Wireless network is more vulnerable to denial of service (DoS) attacks than wired one. In this paper we propose two methods to resist DoS attacks in UMTS-WLAN network, one is active and the other is passive. These two are complementary to each other. In active DoS defense method, a mobile terminal generates an authorized anonymous ID (AAI) using its true ID such as IP address, MAC address, PIN code, or asymmetric key, and substitutes its true ID with the authorized anonymous ID. The mobile terminal may be authenticated by the UMTS-WLAN, however its true ID is anonymous to intending attackers. This method can be used to isolate de-authenticating/disassociating DoS attackers, spoofing power-save DoS attacker, etc. In passive DoS defense method, we propose a trace back scheme. We use covert channels in the header of mobile IP packets to trace back the malicious nodes by embedding some address information of intermediate nodes, and recovering the embedded information by the victim. After the victim successfully traces the attacking paths from malicious nodes, it can segregate the malicious nodes and protect itself. This method can be used to resist DoS attacks of mass-produced junk message congestions.

In military communications, there exist numerous potential threats to message security. Ultra-wideband (UWB) signals provide secure communications because they cannot, in general, be detected using conventional receivers and they can be made relatively immune from jamming. The security of an UWB signal can be further improved by mixing it with random noise. By using a random noise signal, the user can conceal the message signal within the noise waveform and thwart detection by hostile forces. This paper describes a novel spread spectrum technique that can be used for secure and covert communications. The technique is based on the use of heterodyne correlation techniques to inject coherence in a random noise signal. The modulated signal to be transmitted containing the coherent carrier is mixed with a sample of an ultrawideband random noise signal. The frequency range of the ultra-wideband noise signal is appropriately chosen so that the lower sideband of the mixing process falls over the same frequency range. Both the frequency-converted noise-like signal and the original random noise signal are simultaneously transmitted on orthogonally polarized channels through a dual-polarized transmitting antenna. The receiver consists of a similar dual-polarized antenna that simultaneously receives the two orthogonally polarized transmitted signals, amplifies each in a minimum phase limiting amplifier, and mixes these signals in a double sideband up-converter. The upper sideband of the mixing process recovers the modulated signal, which can then be demodulated. The advantage of this technique lies in the relative immunity of the random noise-like un-polarized transmit signal from detection and jamming. Since the transmit signal "appears" totally un-polarized and noise-like, linearly polarized receivers are unable to identify, decode, or otherwise extract useful information from the signal. The system is immune from interference caused by high power linearly polarized signal transmissions since these signals are rejected during the correlation process at the receiver. Dispersive effects caused by the atmosphere and other factors are significantly reduced since both polarization channels operate over identical frequency bands. Several types of modulation schemes and hardware improvements are currently under investigation in order to characterize the performance of this communication system and to enhance its covertness properties.

In military communications, there exist numerous potential threats to message security. Ultra-wideband (UWB) signals provide secure communications because they cannot, in general, be detected using conventional receivers and they can be made relatively immune from jamming. The security of an UWB signal can be further improved by mixing it with random noise. By using a random noise signal, the user can conceal the message signal within the noise waveform and thwart detection by hostile forces. This paper describes a novel spread spectrum technique that can be used for secure and covert communications. The technique is based on the use of heterodyne correlation techniques to inject coherence in a random noise signal. The modulated signal to be transmitted containing the coherent carrier is mixed with a sample of an ultra-wideband (UWB) random noise signal. The frequency range of the UWB noise signal is appropriately chosen so that the lower sideband of the mixing process falls over the same frequency range. Both the frequency-converted noise-like signal and the original random noise signal are simultaneously transmitted on orthogonally polarized channels through a dual-polarized transmitting antenna. The receiver consists of a similar dual-polarized antenna that simultaneously receives the two orthogonally polarized transmitted signals, amplifies each in a minimum phase limiting amplifier, and mixes these signals in a double sideband upconverter. The upper sideband of the mixing process recovers the modulated signal, which can then be demodulated. The advantage of this technique lies in the relative immunity of the random noise-like unpolarized transmit signal from detection and jamming. Since the transmitted signal "appears" totally unpolarized and noise-like, linearly polarized receivers are unable to identify, decode, or otherwise extract useful information from the signal. The system is immune from interference caused by high power linearly polarized signal transmissions since these signals are rejected during the correlation process at the receiver. Dispersive effects caused by the atmosphere and other factors are significantly reduced since both polarization channels operate over identical frequency bands. This paper analyzes in detail various atmospheric propagation effects such as nulls, rain, and forests.

In this paper, we discuss some of the leading issues in through the wall radar imaging (TWRI) problems. We focus on the primary system challenges and deliverables, dealing only with the applications of statistical signal and array processing. Applications of antenna design and electromagnetic propagation are equally important, but they are both outside the scope of this paper. The material presented considers key desirable TWRI system properties and features and provides candidate solutions to achieve them. We focus on research performed at Villanova University and demonstrate some of our recent approaches to address system functionalities and requirements using analyses, computer simulations, and real-data. The paper does not attempt to cover all progress made in the field to date nor does it intend to compare the proposed techniques with alternative and competitive methods. It is written with the primary purpose of bringing to the reader many leading challenges and diverse issues worthy of considerations.

