Future wireless communication systems will utilize the spatialproperties of the wireless channel to improve the spectralefficiency and thus increase capacity. This is realized bydeploying multiple antennas at both the transmitter and receiver.Development and analysis of communication systems utilizing thespatial properties of the channel requires channel models thatproperly reflect these characteristics. Due to the unpredictable nature of the wireless channel, a commonapproach is to model its effects statistically. A few largeworld-wide cooperations, like the third generation partnershipproject (3GPP) or wireless world initiative new radio (WINNER)project, have developed channel models intended for reference andstandardization use. These models are partly based on some bulkparameters that describe the characteristics of the channel overlarger areas of several wavelengths. Such parameters includeshadow fading, angle spread, and delay spread, among others, andare within the WINNER project called large-scale (LS) parameters.In the spatial channel model (SCM) and the WINNER model, theselarge-scale parameters are, however, assumed independent betweenseparate links, i.e., between channels modelling the propagationbetween one mobile and several base stations, or between one basestation and several mobiles. Such assumptions may be valid forsingle-link, singe-cell systems, where each communication link issufficiently separated in either time or frequency. In practice,dependencies between parameters describing separate wirelesschannels is expected. Future systems will allow a dense frequencyreuse, and results from system evaluations based on models withindependent links may be inaccurate. Examples of this may be insystems that exploit the spatial nature of the channel, likemulti-user scheduling using a single carrier, or macro-diversitysystems deploying several base stations. Therefore, it isimportant to analyze multi-node measurements in order to extractand characterize this channel dependence. This thesis focuses on representing the wireless channelstatistically. Through unique multi-site channel measurements andanalysis, key parameters describing the channel namely shadowfading, delay spread and angle spread at both the base station andthe mobile station are extracted. For these parameters, the firstand the second order statistics are derived, and plausibledistributions and models are proposed. Further, the spatial andcorrelation properties of these parameters are analyzed. Moreover,a study highlighting the effect of the independent channelassumption is given, showing the importance of modelling thespatial dependence between the LS parameters when analyzingsystems that utilize the channel's spatial properties.Incorporating the models and correlation properties found hereinfor the LS parameters results in channel models appropriate formulti-node communication analysis and evaluation. / QC20100720 / ACCESS

In the last few years we have been witnessing the beginning of a revolution in the world of wireless communication. The recent launch of advanced smartphones in the mobile market has been welcomed by mobile operators as an opportunity to finally fulfill the promises of 3G and increase the uptake of mobile services. Driven by unprecedented user experience and flat rate pricing, the success of novel mobile platforms for accessing multimedia content and applications has gone far beyond the most optimistic of expectations. Data-hungry services are currently creating significant capacity limitations in many networks, and projections on future content consumption anticipate that with the next generation of mobile devices the stress on cellular infrastructures will increase. Rolling-out additional capacity is not the most profitable of solutions given the current cost structure of mobile infrastructures. Instead, a significant cost reduction can be achieved by revolutionizing the current content provision paradigm. Accordingly, this thesis proposes solutions for making future services and infrastructures more affordable. From an operator perspective, terminal-to-terminal information forwarding has the potential to increase both coverage and capacity, however it introduces opportunity costs and energy losses at the user side. Resource delegation, together with an energy reimbursement scheme, is presented in this thesis to induce user cooperation. The results show that this approach is successful in fostering significant cooperation among users and can create a simultaneous improvement in user utility, data rates and operator revenues. Opening up for public access privately deployed WLANs might radically lower the cost of wireless services. However, since these networks are deployed in an uncoordinated manner, only discontinuous coverage can be provided. To hide the sparsity of the infrastructure to the end-user perception we propose the adoption of opportunistic schemes for content pre-fetching. Results show that already with moderate AP densities a set of archetypical services, including web-browsing, can be provided with sufficient user perceived quality. Epidemic exchange of popular content, and inter-AP cooperation are also shown to further decrease the required AP density. Moreover, to support informed access selection decisions when performing vertical handovers to WLANs, we propose “Word-of-Mouth”, a reputation-based scheme for revealing the QoS of different networks. By