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>

Energy efficiency is essential for many industrial and commercial wireless network applications. In this thesis, we propose an analytical framework to model and design protocols for multi-hop wireless networks for industrial control and automation. We study the mutual interaction among medium access control (MAC), routing, and application layers. Accordingly, we provide three main contributions. First, MAC and routing layers are considered. The carrier sense multiple access (CSMA) of the unslotted IEEE 802.15.4 standard is modeled for multi-hop communications using the specifications of the IETF routing over low power and lossy networks (ROLL). The analysis considers the effects induced by heterogeneous traffic due to the routing mechanism and the node traffic generation patterns, and the hidden terminals due to the reduced carrier sensing capabilities. The interde-pendence between end-to-end performance indicators (reliability, delay, and energy consumption) and routing decisions is described. It is shown that routing decisions based on reliability or delay tend to direct traffic toward nodes with high packet generation rates, with significant negative effects on the energy consumption. Second, we propose TREnD, a cross-layer protocol solution that takes into account tunable performance requirements from the control application. An optimization problem is posed and solved to select the protocol parameters adaptively. The objective is to minimize the energy consumption while fulfilling reliability and delay constraints. TREnD is implemented on a test-bed and it is compared to existing protocols. The protocol model and analysis are validated through experiments. It is shown that TREnD ensures load balancing and dynamic adaptation for static and time-varying scenarios. Eventually, a building automation application is presented by considering the design of a robust controller for under floor air distribution system regulation. The communication performance of an IEEE 802.15.4 network is included in the controller synthesis. We show the impact of reliability and delay on the temperature regulation for synchronous and asynchronous networks. / QC 20110121

Multi-connectivity and network densification are two solutions intended to enhance user throughput and reliability. These solutions are critical since 5G NR uses a wide range of frequency bands, which exhibit different varying radio coverage characteristics. This work studies the user equipment (UE) performance using multi-connectivity within an ultra-dense network (UDN) deployed in an urban city environment. By being connected to more than one access node simultaneously, the UE should benefit from increased reliability and data rates at the expense of a potentially increased power consumption. In this letter, we have constructed an urban city environment and a context-aware UE power consumption model. The performance of the UE is assessed with an uplink power control scheme for multi-connectivity and a novel multi-connectivity scheme is proposed. Our simulation results show that dual-connectivity increases performance by up to 44% and 27% in average downlink and uplink throughput, respectively. Similarly, tri-connectivity does the same by up to 45% and 25%. At mid load (forty-five users), the average increase in UE power consumption compared to single-connectivity is 25% and 60% for dual-connectivity and tri-connectivity, respectively. Dual-connectivity increases global UE energy efficiency by up to 30%. Within an urban environment, dual-connectivity decreases the RLF rate by 20% compared to single-connectivity for high speed users. / <p>Presentation done within the 5th PERCCOM Summer School that took place in Skellefteå. </p> / European Erasmus Mundus programme PERCCOM - PERvasive Computing & COMmunications

In the future of intelligent transport systems, vehicles will communicate with other vehicles and infrastructure. Through this communication, vehicles can cooperate in traffic situations such as intersections and highway merging. This communication can also facilitate sharing of information about other vehicles. ETSI provides a protocol for this communication, the ETSI ITS-G5 protocol. With this protocol, these vehicular ad-hoc networks (VANETs) can decrease traffic congestion, road accidents and travelling time. However, communication also comes with a risk. Communication requires trust, otherwise maliciously intended actors can spread false information which if acted upon might lead to traffic jams, traffic crashes and potential deaths. As a countermeasure ETSI has an architecture for reporting suspect abnormal behaviour: misbehaviour reports. Misbehaviour reports are sent to a centralized authority able to discontinue vehicles' involvement in the VANET. ETSI does not include a warning system between vehicles until the misbehaviour report has been analyzed by the authority. After that, all vehicles will have to experience the abnormal behaviour first-hand without warning. A vehicle-to-vehicle warning system would help vehicles heed extra caution when meeting these misbehaving vehicles, decreasing the risk of interacting with them. The challenges of such a system are protecting the reporter's and reported's integrity, bad actors sending false reports and the system being quicker than the authority. To protect the integrity of vehicles in the VANET, this thesis proposes proving the misbehaviour using non-interactive zero-knowledge proofs, allowing to share of evidence without revealing more than necessary to prove the abnormal behaviour.

