Carrier Access Schemes in Satellite Communications

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Slide Number 1Rev -, July 2001 Section 3.3


Slide Number 2Rev -, July 2001

PART 3.3.1


Slide Number 3Rev -, July 2001 Vol 3: Satellite Communication Principles

Sec 3: Carrier Access Schemes

3.3.1: Satellite Transponders

Satellite Transponder3.3.1.1 Introduction

3.3.1.2 Power Characteristics of a TWTA

3.3.1.3 Transfer Characteristics

3.3.1.4 Conclusion and Example



3.3.1.1: Introduction

IntroductionA satellite transponder could contain a Travelling Wave Tube Amplifier (TWTA) or Solid State Power Amplifier (SSPA). The following bands are commonly used or shared for satellite and land based communications:

• L BAND 1-2 GHZ MOBILE SERVICES

• S BAND 2.5-4 GHZ MOBILE SERVICES

• C BAND 3.7-8 GHZ FIXED SERVICES

• X BAND 7.25-12 GHZ MILITARY

• Ku BAND 12-18 GHZ FIXED SERVICES

• Ka BAND 18-30.4 GHZ FIXED SERVICES

Part 1: Satellite Transponders

Vol 3: Satellite Communication Principles, Sec 3: Carrier Access Schemes



TWTAs continue to be widely used to provide signal power amplification at high frequencies, especially for DBS applications, but have troublesome side effects associated with their performance. SSPAs exhibit similar, but less pronounced, performance characteristics.

The three key manufactures of space qualified TWTAs are:• Hughes• AEG/Thomson• NEC

Introduction

3.3.1.1: Introduction





The most common methods in which TWTAs are specified are:• Peak Power

• P(in) versus P(out) (AM/AM Transfer curve)

• Input Backup-off (IBO) versus Output back-off (OBO)

• Gain

Power Characteristics of a TWTA

Figure 3.3.1.2a Typical AM.AM Transfer Curve

3.3.1.2: Power Characteristics of a TWTA





TWTA non-linearity (AM/AM and AM/PM) is commonly specified by the following Figures of Merit:

• C/3ed order intermod (C/3rdIM)

• NPR (noise power ratio)

• 1 dB compression point

Power Characteristics of a TWTA

3.3.1.2: Power Characteristics of a TWTA


AM/PM TWTA Non-linearity

-40

-30

-20

-10

0

10

Input Drive Level (dB)

Out

put P

hase

Shi

ft (D

egre

es)

Figure 3.3.1.2b AM/PM TWTA Non-Linearity




Transfer CharacteristicsPeak Power is the point whereby the TWT saturates and can no longer provide gain.

TWTAs are always saturated for single carrier applications; however, they need to be backed off for multiple carrier applications in order to avoid the generation of third-order harmonics.

TWTA peak power is seen as the highest point on the curve, and is frequently denoted as power output in dBm.

For example, a TWTA rated to produce an output of 100 Watts will be specified having a 50 dBm output rating (10log[100] + 30 = 50 dBm).

3.3.1.3: TWTA Transfer Characteristics





Transfer CharacteristicsAM/AM transfer curve simply expresses P(in) versus P(out). This curve becomes non-linear as the output power approaches the 1 dB compression point. The 1 dB compression point is the point at which the AM/AM transfer curve departs 1 dB from ideal linearity.

The non-linearity become more pronounced between the 1 dB compression point and the saturation point.

As the input power into the TWTA is increased beyond a certain region of linearity, the output power begins to decline; therefore, any further increase in input power produces no further increase in output power.






Transfer CharacteristicsIBO versus OBO uses the same AM/AM transfer curve except the X and Y axis is referenced to saturation.

The IBO X axis will denote 0 dB as the input value used to saturate the TWTA output. The OBO Y axis will denote 0 dB as the value at which the TWTA output is saturated.

The table provides a typical IBO and OBO relationship:

Gain of a TWTA refers to the small signal gain with reference to the linear part of the AM/AM curve.






C/3rdIM is the third order intermodulation product. Because of the non-linear properties of the TWTA, carriers accessing the TWT will produce undesired intermodulation products. This effect is the result of Amplitude Modulation/Phase Modulation (AM/PM) characteristics.

C/3rdIM is determined by injecting two tones (A & B) of equal power into the TWTA, and then measuring the output power of the fundamental carrier and the 3edIM carrier (2A–B & 2B-A).

The ratio of the fundamental carrier level to the 3edIM carrier level provides the C/3rdIM value.

Transfer Characteristics

3.3.1.3: Transfer Characteristics





C/3rdIM is specified as a function of the IBO per carrier. Therefore, saturation level implies that the two fundamental carriers, A and B, are backed off 3 dB, giving a total backoff of 0 dB. Typically, the C/3rdIM value will drop by 3 dB for every 1 dB drop in IBO once the C/3rdIM value reaches approximately 20 dB. The table provides a typical IBO to C/3rdIM relationship:







NPR, the Noise Power Ratio, is a figure of merit used when many carriers are amplified by the TWTA. SCPC DAMA networks fall into this category.

NPR is a test of non-linearity normally used by Telephony engineers, but is applicable to all in understanding the ramifications of the non-linear characteristics of the TWTA.

NPR is measured by injecting a wide-band noise source at the input of the amplifier, transmitting through a fully equalized test link, and measuring the received noise in a representative test BW. Then, a notch filter exactly matching the test BW is applied to the amplifier input and the representative BW is again measured.


Notch in the LevelPower Signal Noisein LevelPower

NPR






Any noise, in the form of 3ed order harmonics, that spills inside the notch filter is then quantified. This problem occurs as a result of TWTA (AM/PM) non-linearity characteristics.

For FDM applications the NPR is expected to be approximately 40 dB, therefore, it is imperative the IBO value used to drive the TWTA operates within the linear region.

1 dB compression point is the TWTA operating level at which the output power that falls 1 dB below the expected gain. For TWTAs is usually 6 dB below the saturation power.

The Mutliple carrier 1 dB compression point is slightly lower than the single carrier 1 dB compression point because 3rd order components steal energy from the fundamental carriers.







ConclusionA typical IBO operating point for multi-carrier operation is -10 dB, resulting in a -4.5 dB OBO, with a carrier to intermodulation noise ratio of 21 dB.

