. Bent pipe transponders are often used in demand
assignment mode, allowing any configuration of FDMA of MF-TDMA
channels to be adopted.

FDMA-SCPC Operation
• When operated in FDMA-SCPC, the individual inbound RF channels from
the VSATs to the gateway station in Figure 6.26 are 80 kHz wide, to
accommodate a 64 kbps bit stream with QPSK modulation, half rate FEC
and SRRC filters with α = 0.25.
• A guard band of 20 kHz is allowed between each RF channel, so the RF
channel spacing is 100 kHz.
• A bandwidth of 20MHz in the transponder can accommodate 200 of
these channels, but it is unlikely that all are in use at the same time. Two
channels are allocated as CSCs.

• Many VSAT systems are power limited, preventing the full use of the
transponder bandwidth, and the statistics of demand assignment systems
ensure that the likelihood of all the channels being used at one time is
small.
• Considerable backoff is required in a bent pipe transponder with large
numbers of FDMA channels.
• The gateway station receiver has 200 IF receivers with 80 kHz noise
bandwidth and 100 kHz frequency spacing, corresponding to the 200
FDMA VSAT channels.
• When a VSAT station sends a request for a connection, the gateway
station responds by allocating a transmit frequency to the station, and
identifies each transmitting station by its allocated frequency.
• Outbound data is assumed to be delivered using one or more continuous
TDM data streams.

MF-TDMA Operation
• Ten VSAT earth stations are allocated to a TDMA group that occupies
1MHz of the transponder bandwidth.
• The frame duration is 20 ms with a burst rate for each station of 800
ksps.
• Each VSAT burst consists of a packet with a header and traffic symbols,
followed by a cyclic redundancy check (CRC) of four symbols and a guard
time of 80 μs.
• The gateway station has 20 IF receivers with 800 kHz noise bandwidth
and 1MHz frequency spacing, corresponding to the 200 MF-TDMA VSAT
channels.

• When MFTDMA is used, the system has the same capacity as the SCPC-
FDMA system, but the VSAT stations must have transmitters with
approximately 10 times the EIRP of FDMA stations to achieve the same
overall CNR, because the noise bandwidth of the gateway receiver
channels is 10 times larger than for an FDMA receiver.
• This usually means that the VSAT stations require larger antennas,
possibly 2mdiameter, rather than 1mdiameter,- providing a 6 dB increase
in antenna gain. A 4 dB increase in transmit power meets the 10 dB
additional EIRP requirement.

Outbound Link
• The outbound link transmits a continuous bit stream so that receivers
can maintain carrier phase and bit clock synchronization.
• The data is organized into a sequence of packets, addressed to the
receiving stations, and organized into a frame.
• One frame contains one packet for each receiving earth station, similar
to the packet illustrated in Figure 6.22.

• Typical packets are formed with a header that contains the address of the
VSAT and control information, a traffic segment, and a CRC at the end.
• A CRC is similar to a checksum but can detect multiple errors in a packet. If
there is no data to send to a particular VSAT, the packet will have only a
header, or may be omitted from the outbound TDM transmission in a
demand assignment system.
• If the network illustrated in Figure 6.26 is symmetric, the outbound link
must deliver 64 kbps data to each VSAT station.
• With 198 VSAT stations, a 20 ms frame delivers 1280 data bits to each
VSAT to which must be added header bits and a CRC.

• If we allocate 80 bits for a header and 20 bits for the CRC, the gateway
station must transmit 1380 bits per frame to each station giving a
transmission requirement of 13.662Mbps.
• With half rate FEC encoding and QPSK modulation, the symbol rate is
13.662 Msps, and using SRRC filters with α = 0.25 the occupied bandwidth
of the transmission is 17.078MHz.
• This may well be too wide a bandwidth for small VSAT terminals, resulting
in unacceptably low overall CNR.
• The outbound transmission can be divided into a number of FDMA
groups.
• For example, with four FDMA groups, each group transmits at 3.662 Msps,
occupying a bandwidth of 4.578MHz, and links to 50 receiving SAT
stations.

