High-speed Optical Wireless Communications

Fraunhofer Heinrich Hertz Institute

High-speed Optical Wireless Communications Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer Optical Fiber Conference (OFC), March 13, 2014, San Francisco, CA, Tutorial Th1F.5

Fraunhofer Heinrich Hertz Institute, Einsteinufer 37, 10587 Berlin

www.hhi.fraunhofer.de

Fraunhofer Heinrich Hertz Institute

Acknowledgements 

Luz Fernandez del Rosal



Stefan Nerreter



Ronald Freund



Sebastian Randel



Liane Grobe



Joachim Walewski



Kai Habel



Jonas Hilt



Christoph Kottke



Anagnostis Paraskevopoulos



Dominic Schulz

©

Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications

OFC 2014, San Francisco

2

Photonic Networks and Systems

Outline

©



Introduction



Optical wireless channel



Transmitter and receiver properties



Rate-adaptive system concept



Adaptive OFDM with controlled clipping



Realtime implementation



High-speed applications



Summary



3

Wireless optical communications 

Since ever, people used optics to communicate without wires 

©


Example: Optical telegraph in Cologne, Germany, from 1834



Today, visible and invisible light is used



Less photons per bit, much higher speed



Sometimes, light is still an alternative to radio Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


4


Why optics for wireless?

-7

-6

-5

-4

RF

VIS IR-A IR-B

IR-C incl. THZ

Optical spectrum is still unregulated, unlike radio below 100 GHz UV



-3

λ [m] 10

10

10

10

10

f [THz]

300

30

3

0.3

-2

10

-1

10

0

10

VIS & IR-A spectrum (100 THz…1000 THz)



Applications are found where radio is not tolerated or not feasible 

 ©

hospitals, airplanes, manufacturing cells, …

Use of visible light is intuitive for wireless (WYSIWYG) Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


5

Optical wireless domains Light allows connectivity over various distances



Ultra short range 

Short-range 

Medium-/long range Ultra-long range 

©

Inter-building networks, mobile backhaul Tbit/s satellite feeder links, satellite-to-satellite


> 10,000 km





optical WLAN, in-flight communications, car-to-car, car-to-infrastructure, indoor positioning, wireless automation

km



inter-chip interconnects, in-body networks m



mm





6


Focus will be on short-range

©



Using infrared (IR) and visible light communications (VLC)



10 Mbit/s … few Gbit/s per link



Low-cost components



LED, silicon photodiodes, digital signal processing



7


Propagation scenarios  

©

Intensity modulation/direct detection for low cost Wide beams  coverage, robustness, mobility 

Directed LOS



Non-directed LOS



LOS + NLOS



Non-directed NLOS






Switched-beam LOS

Multi-spot NLOS


8


Channel properties BS: base station, MS: mobile station

BS diffuse link directed link

MS 1 

Line-of-sight (LOS)



Non-line-of-sight (NLOS)



direct path  high power



Diffuse reflections  less power



narrow field-of-view  blocking is critical  mobility needs tracking



wide field-of-view



©

MS 2

no multipath  huge bandwidth

 blocking is less relevant  inherent mobility support 

multipath  low bandwidth



9


LOS channel







©

Received signal depends on  the distance and the area of the receiver  directivity of transmitter and receiver No dispersion  propagation yields a Dirac pulse  Bandwidth is limited by e/o and o/e components Polarization is useful Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


10


NLOS channel 

More complicated, can be modeled in two steps



Single diffuse bounce



Multiple diffuse bounces



In each roundtrip, energy is lost

J.B. Carruthers, J.M. Kahn, "Modeling of nondirected wireless infrared channels," IEEE Trans. on Communications, vol.45, no.10, pp.1260,1268, Oct 1997 ©



11

Model for multiple diffuse multipath diffusely reflecting surface

Tx




Path loss and decay time can be estimated by using formulas from integrating sphere



