While wireless communication technology today has become part of our daily life, the idea of wireless undersea communications may still seem far-fetched. However, research has been active for over a decade on designing the methods for wireless information transmission underwater. Significant progress has been made in terrestrial sensor networks to revolutionize sensing and data collection. To bring the concept of long-lived, dense sensor networks to the underwater environment, there is a compelling need to develop low-cost and low-power acoustic modems for short-range communications. This post explains about Aqua communication using a modem and presents designing and developing such a model.
Sensor networks are beginning to revolutionize data collection in the physical world, relatively little work has been done to explore how sensor networks apply underwater. wireless communication, dense deployments (each sensor may have eight or more neighbors), self-configuration and local processing, and maximizing the utility of any energy consumed. Our primary application is seismic monitoring, with alternative applications including assistance during underwater construction, pipeline and leak monitoring, biological data collection, or underwater robot communication. Sensor networks typically consist of many battery-powered nodes, densely deployed in an area for close observation and long-term monitoring.
The underwater acoustic channel presents strong challenges to the design of data communication networks. Besides severe multi-path reflections, there can be curved propagation paths due to uneven temperature distribution and various interference, such as bubbles and noise from man-made objects. However, a potential penalty of this approach is that individual modems become quite expensive and power-hungry, making use of hundreds of modem-equipped sensors economically infeasible. We, therefore, explore a complementary path that emphasizes simple but numerous devices that benefit from dense sensing (e.g., eight or more neighbors per node, rather than one or two) and shorter-range communication. In addition to simpler node-to-node channels due to shorter range, higher-level approaches can compensate for channel problems through approaches such as routing, link-layer retransmission, and application-layer coding.
Our overall goal in the design of our underwater modem is to bring the characteristics that are being exploited in terrestrial sensor networks underwater. Our primary goal is that the modem be inexpensive to make it feasible to purchase and deploy many sensor nodes. A corollary is that we need only short-range communication since long-range communication can be accomplished by multi-hop routing over many individual nodes. Fortunately, these choices reinforce each other, because focusing only on short range communication means we expect to avoid many of the challenges of long-range communication (for example, acoustic ducting and multi-path effects due to surface reflections and temperature gradients), greatly simplifying the modem design. Our target communication range is 50500m. The low-power operation to allow long-lived monitoring, support for higher level protocols in software, and design for expected channel characteristics. Our design uses several techniques to accomplish low power operation. To trigger the more expensive data receiver. When there is no communication activity, nodes can turn off most components, and only leave the wake-up receiver on.
Finally, we provide both analog and digital signal output from the modem to allow high precision time synchronization. Finally, we, of course, match our design to the expected characteristics of the underwater acoustic channel. Since our modem is designed for short-range, dense sensor networks, it does not directly apply to applications that require long-range, reliable, point-to-point communications. For such applications, one should either use existing work on more powerful acoustic modems or use our modem with complementary, multi-hop communication.
Circuit Design and Implementation
The modem hardware is split into three main portions: a wake-up receiver, a data receiver, and a single transmitter. The transmitter has three output frequencies, which correspond to the data mark, data space, and wake-up tone. It is not possible to transmit data and the wake-up tone simultaneously. The entire circuit operates from a 5-volt power supply. Level shifters are used to provide compatibility with CMOS logic levels between 2.8 and 5.0 Volts. Our current prototype contains all the hardware on a single printed circuit board measured as 4 by 5 inches. Figure 2 is a picture of the board with the wake-up receiver and data receiver installed. We next describe the details of each major part of the modem.
1 Wakeup Receiver
The principal goals for the wakeup receiver are good sensitivity and very low power consumption. The only purpose of the receiver is to monitor the total energy level present in a narrow band of frequencies and to produce an interrupt.
We have chosen 18 kHz as the frequency for the wakeup tone.
This is an attractive frequency based on the background noise levels, as well as the attenuation characteristics in the ocean; both factors are frequency dependent.This frequency also lies in the normal audio band (20-20kHz) and allows the use of standard audio hardware and software. Our chosen bandwidth for the wakeup receiver is about 300 Hz. There are several possible ways to produce such a filter L/C with passive inductors and capacitors Active RC using operational amplifiers Digital an ADC followed by a DSP. The need for very low power argues against the active RC and digital designs.
2 Data Receiver
The data receiver is a conventional design based on a commercial FM intermediate frequency demodulator chip, the Philips SA604A. Whenever the data receiver is turned on, the first stage of the wakeup receiver is also powered. Due to the channel characteristics in the underwater environment, we are sending wideband FM. This requires several changes in the way we apply the SA604A. First, we use a simple, single pole low pass and single pole high pass filter to couple between the stages of the SA604A. A narrow band design typically uses an LC resonator or ceramic band pass filter.
The transmitter uses a Linear Technology LTC6900 low power oscillator as a voltage controlled oscillator (VCO).The circuit design is based on Linear Technology Design. The oscillator output feeds into a Texas Instruments TPA2000D1 Class-D Audio Power Amplifier. This is capable of delivering 2 watts into a 4 Ohm load. By selecting lower gains we reduce the output power level but extend battery life. We hope that the combination of RSSI and variable output power will encourage the development of energy efficient communication protocols. The transmitter efficiency ranges from 80 to 90 percent.
In the ultimate application of underwater communications, we will use piezoelectric transducers. These are high impedance devices, and the modem circuitry is designed for high impedance operation. At the present time, we are using Audax brand hi-fi tweeters, both as a transmitter and as microphones. Switching over to hydrophones will only require changing the input and output impedance matching networks
5 Power Control
The modem operates from a single 5 volt supply. The choice of the supply voltage is driven by the dual gate FETs used in the wakeup receiver. These are operated from a 12 volt supply in their intended application. While the modem is basically a 5-volt design we need to interface with microcontrollers such as the Mica2 mote. The modem design includes two features to allow interfacing to any voltage level from 2.8 to 5 volts. Digital input and outputs are tied through a Texas Instruments SN74TVC3010 voltage clamp which limits all digital output signals to the microcontroller supply voltage.
In shallow water, multi-path occurs due to signal reflection from the surface and bottom, as illustrated in Fig. 4. In deep water, it occurs due to ray bending, i.e. the tendency of acoustic waves to travel along the axis of lowest sound speed. Figure 2 shows an ensemble of channel responses obtained in deep water.With limited bandwidth, the signal is subject to multi-path propagation, which is particularly pronounced on horizontal channels. In shallow water, multipath occurs due to signal reflection from the surface and bottom, as illustrated in Figure 4. In deep water, it occurs due to ray bending, i.e. the tendency of acoustic waves to travel along the axis of lowest sound speed. Figure 5 shows an ensemble of channel responses obtained in deep water. The multi-path spread, measured along the delay axis, is on the order of 10 ms in this example. The channel response varies in time, and also changes if the receiver moves. Regardless of its origin, multipath propagation creates signal echoes, resulting in inter symbol interference in a digital communication system.
In addition to serving as stand-alone systems, underwater acoustic networks will find application in more complex, heterogeneous systems for ocean observation. Below figure shows the concept of a deep-sea observatory. At the core of this system is an underwater cable that hosts a multitude of sensors and instruments, and provides high-speed connection to the surface.
This post describes work on designing and developing a low-power acoustic modem for underwater sensor networks. The rationale behind design is to support large scale, long lived, and dense sensor networks powered by batteries. However, the entire work is still in progress. Especially, current work only uses transducers for in-air communication. The plan is to test this modem with real underwater communication in the near future.