Cost 297HAPCOS Meeting, Friedrichshafen, Germany

Oct. 8 – 10, 2008

Communications to and from HAPs –with laser beams?

Walter Leebwalter.leeb@tuwien.ac.at

Vienna University of TechnologyInstitute of Communications and Radio-Frequency Engineering

Gusshausstrasse 25/389, 1040 Vienna

2W. Leeb Oct. 8, 2008

Overview

• Introduction

• Building blocks

• PAT

• Influence of channel (= atmosphere)

• Bandwidth offered by optical and microwave links

• Summary

3W. Leeb Oct. 8, 2008

Motivation for optical links

transmission bandwidth f

(small) percentage of carrier frequency f

f = 200 to 350 THz

f 300 GHz

beam divergence proportional to 1/f

(antenna gain G proportional to f2)

10 rad, G 130 dB

small antenna diameter

expecting:

low terminal mass

low power consumption

4W. Leeb Oct. 8, 2008

Basic differences to microwave links

so far no frequency regulations

no electromagnetic interference

difficult eavesdropping

quantum nature dominates (hf >> kT)

dimension of devices (D >> )

antenna pointing, terminal acquisition, mutual tracking (PAT)

( two-way optical link)

influence of atmosphere

background radiation (Sun, Moon, etc.)

h ... Planck's constantk ... Boltzmann's constantT ... system temperature

5W. Leeb Oct. 8, 2008

Scenarios

GEO ... geostationary orbitLEO ... low earth orbitISS ... International Space Station

distance L = 45 000 to 83 000 km

data rate R = 3 Gbit/s

distance L > 1 000 000 km

data rate R = 2 Mbit/s

6W. Leeb Oct. 8, 2008

HAP – HAP – GEO Scenario

GEO ... geostationary orbitHAP ... high altitude platform

HAP HAP L = 5 ... 100 kmHAP GEO L = 50 000 km

R = 1 Gbit/s

G EO

H AP H AP

ground station

7W. Leeb Oct. 8, 2008

LEO-GEO link

2001

European Space Agency

ARTEMIS (GEO) SPOT-4 (LEO)

mean distance: 40 000 km

= 0.85 µm

R = 50 Mbit/s [2 Mbit/s]

2005

ARTEMIS OICETS (LEO, Japan)

SILEX ... Semiconductor Laser Intersatellite Link Experiment

ARTEMIS

SPOT 4

8W. Leeb Oct. 8, 2008

Balloon-to-ground link

2005German Aerospace Centre (EU project CAPANINA)

STROPEX

balloon (at 22 km) to ground, distance = 64 km

= 1.5 µm (InGaAs diode laser)

R = 622 Mbit/s and 1.25 Gbit/s

9W. Leeb Oct. 8, 2008

Airplane to GEO satellite

2006European Space Agency, France

"LOLA"

airplane (10 km height) to ARTEMIS (GEO)

= 0.85 µm, diode laser

successful pointing and tracking, video transmission

10W. Leeb Oct. 8, 2008

LEO-LEO link

2008

intersatellite laser communication:

TerraSAR-X (LEO, Germany) NFIRE (LEO, USA), 5 000 km

= 1.06 µm (Nd:YAG laser)

coherent receiver (homodyne)

BPSK (binary phase shift keying)

R = 5.5 Gbit/s

11W. Leeb Oct. 8, 2008

Overview

• Introduction

• Building blocks

• PAT

• Influence of channel (= atmosphere)

• Bandwidth offered by optical and microwave links

• Summary

12W. Leeb Oct. 8, 2008

Optical transceiver for space missions

transm itter(laser +

m odulator)

receiver

te lescope(antenna)

inTX

data

R Xdata

electrica l s ignal

fine poin ting

coarsepointing

aquisition andtracking sensor

pointahead

contro ll s ignal

optica l output s ignal

optica l input s ignal

13W. Leeb Oct. 8, 2008

TX, RX for = 0.85 µm

direct modulation

APD ... avalanche photodiode

diode laser0.85 µm

TX data

optica l output power PT

optics

APD photodiodem odule

decisionlogic

data

optica l input pow er P R

optica lbandpassoptics

14W. Leeb Oct. 8, 2008

TX, RX for = 1.5 µm

EDFA ... Erbium doped fiber amplifier

diode laser1.55 µm

externalm odulator

boosterED FA optica l output

pow er PT

optics

TX data

pream plifierED FA

optica lbandpass

PIN photodiodem odule

decis ionlogic

data

optica l input pow er P R

optics

15W. Leeb Oct. 8, 2008

Input-output multiplexing (1)

duplexing: spectrally, or via polarization, or both

to keep crosstalk TX RX low: high isolation within duplexer

(e.g. PT = 1 W, PR = 10 nW) 95 dB

duplex operation between two moving terminals required,at least for acquisition and tracking

receiver

transm itter

duplexer

te lescope(antenna)

optical beam in

optical beam out

(PR)