Windows and losses  mean reflectivity ρ

Rx



Path loss



Decay time

window

l= length w= width h=height

V. Jungnickel, V. Pohl, S. Nonnig, C. von Helmolt, "A physical model of the wireless infrared communication channel," IEEE Journ..Selected Areas in Communications (JSAC), Vol.20, No.3, pp. 631-640, April 2002 ©



Mean time-of-flight



Good agreement with measurements



12


Superimposed LOS and NLOS 

Typical case



Channel impulse response depends on



©



K-factor (Rice)



delay ∆Τ between LOS and NLOS

Frequency-selective channel 

“optical fading”



where photocurrents of LOS and NLOS contributions have similar amplitude but opposite phase



13

Summary on channel and propagation



©


The optical wireless channel is a superposition of LOS and NLOS signals 

LOS channel is more suitable for high data rate



NLOS channel is better for mobility



In mobile scenario, the channel is frequency-selective and time-variant



Altogether, the system concept should be made robust against multipath



14


Transmitter



Optical wireless was limited for a long time due to insufficient optical power



Recently, low-cost high-power LEDs became available using infrared and visible light



For data transmission, LED can be modulated at high speed



©

Flicker is not visible



15

LED design and modulation bandwidth




Blue LED + phosphor

Normalized Gain (dB)

0

-5

-10

-15



Blue LED is fast (~20 MHz)



Phosphor is slow (~2 MHz)



Low-cost, simple driving

-20

-25 10

6

7

10 Frequency (Hz)

10

8



©

R+G+B type 

Enables wavelengthdivision multiplex (WDM)



~15 MHz per LED chip



Higher cost



16

3m

1.65 m

The potential of high-power LEDs




5 x 5 x 3 m room



4 LED arrays, 400-800 lux



Very high SNR (60-70 dB)



High spectral efficiency: 12-16 bps/Hz



Using only blue part of phosphor-type LEDs

A 5m Scenario B, brightness level [lux] 5 400 300

500

Room width, y [m]

4.5 600

3.5

800

3

2.5 2.5

©

to have ~20MHz bandwidth

700

4

3

4 3.5 Room length, x [m]

4.5



400-800 Mbit/s with phosphor-LED



> 1 Gbit/s with RGB

5



17


LED driver

©



Conversion efficiency is < 1W/A



P=R*I²  with 50 Ω: 1 W optical power  50 W RF for modulation



RF leakage can be stronger than the received signal over the optical path



Impedance matching is mandatory for high bandwidth and energy efficiency



Recent results 

>100 MHz modulation bandwidth



30% more energy



18


Lasers are on the horizon



Application is already intended for head lights in luxury cars 

higher energy efficiency



higher bandwidth?



much higher cost



~100 € instead of 1 €

Source: BMW

©



19


Photodetectors 





©

PIN photodiode 

low cost, large area



limited sensitivity

Avalanche photodiode (APD): 

higher sensitivity, smaller area



high reverse bias  higher cost

Image sensors: 

CCD type: low cost due to high volumes, slow due to serial read-out



Array type: pixels are operated like parallel photodiodes  fast but high price, mass market would be revolutionary for optical wireless



20


Receiver design



Wide aperture  optical concentrator



Antireflection and color filter are possible



Impedance matching is critical as well



PD can have 10 dB higher sensitivity using transimpedance amplifier (TIA) compared to 50 Ω design



APD gain can be small

J. Vucic, Ph.D. thesis, 2009 ©



21

Summary on Tx and Rx

©




High-power high-speed light sources became recently available



Very high SNRs  very high spectral efficiency



High data rates despite limited bandwidth



Tx bandwidth and energy efficiency depend on careful impedance matching



Both, PIN-PD and APD receivers may be useful



Impedance matching is very important for PIN receivers (TIA)



22


Rate-adaptive system concept 

We want to be mobile, channel is frequency-selective and time variant



Rate-adaptive system concept based on feedback over the reverse link Channel quality information Ambient light Data in