Direct-sequence spread spectrum (DSSS) modulation offers many properties that make it well suited for a mobile environment including some inherent narrowband interference or jamming (NBJ) suppression capability and resistance to multipath fading. The estimation and filtering of unwanted narrowband signals in DSSS systems has been extensively addressed in previous work but has given limited insight to system performance when multipath fading is introduced and a diversity solution such as the ubiquitous Rake receiver is implemented. In this case, multiple correlators (or fingers) are used to extract the desired signal replicas from the individual delay path components. For the maximum ratio combiner (MRC) version of the Rake receiver, the signal replicas from each finger are then combined in some weighted sense to formulate the final decision threshold. The focus of this study is twofold: to investigate the inaccuracies incurred on path delay estimation due to the presence of NBJ and its impact on the system Bit Error Rate (BER). In order to reduce the impact of NBJ, some adaptive NBJ suppression filters are suggested.

A microcell is a cell with 1-km or less radius which is suitable not only for heavily urbanized area such as a metropolitan city but also for in-building area such as offices and shopping malls. This paper deals with the microcell prediction model of propagation loss focused on in-buildng solution that is analyzed by probabilistic techniques. The RSL (Receive Signal Level) is the factor which can evaluate the performance of a microcell and the LOS (Line-Of-Sight) component and the blockage loss directly effect on the RSL. Combination of the probabilistic method is applied to get these performance factors. The mathematical methods include the CLT (Central Limit Theorem) and the SSQC (Six-Sigma Quality Control) to get the parameters of the distribution. This probabilistic solution gives us compact measuring of performance factors. In addition, it gives the probabilistic optimization of strategies such as the number of cells, cell location, capacity of cells, range of cells and so on. In addition, the optimal strategies for antenna allocation for a building can be obtained by using this model.

This paper investigates the performance of single and multiple TCP connections sharing a bandwidth-delay link. The link employs block-based error correction codes ECC, along with interleaving which is first shown to affect the performance of a single TCP connection. We examine impact of using ECC and interleaving on multiple connections on an ideal two-state fading channel model. Results are obtained by numerical simulation of both single and multiple TCP connections over the channel. Single TCP connection results show that the delay introduced by interleaving significantly affects TCP performance on a lossy channel. Without ECC, the aggregate channel utilization of multiple instances of TCP is higher than that of a single TCP instance. We find that this advantage dissipates when significant ECC is introduced.

Orthogonal Frequency Division Multiplexing (OFDM) is a very popular technique used for data transmission on multipath fading channels. The multipath component of these types of channels causes a phenomenon known as frequency selective fading. This type of fading can severely degrade or completely eliminate the signal energy of many of the OFDM tones producing an irreducible error rate, even when no noise is present. In the early 1990's, researchers combined some of the characteristics of Code Division Multiple Access (CDMA) and Spread Spectrum (SS) with OFDM in order to create a more robust modulation scheme capable of surviving frequency selective fading without the need for forward error correction (FEC) techniques and thus OFDM-CDMA was born. This paper will investigate the performance of uncoded and coded OFDM and OFDM-CDMA waveforms on various HF multipath/fading channels.

Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. For this architecture, we developed a channel model integrating both the RF and FSO channels. Based on the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link is used by itself, clouds hamper availability, due to introduction of attenuation and temporal dispersion. On the contrary, RF signals are relatively immune to the cloud effects, thus improve the availability in a hybrid of RF and FSO links, significantly.

This paper will investigate the performance of a dual transmit antenna, single receive antenna, Alamouti differential Space Frequency (SF) Coded Orthogonal Frequency Division Multiplexed (COFDM) system on a multipath fading High Frequency (HF) radio channel. Prior work demonstrated that the multi-antenna Alamouti system without forward error correction did not perform as well on the HF Channel as a novel uncoded single antenna Code Division Multiple Access (CDMA) system. By adding additional convolutional or Low Density Parity Check (LDPC) error correction coding, the Bit Error Rate (BER) performance will be shown to improve and exceed that of a similar single antenna COFDM differential system. Numerical results will be shown for the CCIR poor (2ms, 1Hz) and extra poor (2ms, 2Hz) channel conditions for both a constraint length 7 convolutional code and a 7680 block length regular LDPC code. BER behavior for the SF-COFDM system will be plotted as a function of channel fade rate and interleaver size. BER comparisons will also be made between the coded system and a coded single transmit and receive antenna HF COFDM CDMA scheme.

An approximately multi-user OFDM transceiver was introduced to reduce the multi-access interference (MAI ) due to the carrier frequency offset (CFO) to a negligible amount via precoding by Tsai, Lin and Kuo. In this work, we investigate the performance of this precoded multi-user (PMU) OFDM system in a time-variant channel environment. We analyze and compare the MAI effect caused by time-variant channels in the PMU-OFDM and the OFDMA systems. Generally speaking, the MAI effect consists of two parts. The first part is due to the loss of orthogonality among subchannels for all users while the second part is due to the CFO effect caused by the Doppler shift. Simulation results show that, although OFDMA outperforms the PMU-OFDM transceiver in a fast time-variant environment without CFO, PMU-OFDM outperforms OFDMA in a slow time-variant channel via the use of M/2 symmetric or anti-symmetric codewords of M Hadamard-Walsh codes.

A subspace-based algorithm to estimate carrier phase for PSK modulation is described. The performance of the algorithm is close to the optimum maximum-likelihood estimate. The approach is blind and based on a subspace decomposition of the received data. Comparisons of our proposed approach are made with other techniques available in literature. The performance of the algorithm with respect to the Cramer-Rao lower bound (CRLB) for the estimation of the carrier phase is investigated. Computer simulation results for BPSK, 4PSK and 8PSK are presented.

