| Abstract |
Wireless ad hoc networks serve as the transport mechanism among devices or between devices and traditional backbone networks allowing users to access information from anywhere a wireless connection is feasible, without relying on any kind of infrastructure. In a wireless network, with nodes sharing the same spectrum, each transmission is affected from, and affects, all other transmissions in range. When multiple uncoordinated links share a common medium the effect of interference is a crucial limiting factor for network performance. A medium access mechanism should be able to control the amount of interference experienced by receivers and, in certain cases, to enforce concurrent transmissions, by tightly coupling both physical and medium access layers to maximize network performance.
The general objective of this thesis is to present an in-depth analysis of how cross layer techniques can be used in the design and study of wireless ad hoc networks. More specifically, we focus on finding how adapting various parameters of the telecommunication system can allow concurrent transmissions, minimize interference, enhance network throughput, maximize individual link data rates, and optimally utilize the network resources for all competing transmissions.
Initially, we define and study the transmission rate regions for a simplified wireless network with a given degree of interference, considered as noise, and individual power constrains. We define the necessary conditions that maximize the system’s aggregate rate for a simplified two-link interference wireless channel and provide criteria under which simultaneous link operation outperforms timesharing. We identify critical points in the rate region where higher aggregate rates can be achieved at the expense of higher power expenditure. In case of light interference the relation between the maximum achieved rate and transmission power is shown to be almost linear, but in case of strong interference, there is need for disproportionally high total power. Finally, for higher order modulations, we give the condition on the maximum individual transmission power for switching to the next higher modulation level in order to achieve higher aggregate rate.
Then, we study the interference exhibited at the center of a circular networking area when interfering nodes are randomly distributed or have specific network topology. We define the interference limited communication range dILR to be the critical communication region around a receiver, with a large number of surrounding interfering nodes, within which a successful communication link can be formed. By adapting the transmission rate we can allow more transmitters to have a chance for a successful communication. We study how interference levels and the interference limited communication range dILR values are affected by the number of the surrounding interfering transmitters, the path loss exponent and most importantly on the selected mode and rate of operation. The results obtained through extensive simulations indicate that our proposed model for the estimation of the interference and of the interference limited communication range is very accurate, even when we relax our assumption on the receiver’s position at the center of the networking area
Finally, for a multicast group we study the multicast throughput which is the product of all successful multicast receptions times the transmission rate. We show that the multicast throughput does not depend on the transmission rate when the path loss exponent is equal to two but it increases with the transmission rate when the path loss exponent is greater than two. By changing the transmission rate we can control the number of receivers able to receive a multicast transmission. For the simple case of two multicast transmitters our results demonstrate that there exists a specific transmission rate and a corresponding percentage of successful receivers that maximizes the multicast throughput.
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