In backscatter communication (BC), a passive tag transmits information by just affecting an external electromagnetic field through load modulation. Thereby, the feed current of the excited tag antenna is modulated by adapting the passive termination load. This paper studies the achievable information rates with a freely adaptable passive load. As a prerequisite, we unify monostatic, bistatic, and ambient BC with circuit-based system modeling. We present the crucial insight that channel capacity is described by existing results on peak-power-limited quadrature Gaussian channels, because the steady-state tag current phasor lies on a disk. Consequently, we derive the channel capacity for the case of an unmodulated external field, for general passive, purely reactive, or purely resistive tag loads. We find that modulating both resistance and reactance is important for very high rates. We discuss the capacity-achieving load statistics, rate asymptotics, technical conclusions, and rate losses from value-range-constrained loads (which are found to be small for moderate constraints). We then demonstrate that near-capacity rates can be attained by more practical schemes: (i) amplitude-and-phase-shift keying on the reflection coefficient and (ii) simple load circuits of a few switched resistors and capacitors. Finally, we draw conclusions for the ambient BC channel capacity in important special cases.

In the growing domain of e-health and technology-assisted living, posture monitoring is an essential component. Wearable systems are favorable for ubiquitous operation, and radio signals between on-body nodes have shown to be a suitable means for detecting human body postures and enabling low-cost and low-complexity systems. In order to minimize overall power consumption and increase the comfort for the user, reducing the number of nodes and optimizing their placement on the body is essential. As an extensive analysis on the topology of the wireless body area network (WBAN) has not been performed yet due to a lack of comprehensive data, this work takes a measurement-based approach to the selection of suitable on-body links for posture detection. We analyze and compare the posture classification performance based on low-complexity UWB RSS measurements using Random Forest classifiers. Ranking links and on-body nodes according to their respective importance for the classification task provides a basis for selecting topologies for three different architectures, which consist of (i) the most important links, (ii) selected TRX nodes and (iii) hierarchical TX-RX topologies. With proper selection of only 8 ultra-low- complexity TX and 4 simple RX nodes, a classification accuracy of 91% can be achieved on a very diverse posture set. The influence of measurement time and system bandwidth is analyzed for selected topologies of all three architectures and discussed in the context of implementation with UWB Impulse Radio.

Recent works have demonstrated the suitability of magnetic induction for future wireless and wearable posture detection systems. They showed that such systems can work with a low complexity by relying on purely passive resonant agent coils on all limbs while multiple anchor coils are centralized on the torso with their input impedance measured at a single frequency.In this work, we experimentally study the practical robustness of such a low-cost, low-complexity posture detection approach by introducing posture variations and unintentional, random displacements of the coils. We find that the prior supervised single-frequency classifiers degrade substantially with these additional disturbances unless they are also present in the training data. Motivated by an in-depth consideration of the degradation mechanism, we propose to measure the complex anchor impedance at multiple frequencies and provide numerical experimental evidence of the improved robustness. To facilitate system design, we further quantify the trade-off between classification accuracy, center frequency and bandwidth. We conclude that a recommendable operating point is a carrier frequency of 510 kHz with a measurement bandwidth of 40 kHz. With these specifications, a classification accuracy of 91 % is retained even in the case of reduced training data, that does not comprise the additional disturbances of the test data. In order to mitigate the inherent complexity increase of the multi-frequency operation, we further propose to use the magnitude of the anchor impedances as features for the deployed classifiers. This simplification results in a negligible accuracy degradation of less than 2 %.

This paper studies achievable information rates of backscatter communication systems where the tag performs load modulation with a freely adaptable passive termination. We find that the complex phasor of the tag current is constrained to a disk and that the capacity problem can therefore be described with existing results on peak-power-limited quadrature channels. This allows us to state the channel capacity and the capacityachieving distribution of the load impedance, which is described by non-concentric circles in the right half-plane. For the low-SNR case (SNR < 4.8 dB) we find that channel capacity is achieved by a purely reactive load with Cauchy-distributed reactance. The exposition is based on a system model that abstracts all relevant classes of backscatter communication systems, including RFID. To address practicality, we construct a symbol alphabet that allows for a near-capacity information rate of more than 6 bit per load-switching period at reasonably high SNR. We also find that the rate hardly decreases when typical value-range constraints are imposed on the load impedance.

We designed a wireless and wearable posture detection system for the human body, which uses four measuring coils (anchors) placed on the torso and additional purely passive resonant coils (tags) placed on each limb and the head. Due to low-frequency magneto-inductive coupling between all coils, each different sensor topology, as a result of the user’s posture, leads to a different and possibly unique set of input impedances at the anchors. Learning the relationship between each posture and its corresponding impedance pattern via supervised classifiers thus enables low-cost and low-complexity posture detection.Previously, this concept was only investigated via simulation, which showed promising results. We now take the next step by presenting the first experimental study of this concept. As such, we propose a simple design for testing purposes and use it to assess the system’s real world posture detection capabilities for 14 different everyday postures. Moreover, we examine the system’s robustness against additional disturbances which would occur when using cheaper and simpler measurement equipment. Lastly, we investigate the impact of each individual anchor on the full system performance. Overall, the proposed system classifies all test cases correctly using the direct measurements and also proves to be resistant against additional Gaussian disturbances.

We propose a novel low-cost and low-complexity approach that uses the magnetic near-field detuning caused by passive coils in order to detect and classify movements. This idea can enable ubiquitous health monitoring applications on or within the body. In this work, we employ our idea with the specific goal of detecting human body postures with only a few measuring nodes. More precisely, we place resonantly loaded passive coils (tags) on a person's limbs and investigate how the sole presence of these coils affects the input impedances of secondary coils (anchors) which are located on the torso. Due to the strong magnetic near-field coupling of all coils, each different posture changes the inter-coil links and hence leads to a unique set of impedance measurements. These impedance patterns can conversely be learned to use them for classification purposes. In order to verify this idea, we simulate a joint-based human body model and use numerical approximations for the magnetic-near field coupling to generate realistic noisy data sets of impedance measurements. Based on these data sets, we train support vector machines, which we subsequently use to classify the postures on a separate testing set. We investigate how robust this procedure is against increasing noise levels and variations of the postures. In this process, we identify appropriate operating points that lead to an average classification accuracy of 94 percent.

We consider the class of wireless links whose propagation characteristics are described by a dipole model. This comprises free-space links between dipole antennas and magneto- inductive links between coils, with important communication and power transfer applications. A dipole model describes the channel coefficient as a function of link distance and antenna orientations. In many use cases the orientations are random, causing a random fading channel. This paper presents a closed-form description of the channel statistics and the resulting outage performance for the case of i.i.d. uniformly distributed antenna orientations in 3D space. For reception in AWGN after active transmission, we show that the high-SNR outage probability scales like pe∝ SNR−1/2in the near- or far-field region, i.e. the diversity exponent is just 1/2 (even 1/4 with backscatter or load modulation). The diversity exponent improves to 1 in the near-far-field transition due to polarization diversity. Analogous statements are made for the power transfer efficiency and outage capacity.

Due to the ever-increasing life expectancy, extending the autonomy of elderly people is of great social importance. Posture detection can be a crucial element here. An example of particular significance is fall prevention, whereby critical postures are detected before a fall occurs – in any environment.An on-body posture detection system based on wireless signaling is particularly attractive because it is lightweight, not obstructive, and promotes a ubiquitous use as it does not require any off-body infrastructure. Due to the manifold of human physiques and living environments (i.e. signal propagation and multipath conditions), posture detection based on wireless signals is very challenging. The feasibility of such a system for an extensive set of postures has not been demonstrated and appropriate comprehensive wideband on-body matrix channel measurements are missing. This paper tries to fill this void. We first define an extensive set of postures related to fall prevention. For these 43 postures we perform a large-scale measurement campaign of (18×18) channel impulse response (CIR) matrices between 18 on-body nodes. In order to make the results as representative as possible, we intentionally include variation in each posture and consider various test subjects and indoor environments. The posture detection performance is evaluated for two metrics: (i) total energy of the CIR (ultra low complexity), and (ii) magnitude of CIR (very low complexity). We investigate suitable choices of carrier frequency and bandwidth, and demonstrate that ultra low transmit power wireless posture detection is feasible under real-world constraints for an extensive set of postures.

Wireless localization is a key requirement for many applications. It concerns position estimation of mobile nodes (agents) relative to fixed nodes (anchors) from wireless channel measurements. Cooperative localization is an advanced concept that considers the joint estimation of multiple agent positions based on channel measurements of all agent-anchor links together with all agent-agent links. In this paper we present the first study of cooperative localization for magneto-inductive wireless sensor networks, which are of technological interest due to good material penetration and channel predictability. We demonstrate significant accuracy improvements (a factor of 3 for 10 cooperating agents) over the non-cooperative scheme. The evaluation uses the Cramér-Rao lower bound on the cooperative position estimation error, which is derived herein. To realize this accuracy, the maximum-likelihood estimate (MLE) must be computed by solving a high-dimensional least-squares problem, whereby convergence to local minima proves to be problematic. A proposed cooperative localization algorithm addresses this issue: first, preliminary estimates of the agent positions and orientations are computed, which then serve as initial values for a gradient search. In all our test cases, this method yields the MLE and the associated high accuracy (comprising the cooperation gain) from a single solver run. The preliminary estimates use novel closed-form MLE formulas of the distance, direction and orientation for single links between three-axis coils, which are given in detail.

