Ensuring covert and secure communication remains a challenge in the evolving landscape of wireless communications. This paper presents a covert cognitive mobile edge computing network (CCMEC) in which secondary nodes (Alice) aim to transmit securely and offload computing tasks to secondary edge computing nodes (Bob) in the presence of multiple primary wardens (Willie). A two-stage connectivity probing and activation scheme is developed to maximize the data transmitted under covertness and minimize the energy consumption within a latency bound. The proposed scheme combines a distributed probing phase to find secure available connections based on the activity of primary and secondary nodes and a centralized activation phase to jointly optimize power allocation, channel, and Bob selection. The problem is solved by a Restless Multi-Armed Bandit (RMAB) framework with Whittle index optimization. Simulation results show the effectiveness of our approach in achieving covert communication compared to existing solutions.

Heterogeneous Internet of Things (IoT) networks enabled by Unmanned Aerial Vehicles (UAVs) operate in various protocols and spectrum bands (e.g., WiFi, LoRa, Zigbee) to collect and offload data generated from heterogeneous sensors. However, achieving timely and secure communications is challenging due to uncertain data generation, mobility, and location of wardens. This paper presents a collaborative framework that exploits cross-technology communications to achieve covertness constraints. The aim is to minimize the age of covert information (AoCI) and energy consumption by jointly optimizing data scheduling, power allocation, and offloading decisions by collaborating with UAVs. A multi-agent actor-critic algorithm that incorporates federated learning and attention mechanisms (cluster-MAAC-attention) is presented to solve the previous problem. Our simulation results show that our algorithm reduces the worst AoCI by 4 times, and reduces the penalty by 20 times compared to existing schemes.

Unmanned aerial vehicles (UAVs) are well known for violating citizen’s privacy either inadvertently or deliberately. However, UAVs could be victims of privacy violations themselves in the sense that an adversary observing a UAV can infer its destination. This article proposes several privacy-preserving mechanisms (PPMs) for protecting a UAV’s location privacy. In particular, we address the privacy protection problem in two major UAV applications that require significantly different measures: 1) package delivery and 2) Internet of Things (IoT) data collection. In the package delivery application, we propose two different PPMs to randomize the UAV’s trajectory such that the observing adversary is confused about the UAV’s destination; we provide privacy guarantees and analyze the tradeoff with energy consumption. In the IoT data collection scenario, the UAV is not necessarily required to hover exactly above the IoT device; hence, we propose a different PPM according to which the UAV chooses a random spot around the IoT device for data collection. Then, considering a minimum mean squared error (MMSE) criterion, we obtain the privacy leakage to the adversary. We also analyze the mean Peak Age of Information (PAoI) of the network and show that the proposed method does not degrade the mean PAoI significantly. Finally, considering the limitations of the MMSE approach for some applications, we also develop a differential privacy (DP)-based counterpart for this PPM. We observe that the mean PAoI degrades significantly in Laplacian DP but is acceptable in Gaussian DP.

Instant Messaging (IM) applications such as Signal, Telegram, and WhatsApp have become tremendously popular in recent years. Unfortunately, such IM services have been targets of governmental surveillance and censorship, as these services are home to public and private communications on socially and politically sensitive topics. To protect their clients, popular IM services deploy state-of-the-art encryption. Despite the use of advanced encryption, we show that popular IM applications leak sensitive information about their clients to adversaries merely monitoring their encrypted IM traffic, with no need for leveraging any software vulnerabilities of IM applications. Specifically, we devise traffic analysis attacks enabling an adversary to identify participants of target IM communications (e.g., forums) with high accuracies. We believe that our study demonstrates a significant, real-world threat to the users of such services. We demonstrate the practicality of our attacks through extensive experiments on real-world IM communications. We show that standard countermeasure techniques can degrade the effectiveness of these attacks. We hope our study will encourage IM providers to integrate effective traffic obfuscation into their software. In the meantime, we have designed a countermeasure system, called IMProxy that can be used by IM clients with no need for any support from IM providers. We demonstrate the effectiveness of IMProxy through simulation and experiments.

Recently, we have proposed a model-free privacy-preserving mechanism (PPM) against attacks that compromise user privacy by matching patterns in data sequences to those that are unique to a given user [1]. Because the PPM is model-free, there are no requirements on the statistical model for the data, which is desirable when the model is not perfectly known. However, the proposed PPM did not enforce any constraints on the value to which a data point might be obfuscated, hence allowing an unlikely pattern that would make it easy for the adversary to detect which values have been obfuscated. In this paper, we consider a constrained PPM that enforces a continuity constraint so as to avoid abrupt jumps in the obfuscated data. To design such, we employ a graph-based analytical framework and the concept of consecutive patterns. At each point, the obfuscated data should be chosen strictly from that point’s neighbors. Unfortunately, this might undesirably increase the noise level employed in data obfuscation and hence unacceptably reduce utility. We propose a new obfuscation algorithm, namely the obfuscation-return algorithm, and characterize its privacy guarantees under continuity and noise level constraints.

Because of their potential ubiquity, unmanned aerial vehicles (UAVs) are often viewed as a threat to people’s privacy. However, the users of UAVs for applications such as package delivery can also have their own privacy compromised by observations of UAV behavior by an adversary. Hence, this paper looks at privacy-preserving path-planning for a UAV. In particular, we consider a UAV which is delivering a package or operating a service, e.g. a health-emergency service, for users while an adversary tries to infer the UAV’s destination by observing its trajectory. We consider two models for the UAV motion for which we provide privacy-preserving path-planning mechanisms (PPPMs) while taking into account the UAV’s energy consumption as well. We obtain the tradeoff between privacy and energy consumption guarantees and show that the proposed PPPMs not only satisfy the privacy guarantees but also meet the energy efficiency criteria.

User privacy can be compromised by matching user data traces to records of their previous behavior. The matching of the statistical characteristics of traces to prior user behavior has been widely studied. However, an adversary can also identify a user deterministically by searching data traces for a pattern that is unique to that user. Our goal is to thwart such an adversary by applying small artificial distortions to data traces such that each potentially identifying pattern is shared by a large number of users. Importantly, in contrast to statistical approaches, we develop data-independent algorithms that require no assumptions on the model by which the traces are generated. By relating the problem to a set of combinatorial questions on sequence construction, we are able to provide provable guarantees for our proposed constructions. We also introduce data-dependent approaches for the same problem. The proposed obfuscation methods are evaluated on synthetic data traces and on the Reality Mining Data set to demonstrate the performance of the proposed algorithms relative to alternatives.

The fundamental limits of covert communication, where a message is sent from transmitter Alice to intended recipient Bob without detection by an attentive adversary warden Willie, has been considered extensively in recent years at the physical layer. The covert throughput depends critically on the warden's understanding of the characteristics of the radio environment and the type of receiver that he employs, and, as expected, the throughput increases when the warden has some uncertainty about the environment or some non-idealities in his receiver. In this paper, we consider the covert throughput when the adversary is only able to observe the medium access control (MAC) layer in a wireless communication system. In particular, given that the system has a rate of$\lambda$packets per slot transmitted over$n$channels by allowable system users, we study the allowable rate$\lambda _a$by covert users while maintaining covertness from an attentive warden observing the channel status in a slotted ALOHA system. We characterize performance for wardens with different abilities to discern the number of packets on a given channel, ranging from simple receivers that detect only whether there was a packet present to complicated receivers that can determine the number of packets involved in any collision, and also consider intended recipients Bob with varying abilities to perform multi-packet reception. In contrast to prior work in covert communications, the application considered motivates the consideration of results for finite (often small) observation vector lengths$n$at the adversary. Numerical results are provided both to illustrate the tightness of our achievability regions for the packet transmission rate of the covert transmitters and to demonstrate the covert throughput of the system as a function of$\lambda$and$n$.

Covert communication considers the ability of transmitter Alice to communicate reliably to receiver Bob without being detected by warden Willie. Previous work has generally considered a discrete-time model, and, in the standard Alice-Bob-Willie scenario, it has been shown that a discrete-time model captures the salient aspects of the underlying continuous-time covert communications system. However, in the presence of a jammer assisting Alice, where it has been shown in previous work that Alice can achieve a positive covert rate on a discrete-time model, we demonstrate here that a straightforward extension of the discrete-time construction to the continuous-time system does not guarantee a positive covert rate, hence indicating that the discrete-time model does not capture the salient aspects of the continuous-time system in such a scenario. This is because Willie is able to exploit excess bandwidth for co-channel interference suppression, as we demonstrate with an interference cancellation receiver. Hence, for the case when Alice is assisted by an uninformed jammer, we consider the continuous-time channel directly and study whether efficient covert communication can still be achieved against any possible receiver that Willie might employ. By presenting and characterizing an approach much different than that suggested by previous work for discrete-time systems in such a scenario, we establish that$\mathcal {O}(WT)$information bits can be transmitted covertly and reliably on a continuous-time channel of asymptotic bandwidth$W$in$T$seconds, regardless of the receiver that Willie employs. This is done for two separate scenarios: 1) when there is perfect frame synchronization between Alice and the jammer’s signals; 2) when there is no frame synchronization between Alice and the jammer’s signals. Numerical results are provided to validate the theory.

There is significant enthusiasm for the employment of Deep Neural Networks (DNNs) for important tasks in major wireless communication systems: channel estimation and decoding in orthogonal frequency division multiplexing (OFDM) systems, end-to-end autoencoder system design, radio signal classification, and signal authentication. Unfortunately, DNNs can be susceptible to adversarial examples, potentially making such wireless systems fragile and vulnerable to attack. In this work, by designing robust adversarial examples that meet key criteria, we perform a comprehensive study of the threats facing DNN-based wireless systems. We model the problem of adversarial wireless perturbations as an optimization problem that incorporates domain constraints specific to different wireless systems. This allows us to generate wireless adversarial perturbations that can be applied to wireless signals on-the-fly (i.e., with no need to know the target signals a priori), are undetectable from natural wireless noise, and are robust against removal. We show that even in the presence of significant defense mechanisms deployed by the communicating parties, our attack performs significantly better compared to existing attacks against DNN-based wireless systems. In particular, the results demonstrate that even when employing well-considered defenses, DNN-based wireless communication systems are vulnerable to adversarial attacks and call into question the employment of DNNs for a number of tasks in robust wireless communication.

Covert communication considers the ability of transmitter Alice to communicate reliably to intended receiver Bob without being detected by adversary warden Willie. One collection of approaches to covert signaling is for Alice to alter the state of a system in such a way that the altered state conveys information to Bob. Motivated by recent work on the foundations of covert communications that has largely considered the physical layer, we provide a fundamental characterization of one approach to covert signaling via activity: employing a codebook pre-shared with Bob, Alice encodes a message by selecting from the codebook the appropriate pattern of slots to insert innocuous packets into a slotted ALOHA system in the presence of other users. The intended recipient Bob detects patterns in the activity of the slotted ALOHA system to determine which codeword was sent. We provide a fundamental analysis of the performance of such a system under a covertness constraint. First, we consider signaling schemes derived specifically for the proposed channel when Bob or Willie has various abilities to discern the number of packets in a given slot. Given the challenges in such design, we next recognize that techniques from optical communications, although designed for a different channel, can potentially be employed and thus yield a large class of schemes that provide lower bounds on the achievable rate. Numerical results are provided to support the analytical development and to demonstrate the potential of covert signaling through such an approach.

Suppose we are given a large number of sequences on a given alphabet, and an adversary is interested in identifying (de-anonymizing) a specific target sequence based on its patterns. Our goal is to thwart such an adversary by obfuscating the target sequences by applying artificial (but small) distortions to its values. A key point here is that we would like to make no assumptions about the statistical model of such sequences. This is in contrast to existing literature where assumptions (e.g., Markov chains) are made regarding such sequences to obtain privacy guarantees. We relate this problem to a set of combinatorial questions on sequence construction based on which we are able to obtain provable guarantees. This problem is relevant to important privacy applications: from fingerprinting webpages visited by users through anonymous communication systems to linking communicating parties on messaging applications to inferring activities of users of IoT devices.

