Assuming the hybrid mmWave architecture and the 3GPP antenna model, we propose a new ULA-like array configuration called array of slanted subarrays (AoSSA) by allowing a non-zero relative angle between subarrays on a terminal. Considering 2-D (azimuth-only) beamforming, two sub-arrays per terminal and a range of base station and user equipment relative rotations, we compare the linear array of subarrays and the AoSSA under four different channel models. We find the slant angle of about 31 degrees optimizes the average spectral efficiency of the link for three of the four channel models and we explain why the result for the fourth channel model should be disregarded.

This paper studies the effects of millimeter-wave (mm-wave) beam alignment errors on the downlink achievable rate of a heterogeneous network (HetNet), which consists of sub-6 GHz macro-cells and mm-wave small-cells. The alignment error is modeled as a function of the underlying mm-wave link parameters. The conventional maximum biased received power criterion, where the bias is used for mm-wave small-cells, is adopted for cell associations. By varying the value of the bias factor, we investigate the changes in the downlink rate coverage probability. Our simulation results indicate that high values (of the order of 30 dB) for the bias, while beneficial in the case of perfect alignment, are actually disadvantageous for the low-rate users in the case of imperfect beam alignment. The low-rate users are better served by a moderate value (of the order of 20 dB) of the bias when the beam alignment errors are accounted for. We also show that the above disparity can be narrowed down by increasing by mm-wave base station (BS) antennas$a$nd/or the mm-wave BS density.

Beam alignment is a critical aspect in millimeter wave (mm-wave) cellular systems. However, the inherent limitations of channel estimation result in beam alignment errors, which degrade the system performance. For systems with a large number of antennas at the base station, downlink channel estimation is performed using uplink pilot signals. The beam alignment errors, thus, depend on the user equipment (UE) transmit power, which needs to be managed properly as the UEs are battery powered. This paper investigates how the use of uplink power control for the transmission of pilot signals in a mm-wave network affects the downlink beam alignment errors, which depend on various link parameters. We use stochastic geometry and statistics of the Student's$t$-distribution to develop an analytical model, which captures the interplay between the uplink power control and downlink signal-to-noise ratio (SNR) coverage probability. Our results indicate that using uplink power control significantly reduces UE power consumption without adversely affecting the downlink SNR coverage.

The sensitivity to blockages at millimeter-wave (mm-wave) frequencies is very different from that at sub-6 GHz frequencies. The blockages affect the user-to-base station (BS) associations and the resulting association regions of the BSs in the network. This in turn alters the load, i.e., the total number of users associated to a BS. In this paper, we use a stochastic blockage model to analyze such effects. We characterize the variation in the load as a function of the blockage environment in a stochastic geometric setting. Our analysis indicates that in the extreme cases of total blocking and no blocking, the mean load on the tagged mm-wave BS is identical to that of a sub-6 GHz BS for a given BS and user density. For intermediate blockage environments, the mean load on the tagged mm-wave BS is found to be less than that on a sub-6 GHz BS. Using Monte-Carlo simulations, we establish that the existing analytical models for load characterization in mm-wave networks result in overestimation of the load per BS and underestimation of the achievable rate.

The auxiliary beam pair (ABP) method processes the outputs of a pair of beams to estimate the angular location of the desired receiver in a line-of-sight (LOS) millimeter wave wireless link. We show that wrong beam selection occurs about 50% of the time, even at high signal-to-noise ratio (SNR), when the desired receiver is at or near boresight of one of the beams, when certain constraints on beam separation and isotropic antennas are used. We further show that, depending on the SNR, the angle errors induced by wrong beam selection can range from 5 to 45 degrees. While the constraints enable analytical inversion of the discriminant function, they cause coincident nulls of the beams. These coincident nulls contribute to wrong beam selection. We show that slight reductions in the beam separations in the ABP method eliminate coincident nulls and reduces the root mean squared error in estimated angle by as much as 54.88%, at the cost of a small reduction in the field of view of the scanned beams and the requirement of a look-up table.

Narrow beamwidth and inaccurate angle-of-arrival (AoA) estimation make perfect beam alignment difficult in millimeter-wave (mm-wave) systems. The extent of beam misalignment depends on the uplink received signal-to-noise ratio (SNR) at a single antenna element of the array. Using stochastic geometry, this paper analyzes and quantifies the loss in downlink SNR coverage probability due to beam misalignment. The standard deviation of the beam alignment error is obtained through the Cramér-Rao lower bound (CRLB) of AoA estimation. The analytical results are verified through Monte-Carlo simulations and it is shown that the beam alignment errors affect the system coverage significantly when the downlink SNR threshold (with array gain) is less than 5 dB. It is also illustrated that increasing the number of antennas alone cannot counter the effects of beam alignment errors.

This Research Category Work in Progress paper presents the first phase of an assessment of the EE and CmpE undergraduate curriculum in terms of its effectiveness in providing clear paths and expectations for students pursuing an EE or CmpE degree. The overarching goal of this study is to build a framework for the potential redesign of the EE and CmpE curriculum that can accommodate future changes while providing clearly defined pathways that correlate to careers students are passionate about. At Georgia Tech, EE and CmpE disciplines are structurally organized into eleven relatively distinct research categories that are useful for internal structure and management of courses, but are less helpful for students navigating through the EE and CmpE undergraduate curriculum. Data are provided that show the enrollments in the current categories over the last six years. Of particular interest are: (1) the most common paths students are currently following and how/why chosen (2) student/faculty expectations of the educational paths that should exist given the current industry climate and (3) the integration of design through the new Interdisciplinary Commons (IDC) (e.g., a new makerspace). This paper will present the strategy for the continuation of this study as well as outline expected outcomes that could be of use to other programs assessing EE and CmpE undergraduate curricula.

Interference alignment (IA) is a well known technique to boost the system capacity of MIMO interfering broadcast channels (IBCs) by aligning interferences into spatial dimensions, which are almost orthogonal to the dimensions occupied by the desired signals. However, the conventional IA algorithms with iterations suffer from large convergence times. We propose a novel hybrid method of nonlinear precoding (NLP) and IA for more efficient interference cancellation. Firstly, we modify the minimum mean squared error IA scheme by ignoring some interference. Then, we selectively apply the Tomlinson-Harashima Precoding (THP) NLP on top of selected IA beams. It is shown through the simulations that the bit error rate performances for all MIMO links with or without NLP are improved dramatically. Meanwhile, the computational complexity in terms of number of iterations is reduced significantly. In addition, to maximize the capacity and overcome the unfairness inherent in THP, that is the latter coded stream has smaller diversity gain, we propose to repeat the low-cost modified IA with random initializations. The optimal precoders and decoders are selected among the repetitions with the largest determinant factor related to the effective channel over the selected IA beams.

We propose a novel time synchronization algorithm based on cooperative-consensus that replaces one- by-one aggregation in a consensus approach with one-shot averaging of many node responses in the physical layer. Through simulation, the algorithm is shown to achieve both skew and offset compensation in a multi-hop wireless sensor network. The proposed algorithm has the capability (a) to significantly improve the convergence rate of consensus-based distributed time synchronization protocols (while keeping the computational complexity low) by employing concurrent cooperative transmission and (b) to reduce the time synchronization error by crossing the network in fewer hops. The proposed algorithm is compared with the Average TimeSynch (ATS) algorithm running a media access control (MAC) protocol based on the IEEE standard 802.15.4 unslotted carrier sense multiple access with collision avoidance (CSMA-CA) for two root locations.

