Transcript Tripping the light fantastic-Optical Wireless Communications

Tripping the light fantastic:
Optical Wireless Communications
Sarah Kate Wilson
Santa Clara University
Outline
• Some history
• What’s different now?
• Challenges with Optical Wireless
•
•
How does it work?
Research challenges
• Summary
Indoor lighting and
communication
History
• In the beginning, there
was light
Optical communications
• TV remotes
• 1979, Gfeller and Bapst, Wireless diffuse
indoor communications
• Joe Kahn
• Mohsen Kavrehad
• And many others
Late 80’s early 90’s
• Wireless optical
research
• Mostly infrared
• Why didn’t we do it
then?
20 years ago –
Requirements and Background
• Wireless LAN was new
• Data needs were meager
• Mosaic and Netscape 1994
• Data needs opened up only
after Internet Browsers were
developed
What we know now about
Wireless LANs Then and Now
•
Then:
• Had to justify the need for wireless LANs
• One technology at a time
• Limited data rates
• Voice was the key technology; not data
•
Now
• Wireless LAN is everywhere
• Cellular providers using Wireless LAN to
supplement service
• Text, data, videos etc.
Technology 20 years ago
• CDMA and TDMA were the technologies
of choice
• Orthogonal Frequency Division
Multiplexing (OFDM) was just being
investigated for Wireless LAN
• Choices for Wireless LAN were radio
frequency or infrared
• LEDs existed, but…
Outline
• Some history
• What’s different now?
• Challenges with Optical Wireless
•
•
How does it work?
Research challenges
• Summary
What’s different now?
The need for speed and data
What’s different now
technically?
• LEDs brighter, faster, better
• New types of modulation
• 5G mm wave push
• Heterogeneity is key
• More avenues for data
5G mm wave
• Data needs are massive
• Spectrum is limited
• Push to use millimeter wave lengths
• Lots of spectrum
• Short reach
• Will require heterogeneity
•
Current methods and spectrum working with new
methods and spectrum
Indoor lighting can now use
LEDs: No additional
transmission energy
needed
Outline
• Some history
• What’s different now?
• Challenges with Optical Wireless
•
•
How does it work?
Research challenges
• Summary
Optical Wireless data
Visible Light versus Infrared
Visible light
Infrared
Can use light fixtures Additional
transmission energy
Downlink only?
Uplink and downlink
Light levels safe
Need to restrict
maximum level
How does optical stack up with RF?
Parameter
5GHz
60 GHz
Optical
10 meter free-space loss
44 dB
66 dB
86 dB
Additional loss due to oxygen
0
5-12 dB
0
Range (10 mW)
~35 meters
~5.5 meters <5
meters
Delay spread
250 ns (in a
big arena)
< 20 ns
<20 ns
How fast can the signal
change? (at 3 m/sec trot)
50 Hz
600 Hz
a few Hz
Visible Light Communications
• Does not replace radio frequency
communications
• Complementary
• An additional way to satisfy the need for
more data
Visible Light Communications
• Intensity Modulation (IM) not coherent
• Direct Detection (DD)
• Bandwidth limited by the
• Response of the LEDs and detectors
• The arrangement of the source and user
• Line-of-sight versus non-line of sight
Directed transmission –
sight but fails if the
Diffuse transmission – received
electrical power falls off as the
fourth power of distance (because
of intensity modulation)
transmitter and receiver
Dispersion due to reflected signals
works well with line of
are not aligned
Indoor lighting and
communication
Concept
Figure from
the Boston
University
Smart
Lighting
Group
Some scenarios
Some people who have contributed
to Optical Wireless
• Including but not limited to:
•
•
•
•
•
Joseph Kahn
Dominic O’Brien
Harald Haas
Steve Hranilovic
TDC Little
How did I get interested?
•
•
•
•
2006 Globecom; Jean Armstrong
OFDM
Physical Layer Fatigue
Opens up a whole new way of
thinking
My interest in OFDM:
Started in 1993
•
•
•
•
Channel Estimation
Wireless LAN
Synchronization
Scheduling
Outline
• Some history
• What’s different now?
• Challenges with Optical Wireless
•
•
How does it work?
Research challenges
• Summary
What makes it fun?