Falls are a major health risk for elderly people living independently. In contrast to fall detection systems, which raise an alarm after the fall has occurred, fall prevention aims to avoid fall-related injuries by initiating countermeasures before the fall occurs. This requires early and fast detection of critical body postures which may lead to a fall. In this work, we evaluate the feasibility of posture detection based on ultra wideband (UWB) communication links between wireless on-body nodes. Our proposed wireless body area network (WBAN) features a hierarchical architecture with low-complexity agent nodes, medium-complexity relays and a processing unit, which can be located off-body. We demonstrate the feasibility of the concept by performing posture detection on measurements from 18 on-body antennas. Our approach proves to be robust towards external disturbances and performs well even under low requirements for synchronization.

Magneto-inductive systems are extensively used for communication and wireless power transfer. However, they are equally advantageous for close range localization because they provide a high channel gain over short ranges and hardly suffer from non-line-of-sight conditions, synchronization problems, and interference. Their inherent near-field operation also introduces an exciting opportunity, which has hardly been explored to date in a localization context: The use of passive relays in the form of passive resonant coils with known locations. In this paper we systematically investigate the potential of passive relays for magneto-inductive localization with a circuit model which accounts for arbitrarily arranged system nodes. Based on the Cramér-Rao lower bound, we show that with carefully placed relay nodes the position root-mean-square error (RMSE) can be improved by an order of magnitude. However, in order to also benefit from randomly placed relays, we slightly extend their functionality by including the possibility to switch off their loads (binary load switching). With this extension we can exploit the presence of multiple randomly placed relays to improve the RMSE by orders of magnitude and also drastically extend the supportable localization range.

We study the estimation of distance d between two wireless nodes by means of their wideband channels to a third node, called observer. The motivating principle is that the channel impulse responses are similar for small d and drift apart when d increases. Following this idea we propose specific distance estimators based on the differences of path delays of the extractable multipath components. In particular, we derive such estimators for rich multipath environments and various important cases: with and without clock synchronization as well as errors on the extracted path delays (e.g. due to limited bandwidth). The estimators readily support (and benefit from) the presence of multiple observers. We present an error analysis and, using ray tracing in an exemplary indoor environment, show that the estimators perform well in realistic conditions. We describe possible localization applications of the proposed scheme and highlight its major advantages: it requires neither precise synchronization nor line-of-sight connection. This could make wireless user tracking feasible in dynamic indoor settings.

The Received Signal Strength Indicator (RSSI) of Bluetooth Low Energy (BLE) is a popular means for indoor user localization and tracking as it reflects the transmitter-receiver distance and is readily available in all current smartphones. Since fading, shadowing and antenna patterns cause severe RSSI fluctuations, many RSSI-based localization systems use fingerprinting instead of parameter estimation based on a channel model (e.g. trilateration from distance estimates). Fingerprinting however requires a large effort for training data acquisition and frequent updates in dynamic environments. In this paper we focus on wireless access control with BLE. We demonstrate that a practical implementation of such a tracking system can meet the typical demands of generic access control problems with low- complexity parameter estimation techniques, namely trilateration and optional Kalman filtering. Thereby, satisfactory accuracy is enabled by diversity (averaging in space, time and frequency), calibration and appropriate observation space modeling. We find that including the RSSI directly in the observation space renders trilateration obsolete, which reduces complexity even further.

Due to their low complexity, RSSI-based solutions for indoor localization have become increasingly popular in recent years despite lacking the accuracy of more sophisticated localization solutions. One of the main reasons for this lack of accuracy is the highly fluctuating nature of RSSI values as a result of indoor channel characteristics, hardware imperfections and varying antenna radiation patterns. In this paper, we thus critically analyze the log- normal path loss model and its validity with the focus on indoor localization. We show how chip antennas of typical consumer devices affect the RSSI measurements and clarify in what manner this effect can be incorporated into the log-normal model. In this process, we are also able to quantify the impact of small-scale fading and diversity on RSSI-based distance estimation. We furthermore propose a novel calibration scheme that estimates path loss exponents based on a simple training walk and outperforms linear regression in all our use cases. At last, we combine our findings in an implementation of an indoor localization system for a 14×3 m office corridor. On average, our measurements yield a significantly decreased position RMSE of 1.36 m, which compares to the Cramer- Rao lower bound on the position RMSE of 0.71 m in this environment.

This paper studies magnetic induction for wireless powering and the data uplink of microsensors, in particular for future medical in-body applications. We consider an external massive coil array as power source (1 W) and data sink. For sensor devices at 12cm distance from the array, e.g. beneath the human skin, we compute a minimum coil size of 150μm assuming 50nW required chip activation power and operation at 750 MHz. A 275μm coil at the sensor allows for 1 Mbit/s uplink rate. Moreover, we study resonant sensor nodes in dense swarms, a key aspect of envisioned biomedical applications. In particular, we investigate the occurring passive relaying effect and cooperative transmit beamforming in the uplink. We show that the frequency- and location-dependent signal fluctuations in such swarms allow for significant performance gains when utilized with adaptive matching, spectrally-aware signaling and node cooperation. The work is based on a general magneto-inductive MIMO system model, which is introduced first.

In this paper we propose a novel approach for the performance prediction of multistage cooperative broadcast. It is based on inter-node distance distributions, considers each stage independently and employs the expected values of the coverage area and the number of nodes reached. The required distance distributions are derived and a simple but accurate approximation of their expected values is discussed. The proposed approach is then evaluated for a large range of node densities and compared to Monte Carlo simulations. We thereby show that it leads to very accurate performance predictions for the coverage area and the number of nodes reached in each stage, even for low node densities.

We consider a wireless sensor network that uses inductive near-field coupling for wireless powering or communication, or for both. The severely limited range of an inductively coupled source-destination pair can be improved using resonant relay devices, which are purely passive in nature. Utilization of such magneto-inductive relays has only been studied for regular network topologies, allowing simplified assumptions on the mutual antenna couplings. In this work we present an analysis of magneto-inductive passive relaying in arbitrarily arranged networks. We find that the resulting channel has characteristics similar to multipath fading: the channel power gain is governed by a non-coherent sum of phasors, resulting in increased frequency selectivity. We propose and study two strategies to increase the channel power gain of random relay networks: i) deactivation of individual relays by open-circuit switching and ii) frequency tuning. The presented results show that both methods improve the utilization of available passive relays, leading to reliable and significant performance gains.

The magnetic near-field provides a suitable means for indoor localization, due to its insensitivity to the environment and strong spatial gradients. We consider indoor localization setups consisting of flat coils, allowing for convenient integration of the agent coil into a mobile device (e.g., a smart phone or wristband) and flush mounting of the anchor coils to walls. In order to study such setups systematically, we first express the Cramer-Rao lower bound (CRLB) on the position error for unknown orientation and evaluate its distribution within a square room of variable size, using 15 × 10 cm anchor coils and a commercial NFC antenna at the agent. Thereby, we find cm-accuracy being achievable in a room of 10 × 10 × 3 meters with 12 flat wall-mounted anchors and with 10 mW used for the generation of magnetic fields. Practically achieving such estimation performance is, however, difficult because of the nonconvex 5D likelihood function. To that end, we propose a fast and accurate weighted least squares (WLS) algorithm which is insensitive to initialization. This is enabled by effectively eliminating the orientation nuisance parameter in a rigorous fashion and scaling the individual anchor observations, leading to a smoothed 3D cost function. Using WLS estimates to initialize a maximum-likelihood (ML) solver yields accuracy near the theoretical limit in up to 98% of cases, thus enabling robust indoor localization with unobtrusive infrastructure, with a computational efficiency suitable for real-time processing.

In this paper, we consider the resource allocation problem of a quantize-and-forward virtual MIMO receive cooperation scheme, where N sources simultaneously communicate over N relays to a destination. Different to previous work, the second hop links are considered to be non-orthogonal (i.e. a shared backhaul) but with individual rate constraints. This requires a careful resource allocation, leading to an involved non-convex optimization problem. The goal of this paper is to provide and evaluate computationally simple and robust resource allocation algorithms. In this context, we devise two low complexity schemes which simply select a subset of relays and assign equal resources to them. In thorough numerical simulations we then demonstrate that for moderate backhaul rates (as to be expected in wireless virtual MIMO), it is beneficial to serve only a subset of the relays. Furthermore, while large gains are achievable by selecting the optimal relays and assigning equal resources to them, only minor additional gains can be achieved by further optimizing the resource allocation among them, e.g., with a gradient search. Hence, the proposed relay selection schemes lead to very good performance and are a reasonable alternative to costly optimization techniques if low complexity is required.

