Accurate channel knowledge is indispensable to the practical success of channel-aware wireless communication technologies. However, channel estimates obtained from pilot symbols rapidly become outdated due to fast time variations of multipath fading channels. To reduce system cost, antenna subset selection reduces radio frequency (RF) chain components. For systems selecting a subset of a plurality of antennas for reception, this outdated channel information is a significant impediment to selection and data decoding reliability. In this thesis, training-based schemes for antenna selection (AS) for time-varying channels which account for practical constraints such as training, packetization and antenna switching time are proposed based on discrete prolate spheroidal sequences. They only operate with knowledge or estimates of the Doppler frequency and the channel signal-to-noise ratio (SNR), but do not require detailed statistical correlation knowledge. A pilot-based AS scheme for time-varying frequency-flat channels for single input-multiple output (SIMO) systems selecting one of a plurality of antennas using packet or symbol-rate antenna switching is first proposed. It is demonstrated that the presented scheme provides significant performance gain over AS methods using Fourier-based orthogonal training as well as over single antenna systems with perfect channel knowledge. Analytical expressions for the symbol error probability (SEP) of M-ary phase-shift keying (MPSK) for systems employing the suggested techniques are provided. The second part of this thesis investigates the more general case of selecting a subset of a plurality of receive antennas. A new pilot-based receive antenna subset selection algorithm for time-varying frequency-flat channels is presented. The proposed AS algorithm is shown to outperform AS methods based on Fourier prediction/estimation as well as SIMO systems with perfect channel knowledge. Analysis of MPSK and quadrature amplitude modulation (MQAM) SEP for systems with receive AS is provided. The combination of AS and orthogonal frequency division multiplexing (OFDM) over the more realistic time-varying and frequency-selective fading scenario is examined in the final part. Training schemes for receive AS using packet-rate antenna switching for SIMO and multiple input-multiple output (MIMO) OFDM systems are developed. The suggested schemes exhibit a superior performance over AS methods using either linear interpolation/extrapolation or Fourier prediction/estimation techniques. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2013-01-30 16:47:06.057

In the modern era of wireless communications, radios are becoming increasingly more cognitive. As the complexity and robustness of friendly communications increases, so do the abilities of adversarial jammers. The potential uses and threats of these jammers directly pertain to fourth generation (4G) communication standards, as well as future standards employing similar physical layer technologies. This paper investigates a number of threats to the technologies utilized by 4G and future systems, as well as potential improvements to the security and robustness of these communications systems. The work undertaken highlights potential attacks at both the physical layer and the multiple access control (MAC) layer along with improvements to the technologies which they target. This work presents a series of intelligent, targeted jamming attacks against the orthogonal frequency division multiplexing (OFDM) synchronization process to demonstrate some security flaws in existing 4G technology, as well as to highlight some of the potential tools of a cognitive electronic warfare attack device. Performance analysis of the OFDM synchronization process are demonstrated in the presence of the efficient attacks, where in many cases complete denial of service is induced. A method for cross ambiguity function (CAF) based OFDM synchronization is presented as a security and mitigation tactic for 4G devices in the context of cognitive warfare scenarios. The method is shown to maintain comparable performance to other correlation based synchronization estimators while offering the benefit of a disguised preamble. Sync-amble randomization is also discussed as a combinatory strategy with CAF based OFDM synchronization to prevent cognitive jammers for tracking and targeting OFDM synchronization. Finally, this work presents a method for dynamic spectrum access (DSA) enabled radio identification based solely on radio frequency (RF) observation. This method represents the framework for which both the cognitive jammer and anti-jam radio would perform cognitive sensing in order to utilize the intelligent physical layer attack and mitigation strategies previously discussed. The identification algorithm is shown to be theoretically effective in classifying and identifying two DSA radios with distinct operating policies. / Ph. D.

