As networked systems grow in size and dynamicity, management systems must become adaptive to changing networking conditions. The focus of the work presented in this thesis is on developing engineering principles for adaptive management systems. We investigate three problems in the context of adaptive management for networked systems. First, we address the control of the performance of an SMS system. We present a design for policy-based performance management of such systems. The design takes as input the operator's performance goals, which are expressed as policies that can be adjusted at run-time. The system attempts to achieve the given goals by periodically solving an optimization problem that takes as input the policies and traffic statistics and computes a new configuration. We have evaluated the design through extensive simulations in various scenarios and compared it with an ideal system. A prototype has been developed on a commercial SMS platform, which proves the validity of our design. Second, we address the problem of decentralized continuous monitoring of network state variables with configurable accuracy. Network state variables are computed from device counters using aggregation functions, such as SUM, AVERAGE and MAX. We present A-GAP, a protocol that aims at minimizing the management overhead for a configurable average error of the estimation of the global aggregate. The protocol follows the push approach to monitoring and uses the concept of incremental aggregation on a self-stabilizing spanning tree. A-GAP is decentralized and asynchronous to achieve robustness and scalability. We evaluate the protocol through simulation in several scenarios. The results show that we can effectively control the fundamental trade-off in monitoring between accuracy and overhead. Third, we aim at improving the performance of the policy distribution task: the mechanism that provides the right policies at the right locations in the network when they are needed. Policy distribution is a key aspect for developing policy-based systems that scale, which is a must for dynamic scenarios. We present a scalable framework for policy distribution. The framework is based on aggregating the addresses of the policies and applying multipoint communication techniques. We show the validity of the framework in a case study. / QC 20101115

In this work, a method to achieve reduced beam-squint in fully metallic leaky-wave antennas is proposed and its applicability for high frequency, high gain, base station antennas in future communication networks is indicated. The antenna is built in gap waveguide technology, where the fundamental mode is allowed to leak by removal of one of the waveguide walls. The leakage is varied along the structure for reduced side lobes and a stable radiation pattern is obtained by coupling the leaked energy from the waveguide through a dispersive prism-lens. The gap waveguide is formed as a groove, sided by three rows of a glide-symmetric holey EBG-structure on one side, suppressing propagation of waves in that direction, and one row of vertical square pins, with tailored heights for optimal leakage, on the other. Beyond the single row of tailored pins, a prism-lens is placed. The prism lens is made of multiple rows of equally spaced and dimensioned, vertical square pins. The dispersive nature of the TE10-mode inside the waveguide is canceled by the oppositely dispersive prism-lens and a stable radiation beam (<1 degree beam-squint) is achieved over a 20% bandwidth. Two antennas are realized, both operating with a center frequency of roughly 60 GHz. The first design is optimized for single-beam operation such that the achieved efficiency is close to 90% across the band and the side lobe levels are below -20 dB. The second design is optimized for dual-beam operation such that two highly directive beams can be obtained, simultaneously or separately. The second design is placed in a 1D-array configuration for electrical beam-steering in one plane, and beam-switching in the orthogonal plane. The two antennas are simulated using CST Microwave Studio. / I det här arbetet föreslås en metod för att reducera skanningen av strålnings-riktningen i helt metalliska läckande-vågantenner. Dess applicerbarhet till hög-frekventa, direktiva, basstationsantenner i framtida kommunikationsnätverk är indikerad. Antennen består av en "gap"-vågledare, där den fundamentala moden tillåts läcka genom att den ena sidoväggen tagits bort. Läckaget varieras längs strukturen för att minska sidlobs-nivån och ett konstant strålningsfält uppnås genom att låta den läckta energin brytas genom en dispersiv prism-lins. Vågledaren formas som en fåra, brevid vilken, på ena sidan, tre rader av glidsymmetriska håliga EBG-strukturer placeras för att undertrycka propagering av vågor i den riktningen, och på andra sidan, en enstaka rad av vertikala, fyrkantiga, pinnar placeras, vilkas höjd är skräddarsydd för optimalt läckage. Bakom den enstaka raden med pinnar placeras prism-linsen. Linsen består av flera rader av lika stora, jämnt fördelade, vertikala fyrkantiga pinnar. Frekvensberoendet hos den fundamentala TE10-moden i vågledaren motverkas av det omvända frekvensberoendet i linsen och ett stabilt strålningsfällt (<1 grad skanning) uppnås med 20% bandbredd. Två antenner realiseras, båda med centerfrekvensen 60 GHz. Den första designen är optimerad för en enkel stråle, så att nästan 90% effektivitet och mindre än -20 dB sidlober uppnås över hela bandet. Den andra designen är optimerad för att tillåta två direktiva strålar, samtidigt eller enskilt. Den andra designen staplas också för att forma en 1-dimensionell gruppantenn, vilket tillåter elektrisk utstyrning i ett plan, och strålningsväxling i det ortogonala planet. De två antennerna simuleras med hjälp av CST Microwave Studio.

