Multi-Element Array Antennas for Free-Space Optical (FSO) Networks

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Multi-Element Array Antennas for Free-Space
Optical (FSO) Networks
D/N

Node 1
Node 2
Repeater 1
Repeater 2
Repeater N-1

• Jayasri Akella, Murat Yuksel, Shiv Kalyanaraman
• Rensselaer Polytechnic Institute (RPI), ECSE
• akellj_at_rpi.edu, yuksem_at_ecse.rpi.edu,
  shivkuma_at_ecse.rpi.edu

shiv rpi
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Outline

• Background and Motivation
• Introduction to free-space optical communication
  using 2-dimensional arrays
• Inter-channel interference in 2-dimensional
  arrays
• Channel capacity between arrays
• Bandwidth-Volume product
• Audio Mixing Experiment with a 2-Channel FSO
  system
• Conclusions

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Why FSO Arrays?

• High aggregate bandwidth spatial reuse
  (multiplexing space) to RF-MIMO
• Link robustness due to spatial diversity leverage
  spatial channel coding
• Optical transceivers are capable of operating at
  bandwidths greater than 100 Mbps.
• With each transceiver operating at a speed of 100
  Mbps, a 1010 array will give 10 Gbps in
  aggregate capacity.
• Use inexpensive off the-shelf opto-electronic
  components
• But cross talk due to inter-channel interference

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Laser/HBLED Beam Profile

• Lateral distance Y, from the axis of the laser
  beam
• Horizontal distance Z
• Received Intensity here is given by I(Y).
• Thus intensity drops off exponentially with Y.
• gt motivation to closely pack transceivers on the
  2-d array.

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Eg Indoor Music Transfer w/ FSO
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2-Dimensional FSO Arrays Parameters

• Parameters of the array
• Package density of the optical transceivers ?
• Distance between the arrays d
• Angle of the transceiver ?

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2-D FSO arrays (contd)

• Consider the transmission from the transceiver T0
  on the array A, TA0 to T0 on the array B, TB0
• A cone from the transceiver TA0 onto the array B
  defines field of view.
• The cone not only covers the intended receiver
  TB0 , but also TB1 , TB2 , TB4 , TB7 .
• gt possible interference, cross-talk

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2-D FSO arrays (contd)

• Similarly TB0 receives the signal from TA1 , TA2
  , TA4 , TA7.
• gt potential interferers.
• For N interferers, cross talk occurs at TB0 if
  the intensity from them exceeds IT .
• Because the intensity across the laser is
  Gaussian distributed, all the potential
  interferers may not be contributing to cross talk.

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Fresnel Lens At Xmit/Rcvr ? ?, ??
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Inter-channel interference in 2-dimensional arrays

• Define a lateral distance on the array
• The transceivers on the array must be spaced more
  than .
• If is
• Distance is 100 meters,
• Transceiver angle of 1 mrad
• gt is about 40 cms.
• gt We cannot pack the optical transceivers too
  closely on a compact array.

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Inter-channel interference continued

• Let the spacing on the arrays be
• Let there be N potential interferers for the
  transceiver TB0
• Crosstalk occurs at TB0 when

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Inter-channel interference continued

• Interference happens when a subset of these
  potential interferers transmit when TA0 is
  transmitting. The probability that such an event
  occurs gives the error probability due to
  crosstalk.
• Where p0 is the probability that a ZERO is
  transmitted.

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Package density (?) ? gt Error Probability ?
But, distance (d) ? allows ? package density (?)
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Tighter beams (? divergence angle ?) gt ? package
density ?
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Channel capacity Between Arrays

• Error due to inter-channel interference occurs
  only when a ZERO is being transmitted by TA0 to
  TB0, so the interference channel is asymmetric.
  ON/OFF Keying
• Thus, the cross talk for the channel between two
  arrays can be modeled as a Binary Asymmetric
  Channel (BAC).

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Capacity of Binary Asymm. Channel (BAC)

• The capacity of such a BAC is given by
• The maximizing input distribution can be found by
  plotting the above for various pe.

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• The capacity of array drops with increasing
  package density.
• The drop is more rapid at larger distances.

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The capacity of arrays drops with package density
and divergence angle of the transceivers.
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All is not gloomy

• Though per-channel capacity decreases with (?
  package density, ? distance and ? divergence
  angle)
• For specific points on the capacity curve,
• the aggregate capacity of array is higher than
  a single channel
• Example
• 5 channel array, each channel _at_ 100 Mbps gt an
  aggregate bandwidth of 0.5 Gbps.
• In same space, 10 channels, each operating at
  3/4ths of its capacity and with an aggregate
  bandwidth of 0.75 Gbps.
• We introduce a metric to measures the
  effectiveness of the 2-D array Bandwidth-Volume
  Product.