• Modulation is totally
different
• Real and positive
signal
• Handover issues
• Interference issues
Visible Light Communications
• Short range
• Intensity-modulated (IM) Direct Detection
– Baseband modulation
– Rate of change NOT a function of carrier
frequency
– Channel and signal is real and positive
Optical Wireless
• Single-carrier
• Pulse amplitude modulation: no Light, Light,
brighter, brightest – 2 bits
• OFDM solutions:
• Must modify to fit the real, positive channel
Single-carrier
Power
frequency
OFDM
Power
frequency
Single-Carrier
Multicarrier: conceptually a
series of chords
OFDM
• Uses an FFT to modulate the tones
• Cyclic prefix
– Preserves orthogonality
– Prevents interblock interference
• Power assignment, bit-loading
OFDM and optical wireless
• Must be real:
• Hermitian symmetric
• Must be positive
• DC –biasing
• Reserve carriers to mitigate negative parts
and bias
• Asymmetrically-Clipped Optical (ACO)OFDM
• Consider bandwidth versus power
ACO-OFDM
• Asymmetrically-Clipped Optical (ACO) Wireless
OFDM
• Armstrong and Lowery, “Power Efficient Optical
OFDM,” Electronics Letters, 2006.
• Only modulate on the odd tones; then clip.
• More power efficient than DC biasing!
• BUT half the bandwidth of DC biasing.
ACO-OFDM 32 tones
ACO-OFDM 16 QAM,128
tones
ACO-OFDM
Why does ACO-OFDM work?
Or just read the paper by Wilson and Armstrong
2009
• Odd tones implies odd symmetry
– If x_n >0 then mod(n+N/2) points away we have -x_n
• Complex exponential of 2(k+1)(n+N/2) is equal to the
negative of the complex exponential of 2(k+1)n
• Adding a negative to a negative makes a positive!
•
the DFT of the clipped signal is equal to the DFT of its
complement.
Rate vs. Power
• Want fast rate
• Want low power
• Traditionally increasing power by 6 dB will add
2 bits per dimension
• E.g. 2 bits – QPSK, need an additional 7 dB
for 4 bits – 16 QAM
• Increasing bandwidth by 2 doubles the rate
Comparing Single Carrier and ACO-OFDM:
Joint work with D.F. Barros and J.M. Kahn – 1 bit per symbol
Required Electrical SNR (dB)
30
25
ACO-OFDM (Rs)
20
15
OOK MMSE-DFE (Rs)
10
5
0.001
0.01
0.1
DT 
RMS Delay Spread
Bit Duration
1
10
Why do people like OFDM?
• Flexibility
• No equalization
• Simpler filters at the receiver
Challenges with OFDM
• Peak-to-average power ratio
• LEDs
• Detectors
• Dimming
• Driving the LEDs
• Different levels changing quickly
• Drive all LEDs together?
• Treat each LED like a bit and turn on/off
individually
ACO-OFDM 16 QAM,128
tones
LED configurations
Change intensity via individual
LEDs
Things to consider
• Physical layer modulation
• Efficiency; Peak-to-average power
• Intensity modulation restrictions
• Bandwidth
• Heterogeneity
• Multiple light sources
• Handover
• Scheduling
Some recent work
• Joint work with Lam, Elgala and Little of
Boston University
• Find a way to use the “un-used” carriers
in ACO-OFDM
• Spectral and Energy Efficient (SEE)
OFDM
SEE OFDM
• Given N subcarriers
• ACO modulates N/4 subcarriers
• SEE OFDM then adds additional
components
• 1st: Modulate N/8 of the unused subcarriers
• 2nd: Then add N/16 of the rest of the unused
subcarriers
• Etc
• Use successive decoding at the receiver
SEE OFDM
Scheduling for OFDM
• Multiple users in a cell – all want access
• Scheduling for RF OFDMA systems
– Multi-user diversity
– Fairness
• But real and positive channel changes
how we approach it
Single-carrier Scheduling
4
User 1
User 2
User 3
User 4
Maximum
3.5
3
SNR
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
60
time (ms)
70
80
90
100
Scheduling with RF-OFDM
one time instant…
User 1
User 2
User 3
Frequency
Scheduling in RF OFDM versus
optical OFDM
• RF
• Users send their best clusters
• The base-station scheduler assigns clusters
of spectrum based on reported SNRs
• Very little chance of collision
• Optical
• Optical wireless channel is low-pass
• All users will pick their lowest frequency
• Collisions!!
What to do?
Pick clusters semi-randomly
– The user has a certain desired rate
– Find all clusters that can deliver that rate
– Send best two cluster indices that are
‘good enough’
Scheduling for Optical-OFDM
• In RF scheduling
– Cluster subcarriers
– Send back index of cluster with best SNR
and/or most need
• In Optical-OFDM
– The best cluster is most likely the lowest
frequency cluster
– All users will be trying to send the same
cluster
Single-carrier Scheduling
4
User 1
User 2
User 3
User 4
Maximum
3.5
3
SNR
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
60
time (ms)
70
80
90
100
Scheduling with RF-OFDM
one time instant…
User 1
User 2
User 3
Frequency
What to do?