Magneto-inductive (MI) systems for wireless communications or power transfer usually suffer from coil mis-alignment, because of mobility or deployment. The impact on performance is hard to analyze because mutual inductance is an involved function of geometry. We focus on links whose coil separation exceeds coil dimensions, which is characteristic for communications links and becoming increasingly desired for wireless powering. In this regime, the dipole approximation allows for an accurate closed-form description of misalignment. In order to analyze fully unaligned links, we propose a stochastic misalignment model that assumes uniformly random coil orientation(s). To this effect, we derive the probability density function (PDF) of mutual inductance for practically relevant setups, comprising single coils and collocated coil arrays. This gives rise to a misalignment comparison of beamforming, coil selection, and single-input single-output (SISO) links. The PDFs lead to the expected attenuation due to misalignment. We relate misalignment to communications performance by means of a basic circuit. A byproduct of the stochastic model is the maximum likelihood range estimate for a noiseless mutual inductance measurement under unknown arrangement.

In this paper, we propose to serve the mobile stations in an urban traffic hotspot by combining user cooperation with traffic offloading to the vast amount of residential WLAN access points. Different to existing virtual MIMO approaches, we include local users, which are assigned to the access points and compete for the physical resources. Furthermore, all signal processing shall be done on the mobile station side. This makes the protocol fully transparent for the access points and allows it to be overlaid on the existing infrastructure and protocols. We thereby discuss the necessary coordination with the local users and suggest a hybrid protocol implementation with distributed spatial multiplexing for the WLAN access at 2.4 GHz and a local exchange with message flooding at 60 GHz, all with single omnidirectional antennas. Using a numerical simulation framework with realistic parameters and channel models, the protocol is thoroughly evaluated, considering multiple clusters and spatial reuse. It is thereby shown that the 60 GHz exchange is very efficient even with omnidirectional antennas. For a moderate cluster size of 16 nodes per cluster, a 10 fold increase in the sum rate is achieved. By scaling the bandwidth for the user cooperation this gain can be further scaled.

In this paper, we propose and investigate a self-calibration method which estimates the locations and clock offsets of the anchors based on TOA measurements. The agent locations are not surveyed and hence they can be realized by a single moving agent, which makes the calibration procedure simple and cost effective. Instead of considering a specific clock synchronization scenario, which was the case with previous related works, we systematically classify different clock synchronization cases that may arise in practice and provide a generalized formulation of the self-calibration problem that covers all the synchronization classes. The maximum likelihood (ML) solution of the calibration problem, which jointly estimates all the unknown parameters, is shown to be a non-convex optimization problem. We propose to relax the ML estimator to a semi-definite programming (SDP) problem by applying semi-definite relaxation. To improve the accuracy of the SDP solution, we further refine it with the ML estimator. Simulation results confirm that the proposed method is capable of accurately calibrating TOA based localization systems.

In this paper we present a novel way of jointly estimating the coupling matrix of closely spaced receive antennas as well as the spatial channel of compact MIMO systems. Unlike conventional approaches, we do not assume any knowledge of the types and geometries of the antennas, or their separations, or having line of sight spatial channels. Our algorithm is based only on changing the load impedances of the antennas and measuring the voltages across such loads. We use the nonlinear relation between the load impedances and the compound channel estimates in order to estimate both the spatial channel and the coupling matrix. Our algorithm is of relevance to future communication systems incorporating heterogeneous networks in dense environments. In such networks different users/nodes may occasionally be closely spaced and hence, mutually coupled. Using our algorithm, they can estimate both the coupling matrix and the spatial channel matrix in real-time and thereby optimize their matching networks and signal processing in order to enhance their performance.

Future MIMO terminals are expected to be equipped with a higher number of antennas. A possible intermediary solution between using the optimal but complex SIC decoder and the simple but poor performing MMSE decoder, is to consider different group decoders, namely, group SIC (GSIC) and group parallel decoding (GPD). The performances of such decoders strongly depend on the grouping strategy. In this paper, we introduce a unified framework to handle different group decoders. We use tools from random matrix theory to present a tight approximation of the average sum rate achieved in the case of adopting any kind of these decoders using only statistical CSI. For large number of data streams and fast changing channels, finding the optimal grouping for each channel realization is very complex. We formulate an optimization problem in which we use our developed approximations to find a static grouping which is shown to lead to a performance near to the optimal SIC and much better than the MMSE, especially for high transmit correlation. We also show how the performance depends on different system parameters, such as correlation strength and number of groups.

We study wireless ultra wideband (UWB) sensor networks operating in the environment of marine engines for telemetry purposes. In the harsh propagation environment posed by the engine room, UWB enables robust communication without the need for complex sensor node implementations. For this work, we therefore present the first UWB channel measurements performed around, on, and inside a marine diesel engine, and use these measurements as basis for the design and reliability analysis of a low- complexity UWB communication system.

For a network consisting of inductively coupled nodes, we experimentally demonstrate circuit-based localization of a passive and chipless agent device. By using measurements of the input reflection at multiple anchor nodes, we reconstruct the unknown position of an agent node in a coplanar arrangement for both random and optimized anchor arrangements. A key step for applying this localization scheme in a practical setup is the introduction of a calibration phase which, in contrast to previous work, eliminates the need for explicit knowledge of the involved circuits. Furthermore, we improve localization performance by investigating imperfections in the measurement device and applying appropriate compensation methods. In the employed setup, we demonstrate a median localization RMSE of 1.1 mm, making the proposed scheme a promising method for precise localization in clouds of RFID-like wireless devices.

In this paper we jointly consider the normally distinct worlds of communication system design and circuit theory. We introduce the - up to our knowledge - first algorithm to optimize the uncoupled matching network (UC-MN) at a receiver that uses a linear MMSE equalizer in a MU-MIMO system with compact antenna array at the receiver. We design our adaptive UC-MN in order to either maximize the minimum rate of all users or to maximize the total sum rate. We compare to the conventional matching, which ignores antenna coupling, and to the cases of choosing one of the MNs that either maximizes the achievable rate or maximizes the minimum user rate for joint decoding of all data streams. In all cases, we observe a considerable performance boost. We view our results as an example of the potential of jointly considering circuit theoretical aspects and communication theory, especially for systems adopting large antenna arrays in a confined space.

We consider the downlink of a "post-cellular" network in which base stations (BSs) are no longer evenly spread in the entire area of service but rather placed where they can easily be installed. Accordingly, the BSs are not necessarily close to the users anymore but have to serve distant mobiles with aggressive spatial multiplexing, possibly with the help of additional low-complexity relays. In such networks, it is important to allow for flexible and dynamic cooperation between different infrastructure nodes to achieve high data rates and coverage. To this end, we develop a framework that optimizes the performance by dynamic BS clustering with joint transmission for one- and two-hop communication under practical conditions such as backhaul rate limitations and power control. The network performance is maximized by an evolutionary algorithm that optimizes the BS clustering, relay routing, and power allocation for a beamforming scheme with closed-form solution. We show that a very high performance can be achieved in an efficient way with small overhead and that high data rates can be delivered to areas without wired backhaul access.

In this paper, we address the problem of sensor network localization based on Time-of-Arrival (TOA) measurements under the presence of anchor location errors. We consider the practically appealing setup where the agents are low-complexity transmit-only nodes whose clocks' are not synchronized with that of the anchors. The maximum likelihood (ML) solution of the localization problem is formulated and shown to be a non-convex optimization problem. The relaxation of the ML solution to a convex optimization problem based on semi-definite programming (SDP) is proposed. The proposed localization method is compared with the Cramér Rao lower bound (CRLB) and an existing related approach. Simulation results show that the proposed localization method outperforms the existing scheme, notably when the anchor location uncertainties are the dominant source of errors.

We consider the problem of locating a sensor in a network of nodes which use inductive coupling as means of interaction. Conventional approaches in this setting require the located node to have a power supply or complex circuitry. By analysis of the circuit model underlying inductive coupling, we develop a localization method which allows for passive nodes with a simple layout. The proposed method reconstructs the unknown position from measurements of the input impedance at a set of reference nodes via the Nelder-Mead simplex algorithm. We show that the accuracy of the localization depends highly on the initialization value used for optimization, and provide a heuristic grid search algorithm to increase the quality of the initial position and thus of the localization process. A lower bound on the performance is provided for a two-dimensional setup; it can be closely met by the proposed localization algorithms in a realistic setup. The results show that the discussed approach can provide very high localization accuracy with a low-complexity node configuration.

