Plenoptics for optical wireless sensor networks

Victor Guerra, Cristo Suarez-RodriguezRafael Perez-Jimenez

Institute for Technological Development and Innovation in CommunicationsUniversity of Las Palmas de Gran Canaria, Spain

Email: vguerra@idetic.eu

Silvestre RodriguezJose Manuel Rodriguez-Ramos

CAFADIS GroupUniversity of La Laguna, Spain

Email: jmramos@ull.es

Abstract—In this paper, a new approach for ad-hoc wirelessoptical sensor networks is presented. It is based on using simplecameras as receivers for non-addressable, low baud rate sensors.We also propose the use of plenoptic cameras so as to reducethe near-far induced penalty present on this kind of networks.This architecture reduces receiver complexity and simplifies thediscovery procedure. This network strategy can be combined withVLC or RF systems so as to be used in conventional-bidirectionalnetworks.

I. INTRODUCTION

Although most used wireless sensor networks are currentlybased on radio frequency systems [1] [2], Wireless OpticalCommunications are becoming an alternative to RF technologyin some specific applications scenarios [3]. It is not only dueto data security considerations, but also to their capabilityto work where the use of EM emissions is restricted (e.g.planes and industrial facilities), where severe security consider-ations limit the use of interference sensitive devices (as insidemilitary vehicles) or simply where RF bands are becomingsaturated (hotels or administrative areas). There are even someenvironments where and optical signal simply presents bettertransmission behaviour than RF ones, as underwater channels.

In this paper, the feasibility of implementing an ad-hoc, lowcost optical sensor network using non-addressable devices isstudied. The receiver will be based on commercial, COST CCDcameras and image processing techniques. We also explore thepossibility of using advanced cameras (e.g. plenoptic systems)as a way of avoiding the near-far and shadowing problem atthe receiver. This proposal is not currently covered by thecommercial standards under development but promises to bea very accurate alternative in several application scenarios.

The proposed wireless optical sensor network architecturereduces the node complexity and power consumption. Themain functions of the network now rely on the access point(node discovery and data signal reception and storage), re-ducing the required storage availability in the nodes. In thisarchitecture only the uplink (based in an infrared or visible-light link) is implemented without synchronization require-ments. This system allows simultaneous transmissions overmultiple sensors and presents lower power consumption, dueto the lower complexity required by the sensor nodes.

This paper is organized as follows: in the following sectionthe fundamentals of ad-hoc non-addressable networks arepresented from an optical wireless sensor network point ofview, introducing some potential advantages and drawbacks.The use of simple cameras as receivers for low-rate optical

sensor networks is also discussed in section II, and the po-tential improvements in their behaviour induced by the useof plenoptic systems is commented in section III. Section IVpresents application scenarios in which nowadays there is agrowing interest on this kind of solutions (domotics, hospitals,military vehicles, etcetera), then some conclusions are finallygiven

II. OPTICAL WIRELESS SENSOR NETWORKS

Sensor networks are known as a group of devices whichhave detection, processing and communication capabilites.Wireless optical links are commonly defined as IM/DD inwhich optoelectronic conversion modules are needed. Focusingon the receiver, silicon-doped semiconductors have gainedpopularity over a broad range of applications.

Moreover, ad-hoc networks do not need any pre-existentinfrastructure so data is forwarded among nodes. These net-works are suitable for applications where quick deployment iscrucial like in emergencies.

In addition, non-addressable nodes are autonomous enoughfor sending information when new data is collected, whichmakes them highly recommended for camera-based networkswhere the main node does not need to poll sensor nodes.

Finally, optical wireless communications offer a straight-forward way to combine quick deployment nodes and ad-hocnetworks. There are different ways to implement the main nodewhich has to handle the uplinks and the medium access. Figure1 depicts the reference scenario in indoor optical wirelesssensor networks.

Fig. 1. Optical wireless sensor network scenario

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1) Conventional photodetectors: Traditionally there aretwo options, PIN and APD photodiodes. The main advantageof the latter is a higher photocurrent due to the avalanchebreakdown even though needs temperature compensationschemes and higher inverse polarization.

Both devices are faster than their counterparts such asphototransistors, image sensors, etcetera. However, as theoptical power is integrated over the photodiode’s surface,there is no option to distinguish among different sources.Therefore, media access techniques are applied for multi-userwhen needed. For instance, THSS [4].

