Paving the way for a future underwater omni-directional wireless optical communication systems

ARTICLE IN PRESS Ocean Engineering 36 (2009) 633–640

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Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng

Paving the way for a future underwater omni-directional wireless optical communication systems Greg Baiden a, Yassiah Bissiri a,b,, Andrew Masoti a a b

Laurentian University, Sudbury, Canada Penguin Automated Systems Inc., Sudbury, Canada

a r t i c l e in fo

abstract

Article history: Received 24 November 2007 Accepted 3 March 2009 Available online 29 March 2009

To lay down the foundation for an underwater omni-directional optical communication system for teleoperation, we tested a point-to-point optical communication system, using laser-emitting diodes (LEDs). The LEDs used in the test emitted light in the green and blue light spectrum and were tested in a pool and in a tank filled with lake water. The primary objective of these tests was to get profiles of the behaviors of such communication systems with respect to water characteristics such as turbidity levels, prior to building the proposed omni-directional optical communication. The results of the tests indicated that turbidity level, viewing angle and separation distance plays a significant role in the behavior of blue light in water. Furthermore, it was possible to graph the profile of the behavior of light with respect to the parameters of interest. The results of the tests and related research are discussed in this paper. Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.

Keywords: Optical communication Wireless Scatter Attenuation Turbidity Data rate Viewing angle Ocean Exploration Underwater construction

1. Introduction This research was inspired by the need to explore different options with respect to the future of the mining industry, but it is also applicable to other industries such as construction, military and underwater inspections. Current mining activities occur on dry surface, where exploration is conducted and mining operation for extraction follows as soon as it is proven that a given deposit can be economically exploited. Recent technological advances (such as equipment size, automation and better communication systems) have made it possible for mining companies to mine larger mineral deposits at a faster rate to respond to an ever increasing worldwide demand. It is clear that at this pace, the world will soon be faced with severe shortages of critical resources needed to sustain the world growing economies if nothing is done in the long run to address this critical issue (Farrell, 2008). Several solutions ranging from responsible consumption and sustainability to space exploration for natural resources have been proposed in the past (Dasgupta and Heal, 1979). If the first proposed solution is virtually impossible to take, the second is far from being achieved due to lack of appropriate advanced technologies (Muff et al., 2004).

 Corresponding author at: Laurentian University, Sudbury, Canada.

E-mail address: [email protected] (Y. Bissiri).

While these solutions are being investigated, it is urgent that accessible solutions be implemented to deal with the problem in the near future. Water covers nearly three quarters of the earth’s surface in oceans as well as rivers and lakes (Hirvonen, 1993) and therefore, developing a technology that will allow exploration, construction and mining underwater may be the immediate step to take while waiting for other solutions to be seriously investigated. Any underwater communication technology for exploration and mining (which will certainly require a swarm of tele-operated equipment) should take into account the following four important factors:

 real-time remote operation with equipment moving freely underwater,

 high data transfer rate for reliable information exchanges,  high bandwidth to handle several video channels and physical control parameters transmitted simultaneously

 low bit errors rate for data integrity Early underwater remotely operated vehicle (ROV) required umbilical cords to transmit power and relay information between an operator and a ROV. However, the length of the cable limits excursion distance and the fact that several ROV may be working in the same area produces the risk of entanglement, thereby limiting the ability of freedom of movement crucial to accomplishing tasks in exploration, construction or mining. Recent

0029-8018/$ - see front matter Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2009.03.007