Covert communication conceals the existence of the transmission from a watchful adversary. We consider the fundamental limits for covert communications via packet insertion over packet channels whose packet timings are governed by a renewal process of rate λ. Authorized transmitter Jack sends packets to authorized receiver Steve, and covert transmitter Alice wishes to transmit packets to covert receiver Bob without being detected by watchful adversaries Willie 1 and Willie 2. Willies cannot authenticate the source of the packets or collaborate. Hence, each Willie looks for statistical anomalies in the packet stream from Jack to Steve to attempt detection of unauthorized packet insertion. First, we consider a special case where the packet timings are governed by a Poisson process and we show that Alice can covertly insert O(√λT) packets for Bob in a time interval of length T; conversely, if Alice inserts ω(√λT) packets, she will be detected by Willie 1 or Willie 2 with high probability. Then, we extend our results to general renewal channels and show that in a stream of N packets transmitted by Jack, Alice can covertly insert O(√N) packets; if she inserts ω(√N) packets, she will be detected by Willie 1 or Willie 2, with high probability.

Network flow fingerprinting can be used to de-anonymize communications on anonymity systems such as Tor by linking the ingress and egress segments of anonymized connections. Assume Alice and Bob have access to the input and the output links of an anonymous network, respectively, and they wish to collaboratively reveal the connections between the input and the output links without being detected by Willie who protects the network. Alice generates a codebook where each codeword is a unique fingerprint indicating a sequence of interpacket delays, and shares it only with Bob. To trace each flow, Alice selects a fingerprint and manipulates the packet timings of the flow to follow the packet timings suggested by the fingerprint, and Bob extracts the fingerprints from it after it passes through the network. We model the network as parallel M/M/1 queues where each queue is shared by a flow fifrom Alice to Bob and other flows independent of fi. Packet timings of the flows are governed by independent Poisson processes. Assuming all input flows have equal packet rates and that Bob observes only flows with fingerprints, we first present two scenarios: 1) Alice fingerprints all the flows and 2) Alice fingerprints a subset of the flows, unknown to Willie. Then, we extend the construction and analysis to the case of arbitrary flow rates and the case where Bob observes flows with and without fingerprints. For each scenario, we derive the number of flows that Alice and Bob can trace by fingerprinting.

Modern applications significantly enhance user experience by adapting to each user's individual condition and/or preferences. While this adaptation can greatly improve a user's experience or be essential for the application to work, the exposure of user data to the application presents a significant privacy threat to the users-even when the traces are anonymized-since the statistical matching of an anonymized trace to prior user behavior can identify a user and their habits. Because of the current and growing algorithmic and computational capabilities of adversaries, provable privacy guarantees as a function of the degree of anonymization and obfuscation of the traces are necessary. Our previous work has established the requirements on anonymization and obfuscation in the case that data traces are independent between users. However, the data traces of different users will be dependent in many applications, and an adversary can potentially exploit such. In this paper, we consider the negative impact of dependency between user traces on their privacy. First, we demonstrate that the adversary can readily identify the association graph of the obfuscated and anonymized version of the data, revealing which user data traces are dependent. Next, we demonstrate that the adversary can use this association graph to break user privacy with significantly shorter traces than in the case of independent users, and that obfuscating data traces independently across users is often insufficient to remedy such leakage. In other words, we have shown that inter-user dependency is disastrous to privacy, and any non-negligible dependency between users significantly reduces the effectiveness of anonymization and obfuscation schemes. Finally, we discuss how users can improve privacy by employing joint obfuscation that removes or reduces the data dependency.

In this work, we study the optimal configuration of the physical layer in wireless networks by means of Semi-Markov Decision Process (SMDP) modeling. In particular, assume the physical layer is characterized by a set of potential operating points, with each point corresponding to a rate and reliability pair; for example, these pairs might be obtained through a now-standard diversity-multiplexing tradeoff characterization. Given the current network state (e.g., buffer occupancies), a Decision Maker (DM) needs to dynamically decide which operating point to use. The SMDP problem formulation allows us to choose from these points. A solution to the SMDP problem is an optimal selection of operating points, which is expressed by a decision rule as a function of the number of packets in the source's finite queue, the channel state, and the size of the packet to be transmitted. We derive a general solution to the SMDP which covers various model configurations, packet size distributions and channel dynamics. For the specific case of exponential transmission times, we analytically prove the optimal policy has a threshold structure. Numerical results validate this finding, as well as depict muti-threshold policies for time varying channels such as the Gilbert-Elliott channel.

Covert communication is achieved if a transmitter (Alice) sends a message to a legitimate receiver (Bob) without being detected by an attentive warden (Willie). Recent work has taken an outage approach that considers an infinite blocklength and analyzes the probability that channel conditions are such that communication is reliable and/or covert. Power adaptation via truncated channel inversion (TCI) has been examined in previous work in an attempt to minimize outage under the covertness constraint. However, the optimality of TCI was not established, and the associated parameters in the TCI scheme were only found numerically. We provide the exact optimal power adaptation schemes under different covertness constraints and with different channel models: 1) when the channel between Willie and an uninformed jammer is AWGN; and 2) when the channel is a Rayleigh fading channel. We also examine the performance improvement of these schemes over standard approaches. This establishes that TCI is optimal in some, but not all, scenarios of interest.

The literature has extensively studied various location privacy-preserving mechanisms (LPPMs) in order to improve the location privacy of the users of location-based services (LBSes). Such privacy, however, comes at the cost of degrading the utility of the underlying LBSes. The main body of previous work has used a generic distance-only based metric to quantify the quality loss incurred while employing LPPMs. In this paper, we argue that using such generic utility metrics misleads the design and evaluation of LPPMs, since generic utility metrics do not capture the actual utility perceived by the users. We demonstrate this for ride-hailing services, a popular class of LBS with complex utility behavior. Specifically, we design a privacy-preserving ride-hailing service, called PRide, and demonstrate the significant distinction between its generic and tailored metrics. Through various experiments we show the significant implications of using generic utility metrics in the design and evaluation of LPPMs. Our work concludes that LPPM design and evaluation should use utility metrics that are tailored to the individual LBSes.

We address the critical problem of hiding unmanned aerial vehicles (UAV) for wireless transmissions by the emerging covert communication technology, since in military surveillance scenarios the disclosure of a UAV's location information may lead to an attack. Specifically, we jointly optimize the UAV's transmit power and height in order to maximize the communication quality to a legitimate receiver subject to a covertness constraint, a maximum transmit power constraint, and a lower bound and an upper bound on the UAV's height. To this end, we first derive the UAV's optimal height for maximizing the legitimate communication quality without any constraint and then we address this problem under constraints in particular the covertness constraint. Our solution explicitly shows the impact of these constraints and reveals the tradeoff among the legitimate communication quality, covertness requirement, and surveillance cost. For example, our examination demonstrates that the legitimate communication quality increases with the surveillance cost represented by the quality of the camera used for conducting surveillance.

Covert communications, where a transmitter Alice wishes to hide the presence of her transmitted signal from a watchful adversary Willie, has been considered extensively in recent years. Those investigations have generally considered physical-layer models, where the adversary has access to a sophisticated (often optimal) receiver to determine whether a transmission has taken place, and have addressed the question of what rate can information be communicated covertly. More recent investigations have begun to consider the change in covert rate when Willie has uncertainty about the physical layer environment. Here, we move up the protocol stack to consider the covert rate when Willie is watching the medium-access control (MAC) layer in a network employing a random access MAC such as slotted ALOHA. Based on the rate of collisions and potentially the number of users involved in those collisions, Willie attempts to determine whether unauthorized (covert) users are accessing the channel. In particular, we assume different levels of sophistication in Willie's receiver, ranging from a receiver that only can detect whether there was a collision or not, to one that can always tell exactly how many packets were on the channel in the random access system. In each case, we derive closed-form expressions for the achievable covert rates in the system. The achievable rates exhibit significantly different behavior than that observed in the study of covert systems at the physical layer.

The rapid growth of the Internet of Things (IoT) necessitates employing privacy-preserving techniques to protect users' sensitive information. Even when user traces are anonymized, statistical matching can be employed to infer sensitive information. In our previous work, we have established the privacy requirements for the case that the user traces are instantiations of discrete random variables and the adversary knows only the structure of the dependency graph, i.e., whether each pair of users is connected. In this paper, we consider the case where data traces are instantiations of Gaussian random variables and the adversary knows not only the structure of the graph but also the pairwise correlation coefficients. We establish the requirements on anonymization to thwart such statistical matching, which demonstrate the significant degree to which knowledge of the pairwise correlation coefficients further significantly aids the adversary in breaking user anonymity.

Many popular applications use traces of user data to offer various services to their users. However, even if user data are anonymized and obfuscated, a user's privacy can be compromised through the use of statistical matching techniques that match a user trace to prior user behavior. In this paper, we derive the theoretical bounds on the privacy of users in such a scenario. We build on our recent study in the area of location privacy, in which we introduced formal notions of location privacy for anonymization-based location privacy-protection mechanisms. Here, we derive the fundamental limits of user privacy when both anonymization and obfuscation-based protection mechanisms are applied to users' time series of data. We investigate the impact of such mechanisms on the tradeoff between privacy protection and user utility. We first study achievability results for the case where the time-series of users are governed by an independent and identically distributed (i.i.d.) process. The converse results are proved both for the i.i.d. case as well as the more general Markov chain model. We demonstrate that as the number of users in the network grows, the obfuscation-anonymization plane can be divided into two regions: in the first region, all users have perfect privacy; and, in the second region, no user has privacy.

Modern applications significantly enhance user experience by adapting to each user's individual condition and/or preferences. While this adaptation can greatly improve utility or be essential for the application to work (e.g., for ride-sharing applications), the exposure of user data to the application presents a significant privacy threat to the users, even when the traces are anonymized, since the statistical matching of an anonymized trace to prior user behavior can identify a user and their habits. Because of the current and growing algorithmic and computational capabilities of adversaries, provable privacy guarantees as a function of the degree of anonymization and obfuscation of the traces are necessary. Our previous work has established the requirements on anonymization and obfuscation in the case that data traces are independent between users. However, the data traces of different users will be dependent in many applications, and an adversary can potentially exploit such. In this paper, we consider the impact of correlation between user traces on their privacy. First, we demonstrate that the adversary can readily identify the association graph, revealing which user data traces are correlated. Next, we demonstrate that the adversary can use this association graph to break user privacy with significantly shorter traces than in the case when traces are independent between users, and that independent obfuscation of the data traces is often insufficient to remedy such. Finally, we discuss how the users can employ dependency in their obfuscation to improve their privacy.

In covert communication, Alice tries to communicate with Bob without being detected by a warden Willie. When the distance between Alice and Bob becomes large compared with the distance between Alice and Willie(s), the performance of covert communication will be degraded. In this case, multi-hop message transmission via intermediate relays can help to improve the performance. Hence, in this paper, multi-hop covert communication over a moderate size network and in the presence of multiple collaborating Willies is considered. The relays can transmit covertly using either a single key for all relays or different independent keys at the relays. For each case, we develop efficient algorithms to find optimal paths with maximum throughput and minimum end-to-end delay between Alice and Bob. As expected, employing multiple hops significantly improves the ability to communicate covertly versus the case of a single-hop transmission. Furthermore, at the expense of more shared key bits, analytical results and numerical simulations demonstrate that the multi-hop covert communication with different independent keys at the relays has better performance than the multi-hop covert communication with a single key.

Covert communication conceals the transmission of the message from an attentive adversary. Recent work on the limits of covert communication in additive white Gaussian noise channels has demonstrated that a covert transmitter (Alice) can reliably transmit a maximum of O(√n) bits to a covert receiver (Bob) without being detected by an adversary (Warden Willie) in n channel uses. This paper focuses on the scenario where other “friendly” nodes distributed according to a two-dimensional Poisson point process with density m are present. We propose a strategy where the friendly node closest to the adversary, without close coordination with Alice, produces artificial noise. We show that this method allows Alice to reliably and covertly send O(min{n, mγ/2√n}) bits to Bob in n channel uses, where γ is the path-loss exponent. We also consider a setting where there are Nw collaborating adversaries uniformly and randomly located in the environment and show that in n channel uses, Alice can reliably and covertly send O(min{n, (mγ/2√n/Nwγ)}) bits to Bob when γ>2, and O(min{n, (m√n/Nw2log2Nw)}) when γ=2. Conversely, we demonstrate that no higher covert throughput is possible for γ>2.