(PT)

single antenna for RX and TX

16W. Leeb Oct. 8, 2008

Input-output multiplexing (2)

receiver

transm itterte lescope(antenna)

optica l beam in (PR )

optica l beam out (PT)

m irror

simple duplexing scheme

increased telescope diameter

shared antenna aperture

17W. Leeb Oct. 8, 2008

Overview

• Introduction

• Building blocks

• PAT

• Influence of channel (= atmosphere)

• Bandwidth offered by optical and microwave links

• Summary

18W. Leeb Oct. 8, 2008

PAT

e.g.:

= 1.55 µm, DT = 20 cm

2T = 10 µradT

T D

42

satellite position uncertainty and vibrations ( > 2T) require:

initial pointing of transmit and receive antenna

mutual search and acquisition of terminal position

closed loop tracking of antenna direction (accuracy: 1 µrad!)

beam divergence 2T

(antenna directivity)

PAT

possibly: extra acquisition laser separate tracking beam and tracking sensor (CCD)

19W. Leeb Oct. 8, 2008

Overview

• Introduction

• Building blocks

• PAT

• Influence of channel (= atmosphere)

• Bandwidth offered by optical and microwave links

• Summary

20W. Leeb Oct. 8, 2008

Influence of atmosphere

absorption by molecules attenuation

scattering (molecules, waterdroplets, fog, snow) attenuation

pronounced influence within first 15 km above the Earth's surface,

but relatively small influence above 15 km

turbulence (random variation of index of refraction)

increased beam divergence ("beam spread" & "breathing" of beam) attenuation, fading

random beam deflection ("beam wander") fading

phase front distortion fading, scintillation

21W. Leeb Oct. 8, 2008

Beam spread

r0 ... Fried-Parameter

... wavelength

diffraction limited beam divergence in vacuum T

DL D

4

2

beam divergence including influence of turbulence2

0

2DLturb r

2)2(2

far field:

θ tu rb

w e ff

sp o t-s izew ith tu rb u len ce

x

y

θ D L

w 0D T

tran sm itte r f ie ld

n ea r-fie ld

fa r-fie ld

w D L

sp o t-s izew ith o u t tu rb u len ce

22W. Leeb Oct. 8, 2008

Fried parameter

Fried parameter r0 characterises the degree of turbulence, integrated over beam path

large r0 means little influence of turbulence

examples (medium turbulence, = 1.5 m):

- HAP(at 17 km)-to-satellite link r0 = 10 m

- ground-to-satellite link r0 = 15 cm

for a transmit antenna diameter DT equal to the Fried parameter r0,

the turbulence causes an increase of the divergence by a factor of ,

i.e. a gain reduction by 3 dB

2

- downlink (satellite to HAP): in general negligible influence of turbulence

- uplink: typically < 0.1 dB additional loss due to turbulence-induced beam spread

23W. Leeb Oct. 8, 2008

Beam wander

caused by large-scale turbulence near the transmitter,

leading to deflection of entire beam

y

x

w ith o u ttu rb u len ce

w ithtu rb u len ce

24W. Leeb Oct. 8, 2008

Scintillation

caused by small-scale turbulence, leads to interference between parts of the beam,

disturbance of intensity profile ("speckle") distortion of beam phasefront, mode de-composition ( reduced coupling into single-mode receiver)

scintillation index 2 characterises the temporal behaviour of intensity (I) fluctuations (normalized variance of I(t))

1I

I

2

2

2 typically 2 < 0.025 for HAP-to-satellite link temporal mean

b eam p h ase fro n tw ith o u t

tu rb u len ce

r

b eam in ten s ity

r

w ithtu rb u len ce

25W. Leeb Oct. 8, 2008

Overview

• Introduction

• Building blocks

• PAT

• Influence of channel (= atmosphere)

• Bandwidth offered by optical and microwave links

• Summary

26W. Leeb Oct. 8, 2008

Sensitivity of receivers

rule of thumb for detecting one bit of information:

required is an energy of either 10 hf or 10 kT, whatever is larger

10 hf 10 kT

optical = 1 µm, T = 300 K

210-18 Ws 410-20 Ws

microwavef = 10 GHz, T = 300 K 710-23 Ws 410-20 Ws

h ... Planck`s constantk ... Boltzmann`s constantT ... system temperature

optical regime requires 100 times larger input power!

Optical on-off keying: BEP = 10-9 requires an average of 10 photons per bit

(absolute physical limit)

27W. Leeb Oct. 8, 2008

Background radiation

sources: Sun, Moon, planets (including Earth), scattering atmosphere

received background power PB = NbackBom

Optical links: noise increase due to background

Nback ... power density (in one spatial mode)

e.g. at  = 1.5 m - Nback,Sun = 410-20 W/Hz

- Nback,Earth  = 410-25 W/Hz

- Nback,atm@20 km  = 10-27 W/Hz

Bo ... bandwidth of optical filter [Hz]

m ... number of modes accepted by receiver

28W. Leeb Oct. 8, 2008

Transmission bandwidth - examples

HAP (20 km) GEO satellite (36 000 km)

distance L = 50 000 km (zenith angle 45°)

TX: GaAs laser diode

RX: avalanche photodiode

TX: InGaAs laser diode

RX: EDFA reamplifierRF in K-band

wavelength 0.85 µm 1.55 µm 1.76 cm

carrier frequency 353 THz 194 THz 17 GHz

achievable bandwidth B for optical and RF links = ?