A finite impulse response (FIR) precoder for the multi-input multi-output (MIMO) channel is proposed to eliminate the intersymbol interference (ISI) effect in this research. Two precoders are designed to maximize the signal to noise ratio (SNR) and the signal to interference plus noise ratio (SINR), respectively. At the receiver end, a finite-length MIMO decision feedback equalizer is adopted to minimize the mean squared error (MSE) of the overall system. It is observed that the SINR-maximizing precoder performs the best among all precoders under our examination. For the 2x2 MIMO channel, the simple one-tap SNR-maximizing precoder can perform almost as well. For a heavily dispersive channel, the SINR-maximizing precoder still performs well even its length is much smaller than the channel length.

Space-time transmit structures for multi-antenna systems have received considerable interest. Circulant structures were among the first space-time coding techniques ever used for multiple-input multiple-output (MIMO) systems due to their simplicity and full rate. The fact that a circulant matrix is diagonalized by the discrete Fourier transformation matrix suggests that the circulant structure can be combined with an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver. Using this method, the spatial mixing effect of the MIMO channel is decoupled but the diversity gain is lost. To recover the diversity advantage, we propose to spread the transmitted symbols over the diagonalized channel using an invertable matrix whose entries are selected from $\{1,-1\}$. After spreading, every symbol experiences all the components of the frequency counterpart of the channel vector which makes our scheme provide full symbolwise diversity. The proposed scheme is full rate and can be easily applied to any number of transmit antenna. Our simulation results show that the performance of our scheme is close to the performance of the ideal orthogonal space-time code and much better than the conventional circulant space-time code.

In this work, we develop a new second-order statistics based multiuser multipath channel estimation algorithm for uplink wireless space-time coded CDMA systems. The estimation procedure is based on the parameterization, with respect to the multiuser channel response vector, of the received data covariance matrix. As a side result, we also obtain an improved covariance matrix estimator. Then we utilize both the channel and the covariance matrix estimates to obtain an estimate of the linear MMSE receiver. Simulation studies illustrate the performance improvements of the proposed estimators relative to existing methods in terms of channel estimation mean-square error as well as receiver filter output SINR and receiver BER.

Outdoor channels can be modelled as as a sum of array response vectors of varying gain at different Angles of Departure (AoDs) from different point sources. Based on this characteristics, we derive a hybrid of Beam-Forming (BF) and Space-Time Block Coding (STBC), where the space-time code is transmitted over the beams generated by array response vectors. This is for the practical case where the transmit array may have adequate information on the departure angles of the dominant paths between transmitter and receiver. In the case where the transmitter has knowledge on the associated complex path gains, a power loading scheme is designed when. We compute analytically the Signal-to-Noise Ratio (SNR) of the proposed hybrid for the specific case of a two-path channel model and using the orthogonal Alamouti code, and compare the result to the SNR of optimal Linear Precoding (LP) and the theoretically possible SNR of Orthogonal STBC (OSTBC). Simulation results show that the performance of the BF/STBC hybrid with and without power loading can be very close to LP-under certain conditions-or even better in the practical case where there are phase estimation errors in the path gain estimates employed at the transmitter.

Large scale, mobile ad-hoc networks (MANET) are of great interest for a number of applications including battlesphere dominance and homeland security. ENSCO, Inc. is designing a system for profiling large regions of the atmosphere. The concept, known as Global Environmental Micro Sensors (GEMS), features an integrated system of airborne probes that will remain suspended in the atmosphere and take measurements of pressure, temperature, humidity, and wind velocity as they are carried by atmospheric currents. In addition to gathering meteorological data, the probes could be used for monitoring and predicting the dispersion of particulate emissions, organic and inorganic pollutants, ozone, carbon dioxide, and chemical, biological, or nuclear contaminants. Several functionality requirements are called into question when investigating a scalable mobile network protocol. For instance, periodic reporting may not always be required and can be abandoned in favor of event-driven reports. Similarly, network connectivity may not be required at all times. Instead of constant global connectivity, paths can be formed only when data packets are ready for transmission. For a successful GEMS system, the most important network function is to relay timely data to one or more receiving stations. We will present both the GEMS system and probe design as well as discuss the trade-offs associated with optimizing a three-dimensional, mobile, airborne network comprised of low-cost, low-power probes. We will also analyze and present measured data to determine the performance of a representative MANET under actual environmental conditions and various aspects of mobility.

A growing number of Public Safety agencies have begun leveraging wireless data communication technology to improve tactical response capabilities as well as overall productivity. For years police departments subscribed to CDPD (Cellular Digital Packet Data) services to provide officers with basic dispatch data and criminal database access. Now as cellular carriers have deactivated CDPD and shifted to 2.5G and 3G data services such as 1xRTT, GPRS and EDGE, police departments are scrambling to fill the void. Not surprisingly, the extraordinary investments cellular carriers made to upgrade their infrastructures have been transferred to the customer, with monthly fees running as high as $80 a month per user. It's no wonder public safety agencies have been reluctant to adopt these services. Lost in the fray are those smaller police departments which account for nearly 90% of the nation's total. This group has increasingly sought out alternative data communication solutions that are not predicated on budget-busting monthly access fees. One such example is the Marco Island Police Department (MIPD) in Southwestern Florida that received a Federal grant to augment its existing voice communications with data. After evaluating several different technologies and vendors, MIPD chose a 900 MHz ad hoc mesh network solution based on its ability to provide reliable, high-speed and secure IP-based data communications over extensive distances. This paper will discuss technical details of Marco Island's mobile mesh network implementation; including: coverage area with 900 MHz spread spectrum radios, strategic repeater tower placement, interference, throughput performance, and the necessity for application-persistence software.

