Prof. Brandt-Pearce Lecture 8 Deep-Space Optical Communications

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Prof. Brandt-Pearce

Lecture 8Deep-Space Optical

Communications

Optical Wireless Communications

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Outline

Deep-Space Optical Communications Introduction

Channel Model

System Performance

Optical Deep-Space Network

RF/FSO Hybrid System

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Sending and receiving data from space crafts has been a

challenging problem since 1950s

Communication over deep-space distances is extremely

difficult, much more difficult than satellite communications

Communications beams spread as the square of the distance

between the transmitter and the receiver

Deep-Space Communications

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The distance from Earth to Neptune or Pluto can be on the

order of 4,000,000,000 km. After propagating over such a

distance, the communications beam from a spacecraft will

spread to an area 10 billion times (100 dB) larger in area than

if the beam from the same system traveled from just the GEO

distance (40,000 km).

A system capable of transmitting 10 Gbps from GEO to the

ground would only achieve 1 bps from Pluto/Neptune

distances.

Deep-Space Optical Communications

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Optical communications has lower divergence compared to RF

Comparison of RF and optical beam spreads from Saturn.


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An important factor for a high data-rate deep-space optical link is the laser transmitter

Lasers are required to have High output power Low divergence


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Another key technology component is a thermally stable and lightweight optical spacecraft telescope.

Similar to satellite communications, for a small beam divergence, tracking and pointing plays an important role in the reliability of deep-space optical links

This pointing must be accomplished in the presence of attitude changes of the host spacecraft that are perhaps a thousand times larger than the laser beam divergence.


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Growth of the Deep-Space Comm. Capacity

Hamid Hemmati, “Deep Space Optical Communications”, Jet Propulsion Laboratory, California Institute of Technology, 2005

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Optical deep-space communications can be implemented in two ways: Direct optical link: A direct optical link is set up between

the earth station and space-craft Atmosphere disperses and attenuates the transmitted and

received signals High power transmitter and large receivers can be used

Indirect optical link: the optical signal is sent from a satellite outside the atmosphere Atmosphere effect is mitigated Transmitter and receiver sizes are limited

Deep-Space Communications

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METOL MARS-EARTH Terahertz Optical Link

Critical Event Monitor UHF:

1 - 16 kbps

Small Lander UHF:128 kbps (150 Mb in 20 minutes)

MER-Class UHF:

128 kbps (1 Gb/so

l)

Directi

onal X-band:

1 Mbps (1

0 Gb/sol)

X-band: up to 4 Mbps(28 Gb/2 hrs)

RF Back-up

100 W 1.07 micron Laser 1 - 10 Gbps

5 W 1.54 micron Laser 1 - 10 Gbps

5W 26 GHz 100 Mbps (RF)

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Cloud opacity is an atmospheric physical phenomenon that jeopardizes optical links from deep space to any single ground station

Clearly, when clouds are in the line-of-sight, the link is blocked

Ground receiving telescopes need to be located in sites where cloud coverage is low and statistically predictable

To guarantee continuity of data delivery from deep space to ground, while the Earth is rotating, a global network of telescopes is necessary

The selection of the sites for telescopes belonging to an optical deep space network (ODSN) is driven by considerations based, among other factors, on cloud-cover statistics

Channel Model

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Channel Model: Atmospheric Transmittance

Main Gases composing the Earth Atmosphere

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Channel Model: Atmospheric Transmittance

Earth atmospheric number density profiles for individual species

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Channel Model: Atmospheric Transmittance Transmittance spectrum at sea level with zenith angle of zero.

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Channel Model: Sun Irradiance

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Channel Model: Sky Irradiance Sky radiance spectrum experienced at an observation point at sea level for 23 km

of visibility and Sun zenith angle of 45 deg while observer zenith angle varies as 10, 40, and 70 deg

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Merits of five deep-space communication link wavelengths.

Deep Space Optical Communications


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Data of a NASA optical link between Earth and Mars Modulation scheme: 256-ary PPM

Bit-rate: 1 Mbps

BER: 10-3

Range: 3.59 × 108 km


Deep Space Optical Communications

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To support deep space missions aimed to the exploration of the universe for the last four decades, NASA has designed and operated a global network of radio-frequency ground stations termed the Deep Space Network

A similar network can be used for optical communications called optical deep-space network (ODSN)

Today NASA’s DSN only requires three radio-telescope hubs to successfully operate the network. The DSN stations (located at approximately 120 deg of separation around the Earth: Goldstone, California; Madrid, Spain; and Canberra, Australia) allow continuous coverage of deep space from Earth

Optical Deep Space Network

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Since the laser transmitter beam width from space covers a limited area on Earth it is necessary that the ODSN consists of a number of ground stations located around the Earth as a linear distributed optical subnet (LDOS)

The idea behind LDOS is to have the spacecraft always pointing at a visible station belonging to the LDOS

When either the line of sight is too low on the horizon (20 deg of elevation) or is blocked by atmospheric conditions (e.g., clouds or low transmittance), the spacecraft beam is switched to a different station (or network node) by pointing to the adjacent optical ground station