In recent years, the wireless data traffic grew exponentially, which was further accompanied by an increasing demand for higher data rates. Towards this aim, Terahertz band (0.1-10 THz) communication is envisioned as one of the key wireless technologies of the next decade. The THz band will help to overcome the spectrum scarcity problems and capacity limitations of current wireless networks, by providing an unprecedentedly large bandwidth. In addition, THz band communication will enable a plethora of long-awaited applications ranging from instantaneous massive data transfer among nearby devices in ultra-high-speed wireless personal and local area networks, to ultra-high-definition content streaming over mobile devices in 5G and beyond small cells. The objective of the thesis is to establish the physical layer foundations of the ultra- broadband communication in the THz band. First, a unified multi-path propagation channel is modeled in the THz band, based on ray-tracing techniques. The wideband characterization are analyzed, which include the distance-varying spectral windows, the delay spread, the wideband capacity and the temporal broadening effects. Second, a multi-wideband waveform design for the THz band is proposed to improve the distance and support ultra- high-speed transmissions. Third, two algorithms for timing acquisition in the pulse-based wireless systems are developed, namely the low-sampling-rate (LSR) algorithm, and the maximum likelihood (ML)-based approach. Fourth, the distance-aware bandwidth resource allocation schemes for the single-user and multi-user THz band networks are developed. Fifth, a three-dimensional (3-D) end-to-end model is developed and characterized, which includes the responses of the graphene-based reflectarray antenna and the 3-D multi-path propagation. The provided physical layer analysis in this thesis lays out the foundation for reliable and efficient ultra-high-speed wireless communications in the THz band.

Cooperative Spectrum Sensing (CSS) allows Cognitive Radio Networks (CRNs) to locate vacant spectrum channels and to protect active Primary Users (PUs). However, the achieved detection accuracy is proportional to the duration of the CSS process which, unfortunately, reduces the time of useful communication as well as increases the Co-Channel Interference (CCI) perceived by an active PU. To overcome this, this thesis proposes three CSS strategies, namely the Dual-Threshold CSS (DTCSS), the Maximum CSS (MCSS), and the Max-Min CSS (MMCSS). These strategies reduce the number of reporting terminals while maintaining reliable performance and minimal CCI e ect. The performance of these three methods is analyzed, and the numerical and simulations results illustrate the accuracy of the derived results as well as the achieved performance gains. The second part of this thesis studies the impact of CCI on the performance of a number of transmission techniques used by CRNs. These are Chase combining Hybrid Automatic Repeat Request (HAQR), Fixed Relaying (FR), Selective Relaying (SR), Incremental Relaying (IR), and Selective Incremental Relaying (SIR). The performance of these techniques is studied in terms of the average spectral e ciency, the outage probability, and the error probability. To obtain closed forms for the error probabilities, this thesis proposes a novel accurate approximation of the exponential integral function using a sum of exponentials. / Thesis (Ph.D, Electrical & Computer Engineering) -- Queen's University, 2013-02-05 13:39:22.35

