FDMA, TDMA, CDMA, and DAMA are multiple access techniques that allow multiple users to share access to a satellite for communication. FDMA divides the available bandwidth into different frequency channels. TDMA divides the bandwidth into different time slots. CDMA spreads each user's signal over the entire bandwidth using unique codes. DAMA dynamically assigns bandwidth according to demand rather than using pre-assigned blocks of time or frequency. These techniques allow efficient sharing of satellite bandwidth among multiple users.

1. Multiple Access
Satellite Communications

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.

4. FDMA
3-dimensional plot showing
FDMA technique.

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.

6. FDMA Frequency sharing in FDMA

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.

9. Intermodulation

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.

11. TDMA
3-dimensional plot showing
TDMA technique.

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.

18. CDMA
3-dimensional plot showing
CDMA technique.

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.

20. CDMA Code assignment in CDMA

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.

23. DAMA Depiction of a DAMA System

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.