Congestion in wireless sensor networks (WSNs) has always been a serious problem for all kinds of communication networks. It causes severe information loss and shortens the life time of the sensor nodes. Congestion also leads to excessive energy consumption due to large number of retransmission and packet loss which lowers throughput. In WSNs congestion need to be controlled in order to have high energy-efficiency, to prolong system life time, improve fairness and improve quality-of-service in terms of packet loss ratio with the packet delay and link utilization. Most of the designs of WSN depend on the applications and the environment in which the sensor nodes are deployed. It has important applications such as remote environment monitoring, target tracking, military and surveillance, etc. The placement of the nodes is often done randomly or in a pre-planned manner. These sensors are equipped with wireless interfaces with which they can communicate with one another to form a network. WSNs can be an event driven which can be routed to detect or monitor data at the base station depending on their degree of priority since the data in the sensor networks has different importance. WSNs face challenges in dealing with the issues of congestion due to their constraint nature and complex algorithms. During the last decade, the tremendous congestion control algorithms and schemes have been proposed to solve the issue of congestion by ensuring fair delivery of packets to the base station. This thesis explores the different congestion detection algorithms and schemes, which are especially targeted for the WSNs and presents a survey of congestion control mechanisms used in WSNs. In addition, the study in this thesis would find out the common features which may help in the future research.

This thesis analyzes a scenario where the distribution system operator needs to estimate whether the average power demand in a given period is above a predetermined threshold using a 1-bit memoryless scheme. Specifically, individual smart-meters periodically monitor the average power demand of their respective households to inform the system operator if it is above a predetermined level using only a 1-bit signal. The communication link between the meters and the operator occurs in two hops and is modeled as binary symmetric channels. The first hop connects individual smart meters to their corresponding aggregator, while the second connects different aggregators to the system operator. In the first set of analysis, the decision making only happens by the network operator in the second hop and aggregators in the first hop only work as relay nodes which only forward the information it has received from the smart meters. AND and OR decision rules are studied in this scenario. Moreover, in the second set of analysis, the decision about the power demand happens in two stages based on the received information bit. Meaning that the decision making happens both by the aggregators in the first hop and network operator in the second hop. We consider here three decision rules in the second scenario: AND, OR and MAJORITY. Our analytical results indicate the circumstances (i.e. how frequent the meters experience the consumption above the defined threshold) and the design setting (i.e. decision rules) that a low error probability can be attained. We illustrate our approach with both theoretical and numerical results from actual daily consumptions from 12 households and 3 aggregators. Also, we derive closed-form equations for the average decision error probability as a function of the system parameters (e.g. number of sensors, communication error, sensing error) and the input signal characterization. The first set of simulations are done in Matlab. Since the second set of data are provided in Excel; thus, the simulations are done using Visual Basic.

This thesis is about testing a system’s actual performance under the IEEE 802.15.4-2011 UWB specifications, with a compliant device DW1000. It is of great interest to test the synchronization ability of different preamble lengths, which are applied in the IEEE 802.15.4-2011 UWB standard. It is also urgent to know the system’s actual performance in a multipath propagation environment, since the IEEE 802.15.4-2011 UWB standard allows an energy detection (ED) receiver to take advantage of multipath propagation. Theory predicts that a long preamble length can offer better synchronization ability at long distances, and has a longer working range. On the other hand, a long preamble length requires a longer channel occupancy time and more power consumption. Thus, a long preamble may not be the optimal choice for working at short distances. The channel model of the IEEE 802.15.4-2011 UWB standard, which is based upon the Saleh-Valenzuela (S-V) model, can be regarded as block fading. Although multipath propagation can improve the system’s performance by increasing the SNR, the effects of block fading on the system must be taken into account as well. It is pointed out in various papers that the S-V model is precise in predicting a none-line-of-sight (NLOS) environment, while it is not precise in predicting a line-of-sight (LOS) environment. The results of the first part of the measurements show that longer preambles have longer working ranges. However, the longer preambles’ performance, in the term of the packet reception ratio, is not necessarily better than that of the shorter preambles. The results of the second part of the measurements show that the system can take advantage of multipath propagation. On the other hand, the system’s performance might become instable, due to block fading. The receiver and the transmitter will have difficulty in synchronization, if the channel impulse response changes too much within the unsynchronized time period. A longer preamble length tends to offer a longer stable working range. In addition, the path loss of the channel will suddenly increase and then decrease. This phenomenon of a sudden change in the path loss at a certain distance is quite similar to that of a narrowband system.

