optical wireless is an advantage. A small, low-cost platform to demonstrate the potential of an optical wireless communications interface for underwater sensor ...
5 Mbps Optical Wireless Communication with Error Correction Coding for Underwater Sensor Nodes Jim A. Simpson, William C. Cox, John R. Krier, Brandon Cochenour, Brian L. Hughes and John F. Muth Department of Electrical and Computer Engineering North Carolina State University Raleigh, NC 27695–7914 Email: {jasimpson,wccox,jrkrier,bcochen,blhughes,muth}@ncsu.edu
Abstract—One issue with underwater sensors is how to efficiently transfer large amounts of data collected by the node to an interrogating platform such as an underwater vehicle. It is often impractical to make a physical connection between the node and the vehicle which suggests an acoustic or optical wireless solution. For large amounts of data, the high bandwidth of underwater optical wireless is an advantage. A small, low-cost platform to demonstrate the potential of an optical wireless communications interface for underwater sensor nodes is demonstrated. To enhance the reliability and robustness of the optical wireless communication digital signal processing and error correction techniques are used. The system was tested in 3 and 7.7 meter tanks at 5 Mbps with the turbidity of the water controlled by the addition of Maalox.
I. I NTRODUCTION Underwater sensor nodes can perform a variety of tasks, from collecting environmental and acoustic data to collecting video data. With low cost nonvolatile memory, and very compact, energy efficient hard drives now available very large data sets can be collected. Even if data compression schemes are used, the time to download the data can become lengthly. It is beneficial operationally to minimize the time a diver or underwater vehicle needs to stay in the vicinity of the node. As an alternative to physically retrieving the node or making a physical connection to download the data, or laying cable or fiber optic to the sensor node underwater optical wireless communications potentially offer a high bandwidth, although relatively short range solution. [1]–[6] Inexpensive, high power light emitting diode and diode lasers in the blue/green portion of the spectrum are now readily available. Some of these high power LED light sources can be modulated at high bandwidths with sufficient modulation depth for underwater optical communications operating at rates of 1 to 5 Mbps using a return to zero format and up to 10 Mbps when using a non-return to zero format. Blue diode lasers with even higher bandwidths are also being explored. On the receiver side, photomultiplier tubes and photodiodes have similar bandwidths.The applications need and availability of these sources has led to several groups to investigate the viability of underwater communications in the 1 to 100 meter range with data rates ranging from kbps to 10 Mbps. [7]–[10] This work was supported by the Office of Naval Research via NRL grant N00173-07-1-G904 and STTR N00014-07-M-0308 and by the National Science Foundation under grants CCF-0515164 and ECCS-0636603.
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Fig. 1. Plot of coding improvement with BER at 0.001. A wide variety of error correction codes have been previously implemented at NCSU and approach the theoretical limits for coded communications assuming an additive white gaussian noise channel. [11] These codes provide a significant advantage over uncoded transmissions. To demonstrate a significant coding advantage, within the hardware constrains of the FPGAs used, Reed Solomon encoding was used since it required fewer logic elements than the more advanced codes.
Like RF communication systems, error correction coding can be expected to improve system performance. [11], [12] As shown in Figure 1, we have examined digital signal processing and the use of error correction coding to extend the range and to reduce bit error rates so that the system performance can approach theoretical communication system limits. II. S YSTEM D ESCRIPTION In this paper the goal was to prototype an optical communications interface to provide continuous transmission of Reed Solomon encoded data from a sensor. To mimic the size and power constraints of what might be expected of a sensor node the optical communications interface was connected to a microcontroller collecting low frequency acoustic data and packaged in a small transparent watertight box. As an example sensor system to provide data a microphone and microcontroller were used to acquire low frequency ambient sounds. The system was placed in a transparent perspex box 8x4x2 inches shown in Figure 6. The system is self contained and powered using Lithium-polymer batteries. The communications interface consists of a submersible
Fig. 2. Block diagram of the system. An acoustic signal is acquired by the microcontroller, encoded by the FPGA and optically transmitted by the light emitting diode. The detected optical signal is digitized by the receiver. Automatic gain control is used to ensure the signal is within the dynamic range of the digitizer. A matched filter and synchronizer prepare the signal for downsampling. The down sampled signal is decoded and error corrected on the FPGA and displayed.
transmitter and a receiver. The receiver was not packaged for submerged operation and was located outside the glass tank window. During normal operation, the microphone or analog input signal is continuously digitized and the transmitter sends bursts of packets at a 5 Mbps datarate. The receiver digitizes the signal at 100 MSps to provide 20 times oversampling of the incoming data, synchronizes and then decodes the transmitted packets. The resulting stream of data is realtime with a latency of one packet.
green (525 nm), or white (with a strong peak at 470 nm) wavelength LEDs. These LEDs can be effectively modulated at data rates as high as 5 Mbps for RZ encoded data or 10 Mbps for non-RZ data.
A. Underwater Transmitter The underwater transmitter consists of all the electronics packaged within the submersible enclosure. The transmitter performs three main functions: it acts as an interface to the sensor node, performs error correction encoding, and acts as an interface to the LED transmitter. All three of these are achieved by the use of a Field Programmable Gate Array (FPGA). The FPGA used in this work is from the Altera Devices Cyclone II series. This FPGA is capable of interfacing to any sensor node with a digital interface operating at clock speeds up to 400 MHz. An analog-to-digital converter (ADC) can also be added and interfaced with the FPGA at up to the same clock speed. In this demonstration, the FPGA is interfaced to a microcontroller over an RS-232 serial link operating at 115.2 kbps. The microcontroller uses an on-chip ADC to continuously sample the output of a microphone. The sampled data is then sent to the FPGA where it is buffered. The FPGA then encodes the data using a Reed-Solomon (RS) encoder for error correction coding. The data is further encoded using return to zero (RZ) and sent to the LED Driver. The LED driver uses an array of NMOS transistors to drive a high power LED. The high power LEDs used are from the CREE XR7090 series. Experiments were conducted with a blue (470 nm),
Fig. 3. The assembled board. From left to right; the LED with heat sink, the LED driver stacked on the field programmable gate array, the analog to digital converter and microcontroller.