The interest in wireless communications has grown constantly for the past decades, leading to an enormous number of applications and services embraced by billions of users. In order to meet the increasing demand for mobile Internet access, several high data-rate radio networking technologies have been proposed to offer wide area high-speed wireless communications, eventually replacing fixed (wired) networks for many applications. This thesis considers cross-layer optimization of multi-hop radio networks where the system performance can be improved if the traditionally separated network layers are jointly optimized. The networks we consider have links with variable transmission rates, influenced by the allocation of transmission opportunities and channels, modulation and coding schemes and transmit powers. First, we formulate the optimal network operation as the solution to a network utility maximization problem and review decomposition methods from mathematical programming that allow translating a centralized network optimization problem into distributed mechanisms and protocols. Second, particular focus is given to networks employing spatial-reuse TDMA, where we develop detailed distributed solutions for joint end-to-end communication rate selection, multiple time-slot transmission scheduling and power allocation which achieve the optimal network utility. In the process, we introduce a novel decomposition method for convex optimization, establish its convergence and demonstrate how it suggests a distributed solution based on flow control optimization and incremental updates of the transmission schedule. We develop a two-step procedure for distributed maximization of computing the schedule updates (maximizing congestion-weighted throughput) and suggest two schemes for distributed channel reservation and power control under realistic interference models. Third, investigate the advantages of employing multi-user detectors within a CDMA/TDMA framework. We demonstrate how column generation techniques can be combined with resource allocation schemes for the multi-access channel into a very efficient computational method. Fourth, we investigate the benefits and challenges of using the emerging OFDMA modulation scheme within our framework. Specifically, we consider the problem of assigning sub-carriers to wireless links in multi-hop mesh networks. Since the underlying mathematical programming problem is computationally hard, we develop a specialized algorithm that computes optimal near-optimal solutions in a reasonable time and suggest a heuristic for improving computation at the price of relatively modest performance losses. / <p>QC 20101117</p>

To deliver a reliable communication service it is essential for the network operator to manage how traffic flows in the network. The paths taken by the traffic is controlled by the routing function. Traditional ways of tuning routing in IP networks are designed to be simple to manage and are not designed to adapt to the traffic situation in the network. This can lead to congestion in parts of the network while other parts of the network are far from fully utilized. In this thesis we explore issues related to optimization of the routing function to balance load in the network. We investigate methods for efficient derivation of the traffic situation using link count measurements. The advantage of using link counts is that they are easily obtained and yield a very limited amount of data. We evaluate and show that estimation based on link counts give the operator a fast and accurate description of the traffic demands. For the evaluation we have access to a unique data set of complete traffic demands from an operational IP backbone. Furthermore, we evaluate performance of search heuristics to set weights in link-state routing protocols. For the evaluation we have access to complete traffic data from a Tier-1 IP network. Our findings confirm previous studies that use partial traffic data or synthetic traffic data. We find that optimization using estimated traffic demands has little significance to the performance of the load balancing. Finally, we device an algorithm that finds a routing setting that is robust to shifts in traffic patterns due to changes in the interdomain routing. A set of worst case scenarios caused by the interdomain routing changes is identified and used to solve a robust routing problem. The evaluation indicates that performance of the robust routing is close to optimal for a wide variety of traffic scenarios. The main contribution of this thesis is that we demonstrate that it is possible to estimate the traffic matrix with good accuracy and to develop methods that optimize the routing settings to give strong and robust network performance. Only minor changes might be necessary in order to implement our algorithms in existing networks. / QC 20101105