ARTICLE IN PRESS G. Baiden et al. / Ocean Engineering 36 (2009) 633–640

efforts have involved in developing networking protocols for wireless underwater acoustic communication (Akyildiz et al., 2004). However, this technology comes with severe limitations such as limited bandwidth (critical in operating multiple machines), high bit error rates due to multipath scenarios, impulse noise and higher latency as excursion distances become larger (Stojanovic, 2003). Radio frequency (RF) as a mean of underwater information carrier is limited by its high attenuation rate in water (Butler, 1978). Developed in the 1960s for military and aerospace applications, free space optical (FSO) communications have matured to the point that installations worldwide are on the rise (Garlington and Long, 2005). This growing interest in FSO is justified by the need for greater bandwidth and the security that the technology provides (point-to-point data transfer). FSO technology requires no spectrum licensing, and installation takes only days to complete when compared to fiber systems (Akella et al., 2005). Current FSO systems are capable of transmitting data at rates up to 2.5 Gbps over distances of several kilometers, which is more than enough for most broadband applications (Wee, 2004). These characteristics make FSO an attractive technology for underwater communication systems provided similar characteristics can be exported to underwater applications. The transmitter and receiver for an underwater link can be very similar to a FSO link in air, with the major difference being the wavelength of operation. However, ocean water has widely varying optical properties depending on location, time of day, organic and inorganic content, as well as temporal variations such as turbulence. To construct an optical link it is important to understand these properties. The loss of optical energy while traversing the link arises from both absorption and scattering. Scattering also adversely impacts the link by introducing multipath dispersion. Color and clarity of water, which is generally dependant upon its constituent components, may have a significant impact of the behavior of light in water (Apel, 1987). Individual trajectories of photons in a collimated beam of light passing through a container of water will result in the disappearance of some photons within the water due to absorption. The energy of these photons will be converted into heat. The trajectories of other photons will change suddenly due to scatter while maintaining their level of energy (Arst, 2003). Seawater is composed of primarily H2O, which absorbs heavily towards the red spectrum. It also contains dissolved salts like NaCl, MgCl2, Na2SO4, CaCl2 and KCl that absorb light at specific wavelengths. As shown in Fig. 1, pure

Absorption Coefficient of Seawater

Absorption (m^-1)

3

2

0.1

0.05

0 200

300

400

500 600 Wavelength (nm)

700

800

Fig. 2. Scatter coefficient of seawater (Bohren and Huffman, 1983).

seawater is absorptive except around 400–500 nm wavelength, which is the blue-green region of the visible light spectrum. Fig. 2 shows that minimum scattering occurs around 560–600 nm wavelength. FSO communication systems operating in an underwater environment face a number of challenges. Lasers or optical communication can achieve high data rate with limited distance because of loss due to photons absorption and scatter in water (Fletcher, 2000). The field of view, a small cross-sectional area of a laser point and an omni-directional light source may represent a serious challenge on its own as it dictates the range of operation for any future vehicle wanting to communicate with the light source and remain within a minimum light power limit for light detectors to generate a distinguishable signal. High-power lasers capable of achieving longer distances can consume a large amount of power which can be a problem for underwater remotely controlled systems. If FSO technology is to be used underwater, it is critical that the communication profile with respect to the water characteristic and operational environment be understood. This paper addresses the challenges of FSO technology applied to an underwater environment through tests performed in the laboratory condition. The tests were limited to three critical parameters: distance, turbidity and transmitter–receiver orientation (field of view (FOV)). These variables are easy to control in a laboratory conditions and represent the variables that are most significant during operation. Other parameters, such as water temperature and water chemistry, were not investigated in this paper.

2. Testing design and procedures

1

0 200

Scattering Coefficient of Pure Seawater

Scattering coefficient (m^-1)

634

300

400 500 Wavelength (nm)

600

700

Fig. 1. Absorption coefficient of seawater (Bohren and Huffman, 1983).

For these experiments a pair of optical transceiver, the model 1013C1 High-Bandwidth Underwater Transceiver (as shown in Fig. 3), specially designed for underwater use and allows 10 megabits per second (Mbps) full duplex communication between two fully submerged platforms, was purchased from Ambalux Corporation. The device consists of a series of high-intensity laseremitting diodes (LEDs) operating in the blue light spectrum. Although the device is unidirectional, a fiber optic link closes the communication loop to allow full Ethernet capability. The unidirectionality of the device does not change the objective of these tests which is to profile data rate with respect to water characteristics.

ARTICLE IN PRESS G. Baiden et al. / Ocean Engineering 36 (2009) 633–640

635

Fig. 3. Underwater transmitter and receiver (Ambalux Corporation).

PC (receiving and response))

PC (sending)

CAT5e

Fiber to copper converter

4. Network testing and benchmarking

10 base FL Multimode Fiber

FSO link

Optical transmitter

recorded on a spreadsheet that tracks all the variables in the experiment.

Optical receiver

Fig. 4. Description of the communication system.