• In VSAT systems, the inbound and outbound channels may be symmetric,
offering the same data rate in opposite direction.
• In a symmetric system the outbound TDM channel must transmit at the
same bit rate as all the VSAT added together.
• Internet access systems are often asymmetric, because requests for
information can be short but the resulting replies may be lengthy.
• The packet length of the TDM signal in the outbound direction may be
fixed, which suits a symmetrical network, or variable, which better suits
an internet channel capable of downloading large files or video from the
internet.

Common Signaling Channel
• The CSCs shown in Figure 6.26 are located at the ends of the transponder
occupied bandwidth.
• When a VSAT earth station wants to access the satellite, it transmits a
control packet to the satellite on the CSC frequency and waits for a reply.
The control packet is received by the gateway earth station and decoded.
• The control packet contains the address of the station requesting the
connection, any other relevant data (such as a character, CP, to indicate
that this is a control packet with no traffic data) and a CRC that is used in
the receiver to check for errors in the packet.

• The control station may record both origination and destination station
addresses and measure the duration of the connection in order to
generate billing data.
• In a true demand assignment system, the control station allocates the
VSAT an uplink frequency or a time slot of specified duration in the
outbound TDM frame.
• If the gateway station has a large volume of data to send

• to a particular VSAT station, it can allocate a longer time slot in the TDM
frame to that station.
• This is important in internet access systems where a large file of video or
other multimedia data may have to be sent. The timeslots usually come
in multiples of a fixed minimum duration so that clock rates and buffer
sizes are compatible.
• If the system becomes busy and many stations are requesting large files,
throughput to any one station will slow down toward the standard
minimum rate, exactly as in a terrestrial internet server.

• A block diagram of a gateway receiver for the signals shown in Figure 6.26
is illustrated in Figure 6.27.
• The receiver amplifies and down converts the received signal to an IF of
700MHz and then to a second IF at 70MHz.
• In the hardware FDMA receiver illustrated in Figure 6.27, individual
FDMA-SCPC channels within the band 60–80MHz are down converted to
a standard IF frequency of 2MHz using local oscillators with frequencies
58–78MHz in steps of 100 kHz.
• There are a total of 200 such 2MHz IF receivers to cover all the frequency
slots.

• A microwave frequency synthesizer is needed to generate the 200 local
oscillator frequencies.
• A better alternatively to building 200 hardware IF receivers is a DSP
receiver where- the second IF signal is split into two channels and
sampled by fast ADCs driven in phase quadrature to create I and Q
channels.
• Sampling the second IF signal at 200MHz is required, and SRRC filtering
with FIR filters centered on the 200 channel frequencies is used to
extract the 200 channels.

• Digital QPSK demodulation of the I and Q channels for each received
frequency is followed by the usual baseband processing to create an
output of 200 digital signals.
• One or more FPGAs or ASICs can be used to replace the 200 IF receivers
of the hardware version.
• A digital receiver for the TDMA version of the VSAT signals in Figure 6.26
has the same form as the FDMA receiver, but for 20 RF channels, each
with an occupied bandwidth of 1MHz.
• The output of the TDMA receiver is 20 TDMA signals with packets from
10 VSAT transmitters. The 10 TDMA signals are then separated by time
division techniques to deliver 200 data channels.

Code Division Multiple Access (CDMA)
• CDMA is a system in which a number of users can occupy all of the
transponder bandwidth all of the time.
• CDMA signals are encoded such that information from an individual
transmitter can be recovered by a receiving station that knows the code
being used, in the presence of all the other CDMA signals in the same
bandwidth.
• This provides a decentralized satellite network, as only the pairs of earth
stations that are communicating need to coordinate their transmissions.

• Each transmitting station is allocated a CDMA code; any receiving station
that wants to receive data from that earth station must use the correct
code.
• CDMA codes are typically 16 bits to many thousands of bits in length, and
the bits of a CDMA code are called chips to distinguish them from the
message bits of a data transmission.
• The data bits of the original message modulate the CDMA chip
sequence, and the chip rate is always much greater than the data rate.
This greatly increases the speed of the digital transmission, widening its
spectrum in proportion to the length of the chip sequence. As a result,
CDMA is also known as spread spectrum.

• Direct sequence spread spectrum (DSSS) is the only type currently used
in civilian satellite communication; frequency hopping spread spectrum
(FH-SS) is used in the Bluetooth system for multiple access in short range
local area wireless networks.
• CDMA was originally developed for military communication systems,
where its purpose was to spread the energy of a data transmission across
a wide bandwidth to make detection of the signal more difficult (called
low probability of intercept).

