Location-based Propagation Modeling for Opportunistic Spectrum Access in Wireless Networks

LOCATION-BASED PROPAGATION MODELING FOR OPPORTUNISTIC SPECTRUM ACCESS IN WIRELESS NETWORKS Tugba Erpek, M.S. George Mason University, 2007 Thesis Director: Dr. Brian L. Mark Wireless spectrum has become a scarce resource due to the exponential growth of the number and type of devices that utilize the electromagnetic spectrum. The cost of long term leasing of spectrum has proven to be a major road block in efficient use of frequency spectrum. Moreover, spectrum measurement studies have shown that substantial portions of the allocated wireless spectrum are highly underutilized. Thus, Dynamic Spectrum Access (DSA) devices have been proposed to be allowed to share the spectrum dynamically between users. The idea is that the DSA devices will continuously scan the spectrum and start to transmit in a channel when a licensed user’s operation is not detected in that channel. Detailed path loss models are needed to calculate the propagation loss between a DSA device transmitter and a licensed user receiver in order for the DSA device to avoid causing interference to the receiver by using overly high transmit power levels. This thesis proposes a novel propagation loss model called Location Based Propagation Modeling (LPM) based on the existing TIREM path loss model. The TIREM model gives the median value of the path loss for a given transmitter and receiver pair and the user needs to know the precise locations of the transmitter and receiver to calculate the path loss with the TIREM model. However, for DSA applications, we usually do not know the precise locations of the licensed user receivers. Furthermore, TIREM model requires detailed terrain information stored in the memory to calculate path loss, but DSA devices have limited memory. As a result, we need a compact representation of the TIREM model which gives the path loss without the need to store terrain information in the memory. These were the motivations to develop the LPM. DSA devices require accurate spectral estimation methods to determine whether a channel is occupied in a specific time and location. Two spectral estimation methods: multitaper spectral estimation method and conventional FFT-based spectral estimation method are compared in this thesis using real signal measurements. Our numerical results show that the multitaper approach yields a significant increase in the number of harvested channels, while maintaining a smaller probability of false alarm. Chapter 1: Introduction The electromagnetic radio spectrum is a natural resource and the use of transmitters and receivers is licensed by governments [1]. Currently, wireless spectrum is partitioned into fixed frequency bands by the Federal Communications Commission (FCC). Over the last decade, it has become a highly valued resource due to the exponential growth of the wireless communications industry. Moreover, spectrum measurement studies have shown that substantial portions of the allocated wireless spectrum are highly underutilized [2]. As a result, the approach of partitioning the spectrum into fixed frequency bands causes a waste of wireless resources. Dynamic Spectrum Access (DSA) is a promising approach to increase the efficiency of the spectrum usage. This approach allows unlicensed wireless users to access the licensed bands from the legacy spectrum holders on an opportunistic basis [3]. The idea is that the DSA devices will continuously scan the frequency spectrum and start to transmit in a channel when the licensed users do not transmit at that frequency. Furthermore, DSA devices will abandon the channel that they transmit within a certain time when they sense the legacy user’s activity on the same channel again. DSA technology makes use of various capabilities such as frequency agility, adaptive modulation, and transmit power control. These features allow the DSA devices to