epidemically exchanging grades on their experienced service quality, we show that terminal agents can collectively improve their decision making, avoiding selecting networks not meeting the required QoS. Context-based content delivery is presented in this thesis as means to improve the utilization of wireless resources in cellular systems. Since networks are typically dimensioned for peak hour traffic, the BSs are underutilized for significant portions of the day. By adopting context-based architectures, capable of opportunistically utilizing the instantaneous excess of resources for content pre-fetching, we show that a significant amount of additional traffic can be served in already existing networks. The gains introduced by this content provision paradigm over the current “on-demand” solutions are further expressed in terms of lower requirements on BS density, more users in the system and larger files served while reaching the same level of user experience. / QC20100716

The use of multiple transmit antennas is considered a key ingredient to significantly improve the spectral efficiency of wireless communication systems beyond that of currently employed systems. Transmit beamforming schemes have been proposed to exploit the spatial characteristics of multi-antenna radio channels; that is, multiple-input single-output (MISO) channels. In multiuser communication systems, the downlink throughput can be significantly increased by simultaneously transmitting to several users in the same timefrequency slot, by means of spatial-division multi-access (SDMA). Several SDMA beamforming algorithms are available for joint optimal beamforming and power control for the downlink. Such optimal beamforming minimizes the total transmission power, while ensuring an individual target quality of service (QoS) for each user; alternatively the weakest QoS is maximized, subject to a transmit power constraint. In this thesis, both of these formulations are considered and some of the available algorithms are generalized to enable quadratic shaping constraints on the beamformers. By imposing additional constraints, the QoS measure can be extended to take factors other than the customary signal to interference-plus-noise ratio (SINR) into account. Alternatively, other limitations such as interference requirements or physical constraints may be incorporated in the optimization. The proposed beamforming algorithms are also based on a more general SINR expression than previously analyzed in this context. The generalized SINR expression allows for more accurate modeling; for example, non-zero self interference can be modeled in code-division multi-access (CDMA) systems. A major limiting factor for downlink resource allocation is the amount of channel-state information (CSI) available at the base station. In most cases, CSI can be estimated only at the receivers, and then fed back to the base station. This procedure typically constrains the amount of CSI that can be conveyed. In this thesis, a minimum mean squared-error (MMSE) SINR estimation framework is proposed, which combines partial CSI with channel-distribution information (CDI); the CDI varies slowly and is assumed to be known at the transmitter. User selection (scheduling) and beamforming techniques, suitable for the MMSE SINR estimates, are also proposed. Special attention is given to the feedback of a scalar channel-gain information (CGI) parameter. The CSI provided by CGI feedback is studied in depth for correlated Rayleigh and Ricean fading channels. It is shown, using asymptotic analysis, that large realizations of the CGI parameter convey additional spatial CSI at the transmitter; the proposed scheme is thus ideal for multiuser diversity transmission schemes, where resources are allocated only to users experiencing favorable channel conditions. It is shown by numerical simulations that, in wide-area scenarios, feeding back a single scalar CGI parameter per user, provides sufficient information for the proposed downlink resource-allocation algorithms to perform efficient SDMA beamforming and user selection / QC 20100712

Wireless access to the Internet is expected to be very valuable for both individuals and the society. However, advances in transmission technology alone may not be sufficient to support the anticipated demand for higher data rates and greater traffic volumes. Fortunately, a low cost means of increasing capacity is to match wireless infrastructures to the non-uniform spatial distribution of traffic. Multiple radio access standards and base station classes, having different cost and performance, could be combined to create a heterogeneous wireless access network which provides the required data rates and capacities where needed (or desired). In the case of a non-uniform spatial distribution of traffic, the traditional technical performance measures of coverage and capacity are no longer adequate for comparing the cost effectiveness of different network configurations. Therefore in this dissertation, we propose a general methodology to evaluate the total cost and capacity of heterogeneous networks. Moreover, a few promising capacity expansion paths, including multiple cellular standards as well as wireless local area network technologies, have been evaluated for urban scenarios. While results show that macro cellular systems are the most cost effective solution for a uniform spatial traffic distribution, a complementary hot