This paper presents the architecture design of a high data rate universal lattice decoder for MIMO channels on FPGA platform. A phost strategy based lattice decoding algorithm is modified in this paper to reduce the complexity of the closest lattice point search. The data dependency of the improved algorithm is examined and a parallel and pipeline architecture is developed with the iterative decoding function on FPGA and the division intensive channel matrix preprocessing on DSP. Simulation results demonstrate that the improved lattice decoding algorithm provides better bit error rate and less iteration number compared with the original algorithm. The system prototype of the decoder shows that it supports data rate up to 7Mbit/s on a Virtex2-1000 FPGA, which is about 8 times faster than the original algorithm on FPGA platform and two-orders of magnitude better than its implementation on a DSP platform.

Multiple-input multiple-output wireless systems use multiple antennas in both transmitter and receiver. The huge capacity of this multi-environment system has attracted intensive interests in recent years. Hardware implementation of MIMO lattice decoder becomes a challenging task because of the complexity of the lattice decoding algorithms. This paper compares two typical lattice decoding algorithms in hardware implementations. The data dependency among the iterative closest lattice point search procedure is examined and the possibilities of parallel implementation are explored. Parallel architectures are designed for each algorithm and are prototyped on two different hardware platforms: FPGA and DSP. Decoding rate and bit error rate are compared between the two algorithms. The performance of different hardware platforms is investigated as well. The experimental results show that the FPGA-based AV algorithm decoder supports 17.6Mbit/s decoding rate when mapped on a Xilinx Virtex2 1000 FPGA, and is more than 7 times faster than the VB algorithm decoder on the same hardware. To achieve the compatible bit error rate performance, FPGA based lattice decoder provides an order of magnitude faster decoding rate than the DSP decoder using the same algorithm.

Contemporary wireless communication strategies focus on efficient use of bandwidth in order to allow more users to exploit the RF spectrum through techniques like Frequency-hopping spread spectrum (SS) and Direct-sequence SS. In many real-world applications, these methods are implemented for synchronous communication systems. To achieve synchronous communication, two-way handshaking that requires overhead circuitry is performed between the sender and receiver. In order to use spread spectrum methods for certain unconstrained and low-power applications, such as implantable and remote monitoring devices, it is necessary to refine these methods to support asynchronous communication. We have designed and modeled a SS system, which could be integrated with a custom integrated circuit that would provide elementary multi-user communication. The SS logic generates a gold-code based on address and data bits that is then modulated with a carrier to transmit the information. Because the system is asynchronous, we treat each transmitted code as an independent piece of information. The receiver decodes the information using a full correlation with the ideal known gold-codes. For efficiency purposes, the filter is applied in the frequency domain. A threshold is applied to the output of the filter to determine if a particular code is transmitted as well as the point in time/space from which the signal was sent. The method was simulated and evaluated under several scenarios including different carrier frequencies, multiple targets, and various positions relative to the receiver from the simulated receiver. The results demonstrate the utility of asynchronous SS under many different conditions.

While the field of wireless communication has developed dramatically over the past several decades, there are several notable applications of wireless technologies which impose constraints on power-consumption and form-factor that are not compatible with cutting edge technologies. These applications include implantable devices and remote monitoring devices. Using a well-defined set of functional needs and system restrictions, we have developed an ultra-compact and ultra-low-powered transponder which contains spread spectrum (SS) logic for wireless communications. The transponder chip was designed and built in the Jazz BiCMOS SiGe technology. The device is activated via a pure tone and emits a SS response which is modulated over the carrier with binary phase shift keying (BPSK). The SS signal is a gold-code generated from two 9-bit m-sequence generators. One of the m-sequences is seeded with a fixed value while the other 9-bit register is pinned out and can be a fixed ID or a bus to transmit data from a microcontroller. The data is received and decoded by a standard PC with a high-speed acquisition board. In order to support multiple devices at various distances, asynchronous decoding is applied. When active, the device draws less than 35 mA of current. Because the duty cycle of this device is likely less than 1%, the device can be powered for several hours using a very small coin battery. The device has been tested both in the lab and natural environment testing is underway. Future work will combine the device with a microcontroller in the field to achieve specific monitoring goals.

The proposed research focuses on the communication in an RF-based indoor wireless localization system. In such a system, wireless badges attached to people or devices report positions to wireless gateway units. Badges have very limited communication, energy, as well as processing capabilities. However, gateways are significantly less constrained by battery than the badges. Wireless gateway units route collected badge information hop-by-hop towards one central unit of the system. We assume that each gateway unit has one transceiver antenna and is able to determine its own relative position in the system. The goal of this research is to develop an application-specific scheme for information routing and topology control among gateway units with maximum reliability, flexibility, adaptability and acceptable latency. We implemented two protocols (a robust one and a traffic-aware one), however, we shall show that for large networks, the use of multiple routing algorithms is beneficial. We assume that the topology control is fully centralized and the central unit is responsible for network management. We simulated the feasibility of the proposed novel two-protocol routing scheme and compared this scheme to a well-known dynamic source routing scheme. We demonstrated noticeable improvements in terms of robustness, traffic-awareness, and throughput. We also showed that the use of multiple protocols in our application-specific wireless indoor localization system will enhance the overall system performance.