3.3.1.4: Conclusion



Figure 3.3.1.4 Spectrum Analyzer

Figure 3.3.1.4 Multi-Carrier Data from a Spectrum Analyzer




PART 3.3.2


Slide Number 17Rev -, July 2001 Vol 3: Satellite Communication Principles, Sec 3: Carrier Access Schemes

Part 2: Frequency Division Multiple Access (FDMA)

3.3.2.1: Description

DescriptionThe FDMA access technique is used when more than one carrier is transmitted to a single satellite transponder, usually from many access points.

As the name suggests, access to the transponder’s resources is granted by assigning each contending carrier a different frequency than that of others.

This access technique has proven to be a very economical model allowing hundreds of low cost Very Small Aperture Terminals (VSAT) to access a designated FDMA transponder for the provision of telephony and data applications.

Connectivity assignments can be as simple as a Permanently Assigned Multiple Access (PAMA) point-to-point network, or as sophisticated as a Demand Assigned Multiple Access (DAMA) Star or Mesh network.



DescriptionCorporate customers requiring satellite connectivity from point A to point B normally use a point-to-point network.

The uplink frequency is normally fixed after being assigned, and the bandwidth of the carrier can range from 30 kHz to 128 kHz for VSAT terminals, or several MHz for larger Earth Stations.

The satellite link can be configured as a simplex link, or as a symmetrical or asymmetrical duplex link.

A Star or Mesh network is typically used by telephone companies requiring remote-to-remote or remote-to-gateway connectivity to service their community of interest.






DescriptionA Star or Mesh network is usually managed and controlled by a Network Control Facility (NCF).

The NCF will dynamically assign ground segment resources and FDMA space segment resources based on the availability of ground segment voice/data ports and pre-assigned FDMA carrier slots associated with a pre-configured DAMA bandwidth pool.

An FDMA network can also consist of a combination of Single Carrier per Channel (SCPC) or Multiple Channel per Carrier (MCPC) carriers.






Traffic Bit Rate(kbps)

FECRate

ModulationFormat

SymbolRate

(ksps)

B/WOccupied

NominalEb/No

CarrierSpacing

(kHz)

MinimumBER

RequiredVoiceG.729

8.0 R3/4 BPSK 10.67 13.33 6.1 17.5 1.00E-03

VoiceG/729

8.0 R3/4 QPSK 5.33 6.67 6.7 10.0 1.00E-03

VoiceG.728

16.0 R3/4 BPSK 21.33 26.67 6.1 30.0 1.00E-03

VoiceG.728

16.0 R3/4 QPSK 10.67 13.33 6.7 17.5 1.00E-03

VoiceG.726

32.0 R3/4 QPSK 21.33 26.67 6.8 30.0 1.00E-04

VoiceG.711

64.0 R3/4 QPSK 44.0 59.40 6.8 62.5 1.00E-04

Data 4.8 R1/2 BPSK 9.60 12.0 7.5 15.0 1.00E-06Data 9.6 R1/2 BPSK 19.20 24.0 7.5 27.5 1.00E-06Data 16.0 R1/2 QPSK 16.0 20.0 7.5 25.0 1.00E-06Data 19.2 R1/2 QPSK 19.20 24.00 7.5 27.5 1.00E-06Data 32.0 R3/4 QPSK 21.33 26.67 8.3 30.0 1.00E-06Data 56.0 R3/4 QPSK 37.33 50.40 8.3 55.0 1.00E-06Data 64.0 R3/4 QPSK 42.67 57.60 8.3 62.5 1.00E-06

Highlights of Commonly Used Telephony and Data SCPC Carriers.






DescriptionThe advantages and disadvantages of an FDMA access scheme are as follows:

Advantages Allows a variety of different traffic types to share the

transponder efficiently. Supports a wide range of power and bandwidth

requirements for individual carriers. Low space segment cost for light route traffic models. Low cost Earth Station terminals can be used. Ground resources and space resources can be easily

scaled for network growth.






DescriptionDisadvantage

TWTA and SSPA amplifiers exhibit AM/AM and AM/PM conversion when operated near saturation; therefore, the transponder output power must be backed off due to the production of multi-carrier inter-modulation noise.

Strong carriers can suppress weak carriers when they share a transponder, if saturation is approached.







3.3.2.2: Interference and Guard Bands

Interference and Guard BandsWhenever multiple carriers are uplinked to a satellite transponder, special design considerations must be followed to account for the inter-modulation products that are generated.

The design must trade off a reduction in the level of intermodulation noise against a reduction in transponder power available.

An optimized SCPC FDMA network must therefore consider and weigh the cost of the satellite space segment and the cost of the Earth Station terminals.

Individual SCPC carriers in an FDMA system must be sufficiently separate from each other to allow filtering of the carriers at the downlink Earth Station in order to prevent inter-carrier crosstalk.



Interference and Guard BandsCarrier separation resolution is commonly determined by the modem’s synthesizer step size. Today’s SCPC DAMA Networks will provide 2.5 kHz step-size resolution, or lower, depending on the modem manufacturer.

To determine the occupied bandwidth of an SCPC carrier the components to consider are:

Data Rate Symbol Rate Filter Quality






Interference and Guard BandsGuard band spacing, which is specified to prevent SCPC carriers from spilling over to the adjacent SCPC carrier, is determined by the following criteria:

Maximum center frequency drift before the SCPC carrier becomes difficult to demodulate (a rule of thumb frequency drift permitted by most demodulators is about 10% of the symbol rate)

Synthesizer step-size resolution

Center frequency drift is commonly corrected by and AFC process. However, if the AFC range is exceeded, the SCPC carrier will begin to drift off center.






Interference and Guard BandsThe quality of the Earth Station terminal reference clock then solely determines the amount of frequency drift, which is associated with temperature change and equipment aging.

Assuming a perfectly tuned FDMA network where each SCPC carrier is optimized to be within 1% of the specified center frequency, and the OPBO is automatically controlled by an uplink power control algorithm, there may still be an adjacent channel interference component.

The impact of this component is dependent upon the quality of the demodulator. If this component becomes too high it will manifest itself as impulse noise in the baseband.






