2012 International Symposium on Communications and Information Technologies (ISCIT)
Optical Wireless Communication for Real Time Swimmers Feedback: A review Rabee M. Hagem1, David V. Thiel1, Steven G. O’Keefe1 1
Thomas Fickenscher2 2
Centre for Wireless Monitoring and Applications Griffith University Brisbane, Queensland, Australia,
[email protected]
Abstract—Underwater optical wireless communication applications include robot communication, data monitoring and collection and control applications (SCADA). Underwater optical wireless communication using visible light is used because of the high data rate compared to acoustic technology and low path attenuation compared to radio frequency (RF) technology. The blue-green wavelength has minimum attenuation for absorption and scattering. One application for optical wireless communications is real time swimmers feedback. This system requires short range communication over 1 m between sensors mounted at the swimmer’s wrist and a receiver on his/her goggles. The link was sometimes line of sight (LOS) and sometimes non line of sight (NLOS). Air bubbles generated from a swimmer causes additional link attenuation. Pool lighting may cause interference both above and below the water. The final unit should be low cost, have low power consumption and small size to reduce drag. This paper reviews technology used to give real time feedback to a swimmer during swimming. This new technology should help swimmers improve their performance during training. Different optical wireless systems using LEDs as a light source are presented. The authors investigated in the previous work the effect of air bubbles and they designed, implemented and tested an optical wireless communication system based on light emitting diode (LED) and integrated detector preamplifier (IDP). Both the position and the orientation of the transmitter and the receiver change during the swimmer’s actions. Simulation results for the line of sight (LOS) communication link and the non line of sight (NLOS) with the effect of air bubbles were used to evaluate the effect on the underwater optical signal. Link budgets were calculated for different light sources. This paper reviews research in underwater optical wireless communication system using LEDs as a light source. The applications to swimming monitoring is presented.
Keyword: Underwater optical wireless communication; Real time swimmers feedback. I. INTRODUCTION Many applications for using underwater wireless sensors networks (UWSNs) based on using optical wireless communications were reported in [1]. Optical communication was used because of low cost, low power consumption and high data rate. The main applications were monitoring underwater environment , real time observation of underwater conditions, gathering data from different kinds of sensors, exploring underwater biological life activity and underwater
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Chair, High-Frequency Engineering, Helmut Schmidt University University of the Federal Armed Forces Hamburg, Germany
exploration using unmanned and autonomous underwater vehicles [1]. The new applications based on using underwater optical wireless communications are swimming applications. Providing feedback to athletes using a monitoring system is crucial for athletes and their coaches. Competitive and recreational swimmers swim in the ocean (sea water with sun light) and/or in the pool (chemically treated water with artificial light). This feedback can provide very important information such as swimming style, stroke rate, lap rate and training load [2]. This information can be used to improve swimming performance and to monitor injury recovery and training progress. A wristmounted accelerometer with a communications link to a receiver located on the goggles allows visual signals to be given to the athlete during swimming. As the maximum distance between a swimmer’s wrist and goggles is approximately 1m the communication system must achieve this distance or more, and this varies because of the movement of the swimmer’s arms. To achieve this goal an investigation of the best communications systems was undertaken. Water is a difficult environment for wireless communications. There are three possible technologies: acoustic, RF (Radio frequency) and optical communications. Table I highlights the differences between the three techniques [3]. Radio waves suffer from a high attenuation in water [1, 3-5] and at low frequencies, require large antennas. It is therefore not practical to use these systems underwater [6]. Underwater sound communications is influenced by propagation delay, path loss, multi-path fading, high bit error rate (in the order of 10-2-10-5 ) and limited bandwidth which depends on range and frequency [6]. In addition, acoustic waves could distress swimmers. Therefore, acoustic communication is not the best solution underwater with applications that need high data rates and real time operation [7]. Underwater optical communications is a feasible solution because of the high data rate and high bandwidth. However, the absorption and scattering have an effect on the optical propagation path [3, 7-9]. This paper reported optical wireless communication for swimming applications based on an LED transmitter and photodiode receiver. One of the main goals is to provide a swimmer with understandable real time feedback.
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TABLE I.
