maximizing data volume for direct to ground satellite systems

David Andrew Carek, P.E. at Lewis FieldGlenn Research Center

Maximizing Data Volume for Direct to Ground Satellite Systems

David Carek

Satellite Networks and Architectures Branch

NASA Glenn Research Center


Overview

• Study initiated as part of ACAD (Advanced Communications Architecture Demonstration)– Direct to ground communication system for ISS– Reliable transmission of latency tolerant payload

data

• Objective –maximize data volume


ACAD Link

• Initially examined ACK based protocol - TCP– Requires one ACK for every other packet – Large bandwidth asymmetry requires large packet size

on downlink• E.g. 622Mbps down; 622Kbps up => 1000:1 asymmetry• Required large downlink packet size to prevent ACK

feedback congestion on uplink– > 20Kbyte downlink packet required for 40byte uplink ACK

packet

– Needed to determine implication of bit errors on large packet transfers


Factors Affecting Data Volume

• Contact time (satellites)– Satellite altitude and orbit inclination (fixed)

– Ground station minimum elevation angle (design parameter tied to link budget)

– Ground station latitude (design parameter)

• Transmission Rate – Function of link budget transmitter power, antenna size,

etc.(design parameter)

• Protocol Efficiency


STK SimulationGround Contacts with ISS

ISS Orbit Inclination: 51.6

degAverage Alt: 380 km

Ground StationLattitude: 45 degMin Elevation Angle: 10 deg.

Factors Affecting Data Volume(Contact Time)


ISS Access time vs. Ground Station Latitude

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60

Ground Station Latitude (deg)

Acc

ess

Tim

e (m

in/d

ay)

70 dayAverage

Maximizing Contact Time

White Sands

GRC


• Function of Link Budget with many interrelated factors– Ground Segment

• Antenna size – Goal for small transportable ground terminal (1.2 meter dish)

– Space Segment• Transmit antenna type/beam width (gimbaled horn)• Frequency/Bandwidth (~27GHz/500MHz)• RF amplifier power

Factors Affecting Data Volume(Transmission Rate)


• Protocol algorithm (e.g. TCP)– Congestion control

• degrades efficiency when actual loss is corruption

– Acknowledgment feedback congestion• high bandwidth asymmetry degrades efficiency

• Information efficiency– Amount of end user data carried over link

relative to total data transmitted

Factors Affecting Data Volume(Protocol Efficiency)


• Information Efficiency

– Packet delivery efficiency (driven by bit errors)• Function of error free packets received

• Increased packet size = decreased efficiency

– Header efficiency (for fixed size header)• Function of data allocated to header vs. user data

• Increased packet size = increased efficiency

Factors Affecting Data Volume(Protocol Efficiency)

hpi eee


Types of Bit Errors• Gaussian Bit Errors

– Random RF Noise

• Burst Errors– Random occurrence of multiple bit errors– E.g. rain, snow, particles, etc.

• Systematic Errors– Often caused by internal electronics– Can be periodic distribution of single bit error or burst error

• Pattern Sensitive Bit Errors– Form of Systematic Errors– Influenced by data pattern within stream

*Reference: An Introduction to Error Location Analysis, Are all your errors truly random?, Application Note 1550-2; Agilent Technologies, 2000


Packet 3Packet 2Packet 1



X X X

XXX

X X X

Random

Burst

Periodic

Packet Delivery EfficiencyData Stream with BER = 1x10-1 Bit efficiency = 90%

- Packet Efficiency = 33%




Header EfficiencyQuantities for Illustrative Purposes Only

XXX H H H H H I I I I I H H H H H I I I I IPacket 3Packet 2Packet 1

X = ErrorH = Header bitI = Information bit (user data)

Burst Error Example• BER = 1x10-1; Packet Size = 10 bits; Header Size = 5 bits

– Bit efficiency = 90%– Packet delivery efficiency = 67%– Information efficiency = 33%


p

hpERi S

S

n

SBe 11

Deterministic EfficiencyAssumes n can be determinedn = average # bit errors per errored packet

p

hSERi S

SBe p 11

Probabilistic Efficiency

Information EfficiencyInformation Efficiency


0%

20%

40%

60%

80%

100%

10 100 1000 10000 100000

Packet Size (bytes)

Info

rmat

ion

Eff

icie

ncy

(%

of

tota

l ban

dw

idth

)

Packet loss dominatesHeader loss dominates

p

hpERi S

S

n

SBe 11

Deterministic Efficiency

p

hSERi S

SBe p 11


BER

10-5

10-4

10-3


15%

20%

25%

30%

50 60 70 80 90 100 110

Packet Size (bytes)

Info

rmat

ion

Eff

icie

ncy

(%

of

tota

l ban

dw

idth

)2% Actual Difference

BER = 10-3

Sh = 40 bytes

Deterministic Equation(worse case periodic error distribution; n=1)

Probabilistic Equation(random single bit errors)

Packet Size vs. Information Efficiency

(71, 19%)

(71, 25%)

(93, 27%)


p

hpERi S

S

n

SBe 11

ER

hp B

nSS

Deterministic EfficiencyAssumes n can be determinedn = average # bit errors per errored packet

Optimal Packet Size

p

hSERi S

SBe p 11

)1ln(

411

2 ERh

hp BS

SS


Optimal Packet Size

Information EfficiencyInformation Efficiency


Information Efficiency – Periodic Error Distribution(Sp = variable, Sh = 40 bytes; n = 1)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02

Bit Error Ratio

Info

rmat

ion

Eff

icie

ncy

(%

of

tota

l b

and

wid

th)

65536 byte

1500 byte

128 byte

64 byte

Max. Efficiency


VME Bus

Mass StorageDevice

Main

Pro

cesso

r

An

alo

g/D

iscrete

I/O

TB

D R

/W IF

Dual LVDS

HR

DL/1

55

3/1

00

BT IF

100Mbps

APS

CORto HRFM

to Ku

Payload

HRDL

1553 PL/MDM

Ethernet

RS232

Ethernet

RS232

Gimbal Assembly

ISS Subsystems and Internal PayloadsACAD Processor/Storage Segment

L-band Up

RS232

Data Flow Unreliable

Data Flow Reliable

Data Flow Reliable NAK’s

OMT

Modulator B

Modulator A

Upconverter A

Upconverter B

HPA A

HPA B

RHCP Ka

LHCP Ka

Antenna

Pointing

Controller

Ka DownLNA

Uplink Antenna

Downconverter

Demodulator


Conclusions

• BER alone is not enough to design a link– Bit error distribution must be accounted for

– Upper layer protocol must be considered

• Properly sizing packet sizes can maximize information efficiency and extend link availability

• Auto-tuning protocols based on packet error ratios could extend link availability and efficiency

maximizing data volume for direct to ground satellite systems

Documents