Interference and Guard BandsSpectral efficiency of an FDMA network can be further improved by employing an overlay scheme called Paired Carrier Multiple Access (PCMA). PCMA will only work if the following satellite and network parameters are met:

• The same terminal can also receive the signal transmitted by each terminal

• The satellite uses a bent-pipe transponder

When PCMA is employed as an overlay to FDMA, both sides of a point-to-point link will be assigned the same frequency to uplink their traffic content. Consequently, the PCMA bandwidth needed is only half the FDMA bandwidth.






Interference and Guard BandsPMCA will enable each terminal to suppress the unwanted signal by subtracting the originated signal from the received signal at RF. The desired signal is then demodulated for processing.

Subtraction of the unwanted signal can be achieved because of the distinguishable data payload characteristics between the originating and received signal.

In order to subtract the unwanted signal from the composite downlink the terminal must accurately estimate each of the following unwanted signal parameters from the composite signal:

Downlink Gain Carrier • Phase Downlink Frequency Offset • Symbol Timing Propagation Delay






Interference and Guard BandsSince not all of the originated signal can be suppressed, the remaining unwanted signal appears as another C/I component. This will account for a service Eb/No degradation of approximately 0.5 dB.

The technology underlying PCMA was developed by ViaSat, and has been granted a US patent.







3.3.2.3: Intermodulation

IntermodulationThe operation of the transponder with multiple carriers generates multicarrier intermodulation noise with a level depending on the transponders operating point. Consequently, the available EIRP is lower when operating with multiple carriers.

The two most common effects caused by TWTA non-linearity are as follows:

Signal suppression Intermodulation

Signal suppression occurs when a large FDMA carrier brings down the level of the smaller FDMA carrier.



IntermodulationIntermodulation occurs when third order products are created due to the AM/AM and AM/PM transfer characteristics of the satellite TWTA. Care must also be taken to limit the amount of intermodulation originating from the Earth Station TWTA or SSPA.

Satellite communication engineers will find that a software tool designed specifically to calculate the frequency and level of intermodulation products is an invaluable asset in designing FDMA networks.

Telesat’s IMSHI, for instance, computes the carrier-to-third-order inter-modulation noise ratio for carriers that share a non-linear amplifier. The program can handle up to one hundred (100) carriers of user-defined frequencies and power levels. A data file containing the AM/AM and AM/PM transfer curves specifies the TWTA characteristics.






IntermodulationInterference BudgetA link budget will contain the following uplink and downlink C/I values that contribute to the overall C/No value:Uplink

Thermal C/No

Adj Sat C/Io

Cross-Pol C/Io

VSAT Terminal Intermod (multi-Channel VSAT) C/Io

DownlinkThermal C/No

Intermod C/Io

Adj Sat C/Io

Cross-Pol C/Io

PCMA (if used) C/Io







3.3.2.4: Other Effects of Transponder Non-Linearity

3.3.2.4.1: Group DelayGroup delay response is a difference in transit time versus frequency for signals going through HPAs, LNAs, waveguide, coaxial cables, RF channel filters and RFconverters.

There are three Group Delay parameters that will effect larger FDMA carriers:

Group Delay Parameter Measurement

Linear ns/MHz

Parabolic (ns/MHz)2

Ripple ns peak-to-peak



3.3.2.4.1: Group DelayA microwave link analyzer is normally used to measure the linear and non-linear Group Delay parameters over the entire transponder bandwidth. Equalization filters are normally used to cancel out the linear and non-linear parabolic delay.

Linear delay occurs when RF propagates through waveguide and coaxial medium. Parabolic and Ripple delay occurs when the RF signal passes through an RF bandpass filter.

Parabolic and Ripple delay are characteristics dependent upon the amount of concatenated tuning cavities used to tune a bandpass filter.






3.3.2.4.2: Intermodulation ProductsAs previously discussed, whenever a signal consisting of two or more parallel carriers, or 'tones', has to pass through a non-linear stage, intermodulation products are generated.

Special attention has to be paid to the third order products because they are located relatively close to the original signal components. If a carrier is located close to the edge of a transponder it is possible that its intermodulation products might spill over into adjacent transponders, especially if filtering is inadequate.

AM/PM ModulationIn an FDMA network where multiple SCPC carriers are amplified by a non-linear TWTA, 3rd order components are generated. These components may fall on top of or next to other SCPC carriers resulting in an increased C/I.








Figure 3.3.2.4 Effects of Transponder Non-Linearity

Other Effects of Transponder Non-Linearity



In order to reduce the level of the 3ed order components the TWTA must be driven within its

linear region. In Telesat’s FDMA

network the TWTA

multicarrier IPBO is 11 dB.

The multicarrier OPBO is 7 dB.




Figure 3.3.2.4 Effects of Transponder Non-Linearity



Other Effects of Transponder Non-LinearityThird Order Intercept The Intercept Point is the point where the linear extension of the intermodulation distortion intersects the linear extension of the input vs output line. Therefore, a TWTA with a high intercept point is more linear than one with a lower intercept point.

FDMA systems must operate in the linear region of the amplifiers transfer gain characteristics in order to reduce the level of third order components.

TWTA amplifier will typically operate with an OPBO of 7 dB with reference to saturation in order to achieve intermodulation levels of better than –25 dBc.

An SSPA will operate with an OPBO of 3 dB with reference to saturation in order to achieve the same intermodulation level of better than –25 dBc.








Figure 3.3.2.4 Third Order Intercept

Other Effects of Transponder Non-Linearity




3.3.2.5: Throughput

ThroughputTransponder throughput efficiencies will drop off dramatically when comparing a single or dual carrier multiplexed network to a multicarrier FDMA SCPC network.

For example, a duplex data network transmitting two E3 streams at 36.67 Mbps can support nearly 1000 duplex, toll quality channels. This scenario applies to a point-to-point network employing large antennas.

An FDMA SCPC network transmitting 32 kbps ADPCM carriers using much smaller 1.8M antennas can support no more than 400 duplex, toll quality channels. This assumes the transponder bandwidth and power requirements are balanced.



ThroughputTransponder bandwidth and power is balanced only if the FDMA SCPC Earth Stations are installed within the hot footprint region. In practice, however, many Earth Stations will be installed outside the hot footprint region and will therefore require larger OPBO values and rain fade margins in order to meet the specified link budget availability.



