Wireless Sensor Networks (WSNs) are a type of wireless network systems which consists of many wireless sensor nodes and several gateways. It was first developed to collect battlefield information by the U.S. military, and then continued to be developed and applied for many other purposes, such as scientific investigation, temperature and humidity sensing and control, fire prediction, and security monitoring. Figure 1-1 is an example of traditional WSNs. Each sensor in the WSN collects and sends its sensing data to its up-stream node. All sensing data will eventually be transmitted to the gateway, and the gateway relays them to the remote server or storage device.
Since wireless sensor networks usually consist of many wireless sensor nodes and are often set up remotely at areas inconvenient to reach, maintenance involving humans is difficult to be performed. It is therefore desirable that sensor nodes are low-cost and its on-board battery can last for several years. Since wireless sensor nodes are low-cost and small-size, they are usually energy-constrained. Hence, in wireless sensor networks transmission data rates are often limited; otherwise, high energy consumption will reduce the life time of sensor nodes. As a result, lowering the energy consumption of the system is the design objective for most wireless sensor networks.
Many specific scenarios are suitable for utilizing wireless sensor networks
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Examples include monitoring systems on the bridge, in the tunnel, and in the mine. The appearances or the interior structure of these locations are long, thin, and with few branches. Figure 1-2 shows a bridge with a deployed WSN. A wireless sensor node is attached to every cable on the bridge and monitors the temperature, the humidity, and the natural vibration of the cable. If abnormal vibration of a cable is detected, the bridge administration can examine and repair the cable in time to prevent cable fractures.
Figure 1-3 shows the interior map of a mine before a gas explosion. One can see that most paths in the mine are long and thin. If WSNs could be deployed and utilized to monitor the gas in the mine, the gas explosion could be prevented. As shown in these examples, safety sensor network systems with long-thin topologies are a common and important case. These safety systems, when in urgent or critical situations, often need to report a much larger amount of sensor data to assist in the disaster recovery process, where the objective of the network should be maximizing the effective end-to-end throughput instead of minimizing the energy consumption as during the regular operation.
Media Access Control (MAC) protocol plays an important role in wireless sensor networks. There are three major types of MAC protocols which are often used in wireless sensor networks: Carrier Sense Multiple Access (CSMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). Most MAC
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protocols for WSNs are designed to maximize the network lifetime and would result in poor throughput in these urgent situations, including the most commonly used TDMA-based and CSMA-based MAC protocols. If TDMA is used as the media access protocol for wireless sensor networks, the transmission time durations of adjacent wireless sensor nodes cannot overlap; otherwise, the receiving nodes experience collisions, i.e., signals sent by different nodes would interfere with each other, received signals would be seriously distorted, and the signal-to-interference-and-noise ratio (SINR) would be greatly reduced. In addition, the node density of the area close to the gateway is usually higher, and therefore, in this case, the average waiting time for nodes in this area would be longer and becomes the bottleneck of the entire network, which leads to lower overall network throughput. Clearly, these are not desirable designs for long-thin WSNs. CSMA-based MAC protocols also have similar designs to TDMA-based MAC protocols and thus have the same limitations. The main difference is that CSMA-based protocols use contention-based approaches to deal with the interference other than scheduling approaches. The most intolerable drawback of CSMA is the unbounded delay since nodes could wait an unbounded time interval as collisions happen [15]. Thus, CSMA is not a good MAC protocol choice with time-sensitive applications.
When CDMA is used as the MAC protocol for wireless sensor networks, the
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original message is modulated into a broadband signal with a pseudo-noise (PN) code.
With the characteristic of PN codes, interference between signals spreaded by different PN codes would be greatly reduced, and, as a result, the adjacent sensor nodes can simultaneously transmit signals, increasing the overall throughput substantially. In addition, due to the modulated PN code, transmitted signals in CDMA systems would appear similar to noises, and, therefore, has the benefits of hiding the information. The transmitted information cannot be easily decoded by unauthorized third parties.
In this thesis, we propose that the system uses a CDMA-based MAC protocol when it switches to the emergency mode in these situations. It allows simultaneous transmissions within the receiving range of a node, and could therefore result in higher throughput. We develop a simple heuristic power allocation scheme to be used in single-chain topologies. The developed scheme does not need the knowledge of the path losses between nodes, and is therefore practical to be implemented. We also propose a scheduling principle which can be used in the CDMA networks with multi-chain topologies under the assumption that the gateway can receive different packets spreaded with different PN codes transmitted in the same time slot. The idea is to schedule more chains (nodes) transmitting to the gateway in the same time slot to decrease the duty cycle, and to schedule adjacent chains to transmit in the same time slot to avoid the interference caused by interleaved transmissions and receptions. The error performance
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is then analyzed and compared to TDMA-based wireless sensor networks with the same topology.
The rest of this thesis is organized as follows. Chapter 2 talks about some researches related to this thesis. Chapter 3 introduces the system model of the proposed system, also including channel configurations, topologies, and evaluation metrics, etc.
Chapter 4 talks about the evaluation result of CDMA and TDMA network with the single-chain and multi-chain topologies. Chapter 5 is the conclusion of this thesis, and we propose some future works could be researched based on this thesis.
Figure 1-1 A typical wireless sensor network
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Figure 1-2 A bridge with a WSN to monitor the natural vibration of steel cables on the bridge [1]
Figure 1-3 A map of the interior of a mine [2]
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