Satellite Multiple Access Schemes

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.