Pick clusters semi-randomly
– The user has a certain desired rate
– Find all clusters that can deliver that rate
– Send best two cluster indices that are
‘good enough’
Full-buffer Throughput
Conclusions
•
Visible Light Communication is an exciting new field
•
Opportunities in both the physical and upper layers of
communications
•
Spectrum is limited; demand is growing
• Future unknown applications will demand more
innovation
•
Talk to people at workshops/conferences
• Can lead to exciting research
Peak-to-Average Power ratio reduction
• Peak-to-Average Power Ratio is a problem
in OFDM
• Good Solution: Use Single-Carrier
Frequency Domain Multiplexing with ACOOFDM (Joint work with Acolatse and BarNess)
Peak-to-Average Power ratio reduction
High Peaks can play havoc with powerlimited LEDs
Name
Method
Number of Number
subcarrierr of
s
repetitio
ns
ACOSCFDE
Modulate
odd of half
subcarriers
N/4
1
RCOOFDM
Modulate
half
subcarriers
N/2
2
DQOOFDM
Modulate
All
subcarriers
N
4
SCFDE and ACO-OFDM
• SCFDE combined with ACO-OFDM
ck
FFTN/4
Map to odd
Subcarriers
Hermitian
symmetry
IFFT
Clip
Negative
Values
.1 W Power
1 W Power
Conclusions
• Higher frequency for the last few
meters
• Higher rates; less interference
• Optical -- Real channel; Low Doppler
• Difference in modulation and channel
leads to neat problems
References
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P. Smulders, “Exploiting the 60 GHz Band for Local Wireless Multimedia
Access: Prospects and Future Directions”, IEEE Comm. Magazine, January
2002.
R. Daniels and R. Heath,” 50 GHz Wireless Communications: Emerging
Requirements and Design Recommendations,” IEEE Vehicular Technology
Magazine, September 2007
V. Chandrasekhar, J. Andrews, A. Gatherer, “Femtocell Networks: A Survey,”
IEEE Communications Magazine, September 2008.
C. Anderson and T. Rappaport, “In-Building Wideband Partition Loss
Measurements at 2.5 and 60 GHz,” IEEE Transactions on Wireless
Communications, May 2004.
J. Armstrong and A. Lowery, “Power efficient optical OFDM”, Electronics
Letters, March 2006.
J. Kahn, W. Krause and J. Carruthers, “Experimental Characterization of
Non-Directed Indoor Infrared Channels”
ACO-OFDM
Simulations
• Channels with random number of taps,
random tap positions
• Compare equal constellation to low-frequency
bit-loading over 5000 different channels for
each SNR
• Equal number of bits/word on each channel
Scheduling with RF OFDM
More than one value to choose from
8
6
4
2
0
100
150
50
100
50
time (ms)
0
0
frequency (kHz)
ACO-OFDM Scheduling
4-PAM vs. ACO-OFDM
2 bit/symbol, Equal or Unequal Symbol Rates, With CSI
45
Required Electrical SNR (dB)
40
ACO-OFDM (Rs)
35
30
25
4-PAM MMSE-DFE (Rs)
20
ACO-OFDM (2Rs)
15
10
0.01
0.1
1
RMS Delay Spread
DT 
Bit Duration
ACO-OFDM at 2Rs best on all channels.
ACO-OFDM at Rs worst on all channels.
10
4-PAM vs. Three OFDM Techniques
2 bit/symbol, Equal or Unequal Symbol Rates, With CSI
1.2
1
CDF(SNR)
0.8
ACO-OFDM (2Rs)
PAM-DMT (2Rs)
0.6
DC-OFDM (Rs)
ACO-OFDM (Rs)
PAM-DMT (Rs)
0.4
0.2
0
10
DC-OFDM (2Rs)
4-PAM MMSE-DFE (Rs)
15
20
25
30
35
Required Electrical SNR (dB)
40
45
ACO-OFDM (or PAM-DMT) at 2Rs best on all channels.
ACO-OFDM (or PAM-DMT) and DC-OFDM at Rs worse than OOK on all channels.
Optical wireless scenarios
(courtesy
J. Kahn)
OFDM without Channel State Information
Hmean f  
1
Nrealiz
Nrealiz
H f 
k
k 1
OOK vs. ACO-OFDM
1 bit/symbol, Without Channel State Information
120
100
ACO-OFDM (CB Loading)
Poutage (%)
80
60
ACO-OFDM
(Equal Loading)
40
OOK
MMSE-DFE
20
0
5
10
ACO-OFDM
(Mean Channel Loading)
15
20
25
30
Required Electrical SNR (dB)
35
40
OOK outperforms all OFDM techniques at moderate-to-low outage probabilities.
For OFDM, ceiling bounce loading is best at low outage probabilities.