We investigate the convergence behavior of the novel "CoCo" network time synchronization protocol, which compensates both offset and skew, for a network of two tight clusters of nodes, taking fading, quantization error consistent with practical OFDM modulation, and additive noise into account. CoCo update equations are similar to the consensus-based asynchronous time synchronization (ATS) protocol, but CoCo replaces the "one-by-one" packet exchanges with neighbors in ATS with the approximate sample mean provided by the semi- cooperative spectrum fusion (SCSF) technique of concurrent transmissions, in just one packet interval. SCSF requires that all participating nodes share the same upper and lower bounds on the parameter being transmitted. In this paper, two of the three SCSF-transmitted parameters have deterministic bounds; the third parameter uses optimized adaptive bounding, which is not practical, but used in this paper to learn how the spreads of the parameter vary with time and other parameters. Simulation results show a rapid convergence to very low rms time errors, followed by a slow divergence, suggesting that CoCo should be stopped just after the rapid convergence, for best results.

In this paper we present an improved algorithm for localizing a single planar reflective surface using ultra wide band radios that measure excess delay of the first multipath component. These radios do not require time synchronization and cannot measure angle of arrival. Simulation results show that the proposed method outperforms the existing state of the art method. Experimental results confirm the practically of the proposed method.

This paper considers the polling of a sensor network using a binary integration scheme. Binary integration is the combination of binary decisions from multiple sensors into a single decision at the base station. The proposed approach accomplishes binary integration in the physical layer in just two packet intervals regardless of the number of sensors, as long as the sensors are within the decoding range of the collector. We assume independent Rayleigh or Ricean links from the sensors to the collector. In the proposed scheme, the sensors simultaneously transmit their signals in each channel of a set of orthogonal channels to create diversity. The statistics of the squared envelopes of the received signals, in both line-of-sight and non line-of-sight channels, are used to perform hypothesis testing using the Neyman-Pearson criteria. It has been shown through the receiver operating characteristic (ROC) curves that the detection probability strongly depends upon the number of diversity channels available for transmission.

Interference cancellation (IC) has been proposed to improve bandwidth utilization for the majority of modern wireless networks. We claim that the conventional model for IC residual error, which is that the residual error power is simply a fraction of the power of the packet being cancelled, is overly simplified. In this paper, we evaluate the practical residual error of IC with spread MSK modulation in a flat fading environment for a low power wide area (LPWA) wireless sensor network application. In particular, we address the two-packets-overlapping scenario as a function of the signal to noise ratio (SNR) of each packet and the overlapping degree. Using software-defined radios, we demonstrate that the average realistic residual error power based on packet transmission with a single pseudo-noise preamble is not a constant fraction of the power of the packet being canceled. Rather, it depends on the synchronization and channel estimation errors, which in turn, depend on the SINR of the stronger packet preamble.

In this paper, we propose a complete data-aided (DA) synchronization algorithm for minimum-shift keying (MSK) transmission, which addresses features of the transmit-only Wireless Sensor Network (WSN). To provide reliable packet detection even under a packet overlapping scenario, our frame synchronization metric is based on a pseudo-random (PN) sequence preamble and keeps a constant low false alarm by dynamic thresholding based on real-time error floor level. The carrier frequency offset algorithm exploits the Discrete Fourier Transform (DFT). Meanwhile, the preamble is also used as the training sequence for phase offset estimation based on a least-square (LS) estimator, to reduce the transmit energy. We simulate the packet detection rate, false alarm rate, and the root mean square (RMS) error of all synchronization parameters for the scenarios with and without packet overlapping. The computer simulation shows the robust performance of the synchronization process and reveals the great flexibility and trade-off control between complexity and performance by adjusting system design parameters.

Continuous measurement of cardiac time intervals throughout normal activities of daily living is of interest for both chronic disease management and preventive wellness monitoring. Systolic time intervals in particular — i.e., pre-ejection period (PEP) and left ventricular ejection time (LVET) — have been shown to be relevant to assessing myocardial health and performance, but are challenging to measure with wearable sensors. In this paper, we present novel methods for estimating PEP and LVET from a single three-axis accelerometer placed at the sternum, based on the measurement of cardiogenic vibrations: seismocardiography (SCG) and ballistocardiography (BCG). Although such signals have been examined in the existing literature, the analysis and interpretation has focused mainly on the dorso-ventral components only in the context of systolic time interval estimation. In this paper, we find that features extracted from the head-to-foot accelerations yield better correlations to PEP measured from impedance cardiogram (ICG) than standard approaches based on dorso-ventral components. Additionally, we examine the effects of postural variations on the correlation between PEP estimated from accelerometer and ICG signals and also on correlation between LVET estimated from both sensors. We determine that such correlations are robust to postural changes. Based on these findings, we anticipate that wearable, accelerometer based vibration measurements from standing subjects can be used for robust systolic time interval estimation in a variety of ubiquitous cardiovascular health and fitness sensing applications.

The development of a feedback-based method for evolving the blended class format for large, multiple section courses is presented with the application to two circuits courses taught at Georgia Tech: ECE 2040 is a circuits course for ECE majors and ECE 3710 is a circuits and electronics course for non-majors. These are both multi-section, high-enrollment courses, with a combined enrollment of over 600 students per term. The courses utilize two MOOCs that provide all of the online content. ECE 3710 was first offered in a flipped mode in the Summer of 2013 and has evolved over five semesters based on feedback from the student surveys, focus groups, instructor feedback, discussion forum questions, and performance on tests. ECE2040 has been offered two semesters in the blended mode using a modified version of the blended model developed for ECE3710.

Multi-hop or cascaded distributed multi-input and multi-output (DMIMO) occurs in a series of clusters of singleantenna radios, when the clusters relay to each other, using concurrent transmission one cluster to the next, within a cooperative route. We address the case of no external synchronization, where one cluster derives its transmit synchronization from a packet received from the previous cluster. For DMIMO system based on orthogonal frequency division multiplexing (OFDM) transmission, frequency offsets between cooperating transmitters or cluster members induce inter-carrier-interference (ICI) in the receivers of the next cluster. Also, the timing offsets between transmitters create additional delay spread potentially necessitating a longer guard interval. Rather than estimating each of the multiple offsets, each receiver estimates just one offset, which implicitly is a channel gain-weighted average, known from our previous work to yield statistically stable synchronization from hop to hop. To effectively suppress the timing and frequency offsets between distributed transmitters, we designed a novel preamble which facilitates this one-step estimation without explicit knowledge of channel gains. The performance improvement of the estimation algorithm is evaluated through simulation as well as experiment on GNU Radio and USRPs.

In this work, we apply machine learning to investigate the effectiveness of an Impulse Radio Ultra-Wide Band (IR-UWB) radar panel, in an under-the-mattress configuration, for detecting apnea events in subjects known to have obstructive sleep apnea (OSA). We consider a collection of features, some novel and some inspired by features that worked well for sleep apnea detection using other types of sensors (i.e., not IR-UWB). To extract the features, we collected a total of 25 hours of data from four subjects as they slept through the night. The data included digitized samples of the IR-UWB radar return signal and the scored polysomnograph (PSG), which is the gold standard and measures a large number of physiological parameters in a well-equipped sleep laboratory. Normal and apnea epochs were extracted from the IR-UWB data corresponding to normal and apnea epochs in the PSG data. Statistical features were derived from these extracted epochs and a Linear Discriminant classifier was trained. Using cross-validation, we found that the classifier had an accuracy of around 70% in detection of apnea and normal epochs. The novel aspect of this project involves processing and investigation of different methods for feature extraction on data obtained from real apnea subjects and suggests that the radar, when paired with other under-the-mattress sensors might provide an effective screening device in a convenient form factor.