©

Tx

OW channel

Rx

Data out

Complex dispersion effects are not avoidable 

Orthogonal frequency-division multiplex (OFDM)



Adaptive modulation and coding



23


On the capacity limits 

Z ~ N ( 0, N n )

Parallel flat OFDM sub-carriers

Gaussian channel



Power constraint

  = E X 2 E ∑ X n2  ≤ P n 



Shannon capacity

= = C  s Hzbitdim  max I ( X ;Y ) fX ( x)

{ }

Xn

EX 2 ≤ P



Yn

 Pn  1 log 1 + ∑n 2 2  N  n  

Waterfilling for Gaussian-distributed inputs

X n ~ N ( 0, Pn )

λ= Pn + N n

Pn = P λ − N n , N n ≤ λ ∑ Pn =  = λ const. ∈ R  0, N n > λ


Nn

Pn

N n = N 0 BSC H n

2

f OFC 2014, San Francisco

But intensity modulation is not linear


P 

PO

Beside the mean power contraint

E { x ( t )} ≤ PO

I th ≈ 0

I



waveform has to be strictly non-negative

x (t ) ≥ 0

x (t ) 

Gaussian input distribution and waterfilling are no longer adequate

t



Trigonometric moment space method


R. You and J. Kahn, „Upper-bounding the capacity of optical IM/DD Channels with multiple-subcarrier modulation and fixed bias using trigonometric moment space method“, IEEE Trans. Inf. Theory, Vol. 48, No. 2, Feb. 2002



You and Kahn provided an upper bound on the IM/DD capacity



Based on this result, a practical formula including a frequency-selective channel characteristics Hn can be derived (see J. Vucic, Ph.D. thesis, 2009)

1  2 2  η P 2 N opt 2 O   log 4 2 C  bit B H N ≤  ∑ n SC 2 opt s   N D  n =1   N opt

γn

   

−1

   

γ effective SNR BSC subcarrier bandwidth N opt ≤ N − 1 optimal no. of carriers PO optical power η optical path gain ND detector noise




How many subcarriers are used? 

Depending on the channel, we maximize the bound of You and Kahn



For NLOS, low-frequency subcarriers are used, while all are used with LOS

60 50

= PO 400 = mW, η 1 A/W

N = 64 N = 32 N = 16

40

LOS

C/BSC y [ [bit/s/Hz] ]

70

600

30

400

300

200

20

NLOS

100

10

0 -15

500

K=-15 dB K=-5 dB K=5 dB K=15 dB K=25 dB

p

Optimal numberof of used used channels Nused Optimal number channel Nopt

= B 100 MHz, N − 1 = 63, = BSC B= / N const.,

-10

-5

0

5 10 K-factor [dB]

J. Vucic, Ph.D. thesis, 2009

15

20

25

0

10

20

30

40

50

Number of used subchannels (best channels out of 63)


60



Dynamic rate adaptation J. Vucic, Ph.D. thesis, 2009 800

P = 400 mW, adaptive O



P = 100 mW, adaptive O

C [Mbit/s]

600

P = 400 mW, non-adaptive O

Non-adaptive system realizes worst-case performance only

PO = 100 mW, non-adaptive

P  N −1 B log 2  O2  N  PO1 

400

2



With adaptive rate, capacity depends on the LOS/NLOS ratio

200

0 -15

-10

-5

0

5 10 K-factor [dB]

15

= B 100 MHz, N − 1 = 63,

20



If LOS is free, capacity is higher



But the adaptive link is always on also in low-light/NLOS scenarios

25



Just the data rate is then reduced

= BSC B= / N const., = η 1 A/W Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