In this thesis, a steerable antenna system which is able to maintain high connectivity through beamforming towards the strongest base station signal is presented. Main objective of the thesis is to design an antenna system for vehicles operating in poor coverage region or even in no coverage region, in order to enable these vehicles to make a reliable communication link with far away base stations. To achieve this high performance antenna system, several antenna designs were considered and evaluated for their performance. This thesis provides an overview of different antenna designs considered and their pros and cons. Among these designs, the concept of stacked patch antenna was selected for prototype manufacturing because of its good performance in terms of bandwidth, efficiency, gain, impedance matching and ease of fabrication because of its simple structure. Stacked patch antenna was optimized for lower GSM band covering frequencies from 824 MHz to 960 MHz by using electromagnetic simulation software CST Microwave Studio. Finally the stacked patch antenna was fabricated and measurements were performed. The results for both the prototype measurement and simulations are presented and compared. From the comparison, it is concluded that the manufactured antenna element prototype gives high performance as promised by simulation results and is suitable for the adaptive vehicular antenna system.

The usage of massive antenna arrays coupled with millimeter-wave (mmW) transmissions has emerged as enabling technology of the fifth generation mobile communication standard, the 5G. This solution has great potentials to provide Gb/s data-rate and high cell capacity by leveraging the synergy amongst high resolution spatial filtering, adaptive beamforming and channel sparsity. One of the main challenges, however, is related to the implementation and digital processing as with a conventional transceiver architecture, an increase of the number of antennas implies more analog-to-digital (or digital-to-analog) converters, more power amplifiers and baseband units. Subsequently, the energy, factor-size and computational power requirements become impractical. To counter these effects a hybrid transceiver design has been proposed, in which multiple analog front-ends are combined into a single (or multiple) baseband processing unit allowing the transceiver to reduce the complexity of the digital signal processing as well as the power consumption. In this Thesis we investigate different architecture models and evaluate the trade-off between energy consumption and performance in channel estimation. More specifically, we study a hybrid receiver model with 64 antenna elements, parallel digital paths and, for the channel estimation, we consider the adaptive-least absolute shrinkage and selection operator (A-LASSO) algorithm that leverages channel sparsity into the estimation. Simulation results have shown that a transceiver architecture with only four base-bands performed best over the different cell sizes. Compared to the fully digital receiver this results in tenfold power consumption reduction according to analysis.

Efficient use of wireless spectrum is needed, due to enormous increase in wireless devices during last few years. In this context lot of effort is being done to make an intelligent and cognitive radio system, which can use the spectrum opportunistically. The ratio of the signal average power to the interference plus noise average power is called signal to interference plus noise ratio (SINR). SINR is one of the important parameters that can help in developing cognitive radio systems, because on the basis of its calculation the spectrum can be utilized efficiently. The principle goal of this thesis is to implement a SINR estimation algorithm for a cognitive radio network (CRN) test-bed. The proposed SINR estimation algorithm is second order moment and fourth order moment (M2M4) SINR estimation algorithm, where M2 and M4 are the second order moment and fourth order moments respectively. The M2M4 estimation algorithm is one of the non-data-aided (NDA) estimation algorithms. Hence, the algorithm takes the received signal as input and calculates the second and fourth moments blindly. The average signal power and average interference plus noise power can be calculated from these second and fourth order moments, their ratio yields the SINR. The M2M4 estimation algorithm is first simulated in MATLAB, and then it is designed for system generator model to draw fair comparison between simulations and system generator model. The experimental evaluation revealed that despite of the word length constraint in the system generator model, it performs reasonably well when compared to the ideal (MATLAB) solution. The M2M4 estimation algorithm is tested and verified by different test cases, to ensure its validity. The algorithm is tested for different signal strengths. The result shows M2M4 is an efficient algorithm for the SINR estimation. However, the proposed architecture could not fit into the aimed hardware because of heavy design since it consume more resources than available.