The prevalence of mobile devices and location-based services (LBS) has generated great concerns regarding the LBS users' privacy, which can be compromised by statistical analysis of their movement patterns. A number of algorithms have been proposed to protect the privacy of users in such systems, but the fundamental underpinnings of such remain unexplored. Recently, the concept of perfect location privacy was introduced and its achievability was studied for anonymization-based LBS systems, where user identifiers are permuted at regular intervals to prevent identification based on statistical analysis of long time sequences. In this paper, we significantly extend that investigation by incorporating the other major tool commonly employed to obtain location privacy: obfuscation, where user locations are purposely obscured to protect their privacy. Since anonymization and obfuscation reduce user utility in LBS systems, we investigate how location privacy varies with the degree to which each of these two methods is employed. We provide: (1) achievability results for the case where the location of each user is governed by an i.i.d. process; (2) converse results for the i.i.d. case as well as the more general Markov Chain model. We show that, as the number of users in the network grows, the obfuscation-anonymization plane can be divided into two regions: in the first region, all users have perfect location privacy; and, in the second region, no user has location privacy.

This paper considers secure energy-efficient routing in the presence of multiple passive eavesdroppers. Previous work in this area has considered secure routing assuming probabilistic or exact knowledge of the location and channel-state-information (CSI) of each eavesdropper. In wireless networks, however, the locations and CSIs of passive eavesdroppers are not known, making it challenging to guarantee secrecy for any routing algorithm. We develop an efficient (in terms of energy consumption and computational complexity) routing algorithm that does not rely on any information about the locations and CSIs of the eavesdroppers. Our algorithm guarantees secrecy even in disadvantaged wireless environments, where multiple eavesdroppers try to eavesdrop each message, are equipped with directional antennas, or can get arbitrarily close to the transmitter. The key is to employ additive random jamming to exploit inherent non-idealities of the eavesdropper's receiver, which makes the eavesdroppers incapable of recording the messages. We have simulated our proposed algorithm and compared it with the existing secrecy routing algorithms in both single-hop and multi-hop networks. Our results indicate that when the uncertainty in the locations of eavesdroppers is high and/or in disadvantaged wireless environments, our algorithm outperforms existing algorithms in terms of energy consumption and secrecy.

Recent work has established that when transmitter Alice wishes to communicate reliably to recipient Bob without detection by warden Willie, with additive white Gaussian noise (AWGN) channels between all parties, communication is limited to O(√n) bits in n channel uses. However, this assumes that Willie has an accurate statistical characterization of the channel. When Willie has uncertainty about such and his receiver is limited to a threshold test on the received power, Alice can transmit covertly with a power that does not decrease with n, thus conveying O(n) bits covertly and reliably in n uses of an AWGN channel. Here, we consider covert communication of O(n) bits in n channel uses while generalizing the environment and removing any restrictions on Willie's receiver. We assume that an uninformed “jammer” is present to help Alice, and we consider AWGN and block fading channels. In some scenarios, Willie's optimal detector is a threshold test on the received power. When the channel between the jammer and Willie has multiple fading blocks per codeword, a threshold test on the received power is not optimal. However, we establish that Alice can remain covert with a transmit power that does not decrease with n even when Willie employs an optimal detector.

Recently, the fundamental limits of covert, i.e., reliable-yet-undetectable, communication have been established for general memoryless channels and for lossy-noisy bosonic (quantum) channels with a quantum-limited adversary. The key import of these results was the square-root law (SRL) for covert communication, which states that O(√n) covert bits, but no more, can be reliably transmitted over n channel uses with O(√n) bits of secret pre-shared between communicating parties. Here we prove the achievability of the SRL for a general memoryless classical-quantum channel, showing that SRL covert communication is achievable over any quantum communication channel with a product-state transmission strategy. We leave open the converse, which, if proven, would show that even using entangled transmissions and entangling measurements, the SRL for covert communication cannot be surpassed over an arbitrary quantum channel.

The recent square root law (SRL) for covert communication demonstrates that Alice can reliably transmit O(√n) bits to Bob in n uses of an additive white Gaussian noise (AWGN) channel while keeping ineffective any detector employed by the adversary; conversely, exceeding this limit either results in detection by the adversary with high probability or nonzero decoding error probability at Bob. This SRL is under the assumption that the adversary knows when Alice transmits (if she transmits); however, in many operational scenarios, he does not know this. Hence, here, we study the impact of the adversary's ignorance of the time of the communication attempt. We employ a slotted AWGN channel model with T(n) slots each containing n symbol periods, where Alice may use a single slot out of T(n). Provided that Alice's slot selection is secret, the adversary needs to monitor all T(n) slots for possible transmission. We show that this allows Alice to reliably transmit O(min{(n log T(n))1/2, n}) bits to Bob (but no more) while keeping the adversary's detector ineffective. To achieve this gain over SRL, Bob does not have to know the time of transmission provided T(n)cTn, cT= O(1).

The effectiveness and the simple implementation of physical layer jammers make them an essential threat for wireless networks. In a multihop wireless network, where jammers can interfere with the transmission of user messages at intermediate nodes along the path, one can employ jamming oblivious routing and then employ physical-layer techniques (e.g., spread spectrum) to suppress jamming. However, whereas these approaches can provide significant gains, the residual jamming can still severely limit system performance. This motivates the consideration of routing approaches that account for the differences in the jamming environment between different paths. First, we take a straightforward approach where an equal outage probability is allocated to each link along a path and develop a minimum energy routing solution. Next, we demonstrate the shortcomings of this approach and then consider the joint problem of outage allocation and routing by employing an approximation to the link outage probability. This yields an efficient and effective routing algorithm that only requires knowledge of the measured jamming at each node. Numerical results demonstrate that the amount of energy saved by the proposed methods with respect to a standard minimum energy routing algorithm, especially for parameters appropriate for terrestrial wireless networks, is substantial.

The exploitation of slight imperfections of transmitters' hardware for identification of wireless devices has recently emerged as an effective method for security enhancement in wireless access networks. Previously, we introduced a model-based approach for device identification based on the imperfections of two main wireless transmitter components: 1) the digital-to-analog converter and 2) the power amplifier. Here, motivated by applications with transmit power control mechanisms, we analyze the degree to which a device can be identified from the unique, power mode independent characteristics of a third main component: the RF oscillator. The model-based device identification method introduced here allows for effective device identification even from short time records at relatively low signal-to-noise ratios when exploiting imperfections of commercially used RF oscillators.

There is a rich recent literature on information-theoretically secure communication at the physical layer of wireless networks, where secret communication between a single transmitter and receiver has been studied extensively. In this paper, we consider how single-hop physical layer security techniques can be extended to multi-hop wireless networks. We show that guaranteed security can be achieved in multi-hop networks by augmenting physical layer security techniques, such as cooperative jamming, with the higher layer network mechanisms, such as routing. Specifically, we consider the secure minimum energy routing problem, in which the objective is to compute a minimum energy path between two network nodes subject to constraints on the end-to-end communication secrecy and goodput over the path. This problem is formulated as a constrained optimization of transmission power and link selection, which is proved to be NP-hard. Nevertheless, we show that efficient algorithms exist to compute both exact and approximate solutions for the problem. In particular, we develop an exact solution of pseudo-polynomial complexity, as well as an ε-optimal approximation of polynomial complexity. Simulation results are also provided to show the utility of our algorithms and quantify their energy savings compared to a combination of (standard) security-agnostic minimum energy routing and physical layer security. In the simulated scenarios, we observe that, by jointly optimizing link selection at the network layer and cooperative jamming at the physical layer, our algorithms reduce the network energy consumption by half.

Variations in the RF chain of radio transmitters caused by imperfections of manufacturing processes can be used as a signature to uniquely associate wireless devices with a given transmission. In our previous work, we proposed a model-based approach that allows for identification of wireless devices based on signatures obtained with time domain analysis of a pair of received and decoded signals. Here, we consider strong adversaries who intentionally introduce distortions to the data symbols before the symbols are exposed to the transmitter's inherent nonlinearities, with the intention of faking the signatures of their devices while still allowing for proper data decoding. The method proposed in this work is based on spectral analysis and on the observation that nonlinear components cause in-band distortion and spectral regrowth of the signal that is dependent on the parameters of the nonlinearity. Hence, by analysis of the in-band distortion of the spectrum as well as the spectral regrowth, we show that wireless devices can be successfully identified even when the users are digitally modifying their data symbols. The utility of the proposed identification approach is demonstrated with simulations based on parameters obtained from the measurements of commercially employed WLAN RF transmitters.

Secure communication over a wiretap channel is considered in the disadvantaged wireless environment, where the eavesdropper channel is (possibly much) better than the main channel. We present a method to exploit inherent vulnerabilities of the eavesdroppers receiver to obtain everlasting secrecy. Based on an ephemeral cryptographic key pre-shared between the transmitter Alice and the intended recipient Bob, a random jamming signal is added to each symbol. Bob can subtract the jamming signal before recording the signal, while the eavesdropper Eve is forced to perform these non-commutative operations in the opposite order. Thus, information-theoretic secrecy can be obtained, hence achieving the goal of converting the vulnerable “cheap” cryptographic secret key bits into “valuable” information-theoretic (i.e., everlasting) secure bits. We evaluate the achievable secrecy rates for different settings, and show that, even when the eavesdropper has perfect access to the output of the transmitter (albeit through an imperfect analog-to-digital converter), the method can still achieve a positive secrecy rate. Next we consider a wideband system, where Alice and Bob perform frequency hopping in addition to adding the random jamming to the signal, and we show the utility of such an approach even in the face of substantial eavesdropper hardware capabilities.

The exploitation of slight imperfections of transmitters' hardware for identification of wireless devices has recently emerged as an effective method for security enhancement in wireless access networks. Previously, we introduced a model-based approach for device identification based on the imperfections of two main wireless transmitter components: the digital-to-analog converter and the power amplifier. Here, motivated by applications with transmit power control mechanisms, we analyze the degree to which a device can be identified from the unique, power mode independent characteristics of a third main component: the RF oscillator. The model-based device identification method introduced here allows for effective device identification even from short time records at relatively low signal-to-noise ratios when exploiting imperfections of commercially used RF oscillators.

The effectiveness and straightforward implementation of physical layer jammers make them an essential security threat for wireless networks. In this paper, reliable communication in a wireless multi-hop network in the presence of multiple malicious jammers is considered. Since energy consumption is an important issue in wireless ad hoc networks, minimum energy routing with and without security constraints has received significant attention in the literature; however, energy-aware routing in the presence of active adversary (jammers) has not been considered. We propose an efficient algorithm for minimum energy routing between a source and a destination in the presence of both static and dynamic malicious jammers such that an end-to-end probability of outage is guaranteed. The percentage of energy saved by the proposed method with respect to a shortest path routing benchmark is evaluated. It is shown that the amount of energy saved, especially in terrestrial wireless networks with path-loss exponents greater than two, is substantial.

Unlike standard security methods (e.g. encryption), low probability of detection (LPD) communication does not merely protect the information contained in a transmission from unauthorized access, but prevents the detection of a transmission in the first place. In this work we study the impact of secretly pre-arranging the time of communication. We prove that if Alice has AWGN channels to Bob and the warden, and if she and Bob can choose a single n symbol period slot out of T(n) such slots, keeping the selection secret from the warden (and, thus, forcing him to monitor all T(n) slots), then Alice can reliably transmit O(min{√n log T(n),n}) bits to Bob while keeping the warden's detector ineffective. The result indicates that only an additional log T(n) secret bits need to be exchanged between Alice and Bpob prior to communication to produce a multiplicative gain of √log T(n) in the amount of transmitted covert information.

Motivated by the compelling application of interference mitigation at wideband receivers in wireless communication and sensing systems, we consider the recovery of a frequency-sparse signal from samples of small magnitude. The standard ℓ1-norm minimization results in an inadequate signal-dependent recovery performance, and hence we introduce three techniques to improve the quality of recovery. The performance of each of these three techniques is characterized through numerical simulations, from which we conclude that each of the proposed techniques show the promise of substantially improving recovery performance.

We consider the problem of endhost-based shortest path routing in a network with unknown, time-varying link qualities. Endhost-based routing is needed when internal nodes of the network do not have the scope or capability to provide globally optimal paths to given source-destination pairs, as can be the case in networks consisting of autonomous subnetworks or those with endhost-based routing restrictions. Assuming the source can probe links along selected paths, we formulate the problem as an online learning problem, where an existing solution achieves a performance loss (called regret) that is logarithmic in time with respect to (wrt) an offline algorithm that knows the link qualities. Current solutions assume coupled probing and routing; in contrast, we give a simple algorithm based on decoupled probing and routing, whose regret is only constant in time. We then extend our solution to support multi-path probing and cooperative learning between multiple sources, where we show an inversely proportional decay in regret wrt the probing rate. We also show that without the decoupling, the regret grows at least logarithmically in time, thus establishing decoupling as critical for obtaining constant regret. Although our analysis assumes certain conditions (i.i.d.) on link qualities, our solution applies with straightforward amendments to much broader scenarios where these conditions are relaxed. The efficacy of the proposed solution is verified by trace-driven simulations.

