2. Need of Multiple Access
• Satellites are usually built and employed for the
common good of one or more regions.
• This means a lot of people needs to send and
receive data through a satellite without major
mishaps
• For this purpose, we may use one of the several
multiple access techniques
• Mentioned here are FDMA, TDMA, CDMA and
DAMA.
3. Frequency Division Multiple
Access (FDMA)
• Frequency division, as the name suggests,
operates by dividing the available bandwidth
between users on the basis of frequency.
• A group of users maybe allotted a range of
frequencies and each individual user can use a
limited frequency shifted channels, within that
range.
• Earlier analogue FDMA used FM waves with
slightly different carrier frequencies to get the job
done.
5. How it works
• FDM-FM was transmitted to satellite from ESs.
• These were received by a common transponder.
• Microwave filters were used in ESs to separate
signals within the transponder.
• The corresponding ESs received data back from
the transponder and the data get demuxed.
• If the frequency assignment has to be changed, all
the ESs’ filters must be retuned.
• This can cause long idle times too.
7. Improving on it
• Single Channel Per Carrier (SCPC) technique
means a number of small mobile earth stations
can transmit and receive data over a single
transponder.
• This can enable satellite telephony
• This can also help utilise the whole wide
bandwidth of the transponders
8. What can go wrong
• High power amplifiers used in transponders can
have non-linear operation near saturation.
• Travelling Wave Tube Amplifier can have more
susceptibility to this than a Solid State High Power
Amplifier.
• This can cause reduced
𝐶
𝑁 0
ratio.
• Equalisation at the Earth station can mask this
effect to a certain extent.
10. Time Division Multiple Access
(TDMA)
• In TDMA, ideally, the whole bandwidth is used by
a user for a fixed amount of time.
• All practical TDMA signals are digital and hence
are advantageous over traditional FDMA system.
• Problems of non-linearity are not present here as
at a time the whole bandwidth of a transponder is
used only by a single signal.
• However, need for higher bitrates (requires more
energy) and presence of ISI can be a problem.
12. How it works
• Multiple ESs transmits data in short bursts so they
reach the transponder in a sequence
• The transponder simply outputs all the incoming
data into a continuous stream of serial data with
short guard times.
• All ESs must be synced for their signals to arrive
within the positions allotted to them in a frame.
• The received serial data can be recovered using
sync attained between transmitter.
13. Bits and pieces…
• Digital data is often broken down into bits.
• Symbols are representations of data in a bit.
• Bitrate is the number of its transferred per second.
• Baud rate is the number of symbols transferred.
• For BPSK, the former and later are same.
• Baud rate determines the bandwidth of a signal.
• QAM can be used for transmission, if a higher
than usual
𝐶
𝑁 0
is available.
14. Being framed
• A TDMA frame contains all data from all ESs in a
TDMA network.
STN 1 STN 2 STN 3 STN 4 STN 1
Frame Period T, µs
Preamble
Guard Time Traffic: N bits
Next Frame
15. Bursting with data
• Data in a TDMA network is sent as bursts.
• Proper synchronisation of bursts is required for it
to be framed properly.
• A master station transmits a reference burst on
which the rest of the earth stations start
transmitting.
• Each frame has one, and it’s a preamble, of the
incoming data traffic.
• Reducing this overhead can improve efficiency.
16. Guardians…
• Guard times small empty slots between traffic
• They are important to avoid crashing of multiple
data bits.
• If a collision occurs, multiple data is lost.
• Longer guard times can affect efficiency, but
improve reliability and safety.
• Typical guard times range from 1-5 µs.
17. Code Division Multiple Access
(CDMA)
• In CDMA, all users can use the whole bandwidth,
all the time.
• The data need not be synced or be filtered.
• It can allow users with more needs utilise the
bandwidth more, while conserving it when idled.
• Transmission times or frequencies are irrelevant
here, and there is hardly any spectrum allocation.
• Currently used method is Direct Sequence Spread
Spectrum.
19. How it works
• Any earth station can code their data based on a
CDMA code, obeying a set of rules defined by an
organisation.
• This code can be 16 bits to several thousand bits
long.
• Since multiple stations can saturate the bandwidth
without much effort, this system is highly efficient.
• The receiver needs the same code used by the
transmitter.
21. Spreading it
• DS-SS essentially turns data into pseudo noise.
• This makes the resultant data useless for anyone
without the proper decoding key.
• This can improve security and increase efficieny at
the same time.
22. Demand Assigned Multiple
Access (DAMA)
• This technique is used when the link between the
station and satellite is not a constant one.
• This helps assign bandwidth according to
demand.
• This can be implemented on TDMA, or as a
combination with FDMA and TDMA.
• Commonly used in VSAT (Very Small Aperture
Terminal) systems.
24. How it works
• There are two different types of channels in DAMA
• A Common Signalling Channel (CSC)
• And a Communication Channel (CC)
• A user wishing to enter the CC first calls the
controlling ES using CSC.
• Bent pipe transponders then accept the data and
retransmits.
• Receivers should listen in sync continuously to
retrieve the data.
25. Where it is used
• DAMA is often used in military environments due
to the relative simplicity of implementation and
ease of modelling
• It can operate with bent pipe transponders and
thus require no security on the satellite side.
• The master and slave stations can upgrade
compressions without expensive satellite
replacements.
26. Ends
• Multiple access schemes are unavoidable
considering the public nature of common
communication satellite.
• There are several more proprietary schemes in
practice, which are even more improved than the
ones described here.
• For general purposes though, these will suffice.