Compact arrays are known to be associated with antenna coupling and noise correlation. The noise can be either antenna noise, LNA noise or downstream noise. Due to these effects, it was shown that the matching network affects the performance of MIMO systems with coupled receiver antennas. Since the optimal multiport matching network is of very high complexity as well as very narrow operation bandwidth, development of single-port (SP) matching networks that boost the performance became inevitable. In this paper we develop a gradient-search algorithm to design the matching network for achievable rate maximization of multi user MIMO systems. For any combination of noise sources, we rigorously derive the exact gradient of the achievable rate with respect to the components of the matching network. We assume either full knowledge of the spatial channel or knowing its statistical properties. In the later case we optimize the matching network to maximize the Jensen's bound. Substantial performance enhancement is shown when our algorithms are used. Significant reduction in the array area is gained in comparison to the often used λ/2 antenna spacing without taking coupling into account. This can be vital for future wireless systems adopting massive MIMO arrays. Via eigenvalues distribution simulations at different SNR regimes, we show an intuitive link to the communication theory.

In this paper, we consider relay-aided wireless multicasting where one source transmits the same information to multiple destinations. We assume that there is a half-duplex regenerative relay station (RS) which may cooperatively assist the source. Our underlying problems are two-fold: when and how should the source and the RS cooperate to improve the multicast rate? Firstly, we show the required conditions for using the RS for wireless multicasting and propose opportunistic relaying strategies where the RS, when in use, forwards only partial information of the source. Thus, the proposed opportunistic relaying strategies lead to an efficient use of the RS. Secondly, we explain how we can maximise the multicast rate by optimising both the source transmission rate and the time sharing between the source and the RS. It is shown that the proposed opportunistic relaying strategies improve the multicast rate of one-hop (direct links only) wireless multicasting and they outperform other benchmarked relaying strategies.

It is well known that compact arrays introduce spatial channel correlation, antenna coupling, superdirectivity and noise correlation. While these effects have been thoroughly investigated for point to point MIMO, only isolated results are available on the effect of such phenomena on performance of relaying systems. This paper tries to fill part of this void. Specifically we study the impact of lossless and lossy compact antenna arrays on multiuser amplify and forward (AF) relaying in the presence of different noise sources. We optimize the gain allocation for sum rate maximization under a dissipated power constraint. For compact arrays this constraint is more relevant than the commonly used radiated power constraint. We provide an extensive insight-oriented discussion of our results, some of which may appear counterintuitive at first glance. We show that for compact MIMO relaying the standard model, considering only channel correlation but ignoring antenna coupling and noise correlation, is insufficient and may lead to a wrong system characterization.

We consider the up- as well as downlink of a cellular network with multi-user multiple-input multiple-output (MIMO) communication and propose to turn the entire network into a two-hop network where a large amount of relays assist the bidirectional communication. To this end, many two-way relays are spread over the entire area of the network. In addition to introducing the concept of ubiquitous relaying, different two-way relaying protocols are developed, for amplify-and-forward (AF) as well as decode-and-forward (DF) relays. The goal is to efficiently apply two-way relaying to interference limited cellular networks. To achieve this, relaying is combined with a simple form of base station (BS) cooperation that allows canceling some interference terms. The relays are used to reduce the complexity at the terminal nodes and to improve the overall performance by distributed interference management. Our results show that the proposed concept can offer significant gains compared to a conventional multi-user MIMO strategy without relays.

The sum-capacity$C$of a static uplink channel with$n$single-antenna sources and an$n$-antenna destination is known to scale linearly in$n$, if the random channel matrix fulfills the conditions for the Marcenko-Pastur law: if each source transmits at power$P/n$, there exists a positive$c_{0}$, such that$\lim_{n \ura{} \infty}C/n=c_{0}$almost surely. This paper addresses the question to which extent this result carries over to multi-hop networks. Specifically, an$L+1$-hop network with$n$non-cooperative source antennas,$n$fully cooperative destination antennas, and$L$relay stages of$n_{\cal R}$(cooperative or non-cooperative) relay antennas each is considered. Four relaying strategies are assessed based on the interrelationship between two sequences. For each considered strategy${\rm XF}$, there exists a sequence$(c_{L}^{\rm XF})_{L=0}^{\infty}$, such that$c_{L}^{\rm XF}=\lim_{n \ura{} \infty}R^{\rm XF}_{L}/n$almost surely, where$R^{\rm XF}_{L}$denotes the supremum of the set of sum-rates that are achievable by the strategy over$L\!+\!1$hops. This sequence depends on the sequence$(P_{L})_{L=0}^{\infty}$, where$P_{L}$corresponds to the power of the source stage and each of the relay stages in an$L\!+\!1$-hop network. Results are summarized as follows:Decode & forward (DF): For$n_{\cal R}=n$,$c^{\rm DF}_{L}$is constant with respect to$L$, if also$P_{L}$is constant with respect to$L$.

Future wireless networks based on Ultra-Wideband (UWB) will offer localization capabilities with centimeter accuracy. We propose to use this inherently obtainable location knowledge to adapt the transceiver to the multipath channel conditions. This saves overhead for channel estimation and dissemination and enables low cost, low complexity and low power data transmission. In particular, we study the location-aware adaptation of generalized energy detection receivers for UWB impulse radio communication with binary pulse position modulation. Conventionally, these receivers are very vulnerable to narrowband interference. Therefore, we derive transmitter and receiver optimization schemes based on the Signal-to-Interference-plus-Noise-Ratio (SINR): First, we present the SINR optimization based on full channel knowledge. The location knowledge is then incorporated by means of a statistical channel model, which depends on the position of the nodes. Performance evaluation based on a simple channel model is used to give insight about the fundamental behavior of the derived optimization schemes. Moreover, an extensive measurement campaign in a rich scattering environment proves that location information can improve the data transmission and helps to successfully suppress narrowband interference. Performance gains of 2 to 5 dB compared to conventional energy detection can be obtained.

We investigate the potential of restricted PHY co- operation for the downlink of 4G networks. The cooperation is restricted to a cluster of eNodeBs. We distinguish between low and high mobility users, and propose two appropriate cooperation methods. The goal is to guarantee high user rates even on cell edges. We present a simulation study to analyze the spectral efficiency achieved by cooperation methods for urban micro cells. To investigate their influence on coordinated multipoint (CoMP), we consider different frequency allocation strategies and different sector orientations in a cell. In addition, we compare CoMP transmission to Multiuser MIMO and investigate how cooperation can improve power allocation. Based on the results, we provide important insights into future cell planing aspects.

We present the first implementation of a MIMO envelope detection system. Such systems have so far been theoretically studied in the framework of nonlinear MIMO. The design considered employs an envelope detector on each receive antenna. The simplicity of the receiver enables extreme low-cost low-power implementations, making such systems attractive for wireless sensor networks and alike applications. The test bed presented here is a first implementation of such a system and demonstrates detection of spatially-multiplexed data streams using envelope detectors.

In this paper, we consider localization based on the Time-of-Arrival (ToA) of Ultra-Wideband (UWB) impulse radio pulses. We present a receiver structure for ToA estimation, which can be implemented with very low complexity and low power consumption. Standard approaches for ToA estimation of UWB pulses require very high speed A/D conversion, which makes them impractical for implementation. Although ToA estimation based on phase estimation of narrowband signals circumvents that problem, the estimation performance suffers strongly from multipath propagation. We propose to combine the advantages of both techniques by performing spectral estimation at the output of a UWB energy detection receiver. This system benefits from resolvability of multipath components due to a large signaling bandwidth, but requires only low sampling rates. The impact of bandwidth on the ToA estimation accuracy is derived analytically and a closed form approximation of the estimation variance for a multipath channel is given. Moreover, we present a performance evaluation based on measured channels and show that accuracy up to 20 cm can be reached in strong multipath environments.

We consider a MIMO system for very low-power applications, e.g, sensor networks, where the receiver employs an envelope detector at each receive antenna. Properties of such systems have been studied in [1], [2] and [3], where the channel was considered to be perfectly known at the receiver, or a linear channel estimation model was used. In this paper we propose a very simple channel estimation scheme, which allows the non-coherent MIMO envelope detector to estimate all required channel knowledge necessary for maximum likelihood detection. In contrast to linear MIMO system, complete knowledge of the channel coefficients is not required.

We propose a novel transmission scheme for the broadcast phase of two-way relaying systems. The proposed scheme employs network coding at the relay and is able to transmit with asymmetric data rates to the receivers according to their individual link qualities. The idea is that the weaker link receiver exploits a priori bit information in each transmit symbol, so that it only needs to decode on a subset of the transmit symbol constellation. Subject to the same bit error rate constraint, the weaker link receiver can decode at lower signal-to-noise ratio compared to the stronger link. The signal labeling used for mapping bits to symbols at the relay is shown to be crucial for the performance at the receivers, and we provide the criterion and method for finding the optimized labeling schemes. Simulations show that the proposed transmission scheme can be applied to practical scenarios with asymmetric channel qualities, and the optimized labeling greatly outperforms conventional ones at both receivers.