In a large class of sensor network deployments, a small subset of the sensors covering the sensor field is equipped with special communications capability to communicate with operators outside the sensor field. These sensors play the role of gateways for off-field communication in the sense that all communications to- or out of the field is through these nodes, and the other non-gateway nodes are only capable of sensor-to-sensor communication. This design achieves a lower cost by concentrating expensive communication devices in a small subset of nodes. An important problem in designing such gateway-based sensor networks is determining the number of gateway nodes needed, their location in the sensor field, and the automation of the sensor-to-gateway association for off-field communication. These design considerations are addressed in this paper. In determining the number of gateways the tradeoff is between performance and cost. As the number of gateways increases, less traffic load is placed on each gateway and its surrounding nodes, resulting in longer network lifetime and larger off-field aggregate transmission capacity. However, with a larger number of gateways the network may be too costly to deploy as gateway nodes are more expensive than non-gateway sensor nodes. We develop and analyze models that allow us to determine the optimal number of gateways and their location in the sensor field. We also provide initial results with respect to determining the needed number of fusion nodes. While the presence of multiple gateways offers a higher degree of off-field communication reliability, a sensor will need to select one of the gateways at a time for off-field communication. In this paper, we also propose a dynamic sensor-to-gateway association protocol. Based on current energy levels, the distributed protocol dynamically assigns sensors to gateways in such a way that the overall transmission load is balanced among the different gateway regions over the lifetime of the sensor field.

Sensors have varied constraints, which make the network challenging for communicating with peers. In this paper, an extension, to the physical layer of the previous predictive sensor network model using the ant system is proposed. The tiny and low-cost sensor nodes are made of RF wireless links, where the states of the nodes vary with respect to time and environment. The ant system is a learning algorithm, that can be used to solve any NP hard communication problem and possesses characteristics such as robustness and versatility. The ant system possesses unique features that keep the network functional by detecting weak links and re-routing the agents. The swarm agents are distributed along the network, where the agent communicates with its neighbors (agents) by means of pheromone deposition and tabu list. The transition probability in the ant system includes an objective function, which is influenced by the poset weights. The poset weights on each of the orthogonal communication parameters greatly affects the decisions made by ant system. The agents carry updated information of its previous nodes, which helps in monitoring the strength of the communication links. Through simulation, comparison between DSSS-BPSK and Bluetooth-GFSK signals are shown. This paper demonstrates the robustness of the model under slow/fast fading, and energy loss at node during transmission. Implementation of this algorithm should be able to handle hostile environmental conditions and human tampering of data. The performance of the network is evaluated based on accuracy and response time of the agents within the network.

A spatio-temporal filtering method is proposed to detect outliers in wireless sensor networks in this work. Outliers are assumed to be uncorrelated in time and space, and modeled as an alpha-stable distribution. The proposed algorithm consists of collaborative time-series estimation, variogram application, and principle component analysis (PCA). It is realized on self-organized clusters that can manage the data locally. Conceptually, each node detects any temporally abnormal data and transmits the rectified data to a local cluster-head, which detects any survived spatial outliers and determines the faulty sensors accordingly. As a result, faulty sensors do not burden the sink to achieve the following two goals simultaneously, i.e., enhancing the data quality and reducing the communication cost in wireless sensor networks. It is demonstrated that the maximum outlier detection rate is around 94% when the noise level is alpha=0.9.

A power-aware approach that allows the power consumption to be evenly distributed among network nodes and, thereby, prolongs the network lifetime is proposed in this research. To begin with, each node has the same residual power level, and the algorithm employs the MST (minimum spanning tree)-based power-efficient topology control algorithm to obtain the first topology. Afterwards, when a node is making a decision on whether a wireless link between itself and a reachable neighboring node should be preserved or not in the topology being constructed, the decision is made based on not only the distance from its neighboring nodes but also the residual power levels of itself and its neighboring nodes. In principle, the constructed topology will be the one in which the node of a larger connectivity degree is the one of a higher residual power level. Also, from time to time, based on the residual power level of each node, the topology may be restructured. A node of degree one is a leaf node, and those non-leaf nodes and their attached links form the backbone of the virtual infrastructure.

We present in this paper an energy efficient media access control (MAC) protocol for chain-type wireless sensor networks. The chain-type sensor networks are fundamentally different from traditional sensor networks in that the sensor nodes in this class of networks are deployed along narrow and elongated geographical areas and form a chain-type topology. Recently, we have successfully developed hierarchical network architecture, sensor deployment strategy, and corresponding network initialization and operation protocols for this class of sensor networks. In this paper, we present a novel TDMA scheduling protocol that takes full advantages of the available channel reuse inherent in the chain-type sensor networks to develop energy efficient and high data throughput MAC protocols for sensor data transmission. The synchronized TDMA scheduling allows the nodes to power on only when it is scheduled to send and receive and therefore results in additional energy saving. Within a cluster, parallel transmission is made possible because of the linear distribution of nodes within the chain-type topology and this yields the desired high throughput. Preliminary simulations have been carried out to show that the proposed TDMA scheduling outperforms the well-know SMAC scheme in terms of energy efficiency and data throughput under various duty cycles.

Bluetooth is a promising wireless technology that enables devices to form short-range multihop wireless ad-hoc networks, or personal area networks. However, the Bluetooth scatternet formation is one of the challenges that need to be resolved since the performance of a Bluetooth network depends largely on the scatternet topology used. We first present a height-balanced binary tree, termed ACB-tree for almost-complete-binary tree, that allows two such trees to be combined to create a larger ACB-tree retaining the height-balance requirements. And, then propose a distributed scatternet formation algorithm to create ACB-trees: the generated scatternet is shown to minimize the number of piconets and provide a logarithmic-diameter in the multihop interconnection network. We also present simulations, conducted using Blueware simulator, to provide experimental results to study and compare the performance of the resulting scatternets.