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Bandwidth-Volume Product

• We define the performance of an FSO communication
  channel by three design parameters
• Number of channels per array
• The capacity of each of the channel in bits per
  second (determined by packing density, distance,
  angle etc)
• The distance over which the arrays can
  communicate with that capacity.
• BVP is similar to the Bandwidth-Distance
  Product metric of a fiber-optic link.
• In a fiber-optic link, the fiber dispersion
  adversely effects the aggregate capacity,
• whereas in the multi-channel FSO link, it is
  interference cross-talk

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Bandwidth-Volume Product vs Package density and
Inter-Array Distance
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Bandwidth Volume Product (Contd)

• BVP is a useful metric that integrates all the
  design parameters of the multi-element 2-D array
  system.
• The curve (BVP vs package density) gives us
  design guidance regarding which parameters to
  choose for the 2-D array communication.
• For example, for a 200 meter range,
• the optimal package density
• 25 channels per square meter (2.5Gbits),
• at a transceiver angle of 1.5 m rad.

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Two Channel Experiment Audio Mixing
a) Two transmitters on different channels
b) Single receiver and circuit for both the
channels
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Typical Link with Marktech Photo Diode on the
receiver side
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Typical Receiver circuit operated on batteries as
power supply
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Conclusions

• 2-dimensional arrays give an very good bandwidth
  performance over short range (100s of meters)
  free-space optical communications.
• To use these arrays over long distances outdoors,
  very narrow beams coupled with auto-aligning
  mechanisms are needed.
• We are experimenting with new optical (hardware)
  modules to manage the critical 2-d array
  parameters
• Multi-hop transmission is a natural way to extend
  range.
• Bandwidth-Volume product is a useful metric that
  provides design guidance on the optimal
  implementation of the 2-D arrays.

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Thanks!
Students Jayasri Akella, sri_at_networks.ecse.rpi.e
du Dr. Murat Yuksel (post-doc)
yuksem_at_ecse.rpi.edu
shiv rpi
Ps Online free videos of all my advanced
networking classes
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Details
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A typical FSO communication system

• Free-space as medium of transmission
• ON-OFF keying digital modulation of the light
  beam at the transmitter.
• The receiver is a threshold detector. Outputs a
  ONE if the received intensity I gt IT , and a
  ZERO if I lt I T. where IT is pre-set threshold
  intensity level.
• Typically duplex communication

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Inter-channel interference continued

• We define the package density for which
  there is no crosstalk such that
• The total number of potential interferers for
  a package density is given by
• N transceivers, includes TA0 and N-1 potential
  interferers for TB0.

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• Let us assume that these N-1 transceivers are
  distributed along J circles around TA0. We can
  calculate the error probability in terms of each
  of these J circles either being ON (transmit a
  ONE) or OFF (transmit a ZERO).
• The number of circles for a transceiver spacing
  of YSep is given by
• The radius of the jth circle rj is given by
  
• The number of transceivers on the jth circle is a
  function of the package density and is given by

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• Crosstalk can happen only when the transceivers
  on each of the J circles are such that
  
• Consider the following cases
• TA0 transmits a 0 and Kj transmit a 0
• TA0 transmits a 0 and Kj transmit a 1
• TA0 transmits a 1 and Kj transmit a 0
• TA0 transmits a 1 and Kj transmit a 1
• Interference happens only in the above Case2,
  since only then TB0 receives a false threshold.
  The probability of such an event is given by
• Where p0 is the probability that a ZERO is
  transmitted.

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Design guidelines for 2-D arrays

• As the package density increases, the error
  probability increases and hence the capacity
  decreases.
• The specific package density at which the
  capacity drops is a function of the distance
  between the arrays, and the angle of the
  transceivers and the specific arrangement of the
  transceivers on the array. The figures
  demonstrate the behavior of the capacity for a
  uniformly spaced transceiver configuration.
• We can choose the package density such that each
  channels operates at a full capacity.
  Alternatively, we choose a package density
  wherein each channel operates at a lower capacity
  point and gets a higher aggregate bandwidth due
  to multiple operating channels.
• For example, we can choose an array with 5
  transceivers, each operating at 100 Mbps each,
  with an aggregate bandwidth of 0.5 Gbps.
  Alternatively, we can pack 10 transceivers, each
  operating at 3/4ths of its capacity and with an
  aggregate bandwidth of 0.75 Gbps.

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Bandwidth-Volume Product

• Bandwidth denotes the capacity of a single
  channel, i.e. the unit of Bandwidth is Mbps.
  Volume describes the 2-dimensional nature of
  the array and the distance over which they can
  communicate. The volume is simply multiplication
  of the number of channels on the array and the
  communication distance, i.e. the unit of the
  Volume here is meter. This means unit of BVP is
  Mbps-meter
• The advantage of BVP is that it provides an
  integrated performance evaluation measure to aid
  the decision process for choosing various
  parameters (e.g. d, ?) of the multi-element FSO
  system. The distance of operation, number of
  channels should be carefully chosen to achieve
  the desired capacity.
• Even if each of the channel is not operated at
  full capacity, one can still achieve high bit
  rates due to the presence of multiple
  simultaneous transmissions.