spot layer is for non-uniform traffic distributions required even at a moderate average traffic density. The incremental cost, which is modest as compared to current revenues for operators, is shown to be quite insensitive to the choice of technology used in the hot spot layer. Moreover, if high data rates are demanded on the uplink, then dedicated indoor solutions are required. Which in turn implies that network providers should exploit existing broadband infrastructures to provide the required backhaul connectivity. In order to address non-urban scenarios, especially in sparsely populated areas, where there is insufficient revenue to support multiple independent networks, a multi-operator network sharing network architecture should be employed. This dissertation proposes a priority queuing method to achieve fair sharing of radio resources between operators in such an architecture. / QC 20100730

Wireless technology has revolutionized the world of communications, enabling ubiquitous connectivity and leading every year to several new applications and services embraced by billions of users. To meet the increasing demand for high data-rate wireless services, standardization bodies and vendors released a new generation of standard-based devices capable to offer wide area high-speed and high-quality wireless coverage. More recently, wireless sensor networks (WSNs) have captured the attention of the industry society to migrate substantial parts of the traditionally wired industrial infrastructure to wireless technologies. Despite the increasing appetite for wireless services, the basic physical resource of these systems, the bandwidth, is limited. Therefore, the design of efficient network control mechanisms for optimizing the capabilities of complex networks is becoming an increasingly critical aspect in networking. In this thesis, we explore the application of optimization techniques to resource allocation in wireless systems. We formulate the optimal network operation as the solution to a network utility maximization problem, which highlights how system performance can be improved if the traditionally separated network layers are jointly optimized. The advantage of such cross-layer optimization is twofold: firstly, joint optimization across layers reveals the true performance limits that can be achieved by practical protocols, and is hence useful for network design or performance analysis; secondly, distributed optimization techniques can be used to systematically engineer protocols and signalling schemes that ensure the globally optimal system operation. Within this framework, we consider several challenging problems. The first one considers the design of jointly optimal power and end-to-end rate allocation schemes in multi-hop wireless networks that adhere to the natural time-scales of transport and physical layer mechanisms and impose limited signalling overhead. To validate the theoretical development, we present a detailed implementation of a cross-layer networking stack for DS-CDMA ad-hoc networks in the network simulator ns-2. This implementation exercise reveals several critical issues that arise in practice, but are typically neglected in the theoretical protocol design. Second, we consider networks employing resource scheduling at the data link layer, and we develop detailed distributed solutions for joint end-to-end communication rate selection, multiple time-slot transmission scheduling and power allocation that achieve the optimal network utility. We show with examples how the mathematical framework can be applied to optimize the resource allocation in spatial-reuse time division multiple access (S-TDMA) networks and orthogonal frequency division multiple access (OFDMA) networks. We then make a slight shift in focus, and consider off-line cross-layer optimization to investigate the benefits of various routing strategies in multi-hop networks, and apply these results to a techno-economical feasibility study of cellular relaying networks. Finally, we consider the design of efficient resource scheduling schemes for deadline-constrained real-time traffic in wireless sensor networks. Specifically, we develop theory and algorithms for time- and channel-optimal scheduling of networks operating according to the recent Wireless HART standard. / QC20100726

The use of low cost fixed wireless relays has been proposed as a way to deploy high data-rate networks at an affordable cost. During the last decade, significant academic and industrial research has been dedicated to relays. Protocol architectures for cellular-relaying networks are currently considered for standardization as part of both IEEE 802.16 and 3GPP. Various relaying techniques have successfully been commercialized over the years. This dissertation concentrates on the particular case of large scale use of low cost relays, for which focus is put on signal processing and radio resource allocation, rather than on antenna and radio frequency (RF) design, or on network planning. A key question is how low relay cost is low enough for a relaying architecture to be viable from an economic point of view? We develop a framework for evaluating the viability of relaying solutions. The framework is based on a comparison between the relaying architectures and traditional single-hop cellular architectures. This comparative analysis is done from an operator perspective, and is formulated as a network-dimensioning problem. The