In this paper, the problem of human motion tracking with ultra-wideband radio nodes is addressed. We provide a general maximum likelihood formulation of the positioning problem based on range measurements which can handle synchronous and asynchronous agents. Geometrical constraints on the node topology, which are imposed by the human body, are also taken into account. For a Gaussian ranging error model and the specific problem of arm motion tracking, we derive the maximum likelihood estimation rule and calculate an analytical expression for the unconstrained and constrained Cramér-Rao Lower Bound. With these results, we study analytically and via computer simulations under what circumstances the geometrical constraints lead to performance gains. It is found that the largest benefits are obtained in case of asynchronous agents and for certain arm positions. Intuitive reasons for this phenomenon are given. Finally, we verify these findings and evaluate the position location performance experimentally with range estimates obtained from measured ultra-wideband channel impulse responses including the impact of the human body.

In this work we investigate the impact of imperfect local oscillators (LOs) at the relays in nonregenerative two-hop networks. Due to the specific traffic pattern of multihop networks, hardware imperfections at the relays have a different impact on the system than hardware imperfections at the sources and destinations. Based on the assumption that one LO generates all oscillations in each node, we consider baseband sampling time as well as sampling frequency offsets, RF frequency and phase offsets, timing errors, and phase noise. In the presence of these imperfections, time division duplex (TDD) and frequency division duplex (FDD) relays introduce errors to their forwarded signals. We identify these errors for both types of relays and use Monte-Carlo simulations to compare the performance of a multiuser network employing either TDD or FDD relays with imperfect LOs.

We consider a sensor network, where an access point (AP) communicates with many sensor nodes (SN), which are simple, cheap, low-complexity and low-power communication nodes. Such systems typically use nonlinear modulation and detection, due to their low power consumption. Increasing their performance by means of multiple antennas at the AP and the SNs has not been considered, since this would violate the stringent power and cost constraints at the SN. We consider SNs with MIMO receivers that perform a nonlinear operation on the complex-valued received signal (amplitude or phase detection). These receivers enjoy the low-cost, low-power, low-complexity characteristics that are crucial for a sensor network. Such nonlinear MIMO systems are first introduced and studied here. They bring the high-rate, high-performance world of MIMO systems and the low-cost, low-complexity world of sensor networks together. We only consider the single-user MIMO system between the AP and one SN, and study the fundamental limits of such systems. We compute achievable rates under perfect and noisy CSI at the SN, and observe that these systems also achieve spatial multiplexing gain, albeit different to legacy linear MIMO systems. We quantify and analyze these gains using numerical means, and give insight into the effect of the nonlinearity on the information theoretic limits of nonlinear MIMO systems.

A set of distributed non-cooperating relay antennas is known to be capable of orthogonalizing multiple source- destination pairs in space by coherent amplify-and-forward (AF) relaying techniques. Known relay gain allocation schemes for this setup require either global knowledge of the channel coefficients between all participating nodes or an excess number of relays. The required dissemination of the channel state information (CSI) introduces a major overhead, which in practice may well diminish the spatial multiplexing gain. We introduce a new distributed gradient based gain allocation scheme, which minimizes this overhead. Key to this is the proof, that the gradient of the destination signal-to-interference-plus-noise ratio (SINR) can be calculated in a distributed manner based on local CSI at the relays and very limited feedback from the destinations. In order to minimize the number of iterations in the distributed gradient algorithm we propose two distributed approaches to determine the optimal step size for each iteration. Finally we provide simulation results on the sum rate performance and identify the sequential opening of the spatial channels as a key factor that impacts speed of convergence.

We consider nonlinear MIMO systems that use amplitude-only (envelope) receivers. Such systems are interesting for low-power, low-complexity applications like sensor networks. We study different modulation schemes and the respective diversity order obtained by the maximum likelihood detector in an uncoded system with perfect channel state information. We show that nonlinear MIMO amplitude receivers loose half the available receive diversity when the uncoded transmission rate increases above 1 bit/s/Hz, or when spatial multiplexing is performed.

In this paper, we consider the broadcast (BRC) phase of two-way decode-and-forward (DF) relaying systems. The channel in that phase is called the bidirectional broadcast channel. Its achievable rates are calculated when the self-interference aided channel estimation scheme is applied. We consider a block- fading channel model in the BRC phase and exploit the self- interference that is inherent in two-way relaying techniques to get an initial estimate of the channel at the receiving terminals. This initial channel estimate is utilized to decode the data in the first several time slots of each coherence interval. Data- aided approaches are then employed to improve the channel estimates in the following time slots of each coherence interval. The spectral efficiency improvement for systems employing this self-interference aided channel estimation scheme is quantified by comparing its achievable rates to that of the traditional pilot- aided channel estimation scheme.

Ultra wideband (UWB) communication is a very promising candidate for the use in wireless body area networks (BAN). The high UWB peak data rate allows for medium average data rates in combination with a very low duty cycle, which is the key for a very low power consumption. Devices in a wireless BAN require low complexity. Hence, mainly non-coherent receivers such as energy detector and transmitted-reference receiver are suited. In this paper, the symbol-wise maximum-likelihood (ML) detectors for pulse position modulation (PPM) and transmitted reference pulse amplitude modulation (TR PAM) are derived assuming partial channel state information (CSI) at the receiver. Additionally, also the ML detectors for a combination of PPM and TR PAM are presented. The performance of the derived receiver structures is evaluated using a novel BAN channel model not distinguishing line-of-sight and non line-of-sight situations. This simple channel model is based on 1100 channel measurements in the frequency range between 2 and 8 GHz, which were measured in an anechoic chamber. Using the BAN channel model, performance of the derived receiver structures is evaluated showing that the knowledge of the average power delay profile (APDP) at the receiver improves performance substantially. Requiring only slightly more complexity such receivers are a well suited alternative to non-coherent receivers for the use in a BAN.

In this paper, we propose multiuser multiple-input multiple-ouput (MIMO) one-and two-way relaying protocols. Several wireless MIMO node pairs are establishing either unidirectional or bidirectional transmission links via a single decode-and-forward MIMO relay without causing interference to each other. For both half-duplex two-phase relaying protocols, the relay separates messages of different node pairs spatially by using zero-forcing based methods, whereas, in order to cancel the interference within the bidirectional links, XOR preceding followed by self-interference cancellation is employed. Both sum rate and maxmin fairness optimizations, as well as quality-of-service assurance, are considered for the design of the precoders at the relay. The corresponding optimization algorithms for both one-and two-way relaying have been developed using semideflnite programming, which consider the intersection of multiple access and broadcast phases' rate regions.

In this paper, we propose a novel channel estimation scheme for the broadcast phase of the newly invented two- way relaying technique. Instead of using pilot sequences, we exploit theself-interference, which contains the data known at the receivers, to get a first estimate of the channel. Then a decision- directed iterative estimation process is started to improve the accuracy of the channel estimates. We consider a block fading channel model. The simulation results show that the proposed scheme has similar performance as pilot-aided channel estimation schemes in our simulation environment. Since pilot sequences are no longer needed in our proposed scheme, higher spectrum efficiency is achieved without performance loss.

We consider a transmission system employing orthogonal frequency-division multiplexing (OFDM) with bit- interleaved coded modulation (BICM) and perfect channel state information (CSI) at both transmitter and receiver. For exploiting the transmit CSI, we present adaptive bit loading (ABL) schemes that minimize the bit-error rate at the decoder output. A simple low complexity and a generalized ABL procedure for square lattice quadrature amplitude modulation signal constellations and Gray encoding scheme are proposed. Simulation results show that at low and medium signal-to-noise ratios (SNRs) the simple low complexity ABL scheme performs as well as the general ABL scheme. The benefits of using the generalized procedure are only visible at high SNR.

Multiple input multiple output (MIMO) communication systems that employ nonlinear detectors, i.e., amplitude-only or phase-only detectors, where first introduced. Such systems are low-complexity and low-power alternatives to legacy MIMO systems in applications, e.g. sensor networks, where low power consumption and low cost are very stringent constraints. This work extends the achievable rates study presented, to cover the case where the channel estimation at the receiver contains an additive Gaussian noise term.

We consider coherent wireless MIMO multi-hop communication where nSnon-cooperating source antennas communicate to nDfully cooperating destination antennas via L clusters of nRnon-cooperating relay antennas each. Assuming i.i.d. zero-mean (slow) fading coefficients of finite variance, the sum-capacity of this channel is proven to scale linearly in min{nS, nD} in the limit Lrarrinfin, when the receive SNR is kept constant and nRisinOmega(min{nS, nD}). This capacity scaling is achieved by a quantization based forwarding strategy. Moreover, we show that Slepian & Wolf compression allows to reduce the growth rate of the transmit power per cluster that is necessary to sustain a constant receive SNR at the destination antennas from exponential to linear in L.