Disseminating data among sensors is a fundamental operation in energy-constrained wireless sensor networks. We present a gossip-based adaptive protocol for data dissemination to improve energy efficiency of this operation. To overcome the data implosion problems associated with dissemination operation, our protocol uses meta-data to name the data using high-level data descriptors and negotiation to eliminate redundant transmissions of duplicate data in the network. Further, we adapt the gossiping with data aggregation possibilities in sensor networks. We simulated our data dissemination protocol, and compared it to the SPIN protocol. We find that our protocol improves on the energy consumption by about 20% over others, while improving significantly over the data dissemination rate of gossiping.

Visual information is of vital significance to both animals and artificial systems. The majority of mammals rely on two images, each with a resolution of 10⁷-10⁸ 'pixels' per image. At the other extreme are insect eyes where the field of view is segmented into 10³-10⁵ images, each comprising effectively one pixel/image. The great majority of artificial imaging systems lie nearer to the mammalian characteristics in this parameter space, although electronic compound eyes have been developed in this laboratory and elsewhere. If the definition of a vision system is expanded to include networks or swarms of sensor elements, then schools of fish, flocks of birds and ant or termite colonies occupy a region where the number of images and the pixels/image may be comparable. A useful system might then have 10⁵ imagers, each with about 10⁴-10⁵ pixels. Artificial analogs to these situations include sensor webs, smart dust and co-ordinated robot clusters. As an extreme example, we might consider the collective vision system represented by the imminent existence of ~10⁹ cellular telephones, each with a one-megapixel camera. Unoccupied regions in this resolution-segmentation parameter space suggest opportunities for innovative artificial sensor network systems. Essential for the full exploitation of these opportunities is the availability of custom CMOS image sensor chips whose characteristics can be tailored to the application. Key attributes of such a chip set might include integrated image processing and control, low cost, and low power. This paper compares selected experimentally determined system specifications for an inward-looking array of 12 cameras with the aid of a camera-network model developed to explore the tradeoff between camera resolution and the number of cameras.

In the adaptive beamforming problem it is usual that the covariance matrix is not know and an estimate from training samples is used. When the size of the training samples is limited the performance of the full rank Minimum Variance Distortionless Response (MVDR) beamformer is deteriorated; and a low rank solution of the MVDR problem can yield a better performance. A new adaptive approach, Indirect Dominant Mode Rejection (IDMR), was introduced in previous work to solve the MVDR optimization problem. In the previous work the IDMR beamformer was used to extract the desired signal, while in this work we use the IDMR beamformer to estimate the power spectrum. We also introduce several modifications, which bring more robustness to the basic IDMR. These include root-music, forward/backward averaging of the sample covariance matrix and power window.

This paper proposes an iterative multi-user detection scheme for coded asynchronous orthogonal frequency division multiplexing (OFDM) systems. Although a spatially multiplexed multi-user communication system can achieve high spectral efficiency, multiple access interference deteriorates the quality of signal detection. A linear minimum mean squared error (MMSE) based multi-user receiver for asynchronous OFDM channels has been proposed, and hard interference cancellation schemes were developed for an uncoded system. In this paper, we develop a low-complexity iterative soft interference cancellation technique exploiting the circular convolution property of cyclic prefix based OFDM systems. The proposed algorithm mitigates the residual interference and improves the performance of a channel decoder in a coded system through iterative demodulation and decoding. Simulation results demonstrate that spatial multiplexing of two users with two receiving antennas can outperform a system serving a single user with one receiving antenna by employing the proposed turbo receiver.

We develop a new direction-of-arrival (DOA) estimation procedure that utilizes a modified version of the orthogonal auxiliary-vector (AV) filtering algorithm. The procedure starts with the linear transformation of the array response scanning vector by the input autocorrelation matrix. Then, successive orthogonal maximum cross-correlation auxiliary vectors are calculated to form a basis of the scanner-extended signal subspace. As a performance evaluation example, our studies for uncorrelated sources demonstrate a gain in the order of 15dB over MUSIC, 7dB over ESPRIT, and 3dB over the grid-search maximum likelihood DOA estimator at probability of resolution 0.9 with a ten-element array and reasonably small observation data records. A reduced complexity version of the proposed DOA estimation algorithm is also suggested. Results for correlated sources are reported as well.

The use of GPS has broadened to include mounting on or inside manned or autonomous vehicles which makes it subject to interference generated from motor emissions. Many sources of interference are typically modeled as impulsive noise whose characteristics may vary in terms of power, pulse width, and pulse occurrences. In this paper, we examine the effect of impulsive noise on GPS delay lock loops (DLL). We consider the DLL for the GPS Coarse Acquisition code (C/A), which is used in civilian applications, but also needed in military GPS receivers to perform signal acquisition and tracking. We focus on the statistics of the noise components of the early, late, punctual correlators, which contribute to the discriminator error. The discriminator noise components are produced from the correlation between the impulsive noise and the early, late and punctual reference C/A code. Due to long time averaging, these components assume Gaussian distributions. The discriminator error variance is derived, incorporating the front-end precorrelation filter. It is shown that the synchronization error variance is significantly affected by the power of the received impulsive noise, the precorrelation filter, and the sample rate.

