Transcript Satellite RF Links In The Disadvantaged Marine Environment John

Satellite RF Links In The
Disadvantaged Marine Environment
John Borden
Dr. Jeffrey N. Shaumeyer
Wavix, Incorporated
Offshore Communications 2002
October 2, 2002
Houston, TX
Topics
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Wavix Background
Disadvantaged Marine Applications
Marine Environment Characteristics
Common Protocol Solutions
Wavix Solution
Project Synergies
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Wavix Background
• Concept development work started in 1995
– Initial funding via NASA/GSFC SBIR teamed with
Woods Hole Oceanographic Institution
– Additional funding from Navy, NSF, NASA/SSC
• Incorporated Wavix in January 1999
• Original idea was to commercialize oceanographic
data collection from buoys
• Developed WavSat in response to lack of
affordable, high-throughput communications
• Received financing for satellite system from
Linsang Partners in Nov 2000
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Wavix Satellite System
• Store-and-forward satellite communication service
– E-mail in underserved areas
• Humanitarian workers in developing countries
• Crewmen onboard merchant ships at sea
• Recreational use of the oceans
– Data transmission
• Oceanography/Meteorology
• Oil and gas exploration and production
• Shipping
• Stasia 906 Satellite Groundstation
– Custom-designed in-house for our applications
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Current Projects
• Ocean Data Telemetry Microsat Link
– Phase 1 SBIR with Office of Naval Research
– Conceptual design of a satellite system to support
oceanography
– Leading to ONR-funded design, construction and
launch of a satellite
• Robust Ultra High Frequency (UHF) Satellite
Communications Protocol for UUVs
– Phase 1 SBIR with Naval Undersea Warfare Center
– Advanced satellite-communication protocol for use in
harsh marine environments
– Leading to use on Navy’s next-generation military
satellite communication system
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Disadvantaged Marine Applications
• Applications where the antenna is close to
the water surface
• Data Collection Buoys
– Moored buoys for oceanography, meteorology
– Drifters and profilers
• UUVs/AUVs
– Military systems for reconnaissance
– Research applications
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Typical Applications
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Marine Environment Characteristics
• Marine channels are prone to large scale errors
• Burst errors and signal fading are the major
contibutors
– Wash over of the near-surface antenna causing signal
fading from ground-plane disruptions and possible
transients in conductance will contribute burst errors.
– Wave motions rocking the UUV may contribute to
signal fading
– Shadowing by waves will contribute to signal fading
– High seas may contribute to fading and burst errors by
coating the antenna with sea water or altering local RF
propagation characteristics
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Marine Environment Characteristics
– Surface waves create reflections with dynamic
multipath contributions to signal interference
– Along the path of propagation, changeable atmospheric
conditions contribute to signal fading and burst errors
– The open-air RF channel is noisy with Additive White
Gaussian Noise that contributes to Bit-Error Rates
• If the noise environment is predominantly bursty in nature,
describing a channel as having a high BER can obscure the
fact that the Gaussian noise itself is actually rather small
– Contention for available channel space and interference
from datagram collisions
• Bit-error rates (BER) 10-5 or worse
– compared to 10-9 achieved by many terrestrial systems
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Common Protocol Solutions
• Modify existing protocols at the Network or Transport
layers of the OSI Model.
• TCP/IP has considerable inefficiencies due to the long
round-trip time
– Slow start and congestion avoidance slows link speed
– Window size limits throughput
– Requirement for positive acks slows link speed
• The Space Communications Protocol Standard is an
improvement on the TCP/IP suite of protocols
– Scaled windows and time stamps
– Fast retransmit and recovery
– Selective acknowledgment
• Assumes error-free or very low bit-error link
• Adds overhead - using more of the limited throughput
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Illustration courtesy Catherine Werst
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Wavix Protocol Solution
• Needs to be attacked at the lowest levels
– Physical layer
• the physical characteristics of the RF signaling
– Data-Link layer
• reliable data delivery across the physical layer
• Diverse array of noise-mitigation techniques
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Metaframing
Time-Division, Multiple-Access Operation
Data Backcapture
Bit Interleaving
DC Balance & Scrambling
Forward Error Correction (FEC)
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Wavix Protocol Solution (cont.)
• Metaframing
Metaframe
Header
Frame
Markers
frame 1
frame 2
– imposes structure on the bitstream
frame 3
– structure adds information to mitigate
etc.
synchronization problems
– allows multiple, simultaneous user channels on the RF link
• Time-Division, Multiple-Access (TDMA) Operation
– metaframes are issued by the satellite node at fixed time intervals
– user synchronizes its timing to uplink in the allotted channel
• Data Backcapture
– receiver maintains a buffer of recently received bits
– receiver can locate previously received frames in the alreadyreceived bitstream by counting backwards
– frame synchronization appears to be nearly instantaneous
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Wavix Protocol Solution (cont.)
• Bit Interleaving
receive bits
write frames
read frames
a
b
c
d
e
f
g
h
i
j
k
l
– Frames are written sequentially
into a buffer
xmit bits
– When the buffer is full, bits are
Input bitstream: abcd efgh ijkl
transmitted by column
Transmitted bitstream: aeib fjcg kdhi
– As bits are received they are written into columns
– Frames are read out by rows.
• DC Balance & Scrambling
– avoid low-frequency components in the RF signal
• Forward Error Correction (FEC)
– number of bit errors that can be corrected determines
the size of the FEC code
– FEC mechanisms are best implemented in the frame
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Protocol Advantages
• Metaframing creates a DC-balanced, hierarchical structure
that makes synchronization more robust
• Metaframing and TDMA allows multiple, simultaneous
user channels on the RF link
• Individual channels can operate with independent
transmission protocols
• Metaframing with Data Backcapture can increase datatransmission efficiency
• Priority allocation of channels fits easily into the
metaframe structure
• Additional techniques to increase error immunity in the
bitstream, such as Interleaving, Data Scrambling, and FEC,
are easily accommodated
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Project Synergies
• Oceanography
– Argo profilers
• Navy UUVs
– Mine detection
• Oil exploration
– Current monitoring
• Delay-Tolerant Networking
– Interplanetary Internet
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