In this work we present a demonstrator for coherent linear amplify-and-forward (AF) relaying. Two source/destination pairs communicate with the help of three half-duplex relays in a two-hop manner. We implemented a coherent gain allocation scheme that suppresses inter-user interference at each destination and does not require a global phase reference at the relays. In order to assess the performance of the scheme, we calculated the signal-to-interference ratio (SIR) at both destination nodes. Inter-user interference arises when the local oscillator phases of the relays change independently during one transmission cycle, e.g. due to phase noise. Consequently, the shorter the time between reception and retransmission at the relays, the more accurate the interference suppression. Measurement results show that our demonstrator is able to suppress inter-user interference at both destinations reasonably well.

Two-way relaying, which enables bidirectional simultaneous data transmission between two nodes, is an efficient means to reduce the spectral efficiency loss observed in conventional half-duplex relaying schemes. In this paper, we consider a multiuser cellular two-way relaying scenario with several mobile stations (MSs) at one end of the bidirectional link and a single base station (BS) serving all MSs at the other end. Both the BS and the MSs exchange private messages simultaneously via a single relay node, i.e., concurrent uplink and downlink, in only two time slots, independent of the number of MSs. In the downlink, while the relay separates different MSs spatially, e.g., using either zero-forcing beamforming or zero-forcing dirty paper coding, it benefits from XOR precoding followed by self-interference cancellation, in order to separate messages within a message pair to be exchanged between the BS and each MS. The corresponding sum rate optimization problem is solved with an iterative algorithm based on semidefinite programming.

It has recently been proposed to orthogonalize the data streams of n interfering source-destination pairs by employing an intermediate stage of non-cooperating coherent amplify-and-forward relays. This distributed zero-forcing scheme achieves a spatial multiplexing gain n/2 and does not require multi-antenna nodes. In this paper we generalize the concept from a single to multiple relay stages. Due to the concatenation of relay stages, the gain allocation problem becomes nonlinear. We show that the zero-forcing relay gain coefficients are given by the common roots of a set of polynomials, and we identify the set of network configurations for which such roots exist. In general, there exist multiple common roots. Based on experimental results we study the impact of the root selection and the number of relay stages on achievable rates. Finally, we propose a new scheme to achieve spatial orthogonalization in a single-hop interference network. Our proposal is based on the observation, that the mathematical model of the multihop problem is identical with the one of a single-hop network if we allow signals to be bounced forth and back between sources and destinations.

A wireless body area network with an average throughput of 500 kbps is considered based on ultra-wideband (UWB) pulse position modulation. For a long battery autonomy ultra low power consumption is essential. A FCC compliant ultra low power UWB communication system was presented. By means of a 1% duty cycle at 50 Mbps peak data rate, the power consumption of the system is estimated below 1 mW. To increase inter-symbol interference (ISI) robustness as well as for synchronization, a simple post-detection maximum-likelihood sequence estimator (MLSE) has been presented. In this work, we extend this MLSE approach to over-sampled energy detectors and non-ideal integration windows. Furthermore, we present optimal MLSE metrics based on partial channel state information as a performance benchmark.

We study the potential of cooperative processing in WLAN uplink scenarios where the access points are connected to a backhaul with limited capacity. We evaluate different decoding strategies based on an outage criterion and discuss the influence of the available backhaul rate. Compared to the ergodic analyses in the literature we observe three major differences: Firstly, outage rates are much more robust to the backbone limitation. Secondly, Slepian-Wolf coding (compress & forward processing) does not improve the outage performance significantly compared to pure quantization and forwarding. Finally, quantization based approaches offer substantially higher gains compared to decoding based strategies. Because of the large number of existing WLANs, we conclude that cooperative processing is a key enabler for high rates on future WLAN uplinks: in cases of limiting backhaul capacities (e.g. ADSL broadband connections in residential environments) as well as in cases of large backhaul capacities (e.g. LAN connections in office environments).

In this correspondence, the cumulants of the mutual information of the flat Rayleigh fading two-hop amplify-and-forward multiple-input multiple-output (MIMO) relay channel under independent and identically distributed (i.i.d.) Gaussian input vectors are derived in the large array limit. The analysis is based on thereplicatrickand covers both spatially independent and Kronecker correlated fading. Beamforming at all terminals is restricted to weight matrices that are independent of the channel realization and constant over time. Expressions for mean and variance of the mutual information are obtained. Their parameters are determined by a nonlinear equation system. All higher cumulants are shown to vanish as the number of antennas per terminal,n, grows to infinity. In conclusion, the distribution of the mutual informationIbecomes Gaussian in the largenlimit. In this asymptotic regime, it is completely characterized by the expressions obtained for mean and variance ofI, which are inTheta(n) andO(1), respectively. Comparisons with simulation results show that the asymptotic results serve as excellent approximations for systems with only few antennas at each terminal.

We consider a distributed wireless network where amplify-and-forward relays assist the communication between multiple source/destination pairs. To provide a global phase reference at the relays it suffices for each relay to know the phase difference between its own local oscillator (LO) and the LO of a global master node. Based on a scheme to achieve this, we characterize the phase synchronization inaccuracy caused by additive signal noise and LO phase noise. It turns out that relay phase noise only has a limited impact on the performance. Merely the phase uncertainty during the time between phase estimation and channel measurements at the relays affects the system. Finally, we quantify the impact of phase estimation inaccuracy on multiuser zero-forcing relaying.

A novel cluster-based coherent multi-user (MU) relaying system is proposed, where a number of source/destination pairs communicate concurrently over the same physical channel and a set of amplify-and-forward relays assist them. A secondary short-range wireless system with single/multi-hop unicast or broadcast ability is considered for the channel state information (CSI) dissemination. To reduce the CSI dissemination overhead, the relays are grouped in several independent clusters. Distributed diversity gain is attained at the destinations by applying phase rotations between clusters. The CSI dissemination is analyzed thoroughly, and compared with that of previous works on MU relaying. It is shown that for a given number of clusters and relays, the most uniform distribution of relays to clusters is optimum to minimize the CSI exchange traffic overhead and transmit energy consumption for CSI dissemination; besides it maximizes the average achievable rate for source/destination links. Furthermore, a new solution for a maxmin fairness based relay gain allocation is proposed and formulated as a semidefinite program.

We consider a wireless multiuser network where a number of source/destination pairs communicate concurrently over the same physical channel. A set of amplify-and-forward relay nodes assist the communication by multiplying their received signal with a complex gain factor before retransmission. We propose and compare different strategies for finding the gain factor for every relay terminal, motivated by the chosen figures of merit: average sum rate, diversity, and fairness. A new and practical definition of diversity is provided in the context of our system model. For each strategy we investigate the provided gains in terms of the figures of merit, and the trade-off between them. The existing schemes in the literature act as references to the proposed schemes.

Ultra-wideband (UWB) geo-regioning is an approach to localize UWB transmitters by means of their channel impulse responses. A received channel impulse response is assigned to a geographic region based on a-priori known characteristics of this region. The first approach relying on knowledge of the average power delay profile of the channel was presented and analyzed in (F. Athaus et al., 2005), (C. Steiner et al., 2006). In this paper we extend this method by including the knowledge of the correlation between channel taps. We discuss this new modeling assumption, derive the respective multivariate maximum likelihood decision algorithm, provide an analytic expression for the probabilities of misclassification, and present simulation results based on measured channel impulse responses. A significant performance gain is visible compared to the previous average power delay profile approach, where the taps are assumed to be independent.

Multiple input multiple output (MIMO) communication systems are popular for achieving high rates, but at the same time they suffer from high implementation cost. Hence, their adoption to future distributed systems, e.g., sensor networks, is stalled due to the stringent cost and power constraints of the node design. One solution is the combination of MIMO systems with nonlinear detectors, i.e., amplitude or phase detectors, which are known for their cheap implementation cost and low power consumption. We provide semi-analytical results for achievable rates in MIMO systems that use nonlinear detectors for the fast fading channel, as well as outage probabilities for the same detectors in the slow fading case.

In this paper the ergodic mutual information of the i.i.d. Rayleigh fading amplify-and-forward MIMO relay channel without direct link between source and destination is derived in the large array limit. By means of the replica method an expression is obtained, whose parameters are determined by a nonlinear equation system. This system can be solved in closed form. Comparisons with Monte Carlo simulations show that the asymptotic expression serves as an excellent approximation for systems with only very few antennas at each node.

We consider a low mobility cellular relaying system downlink where two mobile users are served by two neighboring decode-and-forward relays concurrently using the same frequency channel. We propose two cooperative relaying transmission schemes where each relay can choose proper precoding vectors based on its local channel knowledge to transmit data in the second hop. Each user can receive its own data without interference, which simplifies the user receiver design. We show that the diversity of each user's received data signal can be improved by receiving data from multiple relays. In addition, higher array gain can be achieved by the first scheme at the cost of higher synchronization accuracy requirements. Furthermore, we show that the two transmission schemes achieve higher transmission rate than serving different users in separate channels.

