Cost 297
HAPCOS Meeting, Friedrichshafen, Germany
Oct. 8 – 10, 2008
Communications to and from HAPs –
with laser beams?
Walter Leeb
walter.leeb@tuwien.ac.at
Vienna University of Technology
Institute of Communications and Radio-Frequency Engineering
Gusshausstrasse 25/389, 1040 Vienna
Overview
• Introduction
• Building blocks
• PAT
• Influence of channel (= atmosphere)
• Bandwidth offered by optical and microwave links
• Summary
W. Leeb
Oct. 8, 2008
2
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
W. Leeb
Oct. 8, 2008
3
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 constant
k ... Boltzmann's constant
T ... system temperature
W. Leeb
Oct. 8, 2008
4
Scenarios
distance L = 45 000 to 83 000 km
data rate R = 3 Gbit/s
GEO ... geostationary orbit
LEO ... low earth orbit
ISS ... International Space Station
W. Leeb
distance L > 1 000 000 km
data rate R = 2 Mbit/s
Oct. 8, 2008
5
HAP – HAP – GEO Scenario
GEO
HAP
HAP
HAP  HAP L = 5 ... 100 km
HAP  GEO L = 50 000 km
R = 1 Gbit/s
ground station
GEO ... geostationary orbit
HAP ... high altitude platform
W. Leeb
Oct. 8, 2008
6
LEO-GEO link
ARTEMIS
2001
European Space Agency
ARTEMIS (GEO)  SPOT-4 (LEO)
mean distance: 40 000 km
SPOT 4
 = 0.85 µm
R = 50 Mbit/s [2 Mbit/s]
SILEX ... Semiconductor Laser Intersatellite
Link Experiment
2005
ARTEMIS  OICETS (LEO, Japan)
W. Leeb
Oct. 8, 2008
7
Balloon-to-ground link
2005
German 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
W. Leeb
Oct. 8, 2008
8
Airplane to GEO satellite
2006
European Space Agency, France
"LOLA"
airplane (10 km height) to ARTEMIS (GEO)
 = 0.85 µm, diode laser
successful pointing and tracking, video transmission
W. Leeb
Oct. 8, 2008
9
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
W. Leeb
Oct. 8, 2008
10
Overview
• Introduction
• Building blocks
• PAT
• Influence of channel (= atmosphere)
• Bandwidth offered by optical and microwave links
• Summary
W. Leeb
Oct. 8, 2008
11
Optical transceiver for space missions
fine pointing
telescope
(antenna)
TX
data
in
transmitter
(laser +
modulator)
point
ahead
coarse
pointing
RX
data
aquisition and
tracking sensor
receiver
W. Leeb
optical output signal
electrical signal
optical input signal
controll signal
Oct. 8, 2008
12
TX, RX for  = 0.85 µm
diode laser
0.85 µm
optics
optical output
power PT
TX data
direct modulation
optics
optical
bandpass
APD photodiode
module
optical input
power PR
data
decision
logic
APD ... avalanche photodiode
W. Leeb
Oct. 8, 2008
13
TX, RX for  = 1.5 µm
diode laser
1.55 µm
external
modulator
booster
EDFA
optics
optical output
power PT
TX data
optical input
power PR
optics
preamplifier
EDFA
optical
bandpass
PIN photodiode
module
data
decision
logic
EDFA ... Erbium doped fiber amplifier
W. Leeb
Oct. 8, 2008
14
Input-output multiplexing (1)
duplex operation between two moving terminals required,
at least for acquisition and tracking
 single antenna for RX and TX
transmitter
receiver
optical beam in (PR)
duplexer
optical beam out (PT)
telescope
(antenna)
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
W. Leeb
Oct. 8, 2008
15
Input-output multiplexing (2)
 shared antenna aperture
transmitter
telescope
(antenna)
optical beam in (PR)
optical beam out (PT)
receiver
mirror
 simple duplexing scheme
 increased telescope diameter
W. Leeb
Oct. 8, 2008
16
Overview
• Introduction
• Building blocks
• PAT
• Influence of channel (= atmosphere)
• Bandwidth offered by optical and microwave links
• Summary
W. Leeb
Oct. 8, 2008
17
PAT
beam divergence 2T
(antenna directivity)
4
2 T 
 DT
e.g.:
 = 1.55 µm, DT = 20 cm
 2T = 10 µrad
 satellite position uncertainty and vibrations ( > 2T) require:
 initial pointing of transmit and receive antenna
 mutual search and acquisition of terminal position
PAT
 closed loop tracking of antenna direction (accuracy: 1 µrad!)
possibly:  extra acquisition laser
 separate tracking beam and tracking sensor (CCD)
W. Leeb
Oct. 8, 2008
18
Overview
• Introduction
• Building blocks
• PAT
• Influence of channel (= atmosphere)
• Bandwidth offered by optical and microwave links
• Summary
W. Leeb
Oct. 8, 2008
19
Influence of atmosphere
 absorption by molecules
 attenuation
 scattering (molecules, waterdroplets, fog, snow)  attenuation
 turbulence (random variation of index of refraction)
 increased beam divergence ("beam spread" & "breathing" of beam)  attenuation, fading
 random beam deflection ("beam wander")
 phase front distortion
 fading
 fading, scintillation
pronounced influence within first 15 km above the Earth's surface,
but relatively small influence above 15 km
W. Leeb
Oct. 8, 2008
20
Beam spread
far-field
transmitter
field
DT
spot-size
without
y turbulence
near-field
weff
wDL
w0
θDL
x
θturb
spot-size
with turbulence
far field:
4
 DT
diffraction limited beam divergence in vacuum
2 DL 
beam divergence including influence of turbulence
 2 