Ballistocardiography is a noninvasive measurement of the mechanical movement of the body caused by cardiac ejection of blood. Recent studies have demonstrated that ballistocardiogram (BCG) signals can be measured using a modified home weighing scale and used to track changes in myocardial contractility and cardiac output. With this approach, the BCG can potentially be used both for preventive screening and for chronic disease management applications. However, for achieving high signal quality, subjects are required to stand still on the scale in an upright position for the measurement; the effects of intentional (for user comfort) or unintentional (due to user error) modifications in the position or posture of the subject during the measurement have not been investigated in the existing literature. In this study, we quantified the effects of different standing and seated postures on the measured BCG signals, and on the most salient BCG-derived features compared to reference standard measurements (e.g., impedance cardiography). We determined that the standing upright posture led to the least distorted signals as hypothesized, and that the correlation between BCG-derived timing interval features (R-J interval) and the preejection period, PEP (measured using ICG), decreased significantly with impaired posture or sitting position. We further implemented two novel approaches to improve the PEP estimates from other standing and sitting postures, using system identification and improved J-wave detection methods. These approaches can improve the usability of standing BCG measurements in unsupervised settings (i.e., the home), by improving the robustness to nonideal posture, as well as enabling high-quality seated BCG measurements.

Terahertz Band (0.1-10 THz) wireless communication, which escalates the frequency and provides unprecedented data rates, is envisioned as the key picocell wireless technology in the next five to ten years. In this band, the medium access control (MAC) is more about "scheduling transmissions," instead of "fighting for access," thanks to the huge bandwidth. On the one hand, an electronically steerable narrow beam is essential to overcome the high path loss at THz and extend the communication range. On the other hand, impulse radio in THz Band that generates femtosecond-long pulses enables the beam direction to be switched at the "pulse-level," in contrast to the conventional "packet-level" switching as seen in literature. To capture these features, yet keep complexity low, we assume a simple time-spread on-off-keying (TS-OOK) emitter and energy detection receiver. In this paper, we present the first MAC design to enable macro-scale communication at 100 Gbps by exploring THz Pulse-level Beam-switching with Energy control (TRPLE). For the downlink, the data rate of an arbitrary user and the network throughput are analytically investigated.

For multi-hop wireless sensor networks (WSNs), time synchronization (TS) across distributed nodes is important for certain applications (i.e., surveillance, gunshot location system, etc.). In this paper, a method combining two forms of cooperative transmission (CT), digital-based Concurrent Cooperative Transmission (CCT) and analog-based Semi-Cooperative Spectrum Fusion (SCSF), is proposed and demonstrated to achieve fast and accurate time synchronization over large multihop WSNs. CCT, considered in many physical layer system studies and realistically demonstrated, enables range extension. Range extension implies that the source timestamp can be diffused to the network faster (i.e., in fewer hops) than non-CT-based TS protocols. SCSF is an analog CT method where different cooperators transmit correlated information simultaneously. The two methods combine to create a new distributed method of network TS, called the Cooperative Analog and Digital (CANDI) TS protocol, which promises significantly shorter protocol time and smaller time errors in multi-hop networks. This paper presents the algorithm description, and physical layer simulations as well as network experimental results over a line network that show the performance advantage of CANDI over TPSN.

A linear multi‐hop cooperative network is considered where the relay nodes are deployed in a random fashion according to a Bernoulli random process. The transmission from one hop to another is modelled as a discrete‐time Markov process while taking into account the random locations of the participating nodes along with the Rayleigh fading channel model. Quasi‐stationary theory of Markov chains has been applied to study the various properties of the network under consideration, including successful probability of hops as well as network coverage. It is shown that there is a loss of one‐hop success probability when the random deployment is done compared to a regular deployment. However, an additional signal‐to‐noise ratio margin has also been quantified for obtaining a desired quality of service and one‐hop success probability.

The Opportunistic Large Array (OLA) is one type of cooperative transmission, which provides fast and reliable broadcasts and unicasts in multi-hop networks. While the existing literature on OLAs assumes one-shot transmission with a single packet, we analyze multi-packet OLA transmission within a single flow along strip-shaped networks or OLA-based cooperative routes. When multiple packets are transmitted from the same source using the same channel before the previous packet has cleared the network, which is called spatial pipelining, intra-flow interference is produced by OLAs transmitting co-channel packets. Using the continuum assumption (approximated by high density networks), the authors optimize the throughput. This paper theoretically shows spatial pipelining over strip networks is always feasible for path loss exponent α≥2, which is not true for disk-shaped networks. Moreover, the impacts of system parameters are analyzed, and the upper bound of the optimal packet spacing (the lower bound of the optimal throughput) is derived. Also, spatial pipelining in finite networks is studied with numerical results, which confirm the theoretical analysis.

Cooperative transmission (CT), in which spatially separated wireless nodes collaborate to form a virtual antenna array or virtual multiple-input-multiple-output (VMISO) link, is an effective technique to mitigate multi-path fading by spatial diversity. In this paper, we study the disparities in path losses between the randomly placed relay nodes in a transmit cluster and the destination nodes. Many authors assume that the elements in a virtual antenna array are co-located, even though they are spread out. In this paper, we show that a signal-to-noise-ratio (SNR) penalty of up to 3dB should be included when making this assumption. By the high-SNR approximation of the outage rates, we show that the performance degradation caused by the path-loss disparity can be characterized equivalently by log-normal shadowing. Moreover, we derive the upper and lower bounds of the real outage probabilities in closed forms based on the log-normal shadowing model, by which we can estimate the SNR penalty of the co-located assumption.

We explore range expansion along an appointed direction using a fixed number of mobile nodes that do distributed multiple-input-multiple-output (MIMO) in the diversity configuration, also known as cooperative transmission (CT). The emphasis is on optimal groupings and locations of groups to maximize the multi-hop reach along a line; such information may inform the motion of robots that are deployed as far away as possible from a fixed base station yet still maintain connectivity. Also the impact of message-sharing within a cluster before relaying is considered. The simulation results show that for different fixed number of nodes, Rayleigh fading and arbitrary path loss exponent, there are preferred deployments depending on path loss exponent and sharing strategy.

Theoretically optimal performance and behavior of a routing protocol are very important because they can be used to guide the design of practical protocols. Motivated by the promising lifetime performance of an existing online cooperative transmission (CT) routing protocol for wireless sensor networks (WSNs), we formulate the lifetime optimization problem of cooperative routing using linear programming (LP), which requires considerations of CT's unique characteristics and sophisticated variable definitions. By using our LP, one can obtain the optimal lifetime of multi-hop WSNs that use cooperative routing, which can serve as a performance bound or a benchmark to compare to the lifetimes of existing routing protocols. Through the evaluations of our LP, we show the benefit of cooperative routing and usefulness of our LP formulation.

