Passive Internet-of-Things (IoT), a new paradigm based on battery-free devices, is a promising technology to enable several use cases that require connectivity with stringent requirements in terms of cost, complexity, and energy efficiency. These use cases span critical sectors, such as healthcare, transportation, and agriculture. Passive IoT relies on the development of technologies such as radio frequency (RF) energy harvesting, low-power computing, and backscatter communication. Particularly, backscatter communication allows devices to modulate its information on external RF signals that are backscattered to the receiver or reader. BC considers the following elements: a carrier emitter (CE), a reader, and a backscatter device (BD). The main BC configurations are monostatic BC (MoBC), ambient BC (AmBC) and bistatic BC (BiBC). In a MoBC setup, the CE and reader are co-located and share parts of the same infrastructure. A monostatic system suffers from round-trip path loss, and requires full-duplex technology if the same antennas are simultaneously used for transmission and reception. In an AmBC setup, CE and reader are in different locations, while the CE is not considered dedicated. AmBC uses ambient sources to transmit information, such as Wi-Fi, Bluetooth, and TV signals. In BiBC, the CE and reader are also spatially separated from each other, but there is a dedicated CE. In addition, BiBC can operate in half duplex mode, thus avoiding the complexity associated to the full-duplex operation. Due to the double path-loss effect on the two-way backscatter link, the received backscattered signal is typically weak compared to the direct link interference (DLI) from a CE. This requires a high dynamic range of the circuitry in the reader. As a result, a high-resolution analog-to-digital converter (ADC) is required to detect the weak backscattered signal under heavy DLI; this represents a great limitation because ADCs are major power consumers. Nonetheless, the benefits provided by multiple-antenna and distributed multiple-input multiple-output (MIMO) technologies can be explored to circumvent the limitations of BiBC, which is the main focus of this thesis. In this context, the contributions of this thesis are two-fold. First, we propose a novel transmission scheme that includes a protocol for channel estimation at the multiple-antenna CE as well as a transmit beamformer design to suppress the DLI between the two ends of a bistatic link (namely CE and reader) and increase the detection performance of the BD symbol. Further, we derive a generalized log-likelihood ratio test (GLRT) to detect the symbol/presence of the BD and provide an iterative algorithm to estimate the unknown parameters in the GLRT. Simulation results show that the required dynamic range of the system is significantly decreased while the detection performance of the BD symbol is increased, by the proposed algorithm compared to a system not using beamforming at the CE. For the second part, we consider BiBC in the context of cell-free MIMO networks by exploring the optimal selection of CE and reader among multiple access points, leveraging prior knowledge about the area where the BD is located. First, a maximum a posteriori probability (MAP) detector to decode the BD information bits is derived. Then, the exact probability of error for this detector is obtained. In addition, an algorithm to select the best CE-reader pair for serving the specified area is proposed. Finally, simulation results show that the error performance of the backscatter communication (BC) is improved by the proposed algorithm compared to the benchmark scenario.

Digital communication has gone through much development in the past decade. The literature in this field has developed and improved as well, but the educational materials still lack illustrative descriptions. The modulation techniques are a very complex area that needs practical implementation to facilitate intuitive understanding for engineering students. This thesis presents an educational application where users conduct experiments in digital modulation techniques, ASK, PSK, QAM, and OFDM. The product is a simulation application of the transmission of information from transmitter to receiver with a theoretical description. The application is programmed within the MATLAB environment, and the result is presented with a GUI that contains signal diagrams for each step of the modulation and demodulation process. This tool provides an illustrative description of the modulation techniques without any need for more extensive mathematical knowledge. / Digital kommunikation har genomgått mycket utveckling under det senaste decenniet. Litteraturen inom detta område har utvecklats och förbättrats, men utbildningsmaterialet saknar fortfarande illustrativa beskrivningar. Digitala Modulation teknikerna är ett mycket komplext område som kräver praktisk implementering för att underlätta intuitiv förståelse för ingenjörsstudenter. Denna studie presenterar en pedagogisk applikation där användare genomföra experiment inom digital modulerings-tekniker som ASK, PSK, QAM och OFDM. Produkten är en simulerings applikation för överföring av information från sändare till mottagare med teoretisk beskrivning. Applikationen är programmerad i Matlab utvecklingsmiljö, och resultatet presenteras med ett GUI som innehåller signal diagram för varje steg i modulerings- och demoduleringsprocessen. Detta verktyg ger en illustrativ beskrivning av modulerings-teknikerna utan behov av mer omfattande matematiska kunskaper.