We demonstrate the achievability of a square root limit on the amount of information transmitted reliably and with low probability of detection (LPD) over the single-mode lossy bosonic channel if either the eavesdropper's measurements or the channel itself is subject to the slightest amount of excess noise. Specifically, Alice can transmit O(√n) bits to Bob over n channel uses such that Bob's average codeword error probability is upper-bounded by an arbitrarily small δ > 0 while a passive eavesdropper, Warden Willie, who is assumed to be able to collect all the transmitted photons that do not reach Bob, has an average probability of detection error that is lower-bounded by 1/2 - ε for an arbitrarily small ε > 0. We analyze the thermal noise and pure loss channels. The square root law holds for the thermal noise channel even if Willie employs a quantum-optimal measurement, while Bob is equipped with a standard coherent detection receiver. We also show that LPD communication is not possible with coherent state transmission on the pure loss channel. However, this result assumes Willie to possess an ideal receiver that is not subject to excess noise. If Willie is restricted to a practical receiver with a non-zero dark current, the square root law is achievable on the pure loss channel.

Secure communication over a wireless channel in the presence of a passive eavesdropper is considered. We present a method to exploit the eavesdropper's inherent receiver vulnerabilities to obtain everlasting secrecy. An ephemeral cryptographic key is pre-shared between the transmitter and the legitimate receiver and is utilized to induce intentional intersymbol interference (ISI). The legitimate receiver uses the key to cancel the ISI while the eavesdropper, since it does not have the key, cannot do such. It is shown that although ISI reduces the capacity of the main channel, it can lead to a net gain in secrecy rate. The achievable secrecy rates for different ISI filter settings are evaluated and the proposed method is compared with other information-theoretic security schemes.

There is a rich recent literature on how to assist secure communication between a single transmitter and receiver at the physical layer of wireless networks through techniques such as cooperative jamming. In this paper, we consider how these single-hop physical layer security techniques can be extended to multi-hop wireless networks and show how to augment physical layer security techniques with higher layer network mechanisms such as coding and routing. Specifically, we consider the secure minimum energy routing problem, in which the objective is to compute a minimum energy path between two network nodes subject to constraints on the end-to-end communication secrecy and goodput over the path. This problem is formulated as a constrained optimization of transmission power and link selection, which is proved to be NP-hard. Nevertheless, we show that efficient algorithms exist to compute both exact and approximate solutions for the problem. In particular, we develop an exact solution of pseudo-polynomial complexity, as well as an o-optimal approximation of polynomial complexity. Simulation results are also provided to show the utility of our algorithms and quantify their energy savings compared to a combination of (standard) security-agnostic minimum energy routing and physical layer security. In the simulated scenarios, we observe that, by jointly optimizing link selection at the network layer and cooperative jamming at the physical layer, our algorithms reduce the network energy consumption by half.

We present a square root limit on the amount of information transmitted reliably and with low probability of detection (LPD) over additive white Gaussian noise (AWGN) channels. Specifically, if the transmitter has AWGN channels to an intended receiver and a warden, both with non-zero noise power, we prove that o(√n) bits can be sent from the transmitter to the receiver in n channel uses while lower-bounding α + β ≥ 1-ε for any ε > 0, where α and β respectively denote the warden's probabilities of a false alarm when the sender is not transmitting and a missed detection when the sender is transmitting. Moreover, in most practical scenarios, a lower bound on the noise power on the channel between the transmitter and the warden is known and O(√n) bits can be sent in n LPD channel uses. Conversely, attempting to transmit more than O(√n) bits either results in detection by the warden with probability one or a non-zero probability of decoding error at the receiver as n→∞.

Secure communication over a memoryless wiretap channel in the presence of a passive eavesdropper is considered. Traditional information-theoretic security methods require an advantage for the main channel over the eavesdropper channel to achieve a positive secrecy rate, which in general cannot be guaranteed in wireless systems. Here, we exploit the non-linear conversion operation in the eavesdropper's receiver to obtain the desired advantage - even when the eavesdropper has perfect access to the transmitted signal at the input to their receiver. The basic idea is to employ an ephemeral cryptographic key to force the eavesdropper to conduct two operations, at least one of which is non-linear, in a different order than the desired recipient. Since non-linear operations are not necessarily commutative, the desired advantage can be obtained and information-theoretic secrecy achieved even if the eavesdropper is given the cryptographic key immediately upon transmission completion. In essence, the lack of knowledge of the key during the short transmission time inhibits the recording of the signal in such a way that the secret information can never be extracted from it. The achievable secrecy rates for different countermeasures that the eavesdropper might employ are evaluated. It is shown that even in the case of an eavesdropper with uniformly better conditions (channel and receiver quality) than the intended recipient, a positive secrecy rate can be achieved.

In this paper, we investigate how in-network aggregation approach impacts the target tracking quality in multi-hop wireless sensor networks under network delays. Specifically, we use the mean squared error (MSE) of the target location estimate to quantify the target tracking quality, and investigate how in-network aggregation affects the MSE. To obtain insights without being obscured by onerous mathematical details, we assume a Brownian motion mobility model for the target, Gaussian measurement noise for the sensors, and independent per-hop delays. Under the above assumptions, we first propose an aggregation scheme that preserves a sufficient statistic for optimal tracking under data aggregation at the intermediate nodes and arbitrary network delays. We then analytically study the impact of aggregation in three increasingly more complicated scenarios: single task tracking with only transmission delay, single task tracking with both transmission delay and queueing delay at intermediate nodes, and multi-task tracking. Our results demonstrate that in-network aggregation improves tracking quality in all three scenarios. Furthermore, our analysis provides guidelines on how to choose aggregation parameters in practice.

The capability of nodes to broadcast their message to the entire wireless network when nodes employ cooperation is considered. We employ an asymptotic analysis using an extended random network setting under an additive white Gaussian channel model with path loss, and show that the broadcast performance strongly depends on the path loss exponent of the medium. In particular, the probability of broadcasting in a 1-D infinite network is zero for path loss exponents larger than one, and is equal to a nonzero value for path loss exponents less than one. In 2-D infinite networks, the same behavior is observed for path loss exponents above and below two, respectively.

Neighbor discovery is an important first step in the initialization of a wireless ad hoc network. In this paper, we design and analyze several algorithms for neighbor discovery in wireless networks. Starting with a single-hop wireless network ofnnodes, we propose a Θ(nlnn) ALOHA-like neighbor discovery algorithm when nodes cannot detect collisions, and an order-optimal Θ(n) receiver feedback-based algorithm when nodes can detect collisions. Our algorithms neither require nodes to have a priori estimates of the number of neighbors nor synchronization between nodes. Our algorithms allow nodes to begin execution at different time instants and to terminate neighbor discovery upon discovering all their neighbors. We finally show that receiver feedback can be used to achieve a Θ(n) running time, even when nodes cannot detect collisions. We then analyze neighbor discovery in a general multihop setting. We establish an upper bound ofO(Δlnn) on the running time of the ALOHA-like algorithm, where Δ denotes the maximum node degree in the network andnthe total number of nodes. We also establish a lower bound of Ω(Δ+lnn) on the running time of any randomized neighbor discovery algorithm. Our result thus implies that the ALOHA-like algorithm is at most a factor min(Δ,lnn) worse than optimal.

We present achievable scaling results on the per-node secure throughput that can be realized in a large random wireless network of n legitimate nodes in the presence of m eavesdroppers of unknown location. We consider both one-dimensional and two-dimensional networks. In the one-dimensional case, we show that a per-node secure throughput of order 1/n is achievable if the number of eavesdroppers satisfies m = o(n/log n). We obtain similar results for the two-dimensional case, where a secure throughput of order 1/(√n log n) is achievable under the same condition. The number of eavesdroppers that can be tolerated is significantly higher than previous works that address the case of unknown eavesdropper locations. The key technique introduced in our construction to handle unknown eavesdropper locations forces adversaries to intercept a number of packets to be able to decode a single message. The whole network is divided into regions, where a certain subset of packets is protected from adversaries located in each region. In the one-dimensional case, our construction makes use of artificial noise generation by legitimate nodes to degrade the signal quality at the potential locations of eavesdroppers. In the two-dimensional case, the availability of many paths to reach a destination is utilized to handle collaborating eavesdroppers of unknown location.

A wide range of wireless channel models have been developed to model variations in received signal strength. In contrast to prior work, which has focused primarily on channel modeling on a short, per- packet timescale (millisecond), we develop and validate a finite-state Markov chain model that captures variations due to shadowing, which occur at coarser time scales. The Markov chain is constructed by partitioning the entire range of shadowing into a finite number of intervals. We determine the Markov chain transition matrix in two ways: (i) via an abstract modeling approach in which shadowing effects are modeled as a log-normally distributed random variable affecting the received power, and the transition probabilities are derived as functions of the variance and autocorrelation function of shadowing; (ii) via an empirical approach, in which the transition matrix is calculated by directly measuring the changes in signal strengths collected in a 802.16e (WiMAX) network. We validate the abstract model by comparing its steady state and transient performance predictions with those computed using the empirically derived transition matrix and those observed in the actual traces themselves.

Physical layer secrecy in wireless networks in the presence of eavesdroppers of unknown location is considered. In contrast to prior schemes, which have expended energy in the form of cooperative jamming to enable secrecy, we develop schemes where multiple transmitters send their signals in a cooperative fashion to confuse the eavesdroppers. Hence, power is not expended on “artificial noise”; rather, the signal of a given transmitter is protected by the aggregate interference produced by the other transmitters. We introduce a two-hop strategy for the case of equal path-loss between all pairs of nodes, and then consider its embedding within a multi-hop approach for the general case of an extended network. In each case, we derive an achievable number of eavesdroppers that can be present in the region while secure communication between all sources and intended destinations is ensured.

We present a square root limit on low probability of detection (LPD) communication over additive white Gaussian noise (AWGN) channels. Specifically, if a warden has an AWGN channel to the transmitter with non-zero noise power, we prove that o(√n) bits can be sent from the transmitter to the receiver in n AWGN channel uses with probability of detection by the warden less than e for any ϵ >; 0, and, if a lower bound on the noise power on the warden's channel is known, then O(√n) bits can be covertly sent in n channel uses. Conversely, trying to transmit more than O(√n) bits either results in detection by the warden with probability one or a non-zero probability of decoding error as n → ∞. Further, we show that LPD communication on the AWGN channel allows one to send a nonzero symbol on every channel use, in contrast to what might be expected from the square root law found recently in image-based steganography.

Information theoretic security has recently emerged as an effective physical layer approach to provide secure communications. The outage performance of such a secrecy communication system is considered in this paper, since it is an important criterion to measure whether users' predefined quality of service can be met. Provided that the legitimate receiver and eavesdropper have the same noise power, many existing secure schemes cannot achieve outage probability approaching zero, regardless of how large the transmission power is. By introducing cooperative transmission into secrecy communication systems, it will be shown here that outage probability approaching zero can be achieved. In particular, scenarios with single-antenna nodes and multiple-antenna nodes will both be addressed, and the optimal design of beamforming/precoding will be investigated. Explicit expressions of the achievable outage probability and diversity-multiplexing tradeoff will be developed to demonstrate the performance of the proposed cooperative secure transmission schemes, and numerical results are presented.

We introduce a peak mitigation technique for reference based systems that is similar to the tone reservation scheme employed in orthogonal frequency division multiplexing (OFDM) systems but without the cost in data rate. A comparison of reference-based systems under either peak or average power constraints is presented.

Energy efficient secure communication in wireless networks in the presence of eavesdroppers is considered. For a secure transmission to the destination, a set of intermediate "jammer" nodes are chosen to generate artificial noise that confuses the eavesdropper. We consider two jamming strategies: beamforming and cooperative diversity. Previous research has focused largely on cooperative beamforming strategies, but we demonstrate a number of scenarios where a cooperative diversity strategy is desirable. This motivates approaches which selectively switch between the two strategies, from which significant energy savings can often be realized. In our simulations, energy savings of up to 60% are observed in the simulated networks.

We present BlockRate, a wireless bitrate adaptation algorithm designed for blocks, or large contiguous units of transmitted data, as opposed to small packets. Our work is motivated by the observation that recent research results suggest significant overhead amortization benefits of blocks. Yet state-of-the-art bitrate algorithms are optimized for adaptation on a per-packet basis, so they can either have the amortization benefits of blocks or high responsiveness to underlying channel conditions of packets, but not both.