Background: 6G mobile communication is one of the fastest-growing fields of technology. The present 5G mobile networks will not be adequate to meet society’s wireless connectivity demand in the near to mid-term future. A new generation of wireless mobile networks has to be developed to address this demand. With the current spectrum already congested in 5G networks, the future 6G networks will have to be operated in the high mmWave and sub-THz frequency bands. Along with this, the parallel advancement in wireless communication and sensing made the researchers understand that these two fields have a lot of things in common in terms of signal processing algorithms, devices and system architecture. This has motivated research on integrating communication and sensing into the same spectrum and system which is a major focus in 6G. Thus, the integration of mobile sensing and mobile communication known as Joint Communication and Sensing (JCAS) will be a key feature of 6G as it enhances spectral efficiency. The usage of higher frequency bands offers a wider bandwidth for the increasing data rate demands. This also enables transceivers to employ massive antenna arrays coupled with wider bandwidth to aid in high resolution sensing of the target devices.                    Objectives: The present research focuses on JCAS that allows a common transmission signal to be used jointly for both communication and sensing. The need for simultaneous transmission and reception on the same frequency and channel for sensing creates a full duplex problem. The foundation of current communication systems is either based on time division duplexing (TDD) or frequency division duplexing (FDD), which avoids simultaneous transmission and reception at the same frequency due to the significant self-interference that would need to be managed. A basic challenge involved while building a full duplex system is the self-interference reduction. The research addresses self-interference mitigation. Methods: Simulation is done in MATLAB to verify the objectives. Results: The suitable candidate spectrum bands for the potential applications ofJCAS along with their sensing profile were identified. A path loss model suitablefor JCAS applications was developed. NR waveform can be used for sensing andcommunication. The digital self- interference mitigation technique was able to handle the self- interference cancellation budget much greater than the self- interference cancellation budget that was allocated for it and establish full duplex. Conclusions: Thus, the thesis explored the candidate frequency bands suitable forJCAS applications, sensing profile of these frequency bands in terms of use cases that can be addressed. The thesis also developed a path loss model suitable for JCAS applications. NR signal performance was evaluated for sensing capability as well. A mono static sensing environment is modeled to study full duplex problem and a technique to handle self-interference mitigation is developed and evaluated

The growth of wireless networks has been considerable over the last decades. Due to the broadcast nature of these networks, security issues have taken a critical role in today’s communications. A promising direction towards achieving secure communications is information theoretic secrecy, which is an approach exploiting the randomness of the channels to ensure secrecy. Based on this approach, there has been a recent surge of interest in a potential cooperation between users to enhance the secrecy of communications. In this thesis we investigate the interaction between cooperation and secrecy. In particular the contributions of the thesis can be divided into two parts. In the first part, we study cooperative strategies for secrecy for wireless channels. Our goal is to evaluate the effect of fading and limited CSI on the eavesdropper’s channels. In that purpose we consider a scenario where a helper aims at increasing the secrecy of the communication between a source and destination in the presence of an eavesdropper. Several strategies are discussed for the helper, namely decode-and-forward, amplify-and-forward, and cooperative jamming. We introduce the secrecy outage probability, the conditional secrecy outage probability and the secure throughput as secrecy measures. For each measure, we investigate and compare the secrecy performance of cooperation. We furthermore elaborate a system optimization in terms of strategy selection, node positioning, power allocation and rate design. In the second part, we consider cooperation in the 4-node scenario against a more sophisticated adversary: an active eavesdropper, which can either passively eavesdrop, or jam the transmission. A game-theoretic perspective is a natural way to analyze the competitive interaction between the helper and the eavesdropper. Therefore we define several secrecy games, for which we find the Nash and Stackelberg equilibria as well as the corresponding secrecy rate outcomes. Another important consideration in this scenario is the interaction between the source and the helper, which we model and solve as a Stackelberg game, and we illustrate its impact on the achievable secrecy rates. / <p>QC 20120822</p>

Cooperative transmission is considered to be a key-technique for increasing the robustness, the efficiency, or the coverage of wireless communication networks. The basic concept is that the information transmission from a sender to a receiver can be aided by one or several relay nodes in a cooperative manner under constraints on power, complexity or delay. The main part of this thesis is devoted to studies on practical realizations of cooperative communication systems. Coding solutions that implement the decode-and-forward protocol in three-node relay channels are proposed by employing convolutional and Turbo codes. Distributed Turbo coding (DTC) was the first technique to bring parallel code concatenation into relay networks. To complement the research on parallel concatenated codes, we propose distributed serially concatenated codes (DSCCs) which provide a better error-floor performance and an increased robustness compared with DTCs. Thereafter, we present a flexible distributed code design which can be adapted to the channel conditions in a simple way. For both the cases with perfect and limited channel-state information, the adaptive coding scheme outperforms static codes, like DTCs and DSCCs, in terms of transmission rate and application range. The aforementioned implementations of relaying are based on blockwise decoding and re-encoding at the relay. In some applications, however, these techniques are not feasible due to limited processing and storage capabilities of the relay nodes. Therefore, we propose to combine instantaneous relaying strategies with bit-interleaved coded modulation. A significant gain can be obtained by using sawtooth and constellation rearrangement relaying with optimized bit-to-symbol mappings compared with conventional instantaneous relaying strategies and compared with standard mappings optimized for point-to-point communications. Both the parameters of the instantaneous relaying schemes and the bit-to-symbol mappings are optimized to maximize mutual information. / QC20100607