>Magister Scientiae - MSc / This thesis validates that packet aggregation is a viable technique to increase call capacity for Voice over Internet Protocol over wireless mesh networks. Wireless mesh networks are attractive ways to provide voice services to rural communities. Due to the ad-hoc routing nature of mesh networks, packet loss and delay can reduce voice quality. Even on non-mesh networks, voice quality is reduced by high overhead, associated with the transmission of multiple small packets. Packet aggregation techniques are proven to increase VoIP performance and thus can be deployed in wireless mesh networks. Kernel level packet aggregation was initially implemented and tested on a small mesh network of PCs running Linux, and standard baseline vs. aggregation tests were conducted with a realistic voice traffic profile in hop-to-hop mode. Modifications of the kernel were then transferred to either end of a nine node 'mesh potato' network and those tests were conducted with only the end nodes modified to perform aggregation duties. Packet aggregation increased call capacity expectedly, while quality of service was maintained in both instances, and hop-to-hop aggregation outperformed the end-to-end configuration 4:1. However, implementing hop-to-hop in a scalable fashion is prohibitive, due to the extensive kernel level debugging that must be done to achieve the call capacity increase. Therefore, end-to-end call capacity increase is an acceptable compromise for eventual scalable deployment of voice over wireless mesh networks.

In this dissertation, three multiuser detectors are developed for different application scenarios in direct-sequence code-division multiple access systems. The first detector is an overlapping widow decorrelating detector aimed at asynchronous reverse links. In companion with the design of this detector, a study on the decay property of the ideal decorrelating impulse response is presented, resulting in a quantitative description of the decay rate as a function of the Cholesky factors of the cross-correlation matrix of user signature signals. This result can serve as a guide for determining window length of decorrelating or minimum mean-squared error multiuser detection in asynchronous multiuser systems. Based on this result, a signal-adapted window-length determination algorithm is developed for the proposed detector. Several supporting utilities for efficient implementation of the proposed detector are also described. The second detector is a linear multiuser detector that is also aimed at the reverse links. Particularly, it is desirable for cases where the number of users is small and, thus, significant performance gain over the existing linear multiuser detectors is possible. Unlike in the decorrelating and MMSE detectors, minimizing the bit-error rate is taken as the optimization objective in the proposed detector. To avoid undesired local minima of the highly nonlinear BER cost function, a set of convex constraints is proposed for the optimization problem. It is shown that this constrained optimization problem has a unique solution once the decorrelating detector exists. It is also shown that the proposed detector achieves the best performance among linear detectors for most realistic situations. In addition, a Newton barrier method is developed for efficiently calculating the coefficient vector of the proposed detector (i.e., the solution of the constrained optimization problem). The third detector is an adaptive detector that is aimed at the forward link where information about interfering users is often unavailable. The proposed detector consists of a bank of blind adaptive filters, one for each resolvable path, followed by a channel estimator and a coherent diversity combiner. To allow blind adaptation, the impulse response of each filter is decomposed into two orthogonal parts: one part is fixed as the decorrelating coefficient vector for the path in the absence of interfering users and the other is free to be adapted according to the mean-squared error criterion. Assuming perfect adaptation, the performance of the proposed detector is shown to be between those of the decorrelating detector and the minimum mean-squared error detector. Other studies conducted include the effects of fading on the performance of the proposed detector and the behavior of the proposed blind adaptation algorithm. / Graduate

