PhD Defence Sarwar Morshed

energy-efficient medium access control for transmit reference modulation 

A Wireless Sensor Network (WSN) consists of sensor nodes distributed across an area to collect and communicate information. Research in the field of WSNs became interesting in recent times because of the challenges and constraints these WSNs impose, e.g., limited available energy, achieve low complexity and low cost. These research challenges influence the design of the protocols of all the layers of the communication stack, especially the Medium Access Control (MAC) layer protocol. The MAC protocol is responsible for controlling the transceiver operation, which is the most energy consuming part of the wireless sensor node. Due to this fact, the transceiver is assigned to sleeping mode most of the time in a wireless sensor node to minimize the overall energy consumption of the WSN. In this case, the WSN requires an energy-efficient MAC protocol, which can provide reliable communication even though the sensor nodes are sleeping for most of the time.

The applications of WSNs require robust communication with low power consumption in the MAC layer, as well as in the physical layer. Transmit reference (TR) modulation in the physical layer offers a simple low power receiver architecture, achieves fast synchronization with short signal acquisition time, and provides multiple frequency offsets that can be used for multiple simultaneous communications. However, this modulation technique has a performance penalty at the transmitter side because of transmitting the reference signal together with the modulation signal, and at the receiver side because of their cross products as well as the interfering signals of multiple frequency offsets using the same spectrum. Hence, TR modulation is suitable for WSN applications requiring low power operations, if a suitable MAC protocol can be designed together with this modulation technique to exploit its advantages and minimize its drawbacks.

The main contributions of this dissertation are motivated by the research question of how to design a medium access control protocol to exploit the features of the underlying transmit reference modulation to provide energy-efficiency, traffic-adaptive behavior, and robust medium access control in a shared wireless environment.

We introduce a new energy-efficient MAC protocol, called TR-MAC, which exploits the benefits of the TR modulation in the underlying physical layer, and minimizes its drawbacks. We design an analytical model to analyze the energy- efficiency of the TR-MAC protocol, and provide optimization mechanisms using the analytical model to minimize the overall energy consumption of the protocol operation. Further, we design an energy-efficient traffic-adaptive mechanism for the TR-MAC protocol such that it can accommodate traffic-adaptive behavior to deal with higher traffic load generated by sudden event-driven scenarios where a sudden event rapidly increases the traffic load within the network, which needs to be disseminated fast.

We utilize the multi-channel features provided by the underlying TR modulation, and design a multiple access control model to provide a fundamental insight into the performance of a MAC protocol with TR modulation using multiple frequency offsets for multiple communications. Finally, we provide a detailed design of the TR-MAC protocol, and analyze the protocol performance from the multiple access perspective.

The analysis of this thesis improves the understanding of the frequency offset based transmit reference modulation, and proposes a medium access control protocol customized for this unique modulation technique, named as the TR-MAC protocol. The TR-MAC protocol exploits the fast synchronization capabilities of the underlying frequency offset based transmit reference modulation. Energy- efficiency is achieved in this TR-MAC protocol by using transceiver duty cycling mechanism and preamble sampling technique. Furthermore, this TR-MAC protocol achieves traffic-adaptive behavior while maintaining energy-efficiency by using a duty cycle adaptation algorithm. The multi-channel multiple access model using frequency offsets and the performance analysis of the TR-MAC protocol in a multiple access scenario show that the TR-MAC protocol together with frequency offset based TR modulation increases the throughput and efficiency of the system while maintaining energy-efficiency. This performance enhancement is achieved by exploiting the inherent multiple access capability of the transmit reference modulation, which uses multiple frequency offsets for multiple simultaneous transmissions. Because of the inherent power and performance limitations of frequency offset based transmit reference modulation, the TR-MAC protocol is suitable for low data rate and low-duty-cycle sparse WSN scenarios, for example, monitoring and tracking applications.