associated investment decisions are based on financial measures (cost or profit) and taken under technical constraints (throughput, coverage, etc.).First, we consider a large number of traditional dimensioning scenarios, in which the radio network is design for a predefined traffic demand and target quality of service level. We show that the use of low cost relays can indeed be viable, but that the cost savings vary strongly from case to case and often are only modest. Due to the half-duplex nature of the low cost relays, these relays are best suited for providing coverage to guaranteed data-rates, at low end-to-end spectral efficiency, and in environments with strong shadow fading. The type of environment and the placement of relays are more important than the specific protocols and algorithms used in the network. Therefore, traditional network planning remains an essential and challenging task, which is unlikely to be replaced by large-scale (unplanned) use of relays.Second, we suggest a new direction of research in which the viability of relays is judged considering the entire life cycle of a radio network. We give several examples in which the temporary use of relays is economically viable, especially if the service uptake is slow or the uncertainty about the future demand is high. This is particularly relevant if the last-mile cost of a network is dominated by the backhaul transmission cost, and if relaying is implemented as a feature of an access point, rather than as a new device type. / QC 20100812

This thesis mainly studies the downlink of a wireless multiuser system, where the transmitter has limited knowledge about the communication channels of the users. Key techniques to improve the performance of such systems are, for instance, multiple antennas, multiuser diversity and orthogonal frequency-division multiple access (OFDMA). Common for these techniques is that, to exploit them fully, a cross-layer approach has to be adopted. This means that the scheduling and the signal designs are done jointly and based on parameters from several communication layers. Multiuser diversity can be used to significantly increase system throughput in wireless communication systems. The idea is to schedule users when they experience good channel conditions and let them wait when the channels are weak. In this thesis, several aspects of OFDMA systems that exploit multiuser diversity are investigated. An adaptive reduced feedback scheme for OFDMA is proposed. It significantly reduces the total feedback overhead while maintaining a multiuser diversity gain. The scheme uses clusters of sub-carriers as feedback units and feeds back information about the fading peaks only. It adapts to the number of users so that less feedback per user is required if there are many users in the system. With such a selective feedback scheme, there is a risk that the scheduler has no instantaneous channel quality information for some parts of the spectrum. Better uses for these sub-carriers are investigated. In addition, an alternative based on the the channel quality feedback of some uniformly spaced sub-carriers is proposed. The scheduler estimates the channel quality on the other sub-carriers. Channel-aware scheduling is necessary in order to exploit multiuser diversity. A modified proportional fair (PF) scheduler is proposed. It incorporates individual target bit-rates and delays and a tunable fairness level. An opportunistic beamforming scheme for clustered OFDMA is presented and evaluated. A key aspect of the opportunistic beamforming scheme is that it induces artificial frequency selectivity for users with relatively flat channels. Several aspects of the proposed system are evaluated by means of simulations. In the simulations, the clustered beamforming with the modified PF scheduler performs better than three comparison systems. The modified PF scheduler manages to divide the resources according to the user targets, while at the same time exploiting the multiuser diversity as well as the standard PF algorithm. In many scenarios, the largest gains from having multiple antennas at the basei station come from space-division multiple access (SDMA). In the downlink, this means that data is transmitted to several users simultaneously by using several beams. Opportunistic space-division OFDMA is proposed and evaluated. An enhancement that exploits temporal channel correlation is able to boost the throughput significantly. SDMA based on subspace packings is proposed and evaluated. A set of beamforming matrices (a subspace packing) is made a priori available at the base-station and at all users. In each block, one of the matrices is used for multi-beam transmission. The users pick and feed back the index of one preferred column (beam) from one of the matrices, and the corresponding SINR, which includes all potential inter-beam interference. This enables scheduling of spatially compatible users and accurate rate adaptation, with relatively little feedback. Three different subspace packings are considered and evaluated with simulations. For the i.i.d. Rayleigh fading channel, Grassmannian subspace packings were the best choice. Moreover, a method to further reduce the feedback for large packings is proposed and evaluated. It is based on the arrangement of beams in a graph and the feedback of a neighbor index. Numerical results show that the feedback can be significantly reduced with only small performance losses, even for relatively fast fading channels. / QC 20100819

