satellite communications: medium access techniques

Satellite Access Techniques

Francisco J. Escribano

December 11, 2013

Francisco J. Escribano Satellite Access Techniques December 11, 2013 1 / 45

Table of contents

1 Motivation

2 Types of Access Techniques

3 Fixed AssignmentFDMATDMACDMASDMA and PDMA

4 On Demand Assignment

5 Random Access

6 References


Motivation

Motivation


Motivation

Motivation

Sharing resources

The satellite capabilities are limited.

◮ Bandwidth and power are scarce resources which need to be assignedsomehow.

Several strategies to share resources.

◮ Fixed, dynamic; spectrum, time, and so on.

Main references: [1], [2], [3].


Types of Access Techniques




Classification I

1 Fixed assignment, or, more comprehensively, scheduled assignment.

◮ Examples: FDMA, TDMA, CDMA.Frequency Division Multiple Access

Time Division Multiple Access

Code Division Multiple Access

◮ A fraction of the incumbent resource is assigned to each correspondinguser.

◮ Works well in situations with intense and continuous traffic.

Figure 1 : Fixed assignment space.



Classificacion II

2 On demand assignment.

◮ Example: DAMA, as opposed to PAMA (permanent assignment).Demand Assigned Multiple Access

◮ Resources are allocated only when a communication is to be established.◮ After communication ends, the resources are de-allocated.◮ Variable carrier frequency and frame timing, depending on the available

channels.◮ Well adapted to situations with intense, burst traffic.

Figure 2 : DAMA situation.



Classification III

3 Random access.

◮ Example: ALOHA, slotted ALOHA.

◮ When an ES has data to transfer, it initiates transmission.

◮ Well adapted to situations with a large number of stations generatingshort-duration burst traffic.

Figure 3 : Random access and collisions.


Fixed Assignment

Fixed Assignment


Fixed Assignment FDMA

FDMA

Transponder complete BW is divided into channels.Each channel could have different allocated BW.Each ES has an associated channel.

All ES’s transmit simultaneously.

Figure 4 : Model for FDMA communication.



FDMA

From the point of view of quality and performance analysis,

◮ Each channel can be dealt with separately in most cases.

◮ Each channel can transmit different types of information or signals (dig-ital, analog).

◮ It is important to know which fraction of the overall BW power is allo-cated to each channel.

◮ One important issue at the satellite transponder side is the possibility ofintermodulation between different channels.

It is a simple scheme: no synchronisation is necessary at the satellite.

Figure 5 : FDMA channelization example.



FDMA & FDM

1 SCPC, Single Carrier per Channel

◮ Each ES sends different communications through a different car-rier/channel pair.

Figure 6 : SCPC.

2 FDM/FM, Frequency Division Mul-tiplexing

◮ The signals are first multiplexed,then sent through a unique sig-nal that occupies all the availablebandwidth.

Figure 7 : FDM/FM.



FDMA: Eutelsat scheme

Eutelsat defines a QPSK/FDMA scheme for digital transmission, withFEC capabilities (CC) [4].

◮ Specifications are given for an IF of 70MHz/140MHz.◮ At the RF level, separation between same rate carriers is n × 90KHz

(1/2 rate FEC), or n × 60KHz (3/4 rate FEC), where n × 64Kbps is thecustomer’s data rate.

◮ Each carrier transmission has to comply with severe transmission masksat the IF and RF levels.

Figure 8 : 1/2 rate FEC encoder. Figure 9 : 3/4 rate FEC encoder.



FDMA: Eutelsat scheme

Transmission / reception masks.

Figure 10 : Modulator filteramplitude response.

Figure 11 : PSD mask atthe modulator output.

Figure 12 : Demodulatorfilter amplitude response.


Fixed Assignment TDMA

TDMA

Each ES transmits a data burst during a pre-allocated period of time.Each ES burst occupies the whole carrier bandwidth.◮ Intermodulation problem is mitigated.

Periodic frame-based comm: each ES data burst accesses same timeslot.

Each ES’s data is buffered to access periodically the correct time slot.

The system requires complex synchronisation techniques and protocols.

Figure 13 : TDMA setup.



TDMA

A reference ES transmits a reference burst.

◮ This enables time synchronisation for the other ES’s.

A frame comprises the time between two successive reference bursts.

Frame period, TF .

Input data rate for a ES, Rb.

Burst duration, TB .

Transmission data rate, RT

Required buffer size at a ES, M

M = Rb · TF

RT = Rb ·TF

TB

Figure 14 : GNSS example.



TDMA

There is a essential asymmetry between uplink and downlink.

On receiving the bursts with correct synchronisation, the satellite buildsa frame.

◮ This requires guard intervals between consecutive burst slots, dependingon the estimated delay for the different ES’s.

A burst consists on a header or preamble, and a payload field.