29W. Leeb Oct. 8, 2008

Link geometry

RT

2RT

TR L16

DDPP

(B)powernoiseelectrical

powersignalelectricalSNR

variable parameters: antenna diameters, transmit power

L

transm it antennadiam eter D T

G EO sate llite

receive antennadiam eter D R

H AP

transm ited pow er P T carrier

back

gro

und

ra

diat

or

... wavelengthT, R ... terminal troughputSNR ... signal-to-noise ratioB ... bandwidth

30W. Leeb Oct. 8, 2008

Bandwidth

PT = 10 W

L = 50 000 km, SNR = 16 dB

RF: f = 17 GHz, RR = 0.35, noise figure 3 dB,

PT = 10 W

e.g. DT = 2.8 m DR = 2.0 m

= 1 W

product of antenna d iam eters, D ·D [m ]T R

2

0.01 0 .1 1 10

achi

evab

le b

andw

idth

B

10 G H z

1 G H z

100 M H z

10 M H z

1 M H z

31W. Leeb Oct. 8, 2008

Bandwidth

PT = 10 W

Optical: = 0.85 µm, RR = 0.25, MAPD,opt, in.el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm

RF: f = 17 GHz, RR = 0.35, noise figure 3 dB,

PT = 10 W

e.g. DT = 2.8 m DR = 2.0 m

= 1 W

product of antenna d iam eters, D ·D [m ]T R

2

0.01 0 .1 1 10

achi

evab

le b

andw

idth

B

10 G H z

1 G H z

100 M H z

10 M H z

1 M H z

PT = 0.1 W

L = 50 000 km, SNR = 16 dB

32W. Leeb Oct. 8, 2008

Bandwidth

PT = 10 W

Optical: = 0.85 µm, RR = 0.25, MAPD,opt, in,el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm

RF: f = 17 GHz, RR = 0.35, noise figure 3 dB,

Optical: = 1.55 µm, RR = 0.25, in,el = 12 pA/Hz, Nback = 4·10-25 W/Hz, Bopt= 0.5 nm

PT = 10 W

e.g. DT = 2.8 m DR = 2.0 m

e.g. DT = 14 cm DR = 23 cm

= 1 W

product of antenna d iam eters, D ·D [m ]T R

2

0.01 0 .1 1 10

achi

evab

le b

andw

idth

B

10 G H z

1 G H z

100 M H z

10 M H z

1 M H z

= 0.3 W

PT = 0.1 W

PT = 1 W

L = 50 000 km, SNR = 16 dB

33W. Leeb Oct. 8, 2008

Antenna gain and beam spread loss

HAP(at 20 km)-to-GEO uplink, = 1.5 µm

transm it te lescope d iam eter D [m ]TX

ante

nna

gain

[dB

]

105

107

109

111

113

0.1 0.15 0.2 0.25

antenna gain

antenna gain minus beam spread loss, hHAP = 20 km

antenna gain minus beam spread loss, hHAP = 1 km

34W. Leeb Oct. 8, 2008

Sun as background

SNR degradationdue to sun

as background[dB]

APD receiver(large field-of-view)

15

10

5

0

16 dB

EDFA receiver(single transverse mode)

0.7 dB

Nback = 410-20 W/Hz

35W. Leeb Oct. 8, 2008

20 km 20 km

400 km

20 km

10 km

100 km

dow n

up

Beam spread loss (bs) for HAP-to-HAP links

= 1.55 µm, DT = DR = 13,5 cm

bs = 0.3 dB ... up, medium turbulence

bs = 0.7 dB ... down, medium turbulence

bs = 0.3 dB ... weak turbulence

bs = 0.7 dB ... strong turbulence

bs with DT, because ratio DT/diameter of turbulent eddies ... but much less than antenna gain!

36W. Leeb Oct. 8, 2008

Entangled photons for cryptography

Alice

Bob

aim: global distribution of cryptographic keys using a source of entangled photons onboard the International Space Station (ISS)

or on a HAP?

37W. Leeb Oct. 8, 2008

Summary

large bandwidth obtainable with low antenna diameter small prime power (?) compact terminal (?)

challenges mutual acquisition, tracking of terminals

strategies towards implementation adapt demonstrated systems and technologies systems should have potential for further development

very small disturbance by atmosphere for HAP GEO link (zenith angle < 45°) HAP HAP link (hHAP = 20 km)