An On-demand Scheduling Cooperative MAC protocol (OSC-MAC) is proposed to address the energy hole problem in multi-hop wireless sensor networks (WSNs). By combining an on-demand strategy and sensor cooperation intended to extend range, OSC-MAC tackles the spatio-temporal challenges for performing cooperative transmission (CT) in multi-hop WSN: cooperating nodes are neither on the same duty cycle nor are they in the same collision domain. We use orthogonal and pipelined duty-cycle scheduling, in part to reduce traffic contention, and devise a reservation-based wake-up scheme to bring cooperating nodes into temporary synchrony to support CT range extension. Compared with existing MAC protocols, our NS-2 simulations with REACT show that while we explicitly account for the overhead of CT and practical failures of control packets in dense traffic, OSC-MAC still gives almost 80% increase in network lifetime.

Theoretically optimal performance and behavior of a routing protocol are very important because they can be used to guide the design of practical protocols. Motivated by the promising lifetime performance of an existing suboptimal cooperative transmission (CT) routing protocol for wireless sensor networks, we formulate the lifetime-optimization problem using linear programming (LP), which requires considerations of CT's unique characteristics and sophisticated variable definitions. By evaluating LP for various cases, we show the effectiveness of cooperative routing by comparing it with the non-CT case. Also, by analyzing the LP solution, we identify some important factors and behaviors of optimal cooperative routing. More specifically, we conclude that (i) matching the rate of doing CT and matching the candidate pools of cooperators to the LP solution are important factors to achieve near-optimal lifetime performances, and, (ii) for small multi-hop networks, the design of cooperative routing can be simplified because one may use the sink node as a single VMISO receiver and rely on the existing energy-aware routing for the primary routing.

The Opportunistic Large Array (OLA) is a simple form of cooperative transmission, in which a group of single-antenna nodes decode the same packet, then a short time later relay the packet simultaneously in orthogonal or space-time coded channels. The authors have previously shown that OLA transmissions can be adequately synchronized so they appear to a receiver as having come from a conventional array with co-located antennas doing transmit diversity. OLAs have been considered as a basis for rapid single-packet broadcasting in multi-hop networks, however, there are few studies that consider OLA broadcasting of multiple co-channel packets. Using the continuum assumption (approximated by high density networks), we focus on broadcast throughput optimization, as determined by the packet insertion rate at the source. We consider spatial pipelining, which means broadcasting a packet before the previous co-channel packet has cleared the network. We show theoretically that for infinitely large networks the feasibility of spatial pipelining depends on the path loss exponent. For finite networks, we study the propagation behavior of spatially pipelined packets using numerical analysis.

A one-dimensional cooperative network is modeled stochastically, such that the nodes are randomly placed according to a Bernoulli process. A discrete time quasi-stationary Markov chain model is considered to characterize the multi-hop transmissions and its transition probability matrix has been derived. By the Perron-Frobenious theorem, the eigen-decomposition of the matrix gives useful information about the coverage of the network and signal-to-noise (SNR) margin that is required for obtaining a given quality of service or packet delivery ratio. An SNR penalty for the random placement of nodes, compared to regular placement, is quantified.

Pilot-assisted estimation of signal-to-noise ratio (SNR) is considered for an orthogonal non-coherent M-ary FSK receiver having a carrier frequency offset (CFO) at the front end. The received signal is also corrupted by symbol-rate Rayleigh fading and additive white Gaussian noise. A two-step estimation procedure is designed by observing received data in the receiver. In the first step, an asymptotically unbiased estimate of CFO is obtained using a moments based approach. This estimate is then used to derive a maximum-likelihood estimator for the SNR. The Cramer Rao Bound for SNR has been derived to compare the overall estimator performance in terms of its variance.

The Opportunistic Large Array (OLA), a simple form of concurrent cooperative transmission that extends range, is known to provide fast and reliable broadcasting of a single packet on both disk- and strip-shaped networks. This paper studies multi-packet OLA transmission within a single flow along a strip-shaped network, which also models an OLA-based cooperative route in a large multi-hop network. One packet makes interference on many other packets in the same flow, which generally retards OLA sizes for all packets, and can cause some packets to die off. However, by using the continuum and deterministic channel assumptions, which correspond to high node density, the authors show that multi-packet transmission can be made reliable and throughput can be optimized, by selecting the correct inter-packet spacing.

The modeling of energy storage devices such as supercapacitors for wireless sensor networks is important for assessing performance of harvesting-aware routing protocols that could be key to a green energy future. In this paper, we present a circuit-based model (CBM), which is a good fit to the empirical data available in the existing literature. We compare it with a linear energy model (LEM) that is often used in literature, and show that the choice of models implies a significant difference in the throughput per node.

Effective techniques for extending lifetime in multihop wireless sensor networks (WSNs) include duty cycling and more recently introduced cooperative transmission range extension. However a scalable MAC protocol has not been presented that combines both. In this study, we present a new scheduling cooperative MAC protocol (SCT-MAC) for the multi-hop wireless sensor network, which introduces distributed duty-cycle scheduling and enables nodes to wake up on demand in the data period. The protocol provides a wake-up rendezvous for cooperators to reach a CT agreement and reduces contention over adjacent flows, and the on-demand wake-up further reduces idle listening in the data period. We compare SCT-MAC with DW-MAC using NS-2 simulation, and show that SCT-MAC improves the network lifetime by approximately 100%.

In battery-driven multi-hop wireless sensor networks, the most highly burdened relay nodes, or bottleneck nodes, will die first. These deaths eventually partition the network and trap valuable battery energy in the network. In a multi-hop wireless sensor network that must harvest energy from weak energy sources, the nodes that are one hop away from the Sink must sometimes shut down to harvest, so they limit the access that the rest of the network has to the Sink. One way to lengthen the time-to-partition or increase access to the Sink is to exploit some of the would-be-trapped energy to do cooperative transmission (CT) range extension to hop over the bottleneck node and communicate directly with the next-hop node (e.g., the Sink). To do this, approximate range doubling is necessary. In this paper, we investigate the potential for range doubling over Rayleigh faded channels of four CT schemes that are compatible with the IEEE 802.15.4 standard. We consider the effects of path loss disparities and how many cooperators are necessary for each CT scheme.

Cooperative transmission in wireless networks provides diversity gain in multipath fading environments. Among all the proposed cooperative transmission schemes, delay diversity has the advantage of needing less coordination and higher spectrum efficiency. However, in a distributed network, the asynchrony comes from both carrier frequency and symbol timing between the cooperating relays. In this paper, a minimum mean square error fractionally spaced decision feedback equalizer (MMSE-FS-DFE) is developed to extract the diversity with large multiple carrier frequency offsets, its performance approaches the case without multiple carrier frequency offsets. The front end design for the receiver in this scenario is discussed, and a practical frame structure is designed for carrier frequency offsets and channel estimation. A subblock-wise decision-directed adaptive least squares (LS) estimation method is developed to solve the problem caused by error in frequency offset estimation. The purpose of this paper is to provide a practical design for cooperative transmission (CT) with the delay diversity scheme.