COMPARISON OF THE COMMUNICATIONS CHARACTERSTICS OF ACOUSTIC, RADIO AND OPTICAL SYSTEMS [3]
association (IrDA) physical layer with a 3 W high power green and blue LED in the visible spectrum. Underwater optical wireless communication system was implemented to use for robot communications [11]. Results were presented [14] for a remotely controlled underwater robot in a real-time with highthroughput and low latency link using an optical wireless communication system in shallow water. An optical wireless communication system was designed and implemented [15] to send images over 13 m at 1Mbps speed. In [16] underwater data monitoring and collection based on optical wireless communication was presented. An underwater FM optical wireless communication system based on an LED for transmitting speech was reported [17]. In [18] an underwater optical wireless communications system using blue/green LEDs was reported. Table II shows the advantages and disadvantages of using optical wireless communications systems [19].
Feature
Acoustic
Electromagnetic
Optical
Nominal speed (m/s)
∼ 1500
∼3*107 (@ 400 MHz)
∼ 2.25*108
Power Loss
∼0.1 dB/m/Hz
∼ 28 dB/1Km/100MHz
∝ turbidity and attenuation
Bandwidth
∼KHz
∼MHz
∼ 10-150MHz
Antenna size
0.1m
∼0.5m
∼ 0.1 m
Effective range
∼Km
10m
∼ 10-100m
Major hurdles
Bandwidthlimited, interferencelimited
Power-limited
Environment-limited
Data rate
Up to 100 kbps
Up to 10 Mbps
Up to 1 Gbps
Advantages Low power consumption
Antenna complexity
Medium
High
Medium
No strict laws Huge Bandwidth
TABLE II. ADVANTAGES AND DISADVANTAGES OF OPTICAL WIRELESS COMMUNICATIONS [19]
Unregulated spectrum Optoelectronics technology
II. UNDERWATER OPTICAL WIRELESS COMMUNICATION SYSTEMS USING LED Underwater optical wireless communication systems with high data rates was discussed in [4]. A simple analytical method based on Pspice simulator was used to evaluate the underwater optical link and bit error rate by using light emitting diodes (LEDs) with the effect of absorption and scattering in realistic ocean water were used. A point to point optical wireless transceiver system based on a blue LED and a receiver photodiode was developed for underwater wireless sensor network (UWSN) for nodes communication in [1]. A short range underwater optical wireless communication system using a visible light LED as a transmitter and a photodiode as a receiver was reported in [10]. The results showed that using multicolor LEDs was better than using a single color LED in an underwater environment t. An optical wireless communication transceiver was designed [11] to allows an autonomous underwater vehicle (AUV) to communicate to exchange data and control signals. LED and a photodiode were used. A novel system for underwater wireless sensor networks using a mixture of optical and acoustic communications was used to monitor coral reefs and fisheries [12]. An underwater optical wireless communication system with range of 5 to 10 m using inexpensive components and based on digital signal processing chip (DSP) with complex detection algorithms such as signal detection and clock synchronization was designed and implemented [13]. A small size optical communication transceiver for a swarm of submersibles was designed [5] based on using the combination of the Infrared data
Reusability
Disadvantages Blockage for the optical transmit signal Low power source Alignment Signal scattering Light interference
Discussion Less energy requirements and cost savings. Free license operation. Appropriate for high speed applications. Allows virtually unlimited use of spectrum by individual networks. Inexpensive components and little power consumption. Use some communication equipments and wavelengths by nearby systems. Discussion Leads to design challenges. Requires high sensitive receivers. Leads to more operation constrains. Leads to multipath problems. Affect the system performance negatively.
III. DESIGN ISSUES A. Light Source An LED converts an electrical signal to an optical signal. For real time swimmers feedback the LED is more suitable compared with laser because it is diffuse rather than collimated, has low transmit power, has a less hazardous effect and it is usually used for short distance for indoor and outdoor systems [19]. B. Optical Wavelength The attenuation of light in water depends on wavelength and water type. Fig1. shows the water absorption coefficient for pure water for different wavelengths [20, 21]. The minimum absorption coefficient is found in the blue-green wavelength range between (450-570) nm. The optical attenuation is given by Beer’s law and the received power Pr is given by [3, 17].