Low power ad hoc wireless networks operate in conditions where channels are subject to fading. Cooperative diversity mitigates fading in these networks by establishing virtual antenna arrays through clustering the nodes. A cluster in a cooperative diversity network is a collection of nodes that cooperatively transmits a single packet. There are two types of clustering schemes: static and dynamic. In static clustering all nodes start and stop transmission simultaneously, and nodes do not join or leave the cluster while the packet is being transmitted. Dynamic clustering allows a node to join an ongoing cooperative transmission of a packet as soon as the packet is received. In this paper we take a broad view of the cooperative network by examining packet flows, while still faithfully implementing the physical layer at the bit level. We evaluate both clustering schemes using simulations on large multi-flow networks. We demonstrate that dynamically-clustered cooperative networks substantially outperform both statically-clustered cooperative networks and classical point-to-point networks.

The secure transmission of information in wireless networks without knowledge of eavesdropper channels or locations is considered. Two key mechanisms are employed: artificial noise generation from system nodes other than the transmitter and receiver, and a form of multi-user diversity that allows message reception in the presence of the artificial noise. We determine the maximum number of independently-operating and uniformly distributed eavesdroppers that can be present while the desired secrecy is achieved with high probability in the limit of a large number of system nodes. While our main motivation is considering eavesdroppers of unknown location, we first consider the case where the path-loss is identical between all pairs of nodes. In this case, a number of eavesdroppers that is exponential in the number of systems nodes can be tolerated. In the case of uniformly distributed eavesdroppers of unknown location, any number of eavesdroppers whose growth is sub-linear in the number of system nodes can be tolerated. The proposed approach significantly outperforms a power control approach based on standard multi-user diversity.

Variations in the RF chain of radio transmitters can be used as a signature to uniquely associate wireless devices with a given transmission. Previous approaches, which have varied from transient analysis to machine learning, do not provide verifiable accuracy, which is essential for admissibility of the methods in the court. Here we detail a first step toward a model-based approach, which uses statistical models of RF transmitter components that are amenable for analysis. Algorithms based on statistical signal processing methods are developed to exploit non-linearities of wireless transmitters for the purpose of user identification in wireless systems. The decision rules are derived and their performance is analyzed. In order to establish the viability of the proposed approach, the practical variations of transmitter chain components are analyzed based on simulations, measurements and manufacturers' specifications. Results show that the proposed identification methods can be effective, even for short data records and relatively low signal-to-noise ratios, when exploiting imperfections of commercially used RF transmitters.

This paper considers the problem of finding minimum-energy cooperative routes in a wireless network with variable wireless channels. We assume that each node in the network is equipped with a single omnidirectional antenna and, motivated by the large body of physical layer research indicating its potential utility, that multiple nodes are able to coordinate their transmissions at the physical layer in order to take advantage of spatial diversity. Such coordination, however, is intrinsically intertwined with routing decisions, thus motivating the work. We first formulate the energy cost of forming a cooperative link between two nodes based on a two-stage transmission strategy assuming that only statistical knowledge about channels is available. Utilizing the link cost formulation, we show that optimal static routes in a network can be computed by running Dijkstra's algorithm over an extended network graph created by cooperative links. However, due to the variability of wireless channels, we argue that a many-to-one cooperation model in static routing is suboptimal. Hence, we develop an opportunistic routing algorithm based on many-to-many cooperation, and show that optimal routes in a network can be computed by a stochastic version of the Bellman-Ford algorithm. We use static and opportunistic optimal algorithms as baselines to develop heuristic link selection algorithms that are energy efficient while being computationally simpler than the optimal algorithms. We simulate our algorithms and show that while optimal cooperation and link selection can reduce energy consumption by almost an order of magnitude compared to non-cooperative approaches, our simple heuristics achieve similar energy savings while being computationally efficient as well.

In this letter, we study the opportunistic use of relays for secret communications, and propose two transmission schemes that do not require the knowledge of the eavesdropper's channel state information. Both analytic and numerical results are provided.

We consider a scenario where multiple pairs of users exchange information within pair, with the help of a dedicated multi-antenna relay. The protocol integrates the idea of analogue network coding in mixing two data streams originating from the same user pair, together with the spatial multiplexing of the data streams originating from different user pairs. The key feature of the protocol is that it enables both the relay and the users to participate in interference cancellation. We propose several beamforming schemes for the multi-antenna relay and evaluate the performance using information theoretical metrics such as ergodic capacity, outage probability and diversity and multiplexing tradeoff. Analytical and simulation results justify that the ergodic capacity, outage probability and diversity and multiplexing tradeoff of the proposed beamforming schemes outperform comparable schemes.

In this paper, we study the application of cooperative diversity to wireless broadcast channels, a fundamental building block of wireless communication networks. Several cooperative broadcast protocols will be proposed, and information theoretic metrics are developed to facilitate performance evaluation. Provided that there is no direct S-D link, the proposed protocols can achieve a multiplexing gain close to one, whereas the traditional two-hop scheme can only achieve the diversity gain 1/2. Provided that there are direct S-D links, the proposed protocol can still outperform the comparable scheme, particularly at high multiplexing gains.

Neighbor discovery is essential for the process of self-organization of a wireless network, where almost all routing and medium access protocols need knowledge of one-hop neighbors. In this paper we study the problem of neighbor discovery in a static and synchronous network, where time is divided into slots, each of duration equal to the time required to transmit a hello message, and potentially, some sort of feedback message. Our main contributions lie in detailing the physical layer mechanism for how nodes in receive mode detect the channel status, describing algorithms at higher layers that exploit such a knowledge, and characterizing the significant gain obtained. In particular, we describe one possible physical layer architecture that allows receivers to detect collisions, and then introduce a feedback mechanism that makes the collision information available to the transmitters. This allows nodes to stop transmitting packets as soon as they learn about the successful reception of their discovery messages by the other nodes in the network. Hence, the number of nodes that need to transmit packets decreases over time. These nodes transmit with a probability that is inversely proportional to the number of active nodes in their neighborhood, which is estimated using the collision information available at the nodes. We show through analysis and simulations that our algorithm allows nodes to discover their neighbors in a significantly smaller amount of time compared to the case where reception status feedback is not available to the transmitters.

We investigate the scalability of a class of algorithms that exploit the dynamics of wireless fading channels to achieve secret communication in a large wireless network of n randomly located nodes. We describe a construction in which nodes transmit artificial noise to suppress eavesdroppers whose locations are unknown and ensure secrecy of messages transported across the network. Under a model in which eavesdroppers operate independently and under appropriate conditions on the achievable per-node throughput Ψ(n), we show that the network can tolerate Ω((1⁄√n(n))2c) eavesdroppers while ensuring that the aggregate rate at which eavesdroppers intercept packets goes to 0, where c is a constant such that 0 0. We also establish sufficient conditions on the number of eavesdroppers to achieve a non-zero throughput in our construction.

In this paper, we propose a spectrally efficient cooperative transmission protocol for multiple access scenarios. The key feature is to utilize multi-user diversity and fully exploit the dynamic nature of radio propagation. In particular, by carefully scheduling the multiple sources and relays' transmissions, a source with a poor connection to the destination can have higher priority to obtain help from a relay with better channel condition. As a result, the full diversity gain is achievable even though only a fraction of relays is scheduled to help each user. We developed an achievable diversity-multiplexing tradeoff for the proposed transmission protocol to assist performance evaluation. When the number of relays is large, the diversity-multiplexing tradeoff achieved by the proposed scheme can approximate the optimal multiple-input single-output upper bound. Both analytical and numerical results show that the proposed protocol outperform other comparable schemes in most conditions.

Orthogonal frequency division multiplexing (OFDM) systems have been used extensively in wireless communications in recent years; thus, there is significant interest in analyzing the properties of the transmitted signal in such systems. In particular, a large amount of work has focused on analyzing the variation of the complex envelope of the transmitted signal and on designing methods to minimize this variation. In this paper, it is established that the complex envelope of a bandlimited uncoded OFDM signal converges weakly to a Gaussian random process as the number of subcarriers goes to infinity. This shows that the properties of the OFDM signal will asymptotically approach those of a Gaussian random process over any finite time interval. The convergence proof is then extended to two important cases, namely, coded OFDM systems and systems with an unequal power allocation across subcarriers.

In this paper, cooperative transmission protocols are proposed for wireless broadcast channels, a fundamental building block of wireless communication networks. The concepts of cognitive radio and precoding have been introduced to broadcast channels in order to improve system performance. Information theoretic metrics, such as outage probability and diversity-multiplexing tradeoff, are developed to facilitate performance evaluation. In the absence of direct S-D links, the proposed protocols can achieve a multiplexing gain close to one, whereas the traditional two-hop scheme only achieves a diversity gain of 1/2. In the presence of direct S-D links, the proposed protocol can still outperform the comparable scheme, particularly at high multiplexing gains. Regarding to the channel state information (CSI) assumptions, in the absence of direct S-D links, the source does not need to know CSI, but it is assumed that the relays have access to their own incoming and outgoing channel information. In the presence of direct S-D links, the use of precoding requires an extra assumption that the global CSI is available at the source.

In this paper, we propose a novel spectrally efficient cooperative transmission protocol for multiple access scenarios. Different to some existing cooperative multiple access schemes, the proposed scheme can exploit the availability of relays as an extra dimension to increase reception robustness. By carefully scheduling the multiple sources and relays' transmission, a source with a poor connection to the destination can have higher priority to obtain better help from relays. As a result, the full diversity gain can be achievable for each user although only a fraction of the all relays is scheduled to help him. An achievable diversity-multiplexing tradeoff (DMT) is developed for the proposed transmission protocol to assist performance evaluation. With a large number of relays, the DMT achieved by the proposed scheme can approximate the optimal multiple-input single-input upper bound. Both analytical and numerical results show that the proposed protocol can outperform comparative schemes in most conditions.

Neighbor discovery is one of the first steps in the initialization of a wireless ad hoc network. In this paper, we design and analyze practical algorithms for neighbor discovery in wireless networks. We first consider an ALOHA-like neighbor discovery algorithm in a synchronous system, proposed in an earlier work. When nodes do not have a collision detection mechanism, we show that this algorithm reduces to the classical {\em Coupon Collector's Problem}. Consequently, we show that each node discovers all its $n$ neighbors in an expected time equal to $ne (\ln n + c)$, for some constant $c$. When nodes have a collision detection mechanism, we propose an algorithm based on receiver status feedback which yields a $\ln n$ improvement over the ALOHA-like algorithm. Our algorithms do not require nodes to have any estimate of the number of neighbors. In particular, we show that not knowing $n$ results in no more than a factor of two slowdown in the algorithm performance. In the absence of node synchronization, we develop asynchronous neighbor discovery algorithms that are only a factor of two slower than their synchronous counterparts. We show that our algorithms can achieve neighbor discovery despite allowing nodes to begin execution at different time instants. Furthermore, our algorithms allow each node to detect when to terminate the neighbor discovery phase.

In this paper, we employ a stochastic geometry model to analyze transmission capacity in wireless cooperative networks. Assuming that simultaneous transmitters are randomly located in space according to Poisson point process with density ¿, we develop the bound performances on outage probability and outage capacity for both direct transmission and Decode-and-Forward (DAF) cooperative scheme. Due to the nature of multipath propagation of cooperative transmission, we define regional capacity as the multiplied product of average density of successful simultaneous transmissions, achieved outage capacity and transmission distance. It shows that the regional capacity for cooperative transmission scales as ¿(¿(¿)), which is the same as the transport capacity for wireless network. Furthermore, Monte Carlo simulations demonstrate the significant improvement on the transmission capacity by using cooperative transmission.

Gupta and Kumar established that the per node throughput of ad hoc networks with multi-pair unicast traffic scales with an increasing number of nodes n as lambda(n) = ominus(1/radic(n log n)), thus indicating that performance does not scale well. However, Gupta and Kumar did not consider network coding and wireless broadcasting, which recent works suggest have the potential to significantly improve throughput. Here, we establish bounds on the improvement provided by such techniques. For random networks of any dimension under either the protocol or physical model that were introduced by Gupta and Kumar, we show that network coding and broadcasting lead to at most a constant factor improvement in per node throughput. For the protocol model, we provide bounds on this factor. We also establish bounds on the throughput benefit of network coding and broadcasting for multiple source multicast in random networks. Finally, for an arbitrary network deployment, we show that the coding benefit ratio is at most O(log n) for both the protocol and physical communication models. These results give guidance on the application space of network coding, and, more generally, indicate the difficulty in improving the scaling behavior of wireless networks without modification of the physical layer.