In dense radio frequency identification (RFID) systems, the tag collision arbitration is an important issue for fast tag identification. In this paper, we propose a novel transmission control algorithm for RFID systems based on framed slotted ALOHA (FSA) where the number of tags is assumed unknown in advance. The considered algorithm combines the lower bound tag estimation with a new grouping strategy, and uses a collision ratio factor to control the algorithm. The proposed approach maximizes the system efficiency while keeping moderate the time for reading the tags. Simulation results show that the system efficiency using the proposed algorithm is better than using conventional FSA-based algorithms when the number of tags is large. Furthermore, the number of slots to read the tags increases linearly for increasing the number of tags.

A wireless body area network with an average throughput of 1 Mbps is considered based on ultra-wideband pulse position modulation. For a long battery autonomy, a low duty cycle operation of the nodes and thus, a high peak data rate is essential. Due to the moderate path loss, a peak data rate in excess of 50 Mbps is feasible within the Federal Communications Commission's transmit power constraints. However, with current low complexity pulse position detectors, such as the energy detector, the peak data rate is constrained to much lower values, because they are very sensitive to intersymbol interference. To overcome this constraint a simple post-detection maximum-likelihood sequence estimator is introduced which significantly reduces the impact of intersymbol interference. The same maximum likelihood sequence estimator is then used to replace cumbersome synchronization algorithms at the expense of slight performance losses.

Placing relays around the base station (BS) to assist wireless communication is an effective way of extending coverage in cellular systems. This paper provides a quantitative analysis of coverage extension by using decode-and-forward (DF) relays. To describe the relation between the number of relays and the coverage range extension, we introduce the concept of coverage angle and coverage range. We provide analytical upper and lower bounds for the coverage range in a cellular system for any given coverage angle. By means of simulations, we show the tightness of our analytical approach.

We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss

We consider a two-hop MIMO-OFDM communication scheme with a source, an amplify-and-forward relay, and a destination. We examine the possibilities of power allocation (PA) over the subchannels in frequency and space domains to maximize the instantaneous rate of this link if channel state information at the transmitter (CSIT) is available. We consider two approaches: (i) separate optimization of the source or the relay PA with individual per node transmit power constraints and (ii) joint optimization of the source and the relay PA with joint transmit power constraint. We provide the optimal PA at the source (or the relay) with a node transmit power constraint that maximizes the instantaneous rate for a given relay (or source) PA. Furthermore, we show that repeating this separate optimization of the source and the relay PA alternately converges and improves the achievable rate of the considered link. Since the joint optimization of the source and the relay PA is analytically not tractable we use a high SNR approximation of the SNR at the destination. This approximation leads to rates which are quite tight to the optimum.

We consider a two-hop MIMO-OFDM communication scheme with a source, a relay, and a destination. The relay is assumed to be nonregenerative (or amplify-and-forward (AF)). We assume channel state information at each transmitter. (CSIT), source and relay. We present a jointly optimized power allocation (PA) over the subchannels in space and frequency domain at source and relay with a joint transmit power constraint. The PA is based on a high SNR approximation of the SNR expression at the destination. In the low SNR regime, the performance is still very tight to the optimal rate. The presented PA can be computed with low computational complexity and achieves a considerable performance gain compared to a uniform PA at source and relay. To further enhance the performance of the considered scheme, all subchannels, in space and frequency domain, of the source to relay and relay to destination channel are paired according to their actual magnitude.

We consider a two-hop communication scheme using OFDM modulation. The relay is assumed to be nonregenerative (or amplify-and-forward). We examine the possibilities of power allocation (PA) over the frequency subchannels at source and relay, with respect to separate transmit power constraints, if channel state information at the transmitter (CSIT) is given. We provide the optimal PA at the relay (source) that maximizes the instantaneous rate for a given source (relay) PA. Furthermore, we show that alternate, separate optimization of source and relay PA converges to the solution of the joint optimization of source and relay PA. To further enhance the rate of the considered scheme, the OFDM subchannels of the source to relay and relay to destination channel can be paired according to their actual magnitude. In the case of a joint sum power constraint, we propose to allocate the transmit power between source and relay with respect to average channel attenuation of first and second hop. It is shown that this leads to a information rate that is near to the optimal rate achieved by a joint optimization of source and relay PA with joint transmit power constraint.

We study the two-way communication problem for the relay channel. Hereby, two terminals communicate simultaneously in both directions with the help of one relay. We consider the restricted two-way problem, i.e., the encoders at both terminals do not cooperate. We provide achievable rate regions for different cooperation strategies, such as decode-and-forward based on block Markov superposition coding. We also evaluate the regions for the special case of additive white Gaussian noise channels. We show that a combined strategy of block Markov superposition coding and Wyner-Ziv coding achieves the cut-set upper bound on the sum-rate of the two-way relay channel when the relay is in the proximity of one of the terminals

We consider a wireless local area network (WLAN) in the 5 GHz band. The coverage is one of the main challenges in high-frequency wireless systems and we focus on enhancing the coverage range of such a system by relaying. So far no relaying functions are defined in IEEE 802.11 medium access control (MAC). We introduce three simple MAC schemes for multihop communications and classify them based on delay, simplicity, capacity and application. We also point out the deficiency of the actual standard regarding multihop connections. We study the outage behaviour of a single input single output (SISO) two-hop link and improve it by developing a multiple input multiple output (MIMO) system. We investigate the tradeoff between coverage range and number of relays in a two-dimensional scenario. We also discuss the tradeoffs between having relays instead of adding more access points (APs) to the system and give an estimation of the number of required APs/relays to fully cover an area. In other words we show how much range one can gain from decode-and-forward (DF) relaying in a WLAN scenario

Non-coherent receivers such as transmitted reference receivers recently attracted much interest due to the complexity of coherent ultra wideband (UWB) receivers. In this paper, we investigate the impact of channel state information (CSI) on UWB transmitted reference (TR) maximum likelihood (ML) receivers in the presence of a UWB co-channel interferer (CCI). TR ML receivers are derived assuming the following kinds of channel state information: full CSI, knowledge of the magnitude of the channel impulse response, and no CSI. We compare the performance of these ML estimators with and without CCI by means of bit error rate simulations

We consider a heterogeneous wireless ad hoc network. All nodes are within radio range and may employ an arbitrary number of antennas. Our goal is to achieve a distributed spatial multiplexing gain in this scenario without introducing additional delay. To this end we let 2N of the nodes form N source/destination pairs, that concurrently communicate on the same physical channel. Other nodes in the network act as coherent amplify&forward relays. We use relaying to minimize the cochannel interference between different links (coherent multiuser relaying) and to achieve diversity. Some of the relays may be able to exchange received signal information e.g. by utilizing a suitable short range wireless technology such as Bluetooth (partial cooperation). The main contributions of this paper are (i) a unified derivation of coherent multiuser relaying with an arbitrary number of relays and an arbitrary cooperation pattern and (ii) a novel block zero forcing relay gain allocation for this setting. The derivation naturally includes any combination of multi-antenna (MIMO) nodes. The gain allocation is based on a subspace approach and the complexity is essentially independent of the number of relays. Performance results show, that it achieves the full spatial multiplexing gain N. If the number of relays and/or cooperations exceeds a minimum value, we additionally obtain a distributed diversity gain and array gain for each source/destination link

In this paper, we investigate the use of power line communication (PLC) to assist cooperative wireless relaying. We consider a communication scheme that uses the power line to initialize and synchronize wireless amplify-and-forward relays and to broadcast information between the relays. Starting from an analysis of transfer functions and noise measurements of PLC channels in office and residential environments, we propose a power line transmission scheme for the inter-relay-communication and assess the influence of this scheme on wireless relaying. This scheme is based on linear precoded orthogonal frequency-division multiplexing; it is designed to optimally exploit the frequency diversity available on PLC channels. The use of PLC leads to a very flexible way of enhancing wireless communications by plugging in additional relays where they are needed-without additional wiring.

This paper concerns the coverage range of a high quality voice over wireless local area network (VoWLAN) system based on the IEEE 802.11a standard. Techniques for enhancing the coverage range of each access point (AP) using co-located and distributed antenna diversity schemes are proposed and it is shown that the number of required APs and therefore the infrastructure cost may thus be reduced. Comparisons are made between the coverage range of the 802.11a and that of DECT (digital enhanced cordless telecommunication), and simulations and channel measurements devised for predicting the effect of diversity techniques on the range of APs are described. The results of two different measurement campaigns are presented, one with co-located antennas and the other with locally distributed antennas. The coverage enhancement gained within these scenarios by employing diversity techniques is thus deduced

We consider a wireless ad hoc network with single antenna nodes under a two-hop relay traffic pattern. Some of the nodes in the network form source/destination pairs, while the other nodes serve as amplify and forward relays. The relay gains are assigned such that the interference between different source/destination links is nulled (multiuser zero forcing relaying). This essentially realizes a spatial multiplexing gain with distributed single antenna nodes. We introduce a specific zero forcing gain allocation and give comprehensive performance results, which include the impact of noisy channel state information and time variant channel coefficients. One of our most intriguing results shows that in a typical indoor scenario (delay spread<500 ns) multiuser zero forcing relaying at 5 GHz and pedestrian speed achieves a six fold increase in the sum rate of the network, even if we take the overhead for the measurement and the dissemination of the channel matrices into account.