In modern communication systems, the rate of transmitted data is growing rapidly. This leads to the need for more sophisticated methods and techniques of implementation in every block of the transmitter-receiver chain. The weakest link in radio communications is the transmission channel. The signal, which is passed through it, suffers from many degrading factors like noise, attenuation, diffraction, scattering etc. In the receiver side, the modulated signal has to be restored to its initial state in order to extract the useful information. Assuming that the channel acts like a filter with finite impulse, one has to know its coefficients in order to apply the inverse function, which will restore the signal back to its initial state. The techniques which deal with this problem are called channel estimation. Noise is one of the causes that degrade the quality of the received signal. If it could be discarded, then the process of channel estimation would be easier. Transmitting special symbols, called pilots with known amplitude, phase and position to the receiver and assuming that the noise has zero mean, an averaging process could reduce the noise impact to the pilot amplitudes and thus simplify the channel estimation process. In this thesis, a novel channel estimation method based on noise rejection is introduced. The estimator takes into account the time variations of the channel and adapts its buffer size in order to achieve the best performance. Many configurations of the estimator were tested and at the beginning of the research fixed size estimators were tested. The fixed estimator has a very good performance for channels which could be considered as stationary in the time domain, like Additive White Gaussian Noise (AWGN) channels or slowly time-varying channels. AWGN channel is a channel model where the only distorting factor is the noise, where noise is every unwanted signal interfering with the useful signal. The properties of the noise are that it is additive, which means that the noise is superimposed on the transmitted signal, it is white so the power density is constant for all frequencies, and it has a Gaussian distribution in the time domain with zero mean and variance σ2=N. A slowly time varying channel refers to channel with coherence time larger than the transmitted symbol duration. The performance of a fixed size averaging estimator in case of fast time-varying channels is subject to the buffering time. When the buffering time is smaller or equal to a portion of the coherence time the averaging process offers better performance than the conventional estimation, but when the buffering time exceeds this portion of the coherence time the performance of the averaging process degrades fast. So, an extension has been made to the averaging estimator that estimates the Doppler shift and thus the coherence time, where the channel could be assumed as stationary. The improved estimator called Adaptive Averaging Channel Estimator (AACE) is capable to adjust its buffer size and thus to average only successive Orthogonal Frequency Division Multiplexing (OFDM) symbols that have the same channel distortions. The OFDM is a transmission method where instead of transmitting the data stream using only on carrier, the stream is divided into parallel sub-streams where the subcarriers conveying the sub-streams are orthogonal to each other. The use of the OFDM increases the symbol duration making it more robust against Inter-Symbol Interference (ISI), which the interference among successive transmitted symbols, and also divides the channel bandwidth into small sub-bandwidths preventing frequency selectivity because of the multipath nature of the radio channel. Simulations using the Rayleigh channel model were performed and the results clearly demonstrate the benefits of the AACE in the channel estimation process. The performance of the combination of AACE with Least Square estimation (AACE-LS) is superior to the conventional Least Square estimation especially for low Doppler shifts and it is close to the Linear Minimum Mean Square Error (LMMSE) estimation performance. Consequently, if the receiver has low computational resources and/or the channel statistics are unknown, then the AACE-LS estimator is a valid choice for modern radio receivers. Moreover, the proposed adaptive averaging process could be used in any OFDM system based on pilot aided channel estimation. In order to verify the superiority of the AACE algorithm, quantitative results are provided in terms of BER vs SNR. It is demonstrated that AACE-LS is 7dB more sensitive than the LS estimator.

Thesis (M.S.) -- Worcester Polytechnic Institute. / Keywords: distributed beamforming; carrier synchronization. Includes bibliographical references (p.105-106).

No description available.

Next Generation Wireless Networks (NGWNs) are envisaged to be heterogeneous. They will integrate multiple Radio Access Technologies (RATs) on the same platform where the RAT s are expected to work together in a coordinated fashion. Radio resources will therefore be jointly managed. Joint Call Admission Control (JCAC) approach has been adopted to jointly allocate or handle the radio resources amongst the different RATs in an NGWN. A number of JCAC algorithms have been proposed in the literature with the objective of enabling efficient utilisation of the radio resources and ensuring that required QoS requirements are met . However, studies in JCAC have not typically considered the revenue obtained by network operators in NGWNs . With the implementation of NGWNs, user demand for network services is expected to significantly increase. Consequently, one of the challenges envisaged in the NGWNs for network operators is a large decoupling between users' traffic and the revenue obtained . Therefore, it is important to analyse the network operators' revenue in NGWN s and devise means to ensure that sufficient revenue is obtained. This research analyses network operators' revenue in a heterogeneous network environment while maintaining QoS by incorporating pricing in JCAC. An analytical model based on a multi-dimensional Markov decision process is used to model JCAC algorithm in an NGWN. The JCAC algorithm used is load-based whereby calls arriving in the network are admitted to the least loaded RATs. The performance of the algorithm is evaluated using MATLAB. The analysed NGWN consists of two RATs and two service classes. QoS performance is measured with connection-level QoS metrics namely call blocking probability and call dropping probability. Other investigated performance metrics are average number of calls in the systems, user utility and revenue obtained. Performance of the NGWN is carried out with a dynamic pricing scheme incorporated in JCAC. A discount approach is applied to determine new price under the dynamic pricing scheme. User behaviour with price variation is impacted by change of call arrival rate and average call duration. Simulations are also carried out with flat pricing for comparison purposes. The results obtained show that better system performance and high revenue for network operators are obtained with the dynamic pricing scheme. Furthermore, a hybrid pricing model is proposed whereby flat and dynamic pricing schemes are integrated, giving users flexibility of choosing an appropriate pricing scheme. The analysis is conducted with different percentages of users opting for either dynamic or flat pricing schemes. Simulation results show that higher user utility and improved QoS performance are obtained with the hybrid pricing model. Additionally, reasonable operators' revenue is guaranteed with the hybrid pricing model.