2  turb  (2 DL )  

r
 0 
2
2
r0 ... Fried-Parameter
 ... wavelength
W. Leeb
Oct. 8, 2008
21
Fried parameter
Fried parameter r0 characterises the degree of turbulence,
integrated over beam path
 for a transmit antenna diameter DT equal to the Fried parameter r0,
the turbulence causes an increase of the divergence by a factor of 2 ,
i.e. a gain reduction by 3 dB
 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
 - downlink (satellite to HAP): in general negligible influence of turbulence
- uplink: typically < 0.1 dB additional loss due to turbulence-induced beam spread
W. Leeb
Oct. 8, 2008
22
Beam wander
caused by large-scale turbulence near the transmitter,
leading to deflection of entire beam
y
with
turbulence
x
without
turbulence
W. Leeb
Oct. 8, 2008
23
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)
beam
intensity
without
turbulence
with
turbulence
r
beam
phasefront
r
scintillation index 2 characterises the temporal behaviour of intensity (I)
fluctuations (normalized variance of I(t))
2 
I2
I
W. Leeb
2
1
typically 2 < 0.025 for HAP-to-satellite link
 temporal mean
Oct. 8, 2008
24
Overview
• Introduction
• Building blocks
• PAT
• Influence of channel (= atmosphere)
• Bandwidth offered by optical and microwave links
• Summary
W. Leeb
Oct. 8, 2008
25
Sensitivity of receivers
Optical on-off keying: BEP = 10-9 requires an average of 10 photons per bit
(absolute physical limit)
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
210-18 Ws
410-20 Ws
microwave
f = 10 GHz, T = 300 K
710-23 Ws
410-20 Ws
optical regime requires 100 times larger input power!
h ... Planck`s constant
k ... Boltzmann`s constant
T ... system temperature
W. Leeb
Oct. 8, 2008
26
Background radiation
Optical links: noise increase due to background
 sources: Sun, Moon, planets (including Earth), scattering atmosphere
 received background power PB = NbackBom
Nback ... power density (in one spatial mode)
e.g. at  = 1.5 m
- Nback,Sun = 410-20 W/Hz
- Nback,Earth = 410-25 W/Hz
- Nback,atm@20 km = 10-27 W/Hz
Bo ... bandwidth of optical filter [Hz]
m ... number of modes accepted by receiver
W. Leeb
Oct. 8, 2008
27
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 reamplifier
RF 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 = ?
W. Leeb
Oct. 8, 2008
28
Link geometry
GEO satellite
background radiator
HAP
receive antenna
diameter DR
transmit antenna
diameter DT
transmited
carrier power PT
L
2
 D T DR  
  T R
P R  P T 