Two categories of tests were performed to determine the profile of data rate (in Mbps) with respect to distance, field of view and turbidity: pool and tank tests (controlled environment).

3. Communication system design The communication design for these tests, illustrated in Fig. 4, consists of a sending and receiving PC. The sending PC is connected to a fiber-to-copper converter with a CAT5e networking cable through a typical Ethernet connection. A data packet from the copper Ethernet network is transmitted to an optical cable connected to the underwater FSO transmitting device which sends light signal to an optical receiver that contains a concentrating lens and a photo multiplier tube or PMT. Electronic components sent the received data onto a fiber coming from the underwater FSO receiver. The data packet then traveled on another fiber-to-copper converter before being converted to an electrical signal on the copper Ethernet attached to the receiving PC. If the receiving PC needs to respond to a data packet, it will send a data packet to the fiberto-copper converter which delivers the data down a fiber connected to the fiber-to-copper converter at the sending PC end. This fiber closes the communication loop between the two PCs. In this experiment, it is important to note that the transmission underwater is done in one direction as the preliminary objective of this work is to determine the behavior of LEDs in water with respect to parameters mentioned previously. It allows the PCs to send and receive data between each other while tracking the rate at which communication is occurring. As request/response or streaming data rate test is performed, data is

Two computers, equipped with the network performance testing software NETPERF (a freeware network benchmarking utility created by Hewlett-Packard, 1996) are used to run data and transaction rate tests to benchmark the communications system. NETPERF is used to establish a performance rating with TCP and UDP protocols. These are the most common protocols used for and other network communications, and are also supported by the optical communication system and its Ethernet backbone. Benchmarking the communication system using data and transaction rates is an important step to understanding how well the system operates in varying underwater conditions. In a potential tele-operation scenario, a minimum data rate should be obtained to sustain a digital video feed sufficient for tele-operation. This means that a minimum data rate as well as a minimum time delay must be met to determine the fitness of the communications system. Since digital video quality data is both time sensitive and data rate sensitive, the response of these sensitivities to environmental conditions is critical knowledge. Streaming data rate tests were conducted for both the UDP and TCP protocols, and the request and response tests were conducted for the TCP protocol only. A streaming data rate test works by a client computer creating a connection to a server computer using either the TCP or UDP protocol. The client computer then streams data unidirectionally to the server, which responds when the packets are received. The number of bits sent by the streamer and received by the server per second is calculated and reported by the network benchmarking utility as megabits per second. The other important metric adopted in this experiment is the measure of transactions per second (T/s) for a given request and response size. T/s represents the time taken for a request to be made to the server followed by a response. The number of requests and responses that can occur in one second makes up T/s. The size of the request/response packet can be varied to reflect the scenario being analyzed. In this study, the request packet size is 1024 bytes and the response packet size is 256 bytes. These sizes represent a typical packet size for control data. T/s can be used to determine the quantity of packets that can be sent and received in one second. If this number proves to be too low, then slower response times may be experienced. Note that data rate can be derived from transaction rates. For example, in the case of the request response test performed, the maximum transaction rate is approximately 512 T/s for a send size of 1024 bytes (or 8192 bits). The bitrate to match the transaction rate in this case can be derived as follows: Bitrate ¼ 8192 bits=T  512 T=s ¼ 4:2 Mbps

(1)

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Fig. 6. Description of tank test.

Fig. 5. View of the pool test.

The transaction rates being displayed in the figures obtained form the different tests are the TCP request/response transaction rates in transactions per second.

182

102.5

239 326

4.1. Pool tests Pool tests were conducted by changing the transmitter location in a manner to define the communication system operating range through offset distances at varying separation distances as described in the procedure below. The same communication setup previously described was used for this experiment. In pool tests, control on ambient light levels was possible along with fairly accurate control on separation distance and viewing angle. However, varying turbidity was not an option. Therefore, the results fell short on defining how the system responded to varying levels of suspended particulate. The test procedure consisted of spacing the transmitter at an initial distance of one meter in alignment from the receiver (its position remained fixed). The transmitter is incrementally offset from the receiver until communication is lost. To find the optimum offset distance (distance just before communication is lost), the offset distance is incrementally decreased until communication is regained and the corresponding offset distance recorded is the threshold offset distance (TOD). The separation distance between the transmitter and the receiver is increased, and the same offset procedure is repeated to determine the TOD for different separation distances. The procedure is illustrated in Fig. 5.