3.3.2.5: Throughput




PART 3.3.3




DescriptionTime Division Multiple Access (TDMA) is an efficient access scheme that allows multiple users to access a single channel.

Unlike FDMA, which assigns a single user to a single channel, TDMA allocates time slots for multiple users on the same channel.


Part 3: Time Division Multiple Access


Figure 3.3.3.1a FDMA Access Scheme Figure 3.3.3.1b TDMA Access Scheme



DescriptionA TDMA network topology can support star or mesh configurations.

A star configuration normally consists of an expensive Bus Master located at the HUB, and numerous inexpensive remote terminals. This type of network supports integrated data, voice and video communications between the entire remote terminal population and the HUB.

A single TDM outroute signal is normally used to transmit data to the remote terminals. The remote terminals transmit their data to the HUB on an assigned time allocated TDMA channel. One to 32 TDMA inroute channels can be used, depending on the throughput requirements.








Description

Figure 3.3.3.1c TDMA FTDMA/IPA Network





Description

Figure 3.3.3.1d TDMA SCPC Meshed VSAT Configuration



DescriptionA mesh configuration TDMA network is very similar to a mesh SCPC DAMA network. Therefore, outbound and inbound control channels are used to set up single hop peer-to-peer sessions. This type of network is used to support network applications, such as telephony.

Mesh TDMA networks have an SSPA sizing advantage because a single TDMA carrier is used to support four or more time slots and consequently four or more voice or data interfaces. Since a single TDMA RF carrier is transmitted by the SSPA, it can be driven close to saturation. This is an important feature for installations that rely on solar panels to generate electricity.






DescriptionThere are a variety of enhanced TDMA access schemes available that improve the throughput efficiency.

MFTDMA, for instance, assigns different channels based upon the inroute access schemes configured for specific applications.

Other proprietary TDMA access schemes, such as Enhanced TDMA (ETDMA), dynamically assigns time slots upon detection of traffic payload and therefore further increases the throughput efficiency of a TDMA network.

These enhanced TDMA access schemes offer more efficient usage of space segment and therefore provide better response time, high throughput, and higher reliability due to a greater immunity to inter-carrier interference.






Bursts and FramesTDMA BurstsA remote terminal belonging to a star topology can be configured to the following types of inroute access burst and streaming schemes:

Slotted Aloha Access Transaction Reservation Access Stream Access Inroute Switching Flexroute

The Slotted Aloha access method is useful for data with uniform, short message lengths and short average response requirements. A set of TDMA time slots is therefore assigned to many remotes for spontaneous access. Spontaneous access will cause collisions, therefore necessitating retransmission.



3.3.3.2: Bursts and Frames



Bursts and FramesTransaction Reservation is an explicit time slot reservation permitting data to be transmitted without danger of collision.

In this method, bandwidth is reserved via a subset of Aloha slots, and data is transmitted at a time when no other remote user is permitted access to the inroute.

This method of inroute allocation is best suited for data that has long, or widely varying, message lengths or traffic rates.

Stream Access allows a dedicated stream of demand-assigned time slots to be allocated without contention from other remotes. This is suited for applications having a heavy or continuous stream of inbound data.

Inroute Switching extends the TDMA throughput by allowing specific remote clusters to switch to alternate inroute TDMA carriers if bandwidth is not available on the native TDMA carrier.






Bursts and FramesFlexroute access allows a data port to be configured to operate with two inroute TDMA access techniques. A lower and higher bit rate threshold is configured for the data port. If the lower bit rate threshold is crossed, a low rate access technique is automatically selected; conversely, if the higher bit rate threshold is crossed, a high rate access technique is automatically selected.

In addition to this, Flexroute will also invoke Inroute Switching.

TDM and TDMA FramesTypical outroute TDM and inroute TDMA frame formats are divided into superframes, which are further divided into frames. The structure of the packets within these frames differs depending on whether the frame is carrying TDM data packets or TDMA bursts.






Bursts and FramesA TDM outroute carrier is normally large, typically supporting a range from 128 to 512 kbps. The outroute payload will consist of demand-assigned packets.

A TDMA inroute carrier is normally much smaller, ranging from 64 to 256 kbps. However, multiple inroute carriers can be supported based on the traffic-engineering model chosen. The use of multiple inroute carrier supports higher remote terminal aggregate data throughput requirements.

The key feature is being able to scale the inroute traffic load as the network model changes or grows.






Bursts and FramesTDM Outroute Packet StructureIn a star configuration topology where expensive and complex hub equipment is utilized, a single TDM outroute is normally broadcast to all the remotes belonging to specific user groups.

The outroute can address all remotes to perform maintenance and administration tasks, or can address single remotes in response to a session requested by the remote.

The TDM frame format hierarchy consists of a superframe enclosing multiple frames and packets.

A superframe header contains a unique outroute identification number. The remote terminal uses this unique outroute number to ensure that it synchronizes with the correct outroute.






TDM Superfram eForm at

FRAME 0 FRAME 1 FRAME 2 FRAME 3 FRAME 4 FRAME 5 FRAME 6 FRAME 7

SUPERFRAMHEADER

FILLERSUPERFRAM

HEADER

1 S U P E R F R A M E = 3 6 0 m sec

F R A M E 0 F O R M A T

SF HEADERPACKET

REAL-TIMEPACKETS

NON-REAL TIMEPACKETS

F R A M E 1 TO F R A M E 7

REAL-TIMEPACKETS

NON-REAL TIMEPACKETS

BLOCK DATA PACKETFORMAT (BYTES)

P ort C ardA d d ress (2 )

FEC(1)

Length(1)

PacketControl (1)

SessionNumber (1)

ARQControl (2)

DATA0 TO 246

CRC(2)

FRAM E = 45 msec FRAME SIZE = 720BYTES @ 128 kbps

FRAME SIZE = 2880BYTES @ 512 kbps




Figure 3.3.3.2a TDM Superframe Format



Bursts and FramesTDMA Inroute Packet StructureSingle or multiple TDMA inroute carriers are time shared among all the remote terminals in a network. Therefore, all the remote terminals share the TDMA bandwidth capacity.

Time allocation is based upon the type of demand assignment algorithm associated with the remote terminals’ access configuration.

The TDMA frame format hierarchy consists of a superframe, multiple frames and packets.






