Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 Multi-Element Array Antennas for Free-Space Optical (FSO) Networks Jayasri Akella, Murat Yuksel, Shiv Kalyanaraman Rensselaer Polytechnic Institute (RPI), ECSE : “shiv rpi” Node 1 Node 2 … Node 1 Node 2 Repeater 1 Repeater 2Repeater N-1 D D/N

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

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3 Why FSO Arrays? q High aggregate bandwidth: spatial reuse (multiplexing space): ~ to RF-MIMO q Link robustness due to spatial diversity leverage & spatial channel coding q Optical transceivers are capable of operating at bandwidths greater than 100 Mbps. q With each transceiver operating at a speed of 100 Mbps, a 10×10 array will give 10 Gbps in aggregate capacity. q Use inexpensive off –the-shelf opto-electronic components q But… cross talk due to inter-channel interference Node 1 Node 2 D

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4 Laser/HBLED Beam Profile q Lateral distance ‘Y, from the axis of the laser beam q Horizontal distance ‘Z’ q Received Intensity here is given by I(Y). q Thus intensity drops off exponentially with Y. q => motivation to closely pack transceivers on the 2-d array.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5 Eg: Indoor Music Transfer w/ FSO

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 6 2-Dimensional FSO Arrays: Parameters q Parameters of the array: q Package density of the optical transceivers ‘ρ’ q Distance between the arrays ‘d’ q Angle of the transceiver ‘θ’

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 7 2-D FSO arrays (contd) q Consider the transmission from the transceiver T 0 on the array A, T A 0 to T 0 on the array B, T B 0 q A cone from the transceiver T A 0 onto the array B defines field of view. q The cone not only covers the intended receiver T B 0, but also T B 1, T B 2, T B 4, T B 7. => possible interference, cross-talk

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8 2-D FSO arrays (contd) q Similarly T B 0 receives the signal from T A 1, T A 2, T A 4, T A 7. => potential interferers. q For ‘N’ interferers, cross talk occurs at T B 0 if the intensity from them exceeds I T. q Because the intensity across the laser is Gaussian distributed, all the potential interferers may not be contributing to cross talk.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9 Fresnel Lens At Xmit/Rcvr:  , 

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 10 Inter-channel interference in 2-dimensional arrays q Define a lateral distance on the array q The transceivers on the array must be spaced more than. q If is; q Distance is 100 meters, q Transceiver angle of 1 mrad q => is about 40 cms. q => We cannot pack the optical transceivers too closely on a compact array.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 11 Inter-channel interference continued q Let the spacing on the arrays be q Let there be N potential interferers for the transceiver T B 0 q Crosstalk occurs at T B 0 when

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 12 Inter-channel interference continued q Interference happens when a subset of these potential interferers transmit when T A 0 is transmitting. The probability that such an event occurs gives the error probability due to crosstalk. q Where p 0 is the probability that a ZERO is transmitted.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 13 Package density (  )  => Error Probability  But, distance (d)  allows  package density (  )

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 14 Tighter beams (  divergence angle  ) =>  package density 

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 15 Channel capacity Between Arrays q Error due to inter-channel interference occurs only when a ZERO is being transmitted by T A 0 to T B 0, so the interference channel is asymmetric. [ON/OFF Keying] q Thus, the cross talk for the channel between two arrays can be modeled as a Binary Asymmetric Channel (BAC) p e pepe 1 X Y