B. Receiver For the purposes of this demonstration, it is assumed that the receiver is allowed a relaxed set of requirements for space, power and cost. This is analogous to having a receiver on a larger platform that would interrogate the bottom node. In our laboratory environment it allowed us to easily change out photodiode and PMT detectors and provided a flexible platform for data viewing and manipulation. The front-end of the receiver consists of either an amplified photodetector or a photomultiplier tube (PMT). Both
Fig. 4. The timing of the data acquisition is show schematically with an example of the encoded packet shown at the right side of the figure. The continuous acoustic signal is digitized at a rate of about 1.8 kHz. Each 223 byte segment is sent to the FPGA operating with a 48 MHz clock. Encoding the 223 bytes takes approximately 6 microseconds. The 50 byte header and 255 byte Reed Solomon encoded packet is sent at 5 Mbps data rate in an optical burst that is approximately 61 microseconds long. The timing between optical packets is about 120 miliseconds which is limited by the analog signal input bandwidth.
were found to have sufficient bandwidth under clear water conditions. With the addition of scattering agents, in a 3 meter tank for conditions above 4 attenuation lengths the PMT was preferred. In the 7 meter tank the system was only tested with the photodiode in relatively clear water with 1-2 attenuation lengths. The output of either detector is digitized by a National Instruments digitizer operating at 100 MSps. The digitized signal at the receiver is sent over a USB interface to a PC. The PC runs a continuously looping multithreaded C based application that performs the required Digital Signal Processing (DSP) and outputs the data originally captured by the transmitter. The C application consists of an acquisition and a processing thread that runs in parallel. The acquisition thread is responsible for acquiring data from the digitizer. If necessary uses automatic gain control and buffers the collected data. The processing thread implements the DSP algorithms in sequence. A matched filter is used to maximize the signal to noise ratio of the received data by correlating a square pulse with received pulse sequence. An early-late gate symbol synchronizer then finds the optimal point to downsample the signal, followed by a thresholding detector. The detected bits are then sent to a Reed-Solomon decoder, which corrects detected errors in the data. Finally, the data is displayed on the screen for viewing and saved to file. C. Reed-Solomon Error Correction Coding Although more modern and efficient low-density paritycheck (LDPC) codes and turbo codes have been used by the authors in underwater optical links, the Reed-Solomon (RS) code was chosen for it’s robustness and relative ease of implementation. A RS(255,223) code was ultimately chosen for it’s low overhead and low resource utilization. It is a systematic code, where 223 data bytes are used to generate 32 parity bytes, which are concatenated at the end of the data to form a 255 byte coded packet. This limits the coding
Fig. 5. The expected improvements for two Reed Solomon codes are compared with uncoded performance. At a bit error rate of 10−6 the RS (255,129) and RS(255,223) codes provide a signal to noise advantages of about 6 and 4 dB respectively. Note the points on the x-axis represent error rates that are 0 or less than 10−6 .
overhead to 1/8, but also limits the number of errors that can be corrected to 16 byte errors. Over a typical underwater link, where the noise can be modeled as Additive White Gaussian Noise (AWGN), this code provides a 4 dB improvement over the uncoded data at a bit-error-rate of 10−5 . With the 50 byte header attached, and 32 encoded parity bytes, 73 percent of the transmitted packet is data. When not limited by the digitization rate of the acoustic signal, for example if a stored data file was to be transmitted, this would correspond to a data throughput of about 3.65 Mbps when transmitting at 5 Mbps. This is sufficient for video, file transfers and other high bandwidth applications.
The transmitter is capable of transmitting bits at a data-rate as high as 5 Mbps. Each bit is return-to-zero encoded for ease of synchronization at the receiver. Additionally, a (255,129) Reed-Solomon error correction code is also employed to encode the transmitted bits. This allows the system to operate at a lower signal-to-noise (SNR) for a given bit-error-rate (BER). This particular code has a performance gain of ~6 dB at a BER of 10-4 and is capable of correcting up to 63 error bytes in a packet of 255 bytes transmitted.
[3] J. Simpson, “A 1 Mbps Underwater Communications System using LEDs and Photodiodes with Signal Processing Capability,” M.S. thesis, Dept. of Elec. and Comp. Eng., North Carolina State University, Raleigh, NC, 2007. [4] F. Hanson and S. Radic, “High bandwidth underwater optical communication,” Appl. Opt., vol. 47, pp. 277-283, Jan. 2008. [5] C. Pontbriand, N. Farr, J. Ware, et al., “Diffuse high-bandwidth optical communications,” Proc. OCEANS 2008 , vol., no., pp.1-4, 15-18 Sept. 2008.
Fig. 6. The transmitter shown submerged. The hook is attached to a weight since the box is buoyant.
III. C ONCLUSION A 5 Mbps optical wireless communications interface for underwater sensor nodes to offload information to the external world was implemented. The system is improved from previous versions with the addition of digital signal processing techniques including error correction coding. A small, low-cost packaging of the system has been demonstrated in laboratory underwater environments in a 3 and 7 meter tanks at data rates of 5 Mbps. Future work includes testing the system in natural waters and investigating the use of more sophisticated coding schemes. R EFERENCES
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