Receiver

Transmitter Fig. 7. Description of the tank test mechanism.

nication is lost for an ‘‘accurate’’ determination of the optimum viewing angle. The experiment is illustrated in Fig. 7.

5. Turbidity measurement Turbidity measurement was limited to the Secchi method (Potsma, 1961), a simple but effective turbidity measurement method. The tank was filled with water from Ramsey Lake, a lake located in Sudbury, Ontario, Canada. The water in the tank was made more turbid by increasing suspended particulate concentration consisting of fine silt collected from the bottom of Ramsey lake. To increase the significance of the turbidity test, an alternate particulate composed of iron oxide was also used.

4.2. Tank tests

5.1. Notation

The tank test consists of a 21-in. inside diameter, 3.65-m long high-density polyethylene black plastic pipe. The pipe had three sections cut out one third of its length to allow access to the tank as shown in Fig. 6. Four separation distances between the transmitter and the receiver were set up. The position of the receiver was kept constant while that of the transmitter was varied. The viewing angle was determined by incrementally rotating the transmitter by five degree until communication was lost. At this point, the rotating device is rotated back to its previous position and a lower increment angle is applied until commu-

To simplify the representation of the results, the following notation is adopted:

 Separation distances: x1 ¼ 102.5 cm, x2 ¼ 182 cm, x3 ¼ 239 cm and x4 ¼ 326 cm.

 Turbidity level: labeled as t0 to t12, with t0 being the turbidity level for clear water and the remaining turbidity are obtained from adding more silt in the water and, therefore, decreasing the Secchi depth by approximately 8–10 cm. The particulates were collected from the bottom of Ramsey Lake (Sudbury, Ontario) to mimic an experiment close to that of Ramsey Lake.

ARTICLE IN PRESS G. Baiden et al. / Ocean Engineering 36 (2009) 633–640

 A threshold viewing angle (TVA) is defined as the minimum

Note that the TVA is equivalent to the threshold offset distance, since one of them can be derived from the other by the tangent function or its inverse function.

2.3 Threshold Offset Distance - d0 (m)



viewing angle at which communication is lost when one of the communication devices is rotated with respect to a virtual axis that contains the segment represented by the center of gravities of the two devices when they are aligned. Communication data are measured in transactions per second, because it gives a finer resolution for the TCP request response. The transaction rate can be converted to bitrate by multiplying the sent data size (in bytes or bits) by the transaction rate.

637

Threshold Offset Distances vs. Separation Distances

2 1.7 1.4 1.1 0.8 0.5

6. Results and discussion

0

2

4

6 8 10 12 14 16 Separation Distance - d (m)

6.1. Pool tests

18

20

22

Fig. 9. Threshold offset distance as a function of offset distance.

Sample Decay Rates Showing Threshold Viewing Anfgle

600 Transaction Rate (T/s)

The pool test results for separation distances x1 ¼ 3 m, x2 ¼ 3 m, x3 ¼ 5 m and x4 ¼ 21 m are shown in Figs. 6 and 7. To fine tune the test, variability of offset distances for each separation distance were, respectively, 5, 11, 14 and finally 30 cm for separation distance x3. Fig. 8 shows that the data rate profile is almost identical for all the separation distances. A change of offset distance by a few centimeters (relative to the separation distance) causes the data rate to drop from full communication potential (8.2 Mbps) to no communication at all (0 Mbps). The offset value at which communication suddenly drops to zero is called the threshold offset distance, and it is plotted in Fig. 9 against the four separation distances. It indicates that as separation distance increases, so does the threshold offset distance. In the pool tests, only separation distance was considered the variable as we did not have permission from Laurentian University to add particulate in the pool.

500 400 300 t1x0 t1x1 t1x2 t1x3

200 100 0 0

10

20

30

40

50

60

70

80

90

Viewing Angle (Degrees) 6.2. Tank tests

Fig. 10. Transaction rate as function of viewing angle for turbidity t1.