Over the last decade, both the Internet and mobile telephony has become parts of daily life, changing the ways we communicate and search for information. These two distinct tools are now slowly merging. The topic of this thesis is TCP over wireless, and the automatic control that is used both within the system, from the link-layer power control to the end-to-end congestion control. It consists of three main contributions. The first contribution is a proposed split-connection scheme for downloads to a mobile terminal. A wireless mobile terminal requests a file or a web page from a proxy, which in turn requests the data from a server on the Internet. During the file transfer, the radio network controller (RNC) sends radio network feedback (RNF) messages to the proxy. These messages include information about bandwidth changes over the radio channel, and the current RNC queue length. A novel control mechanism in the proxy uses this information to adjust the sending rate. The stability and convergence speed of the proxy controller is analyzed theoretically. The performance of the proposed controller is compared to end-to-end TCP Reno, using ns-2 simulations of realistic use cases. It is shown that the proxy control is able to reduce the response time experienced by users, and increase the utilization of the radio channel. The changes are loalized to the RNC and the proxy; no changes are required to the TCP implementation in terminal or server. The second contribution is the analysis of an uplink channel using power control and link-layer retransmissions. To be able to design the link-layer mechanisms in a systematic way, good models for the link-layer processes, and their interaction with TCP, are essential.The use of link-layer retransmissions transforms a link with constant delay and random losses into a link with random delay and almost no losses. As seen from the TCP end points, the difference between such a link and a wired one is no longer the loss rate, but the packet delay distribution. Models for the power control and link-layer retransmissions on the link are used to derive packet delay distribution, and its impact on TCP performance is investigated. The final contribution considers ways to optimize the link-layer processes. The main result is that TCP performance, over a wireless link with random retransmission delays, can be improved by adding carefully chosen artificial delays to certain packets. The artificial delays are optimized off-line and applied on-line. The additional delay that is applied to a packet depends only on the retransmission delay experienced by that same packet, and this information is available locally at the link. / QC 20101216

Personal mobile multimedia services are rapidly become popular. They leverage the combination of mobile and Internet-based communications, the increased capabilities of the third generation of mobile communications (3G), and the flexibility of a common transport and service platform. Cost and quality are critical factors for the success of IP multimedia services. 3G is based on hetereogeneous networks, characterized by a mixture of wired and wireless links with very different requirements on the communication itself. Furthermore, the "always best connected" paradigm requires the design of the new applications to accomodate different types of access. It turns out that the cellular link is typically the most constrained, e.g. in terms of limited bandwidth and high delay, especially when IP is used as transport. Security is an important aspect of the service. Never before has this awareness been more present, not only among technicians, but also among users. Users want the possibility to protect themselves and their data and private matters, in a way that no one else can interfere and that private communications are guaranteed to remain private. The use of the Internet as a common transport backbone, a notoriously open and adverse environment, and in general the use of the IP technology, which is receiving a bad (media) reputation in terms of the security breaches it opens, calls for an end-to-end security solution for IP multimedia services. There are also scenarios where end-to-end protection of other information is an absolute must, for example public safety communication, catastrophe and emergency communication, corporate, or government communication. This research addresses the design of a security solution for IP multimedia, striving in particular for the promotion of end-to-end security on a large scale. This research builds on the concept of a "transport-friendly" security solution: security always comes at a cost, hence it is important to control its impact on services, otherwise security is generally sacrificed. This research focused on the protection of the media traffic. It first identified the requirements that arise from IP multimedia applications in hetereogeneous networks, examined existing security solutions, and concluded that they did not fullfil the requirements raised by 3G environments. This has motivated the design of two new security protocols that are transport-friend to meet the identified requirements, so as to have a low impact on services. These two protocols have then been integrated to provide a security solution for IP multimedia in hetereogeneous networks. The research leading to this licentiate has been mostly conducted in the IETF (a standardisation body), and its results were also adopted byothers, e.g. 3GPP services. / <p>QC 20101125</p>