Maximizing Data Volume for Direct to Ground Satellite Systems
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Transcript of Maximizing Data Volume for Direct to Ground Satellite Systems
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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
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David Andrew Carek, P.E. at Lewis FieldGlenn 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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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)
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
ISS Access time vs. Ground Station Latitude
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5
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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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
• 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)
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
• 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)
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
• 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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
Packet 3Packet 2Packet 1
Packet 3Packet 2Packet 1
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%
- Packet Efficiency = 67%
- Packet Efficiency = 0%
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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%
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
Probabilistic Efficiency
BER
10-5
10-4
10-3
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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%)
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
Probabilistic Efficiency
Optimal Packet Size
Information EfficiencyInformation Efficiency
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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
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David Andrew Carek, P.E. at Lewis FieldGlenn Research Center
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