satellite multiple access schemes
DESCRIPTION
TRANSCRIPT
Multiple AccessSatellite Communications
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.
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.
FDMA3-dimensional plot showing FDMA technique.
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.
FDMA Frequency sharing in FDMA
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
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 ratio.• Equalisation at the Earth station can
mask this effect to a certain extent.
Intermodulation
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.
TDMA3-dimensional plot showing TDMA technique.
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.
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 is available.
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
PreambleGuard Time Traffic: N bits
Next Frame
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.
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.
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.
CDMA3-dimensional plot showing CDMA technique.
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.
CDMA Code assignment in CDMA
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.
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.
DAMA Depiction of a DAMA System
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.
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.
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.