As the Warfighter progresses into the next generation battlefield, transformational communications become evident as an enabling technology. Satellite communications become even more vital as the battles range over greater non-contiguous spaces. While current satellite communications provide suitable beyond line-of-sight communications and the Transformational Communications Architecture (TCA) sets the stage for sound information exchange, a realizable transition must occur to ensure successful succession to this higher level. This paper addresses the need for a planned escalation to the next generation satellite communications architecture and offers near-term alternatives. Commercial satellite systems continue to enable the Warfighter to reach back to needed information resources, providing a large majority of available bandwidth. Four areas of concentration for transition include encrypted Telemetry, Tracking and Control (or Command) (TT&C), encrypted and covered data, satellite attack detection and protection, and operational mobility. Solution methodologies include directly embedding COMSEC devices in the satellites and terminals, and supplementing existing terminals with suitable equipment and software. Future satellites planned for near-term launches can be adapted to include commercial grade and higher-level secure equipment. Alternately, the expected use of programmable modems (Software Defined Radios (SDR)) enables incorporation of powerful cipher methods approaching military standards as well as waveforms suitable for on-the-move operation. Minimal equipment and software additions on the satellites can provide reasonable attack detection and protection methods in concert with the planned satellite usage. Network management suite modifications enable cohesive incorporation of these protection schemes. Such transitional ideas offer a smooth and planned transition as the TCA takes life.

Future U.S. MILSATCOM will rely on a complex satellite design to implement the TSAT satellite system that was developed on the Transformational Communication Architecture program. TSAT will deliver secured IP based, on-demand communication system to meet the needs of the military and the warfighter and will demand a very large satellite system, program, and an anticipated lengthy deployment time. An alternate system is proposed that can give IP based communications in a significantly shorter time frame. A constellation of commercial-like satellites, with transponded Ka-, Ku- and X-band, would nicely complement the MILSATCOM fleet. The services provided by these satellites would enable widespread communications services such as IP to the theatre, augmenting the DSCS, Wideband Gapfiller, and protected communications systems of MILSTAR and Advanced EHF. The capacity provided by the dedicated commercial satellites would help provide the capacity demanded by the warfighter as MILSATCOM transitions from today's technology to the TSAT system of tomorrow's Transformational Communications Architecture. This paper will discuss the satellite concept design and how it can augment the MILSATCOM fleet and show a more flexible approach of bandwidth management to support fixed and COTM terminals.

With the advent of low-cost launches and the advancement of technology, it is now possible to deploy small military communication payloads which can be used autonomously by nations of modest means for the security of their realm. Such platforms can also be used for bespoke functions, for example, to fill in a gap by providing simple communication channels while the main satellite is being developed.

A recent FCC decision1 has validated a new architecture with the potential to provide all of North America with an unusually broad and powerful wireless communications system. This architecture permits an existing allocation of mobile satellite spectrum to power a hybrid network, comprised of both terrestrial and satellite components. A satellite provides ubiquitous coverage, switching to a terrestrial-based cellular technology when in urban areas. The terrestrial component also provides capacity in areas of high demand. This ultra wide-area hybrid network, in turn, can act as a hub in an interlocking system of networks, incorporating public safety LMRs. The applicability of this system to homeland security should be obvious. It works nearly everywhere. It continues working even when towers are down throughout a wide area. It works with a conventional mobile device in a functionally transparent manner, providing first responders with interoperability, coverage, and redundancy needed to execute both their day-to-day and exigent responsibilities.

The automatic identification system (AIS) signal is of particular interest to the defense and security community because of its capability to identify and classify ships in U.S. waters. Placing an AIS receiver on a satellite provides a low cost solution to enhance security over a wide region. This paper describes the development of an AIS burst-acquisition receiver on a software radio consisting of a field programmable gate array (FPGA) and general purpose processor (GPP). A simple hybrid 1-bit/2-bit differential receiver can be easily implemented on such a software radio platform, and is sufficient to reliably demodulate collision free bursts in a space-borne signal environment.

In 2003, the California Space Authority (CSA) was provided funding by the U. S. Congress through the Defense Appropriations Act to develop a project that would demonstrate the U.S. space enterprise capability that would contribute to the effectiveness of those engaged in Homeland Security. The project was given broad latitude in selecting the area of Homeland Security to be addressed and the nature of the space technology to be applied. CSA became aware of a nascent law enforcement data-sharing project in the San Diego region known as the Automated Regional Justice Information System (ARJIS). First developed by the police departments in San Diego, ARJIS is an innovative system that shares criminal justice information among 50 federal, state, and local agencies. ARJIS was completing a pilot project that enabled officers to receive information on handheld computers, which was transmitted wirelessly through cellular networks. The accessed information came from several databases that collectively contained the entire region's crime and arrest reports, traffic citations, and incidents, as well as state and county wants and warrants. The fundamental limitations that plague all cellular-based devices caught CSA's attention and resulted in a cooperative effort to harden the communications link between the patrol officer and critical data. The principal goal of the SATCOM development task was to create a proof-of-concept application that would use SATCOM links to augment the current ARJIS handheld wireless (cellular) capability. The successful technical demonstration and the positive support for satellite communications from the law enforcement community showed that this project filled a need-both for improved information sharing and for highly reliable communications systems.

This paper describes the current National Weather Service's (NWS) system for providing weather alerts in the U.S. and will review how the existing end-to-end architecture is being leveraged to provide non-weather alerts, also known as "all-hazard alerts", to the general public. The paper then describes how a legacy system that transmits weather and all-hazard alerts can be extended via commercial wireless networks and protocols to reach 154 million Americans who carry cell phones. This approach uses commercial SATCOM and existing wireless carriers and services such as Short Messaging Service (SMS) for text and emerging Multimedia Messaging Service (MMS) protocol, which would allow for photos, maps, audio and video alerts to be sent to end users. This wireless broadcast alert delivery architecture is designed to be open and to embrace the National Weather Service's mandate to become an "" warning system for the general public. Examples of other public and private sector applications that require timely and intelligent push mechanisms using this alert dissemination approach are also given.