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Example of LDOS (star = telescope) architecture for an optical deep space network (ODSN)



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Global Sites for Deep-Space Optical Communications

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Usually the received photon count is very low

PMTs are used to detect signal

The operation temperature of the space-craft is low

Thermal noise is proportional to the temperature:

Hence, shot noise is the dominating noise

Poisson statistics should be used for analysis

System Model

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For OOK: Probability density functions for transmitting “0” and “1” when

=Data average photon count/pulse =Background average photon count/pulse

Then

As discussed before, threshold is where the two pdf’s become equal Threshold = BER = When =0, Threshold=0 and BER =

System Model

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For Poisson distribution

where

In the absence of background light

Performance of Deep-Space Optical Communication

For PPM

Symbol error probability is

𝑃𝑏=12 𝑒

−𝐾 𝑠

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BER versus signal level for uncoded OOK signaling on a Poisson channel, for various background levels

Performance Analysis of OOK

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Performance Analysis of PPM

BER of uncoded PPM on a Poisson channel, versus Ks

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Performance Analysis of PPM

BER of uncoded PPM on a Poisson channel, versus Pav =

Ks /M

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FEC in Deep-Space Optical Comm. Due to the low received power the BER is high BER is usually 0.001 Forward error correction (FEC) is used to decrease BER down to 10-15

Deep-space optical systems use high order PPM since they have high energy efficiency

Reed-Solomon codes are used as FEC High-order PPM modulation (256-PPM) with a high alphabet (8-bit

alphabet) RS code Accumulator (product) codes:

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Outline

Deep-Space Optical Communications Introduction

Channel Model

System Performance

Optical Deep-Space Network


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Radio-Frequency (RF) Communications Low bandwidth

Stable Channel

Relatively immune to cloud blocking

Sometimes affected by heavy rain

Free-Space Optical Communications High Data Rate

2.5 Gbps commercially available (Tbps demonstrated)

Bursty Channel

Must have clear / haze conditions

Less degradation than RF in rain


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Enables FSO Communications bandwidth without giving up RF reliability and “adverse-weather” performance

Improves network availability: Quality of Service (QoS)

More options for adapting to weather Common atmospheric path effects and compensation (directional links) Physical Layer diversity improves jam resistance

Size, Weight and Power Focus Leverages common power, stabilization, etc. Economical use of platform volume

Enables seamless transition of free space optical communications into RF Environment

Combining RF and FSO System

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Average Data-Rate of a Hybrid FSO/RF

0 10 20 30 40 50 60 70 80 90 100

FSO LINK AVAILABILITY (%)

0

1

2

3

AVE

RA

GE

DA

TA R

ATE

(Gb/

s)AVERAGE DATA TRANSFER RATE OF HYBRID FSO/RF LINK

FSO 2.5Gb/s

RF 10Mb/s

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Short range applications:

Mesh networks

Cross-divide links (rivers, canyons, etc.)

Indoor systems

Long-range applications:

Air-to-air links

Satellite links

Wireless basestation connectivity

Applications

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Hybrid RF/FSO Point-to-Point Link

Either switching between technologies or simultaneous use

Joint modulation/coding across two technologies

With channel state information, can optimize throughput

Without channel state information, can use variable-length codes (fountain codes)

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Hybrid FSO/RF Two different modulations are assumed for RF and FSO links

with constellation sizes of M1 and M2

The links are assumed to operate synchronously

R1 and R2 are the data rates

Let C1 and C2 be the capacity of RF and FSO channel

respectively (Ci is a function of Ri)

From Shannon capacity we have

Then the throughput is

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Optimal Joint Modulation/Coding

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Short Range Hybrid RF/FSO Network

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Hybrid RF/FSO Networks

Considering that FSO link has a higher cost, only a given number of FSO links can be used in an RF/FSO system

Assume that an RF network is given

The problem is to find the best choices for replacing RF with an FSO link

This depends on the topology, distances between nodes and the availability of FSO link (depends on the weather condition)

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Formulate the problem as follows

The problem is to maximizes the following function

where

Network is modeled with a directed graph G=(N,L) i ∈ N denote the nodes in the network B is the number of demands lij ∈ L denote the directed link from node i to node j.

f (b)ij represent the flow of traffic on link lij

Dij is an indicator function of an FSO link from node i to node j

One unit time is divided into fractions represented by λk, k = 1,2, ..., K


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The maximization is subject to Input and output flow is equal for intermediate nodes

Input flow is zero for source nodes

Output flow is zero for sink nodes

Flow has to be positive

Sum of the time fractions is one

The maximum number of FSO links is M


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Here RF capacity is

CRFij=100 Mb/s and

CFSOij represent the

capacity of FSO links between nodes i and j

This problem can be solved using mixed integer linear programming (MILP)

Optimal throughput and bounds for the 16 node grid network and 28-node random.


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Prof. Brandt-Pearce Lecture 8 Deep-Space Optical Communications