Extensive research has demonstrated the potential improvement in physical layer performance when multiple radios transmit concurrently in the same radio channel. We consider how such cooperation affects the requirements for full connectivity and percolation in large wireless ad hoc networks. Both noncoherent and coherent cooperative transmission are considered. For one-dimensional (1-D) extended networks, in contrast to noncooperative networks, for any path loss exponent less than or equal to one, full connectivity occurs under the noncoherent cooperation model with probability one for any node density. Conversely, there is no full connectivity with probability one when the path loss exponent exceeds one, and the network does not percolate for any node density if the path loss exponent exceeds two. In two-dimensional (2-D) extended networks with noncoherent cooperation, for any path loss exponent less than or equal to two, full connectivity is achieved for any node density. Conversely, there is no full connectivity when the path loss exponent exceeds two, but the cooperative network percolates for node densities above a threshold which is strictly less than that of the noncooperative network. A less conclusive set of results is presented for the coherent case. Hence, even relatively simple noncoherent cooperation improves the connectivity of large ad hoc networks.

Recently proposed physical-layer network coding (PNC) has demonstrated the promise to significantly improve the throughput of wireless networks whose links can be modeled as additive white Gaussian noise (AWGN) channels. However, the extension to multipath channels is problematic, since the technique would then require both amplitude and phase compensation at each transmitter. Phase compensation requires accurate distributed phase tracking, whereas the required amplitude compensation is even more troubling, as it leads to an inefficient system that yields no diversity even in the presence of perfect channel estimates. Here, a system that avoids these limitations is obtained by reaching up one level higher in the network hierarchy and performing distributed relay selection with cognizance of the PNC technique that we will employ at the physical layer. Since the resulting scheme will achieve a form of selection diversity, we term it ldquonetwork coding with diversityrdquo (NCD). To facilitate performance evaluation, two information-theoretic metrics, the outage and ergodic capacity, are studied. Our analytical and simulation results show that the proposed protocol achieves more robust performance and higher system throughput than comparable schemes. Finally, the proposed network coding is extended to the context of cooperative multiple access channels, which yields a new cooperative protocol with larger outage and ergodic capacity compared with existing transmission schemes.

The dependence of the communication capacity of multiple-input multiple-output (MIMO) wireless systems on the average received signal-to-noise ratio (SNR), assuming the channel is unknown at the transmitter and perfectly known at the receiver, is studied through full wave electromagnetic tools. Although it is commonly accepted that the capacity of a MIMO system increases linearly at high SNRs when plotted versus the SNR expressed in dB, the fact that the number of effective degrees of freedom (DOF) of the system increases with SNR in many practical environments calls this conclusion into question for reasonably high SNRs. Based on a full wave electromagnetic investigation, we are able to analytically predict and then confirm a significant region on the MIMO capacity curve where the capacity grows quadratically when plotted versus the SNR in dB. This gives analytical insight into a portion of the capacity curve that may previously be (incorrectly) attributed to the concavity of the logarithm function rather than the increase in electromagnetic degrees of freedom. The quadratic, rather than linear, growth of capacity suggests that it may be worthwhile to invest more transmit power to achieve higher performance gains. However, to fully take advantage of this second order benefit, the numbers of antennas at the transmitter and the receiver must be close to or slightly larger than the wavevector-aperture-product (WAP) of the corresponding EM system.

Connectivity and capacity are two measures for the performance of mobile ad hoc networks that have been studied extensively under standard point-to-point physical layer assumptions. However, extensive recent research at the physical layer has demonstrated the improvement in performance possible when multiple radios concurrently transmit in the same radio channel. In this paper, we consider how such physical layer cooperation improves the connectivity in wireless ad hoc networks. In particular, with noncoherent cooperation at the physical layer, we consider conditions on the node density λ (or, equivalently, the transmit power) for full connectivity and percolation for large networks in various dimensions and with various path loss exponents α. For one-dimensional (1-D) extended networks, in sharp contrast to noncooperative networks, we demonstrate that full connectivity can be realized under certain conditions. In particular, for any node density with path loss exponent α < 1, or for node density λ > 2 when α = 1, full connectivity occurs with probability one. Conversely, we demonstrate that, under noncoherent cooperation, there is no full connectivity with probability one when α < 1. In two-dimensional (2-D) extended networks with noncoherent cooperation, for any node density with α < 2, or for node density λ Ū 5 when α = 2, full connectivity is achieved. Conversely, there is no full connectivity with probability one when α > 2, but we prove that, for α ≥ 4, the percolation threshold of the noncoherent cooperative network is strictly less than that of the noncooperative network. Analogous results are presented for dense networks. Hence, the main conclusion is that even relatively simple physical layer cooperation in the form of noncoherent power summing can substantially improve the connectivity of large ad hoc networks.

Ultrawideband communications often occur in heterogeneous networks where different receivers have different complexity and energy consumption requirements. In this case it is desirable to have a modulation scheme that works well with coherent receivers as well as simpler receivers, namely transmitted-reference (TR) receivers. In particular, we consider a TR scheme that employs slightly frequency-shifted reference (FSR) signals [15] and thus avoids one of the main drawbacks of conventional TR schemes, namely the need to implement a delay line. We propose and analyze a modulation scheme that works well with both FSR receivers (where it has the same performance as conventional TR modulation), and coherent receivers. Coherent receivers receiving conventional TR modulation suffer a 3 dB penalty, because they cannot make use of the energy invested into the reference pulse. Our proposed scheme avoids this drawback by including a data preprocessor that can be viewed as a nonsystematic rate-1/2 convolutional code with a high constraint length. These codes give 1.5 dB gain over our previously proposed constraint-length-two systematic codes at a BER of 1 times 10-4in 802.15.3a CM4 multipath fading channels.

The design of ultra-wideband (UWB) communication systems has proven to be challenging. Coherent systems require a rake receiver with many taps that is difficult to train, and transmitted-reference (TR) schemes require a wideband analog delay line whose implementation has proven extremely difficult. In response to this challenge, we have recently proposed the frequency-shifted reference (FSR-UWB) system, which leads to the replacement of the delay line in the receiver of the standard TR-UWB system with a mixer, and the resulting system is easily implemented. In this paper, the FSR-UWB idea is extended to provide multiple-access, and a performance analysis is presented. This analysis and the corresponding numerical results reveal that, due to the large bandwidth expansion of the UWB system, the FSR-UWB single-user receiver can still be successfully employed in lightly loaded systems; in other words, successful multiple- access can be achieved without the need for the wideband analog delay line of TR-UWB or the despreading circuitry generally associated with conventional multiple-access spread-spectrum systems. The result is an extremely simple architecture for wideband multiple-access communications in the wireless environment.

Gupta and Kumar established that the per node throughput of ad hoc networks with multi-pair unicast traffic scales (poorly) as lambda(n) = Theta (1 / radic(n log n)) with an increasing number of nodes n. However, Gupta and Kumar did not consider the possibility of network coding and broadcasting in their model, and recent work has suggested that such techniques have the potential to greatly improve network throughput. In [1], we have shown that for the protocol communication model of Gupta and Kumar [2], the multi-unicast throughput of schemes using arbitrary network coding and broadcasting in a two-dimensional random topology also scales as lambda(n) = Theta (1 / radic(n log n))1, thus showing that network coding provides no order difference improvement on throughput. Of course, in practice the constant factor of improvement is important; thus, here we derive bounds for the throughput benefit ratio -the ratio of the throughput of the optimal network coding scheme to the throughput of the optimal non-coding flow scheme. We show that the improvement factor is 1+ Delta / 1+Delta /2for 1D random networks, where Delta > 0 is a parameter of the wireless medium that characterizes the intensity of the interference. We obtain this by giving tight bounds (both upper and lower) on the throughput of the coding and flow schemes. For 2D networks, we obtain an upper bound for the throughput benefit ratio as alpha (n) les 2cDeltaradic(pi = 1+Delta/Delta) for large n, wnere cDelta= max {2, radic(Delta2+ 2Delta)}. This is obtained by finding an upper bound for the coding throughput and a lower bound for the flow throughput. We then consider the more general physical communication model as in Gupta and Kumar. We show that the coding scheme throughput in this case is upper bounded by Theta (1/n) for the 1D random network and by Theta(1/radic(n)) for the 2D case. We also show the flow scheme throughput for the ID case can achieve the same order throughput as the coding scheme. Combined with previous work on a 2D lower bound [3], we conclude that the throughput benefit ratio under the physical model is also bounded by a constant; thus, we have shown for both the protocol and physical model that the coding benefit in terms of throughput is a constant factor. Finally, we evaluate the potential coding gain from another important perspective - total energy efficiency - and show that the factor by which the total energy is decreased is upper bounded by 3.

Wireless links form a critical component of communication systems that aim to provide ubiquitous access to information. However, the time-varying characteristics (or “state”) of wireless channels caused by the mobility of transmitters, receivers, and objects in the environment make it difficult to achieve reliable communication. Adaptive signaling exploits any channel state information (CSI) available at the transmitter to provide the potential to significantly increase the throughput of wireless links and/or greatly reduce the receiver complexity. As such, adaptive signaling has attracted significant research interest in the last decade and has found application in numerous commercial wireless systems, ranging from cellular data systems to wireless local area networks (WLANs). However, one of the great challenges of wireless communications is that it is difficult to obtain perfect CSI due to the inherently noisy and outdated nature of CSI available at the transmitter. Over the last decade, we have championed the idea of choosing the appropriate transmitted signal based on statistical models for the current channel state conditioned on the channel measurements. In this semi-tutorial paper, we first review how this class of methods has been developed for single-antenna systems, and then present novel recent designs for multiple-antenna systems. Key to the development in each case is the development of the error characterization given the outdated estimates and the use of such to allocate data rate and power over time and possibly space. In general, the focus is on rate allocation, while power allocation is done through a pruning method. Numerical results will demonstrate in both the single-antenna and multiple-antenna cases that such an approach provides a robust method for improving system data rate versus the standard practice of employing link margin to compensate for such uncertainties.

A promising ultra-wideband (UWB) radio technique being widely considered for low-data-rate applications, such as those often encountered in sensor networks, is the transmitted reference (TR) UWB scheme. However, the standard TR-UWB scheme, while often motivated by the simplicity of its receiver, is still dogged by implementation concerns. In particular, the receiver requires an extremely wideband delay element, which is difficult to incorporate into low-power integrated systems. In this paper, a TR scheme is proposed in which the separation between the data and reference signals, rather than being a time delay, is a slow rotation over the symbol interval. This provides a (slightly) frequency-shifted reference that, while orthogonal to the data-bearing pulse, still goes through a nearly equal channel. A detailed analysis of the proposed scheme is provided. Simulation results demonstrate the expected result that frequency shifting of the reference in the proposed manner is not effective for high-data-rate systems that experience appreciable intersymbol interference. However, for the targeted low-to-moderate data-rate applications, numerical results demonstrate that the proposed system not only achieves the primary goal of providing a much simpler receiver architecture, but also that it outperforms the standard TR-UWB system

The development of Reed-Solomon (RS) codes has allowed for improved data transmission over a variety of communication media. Although Reed-Solomon decoding provides a powerful defense against burst data errors, the significant circuit area and power consumption of customized RS decoder hardware can be limiting for embedded computing environments. To support enhanced performance decoding with minimal power consumption, a dynamically-reconfigurable FPGA-based Reed-Solomon decoder has been developed. Our errors-and-erasures decoding system uses multiple erasure blocks to identify the location of likely corrupted data and multiple decoders to attempt error correction. The RS decoder design is implemented in reconfigurable hardware to leverage architectural parallelism and specialization. Run-time dynamic reconfiguration of the decoding system is used in response to variations in channel conditions to support the fastest possible data rate while, as a secondary metric, minimizing decoder power consumption. Algorithm parameters for the decoding system have been determined via simulation and the design has been implemented in Altera Stratix FPGAs. Through experimentation using an Altera 1S40 Stratix FPGA, we show that dynamic reconfiguration can result in an 14% performance improvement versus a non-reconfigurable decoder implementation. Comparisons with a Pentium IV microprocessor illustrate five orders of magnitude performance improvement.