We consider code design for Wyner’s wiretap channel. Optimal coding schemes for this channel require an overall code that is capacity achieving for the main channel, partitioned into smaller subcodes, all of which are capacity achieving for the wiretapper’s channel. To accomplish this we introduce two edge type low density parity check (LDPC) ensembles for the wiretap channel. For the scenario when the main channel is error free and the wiretapper’s channel is a binary erasure channel (BEC) we find secrecy capacity achieving code sequences based on standard LDPC code sequences for the BEC. However, this construction does not work when there are also erasures on the main channel. For this case we develop a method based on linear programming to optimize two edge type degree distributions. Using this method we find code ensembles that perform close to the secrecy capacity of the binary erasure wiretap channel (BEC- WT). We generalize a method of M ́easson, Montanari, and Urbanke in order to compute the conditional entropy of the message at the wire- tapper. This conditional entropy is a measure of how much information is leaked to the wiretapper. We apply this method to relatively simple ensembles and find that they show very good secrecy performance. Based on the work of Kudekar, Richardson, and Urbanke, which showed that regular spatially coupled codes are capacity achieving for the BEC, we construct a regular two edge type spatially coupled ensem- ble. We show that this ensemble achieves the whole capacity-equivocation region for the BEC-WT. We also find a coding scheme using Arıkans polar codes. These codes achieve the whole capacity-equivocation region for any symmetric binary input wiretap channel where the wiretapper’s channel is degraded with respect to the main channel. / QC 20110321

Wireless communication is commonly modeled as a stochastic system. This is justified by the fact that the wireless channel incorporates a number of stochastic effects including fading, interference and thermal noise.One example for a stochastic model is the additive white Gaussian noise (AWGN) model, which has been successfully used to analyze the capacity of the point-to-point channel and some multi-terminal networks. However, the AWGN capacity of most networks is still an open problem. This includes small examples like the relay channel, which consists of just three terminals.In order to progress, it was suggested to investigate deterministic channel models as an approximation of the AWGN model. The objective is to find a deterministic model, which is accessible to capacity analysis. Furthermore, this analysis should provide insights on the capacity of the AWGN model.In this thesis we consider two deterministic models, the linear finite-field model (LFFM) by Avestimehr et at. and the discrete superposition model (DSM) by Anand and Kumar.It has been shown that the capacity of the DSM is a constant gap approximation of the AWGN capacity for some networks including the parallel relay network (PRN). We find upper and lower bounds on the DSM capacity of the point-to-point channel, the multiple-access channel, the broadcast channel and the PRN. Our bounds are within a constant gap, hence, they yield a constant gap approximation to the AWGN capacity of the PRN.We also show how the LFFM can be utilized to design transmission strategies for AWGN relay networks. A transmission strategy in the LFFM can be translated into a transmission strategy in the AWGN model if it fulfills certain constraints. We consider two sets of constraints, and we show that in both cases the rate in the AWGN model is at most a constant below the rate in the corresponding LFFM. / QC 20110407

The deployment of large communication networks with many autonomous devices has opened new possibilities for transmission. In particular cooperation among the different nodes has been identified as an enabling technology to satisfy the increasing demand of resources. This thesis studies different coding strategies for cooperation in relay channels in the form of compress-and-forward. In the first part of this thesis we consider the application of the newly introduced polar codes for compress-and-forward relaying in relay channels with orthogonal receivers. First we construct polar codes for compress-and-forward relaying based on Slepian-Wolf coding for the scenario where the capacity of the relay-destination channel is large enough. We then consider the more general picture where the capacity of the relay-destination channel is arbitrary. As for Wyner-Ziv coding, we employ nested polar codes for source and channel coding that allow for compression at any desired distortion and exploit the correlation between the observations of the source transmission to minimize the transmission rate over the relay-destination channel. This construction allows for transmission at the prominent compress-and-forward rate under some additional constraints. In the second part of this thesis we propose a new coding strategy for compress-and-forward relaying for half-duplex Gaussian channels. Our code construction is based on simple code concatenation for joint source-channel coding at the relay and iterative decoding at the destination. Finally, we propose several realizations of the structure at the relay and different iterative decoding algorithms in order to adapt the construction to different scenarios. Our simulation results show remarkable performance gains over other cooperation strategies such as decode-and-forward and amplify-and-forward in the scenarios where both source-relay and relay-destination links have low signal-to-noise ratios. / <p>QC 20101206</p>