2) Camera-based networks: In [5], an image sensor com-munication system is presented relying on a CMOS sensor.Despite of the fact that the frame rate is not as high, eachpixel represents a potential communication channel. In fact,a CMOS sensor can also be viewed as a two-dimensionalarray of photodiodes. Furthermore, inherent spatial separationminimizes interferences and offers positioning capabilities.Nonetheless, it is not cost-effective for low cost deploymentswhere several photodiodes have similar performance.

III. PLENOPTIC SENSORS

A traditional camera uses a convergent lens to concentratethe rays which come from adjacent points in the scene intoa single pixel of the sensor. On the other hand, a plenopticcamera splits those rays which come from the same positioninto bundles of rays that present different angular directions.This effect is achieved by the use of a microlenses array andcan be observed in figure 2.

Fig. 2. Example of a plenoptic frame

As plenoptic cameras capture ’four-dimensional’ light-fieldinformation, it is possible to recreate a volume of imagescovering a range of focusing distances using a single frame [6].This is known as focal stack. Although this kind of cameras arecurrently used in distance measurement and stereo 3D appli-cations, the main advantage from the point of view of opticalsensor networks is the possibility of refocusing the capturedimage at different focal distances. This refocusing allows thereceiver to scatter possible interferences and therefore increasethe signal-to-noise ratio of a point of interest.

A. Contribution to optical wireless sensor networks

As it was commented above, the main advantage of the useof plenoptic cameras as receivers is the possibility of increasingthe signal-to-noise ratio of regions of interest.

Near-far situations, where two emitters are located inthe same direction at different distances, normally imply the

shadowing of one of the nodes. As the projections of the twoemitters over the sensor are located at the same region ofinterest, the nearer node will be considered as the only nodeat that direction. If the shadowed node were intended to bediscovered, a MAC layer protocol should be introduced in thesystem, and a downlink would be needed. In the case of non-addressable networks, a downlink is not present an this effectmust be solved at the receiver.

If a plenoptic sensor were used as the main node, refo-cusing the captured frames at the closest focal distances foreach node would increase the signal-to-interference ratio. Inthis case, it would be possible to discriminate the informationsent by each emitter. Figure 3 shows the result of a conceptexperiment were two WLED are located at close directions butat different distances. After capturing the plenoptic frame andrefocusing at the front WLED it can be observed that the rearone is scattered and its intensity seems to lower. Nevertheless,if the refocusing is made at the rear WLED, it can be observedthat the front WLED power is scattered enough to establish athreshold.

Fig. 3. Near-far situation and refocusing on three different planes located atthe front WLED (upper), central focal distance (center) and rear LED (lower)

Furthermore, a plenoptic sensor allows three dimensionalpositioning. Two dimensions are obtained from the position ofthe pixel inside the sensor and the third coordinate is inferredfrom the focal stack.

In order to establish the processing parameters, during thediscovery stage of the main node protocol, the focal distancesand the regions of interest should be determined.

IV. APPLICATION SCENARIOS

Optical wireless communications are normally associatedto environments where EM emissions are restricted, such as

hospitals, planes, underwater scenarios or military vehicles.Furthermore, optical signals are intrinsically secure againsteavesdropping. The light rays are confined by the walls ofindoor environments, and to capture information the eaves-dropper must be in the middle of the communication link.

Sensor networks are commonly used to gather differenttypes of information from a given scenario. Camera-basedoptical sensor networks take advantage of its inherent spatialdivision to allow quick deployments of nodes inside an indoorenvironment. As it was commented, when near-far situationsoccur, traditional sensors cannot decide the origin of theinformation without a MAC layer protocol. Plenoptic sensorsallow less critical deployment localizations as the signal-to-noise ratios and the adjacent channel interference performancecan be enhanced simultaneously for all the nodes.

Plenoptic-based sensor networks may be used as an alter-native to traditional optical sensor networks in environmentswhere high density deployments are needed. The growinginterest in internet-of-things points at a future scenario of highpopulated networks, where near-far conditions could be usual.Also, plenoptic sensors may be used to avoid shadowing effectsdue to a mobile device by refocusing the acquired image onthe plane of interest.

V. CONCLUSIONS

In this paper, a new concept of main node of an opticalwireless sensor network has been presented. Plenoptic sensorshave been proposed as an enhancement of the current camera-based main nodes. This kind of sensor minimizes the effectof near-far situations as it has been shown in figure 3. Fur-thermore, as the refocusing is made at certain planes, eachsensor node can be localized with three spatial coordinates (2dimensions from the pixel and one from the focal distance)instead of two as common systems allow.

Several application scenarios has been also commented.The main advantage of plenoptic sensors networks lies in highdensity deployments, where shadowing and near-far conditionswould be presumably typical.

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