Multimedia communications over packet networks, and in particular the voice over IP (VoIP) application, have become an integral part of society. However, the unreliable and heterogeneous nature of packet networks has led to a best-effort delivery of services. Delay, limitation of bandwidth, and packet-loss rate all affect the quality of service (QoS). In this thesis, we address two important network impairments in the design of robust multimedia communication systems: packet delay-variation and packet-loss. Paper A considers the mitigation of the effect of packet delay-variation for audio communications by introducing a buffer at the receiver side. A new adaptive playout scheduling approach is proposed to control the buffering length, or, equivalently, the packet playout deadlines, in response to varying network conditions. A Wiener process is used to model the fluctuation of the buffering length without any playout adjustment. The playout scheduling problem is then reformulated as a stochastic impulse control problem by taking the playout adjustment as the control signal. The proposed approach is shown to be the optimal solution to the new control problem. It is demonstrated experimentally that the proposed approach provides improved perceived conversional quality. Papers B, C and D address the packet-loss issue. Paper B focuses on the design of a low-complexity packet-loss concealment (PLC) method that is compatible with existing speech codecs for VoIP application. The new method is rigorously motivated based on the autoregressive (AR) speech model and the minimum mean squared error (MMSE) criterion. The effect of model estimation error on the prediction of the missing speech segment is also considered and an upper bound for the prediction error is derived. Both the theoretical and experimental results provide insight in the performance of the heuristically designed PLC methods. On the other hand, Paper C and D consider an active packet-loss-resilient coding scheme, namely multiple description coding (MDC). In general, MDC can be used for the transmission of any media data. Paper C derives a simple and accurate approximation of the rate-distortion lower bound of a particular multiple- description scenario and then demonstrates that the performance loss of some practical MD systems can be evaluated easily with the new approximation. Paper D studies the performance limit of a vector Gaussian multiple description scenario. An outer bound to the rate-distortion region is derived, and the outer bound is tight when the problem specializes to the scalar Gaussian case. / QC20100830

Wireless sensor networks have been proposed as a cost effective and easy to deploy alternative to traditional wired systems in a multitude of application scenarios ranging from industrial automation to healthcare monitoring. They are expected to enable an unparalleled number of new services that will bring countless benefits to society. However, low power communications of sensor nodes operating in unlicensed bands face several challenges and are easily corrupted by transmissions of other collocated wireless networks. This problem has recently raised reliability concerns which have been tremendously enhanced by the proliferation of wireless devices we have been witnessing during the last years in the few available regions of the spectrum. This dissertation studies how to achieve reliable communications by proposing different ways for the energy aware management of the radio interference problem. The use of wireless sensing technologies has been envisaged in a broad variety of settings: for this reason it is not possible to identify a unifying solution for the problem of interference, but rather it is necessary to tailor the design of communication schemes accounting for the specific communication paradigm adopted by sensors, the traffic pattern generated by the expected application, as well as for the channel conditions experienced by nodes. When packet transmissions are addressed to a single receiver, cognitive access schemes can be utilized and sensors can opportunistically select for their transmissions when to access a certain channel or which channel to access so as to avoid interference. We provide an energy aware design for communication schemes implementing these ideas and evaluate their energy performance by means of experiments using real hardware. Our results indicate that the first approach should be considered only for sporadic packet transmissions over channels presenting limited interfering activities; channel adaptation should instead be preferred for large bulks of data or when the risk of operating in heavily interfered frequency bands is high. We further propose and evaluate a novel adaptive frequency hopping algorithm: this algorithm has been shown to be very efficient in mitigating the negative effects of interference allowing at the same time to avoid the use of the energy costly spectrum sensing algorithms required by cognitive access schemes. However, none of these three approaches may be suitable for scenarios where packet transmissions are addressed to multiple receivers. To deal with the packet losses that nodes may experience over noisy or interfered channels we envisage the use of fountain codes and show how it is possible to engineer such a coding solution so as to reduce the complexity and overhead introduced by the encoding and decoding procedures. The resulting codes provide an efficient way for disseminating data over multi-hop wireless sensor networks. Results obtained in this dissertation can be of great utility for designers of sensor applications who can use them in order to select the most energy efficient way to achieve reliable interference-aware communications. / QC 20101124