FRAME 1 FRAME 2 FRAME 3 FRAME 4 FRAME 5 FRAME 6 FRAME 7 FRAME 8

1 S U P E R F R A M E = 3 6 0 m sec

GUARD &TAIL

P R E A M B L E P A C K E T P A C K E T P A C K E T P A C K E T. . .

B U R S T B U R S T B U R S T B U R S TB U R S T B U R S TB U R S T B U R S T. . .

F R A M E4 5 m s ec = 7 2 0 b ytes

TDM A SUPERFRAM EFORM AT

FEC(1)

Length(1)

PacketControl (1)

SessionNumber (1)

ARQControl (2)

DATA0 TO 246

A D D R E S SC R C




Figure 3.3.3.2b TDMA Superframe Format



SynchronizationSynchronization between remote terminals in the network is required to avoid burst overlap. A burst overlap would generate a level of interference that would prevent the hub receiver from detecting incoming bits properly.

An efficient TDMA system must have no burst overlaps, and should have an optimized guard time between bursts to account for the time difference between to signals reaching a geostationary satellite from two widely separated Earth Stations within the coverage area. The guard time between bursts must however be optimized in order to maximize throughput efficiency.

Typically, when a remote terminal receives and detects a frame sync on the outbound TDM carrier, it starts a countdown timer. This timer is used by the terminal to transmit its payload at the start of its uniquely assigned burst time.



3.3.3.3: Synchronization



SynchronizationMoreover, if the TDMA network is equipped with an expensive hub acting as the Bus Master, a frame sync message is also transmitted on the TDM outroute.

This frame sync serves as a time reference point that is used to resynchronize the time basis of each remote terminal. This synchronization process eliminates cumulative time skew caused by oscillator inaccuracies over the duration of a frame.

Burst synchronization is further exasperated by the residual movements of a geosynchronous satellite. A round-trip propagation delay differential of about 250 microseconds is associated with the figure 8 motion of a GEO satellite in its station keeping box.






SynchronizationThe figure 8 pattern moves within this box having the following values:

X Axis = 75 km

Y Axis = 75 km

Z Axis = 35 km

Doppler effect is associated with the maximum displacement velocity of the satellite, which is about 10 km/h.

Consequently, this causes displacement of the TDMA burst of about 10 nanoseconds/s.






SynchronizationThe following synchronization methods can be implemented to handle the residual movement of geosynchronous satellites:

Closed loop method Open loop method

Closed loop methodIn a closed loop methodology, a newly commissioned remote terminal is initially allocated a TDMA burst. This burst is then compared to a reference burst transmitted from the hub. Both TDMA bursts have the same allocated time slot.

The remote terminal will receive and decode its own burst plus the reference burst.






SynchronizationThe time difference between the reference time burst and the remote terminal time burst is measured and compared. The resultant value is used to offset the remote terminal’s burst allocation time.

This practice is implemented during the commissioning phase, periodically during the operation phase, and following a remote site shutdown.

The objective is to repeatedly calculate the slant range between the remote terminal and the satellite to dynamically optimize the burst allocation time and minimize burst overlap.

Moreover, the hub continually transmits a reference clock to all the remote terminals. The reference clock is used to synchronize the remote terminals’ internal clocks and maintain their timing relationship with the hub.






SynchronizationTherefore, burst synchronization and clock synchronization serve to maintain the correct timing relationship between the hub and remote terminal, this ensuring that the remote will burst accurately into its time slot.

Open loop methodIn an open loop methodology, the remote terminals receive the satellite position directly from the hub in order to determine the exact location of the satellite.

The satellite position is derived from the hub and two designated remote ranging terminals. The hub and the two ranging terminals each transmit a burst to themselves in order to measure the propagation time of their bursts.






SynchronizationThe propagation time measured by the two ranging terminals is subsequently transmitted to the hub. The hub determines the satellite position by triangulation. The satellite position value and reference clock are transmitted to all the remote terminals.

The remote terminals will compare the satellite position value to the value stored during the commissioning phase and modify their assigned burst time accordingly.

The reference clock is used to synchronize the remote terminals’ internal clocks in order to maintain a timing relationship with the hub.






ThroughputThe throughput efficiency of a TDMA network is 85% better than that of single carrier access schemes when up to 50 TDMA carriers are being used. This value continues to improve as newer TDMA access schemes come on line.

Improved throughput efficiencies are realized when the following access and synchronization schemes are employed:

Dynamically assigned multiple SCPC TDMA carriers Payload/voice activated TDMA transmissions Dynamic TDMA burst allocation schemes for a variety of traffic

models Dynamic burst synchronization schemes to drastically reduce

overlap Dynamic clock synchronization schemes Optimized guard bands between bursts and frames



3.3.3.4: Throughput



ThroughputTDMA is typically employed in star configurations where numerous remotes, consisting of numerous network users groups, need too establish connectivity to a central hub.

The hub will break out the data streams belonging to each network user group and interface it to separate terrestrial links. These links will then connect to their respective corporate offices. The payload is typically data.

This type of network dominates the market place.

Mesh TDMA, on the other hand, is used to provide peer-to-peer voice data connectivity, and competes very well against SCPC DAMA networks.



3.3.3.4: Throughput



3.3.4 Code Division Multiple Access (CDMA)



DescriptionCDMA is a spread-spectrum approach to multiple access. In CDMA, many users share the same radio frequency band.

Each user’s spread-spectrum signal occupies a bandwidth much greater that that which is necessary to send the information. This results in many benefits, such as immunity to interference and jamming, while providing multi-user access.

The bandwidth is spread by means of a code, which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which only the data modulation spreads the spectrum.

The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.


Part 4: Code Division Multiple Access (CDMA)




Description

Figure 3.3.4.1 Description






The CDMA ConceptIn CDMA, each user is assigned a unique pseudo-random binary code sequence. This sequence is used to encode the information-bearing signal.

The receiver, knowing the unique code sequence of the user, decodes a received signal after reception and recovers the original data. This is possible because the cross-correlation between the code of the desired user and the codes of other users is small.

Since the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information-bearing signal, the encoding process spreads the spectrum.