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 16 Capacity of Binary Asymm. Channel (BAC) q The capacity of such a BAC is given by q The maximizing input distribution can be found by plotting the above for various p e.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 17 The capacity of array drops with increasing package density. The drop is more rapid at larger distances.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 18 The capacity of arrays drops with package density and divergence angle of the transceivers.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 19 All is not gloomy… q Though per-channel capacity decreases with (  package density,  distance and  divergence angle)… q For specific points on the capacity curve, … q … the aggregate capacity of array is higher than a single channel q Example: q 5 channel array, each 100 Mbps => an aggregate bandwidth of 0.5 Gbps. q In same space, 10 channels, each operating at 3/4 ths of its capacity and with an aggregate bandwidth of 0.75 Gbps. q We introduce a metric to measures the effectiveness of the 2-D array: “Bandwidth-Volume Product”.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 20 Bandwidth-Volume Product q We define the performance of an FSO communication channel by three design parameters: q Number of channels per array q The capacity of each of the channel in bits per second (determined by packing density, distance, angle etc) q The distance over which the arrays can communicate with that capacity. q BVP is similar to the “Bandwidth-Distance Product” metric of a fiber-optic link. q In a fiber-optic link, the fiber dispersion adversely effects the aggregate capacity, … q … whereas in the multi-channel FSO link, it is interference & cross-talk

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 21 Bandwidth-Volume Product vs Package density and Inter-Array Distance

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 22 Bandwidth Volume Product (Contd)… q BVP is a useful metric that integrates all the design parameters of the multi- element 2-D array system. q The curve (BVP vs package density) gives us design guidance regarding which parameters to choose for the 2-D array communication. q For example, for a 200 meter range, q the optimal package density q ~25 channels per square meter (2.5Gbits), q … at a transceiver angle of 1.5 m rad.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 23 Two Channel Experiment: Audio Mixing a) Two transmitters on different channels b) Single receiver and circuit for both the channels

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 24 Typical Link with Marktech Photo Diode on the receiver side

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 25 Typical Receiver circuit operated on batteries as power supply

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 26 Conclusions q 2-dimensional arrays give an very good bandwidth performance over short range (100s of meters) free-space optical communications. q To use these arrays over long distances outdoors, very narrow beams coupled with auto-aligning mechanisms are needed. q We are experimenting with new optical (hardware) modules to manage the critical 2-d array parameters q Multi-hop transmission is a natural way to extend range. q Bandwidth-Volume product is a useful metric that provides design guidance on the optimal implementation of the 2-D arrays.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 27 Thanks! : “shiv rpi” Students: Jayasri Akella, Dr. Murat Yuksel (post-doc): Ps: Online free videos of all my advanced networking classes

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 28 Details

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 29 A typical FSO communication system q Free-space as medium of transmission q ON-OFF keying digital modulation of the light beam at the transmitter. q The receiver is a threshold detector. Outputs a ONE if the received intensity I > I T, and a ZERO if I < I T. where I T is pre-set threshold intensity level. q Typically duplex communication

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 30 Inter-channel interference continued q We define the package density for which there is no crosstalk such that q The total number of “potential interferers” for a package density is given by q N transceivers, includes T A 0 and N-1 potential interferers for T B 0.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 31 q Let us assume that these N-1 transceivers are distributed along J circles around T A 0. 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). q The number of circles for a transceiver spacing of Y Sep is given by q The radius of the j th circle r j is given by q The number of transceivers on the j th circle is a function of the package density and is given by:

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 32 q Crosstalk can happen only when the transceivers on each of the J circles are such that q Consider the following cases: 1. T A 0 transmits a ‘0’ and K j transmit a ‘0’ 2. T A 0 transmits a ‘0’ and K j transmit a ‘1’ 3. T A 0 transmits a ‘1’ and K j transmit a ‘0’ 4. T A 0 transmits a ‘1’ and K j transmit a ‘1’ q Interference happens only in the above Case2, since only then T B 0 receives a false threshold. The probability of such an event is given by: q Where p 0 is the probability that a ZERO is transmitted.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 33 Design guidelines for 2-D arrays q As the package density increases, the error probability increases and hence the capacity decreases. q 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. q 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. q 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 3/4 ths of its capacity and with an aggregate bandwidth of 0.75 Gbps.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 34 Bandwidth-Volume Product q “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 q 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, θ) of the multi-element FSO system. The distance of operation, number of channels should be carefully chosen to achieve the desired capacity. q 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.