We consider a quasi-stationary Markov chain as a model for a decode and forward wireless multi-hop cooperative transmission system that forms successive Opportunistic Large Arrays (OLAs). This paper treats a linear network topology, where the nodes form a one-dimensional horizontal grid with equal spacing. In this OLA approach, all nodes are intended to decode and relay. We derive the transition probability matrix of the Markov chain based on the hypoexponential distribution of the received power at a given time instant assuming that all the nodes have equal transmit power and the channel has Rayleigh fading and path loss with an arbitrary exponent. The Perron-Frobenius eigenvalue and the corresponding eigenvector of the sub-stochastic matrix indicates the signal-to-noise ratio (SNR) margin that enables a given hop distance.

This paper considers how to estimate the average signal-to-noise ratio (SNR) for a communication system employing orthogonal non-coherent M-ARY frequency shift keying (NCMFSK), in white Gaussian noise (AWGN) and over both symbol-by-symbol fading channels and block fading channels. The proposed algorithm finds its application in a variety of applications including a cooperative transmission system, which is the main motivation behind this study. The maximum likelihood estimator and one using data statistics have been derived and simulated for various scenarios including data-aided, non-data aided and joint estimation using both the data and pilot sequences. We also derive the Cramer-Rao bound for the estimators in the case of Rayleigh fading channels. The results show that for a particular region of interest (e.g. high SNR or low SNR) and depending upon the availability of pilot sequence, a particular SNR estimation scheme is suitable.

We consider a quasi-stationary Markov chain as a model for a decode and forward wireless multi-hop cooperative transmission system that forms successive Opportunistic Large Arrays (OLAs). This paper treats a linear network topology, where the nodes form a one-dimensional horizontal grid with equal spacing. In this OLA approach, all nodes are intended to decode and relay. Therefore, the method has potential application as a high-reliability and low-latency approach for broadcasting in a line-shaped network, or unicasting along a pre-designated route. We derive the transition probability matrix of the Markov chain based on the hypoexponential distribution of the received power at a given time instant assuming that all the nodes have equal transmit power and the channel has Rayleigh fading and path loss with an arbitrary exponent. The state is represented as a ternary word, which indicates which nodes have decoded in the present hop, in a previous hop, or have not yet decoded. The Perron-Frobenius eigenvalue and the corresponding eigenvector of the sub-stochastic matrix indicates the signal-to-noise ratio (SNR) margin that enables a given hop distance.

This paper proposes a preamble detection-based cooperative transmit time pre-synchronization technique specially for Concurrent Cooperative Transmission (CCT). CCT enables a collection of power-constrained radios to transmit as a group and achieve a transmit range that is much greater than the range of a single device. In the absence of a GPS system or a globally synchronized clock, each cooperating node can derive its transmit time autonomously by detecting the preamble from a received signal. The paper first compares two methods for estimating the start of packet (SOP) received in the presence of a carrier frequency offset (CFO): the peak method, which is optimal in a single diversity channel without CFO, and a mean method, which outperforms peak with large CFO. Next, the SOP is estimated by combining all diversity channels using an approximated Best Linear Unbiased Estimator (BLUE). Using theory and measurement, we show that the time error statistics are convergent as the number of hop increases.

We consider the irreducible discrete time Markov chain, with one absorbing state, as a potential candidate to model a wireless multi-hop transmission system that does cooperative transmission at every hop. This paper describes the modeling for a special geometry where the nodes are aligned on a one-dimensional horizontal grid with equal spacing and such that the cooperating clusters are adjacent. This model can be considered a precursor to a model for an Opportunistic Large Array broadcast for the finite density case. Assuming all the nodes have equal transmit power, the successive transmissions can be modeled as a Markov chain in discrete time. We derive the transition probability matrix of the Markov chain based on the hypoexponential distribution of the received power at a given time instant. The Perron-Frobenius eigenvalue of that sub-stochastic matrix is used in formulating a bound on how far transmissions can reach with a particular relay transmit power.

This paper addresses the issue of network life during repeated broadcasts originating from the same source in large, strip-shaped wireless sensor networks. The proposed technique would also apply to a unicast over a strip-shaped cooperative route within a network. The proposed medium access control (MAC)-free "alternating sets" transmission strategy is based on a very low- overhead form of cooperative diversity called the opportunistic large array (OLA). The proposed alternating OLA with transmission threshold (A-OLA-T) uses a received power-based threshold to define mutually exclusive sets of nodes, such that the union of the sets includes all the nodes in the network or cooperative route during the initial broadcast. A-OLA-T alternates the set of cooperators (or OLAs) during successive transmissions to reduce the energy consumption and the duty cycles of nodes (measured in terms of the percentage time a node is on and transmitting), thereby avoiding the formation of "hot spots" (regions of overuse, high energy consumption, and traffic) in the network. Under the continuum assumption (very high node density) for two-dimensional strip networks (or routes), we obtain lower bounds (as a function of the number of alternating sets, m) on the node degree for achieving optimal load balancing in the network, and, consequently, prolonging the network life.

This paper deals with the problem of estimating average signal-to-noise ratio (SNR) for a communication system employing non-coherent M-ary frequency shift keying (NCMFSK) over fading channels and white Gaussian noise (AWGN). We derive two estimators; one using maximum likelihood (ML) approach and other using the data statistics. Various scenarios have been taken into account including data-aided (DA), non-data aided (NDA) and joint estimation using both the data and pilot sequences. We also derive the Cramer-Rao bound (CRB) for the estimators. The results show that for a particular region of interest (e.g. high SNR or low SNR) and depending upon the availability of pilot sequence, a particular SNR estimation scheme is suitable.

Estimation of the signal-to-noise ratio (SNR) is considered for a non-coherent M-ary frequency shift keying (NC-MFSK) receiver. It has been assumed that the transmitted symbols undergo a slow flat fading channel where a block of data is corrupted by an independent constant fade and additive white Gaussian noise. Two approaches for SNR estimation are reported in this paper: an approximate maximum likelihood approach and another using the data statistics, both for data aided and non-data aided systems. It has been shown that for a particular SNR region of interest and depending upon the availability of pilot symbols, a particular approach is suitable for SNR estimation.

A self-cancellation scheme is proposed to cope with the sample frequency offset (SFO) problem in OFDM systems in the presence of a carrier frequency offset (CFO). Making use of the symmetry between the phase shifts caused by SFO and the subcarrier index, we put the same constellation symbol on symmetrical subcarriers, and combine the pairs at the receiver coherently. In this way, the SFO effects are approximately cancelled with the price of cutting down the bandwidth efficiency by half. However some array gain and diversity gain are obtained from the symmetrical combining. Our scheme can work well together with the phase tracking for residual CFO, so that both SFO and residual CFO can be removed with low complexity. Simulations show that, our scheme effectively removes the effect of SFO; the proposed system outperforms the ideal normal OFDM systems (with no SFO) under the same energy efficiency at high SNR, so the proposed system will also outperform the normal system that using the same overhead for SFO estimation. Finally, a mixed system is proposed to mitigate the drawbacks of our design and the normal OFDM systems for the SFO compensation. Our scheme may be helpful for the synchronization of multiple SFOs in the cooperative transmission.

The opportunistic large array (OLA) is the basis for a simple cooperative transmission protocol that is suitable for broadcasting in wireless sensor networks and in which every node transmits once per broadcast. OLA with transmission threshold (OLA-T) is an energy-efficient extension of the OLA protocol that limits node participation during broadcasting. Performance of OLA-T has been studied for disc-shaped networks. In this paper, we propose a method to systematically set the transmission threshold and design the OLAs for two-dimensional strip-shaped networks. Theoretical bounds and conditions for achieving sustained OLA propagation and reducing the total energy consumption in the network using OLA-T have been derived in this paper. These results would also apply to arbitrarily shaped networks that have node participation limited to strip-shaped collections.