3.3.4.2: The CDMA Concept



The CDMA ConceptA spread-spectrum modulation technique will therefore consist of the following criteria:

• The transmission bandwidth must be larger than the information bandwidth

• Statistically orthogonal pseudo-random spreading codes must be used to separate the various signals in code space with the objective of providing low cross-correlation






The CDMA ConceptThe ratio of transmitted bandwidth to information bandwidth is called the processing gain of the spread-spectrum system.

Gp = Bt/BiGp = processing gainBt = transmission bandwidthBi = bandwidth of information-bearing signal

The Pseudo-random Number (PN) spreading codes use the following terminology:

• Chipping Frequency: The bit rate of the PN code• Information Rate: The bit rate of the bearing signal• Chip: One bit of the PN code• Epoch: The length of time before the

code starts repeating itself Vol 3: Satellite Communication Principles, Sec 3: Carrier Access Schemes





The CDMA ConceptThe PN code sequence acts as a noise-like, but deterministic, carrier used for bandwidth spreading of the signal energy. The selection of a good code is important because the type and length of the code sets bounds on the system capacity.

The PN code sequence is a pseudo-random sequence of 1’s and 0’s, not a truly random sequence, because random signals cannot be predicted. PN codes will have the following properties:

• Not random, but it looks random to the user who doesn’t know the code

• Deterministic, periodic signal that is known to both the transmitter and receiver. The longer the period of the PN spreading code, the closer the transmitted signal will be to a truly random binary wave, making it harder to detect

• Should exhibit the statistical properties of sampled white noiseVol 3: Satellite Communication Principles, Sec 3: Carrier Access Schemes





The CDMA ConceptIn practice PN codes are not perfectly orthogonal; therefore, cross-correlation between user codes introduces performance degradation, which consequently limits the maximum number of simultaneous users.

PN code generators in use today are as follows:

• Barker Code

• Gold Codes

• Hadamard-Walsh Codes

• Kasami Code






PN Code Randomness PropertiesCDMA transmitters modulate data onto wideband carriers that are distinguishable from each other by different PN sequences. The users share the same band of frequencies, and the receivers can identify their intended data by searching their list of PN sequences.

In order to avoid self-interference, so that one user traffic channel does not interfere with another user traffic channel, the traffic must be spread with synchronized, orthogonal PN sequences.

The number of PN sequences can be chosen from a vast set of low cross-correlation codes generated by a variety of PN code generators highlighted in the previous section.



3.3.4.3: PN Code Randomness Properties



PN Code Randomness PropertiesIn mobile communication applications, a unique PN sequence can move into other footprints or cells without fear of meeting interference from another user.

There are three basic properties that can be applied to any periodic binary PN sequence as a test for the appearance of randomness, and hence to ensure orthogonality.

Balance property

Good balance requires that in each period of the PN sequence, the number of binary ones differs from the number of binary zeros by at most one digit.






PN Code Randomness PropertiesRun Property

A run is defined as a sequence of a single binary digit state: 1 or 0. The appearance of the alternate digit in a sequence starts a new run. The length of the run is the number of digits in the run. Among the runs of ones and zeros in each period, it is desirable that about one-half the runs of each type are of length 1, about one-fourth are of length 2, one-eighth are of length 3, and so on.

Correlation property

If a period of the sequence is compared term by term with any cyclic shift of itself (one clock cycle latter), it is best if the number of agreements differs from the number of disagreements by not more than one count.






PN Code Randomness PropertiesFor example, consider the Simple Shift Register Generator shown below.

Figure 3.3.4.3a Simple Shift Register Generator






PN Code Randomness PropertiesThe Simple Shift Register Generator, otherwise known as the PN sequence generator, contains three basic elements:

• Delay element (in this example there are four shift registers [n])

• Linear combining element (modulo 2 Xor combiner)

• Feedback loop element

The three common characteristics that can be modified in the Simple Shift Register Generator are:

• Tap Placements (common rule is to have even number of taps)

• Sequence Length (2n – 1) n = number of registers

• Initial register fill contents (start off with initial sequence and reload initial sequence following X clock cycles)






PN Code Randomness PropertiesAssume that stage X1 is initially filled with a one and the remaining stages are filled with zeros. The succession of register states will be as follows:

X1 X2 X3 X41 0 0 00 1 0 00 0 1 01 0 0 11 1 0 00 1 1 01 0 1 10 1 0 11 0 1 01 1 0 11 1 1 01 1 1 10 1 1 10 0 1 10 0 0 11 0 0 0






PN Code Randomness PropertiesThe output at X4 will generate the following PN sequence:

0 0 0 1 0 0 1 1 0 1 0 1 1 1 1 0

Since the last state (1 0 0 0 ) corresponds to the initial state, the register repeats the sequence after 15 clock pulses.

The output sequence is therefore:

0 0 0 1 0 0 1 1 0 1 0 1 1 1 1

Since there are seven zeros and eight ones the sequence meets the balance property condition.






PN Code Randomness PropertiesThe natural length of the unique run is 15 digits, which is the run property.

A configuration which generates a sequence whose repeat length is 2n-1 and whose register fills contains all possible fill values (except the all zero fill) after 2n-1 clocks and in a particular order is called Maximal Length configuration (M sequence).

Some PN sequence generator architectures are truncated before reaching their natural length, or are allowed to run several chips (clock cycles) past its natural length (appended) before being reloaded with the initial fill.






PN Code Randomness PropertiesIf the output sequence is compared to the output sequence one clock cycle latter the following output will result:

0 0 0 1 0 0 1 1 0 1 0 1 1 1 11 0 0 0 1 0 0 1 1 0 1 0 1 1 1 d a a d d a d a d d d d a a a

The digits that agree are labeled “a” and those that disagree are labeled “d”.

The correlation property is met because the number of disagreements differs from the number of agreements by not more than one count.






PN Code Randomness PropertiesCorrelation has a specific mathematical meaning. For example, if the transmitted PN code sequence broadcast from the source is compared to a local PN code sequence generated by a receiver and are found to be equal, a match occurs. This equates to a value or 1.

If a match does not occur, a zero value is generated. Intermediate values indicated how much the codes have in common. The more they have in common, the harder it becomes to extract the appropriate signal.

Therefore, if a transmitted PN sequence is cross-correlated to a PN sequence generated by the receiver, alignment of the PN sequence occurs when the fifteen digits match: that is agreements (A) = 15 and disagreements (D) = 0. If alignment does not occur then the result will be A = 7, D = 8.






