This project aims at enabling dynamic spectrum access (DSA) operation over a large swath of the radio spectrum below 6 GHz, including the UHF portion of the TV white spaces (470-698 MHz segment), the 3.5 GHz radar band, as well as other bands in the 1 to 4.4 GHz range that may later become candidates for DSA. By aggregating different (not necessarily contiguous) portions of the RF spectrum at the same wireless device, variations in white/grey spaces can be significantly reduced, allowing for sustained DSA operation over a large geographical area. Enabling such operation while maintaining high link throughput requires advances in the following areas: (1) Electrically small tunable antennas that allow for dynamic access to narrowband (e.g., 6 MHz) channels, but yet support a broad frequency range from 470 MHz to 4.4 GHz; (2) channel/carrier aggregation techniques for noncontiguous narrowband channels; and (3) multi-channel MIMO functionality that allows a given secondary link to boost its throughput (via spatial multiplexing gain) and, simultaneously, minimize interference onto other coexisting systems (via MIMO precoding techniques). [Details]
The open nature of the wireless medium leaves it exposed to adversarial eavesdroppers. Using commodity radio hardware, unauthorized adversaries can easily intercept packet transmissions. Although encryption can be applied to ensure information secrecy, eavesdroppers can still perform low-level RF and traffic analysis, and capture with different degrees of certainty several communication attributes, such as the packet size, its duration, inter-packet times, traffic directionality, channel state information, etc. Through knowledge of the protocol semantics, these attributes can be correlated to create transmission fingerprints, from which an adversary can derive a wealth of contextual information, including the relative significance and locations of the communicating parties, their mobility patterns, the applications they run, their visited web sites, etc. Contextual information can be subsequently used to launch sophisticated attacks (e.g., selective jamming/dropping, targeted node compromise, intelligent DoS attacks, etc.). State-of-the-art privacy research focuses on the leakage of high-layer attributes, typically captured from unencrypted headers. In fact, most efforts in this domain deal with demonstrating the feasibility of privacy attacks, and less with providing remedies. Countermeasures are often limited to specific "point" attacks, without addressing their root cause, i.e., the exposure of transmission signatures. In this project, we aim at advancing the state-of the-art on signature-free secure wireless communications. Our techniques are meant to counter different time scales of eavesdropping. For short-term eavesdropping, we are developing novel PHY-layer techniques for concealing transmission attributes. These techniques exploit advances in MIMO and self-interference suppression to provide integrated transmitter/receiver friendly jamming, with the goal of preventing eavesdropping irrespective of the eavesdropper's location. Novel digital/analog signal combining schemes are also used to obfuscate additional fingerprints. For longer time-scale and/or colluding eavesdroppers, we are developing randomization techniques that aim at countering traffic and frequency inference attacks facilitated by long-term monitoring of transmission activities over several links. [Details]
Establishing communications in a multi-channel wireless network requires the communicating devices to "rendezvous" on a common channel before carrying out normal data transmissions. The rendezvous process enables the exchange and negotiation of critical information, including transmission parameters, connectivity and topological changes, etc. This process takes place at the time a new session is established and following the loss of connectivity between the transmitter and receiver (e.g., due to jamming and/or channel fading). Clock drifts make it difficult for the communicating parties to maintain a common time reference, so the rendezvous process must be robust to misalignment and lack of tight synchronization. Moreover, in a dynamic spectrum access (DSA) environment, rendezvousing may need to be conducted under a heterogeneous opportunistic spectrum setting, i.e., the set of idle channels are generally different for different nodes. This heterogeneity is caused by spatiotemporal variations in spectrum availability as well as interference/jamming conditions. In this project, we are developing novel quorum-based frequency hopping (FH) schemes for unicast and multicast rendezvous in wireless networks. The quorum approach makes it possible to come up with more structured (less random) FH sequences, hence increasing the likelihood of overlap (rendezvous) between two or more nodes. At the same time, this structured approach makes devices inherently vulnerable to "insider attacks," i.e., attacks by compromised nodes that exploit knowledge of the underlying quorum system to prevent the rendezvous process for succeeding. Our research deals with both the performance (speed of rendezvous) as well as security (vulnerability to adversarial attacks) issues of quorum-based FH rendezvous designs, under various spectrum and adversarial settings. [Details]