Multipath fading can be effectively combatted in wireless networks of single-antenna nodes by employing cooperative transmission techniques. In a wide variety of these cooperative techniques, multiple nodes encode a common data stream and then simultaneously put a signal onto the physical channel to exploit spatial diversity in the channel. For such a cooperative system, often it is difficult or undesirable from a complexity perspective to accurately lock the multiple transmitting nodes to a common oscillator, and thus the problem arises of multiple mismatches between the local oscillators of the transmit nodes and that of the receiver. Previously, such a situation has only been addressed through standard channel estimation; however, such techniques are not effective except for very small oscillator offsets. Here, a novel minimum mean square error (MMSE) decision feedback equalizer (DFE) that explicitly accounts for the frequency offsets is proposed for the receiver, and its performance is compared to standard approaches.

In this paper, we introduce a novel differential signaling approach for ultra-wideband (UWB) communications using multiple digital carriers. Unlike the transmitted reference (TR), differential and noncoherent UWB that also bypass explicit channel estimation, our scheme avoids the analog delay element whose on-chip implementation is challenging. Compared with the frequency-shifted reference (FSR) UWB, our multi-carrier differential signaling captures the signal energy more effectively and can achieve the full diversity gain, even in the presence of inter-frame interference. In addition, our approach relies on digital carriers that do not incur any spectrum expansion and can be realized with standard discrete-cosine transform (DCT) or fast Fourier transform (FFT) circuits operating at the frame-rate. Simulations are also carried out to corroborate our theoretical analysis.

Channel state information(CSI) at the transmitter, even imperfect, can improve the capacity of a multiple-input multiple-output(MIMO) communication system and substantially reduce the receiver complexity. Eigenbeamforming has been proven to be the preceding scheme that not only achieves the channel capacity, but also minimizes the mean square error (MSE) at the output of the receiver in this case. This paper presents a novel adaptive modulation scheme that loads power and integer bits over the eigenmodes in a MIMO communication system according to outdated CSI at the transmitter. An un- coded narrowband system is considered in a time-varying and spatially uncorrelated fading channel. Simulations indicate that the proposed scheme provides a much higher spectral efficiency than the standard strategy which simply adds energy margin to achieve the target error performance.

In the standard approach to studying connectivity, a physical layer is assumed that allows direct transmission between neighbors within some fixed distance. The graph resulting from connecting all such pairs of neighbors reveals clusters of nodes within which communication is possible. However, future wireless networks will provide a physical layer where nodes that are connected can collaboratively search for more connections via simultaneous RF transmission and reception, thus adding connections that are not possible in the traditional non-collaborative model. The purpose of this paper is to introduce this collaborative network model and to characterize its asymptotic connectivity properties for one characterization (noncoherent power summing) of the physical layer collaboration. In the case of sparse ad hoc networks, simulations show that an infinite cluster will emerge in the infinite two-dimensional plane at a node density roughly 20% of that required in non-collaborative ad hoc networks. In the case of dense ad hoc networks, the probability for the event that the network is connected goes to one asymptotically if the transmission area of each node is [4pi(4 log N)alpha/alpha+2(log log N+log 2)2/alpha+2]/N no less than , where N is the number of nodes in the network of unit area and a is the pathless exponent. Hence, significant gains in the asymptotic connectivity properties of the ad hoc network are obtained through collaboration.

2517-2527

Cooperative diversity, which employs multiple nodes for the simultaneous relaying of a given packet in wireless ad hoc networks, has been shown to be an effective means of improving diversity, and, hence, mitigating the detrimental effects of multipath fading. However, in previously proposed cooperative diversity schemes, it has been assumed that coordination among the relays allows for accurate symbol-level timing synchronization at the destination and orthogonal channel allocation, which can be quite costly in terms of signaling overhead in mobile ad hoc networks, which are often defined by their lack of a fixed infrastructure and the difficulty of centralized control. In this paper, cooperative diversity schemes are considered that do not require symbol-level timing synchronization or orthogonal channelization between the relays employed. In the process, a novel minimum mean-squared error (MMSE) receiver is designed for combining disparate inputs in the multiple-relay channel. Outage probability calculations and simulation results demonstrate the not unexpected significant performance gains of the proposed schemes over single-hop transmission, and, more importantly, demonstrate performance comparable to schemes requiring accurate symbol-level synchronization and orthogonal channelization.

Error-correcting convolutional codes provide a proven mechanism to limit the effects of noise in digital data transmission. Although hardware implementations of decoding algorithms, such as the Viterbi algorithm, have shown good noise tolerance for error-correcting codes, these implementations require an exponential increase in very large scale integration area and power consumption to achieve increased decoding accuracy. To achieve reduced decoder power consumption, we have examined and implemented decoders based on the reduced-complexity adaptive Viterbi algorithm (AVA). Run-time dynamic reconfiguration is performed in response to varying communication channel-noise conditions to match minimized power consumption to required error-correction capabilities. Experimental calculations indicate that the use of dynamic reconfiguration leads to a 69% reduction in decoder power consumption over a nonreconfigurable field-programmable gate array implementation with no loss of decode accuracy.

Previous authors have shown that the asymptotic capacity of a multiple-element-antenna (MEA) system with N transmit and N receive antennas [termed an (N,N) MEA] grows linearly with N if, for all l, the correlation of the fading for two antenna elements whose indices differ by l remains fixed as antennas are added to the array. However, in practice, the total size of the array is often fixed, and thus the correlation of the fading for two elements separated in index by some value l will change as the number of antenna elements is increased. In this paper, under the condition that the size of an array of antennas is fixed, and assuming that the transmitter does not have access to the channel state information (CSI) while the receiver has perfect CSI, the asymptotic properties of the instantaneous mutual information I/sub N,N/ of an (N,N) MEA wireless system employing uniform linear arrays in a quasi-static fading channel are derived analytically and tested for accuracy for finite N through simulations. For many channel correlation structures, it is demonstrated that the asymptotic performance converges almost surely, implying that such MEA systems have a certain strong robustness to the instantiation of the channel fading values.

The development of turbo codes has allowed for near-Shannon limit information transfer in modern communication systems. Although turbo decoding is viewed as superior to alternate decoding techniques, the circuit complexity and power consumption of turbo decoder implementations can often be prohibitive for power-constrained systems. To address these issues, we have developed a reduced-complexity turbo decoder specifically optimized for contemporary FPGA devices. Our key power-saving technique is the use of decoder run-time dynamic reconfiguration in response to variations in the channel conditions. If less favorable channel conditions are detected, a more powerful, less power-efficient decoder is swapped into the FPGA hardware to maintain a fixed bit error rate. More favorable channel conditions result in the opposite effect. Through experimentation on a stratix-based NIOS development board, we show that dynamic reconfiguration can result in a 52% power reduction versus a static decoder implementation. Comparisons with contemporary microprocessors illustrate a 100/spl times/ performance improvement.

Recently, there has been significant interest in employing space-time codes in a distributed fashion, where a codeword is spread across antennas at large geographical separations. In such situations, the differences in path delays between disparate transmit antennas and the receiver often requires the consideration of space-time codes that are robust to such offsets. In this paper, this problem is motivated from the application of public safety radio. The optimal receiver is derived, and a number of codes are obtained. Finally, a matched filter bound analysis and numerical results are presented to demonstrate the performance gains of the derived codes relative to other approaches.

Adaptive-modulation schemes are designed that yield the minimum outage probability for wireless systems with strict delay constraints, under the assumption of perfect causal channel state information at the transmitter and receiver. Numerical results indicate that the proposed schemes significantly outperform adaptive schemes designed to maximize the average rate.

There has been significant interest in the capacity of multiple element antenna (MEA) wireless systems. Previous authors have shown that the asymptotic capacity of a system with N transmit and N receive antennas (termed an (N,N) MEA) grows linearly with N if, for all l, the correlation of the fading for two antennas whose indices differ by l remains fixed as antennas are added to the array. However, in practice, the total size of the array is often fixed, and thus the correlation of the fading for two elements separated in index by some value l changes as the number of antenna elements is increased. In this paper, under the condition that the length of a linear array of antennas is fixed, the asymptotic properties of the instantaneous mutual information I/sub N,N/ of an (N,N) MEA wireless system are derived analytically and tested for accuracy for finite N through simulations. Two different cases are considered: (1) when the fixed array size constraint is imposed at the mobile unit, and (2) when the fixed array size constraint is imposed at both the base station and the mobile unit. For the first case, simulation results indicate that the analytical approximations are very accurate for moderate values of N, especially at high signal-to-noise-ratios (SNR). For the second case, the predicted non-convergence of I/sub N,N/ is observed in simulations, as well.

The use of error-correcting codes has proven to be an effective way to overcome data corruption in digital communication channels. Although widely-used, the most popular communications decoding algorithm, the Viterbi algorithm, requires an exponential increase in hardware complexity to achieve greater decode accuracy. In this paper, we describe the analysis and implementation of a reduced-complexity decode approach, the adaptive Viterbi algorithm (AVA). Our AVA design is implemented in reconfigurable hardware to take full advantage of algorithm parallelism and specialization. Run-time dynamic reconfiguration is used in response to changing channel noise conditions to achieve improved decoder performance. Implementation parameters for the decoder have been determined through simulation and the decoder has been implemented on a Xilinx XC4036-based PCI board. An overall decode performance improvement of 7.5X for AVA has been achieved versus algorithm implementation on a Celeron-processor based system. The use of dynamic reconfiguration leads to a 20% performance improvement over a static implementation with no loss of decode accuracy.

Wireless links are characterized by high bit error rates and intermittent connectivity. This can result in significant degradation in the performance (goodput) of TCP over wireless networks since non-congestion related packet losses can be misinterpreted by TCP as indications of network congestion, resulting in unnecessary congestion control. In this paper, we propose a technique, TCP with adaptive forward error correction (TCP-AFEC), to improve TCP performance over wireless networks. TCP-AFEC combines the well-established performance characterization of TCP with an understanding of the link layer error control scheme to dynamically select the forward error correction (FEC) that maximizes TCP goodput according to the current channel condition. The benefit of coupling a characterization of TCP performance with link layer FEC to improve TCP goodput is demonstrated by comparing the performance of TCP-AFEC against those of TCP-SACK and Snoop. Simulation results show that TCP-AFEC outperforms TCP-SACK and Snoop for a wide range of wireless channel conditions.

Orthogonal frequency division multiplexing (OFDM) is a promising framework for future wireless communication systems. One of the main impediments that has limited the applicability of OFDM systems in low-power wireless communication systems is the highly variable amplitude of the baseband transmitted signal; thus, a number of previous analyses have characterized this variation. These analyses have generally employed the following two components: (1) the assumption that the complex envelope of the OFDM signal converges to a Gaussian random process in some sense as the number of subcarriers becomes large, and (2) Rice's (1945) classical results on level-crossing rates for the envelope of Gaussian random processes. In this work, we improve on both of these components to arrive at a simple, accurate, and rigorously-established expression for the peak distribution of the OFDM envelope. In particular, using a rigorous (and non-trivial) proof establishing the convergence in (1) above as justification, the modern extreme value theory for chi-squared processes is applied to the problem. Numerical results for both uncoded and coded systems establish that the simple expression obtained for the distribution of the peaks of the envelope process is extremely accurate, even for a modest number of subcarriers.

This article presents a blind predictive decision-feedback equalization (DFE) scheme motivated by the work of Labat et al. (see idid., vol.46, p.921-30, July 1998). The proposed scheme outperforms another previously proposed blind predictive DFE scheme, and also eliminates the filter interchange required in the scheme of Labat et al.

An orthogonal frequency division multiplexing (OFDM) system operating over a wireless communication channel effectively forms a number of parallel frequency-nonselective fading channels, thereby obviating the need for complex equalization and thus greatly simplifying equalization/decoding. However, the OFDM system also exhibits two weaknesses relative to its single-carrier counterparts: (1) the diversity achieved by the OFDM system can be less than a single-carrier system employing the same error control code in a signaling environment rich in diversity and (2) the baseband transmitted signal can exhibit significant amplitude fluctuation over time, thereby precluding efficient transmit amplifier operation. In this paper, nonstandard multidimensional signal sets matched to the OFDM framework are prescribed that address both of these issues. The proposed signal sets are chosen to maximize the diversity achieved by an uncoded system under a constraint to control the peak-to-mean envelope power ratio (PMEPR) of the baseband transmitted waveform. The cost of employing the proposed signal sets is an increase in decoding complexity, as essentially a small amount of controlled equalization has been added to the receiver; thus, the resulting system can be viewed as a hybrid between an OFDM system and a standard single-carrier system. Numerical results are presented which suggest that: (1) the system can provide an attractive alternative to a standard OFDM system in terms of required average transmitted SNR versus receiver complexity and (2) the system yields a modest reduction in PMEPR versus a standard OFDM system.