Summary on rate adaptive concept

©




Optical wireless channel suggests a rate-adaptive system concept



Gaussian input distribution and waterfilling are not appropriate



Strictly non-negative waveform  Upper bound using TMS



Adapt the link according to SNR and LOS/NLOS available



Adaptive approach enables robust operation in mobile scenario



29

Implementation using adaptive OFDM




Now we consider a practical optical wireless link



In 2005, we proposed OFDM with adaptive bit and power loading J. Grubor, V. Jungnickel, K.-D. Langer, and C. von Helmolt, “Dynamic data-rate adaptive signal processing method in a wireless infra-red data transfer system,” Patent EP1897252 B1, 24 June 2005. J. Grubor, V. Jungnickel, K.-D. Langer, „Capacity Analysis in Wireless Infrared Communication using Adaptive Multiple Subcarrier Transmission, ICTON We C2.7, 2005.

©



30


OFDM



Cyclic prefix insertion (CPI) transforms Toeplitz channel into a circulant matrix



Becomes orthogonal using IFFT/FFT



Often explained as parallel transmission over multiple orthogonal sub-carriers Graphs from Falconer et al. and Siemens

©



31


Discrete multi-tone 

OFDM yields a complex-valued waveform 

Use double-sized IFFT



Subcarriers in the upper side-band are complex conjugated and used again in the lower sideband



Yields a real-valued waveform



Complex-valued symbol-constellations with variable spectral efficiency can be used on each subcarrier (QPSK, 16-QAM, 64-QAM, …)

©



32

Adaptive bit- and power loading




Depending on the channel, sub-carriers are loaded with suitable modulation

R



6 2

Power is modified to adapt the SNR to the switching thresholds between the

4

modulation schemes 16QAM

f



64QAM QPSK

Loading: Hughes-Hartogs, Chow-CioffiBingham, Fischer-Huber, Krongold



Krongold is optimal, has low complexity

B.S. Krongold et al.,“Computationally Efficient Optimal Power Allocation Algorithms for Multicarrier Communication Systems,“ IEEE Trans. Commun., Vol. 48, No. 1, 2000

©



33


Controlled clipping 

Research tends to avoid clipping for OFDM



But many practical systems tolerate it and correct eventual errors





forward error correction (FEC)



retransmissions

Samples are clipped in the digital domain x (k)

Graph from NSN

[ X ]( N −1)×1

CS IFFT

X0

CL

GCP

LD

2X 0 X0 0



©

k

Link adaptation with controlled clipping 

Inner loop: Bit and power loading using a fixed modulation power



Outer-loop: Adapt the modulation power until a desired error rate is reached Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


34


Theoretical results J. Vucic, Ph.D. thesis, 2009



Red is the upper bound using TMS



Blue: 10% clipping probability yields gap ~2 dB to TMS



Green: Clipping is nearly avoided



Gaussian input distribution and waterfilling for all curves (not red)

Popt=400 mW, η=1A/W, B=100 MHz, N=64

©



35


Practical results J. Vucic, Ph.D. thesis, 2009



Gap is larger because less clipping is tolerated



Waterfilling with Gaussian input



Discrete bitloading with M-QAM



Powerful FEC + retransmissions are important

Popt=400 mW, η=1A/W, B=100 MHz, N=64

©



36

Summary on AOFDM with clipping

©




OFDM with adaptive bit- and power loading



Clipping is tolerated, while the probability is kept under control



At 10% clipping probability, gap to the bound is only 2 dB



Discrete modulation and less clipping tend to increase the gap



Efficient error correction is important (FEC, HARQ)



37

1st implementation: Transparent link

LED driver


Rx AMP 10BaseT

10BaseT

LED

VLC / IR channel Lighting / Power supply

 

Photodetector

Bidirectional link: White-LED (downlink) and infrared LED (uplink) LED drivers and receivers are optimized, but – bandwidth is not fully exploited, no rate adaptation, limited spectral efficiency of the Manchester code Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


38


On-off keying Error counter PRBS generat or

LPF dc

AMP

Tx

white LED

VLC channel

„blue“ filter AMP PD lens

Rx



Phosphor-type white LED: Blue is filtered out



Coverage is limited by color filter



125 Mbit/s with PIN, 230 Mbit/s with APD

J. Vucic, C. Kottke, S. Nerreter, K. Habel, A. Buettner, K.D. Langer, J. W. Walewski, „125 Mbit/s over 5 m wireless distance by use of OOK-modulated phosphorescent white LEDs,“ ECOC 2009.