Common handheld wireless radios operate in half-duplex (HD) mode, with one transceiver alternating between transmission and reception. Full-duplex (FD) operation in which a device is capable of simultaneous transmission and reception (STAR) can be achieved by employing multiple transceivers at the same device or (more recently) through orthogonal frequency division multiple access (OFDMA) technology. In both cases, however, STAR is performed over different transmit and receive channels (henceforth called STAR-D capability). Sufficient separation (i.e., guard bands) is needed between the transmit and receive channels to prevent side-lobe interference and spectrum leakage from corrupting the received signal. Until recently, performing STAR over the same channel (a mode that we refer to as STAR-S) was deemed impossible due to strong self-interference. This common belief was recently challenged by several research groups, which have successfully demonstrated the possibility of FD communications using a combination of self-interference suppression (SIS) techniques, including RF analog cancellation, digital baseband interference cancellation, circulators, phase shifters, etc. In fact, it has been shown that a node's own transmission can be suppressed by 73 to 110 dB (noise floor), depending on the underlying SIS schemes. Such a level of SIS has significant ramifications on network protocols, which are often designed with the assumption of HD operation. In this project, we aim at exploiting SIS capabilities in designing efficient protocols for wireless networks. Two main thrusts are being pursued:
Dynamic spectrum access (DSA) and multi-input multi-output (MIMO) technologies have received great attention in recent years. While the former is viewed as a new communications paradigm to improve the utilization of the licensed spectrum, the latter has already proven itself as a powerful signal processing technique for boosting spectral efficiency. Through channel sensing and/or database access, DSA devices, a.k.a. secondary users (SUs), can opportunistically communicate on temporarily available spectrum bands while avoiding interference with licensed users. By exploiting spatial diversity, MIMO enables two communicating devices to extend their reach, reduce their energy consumption, and/or increase the throughput. In a multi-user (multi-link) setting, MIMO offers significant benefits related to interference avoidance, beamforming/directionality, anti-jamming, and spatial reuse. The main goal of this project is to tightly integrate MIMO into DSA systems, considering optimizations at both the signal level (via adapting the precoding matrices) as well as the antenna level (via cognitive and reconfigurable antennas). Our research addresses the technical challenges associated with operating MIMO/DSA in both centralized and distributed environments. The research agenda includes: (i) Novel cognitive MIMO (CMIMO) adaptation and resource allocation techniques, (ii) reconfigurable and cognitive antennas for supporting simultaneous transmit and sense (STAS) and full-duplex CMIMO functionalities, and (iii) game theoretic pricing and incentive mechanisms for managing MIMO-related interference in a networked (multi-link) setting. Our CMIMO adaptation approach is based on dynamic tuning of the precoding matrices at various CMIMO transmitters, with the goal of optimizing a given network utility function (e.g., network throughput, proportional fairness, total energy consumption) subject to different scheduling and power/rate constraints.
The main goals of this project are to investigate optimal resource allocation policies for opportunistic and collaborative cognitive radio networks (CRNs), and to use such policies in the design of distributed and adaptive cross-layer protocols for CRNs. Through opportunistic access to the licensed spectrum, CRNs aim at improving spectrum efficiency, hence providing higher spatial reuse, programmable connectivity, and increased network availability. Our research agenda includes analytical formulations that aim at joint optimization of transmission powers, transmission rates, and spectrum in an opportunistic and distributed ad hoc CRN that is co-located with several primary (legacy) radio networks (PRNs). The presence of the PRNs impose frequency-dependent power masks on the transmissions of the CRN. Depending on channel dynamics (i.e., channel coherence time relative to the optimization window), we study a deterministic-control formulation for slowly varying channels as well as a stochastic-control formulation for fast varying channels. In both cases, the setup is general enough to allow for multi-channel, multi-path optimization at either the packet level or the session level. The optimized resource allocation strategies are used to develop distributed MAC and routing protocols for opportunistic CRNs under various settings and system constraints ... [click here for more details]
A mobile ad hoc network (MANET) is an autonomous system of wirelessly interconnected mobile terminals. The interest in such networks stems from their ability to provide temporary wireless connectivity in situations where a fixed infrastructure is lacking or is expensive (or infeasible) to deploy (e.g., disaster relief efforts, battlefields, etc.). Our research is focused on the development of power-controlled medium-access (MAC) and routing protocols for MANETs. Theoretical studies and simulations have demonstrated that transmission power control can provide significant improvements in network throughput (i.e., spatial reuse) and/or reduction in energy consumption. Our main goal is to design fully distributed protocols that enable nodes to coordinate their transmission powers and rates so as to improve the spatial throughout of the network. So far, we have developed several power-controlled MACs for MANETs, including PCDC, POWMAC, and GMAC. PCDC demonstrates the efficacy of