 16  L 
SNR 
electrical signal power
electrical noise power (B)
variable parameters: antenna diameters, transmit power
 ... wavelength
T, R ... terminal troughput
SNR ... signal-to-noise ratio
B ... bandwidth
W. Leeb
Oct. 8, 2008
29
Bandwidth
L = 50 000 km, SNR = 16 dB
RF:
f = 17 GHz, RR = 0.35, noise figure 3 dB,
achievable bandwidth B
e.g. DT = 2.8 m
DR = 2.0 m
10 GHz
1 GHz
PT = 10 W

100 MHz
=1W

10 MHz
1 MHz
0.01
0.1
1
10
2
product of antenna diameters, DT·DR [m ]
W. Leeb
Oct. 8, 2008
30
Bandwidth
L = 50 000 km, SNR = 16 dB
RF:
f = 17 GHz, RR = 0.35, noise figure 3 dB,
Optical:  = 0.85 µm, RR = 0.25, MAPD,opt, in.el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm
achievable bandwidth B
e.g. DT = 2.8 m
DR = 2.0 m
10 GHz
1 GHz
PT = 10 W
PT = 0.1 W

100 MHz

10 MHz
=1W

1 MHz
0.01
0.1
1
10
2
product of antenna diameters, DT·DR [m ]
W. Leeb
Oct. 8, 2008
31
Bandwidth
L = 50 000 km, SNR = 16 dB
RF:
f = 17 GHz, RR = 0.35, noise figure 3 dB,
Optical:  = 0.85 µm, RR = 0.25, MAPD,opt, in,el = 12 pA/Hz, Nback = 2·10-25 W/Hz, Bopt= 1nm
Optical:  = 1.55 µm, RR = 0.25, in,el = 12 pA/Hz, Nback = 4·10-25 W/Hz, Bopt= 0.5 nm
achievable bandwidth B
e.g. DT = 14 cm
DR = 23 cm
10 GHz
e.g. DT = 2.8 m
DR = 2.0 m
PT = 1 W
= 0.3 W

1 GHz

100 MHz
PT = 10 W
PT = 0.1 W


10 MHz
=1W

1 MHz
0.01
0.1
1
10
2
product of antenna diameters, DT·DR [m ]
W. Leeb
Oct. 8, 2008
32
Antenna gain and beam spread loss
HAP(at 20 km)-to-GEO uplink,  = 1.5 µm
113
antenna gain [dB]
antenna gain
antenna gain minus
beam spread loss, hHAP = 20 km
111
antenna gain minus
beam spread loss, hHAP = 1 km
109
107
105
0.1
0.15
0.2
0.25
transmit telescope diameter DTX [m]
W. Leeb
Oct. 8, 2008
33
Sun as background
SNR degradation
due to sun
as background
[dB]
Nback = 410-20 W/Hz
15
16 dB
10
5
0.7 dB
0
APD receiver
(large field-of-view)
W. Leeb
Oct. 8, 2008
EDFA receiver
(single transverse mode)
34
Beam spread loss (bs) for HAP-to-HAP links
 = 1.55 µm, DT = DR = 13,5 cm
400 km
20 km
20 km
bs = 0.3 dB ... weak turbulence
bs = 0.7 dB ... strong turbulence
up
down
100 km
20 km
bs = 0.3 dB ... up, medium turbulence
10 km
bs = 0.7 dB ... down, medium turbulence
bs with DT, because ratio DT/diameter of turbulent eddies  ... but much less than antenna gain!
W. Leeb
Oct. 8, 2008
35
Entangled photons for cryptography
aim: global distribution of
cryptographic keys using
a source of entangled
photons onboard the
International Space
Station (ISS)
or on a HAP?
Bob
Alice
W. Leeb
Oct. 8, 2008
36
Summary
very small disturbance by atmosphere for
 HAP  GEO link (zenith angle < 45°)
 HAP  HAP link (hHAP = 20 km)
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
W. Leeb
Oct. 8, 2008
37