The measurement of the average received power is essential for power control and dynamic channel allocation in wireless communication systems. However, due to the effects of multipath fading and additive noise inherent to the wireless channel, there can be significant errors in such measurements. In this paper, the error statistics for average power measurements are considered; in particular, the probability distribution of the value of the average received power at the time of interest conditioned on an outdated measurement is obtained. The resulting expression should have high utility in the analysis of wireless communication systems. However, in this paper, the design of power control algorithms that minimize the average transmitted power required to achieve a desired outage probability for the link is considered. A number of novel power control algorithms based on various models for the error in the average power measurement are derived. Numerical results indicate that power control algorithms based on the accurate expression derived in this paper can demonstrate significant gains over those based on previous approximate models.

In this paper, adaptive modulation schemes are designed that yield the minimum outage probability for wireless systems with strict delay constraints, under the assumption of perfect causal channel state information (CSI) at the transmitter and receiver. Using the channel characterization and delay constraint parameters imposed by the application, a recursive set of equations is derived for the outage probability. This set of equations is used to derive an optimal adaptive uncoded quadrature amplitude modulation scheme, and its performance is compared against that of traditional variable rate techniques designed to maximize average rate subject to a bit error probability constraint. A suboptimal scheme is also presented. Numerical results indicate that the proposed schemes significantly outperform adaptive schemes designed to maximize average rate.

Signal processing algorithms and architectures can use dynamic reconfiguration to exploit variations in signal statistics with the objectives of improved performance and reduced power consumption. Parameters provide a simple and formal way to characterize incremental changes to a computation and its computing mechanism. This paper examines five parameterized computations which are typically implemented in hardware for a wireless multimedia terminal: (1) motion estimation, (2) discrete cosine transform, (3) Lempel-Ziv lossless compression, (4) 3D graphics light rendering and (5) Viterbi decoding. Each computation is examined for the capability of dynamically adapting the algorithm and architecture parameters to variations in their respective input signals. Dynamically reconfigurable low-power implementations of each computation are currently underway.

Blind equalization, which refers to the technique by which an unknown dispersive channel is equalized without the aid of a training sequence, has generated a lot of interest in previous years. However, blind equalizers are notorious for the slow convergence of their filter coefficients to the optimal values, which has prompted a flurry of activity into the development of numerous algorithms and structures to speed up their convergence. This paper is motivated by the approach presented by Labat, Macchi and Laot (see IEEE Transactions on Communications, p.921-30, vol.46, no.7, 1998) and presents a scheme to hasten convergence of the coefficients of the filters of a predictive decision feedback equalizer (PDFE). The proposed scheme is seen to ameliorate many of the drawbacks of the scheme proposed by Labat et al. Numerical results are presented that demonstrate the superiority of the proposed scheme versus the scheme suggested by Tong and Liu (see Proceedings of the IEEE International Conference on Acoustics, Speech, and Signal Processing, p.3901-3904, no.5, 1997).

In wireless systems supporting slowly moving users, adaptive trellis-coded modulation (TCM) schemes have demonstrated large bandwidth efficiency gains over their nonadaptive counterparts. In systems with highly mobile users, the adaptive bit-interleaved coded modulation (BICM) achieves a moderate bandwidth efficiency gain over previously proposed adaptive schemes and nonadaptive schemes with similar complexity. However, adaptive BICM requires a bit interleaver, which results in long latency. In this paper, adaptive coded modulation (ACM) schemes which do not employ interleaving and do not use uncoded bits are considered for time-varying channels. Two such ACM schemes are proposed. One of the ACM schemes uses a forward trellis search algorithm (FTS) to adapt to the current channel fading. Numerical results demonstrate that the proposed FTS-ACM scheme achieves a comparable bandwidth efficiency gain to adaptive BICM. FTS-ACM is particularly attractive for low latency transmission applications.

The measurement of the average received power is essential for power control and dynamic channel allocation in wireless communication systems. In this paper, the error statistics for average power measurements are considered; in particular, the probability distribution of the value of the average received power conditioned on a noisy measurement is obtained. The resulting expression should have high utility in the analysis of wireless communication systems; however, in this paper, this expression is employed in the design of power control algorithms that minimize the average transmitted power required to achieve a desired outage probability for the link. It is demonstrated that power control algorithms based on the accurate expression derived in this paper demonstrate significant gains over those based on previous approximate models.

Adaptive coded modulation is a powerful method for achieving a high spectral efficiency over fading channels. Previously proposed adaptive schemes have employed set-partitioned trellis-coded modulation (TCM) and have adapted the number of uncoded bits on a given symbol based on the corresponding channel estimate. However, these adaptive TCM schemes do not perform well in systems where channel estimates are unreliable, since uncoded bits are not protected from unexpected finding. In this paper, adaptive bit-interleaved coded modulation (BICM) is introduced. Adaptive BICM schemes remove the need for parallel branches in the trellis-even when adapting the constellation size, thus making these schemes robust to errors made in the estimation of the current channel fading value. This motivates the design of adaptive BICM schemes, which will lead to adaptive systems that can support users with higher mobility than those considered in previous work. In such systems, numerical results demonstrate that the proposed schemes achieve a moderate bandwidth efficiency gain over previously proposed adaptive schemes and conventional (nonadaptive) schemes of similar complexity.

Previous work in sequential multihypothesis testing has considered the goal of minimizing the expected number of observations required to choose a hypothesis with a desired level of accuracy. Motivated by reduced-complexity decoding applications, we consider sequential multihypothesis testing techniques that remove individual hypotheses from consideration as they become unlikely in order to minimize the expected aggregate number of hypotheses tested. In the limit of small decision error probabilities, the optimal sequential test that rejects a single hypothesis is characterized. A full minimum complexity sequential multihypothesis testing scheme which assumes the same error probability at each drop of a hypothesis then follows in a straightforward manner. Numerical results are presented that demonstrate the complexity savings via this approach for two simple examples.

For pt.I see ibid., vol.47, no.1828-36 (1999). In Part I, a class of multicarrier systems was proposed to study the effect of the method of diversity allocation on the performance of coherent multiuser communication systems operating over fading channels. In this paper, optimization over the proposed class of systems is considered for a fixed number of users per unit bandwidth. The first case studied is a system where the only noise not attributable to users in the system is additive white Gaussian noise. It is observed that either a system employing exclusive allocation, where users are allocated time-bandwidth resources that are not simultaneously shared with other users, or a system employing maximum resource sharing, where all users simultaneously share time-bandwidth resources, is optimal. Next, the preferable of these two extreme forms of resource allocation is determined. For any reasonable signal-to-noise ratio (SNR) and user density, it is shown that the system employing exclusive resource allocation is optimal in a single-cell environment with perfect subchannel separation at the receiver. Finally, the optimization is repeated in the presence of partial-band interference (PBI). Once again, either a system employing exclusive resource allocation or a system employing a maximum resource sharing scheme is observed to be optimal. The presence of the PBI increases the range of user densities and SNRs where a system employing a maximum resource sharing scheme is optimal, particularly when the probability of a particular time-bandwidth slot experiencing interference is high.

The Shannon capacity of a fading channel under an average-power constraint with channel side information at the transmitter and receiver is only negligibly larger than the capacity of the same channel when constant-power transmission is employed. However, power adaptation has been shown to be quite useful in practical systems, where it has been conjectured that it allows for compensation of the effect of rate quantization. Here, an average bit-error probability constraint is employed instead of the conventional instantaneous bit-error probability constraint. When the set of rates available to the transmitter is unrestricted in practical systems, necessary conditions for jointly optimal power and rate allocation are derived and used to demonstrate that power adaptation is of limited utility. However, when the rates available to the transmitter are restricted to the nonnegative integers for the example of uncoded quadrature amplitude modulation over frequency-nonselective Rayleigh fading channels, a 0.5-0.75 dB loss in power efficiency is incurred when employing only a single power level for each constellation, and a 0.5-bits/symbol loss in rate is incurred when constant power transmission is employed.

This paper proposes a scheme that trades complexity for increased diversity and a reduction in the peak-to-mean envelope power ratio (PMEPR) in wireless orthogonal frequency division multiplexing (OFDM) systems. The key idea is to employ non-standard signal sets with small dimension matched to the OFDM framework; these signal sets are chosen to maximize the diversity of an uncoded system under a constraint to control the system PMEPR. These signal sets allow coded modulation schemes to achieve diversity equal to the product of the Hamming distance of the scheme and the frequency diversity of the channel, while still allowing optimal combined equalization/decoding of reasonable complexity in symbol-interleaved (as opposed to bit-interleaved) coded modulation schemes. Numerical results are presented both for systems employing uncoded modulation and for systems employing bit-interleaved coded modulation (BICM). We conclude that the proposed BICM system can be viewed as a hybrid of a coded OFDM system and a coded single-carrier system employing a soft-output equalizer, thus providing the system designer a possible implementation for the correct combination of the two systems for a given application.

When operating in a wireless transmission environment with available diversity greater than the minimum Hamming distance of its error control code, orthogonal frequency division multiplexing (OFDM) systems can suffer a diversity loss relative to their single-carrier counterparts when both systems are employing identical error control codes. A method of trading complexity for increased diversity in wireless OFDM systems is considered. The method employs a multidimensional signal set, where each signal point in the set is a linear transform of a signal point drawn from a multidimensional signal set that is the product of standard two-dimensional signal sets, each rotated about the origin. The peak-to-mean envelope power ratio (PMEPR) of the proposed system is considered, and it is shown that the PMEPR can be reduced further at the sample points, albeit moderately, by a modification of the proposed signal sets on certain subsets of the subcarriers. Finally, performance curves for the proposed system are given for environments with a varying amount of frequency diversity available in the channel.

The idea of using knowledge of the current channel fading values to optimize the transmitted signal in wireless communication systems has attracted substantial research attention. However, the practicality of this adaptive signaling has been questioned due to the variation of the wireless channel over time, which results in a different channel at the time of data transmission than at the time of channel estimation. By characterizing the effects of fading channel variation on the adaptive signaling paradigm, it is demonstrated here that these misgivings are well founded, as the channel variation greatly alters the nature of the problem. The main goal of this paper is to employ this characterization of the effects of the channel variation to design adaptive signaling schemes that are effective for the time-varying channel. The design of uncoded adaptive quadrature amplitude modulation (QAM) systems is considered first, and it demonstrates the need to consider the channel variation in system design. This is followed by the main contribution of this paper; using only a single outdated fading estimate when neither the Doppler frequency nor the exact shape of the autocorrelation function of the channel fading process is known, adaptive trellis-coded modulation schemes are designed that can provide a significant increase in bandwidth efficiency over their nonadaptive counterparts on time-varying channels.

A class of multiuser multicarrier communication systems is introduced to study the influence of resource allocation on the performance of multiuser communication systems operating over fading channels. This class of systems includes both systems that employ exclusive allocation schemes, where users are allotted time-bandwidth slots without interference from other users, and systems that employ shared allocation schemes, where users are allotted time-bandwidth slots that are also employed by other users. The optimal weighting factors used in the combining of the received signals from the slots of a single user for the conventional receiver is derived, and the performance of systems in the class is characterized. For each of a number of popular multiuser architectures, it is shown that there exists a system in the class with nearly identical performance. Based on these relations, it is concluded that a class of systems has been introduced that allows the study of the merits of different types of time-bandwidth allocation under a single framework.

In this paper, the development of robust adaptive signaling schemes is considered for communication systems that have outdated estimates of a fading channel available at the transmitter. After briefly reviewing pertinent design criteria for both systems employing adaptive trellis coded modulation and systems employing adaptive uncoded quadrature amplitude modulation (QAM), results are presented for these systems when only a single outdated fading estimate is available. The desire to extend the observed bandwidth efficiency gains to systems operating over channels that exhibit higher rates of variation motivates the non-trivial consideration of robust adaptive signaling schemes for systems that employ multiple outdated fading estimates. Under each of two adaptive signaling schemes, results are presented for a system that employs adaptive uncoded QAM and multiple outdated fading estimates; the latter of the two schemes explicitly guarantees the desired strong robustness criterion and demonstrates for a simple uncertainty class the ability to extend bandwidth efficiency increases to systems operating over channels that have higher rates of variation.