cross-layer design, as one aspect of this protocol involves controlling the transmission powers of the route-request (RREQ) packets, hence indirectly impacting the path selection process performed by the routing layer. POWMAC uses a single-channel, single-transceiver architecture (to maintain compatibility with COTS 802.11 hardware). Channel access is performed through a succession of contention slots (together called an access window) during which control packets are exchanged between multiple pairs of terminals. At the end of the access window, concurrent, interference-limited data transmissions can commence in the same vicinity of a receiving terminal at appropriately selected transmission powers and rates. While POWMAC achieves impressive performance gain over the standardized 802.11 approach (i.e., classic CSMA/CA), the heuristic manner by which POWMAC sets the "interference margin" (which is used to accommodate interference from future transmissions) leaves room for improvement. Accordingly, we have recently introduced GMAC (game-theory inspired MAC), in which the determination of transmissions powers is inspired by game-theoretic analysis that is aimed at maximizing a network utility subject to constraints ...[click here for more details]
In this project, we aim at designing resource management schemes and adaptive protocols for wireless networks that are capable of directional transmission/reception. In contrast to an isotropic antenna, which transmits the same amount of power is all directions, a directional antenna has preferred direction(s) for transmission and reception; while transmitting, the antenna concentrates the power in certain direction(s), and while receiving the antenna has greater sensitivity for electromagnetic radiation in certain direction(s). Compared with an isotropic antenna, directional antennas have the potential to significantly improve the network throughput and/or reduce the required per-bit energy consumption. For instance, sectoring provided by directional antennas enables a base station to serve more than one cell at a time, thus improving the capacity of a cellular network. Because of these advantages, directional antennas have been adopted in IS-95 and 3G cellular systems. Recently, directional antennas have been suggested for mobile ad hoc networks (MANETs) as well as wireless mesh networks (WiMAX). However, classic MAC and routing protocols in such networks have been designed for omni-directional communications, and extending these protocols to handle directional communications is fraught with challenges. Chief among them is new forms of the hidden-terminal problem (attributed to the asymmetry of the communications), side-lobe interference (due to energy leakage), and transmitter deafness. To address these challenges, we are developing novel, power-controlled MAC and routing protocols for MANETs with directional antennas. Two MAC protocols (called DMAP and LCAP) have been developed and evaluated using simulations and analysis. The latter protocol holds greater promise for practical implementation. Its novelty lies in using an elaborate packet-based power control strategy that is aimed at increasing the channel's spatial reuse by allowing interference-limited, concurrent directional transmissions to take place in the same vicinity. By employing a separate control channel and by accounting for minor-lobe interference, LCAP alleviates many of the channel access problems that afflict commonly used MAC protocols ...[click here for more details]
MAC and routing protocols are often designed for single-input-single-output (SISO) wireless systems, where each node is equipped with a single antenna for transmission and reception (typically, operated in a half-duplex manner). Significant improvement in performance can be achieved by employing multi-input multi-output (MIMO) techniques, whereby multiple transmit and/or receive antennas are used to provide spatial diversity. MIMO offers three types of gains: array, diversity, and multiplexing. The array gain is achieved either at the transmitter through directional alignment of the transmitted signal or at the receiver by coherently combining independently received copies of the signal. Diversity gain is interpreted as the slope of the average BER curve versus SNR, which is proportional to the number of independent paths. Multiplexing gain is obtained when different signals are transmitted over various transmit antennas for the purpose of increasing the total transmission capacity of the link. In this project, we are investigating the feasibility of adapting the number of transmit/receive antennas (on a per-packet basis) in multi-antenna wireless devices for the purpose of minimizing energy consumption and/or maximizing network capacity. Our initial setup considers networks with 2-antenna nodes, allowing for four possible transmit/receive antenna configurations (also called "antenna modes") per active link: 1-by-1 (SISO), 2- by-1 (MISO), 1-by-2 (SIMO), and 2-by-2 (MIMO). Depending on the type of MIMO gain (diversity vs. multiplexing) and the primary performance objective (energy reduction vs. throughput), we have developed several MIMO-adaptive MACs for MANETs and wireless LANs. These include E-BASIC, MIMO-POWMAC, and CMAC. E-BASIC targets MIMO systems that are capable of providing diversity gain. It allows nodes to distributively adapt their antenna modes, transmission powers, and modulation orders (for multi-rate systems) on a per-packet basis such that the total per-bit energy requirement (transmission plus circuit) is minimized. The protocol was integrated into the design of a power-aware routing (PAR) scheme for MIMO-capable ad hoc networks. In contrast of E-BASIC, MIMO-POWMAC is aimed at multi-antenna systems that offer multiplexing gain...[click here for more details]