We consider a wireless network, which consists of several active source/destination pairs and a number of idle nodes. Multiple idle nodes volunteer as relays to facilitate the communication. All nodes have low mobility and the fading channel is essentially constant over the latency time scale of interest (block fading). Conventional adaptive scheduling techniques are either unfair or inefficient in this context. We introduce a new approach to jointly utilize adaptive scheduling and cooperative diversity in the low mobility regime. In contrast to known approaches the new scheme does not compromise the quality of service of individual source/destination links (fairness) if the number of relay nodes is sufficiently high. In a typical setup the 1%-outage aggregate throughput is improved by a factor of nine, if six source/destination pairs are considered jointly. We conclude that the joint cooperative diversity and scheduling scheme extends the benefits of adaptive scheduling to the low mobility regime.

Future wireless personal and sensor networks will be established by a huge number of mobile nodes with transceiver capabilities. In this context, cost aware and power efficient technologies have attracted considerable attention. A proven technology for physical layer cost reduction is the use of continuous phase modulation (CPM) and hard (amplitude) limiting receiver structures that substantially reduce the cost for the analogue domain of the receivers. The major penalty of those systems is their failure in severe intersymbol-interference (ISI) environments even in combination with maximum likelihood sequence estimation (MLSE) decoders. In this paper we present a family of new MLSE based decoders that improve the ISI performance to such an extent that receiver structures with hard limiter become an attractive alternative to the expensive linear receivers. The most powerful equalizers for such nonlinear receivers are phase-space equalizers: MLSE decoders for phase detection generally use a look-up-table (LUT) consisting of the phase values of the desired signal. The key idea of the new decoder is to use a LUT that additionally includes amplitude values associated with the phase values. Based on this information metrics with different and feasible complexity are derived. By means of simulations the performance of a GMSK transmission system is examined for a set of representative channels. Resulting bit error ratios (BER) are compared with BER for state-of-the-art decoders.

Future wireless indoor network applications require technologies that allow high data rate transmission in multipath propagation environment. The acceptance of new technologies will depend essentially on the expense of its realization. A key technology for link level cost reduction is the use of hard (amplitude) limiting receiver structures that substantially save the costs for the analogue domain of the receivers [J.P. Fonseka, 1996][A. Wittneben and U. Dettmar, 1997]. Technologies suited for multipath propagation, such as ML based detection or smart antennas training, require the knowledge of the complex channel impulse response (CIR) [J.G. Proakis, 1995][M. Kuhn, 2002]. A method on how to estimate the complex CIR in case of a hard limited receiver structure is needed. In this paper, an estimation method is presented that allows an estimation of the complex CIR on basis of phase samples. The new estimation technique is a two level approach, which exploits the finite-state nature of the transmission system. With the help of an appropriate training sequence an estimate of the desired phase and amplitude is performed. The second level procedure performs a subspace approach to estimate the CIR, which yields a well treatable overdetermined linear equation system. Simulation results for a GMSK transmission system highlight the suitability of this estimation method.

Power line communication (PLC) uses the highly developed infrastructure of the electrical energy distribution network for data transmission. Measurements are showing that a broadband use of PLC channels is possible. So, PLC is of interest to future broadband communication systems. Such systems will be heterogeneous in several respects, for example relative to the used transmission channel but also relative to the complexity of the participant nodes. Wittneben and Kuhn (see block VTC 2002 - Spring) presented a new class of space-time block codes for radio channels. These codes meet the requirements of future communication systems. They are highly flexible and can be adapted to the particular requests of the transmission. In this paper a PLC application of these codes is investigated. They are used as a basis for a scalable and efficient PLC channel coding scheme. This shows that this class of codes can be used as a generic coding scheme for heterogeneous networks. Alternatively, the PLC application can be considered as an example of the usage of the codes on a frequency selective channel with coloured noise. The applied linear block code optimally uses the diversity of the frequency selective channels in combination with OFDM. This leads to significant gains in performance in case of very frequency selective channels; for barely frequency selective channels or for an AWGN channel the performance is slightly better or not affected respectively. These performance results are presented for measured PLC channels.

The integration of power line communication (PLC) is of interest to future broadband communication systems. These communication systems will be mostly wireless but the use of non-dedicated wired infrastructure will help to reduce costs. PLC uses the highly developed infrastructure of the electrical energy distribution network for data transmission. So it is possible to enlarge the capacity of communication systems without additional wiring and additional costs for this wiring-for outdoor applications, for example to bridge the last mile, as well as indoors, for example to establish or enlarge LANs without new data cables. To evaluate the potential of PLC an analysis of the channel capacity of the power line at higher frequencies is needed. In this paper a detailed analysis of the link level channel capacity of indoor PLC channels is presented at frequencies between 1 MHz and 30 MHz. This analysis is based on extensive measurements of the transfer function and the noise of these channels-for an office environment as well as for a home environment.

Decision feedback equalization (DFE) and antenna diversity combining are two practical and well-known techniques for combating multipath ISI due to frequency-selective fading. We propose a scalable very low complexity combination of antenna combining and decision feedback equalization, which considers the characteristics of mobile communication channels. The performance of this new equalization method is compared to state-of-the-art equalizers by means of simulations. The complexity is evaluated by estimating the total computing expenditure. In addition a very efficient low-cost hardware structure for implementation is proposed. The algorithms are adapted for this hardware structure and the performance of this optimization is illustrated by simulation results.

Space-time codes represent a key technology for future broadband wireless communication systems. A class of space-time block codes according to Wittneben and Kuhn (see VTC 2002 - Spring) is used which can lead to intersymbol interference (ISI) due to an optimized diversity performance. Therefore ISI compensation is an important task of the decoding process of these codes. Several known ISI compensation methods can be applied for example MMSE equalization or parallel or serial ISI cancellation. In this paper a new scalable ISI cancellation method is presented, that by using a posteriori information achieves almost the performance of the maximum likelihood decoder but with a much lower complexity. The key idea is to use estimated a posteriori probabilities to determine the order of the cancellation process. In each iteration the decoder jointly decodes a variable number of symbols, which meet a specified probability threshold. By varying the threshold, the decoder is scalable in such a way that the complexity of the decoder - and consequently the available data flow rate - can be adapted in a wide range to the requests of the transmission, to a given node complexity or a required quality of service.

Future wireless communication systems are characterized by heterogeneity in terms of node capabilities, link level requirements, propagation environments, cell designs and subnet structures. High carrier frequencies and large system bandwidth will make extreme diversity factors feasible. This imposes new requirements on space-time codes. We present a new concatenated linear space-time block coding scheme, which meets these requirements. The code consists of a linear time variant inner code and a linear time invariant outer code. We intentionally decouple the designs of each constituent code in order to achieve the required adaptivity. The inner code is optimized with respect to the variation of the instantaneous channel capacity over the block length of the outer code. Efficient inner coding matrices are given for pure TX diversity and joint spatial subchanneling and TX diversity. The linear outer code is optimized for diversity performance. We describe an efficient approach for the optimization of an unitary coding matrix for any given block length. Simulation results for different system parameters are presented. It is concluded that the proposed concatenated linear high rate space-time block coding scheme is a promising candidate for heterogeneous future mobile communication systems.

In high speed digital mobile communication systems (e.g. wireless LANs) adaptive antennas are a key technology to low cost/low power systems, because they can replace the computationally demanding (temporal) equalizer. To this end it is very important however to understand the impact of combiner imperfections on the performance, because any over-specification will increase hardware cost and power consumption. The main contribution of the paper is a new analysis of the impact of combiner imperfections on the worst case signal to noise plus intersymbol interference ratio (SINR). This analysis provides deep insight into the degradation mechanism. We give an exact analytical expression for the worst case SINR of a zero forcing combiner and an extremely tight approximate bound for the minimum mean squared error combiner. The analysis serves as starting point to optimize the combiner for robustness.

In a time division duplex personal communications system uplink and downlink use different time slots on the same carrier. This makes transmit antenna selection diversity possible to combat performance loss in slow fading channels. The best transmit antenna is selected on the basis of signal measurements in preceding receive time slots. Previously proposed selection algorithms have poor performance, if the signal measurements are corrupted by noise. The structure of the optimum selection algorithm for noisy coherent signal measurements is reviewed. It involves an adaptive linear predictor. The design of an optimum nonadaptive predictor is described and the error performance of a system with DECT-like parameters is given. Almost the full diversity gain can be maintained for mobile speeds up to 12 km/h.>