PN Code Randomness PropertiesIn conclusion, the CDMA spread-spectrum receiver will first invoke cross-correlation to separate the wanted signal from unwanted signals, and then invoke auto-correlation to maintain signal lock and reject multi-path interference.

The diagram below graphically depicts the auto-correlation function. There is one instance whereby all 15 digits are correlated (A-D=15), and all other instances whereby there is no correlation (A-D=-1).

Therefore, the auto-correlation function [Ra(t)] for a specific PN sequence is defined as the number of agreements less the number of disagreements in a term by term comparison of one full period of the sequence.






PN Code Randomness Properties

Figure 3.3.4.3b Auto-Correlation Function






PN Code Randomness Properties

Figure 3.3.4.3c Cross-Correlation Function for Two PN Sequences






PN Code Randomness PropertiesTransfer function (black) = good cross-correlation

Transfer function (red) = poor cross-correlation

Cross-correlation is the measure of agreement between different PN sequence codes transmitted for different users occupying the same frequency and time domain. In theory, when the cross correlation function [Rc(t)] is zero for all value of t, the codes are said to be perfectly orthogonal.






PN Code Randomness PropertiesIn practice, the PN sequence codes are not perfectly orthogonal; therefore, cross-correlation between user codes introduces performance degradation (increased noise power after despreading), which limits the maximum number of simultaneous users.

This problem becomes further exacerbated if a user transmits too much power. Consequently, CDMA network must implement very strict power control algorithms in order to maximize and maintain a satisfactory amount of users sharing the same frequency and time domain.

The key objective is to have very little cross-correlation between PN sequences whereby multiple users occupy different code space.






Two TechniquesThere are two principle CDMA modulation techniques which are commonly used: Direct Sequence CDMA (DS-CDMA) and Frequency Hopping CDMA (FH-CDMA).

3.3.4.4.1: Direct Sequence (DS) CDMA

In DS-CDMA the bearing signal (data signal) is directly modulated by a digital code signal. To obtain the desired spreading of the signal, the chip rate of the code signal must be much higher than the data rate of the bearing signal. Various modulation techniques can be used in the form of phase shift keying (PSK), like BPSK or QPSK.

The rate of the code signal is called the chip rate. If 10 code chips per information symbol are transmitted whereby the code chip rate is 10 times the data rate, the processing gain is equal to 10.



3.3.4.4: Two Techniques




After transmission of the signal, the receiver uses coherent demodulation to despread the spread spectrum signal, using a locally generated PN code sequence.

The DS-CDMA properties are as follows:

• Multiple Access: Multiple users can share the same channel at the same time. If the cross-correlation between the PN code of the desired user and the codes of the interfering users are small, coherent detection will only put a small part of the power of the interfering signals into the information bandwidth.







• Multipath Interference: If the PN code sequence has an ideal auto-correlation function, then coherent demodulation will treat the delayed version as an interfering signal, putting only a small part of the power in the information bandwidth. The use of a RAKE receiver will combine the received multipath signals in order to improve performance.

• Narrowband Interference: The user data rate is multiplied by the PN code sequence thereby exhibiting processing gain. Consequently, the power in the information bandwidth decreases by a factor equal to the processing gain; therefore, susceptibility to interference is substantially lowered.







• Low Probability of Intercept (LPI): The direct sequence signal uses the whole signal spectrum all the time; therefore, it will have a very low transmitted power per hertz. This makes it very difficult to detect a DS-CDMA signal.

3.3.4.4.2: Frequency Hopping (FH) CDMA

In FH-CDMA the carrier frequency of the modulated information changes periodically. During time interval “T”, the carrier frequency will remain the same; however, after each time interval the carrier hops to another frequency. The hopping pattern is decided by the code signal. The set of available frequencies the carrier can attain is called the hop-set.






3.3.4.4.2: Frequency Hopping (FH) CDMA

Fast Frequency Hopping (F-FH) occurs if the hopping rate is much faster than the symbol rate. In this case the carrier frequency changes a number of times during the transmission of one symbol, so that one bit is transmitted in different frequencies.

Slow Frequency Hopping (S-FH) occurs if the hopping rate is lower than the symbol rate. That is, multiple symbols are transmitted at the same frequency.

FH-CDMA has the same properties as DS-CDMA. However, there are some key advantages:

• Multiple Access: Multiple users can be assigned different frequency hop channels therefore further reducing the cross-correlation between multiple users on the same frequency hop channel.






3.3.4.4.2: Frequency Hopping (FH) CDMA• Multipath Interference: Frequency hopping changes the

mutipath effect for every frequency. Therefore, the receiver will average out the received multipath fading. On average FH-CDMA multipath interference is slightly better DS-CDMA.

• Narrowband Interference: An interfering signal would probably affect only one of the frequency hop channels. Consequently, only a small percentage of the total spectrum is affected. On average FH-CDMA narrow band interference is slightly better than DS-CDMA.

• LPI: Although the FS-CDMA signal uses much more power (dBW/Hz) than a DS-CDMA signal, it is still very difficult to intercept. The frequency of the signal is unknown and the duration of the signal is quite low, so that intercepting the signal becomes difficult.






ThroughputIn CDMA networks, two principle parameters determine the throughput capacity of a network:

• Low cross correlation of multiple sequences

• Very strict dynamic power control

Poor cross correlation of multiple sequences has the affect of adding noise to the users signal following despreading of the spread spectrum signal.

Unregulated power control will contribute to the overall white noise floor, thereby reducing the Eb/No of the user signal following despreading.



3.3.4.5: Throughput



user 1user 2

u ser 1 s p readsp e c tru m

u ser 2 s p readsp e c tru m

user 1 signal

user 2noise

appearslike interf.

u ser 2 ap p earsas w h ite n o ise

u ser 2 ap p earsas wh ite n o is e

Principle ofSpread

Spectrummultipleaccess

u se r 1 + u s er 2sp read

sp e c tru m

Figure 3.3.4.5 Principle of Spread Spectrum Multiple

Access



3.3.4.5: Throughput



ThroughputIn terrestrial CDMA networks the use of unique low cross-correlated multiple sequences provides excellent orthogonal discrimination, thereby improving the signal to interference ratio (S/I) in a multiuser environment.