Wireless sensor networks (WSNs) are expected to play an important role in a wide range of civilian and military applications, including environment monitoring (e.g., soil and air contaminants), seismic-structure analysis, marine micro-organisms research, military surveillance and reconnaissance, etc. In many deployment scenarios, sensors must be powered by batteries, which makes energy consumption a critical factor in the design of a WSN and calls for efficient and distributed energy management solutions that maximize the operational lifetime of the network. In this project, we are developing such solutions through intelligent topology management, adaptive sensing, clustering, cooperative (virtual) MIMO communications, and energy harvesting. Our agenda includes:
Dynamic Activation of Sensors in Location-Unaware WSNs. To improve their reliability, sensors are typically redundantly deployed to account for unexpected failures and improve the fidelity of the received measurements. Redundancy means that some parts of the monitored area are covered by more than one sensor at the same time. While redundancy achieves better reliability, it does not necessarily improve the coverage time, defined as the time until a certain percentage of the area is no longer monitored by any sensor. To prolong coverage time, the network topology can be controlled by selecting a subset of nodes that actively monitor the field and putting the remaining nodes to sleep. We are developing and implementing distributed algorithms that enable GPS-less sensors to determine their active/sleep patterns (including the duration of the active/sleep period) based only on local information gathered from neighboring sensors or from "anchor" nodes (e.g., fixed access points with GPS capability). Our work is motivated by many practical scenarios in which sensors are required to operate without line-of-site communications to satellites (hence, the inability to acquire GPS coordinates).
Maximizing Quality of Coverage in Active WSNs with Energy Harvesting Capabilities. As the cost of photovoltaic (solar) panels continues to go down, energy harvesting is becoming an attractive solution for operating/maintaining a WSN and reducing the overhead of replacing the chemical batteries of its sensing nodes. However, proper utilization of energy harvesting can be burdensome and complex. The unpredictable nature of the solar profile (cloud cover, shadows of buildings, etc.) introduces a high degree of uncertainty and difficulty in the management of sensors (adjusting their sensing ranges, sampling intervals, etc.). Consequently, the problem of guaranteeing a minimum coverage for targets, i.e., providing a guaranteed quality of coverage (QoC), becomes more acute. This problem is particularly relevant to active sensor networks (e.g., surveillance applications, radar sensors), where periods of low coverage represent times of vulnerability to outside influences. As such, it is important to eliminate these periods by ensuring that the minimum coverage experienced throughout the operational time is maximized. In this project, we are developing novel approaches for exploiting harvested solar energy, with the goal of maximizing the QoC of an active WSN. More specifically, we consider solar-powered active WSNs in which each sensor’s range is controllable. In this context, QoC is defined as the minimum number of targets that can be simultaneously monitored during the period of network operation. Considering an accurately modeled profile for the solar power flux (which can be deterministic or random) and a (re)charge/discharge battery model, we are investigating the joint optimization of sensor radii (adapting the sensing ranges) and the routing matrix (relaying traffic between sensor nodes) in a WSN that is comprised of N solar-powered sensing nodes and M monitored targets. We are also investigating efficient sensor scheduling mechanisms for supporting fault-tolerant WSN communications (e.g., guaranteeing k-coverage, where k is the redundancy factor), at the minimal possible energy overhead. Computationally efficient algorithmic solutions are being developed, along with testbed implementation.
Clustering Algorithms for WSNs. When sensors are deployed in large numbers, it is extremely inefficient to operate them as a flat (ad hoc) topology, where each sensor acts as a data source and as a relay to other sensors. In such scenarios, clustered designs are known to provide better network manageability and improved network lifetime. In a clustered WSN, sensors are grouped into dynamically formed clusters, and each cluster is assigned one of its members to act as a cluster head (CH). The CH is responsible for collecting data from the members of its own cluster, fusing (e.g., summarizing, aggregating) the collected data, and then transmitting a summary report to a command-and-control center (CCC). The key questions that we seek to answer here is how to allow sensors to self-cluster with no or little intervention from the CCC, how to dynamically elect and re-elect the CHs, and how to design energy-efficient routing protocols in such a clustered architecture.
QoS provisioning over wireless networks.
Pseudo self-similarity and chaoticity in network traffic.
Encapsulation mechanisms for real-time and non-real-time packet services over satellite links.
Network support for interactive video-on-demand using time-varying traffic envelopes.