TCP Datarate Response to Offset Distance for all separation distances

Data Rate (Mbps)

8

Request/Response Decay Rates at t0 (clear Water Tank) 500 Transaction Rate (T/s)

The experiment in the tank was designed to offset our limitation of controlling the parameters of interest in the pool. The results of the tank tests are shown in Figs. 10–14. Fig. 10 shows how the threshold viewing angle for a step t1 turbidity lever at different separation distances x1, x2 and x3. The angle at which communication is the strongest followed by sudden decay when slightly incremented correspond to the TVA. The profiles of the responses with respect to the viewing angle parameter are very similar for all three separation distances. It shows a sudden decay

400 300 t0x0 t0x1 t0x2 t0x3

200 100 0 50

6

60 70 Viewing Angle (Degrees)

80

Fig. 11. Transaction rate as a function of viewing angle for turbidity t0.

4 x0 -2m x1 -3m x2-5m x3-21m

2 0 0

0.5

1 1.5 Offset Distance - d0 (m)

2

2.5

Fig. 8. TCP data rate response to offset distance at different separation distances.

of transaction rate from full communication to no communication at a given viewing angle. For clarity, the request/response decay rates are plotted for turbidity t0 (clear water) and t1, respectively, in Figs. 11 and 12. Although Figs. 11 and 12 show similar graph profiles, it is clear that increasing the turbidity from t0 to t1 decreases the TVA. The TVA is recorded for the all the turbidities

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(t0 to t12) and all separations distances are shown in Fig. 10 which represents the summary of all the experiments performed for the tank test and a replicate to confirm the results. The replication of data has shown consistency in our measurements with little variation observed that could be attributed to the quality of mixing particulate in the water (Masoti, 2005). It is however clear that the trends are consistent. The plot indicates that there exists an angle, the threshold viewing angle, at which communication is lost abruptly at given water characteristic. Fig. 12 shows the plots of the transaction with respect to all the separation distances and the turbidity levels. These graphs show similarities in their profile with different TVA. It is clear from Figs. 11 and 12 that turbidity and viewing angle have a significant impact on transaction rate. Fig. 13 represents the plots of the TVA with respect to Secchi depth (or turbidity level) at all the different separation distances with replication. To show the consistency of the tests, each set was duplicated. It appears from the graphs that TVA decreases for increasing turbidity level (increasing Secchi depth value). For example at a separation distance of x0, the TVA is about 601 at a turbidity level of t0 whereas the TVA is about 801 for a turbidity level of t6. The same profile is observed for the remaining separation distances. The plots show a rapid decay in TVA as turbidity levels increase. Finally, the TVA was plotted as a function of two variables that are the turbidity level and the separation distances and the graph is shown in Fig. 14. The plot shown in Fig. 8 indicates a clear pattern of the response of the TVA with respect to the two variables. This 3-D plot indicates that an increase in separation distances and turbidity levels decreases the TVA.

Request/Response Decay Rates at 1st Turbidity Step (t1)

Transaction Rate (T/s)

500 400 300 200

t1x0 t1x1 t1x2 t1x3

100 0 40

50

60 70 Viewing Angle (Degrees)

80

Threshold Viewing Angle (degrees)

Fig. 12. Transaction rate as a function of viewing angle for turbidity t1.

Ingluence of Turbidity Level on TVA for all the separation distances

90 80 70 60 50 40 30 20 10

6.3. Significance of the TVA

0 5

15

25

35 45 55 Secci Depth (cm)

65

75

1st Testx0

2nd Testx0

1st Testx1

2nd Testx1

1st Testx2

2nd Testx2

1st Testx3

2nd Testx3

85

Fig. 13. Influence of turbidity level on TVA for all the separation distances.

100

For this study, the threshold viewing angle is defined as the minimum angle at which communication is lost when one of the devices is rotated with respect to their alignment axis. The TVA and/or TOD are very important if LEDs are to be used to transfer data wirelessly under water for UUV. The TVA indicates the region where data transfer is still possible as shown in Fig. 15. For a given turbidity and separation distance, a vessel has to remain within +TVA and –TVA to receive or send data through an LED. It is important to point out that the TVA is different from the incidence angle of the transmitter beam. The difference is explained in the following section.