• CDMA has become popular in cellular telephone systems where it is used
to enhance cell capacity. However, it has not been widely adopted by
satellite communication systems because it usually proves to be less
efficient, in terms of capacity, than FDMA and TDMA.
• The Globalstar LEO satellite system was designed to use CDMA for
multiple access by satellite telephones; one advantage of CDMA in this
application is soft handoff in which the same signal is received from two
satellites during the period that one satellite is about to disappear below
the horizon and another satellite has just appeared above the horizon.

• This technique increases the CNR in the receiver when the satellites are
at their maximum range and the signals are weakest.
• The GPS navigation system uses DSSS CDMA for the transmission of
signals that permit precise location of a receiver in three dimensions.
• Up to 14 GPS satellites may be visible to a receiver close to the earth’s
surface at any one time.
• CDMA is used to share a single RF channel in the receiver between all of
the GPS satellite transmissions.

Spread Spectrum Transmission and Reception
• This discussion of CDMA for satellite communications will be restricted to
direct sequence systems, since that is the only form of spread spectrum
that has been used by commercial satellite systems to date.
• The spreading codes used in DSSS CDMA systems are designed to have
good autocorrelation properties and low cross correlation.
• Various codes have been developed specifically for this purpose, such as
Gold and Kasami codes (Pseudorandom noise 2018; Pseudo-random noise
codes 2013).
• The following discussion is based on the C/A spread spectrum codes used
in civil receivers of GPS position location signals, which are all Gold codes.

• GPS satellites transmit pseudo-random sequence (PRN) codes, also
known as pseudo noise codes.
• All GPS satellites transmit a C/A (course acquisition) code at the same
carrier frequency, 1575.42MHz, called L1, using BPSK modulation.
• The C/A code has a clock rate of 1.023MHz and the C/A code sequence
has 1023 chips, so the PRN sequence lasts exactly 1.0ms.
• A second spread spectrum signal, the P code is also transmitted by GPS
satellites. Its use is restricted to authorized users, primarily military.
• The C/A code is transmitted as BPSK modulation of the L1 frequency RF
signal, and is also modulated by a 50 bps navigation signal.
• The navigation signal contains information essential to the calculation of
the location of a GPS receiver.

• Figure 6.30 shows the way in which the C/A code is generated on board
a GPS satellite. There are two 10 bit shift registers known as G1 and G2
that are clocked at 1.023MHz.
• Each shift register generates a 1023 chip PRN code sequence using
feedback loops not shown in Figure 6.30. The position of the S1 and S2
output taps of the G2 shift register determine which version of the C/A
code is generated.
• The outputs of the G1 and G2 shift registers are added (modulo 2) to
create a 1023 bit long Gold code sequence, which is the C/A code for
one satellite. There are a total of 37 C/A codes available to GPS satellites,
determined by the S1 and S2 settings. Every GPS receiver contains an
identical C/A code generator.

• In a GPS satellite, the C/A code is modulated with 50 bps navigation data.
The C/A code sequence lasts exactly 1.000 ms, so there are 20 repetitions
of the C/A code within each bit of the navigation message.
• When the navigation message bit changes from a 0 to a 1, or a 1 to a 0,
the next 20 C/A sequences are inverted.
• The C/A code is modulated onto the L1 carrier using BPSK and sent to the
satellite L1 transmitter.

• The C/A code for a particular satellite is created with an algorithm that
includes the identification number of the GPS satellite, creating a unique
code with a signal number that is the same as the GPS satellite number
(space vehicle, SV number).
• The algorithm for generating a C/A code for SV number i is
• Ci(t) = G1(t) G2 (t + n
⊕ iTc)
• where ni is a unique value for each C/A code sequence and Tc is the C/A
code chip period.
• The symbol is the exclusive OR function.
⊕

Thank You.
Reference Text Book:
Satellite Communications, Third Edition.
Timothy Pratt and Jeremy Allnutt.© 2020
JohnWiley & Sons Ltd. Published 2020 by
JohnWiley & Sons Ltd.

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Unit 4 Multiple Access.ppt presentation for Satellite communication