XTAR, LLC is a new telecommunications company exclusively to provide government users with satellite capacity in the X-Band. A description of the company, the services it intends to provide and the infrastructure it has built, the satel-lite it has launched and the systems it intends to launch in future are described in detail.

A reprogrammable digital transponder architecture allows a common product design to be scaled to meet varying user capacity requirements, match the transponder to the spacecraft resources, and maximize affordability by minimizing non-recurring costs. On-orbit frequency programmability permits the design and hardware development to proceed in parallel with the frequency coordination process, reducing schedule risks and providing operational flexibility. Leverage of digital processing technologies achieves improved channel performance characteristics compared with traditional implementations while also allowing channel characteristics such as selectivity, adjacent channel rejection, and channel frequency plans to be altered in response to the on-orbit interference environment. Channel passband shapes can also be altered for higher capacity waveforms that require different passband shapes or that need improved phase linearity over wider passbands than legacy waveforms. The ability to change these parameters on-orbit in response to upgraded ground terminal technology made possible by software-based radios will allow extended mission life without compromising communications capabilities. An ultra-high frequency (UHF) transponder with a scalable, expandable (or contractible) modular architecture, on-orbit frequency selection over entire communications bands, and functional reprogrammability through digital signal processing capabilities is described.

Turbo-code encoders are one of the spreadest family of error correcting codes used in the communication's world, especially in space transmissions. This paper presents an efficient technique to reconstruct turbo-code encoders which allows a passive adversary, with only few bits of an intercepted message encoded by the target turbocode encoder, to determine the parameters of the turbo-code encoder used, and therefore to decode online the communications. Thereby, our results confirm that keeping secret the parameters of turbo-code encoders can not be considered as a cryptographically way to ensure confidentiality. The starting point of our work is algorithms due to Filiol which enable to find the parameters of each convolutional encoder in the turbo-code encoder. Then, we recover the interleaver with two new algorithms, the first one based on the dynamic trie structure and the second one on a first order statistical test. The first algorithm is dedicated to noiseless channels. The asymptotic complexity of the complete process is O(n⁴) when a n²-bit message is available to attack a n-bit turbo-code encoder. The second algorithm works for every kind of channel and the noise does not matter much. Additionally, we present experimental results which underline the right detection threshold to use to recover the interleaver with a high probability. Furthermore, this method also works for turbo-code encoders composed of punctured convolutional encoders.

Through the use of recent technological developments, it is now feasible to establish free-space optical (FSO) communication links over ultra-long distances. Recent research has shown that FSO systems could be deployed to establish high-rate data links to deep space. This study analyzes beam steering tolerances, beam divergence, and geometric loss for different distance ranges of interest for FSO communication links based on a mechanical gimbaled beam steering mechanism. The tolerance, divergence and geometric loss calculations are performed to evaluate the feasibility of establishing FSO links between the Earth and satellites, the Earth and aircraft, aircraft and satellites, the Earth and moon, the Earth and Mars, and the Earth and the edge of the solar system. The analysis and calculations performed take into consideration the availability of new technology such as low noise photon-counting detectors and fiber lasers and amplifiers. The beam steering tolerance and divergence calculations provide beneficial information for determining the extent to which future FSO systems could be deployed for both commercial, military and space exploration applications. Recommendations on the suitability of an FSO communication link for various applications are then made based on the beam steering tolerance and divergence calculations.

An important consideration when deploying free-space optical (FSO) communication links over ultra long distances is the ability to actively steer the laser beam. FSO links are currently being researched as an attractive option for deep-space communication links or as a link to provide broadband communications to aircraft in-flight. In order to establish ultra long FSO links or to actively track FSO links between moving platforms, an active tracking system based on hybrid technology is essential. These hybrid systems are usually a combination of a mechanical gimbal and some array of active optical components. The presence of active optical components in a beam steering device is necessary to provide a high bandwidth while offsetting vibrations present on the mounting platform. This study compares three active beam steering elements that can be used in FSO transmitters and receivers. Performance characteristics of MEMS-based micro mirror arrays, acousto-optic modulators and steerable mirrors are analyzed and compared. A comparison of aperture size, range of motion, resolution and scanning speed is performed. Simulations in order to show the effects of vibration on various different length FSO links are run. A simulation in order to verify the ability of a fast steering mirror to offset vibration effects in a ground-to-UAV link is performed.

This paper will provide the reader with an in-depth description of the Army Research, Development and Engineering Command's (RDECOM) Joint SATCOM Engineering Center (JSEC) located at Fort Monmouth, NJ. The JSEC is a one-of-a-kind, state-of-the-art facility that has been in existence for over 30 years and has expanded to become a well respected and versatile engineering and research facility that supports all of the military services, Department of Defense (DoD) agencies, special users, as well as private industry and academia. The JSEC is divided into four functional areas: the Control Systems Laboratory (CSL), the Strategic Systems Laboratory (SSL), the Tactical Systems Laboratory (TSL) and the DoD Teleport Testbed (DTT). This paper will touch on the JSEC and the four functional areas in general and it will highlight the most recent addition to the JSEC, the DoD Teleport Testbed. Each functional area with its own mission, expertise and unique assets support the research, development, performance evaluation, system and equipment certification testing and anomaly resolution of space dependent and space based communications systems and equipment in the EHF, Ka, SHF and UHF frequency bands. The JSEC is also utilized to help mitigate program risk by using satellite simulation and participation in annual joint service communications exercises that serve as a platform to prove out real world requirements in a more controlled atmosphere.