The use of closed loop or open loop power control methodologies keeps the overall white noise floor dynamically optimized, maximizing throughput capacity.

Moreover, very efficient 8 kbps enhanced variable rate codec (EVRC) voice coders, defined in IS-95, can be implemented to further improve throughput capacity.



3.3.4.5: Throughput



ThroughputIn a multi-cell terrestrial mobility system, CDMA networks continue to outpace FDMA and TDMA technologies for the following reasons:

• Multiple Access Capability

• Protection Against Multipath Interference

• Privacy

• Interference Rejection

• Low Probability of Jamming (LPJ)

• Low Probability of Interception (LPI)

• Improved Throughput Capacity



3.3.4.5: Throughput



ThroughputIn very simplistic terms, CDMA network throughput capacity can be expressed as

Cmax = Gp / Eb/No

Cmax = System Capacity

Gp = Processing Gain (spreading factor)

Eb/No = Eb is the energy of 1 bit of information

No is the total noise power spectral density (white noise and interference).

The throughput capacity of a CDMA network is approximately 10% that of FDMA.



3.3.4.5: Throughput



ThroughputThe disadvantages of CDMA in the satellite environment are as follows:

• Power control cannot be as tight as it is in a terrestrial system because of the long round-trip delay (Eb/No is not optimized throughout the duration of a call).

• A fade margin must be included to account for rain fade (Eb/No is increased by 2 to 3 dB).

The lack of a very tight uplink power control system, plus the consumption of power to overcome rain fade, will reduce the overall throughput capacity of a satellite-based (Geo Satellite) CDMA network.



3.3.4.5: Throughput




3.3.5 Short Duration Access



Part 5: Short Duration Access

3.3.5.1: Newsgathering-style Access

Newsgathering-style Access (Occasional Use)Occasional Use access of satellite resources is employed to provide customers with short-term access to space segment resources on a time-required basis.

Access periods can be as short as a few minutes. Uplink stations are often mobile facilities.

Occasional Use satellite traffic is generated by the following events:

• Newsgathering (collection and distribution)

• Emergency restoration services (isolated communities)

• Corporate video conference services (on-demand basis)

• Trial technology services (testing new access schemes)



Newsgathering-style Access (Occasional Use)Occasional Use services can be provided in a manual fashion by the satellite operator, or can be part of an elaborate bandwidth-on-demand (BOD) database entry system.

BOD systems typically allow the customer to specify the uplink frequency and bandwidth, and the time and duration of the required service. The database would validate the new entry against previous entries to prevent service overlaps and generate the appropriate billing information.

The BOD systems are normally interfaced to an automated carrier monitoring system, which is used to proactively monitor and measure the output backoff values and cross-polarization interference levels of all Occasional Use carriers.






Newsgathering-style Access (Occasional Use)An access agreement is normally provided to all Occasional Use customers. The agreement specifies the Earth Station operating parameters and standards, including the all-important cross-polarization specifications.

Another essential element is the provision of a Supervisory Control And Data Acquisition (SCADA) network, which will allow the satellite operator to access the Occasional Use Earth Station and regulate EIRP output, or switch off the transmitter if the Earth Station transmits rogue carriers.

The SCADA network could be interfaced terrestrially, or be part of the space segment payload.






Demand Assigned Multiple Access (DAMA)A corporate customer can also manage and control the exclusive use of dedicated Occasional Use transponders to perform uplink source transfers between major metropolitan areas. Some of these corporate customers will also resell their space segment services to third parties.

The employment of DAMA technology allows the satellite operator, or private customer, to maximize the use of finite space segment and therefore maximize their return on investment for the service provided.

DAMA technology essentially manages the Earth Station and space segment resources on a call-by-call basis. The resources are usually defined in a configuration database, which incorporates elaborate validation software that assigns the resources when requested, and generates the billing information following the termination of the service.



3.3.5.2: Demand Assigned Multiple Access



Demand Assigned Multiple Access (DAMA)The DAMA software resides in a DAMA Processing Unit (DPU), which is the kernal software that runs in an industrial personal computer. A complete DAMA network, known as a Network Control Facility (NCF), will consist as a minimum the following software elements:

• Configuration Database, proprietary or commercial (Oracle®)• Graphic User Interface (GUI)• DPU (assigns Earth and “multiple transponder” space resources)• Outbound and Inbound control software (designate Earth and

space resources)• Report generation software (configuration and system

performance reports)• Billing generation software (revenue)• Alarm and Control software (maintenance and diagnostics)






Demand Assigned Multiple Access (DAMA)The NCF is normally used to manage and control SCPC or TDMA meshed FDMA networks. Enhanced versions can also perform BOD functions.






Random Access (Aloha)Aloha schemes are used in Occasional Use applications, particularly for burst traffic. Examples of random access protocols are:

• Aloha• Slotted Aloha• CSMA• CSMA/CD

AlohaThis access scheme is also known as pure random aloha.

Nodes transmit whenever they have data to send. No attempt is made to synchronize transmission to avoid overlap.

If overlap occurs between two transmissions, the receiver will not detect or acknowledge either one.



3.3.5.3: Random Access (Aloha)



Random Access (Aloha)If an acknowledgement is not received within a specified time-out period, the node waits a random period of time and then retransmits.

Slotted AlohaTime is divided into equal size slots.

A node will transmit at the beginning of the next slot.

If an acknowledgement is not received with a specified time-out period, the node will retransmit its payload on the next upcoming slot until successful.

Carrier Sense Multiple Access(CSMA)The node listens before transmitting its payload.

If the line is idle the node will transmit, if it is busy it will not transmit.






Random Access (Aloha)Collisions may still occur because two nodes may sense the link idle at the same time.

Carrier detection and propagation delay are the two important elements that affect throughput capacity.

A node at one end of the link cannot detect that another node has begun transmission if the transmitting node is located at the other end of the link.






Random Access (Aloha)Carrier Sense Multiple Access/Collision Detection (CSMA/CD)In this method, the node listens before transmitting its payload.

If the line is idle, the node will transmit its payload and read back its own content.

If there is a mismatch a collision is assumed.

The node will retransmit its payload following a random back-off time period.




Engineering

Carrier Access Schemes in Satellite Communications