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The feasibility of analyzing an optical communication system using standard electronic circuit analysis programs such as SCEPTRE and ASTAP is demonstrated in this thesis. The transmitter (laser and modulator) and receiver (detector and amplifier) are modelled using equivalent circuits for the various components. The transmission medium (optical fiber) is modelled using an impulse response function. Two fibers are investigated--a step-index fiber and a square-law (parabolic index) fiber.

A formula for the average fade time of the intensity of a plane optical wave traveling through atmospheric turbulence is developed. The model employed involves isotropic, homogeneous statistics using a lognormal distribution for the channel. The analysis is based on the fact that the logarithm of the irradiance is normally distributed and uses the work of S. O. Rice who developed such an expression for a zero mean, Gaussian process. The analysis employs the covariance function and the Taylor frozen turbulence hypothesis which results in an expression for the autocorrelation function.

The average fade time of the fluctuations in the intensity of laser light propagation in the atmosphere is analyzed. Experimental results show that the average fade time for a propagation distance of 208 meters, in the weak turbulence regime, is well described by formulas developed from a Log-Normal distribution. A circuit that calculates real time statistics of fade time is designed, then validated using a sine wave input whose fade time statistics are predetermined. The experimental signal is then tested for stationarity, and the average fade time is measured. A curve fitting is then performed for the experimental data and compared theoretical results.

This thesis evaluated the performance of four different virtual private networks (VPNs): IP security (IPsec), OpenVPN, SSH port forwarding and SSH using virtual interfaces. To evaluate these VPNs, three comparative performance tests were carried out in which the maximum throughput of each VPN was measured. In every test, a specific parameter was varied to observe how it affected the VPNs throughput. The parameters varied were the type of transport layer protocol used, the encryption algorithm used and whether the VPN used compression or not. The results showed, among others, that when TCP traffic was transferred through the VPN and AES-128 was used as encryption algorithm in a Gigabit Ethernet network, the throughput for SSH port forwarding was 168 Mbit/s, 165 Mbit/s for IPsec, 95,0 Mbit/s for SSH using virtual interfaces and 83,3 Mbit/s for OpenVPN. These results are to be compared to the through put measured when no VPN was used, 940 Mbit/s. Three conclusions are drawn from the results of the performance tests. The first conclusion is that the throughput of a VPN depends on the technology the VPN solution is based on, the encryption method that is used and the type ofdata that is sent over the VPN. The second conclusion is that IPsec and SSH port forwarding are the most effective VPNs of the ones compared in this thesis, while OpenVPN and SSH using virtual interfaces are less effective. Lastly, it is concluded that although the different parameters affected the throughput of each VPN, the relation between the VPNs is the same in almost every test. In other words a VPN that performs well in one test performs well in every test.

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Abstract This thesis presents a comparative analysis of two network automation tools, Ansible and Netmiko, within a virtualized environment. The primary objective is to evaluate and contrast the performance, scalability, and user complexity associated with each tool, providing insights that can help network engineers make informed decisions regarding their automation strategies. The study was conducted by setting up a controlled virtual network topology in which both network automation tools could be assessed. GNS3 and VirtualBox were used to set-up the virtual network topologies and simulate real-world network scenarios. Findings from the experiments indicate that Ansible, while slower in configuring a sole router, scales more efficiently as network complexity increases, benefiting from its parallel execution capability. In contrast, Netmiko shows strengths in environments where processing power is a significant consideration and for when the network is extremely congested. In terms of user complexity, configuring small-scale environments with the same tasks requires fewer symbols for Netmiko than Ansible though this result represents only a narrow use-case and further research is needed.