In this thesis, we propose a number of distributed schemes for wireless communications in the cross layer design context, considering an uplink random access network in which multiple users communicate with a common base station. In addition, we perform a comprehensive study on a splitting based multiuser selection algorithm which is simple, effective, and scales with the network size. First, we investigate a reservation-type protocol in a channel aware ALOHA system. Various Markovian models are used to describe the system and to capture the temporal correlation of the channel evolution. The average throughput of the system is obtained using the Markov Analysis technique and we show that the reservation protocol can achieve better performance than the original channel-aware ALOHA by reducing the collision probability. Second, for better resource utilization in the Opportunistic Multichannel ALOHA scheme, we propose a simple extension to the transmission policy that exploits the idle channels. Performance analysis shows that, theoretically, the maximum system throughput can be improved by up to 63% in the asymptotic case. Through numerical results, it can be seen that a significant gain is achieved even when the system consists of a small number of users. Third, we consider a splitting based multiuser selection algorithm in a probabilistic view. Asymptotic analysis leads to a functional equation, similar to that encountered in the analysis of the collision resolution algorithm. Subject to some conditions, the solution of the functional equation can be obtained, which provides the approximations for the expected number of slots and the expected number of transmissions required by the algorithm in a large system. These results shed light on open design problems in choosing parameters for the algorithm when considering the delay and the overhead jointly. A typical example is to optimize the parameters that minimize the weighted sum of these measures of interest.

The primary focus of this dissertation is on the analyses of self-resonant bent antennas. The need for the accurate characterization of such antennas due to their growing importance in present day wireless communications is the motivation for this work. To this end, several self-resonant bent antennas are analyzed which includes an inverted-L antenna (ILA), a meander-line dipole (MLD) antenna, a meander-line bow-tie (MLBT) antenna, a dual meander antenna, and a printed meander antenna. A simple analytical model, based on the induced EMF method, is presented to compute the input impedance of the ILA. First, a sinusoidal distribution of current on the antenna, with zero current at the end is assumed, and then an expression for the input impedance is derived using the near-fields of the antenna. The accuracy of the formulation is verified by comparing the results computed using it with that from NEC [1] computation. Unlike the analytical solutions available in the literature, our proposed solution is not restricted to antennas that are electrically small. In addition the new formulation can be extended to treat other antennas, such as the T-antenna, the folded unipole antenna, and the loop-loaded monopole antenna. The input impedance, radiation pattern, and gain of the MLD and MLBT antennas are computed and correlated with their parameters. Input impedances of both antennas are computed using NEC. Simple analytical models are presented to compute the radiation patterns of the MLD and the MLBT antennas. For each antenna, a sinusoidal distribution of current is assumed and closed-form expressions for the radiation fields are derived. The results computed using the analytical models are verified by comparing them with the results from the NEC computation. Since in each model the radiation pattern of an antenna is expressed in terms of ready to evaluate algebraic expressions, the computation of such pattern is fast and easy. The input impedance and radiation characteristics of a dual meander antenna are computed using NEC. Similarly as before the input impedance, radiation pattern, and gain of this antenna are also correlated with its parameters. The input impedance and radiation pattern of a planar printed meander antenna are investigated using the Finite-Difference Time-Domain (FDTD) technique. The antenna is modeled on a dielectric substrate both in the presence and absence of a metallic ground plane. Characteristics of the antenna are examined as function of dielectric constant, and substrate thickness. New results of input impedance, radiation pattern, and gain are presented which are vital for the design of such antennas. Several novel applications of self-resonant bent antennas are described. First, a wide-band dual meander-sleeve antenna is designed, manufactured, and measured for application in dual frequency vehicular personal communication. The antenna can operate simultaneously in the 824-894 MHz and 1850-1990 MHz bands of the PCS system. Second, an MLBT dipole is introduced as a feed for plane sheet reflectors. Numerical results computed using NEC show that the feed when used in front of a plane sheet reflector, results in superior radiation characteristics than a conventional dipole feed, namely, it reduces the reflector dimension by 46% for the same front to back ratio, beam width and gain. Finally, a compact plane sheet reflector antenna is described that uses an MLBT monopole feed. Since the antenna uses a monopole, a balun is not required. This antenna has a gain and half-power beam width of 8.4 dBi and 94° respectively. / Graduate