39


First DMT experiments PC

AWG

OSC LPF

EOE channel lens dc AMP

Tx

white LED

Phosphor LED



APD Rx



35 MHz 3-dB

AMP

VLC channel

APD „blue“ filter

Information (bit) distribution [bit/s/Hz]



Rx

bandwidth 10

5

0 0

20

40

60 80 Subcarrier index

100

120

Measurements (R=513 Mbit/s) Simulations (R=604 Mbit/s) upper bound (C=757 Mbit/s)



128 subcarriers 100 MHz bandw.



513 Mbit/s

J. Vucic, C. Kottke, S. Nerreter, K. Langer, and J. Walewski, "513 Mbit/s Visible Light Communications Link Based on DMT-Modulation of a White LED," J. Lightwave Technol. 28, 3512-3518 (2010).



40


The potential of WDM AWG out 2 out 1

PC

OSC

RGB luminary

LPF

Figure shows red channel of the LED under test

dc AMP

R/G/B WDM filter

R dc

AMP

G AMP

coupler

dc

B

APD

VLC channel 1000 lx

AMP: amplifier AWG: arbitrary wave generator OSC: oscilloscope LPF: low-pass filter

lens



Commercially available RGB-type white LED (3 WDM channels)



Commercially available WDM bandpass filters



41


Bit and power loading for WDM

Frequency [MHz]

Frequency [MHz] 7.8

15.6

23.4

31.3

39.1

1.5

48.4

7.8

15.6

23.4

31.3

39.1

48.4

5

Power distribution [%]

Bit-loading mask, Rn [bit / subcarrier]

1.5 10 8

6

4 Red LED chip Green LED chip

2

1

5

10

3 dB 3 2 Red LED chip Green LED chip 1

Blue LED chip 0

4

15

20

25

31

Blue LED chip 1

5

DMT subcarrier index, n

10

15

20

25

31

DMT subcarrier index, n



Bit- and power-loading using uncoded BER ≤ 2∙10-3



~293+ Mbit/s (R), ~223+ Mbit/s (G), ~286+ Mbit/s (B)



WDM almost triples the throughput: 803 Mbit/s



1.25 Gbit/s at ECOC 2012

C. Kottke, J. Hilt, K. Habel, J. Vucic, and K. Langer, "1.25 Gbit/s Visible Light WDM Link based on DMT Modulation of a Single RGB LED Luminary," in Proc. ECOC 2012, We.3.B.4. Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


42


Recent record results 

Beyond 1 Gbit/s possible 

using WDM and DMT



3.4 Gbit/s is latest record

Optical Wireless Bitrates 100

?

G. Cossu et al., “3.4 Gbit/s visible optical wireless

Single Channel

transmission based on RGB LED,“ Optics Express, Dec.

Gbit/s

10 Multi-Channel

2012

Expon. (Max)



10 Gbit/s is on the horizon

1



higher bandwidth per color

D. Tsonev et al. “3-Gb/s Single-LED OFDM-based Wireless VLC Link Using a Gallium Nitride μLED",

0,1

PTL, Jan. 2014 0,01 Jan. 05

 Jan. 07

Jan. 09

Jan. 11

Jan. 13

Date of publication

Jan. 15

Jan. 17

MIMO

Jan. 19



Most results use off-line signal processing

©



43


Realtime DMT

K.D. Langer, J. Vučić, „Optical Wireless Indoor Networks: Recent Implementation Efforts,” ECOC 2010, WE.6.B.1