◮ The header or preamble, apart from signalling the beginning of the burstitself, enables the carrier and bit time recovery at the receiver.

◮ The payload field carries the useful data.

In a standardized system (such as INTELSAT), there could be twokinds of bursts:

◮ A traffic ES data burst (e.g. 280 symbol header, and 64-multiple payloadfield).

◮ A reference ES burst (e.g. 288 symbol header and no payload field).



TDMA: INTELSAT frame

Example of the INTELSAT TDMA frame [5].

Figure 15 : INTELSAT frame fields.

TB = Rb ·TF

RT=

= 2048·106·2·10−3

128832·106 =

= 39µs



TDMA: synchronisation

Two sides:

1 Receiver acquisition.

Figure 16 : SORF marker.

2 Transmitter synchronisation.

Figure 17 : TX synchro.

Open loop timing control (10%losses).Closed loop timing control.Feedback timing control.



FDMA vs. TDMA

1 Information transmission capabilities. Bits per user, b.

◮ FDMA with M orghogonal frequency bands.

◮ TDMA with M time slots.

RFDMA = M ·b

TFRTDMA = b

TF /M= M ·

bTF

= RFDMA

2 Average message delay.

D = w + τ

DFDMA = 0 + TF

wTDMA = 1M

·∑M

n=1(n − 1)TF

M= TF

2

(

1 −1M

)

τTDMA = TF

M

DTDMA = DFDMA −TF

2

(

1 −1M

)

◮ TDMA is better than FDMA from the point of view of message delay.

◮ In TDMA, the average delay is lower because user could transmit inother slots.


Fixed Assignment CDMA

CDMA

Technique based on spread spectrum.

◮ Low correlation spread sequences are required.

Each ES is assigned a unique code.

◮ This ensures some degree of privacy.

Figure 18 : CDMA setup.



CDMA

Remember the different ways of using Spread Spectrum techniques:

◮ Direct Sequence (DS).s (t) = c (t) · d (t)

where c (t) is the spreading sequence at a Tc chip period, and d (t) isthe data sequence, at a Ts symbol period.

◮ Frequency Hopping (FH).s (t) = ℜ

[

d (t) · e j2πfi t]

where d (t) is the narrowband baseband data sequence, and fi is a carrierfrequency which changes value at each i instant according to a predefinedspreading sequence.

◮ Time Hopping (TH).s (t) = d (t) ∗ p (t − ni · Ts/N)

where d (t) is the data sequence in the form of a Dirac delta train ofsymbol period Ts , p (t) is a unit amplitude pulse shape of duration Tc ,and ni is an integer between 0 and N −1, N = Ts/Tc , chosen accordingto a predefined spreading sequence of index i .



CDMA

Be it DS, FH or TH based CDMA, it is important to characterize:

◮ Spreading gain given by the SS scheme.

◮ How resources (bandwidth, data rate, SNR...) are shared as a functionof Tc , Ts , Tb, according to the specific SS scheme.

◮ The effect of the interference of concurrent users, and the evaluation ofthe corresponding degradation of the Eb/N0 per user.

For example, in a simple DS-CDMA scheme where M users interferewith the same received signal power S each, we would have an Eb/N0:

Eb

N0= B/Rb

(M−1)+η/S

where η is the amount of noise power, B is the spreading bandwidthand Rb is the data bit rate.

Usage of CDMA may require sofisticated power control techniques.



CDMA: Globalstar example [6]

GLOBALSTAR is a satellite-based cellular telephone system that allowsusers to talk from anyplace between 70o N and S latitudes.

Avoids outages caused by blockage of signals by using diversity signalsfrom two satellites.

Consists of a Walker 48 − 8 − 1 constellation:

◮ 48 low-orbiting (1400Km altitude) satellites in 8 orbits, inclined 52o withrespect to the Equator. 6 satellites in each orbital plane.

Users transmit and receive on the 1.6GHz band and 2.5GHz band.

Satellites communicate with the gateway ground antennas on the 5GHzand 7GHz bands.

CDMA provides for extensive frequency reuse through the use of or-thogonal codes in 1.23MHz channels.

Capacity to serve up to 30 million subscribers (not simultaneously).



CDMA: Globalstar example

Figure 19 : Globalstar system overview.




Figure 20 : Nominal link parameters.




Figure 21 : 16 beam user TX antenna.

Figure 22 : Antenna amplifier performance.




Figure 23 : Example of forward traffic channel structure.




Figure 24 : Forward link spreading and modulation.


Fixed Assignment SDMA and PDMA

SDMA, PDMA

Spatial Division Multiple Access (SDMA).◮ Satellite reuses frequencies, but serving non-overlapping Earth areas.

◮ Also called Multiple Beam Fequency Reuse.

Polarization Division Multiple Access (PDMA).◮ Communications overlap in frequency and space, but are separable: orthogonally polarized beams.

◮ PDMA is highly sensitive to cross-polarization issues in atmospheric propagation.