This paper deals with the problem of estimating the average signal-to-noise ratio (SNR) for a communication system employing non-coherent binary frequency shift keying (NCBFSK) over fading channels with white Gaussian noise (AWGN). The maximum likelihood (ML) estimator and one using data statistics have been derived and simulated for various scenarios including data-aided (DA), non-data aided (NDA) and joint estimation using both the data and pilot sequences. We also derive the Cramer-Rao bound (CRB) for the estimators. The results show that for a particular region of interest (e.g. high SNR or low SNR) and depending upon the availability of pilot sequence, a particular SNR estimation scheme is suitable.

A wireless MIMO link is considered in a LOS configuration with multiple SISO wireless repeaters assisting. A theoretical analysis is presented in which repeater positions are considered and sufficient conditions proposed and evaluated for maximizing the MIMO multiplexing gain. The assisting repeaters would be useful when space constraints, for example, on a building top, would prohibit the wide element spacings needed by a MIMO terminal to reach full capacity over a long range LOS link. Sufficient conditions include, among other things, mutual orthogonality of steering vectors from one MIMO terminal pointing toward the opposite terminal and toward each of the repeaters. Once n - 1 such repeaters have been properly placed, where n represents the smaller dimension of the channel matrix, the matrix becomes full rank and the multiplexing gain is maximized.

We propose a broadcast protocol that is based on a form of cooperative transmission called the Opportunistic Large Array (OLA). Multiple SNR (or transmission) thresholds are used to define mutually exclusive sets of OLAs, such that the union of the sets includes all the nodes in the network. The new protocol, termed Alternating OLA with Transmission Threshold (A-OLA-T), exercises a different set of OLAs on each consecutive broadcast from the same sink until all sets have transmitted once. Then the sequence repeats. Thus, broadcasts consume energy efficiently and uniformly over the network, and A-OLA-T is especially well suited for static networks. The transmission thresholds are optimized to maximize the network life if broadcasts were the only transmissions. In this paper, we first optimize triples of broadcasts, and then extend the optimization for a higher number of broadcasts.

A single on-frequency, full duplex wireless repeater is considered for enhancing the capacity of a long-distance 2 times 2 multiple-input multiple-output (MIMO) wireless link with a dominant line-of-sight (LOS) component. Such links might be used for high-speed communication between buildings or towers. For practical reasons, the aperture sizes of the transmit (TX) and receive (RX) arrays may be limited, which tends to make the rank of the long-distance MIMO channel matrix close to unity. The paper shows that the addition of just one repeater can bring the channel to full rank, approximately doubling the capacity of the link. Typical values of repeater isolation are assumed. For small TX and RX array element spacings, there is considerable robustness in optimal repeater location. Even with some multipath, most of the capacity improvement is retained.

We propose a protocol for broadcasting in wireless multihop networks that is based on a form of cooperative transmission called the opportunistic large array (OLA). An SNR ("transmission") threshold is used to define two mutually exclusive sets of OLAs, such that the union of the sets includes all the nodes in the network. The broadcast protocol then alternates between the sets for each broadcast and is called Alternating OLA with Transmission Threshold (A-OLA-T). Under A-OLAT, all participating nodes transmit with the same low power, therefore the energies of the nodes in the network drain efficiently and uniformly, extending the network life relative to broadcasts that use simple OLA or non-alternating OLAs with a transmission threshold. In this paper, we optimize the A-OLA-T protocol under the continuum assumption (very high node density).

We propose a broadcast protocol that is based on a form of cooperative diversity called the opportunistic large array (OLA). In the initial broadcast, an SNR ("transmission") threshold is used to define mutually exclusive sets of OLAs, such that the union of the sets includes all the nodes in the network. The broadcast protocol then basically alternates the transmitting set of nodes (or OLAs) during each broadcast cycle and is called alternating OLA with transmission threshold (A- OLA-T). Under A-OLA-T, broadcasting drains the energies of the nodes in the network efficiently and uniformly, extending the network life relative to broadcasts that use simple OLA or non-alternating OLAs with a transmission threshold. In this paper, we optimize the A-OLA-T protocol under the continuum assumption (very high node density).

Orthogonal space-time block codes (OSTBCs) are known to provide transmit diversity gain with negligible decoding complexity. In this paper, we present a single-stage MMSE multi-user interference suppression technique that exploits the temporal and spatial structure of OSTBCs, leading to simple linear processing. In particular, we show that MMSE-based detection can be implemented for real OSTBCs as well as the derived rate-1/2 complex OSTBCs, using simple linear processing that avoids computationally intensive matrix inversions.

An opportunistic large array (OLA) is a form of cooperative diversity in which a large group of simple, inexpensive relays or forwarding nodes operate without any mutual coordination, but naturally fire together in response to energy received from a single source or another OLA When used for broadcast, OLAs form concentric rings around the source, and have been shown to use less energy than conventional multi- hop protocols. This paper proposes two schemes, one for general OLA transmission and one specifically for upstream routing in the topology of wireless sensor networks, that each save over 50% of the energy of flooding. The OLA-T scheme uses a transmission threshold to suppress nodes that would make weak contributions. The OLA Concentric Routing Algorithm (OLACRA) takes advantage of the concentric ring structure of broadcast OLAs to limit flooding on the upstream connection. Combination of these two schemes and variations of OLACRA are evaluated in terms of energy savings and probability of successful upstream routing.

In this paper, the optimal form of a channel matrix is derived to maximize the MIMO capacity in a LOS environment. Antenna response functions are proposed to achieve the optimal form and suboptimal results are considered. In designing an antenna to maximize the capacity, the objective is to increase the slope of the far-field phase response without sacrificing too much antenna gain. Such a constraint has not typically been considered in antenna design and should be further explored.

Three vehicle-to-vehicle (VTV) models and three roadside-to-vehicle (RTV) models, each suitable for RF channel emulation and based on measurements at 5.9 GHz, are presented. Each model captures the joint Doppler-delay characteristics of a different environment. The packet error rate (PER) for each model, measured with an emulator and a 802.11p Wireless Access in Vehicular Environments (WAVE) prototype, is presented.

Antenna selection combined with dual-polarized antennas offers an attractive alternative for realizing higher order multiple-input multiple-output (MIMO) configurations in compact, low-complexity devices. In this paper we analyze the performance of antenna selection for narrowband dual- polarized MIMO channels with linear minimum mean square error (MMSE) receiver processing. We analytically study the impact of cross-polar discrimination (XPD) on the achieved selection gain. We use channel measurement data, collected at 2.4 GHz in a typical office environment, to compare the performance of spatial and dual-polarized MIMO channels with respect to antenna selection. Our bit-error-rate (BER) results indicate that antenna selection with dual-polarized antennas can achieve significant performance gains for compact configurations.