Real-time is mandatory for mobility



PHY: Synch. over the air, DMT with FEC and 100BaseT network interface



System running at 125 Mb/s (gross), 100 Mb/s (net), realtime video demo

©



44


Realtime demo



16 LED lamps covering an area ~10 m2



Broadcast of 4 HD videos (~80% utilization)



demonstrated at ORANGE Labs, Feb. 2011



©

see http://www.youtube.com/watch?v=AqdARFZd_78 Volker Jungnickel, Jelena Vucic, Klaus-Dieter Langer High-Speed Optical Wireless Communications


Reducing the form factor

©




Attract industry for VLC with realtime bidirectional DMT link



Entirely based on off-the-shelf components: Small volumes are available




Throughput vs. intensity



Adaptive DMT 

Throughput depends on the intensity



Robust against multipath & shadowing



First mobile optical wireless experience

L. Grobe et al. "High-speed visible light communication systems." IEEE Communications Magazine, Dec. 2013: 60-66.

©



47


Recent results

Throughput versus distance





Variable optics for different scenarios



2’’ lens at Tx: 200 Mb/s over 15 m



1’’ lenses: same over 2 m



Diffusely reflected NLOS works!



NLOS configuration

K.D. Langer et al. „Rate-adaptive visible light communication at 500Mb/s arrives at plug and play,” SPIE Newsroom, Nov. 2013 48 ©




LOS is no longer needed

K.D. Langer et al. „Rate-adaptive visible light communication at 500Mb/s arrives at plug and play,” SPIE Newsroom, Nov. 2013 49 ©



Summary on implementation

©




Higher-order modulation and DMT exploit the high SNR that is available



WDM triples the rate  VLC approaches technology limits at 3 Gbit/s



Higher rates are possible using more bandwidth and MIMO



Realtime implementation enables first mobile optical wireless experience



50

Future high-speed applications



Multiuser multicell MIMO 

Coverage in larger areas



Multiuser support



Mobility: hand-over btw. cells



Efficient interference coordination



Challenges:



©




Reduce complexity but to keep the performance high



Similar to 5G mobile networks

Optical wireless as playground for next generation cellular 

Ideal properties, simpler implementation, no license



Large continuous modulation bandwidth  high speed, low latency



51


Indoor navigation

S.Y. Jung et al., „Optical wireless indoor positioning system using light emitting diode ceiling lights,“Microwave and Optical Technology Letters, Vol. 54, No. 7, pp. 1622–1626, July 2012.



Exploits wide modulation bandwidth of LED lighting



Multiple transmitters are identified with high-speed sequences



Correlation + positioning algorithm at Rx side, 200 Mbit/s over 50 m with IR LED demonstrated with further potential



54

Machine-type communications



©


Flexible manufacturing cells 

Personalized production needs higher flexibility



2.4 GHz is only useful spectrum: Interference and jamming are harmful



Optical wireless is an interesting alternative



Main requirements are high speed, low latency, reliable connections



55

Summary on applications




Recent achievements turned optical wireless into a mobile technology



New applications where high speed is required 

Optical WLAN with integrated navigation



Low-cost backhaul for small cells



Industrial automation



Optical wireless offers high-speed, low latency and reliable connections



May be it is an alternative physical layer in 5G



COST Action 1101: OPTICWISE

©



56


Summary 

Robust optical wireless is possible with combined LOS and NLOS link



High-power LEDs became available at low cost  sufficient coverage



High-speed transmission using adaptive OFDM with controlled clipping



Potential for Gbit/s was demonstrated

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Realtime implementation enabled first mobile optical wireless experience

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Many useful applications besides optical WLAN

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Indoor navigation integrated with communications

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Car-to-car and machine-type communications

Optical wireless may be a physical layer in 5G



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Thank you very much for your attention. I am looking forward to answer your questions! Dr. Volker Jungnickel, Fraunhofer HHI Metro-, Access and Inhouse Systems Group [email protected] http://www.hhi.fraunhofer.de/pn Tel. +49 30 31002 768

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