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Pr = Po exp[− c(λ ) s]
(1)
Absorption coefficient for pure water (m-1)
3.5
TABLE III. COMPARISON BETWEEN DIFFERENT MODULATION TECHNIQUES WHERE P REPRESENTS THE SMALLEST PULSE WIDTH [22]
3
2.5
2
1.5
OOK
FSK
DPSK
4-PPM
8-PPM
Maximum rate
1/(2P)
1/(2P)
1/(2P)
(1/2P)
(3/8P)
Transmit power
Middle
Higher
Highest
Low
Lowest
Modulation complexity
Low
Higher
Highest
Lower
Lower
1
0.5
0 200
300
400
500 Wavelength (m)
600
700
800
Figure 1. Absorption coefficient for pure water as a function of wavelength [20, 21] where Po is the initial power, s is the distance of propagation and c(λ) is the wavelength dependent attenuation coefficient. The propagation loss factor Lpr(λ,s) is related to the path length s [7].
L pr (λ , s ) = exp [− c (λ ) s ]
In [23] a modified PPM transceiver system called shorten pulse position modulation (SPPM) for image transmission based on using underwater optical wireless communication was reported. The 4-PPM represents each 2 bits of the information signal by 4 bits while the modified PPM represents it by 3 bits. Table IV shows how to represent each 2 information bits in 4-PPM and SPPM. The results showed that the modified PPM has the same performance of the PPM relating to error control. However, the modified PPM improved the bandwidth utilization.
(2) TABLE IV.
Where the attenuation coefficient (or the extinction coefficient) c(λ) is given by [7]. (3) c (λ ) = α (λ ) + β (λ ) where α(λ) is the absorption coefficient and β(λ) is the scattering coefficient. The attenuation coefficient for clean ocean water is 0.1514 m-1 and for coastal ocean water is 0.30 m-1 at wavelength 520 nm [7]. C. Optical Modulation The advantages and limitations for different modulation techniques for underwater optical wireless communication were reported [22] based on modeling and simulation. Intensity modulation and direct detection have been used in the underwater environment. By changing the pulse rate, pulse width and frequency, different modulation techniques can be achieved. The detection techniques are usually classified into two types. (a) Non-coherent detection methods depend on the presence or absence of a signal and there is no information about the phase such as amplitude shift keying (ASK), on-off keying (OOK) and (PPM). (b) Coherent detection such as frequency shift keying (FSK) and Phase shift keying (PSK). It has been proven that the (PPM) is better for low power underwater applications while, PSK gives good performances in terms of bandwidth, error performance but with poor power efficiency. In addition, OOK and PPM are usually used in a simple direct detection system with lower complexity while FSK and PSK are used with high complexity. Table III shows the difference between different modulation techniques.
Information 00 01 10 11
REPRESENT THE INFORMATION DATA USING 4-PPM AND SPPM [23] PPM modulation (b0 b1 b2 b3) 1000 0100 0010 0001
SPPM modulation (b0 b1 b2) 010 001 110 101
D. Link Budget Link budget calculations are very important in any communication system in order to evaluate the success of this system. The more received power at the receiver and the greater the receiver sensitivity, the higher will be the reliability of the communication system at the expense of battery power [19, 24, 25]. Three communication links, line of sight, retro reflector and reflective link were discussed in [7, 8, 26]. The authors used Matlab in [7, 8] and Matlab and OptiSystem in [26]. They found that the line of sight link is more effective than the other two. They suggested a hybrid communication link based on using acoustic with optical, or radio frequency (RF) with optical in order to keep the availability of the communication link. The effect of air bubbles was presented in [27]. Underwater wireless optical communication using IrDA transceivers was presented in [28]. Timing jitter was used as a measure of the link performance. The effect of air bubbles on the optical signal and transmission through the water surface were discussed. Different experiments were done in air, water with and without the effect of air bubbles. The distance achieved in air was up to 1 m. In water without the bubbles the distance achieved was 47.5 cm and this decreased to 27.5 cm in water with bubbles. The authors suggested real time feedback using smart sensor and an LED
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matrix. The authors discussed and calculated the link budget calculation for different light sources in [29]. E. Optical Receiver A photodiode converts an optical signal to an electrical signal or current. There are many factors that could be taken into account in choosing the photo detector such as sensitivity, speed, size, power consumption. Table V shows the comparison between different photo detector technologies: phototransistor, photoresistors, pin photodidodes and avalanche photodiodes [15]. TABLE V. COMPARISON BETWEEN DIFFERENT PHOTO DETECTORS [15] Features
Photo_ resistors
Photo_ transistors
pin photodiodes
Avalanche Photodiodes
Size
Small
Small
Small
Small
Linearity
Over small regions
Good
Excellent
Not Linear
Speed
Slow