Figure 25 : Examples of SDMA and PDMA.


On Demand Assignment




DAMA

DAMA, Demand Assignment Multiple Access, as opposed to PAMA,Permanent Assignment Multiple Access.

◮ As its name implies, resources are provided following on-demand princi-ples.

◮ As a contrast to fixed/scheduled assignment techniques, performanceand results are highly dependent on traffic profiles.

◮ Main difference with respect to xDMA fixed methods:

Pure xDMA systems divide a single physical channel into a numberof virtual channels, and rely on near-real-time protocols to adjust de-mand/throughput.

DAMA philosophy is adequate when resources are sparsely utilized andthere is no need for heavily planified protocols to share them.

DAMA can be used along with any xDMA scheme, depending on thesparsity and traffic usage.



DAMA

DAMA can be applied according to a number of flavours:

1 Variable Capacity DA.

2 Per-Call Variable Capacity DA.

3 Per-Call DA.

4 Fully Variable DA.

Traffic and demand analysis are required to ground the correct flavour

choice and the appropriate system dimensioning.

DAMA is used in military sat networks due to the simplicity and ro-bustness of implementation.

A DAMA-based system is usually affected by collisions.



DAMA

Variable Capacity DA.

◮ Employed in TDMA sat networks.

◮ Traffic profile: slow variations and a few destinations covering a largearea.

◮ System enjoys a medium DAMA gain.

◮ Useful when the traffic peak bursts are known or well characterized.

◮ Reference TS manages the whole burst planning.



DAMA

Per-Call Variable Capacity DA.

◮ Employed in TDMA sat networkds.

◮ Each station serves several destinations.

◮ Low traffic intensity, but fast varying.

◮ Maximum DAMA gain.

◮ Reference TS changes the burst length and position.



DAMA

Per-Call DA.

◮ Employed in TDMA and FDMA sat networks.

◮ Each station serves many destinations.

◮ Low traffic intensity.

◮ Medium DAMA gain.



DAMA

Fully Variable DA.

◮ Employed in TDMA and FDMA sat networks.

◮ Each station serves large number of destinations.

◮ Fast changing traffic intensity.

◮ The total traffic intensity at a station is low.



SPADE example

SPADE, SCPC PCM Multiple Access DA Equipment.

◮ As the name implies, it is the integration into one network architectureof known principles.

Figure 26 : SPADE channeling scheme.

Figure 27 : SPADE net diagram.



SPADE example

Figure 28 : Possible SPADE QPSK SCPC configuration.


Random Access

Random Access


Random Access

ALOHA

Purely random access makes sense for packet-oriented networks.

ALOHA is a protocol to share a common channel with available rateRT among M possible users:

◮ As soon as a terminal has data to send, it starts transmission.

◮ If a collision is detected at the satellite, the packet is retransmitted(NACK).

◮ System useful when the access to the channel is sporadic and packetsare short.

Figure 29 : ALOHA channel throughput.


Random Access

ALOHA

Figure 30 : Delay versusthroughput comparison.

Figure 31 : Illustration of the ALOHA protocol.

In an ALOHA system, there is a great deal idle time inthe channel.

ALOHA naturally accomodates traffic profiles of low in-tensity, short bursts.

The main advantage is flexibility and simplicity of imple-mentation.


Random Access

S-ALOHA

Slotted-ALOHA.

◮ Similar to pure ALOHA, but transmissions can only happend at givenpredefined time slots.

◮ Throughput is higher.

◮ Delay is reduced.

Figure 32 : Throughput versus load comparison.


References

References


References

Bibliography I

G. Maral, Satellite Communications Systems: Systems, Techniques and Technology. Chichester: John Wiley & Sons,

Inc., 1998.

T. Pratt, Satellite Communications. New York: John Wiley & Sons, Inc., 2003.

G. E. Corazza, Digital Satellite Communications. New York: Springer, 2007.

Eutelsat Company Web Site, “SMS QPSK/FDMA System Specification,” EESS 501 G Issue 3. [Online]. Available:

http://www.eutelsat.com/files/contributed/satellites/pdf/eess501.pdf

A. S. Oei, R. J. Colby, R. Parthasarathy, and A. L. Stimson, “Alignment, testing and maintenance principles in

the intelsat tdma/dsi network,” International Journal of Satellite Communications, vol. 3, no. 1-2, pp. 161–166, 1985.[Online]. Available: http://dx.doi.org/10.1002/sat.4600030118

F. Dietrich, P. Metzen, and P. Monte, “The Globalstar cellular satellite system,” Antennas and Propagation, IEEE Trans-

actions on, vol. 46, no. 6, pp. 935–942, 1998.


http://www.eutelsat.com/files/contributed/satellites/pdf/eess501.pdf

http://dx.doi.org/10.1002/sat.4600030118

satellite communications: medium access techniques

Education