Wireless Sensor Networks (WSNs) provide a way to bridge the gap between the physical and the virtual worlds. They promise unprecedented abilities to observe and understand large-scale, real-world phenomena at a fine spatial-temporal resolution. Their application in Developing Countries is evenmore interesting: they can help solve problems that affect communities. The number of potential applications in such an environment is huge: water monitoringand crop modeling are just two examples. This thesis will analyze the potential of WSNs in Developing Countries, and tackle three of the main problems that their deployment poses: 1. Power. Power consumption is an important issue in deployments. For WSN nodes this is a well-addressed issue. Most commercial solutions today assume that WSN gateways (the devices that provide the interface between the nodes and the network infrastructure) will encounter ideal scenarios in terms of power when deployed. In a Developing World scenario, the gateway must operate with bounded energy supplies. The gateway should have sufficient stored power to save sensed data with high probabilities of service interruptions due to power loss. 2. Connectivity. Network connectivity in many Developing Countries is unreliable. Data gathered by the sensor nodes (motes) must therefore be stored safely in the gateway and transferred when a network connection is available. WSN deployments can encounter different network topologies such as wired, wireless and mesh, and should be flexible enough to interact with each of them. A two layer architecture with long wireless links above a wireless sensor network is a practical alternative for providing connectivity in the access network. 3. Quality of WSN links. To implement reliable and robust sensor networks, we need to understand the variation of link quality and battery behavior in a real world environment. Low-power transmitters have a limited range, and it is important to understand communication patterns. Energy is the scarcest resource of WSN motes, and it determines the lifetime of WSNs. Motes are meant to be deployed in various environments, including remote and hostile regions; consequently, they must use little power and one need to make sure that all batteries last the same amount of time. Also, battery level has an impact on routing. There is still research to be carried out to make WSNs suitable for deploymen tin Developing Regions. Following the most recent developments of sensor networks, this thesis discusses what ICT4D researchers could do to accelerate the dissemination of this new technology and proposes prototype solutions for some of the three problems mentioned above. These include: • Efficient link quality models. Link quality models are important tools upon which the deployment of wireless sensor networks depends. They allow the selection of efficient working parameters that enable the information collected by sensor networks to be routed efficiently from WSN nodes to a sensor base station. Using two testbed WSNs based on two different technologies, we analyzed spatial and temporal behavior of link quality and derived good working parameters for sensor network deployments. These parameters have been used in related works to design new routing protocols for sensor networks. • Robust and flexible gateways. Building upon the assumption that a WSN gateway to be deployed in developing regions should be designed at low cost with battery backup to maintain a continuous supply of electric power in absence of power grid, we proposed in this thesis two low-cost solutions that 1) meet low-power consumption and high storage capabilities constraints 2) are based on web technologies and (3) allow long range deployment for information dissemination. These solutions are developed around two smart board systems. • Long wireless links. Besides power energy limitations, limited range is one of the main factors which has delayed large scale deployments of wireless sensor networks. Building upon the assumption that next generation sensor networks will be employed in multilayer network environments with a WiFi gateway network layered above islands of sensor networks, we presented the deployment of several long wireless links. These prototypes revealed that long distance links can be a practical, inexpensive alternative for connecting wireless sensor networks while providing access to Internet in Developing Countries. • Water quality management. We developed a water quality monitoring system to de deployed in Malawi. Water quality measurementadds another dimension to the issue of power consumption of the WSN system since one has to take into account the contribution of the energy consumed by the water quality sensors in the overall energy consumption of the water monitoring system. As low-power sensors for water quality are not yet commercially available, we proposed an energy consumption minimization strategy where a wake-up mechanism that triggers sleeping/wake-up modes is used to reduce energy consumption. / QC 20101207