TVA (degrees)

80 + TVA 60 40 Tx

20 0 100

80

60 i Dep

Secch

40 th (cm )

20

100 150 m) 200 ce (c n sta 250 Di n 300 io rat 350 pa e S

Fig. 14. TVA as a function of separation distances and turbidity levels.

Communication Zone

Rx

- TVA Separation distance Fig. 15. Illustration of the concept of TVA and communication zone with respect to viewing angle.

ARTICLE IN PRESS G. Baiden et al. / Ocean Engineering 36 (2009) 633–640

7. Discussion

water. Scattered photons create a multipath system that injects a lot of noise in the communication if not properly filtered.

Light is an ensemble of photons that are absorbed and scattered by water, suspended particles and dissolved matter as they travel through a sample. The absorption coefficient, a(l), is a measure of the conversion of radiant energy to heat and chemical energy. It is numerically equal to the fraction of energy absorbed from a light beam per unit of distance traveled in an absorbing medium (Smith and Baker, 1981). Light scattering changes the direction of photon transport, ‘‘dispersing’’ them as they penetrate a sample, without changing their wavelength. The scattering coefficient, b(l), is equal to the fraction of energy dispersed from a light beam per unit of distance traveled in a scattering medium, in cm1. The attenuation coefficient, c(l) ¼ a(l)+b(l), is a measure of the light loss from the combined effects of scattering and absorption over a unit length of travel in an attenuating medium. The Beer–Lambert law gives a relation between the attenuation coefficient, separation distances and irradiance for a light beam with a given wavelength as follows (Killinger, 2002): IðzÞ ¼ I0 expðcðlÞ zÞ

(2)

where, I0 is the irradiance_at_surface; I(z) the irradiance_at_ depth_z; z the depth; c(l) the attenuation_coefficient_ for_wave_length_l The intensity of irradiance is a function of the photons arrival rate at a given cross section of the medium in which they travel. Eq. (2) shows that as distance increases the intensity, the number of photon decreases at a given cross section. As photons are absorbed and scatter, their ‘‘detectability’’ become probabilistic at any given point for a given period of time. The generation of photon in an optical transmitter for any given of time follows a Poisson process (Alexander, 1997) as described in Eq. (3) ðrTÞn erT n! r ¼ mean arrival rate in photons=second Pðn=TÞ ¼ probability of counting n photons in a T seconds Pðn=TÞ ¼

observation time

(3)

Fig. 16 illustrates the difference between the angle of incidence when the light beam leaves the transmitter due to scatters and the TVA, which represent the minimum angle at which communication is lost or the probability of detecting photons at the detector (optical receiver) is at its lowest. Usually an incidence angle is made up of photons that traveling straight and the ones that are deflected because of particles in the

8. Conclusion and future work This study has established that turbidity level, viewing angle and separation distance plays a significant role in the behavior of blue light in water. Further more, it was possible to graph the profile of the behavior of light with respect to the parameters of interest. TVA and TOD have been defined for future application of underwater navigation. If a transceiver capable of covering a 3-D environment is to be built, TVA values need to be taken into consideration with respect to its geometric form. To this date, an omni-directional optical system was built (inspired by this study), tested, and the test results will be published in a future publication. A brief description of phase one of the omnidirectional transceiver and the testing mechanism are described in the following sections. The optical transmitter designed was constructed from multiLED PCB panels that share the same input signal and power supply. The optical transmitter system comprises four key design features: (a) a geometric form that allows for the omni-directional propagation of light while still relying on planar printed circuit boards; (b) a modular design capable of easily repairing inoperative LED panels; (c) wide angle surface mount bright blue LEDs (468 nm central wavelength); (d) high-speed switching transistors; and (e) TTL compatible signal input for ease of interfacing to various digital signal sources. Two geometric forms were modeled to assess their relative merits against the above design criteria. Although two geometrical forms (icosahedrons and spherical hexagon) were considered, the icosahedrons was retained because of its simplicity in geometry and its ability to provide complete free space coverage using the selected LED as shown in Fig. 17.

+ i/2

+ TVA

Tx

639

Communication Zone

Rx

-TVA Separation distance -i/2 Fig. 16. Illustration of the difference between incidence angle and TVA.

Fig. 17. First built underwater omni-directional optical transmitter.

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