WSN Page 1 of 23 ECE 4512: Design I December 4, 2003 2. DESIGN REQUIREMENTS The goal of this project is to design elements of a wireless sensor network (WSN) that communicate with each other over a self-configuring ad hoc network. Each element consists of a remote sensor (or “mote”). One mote functions as a base station in conjunction with a PC. This base station is used for communication as well as data storage. The mote’s microcontroller employs an operating system (TinyOS) and software specifically tailored for embedded systems networking and data management. Applications of wireless sensor networks include data logging, monitoring, and communication over large or small areas. To meet these goals, the following technical and practical design requirements have been applied. 2.1. Technical Design Constraints Our technical design constraints are shown in Table 1. These rules constrain the circuit and software design of the system. As an embedded system, each mote is expected to perform self-sufficient communication for an indefinite period of time. This entails automatic coordination of transmission, compatibility between software and hardware among motes, power efficiency, and RF frequency. 2.1.1. RF Frequency The mote will transmit data on a carrier frequency of 433 MHz. Current state-of-the-art motes, Crossbow’s MICA series (developed at the University of California, Berkeley), operate at either 433 or 868/916. These frequencies were chosen based on standards set forth in the FCC Rules and Regulations Parts 2 and 15. Each of these frequencies falls in the band of Industrial Medical and Scientific (IMS) bands, which are available to use license-free. While there are advantages to operating at the higher frequency, we chose to design our system for the lower frequency mainly to save power. The lower frequency will also experience less attenuation. Unfortunately, this will render the mote slightly more susceptible to interference and make the antenna larger. 2.1.2. Efficiency Several factors must be taken into account regarding the efficiency of the system. The intensity of the signal will affect the power dissipation of the system. However, lower intensity transmissions will result in errors. When an error occurs, the data must be retransmitted causing additional power consumption and medium usage. This retransmission of lost data wastes energy. The minimum transmission intensity can be determined using the received signal strength indicator during network discovery. Network discovery packets will be transmitted at maximum intensity and the received intensity will determine the necessary transmission intensity of data packets. Since more power is used in the transmission of a signal than in any other stage of the communications process, the goal is to transmit as seldom as possible by only transmitting meaningful data. By placing more processing responsibility on each sensor, the amount of raw data transmitted is reduced, thus Name Description RF Frequency Transmits at 433 MHz Efficiency 80% (Radiated Power) / (Battery-Supplied Power) Transmission Distance / BER Maximum distance with optimum BER is 300ft. MCU Frequency Operates at 3 MHz Software Will run a modified TinyOS Table 1 Technical design constraints for the WSN system.WSN Page 2 of 23 ECE 4512: Design I December 4, 2003 maximizing power efficiency. These methods can be tested by measuring the power consumption and comparing the consumption to that of a network that sends all raw data to the base station. In addition, a channel coding scheme will be used to make the data transmission more reliable, thus reducing retransmission of data. A balance between redundant data and error rate must be reached to minimize the number of transmitted bits. This can be tested using communications simulation software. 2.1.3. Transmission Distance and Bit Error Rate The system is designed to be used over a relatively short range. Each mote will have a maximum outdoor transmission distance of 300 ft. By using channel coding, the occurrence of errors should reduce dramatically. Therefore, we hope to achieve a bit error rate (BER) of 10-6. This figure is three orders of magnitude better than the present top-of-the-line Mica motes from Crossbow. 2.1.4. MCU Frequency The operating frequency of the MCU will be 3 MHz. Assuming the motes collect 16-bit data at a maximum rate of 44,100 samples per second and an average of 2.5 clock cycles per instruction, the required clock speed is about 1.76 MHz. However, the MCU we will use supports clock speeds of 3 - 24 MHz. To reduce power consumption, we chose the lowest speed available to us. 2.1.5. Software The mote will be designed to run a modification of the TinyOS operating system developed at Berkeley. This operating system allows data transfer protocols to be implemented in embedded systems with limited memory. Protocols may be updated based on the need. The operating system itself will undergo minor modifications since it was originally developed for the Atmel Atmega 128L microcontroller. 2.2. Practical Design Constraints The practical design constraints of this project are shown in Table 2. These constraints define how the motes will be used. 2.2.1. Power/Current Consumption Power consumption is one of the key design elements that will be addressed in the design of the wireless Type Name Description Environmental Power 2 AA batteries will provide the supply voltage between 2.7 and 3 V. Economic Cost The total cost to build the mote will not exceed $35. Manufacturability Size The mote will have a length of 3 in, width of 2 in, and height of .8 in. Sustainability Reliability The mote will operate continuously for 6 months on two AA batteries Environmental Temperature The mote will operate at temperature between -20ºC to 60ºC. Health and Safety FCC/UL Regulations The mote will be compliant with FCC CFR47 Part 15 specification for the USA ISM bands. Table 2 Practical design constraints for the WSN system.WSN Page 3 of 23 ECE 4512: Design I December 4, 2003 sensor module. The module along with many more modules will be implemented into a wireless sensor network to monitor data of a wide range (defined in the transmission distance section of this document) of area. Therefore, changing the batteries in every module on a regular basis is impractical. The battery life of the module correlates directly with the amount of power consumed
View Full Document
Unlocking...