Statistical channel models for small-scale fading are presented for a frequency selective vehicle-to-vehicle or mobile-to-mobile wireless communications link in an expressway environment in Atlanta, Georgia, where both vehicles travel in the same direction. These doubly selective models were developed from measurements taken using the direct sequence spread spectrum (DSSS) technique at 2.45 GHz. The models imply a non-separable channel with a persistent Ricean behavior across multiple model taps. We also explore model dynamics by comparing BER statistics for an overall model that is fit to the entire data set with those of a collection of models, each fit to smaller, non-overlapping intervals

In this paper, we analyze the impact of polarization diversity on the capacity of multiple-input multiple-output (MIMO) channels in indoor environments. A channel measurement campaign was conducted at 2.4 GHz to measure the co-polarized and cross-polarized subchannels under line-of-sight (LOS) and non-line-of-sight (NLOS) channel conditions. We analyze the measured data in terms of Ricean K-factor, cross-polar discrimination (XPD) and subchannel correlations. A major contribution of this paper is that in these measured channels, we observe a coincidence of low K factors and high XPD. In such channels, MIMO systems employing polarization diversity incur SNR and diversity deficits, when compared to spatial configurations. On the other hand, our results indicate that polarization diversity can substantially lower the subchannel correlations for compact configurations, even in a LOS scenario. We draw a fair comparison in terms of capacity, between spatial MIMO configurations and systems using polarization diversity. We analyze the performance of 2times2 and 4times4 MIMO configurations for a range of values of inter-element spacing

The plane-wave assumption has been used extensively in array signal processing, parameter estimation, and wireless channel modeling to simplify analysis. It is suitable for single-input single-output and single-input multiple-output systems, because the rank of the channel matrix is one. However, for short-range multiple-input multiple-output (MIMO) channels with a line-of-sight (LOS) component, the plane-wave assumption affects the rank and singular value distribution of the MIMO channel matrix, and results in the underestimation of the channel capacity, especially for element spacings exceeding half a wavelength. The short-range geometry could apply to many indoor wireless local area network applications. To avoid this underestimation problem, the received signal phases must depend precisely on the distances between transmit and receive antenna elements. With this correction, the capacity of short-range LOS MIMO channels grows steadily as the element spacing exceeds half a wavelength, as confirmed by measurements at 5.8 GHz. In contrast, the capacity growth with element spacing diminishes significantly under the plane-wave assumption. Using empirical fitting, we provide a threshold distance below which the spherical-wave model is required for accurate performance estimation in ray tracing.

Three-dimensional (3-D) ray-tracing and analysis is used to characterize the path angles in a rectangular building in which all interior walls are parallel or perpendicular to the exterior walls. Path angles are recorded for 500 random transmitter and receiver locations, maintaining constant range throughout a particular building model. The main contribution of this paper is the observation that when all of the path angles from all 500 trials are measured relative to one building wall and collectively analyzed in a signal strength distribution, clustering is clearly apparent. While this angle clustering in the ensemble of channels is not the same as clustering in a single channel trial, it may partially account for the clusters observed in single trials. As a practical matter, these results may impact the use and placement of directional antennas for wireless LANs in buildings of this type.

Measured per-tap Doppler spectra are presented for a frequency selective vehicle-to-vehicle or mobile-to-mobile wireless communications link in various multipath environments in Atlanta, Georgia. The measurements were taken using the direct sequence spread spectrum (DSSS) technique at 2.45 GHz. The environments, chosen for their exceptionally long delay spreads, include an expressway, an urban T-intersection, and an exit ramp. The different environments produced quite different spectra. Also, for a given channel, the spectra corresponding to different delays were different, implying a non-separable channel model.

Recent work has shown that multiple-input an multiple-output (MIMO) systems hold great promise to help achieve the high data rates required for next generation wireless local area networks (WLANs). This paper analyzes the advantages of using optimal antenna selection to aid the stream control process for the open-loop MIMO link with a limited-feedback channel. It is observed that stream control with optimal antenna selection renders significant throughput gain relative to stream control with deterministic selection for both simulated data and measured data. In high-interference zones, the gap between the open- and closed-loop MIMO throughput is significantly reduced if optimal antenna selection is used. Lastly, open-loop MIMO with optimal antenna selection provides better protection against correlated interference than open-loop MIMO with deterministic antenna selection.

We present a new medium access control (MAC) protocol for ad-hoc networks with multiple input multiple output (MIMO) links. Links that use multiple element arrays (MEAs) at both ends are referred to as MIMO links. MIMO links are known to provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad-hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm that has the optimization considerations incorporated in its design. Finally, we present a distributed protocol that approximates the centralized algorithm, and compare its performance against that of baseline protocols that are variants of the CSMA/CA protocol.

We present an accurate, robust, and computationally efficient number-of-source detection method, which utilizes the variance of transformed rotational submatrix (VTRS) as the criterion, Unlike the traditional methods that use the eigenvalues, the VTRS detector determines the number of sources by exploiting the eigenvectors of two subarrays. This method applies to arbitrary subarray geometries. In addition, no subjective threshold or extra parameter Is required. In conjunction with spatial smoothing, this method can also detect coherent signals. One novelty of the VTRS method is that not only can it detect the number of sources, it can also indicate the quality of measured data and determine the optimum smoothing array size for parameter estimation, because of the low computational complexity, the VTRS number detector is suitable for adaptive or real-time applications.

The use of multiple input and multiple output (MIMO) antennas is a promising technique for increasing the capacity of wireless communication systems. Huge capacities are guaranteed only when channels between different transmitting and receiving antenna pairs are statistically independent. In this paper, we investigate the capacities of MIMO systems in a mobile-to-mobile environment by employing a "two ring" model which is simple extension of previous "one ring" model. With this model, we investigate the spatial correlation and information theoretic channel capacity of MIMO systems that use omni-directional antennas as well as MIMO systems with directional antennas. We show that MIMO systems with directional antennas yield good spatial decorrelation between antenna pairs even in a scarce scattering environment, while MIMO systems with omni-directional antennas suffer capacity degradation either when antennas are located closely each other, or when there is not enough multipath richness. Directional antennas guarantee the independence between different antenna pairs by not sharing scatterers, and the capacity is maximized by exploiting the independence between antenna pairs.

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.

we present a medium access control (MAC) protocol for ad hoc networks with multiple input multiple output (MIMO) links. MIMO links provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links coupled with several key optimization considerations, necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm called stream-controlled medium access (SCMA) that has the key optimization considerations incorporated in its design. Finally, we present a distributed SCMA protocol that approximates the centralized algorithm and compare its performance against that of baseline protocols that are CSMA/CA variants.

A distributed algorithm, which exploits channel state information (CSI) at the transmitter, is presented for determining the maximum number of independent data streams for each transmitting node in a network of interfering multiple-input multiple-output (MIMO) links. Simulated throughputs for two simple network topologies show that the algorithm yields nearly optimal stream control. These closed-loop MIMO throughputs are compared to those of open-loop MIMO, with and without optimal stream control, and to the throughput when the links operate in a non-interfering, TDMA fashion.

This paper considers joint optimization of power and array weights for a set of interfering cochannel links, given the desired capacity for each link. All transmitters and receivers are assumed to have array antennas. An iterative method for determining the link parameters is given. In this method, the desired capacities for the interfering links are the control parameters. It is also shown that the iterative method does not maximize the network throughput. Reducing the number of independent data streams for each link may enhance the network performance.