Yes / In this research work, a planar crossed exponentially tapered slot antenna with a multi-resonance function is introduced. The presented antenna design is ascertained on a low-cost Rogers 5870 dielectric with a circular schematic. The antenna is designed to support several frequency spectrums of the current and future wireless communications. The configuration of the design contains a pair of crossed exponentially tapered slots intersected by a star-shaped slot in the back layer and a bowtie-shaped radiation stub with a discrete feeding point extended among the stub parts. The crossed exponential slots exhibit a wide impedance, and the star slot generates an extra resonance at the upper frequencies. For S11 ≤ -6, the antenna provides a wide operation band of 1.7 to 5.9 GHz supporting several frequency bands of 3G, 4G, and 5G communication. The fundamental characteristics of the proposed slot radiator are studied, and good performances have been achieved. / European Union’s Horizon 2020 research and innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424.

Ultra-Wideband (UWB) wireless communication systems employ large bandwidths and low transmitted power spectral densities, and are suitable for operation as underlay systems which reuse allocated spectrum. The subject of this dissertation is Multiband Orthogonal Frequency Division Multiplexing (MB-OFDM) UWB for high data-rate communication. We address four main questions: (1) What are the theoretical performance limits and practical system performance of MB-OFDM? (2) What extensions can be used to increase the system power efficiency and range? (3) Is it possible to estimate the system error rate without resorting to time-consuming simulations? and (4) What is the effect of interference from narrowband systems on MB-OFDM, and can this interference be mitigated? As for questions 1 and 2, we investigate the MB-OFDM performance, and propose system enhancements consisting of advanced error correcting codes and OFDM bit-loading. Our methodology includes the development of information-theoretic performance measures and the comparison of these measures with performance results for MB-OFDM and our proposed extensions, which improve the power efficiency by over 6 dB at a data rate of 480 Mbps. To address question 3, we develop novel analytical methods for bit error rate (BER) estimation for a general class of coded multicarrier systems (of which MB-OFDM is one example) operating over quasi-static fading channels. One method calculates system performance for each channel realization. The other method assumes Rayleigh distributed subcarrier channel gains, and leads directly to the average BER. Both methods are also able to account for sum-of-tones narrowband interference. As for question 4, we first present an exact analysis of the uncoded BER of MB-OFDM in the presence of interference from incumbent systems such as IEEE 802.16 ("WiMAX"). We also present a Gaussian approximation for WiMAX interference, and establish its accuracy through comparison with exact analysis and simulations. We then propose a two-stage interference mitigation technique for coded MB-OFDM, consisting of interference estimation during silent periods, followed by metric weighting during decoding, which provides substantial gains in performance in return for modest increases in complexity, and without requiring any modifications to the MB-OFDM transmitter.