We show that the multiple-input, multiple-output (MIMO) capacity of measured, indoor flat-fading channels may be improved by adaptively changing the locations of the antenna elements. Moving the antenna elements is a way to provide spatial diversity to a MIMO link without decreasing the number of parallel data streams. First, we demonstrate that significant variations in equal-power capacity are possible using measured data from finely sampled virtual arrays at both ends of a MIMO link. Second, using steepest descent and a simulated path-type channel derived from measured data, we adapt the element locations. In line-of-sight (LOS) and obstructed-line-of-sight (OLOS) examples, the local maximum capacity is reached within 5 and 10 iterations, respectively. The improvements relative to the equally spaced arrays are approximately 22% and 19%, respectively.

A beamforming network is considered for a space-time block-coded orthogonal frequency-division multiplexing system in a high-speed indoor wireless network. We find that choosing the most powerful beams for transmission provide the best performance in the absence of interference. In the presence of interference, an iterative two-metric beam-selection method is proposed.

Explicit clustering in a statistical indoor propagation model is found to have significant effects on the capacity of an array-to-array link or matrix channel. Effects of antenna spacing, Poisson homogeneity, block size, and transmit power allocation strategies are also considered.

A multibeam beamformer in combination with a decision feedback equalizer is considered for the base station in a single-cell 100-Mb/s TDMA/TDD QPSK indoor wireless network at 24 GHz. The outage rate in terms of required SNR/bit/antenna is estimated using a statistical, clustered propagation model and for beam selection diversity and two-beam combining.

Modulated backscatter is an RF transmission technique useful for short-range, low-data-rate applications constrained to have extremely low power requirements, such as electronic shelf tags, RF tags, and some sensor applications. The small-scale fading observed on the backscattered signal has deeper fades than a signal from a traditional one-way link of the same range in the same environment because the fading on the backscattered signal is a product of the fading on the off-board generated carrier times the fading on the reflected signal. We present the first published reports of measured cumulative distribution functions for the small-scale fading at 2.4 GHz over this type of link.

An iterative algorithm is presented to jointly optimize the capacities for a number of interfering array-to-array or MIMO flat-fading links. All signals are co-channel. At each iteration for a given link, water-filling is used to calculate the optimum transmitter signal vector correlation matrices and receiver transformations under a constraint of total transmitted power for that link. Thus, by controlling the relative transmit powers at each of the interfering links the relative capacities can be controlled. A Monte-Carlo simulation was performed for an example three-link geometry. Several statistics for the three capacities are given for various power control conditions.

Clustering of paths in angle in RF multipath scenarios has been observed in measured data, explicitly used in stochastic propagation models, and shown to have a significant impact on the capacity of MIMO links. A 3D ray tracing is used to characterize the path delays and angles in a rectangular building in which all interior walls are parallel and perpendicular to the exterior walls. Path delays and angles are recorded for 200 random transmitter and receiver locations, maintaining constant range and throughout a particular building model. The main contribution of this paper is the observation that when all of the path angles from all 200 trials are measured relative to one building wall and collectively analyzed in a histogram, clustering is clearly apparent. Furthermore, the mean angles of the clusters are a function of the range between the transmitter and receiver. While this angle clustering in the ensemble of channels is not the same as clustering in a single channel trial, it may partially account for the clusters observed in single trials. As a practical matter, these results may impact the use and placement of directional antennas for wireless LANs in buildings of this type. By simply measuring the angles relative to the line-of-sight (LOS) path rather than the building wall, the data is shown to be consistent with the joint probability density function (PDF) of delay and angle based on the geometrically based elliptical model (GBEM).

Automatic repeat-request (ARQ) protocols using transmit diversity are investigated. The performance of space-time codes (STC) in a pure ARQ protocol (ST-ARQ) is first examined. Two hybrid ARQ schemes using STC are then proposed: space-time hybrid ARQ (ST-HARQ) and turbo space-time hybrid ARQ (TST-HARQ). For the ST-HARQ scheme, the previous transmitted packets are combined with the current received packet. In this scheme, the diversity gain is increased with every retransmission resulting in a higher probability of an accepted packet. For TST-HARQ, the transmitted packets are code combined and iteratively decoded. Performance analysis of ST-ARQ and ST-HARQ are approximated and shown to agree well with simulated results. TST-HARQ is shown to have the highest throughput efficiency compared to ST-HARQ and ST-ARQ in quasi-static and fast flat Rayleigh fading channels.

A method for designing near-optimal, tapered subarrays for adaptive interference cancellation is proposed. The limited aperture or limited element feature of these subarrays enables a low-complexity hardware implementation of a partially adaptive array. This approach optimizes canceller performance for a given number of beams and a given number of elements per beams.

This paper first presents a Markov chain that exactly models an optical shared-memory packet switch. Without loss in model accuracy, this Markov chain state size is greatly reduced to form a reduced Markov chain (RMC). A simplified construction method is given to make the RMC tractable. Throughput and probability of packet loss derived using the RMC are also presented.

Performance of shared memory architectures for optical switching is typically obtained by simulation, and in low packet loss regions is subject to statistical error or excessive computer run times. To obtain a reliable measure of performance, a Markov chain with a small number of states has been developed to model the switch's behavior. This reduced chain is derived from a full Markov chain which more directly models the switch. Equations to count the number of states for both Markov chains are derived. Packet loss results derived from the reduced Markov chain are compared with simulation results.

We show that the equalizer weights that cause the overall channel-equalizer response to approximate a pure delay (in a mean square error sense) are within the region of convergence of a stable local minimum of the Bussgang cost function. For any nontrivial channel the approximations for some delay values are poor. If a Bussgang algorithm is initialized with these weights it misconverges. A Monte-Carlo simulation created misconvergence as predicted. We propose the variance constraint (VC) algorithms that have more robust convergence properties than the Bussgang algorithms. The VC cost functions are similar to the well-known Godard cost functions. They incorporate estimates of the channel-equalizer overall gain and use a linear constraint on the weights. These two modifications push the bad stable points farther away in weight space from the stable points with good performance (i.e., low intersymbol interference). A Monte-Carlo simulation of nearly 2000 channels shows no misconvergence.

The objective of partially-adaptive array design is to reduce the number of adaptive weights without significantly degrading the performance of the adaptive array. Previous work includes numerical techniques for approximately minimizing the average generalized sidelobe canceller (GSC) output power for a desired number of adaptive weights, where the average is over a range of jammer parameters. Our new approach also minimizes the average GSC output power, but under the constraint that the reduced-dimensional solutions for all of the scenario trials lie in the same subspace. This constraint allows us to use the singular value decomposition to get the solution, thereby simplifying the optimization problem. Although the subspace constraint yields a theoretically suboptimum solution, the solution yields lower output power in comparison with a previously reported example that used the numerical methods. Also, the simplicity of the method allows large ranges of jammer parameters to be considered. Computer simulations are given to demonstrate effectiveness.

This paper considers the steady state behavior of adaptive processors. The use of a time-varying, minimum mean-square error searching, tree-structured filterbank is introduced to select the best subspace, in a least squares sense, for the adaptive processor with a predetermined number of adaptive coefficients. With the realistic assumption that the quantity of adaptive coefficients is the major computational restriction on the adaptive processor, it is shown that the time-varying filterbank allows the processor to achieve a lower minimum mean-square error than that which could be realized otherwise. The simple transform-domain least mean-square algorithms may be utilized to improve the dynamic behavior of the adaptive processor. These ideas are extended to the multidimensional filter case, and an example is presented via simulation for sensor array processing utilizing the generalized sidelobe canceller.>