We present several novel techniques to track (unassociated) mobile devices by abusing features of the Wi-Fi standard. This shows that using random MAC addresses, on its own, does not guarantee privacy. First, we show that information elements in probe requests can be used to fingerprint devices. We then combine these fingerprints with incremental sequence numbers, to create a tracking algorithm that does not rely on unique identifiers such as MAC addresses. Based on real-world datasets, we demonstrate that our algorithm can correctly track as much as 50% of devices for at least 20 minutes. We also show that commodity Wi-Fi devices use predictable scrambler seeds. These can be used to improve the performance of our tracking algorithm. Finally, we present two attacks that reveal the real MAC address of a device, even if MAC address randomization is used. In the first one, we create fake hotspots to induce clients to connect using their real MAC address. The second technique relies on the new 802.11u standard, commonly referred to as Hotspot 2.0, where we show that Linux and Windows send Access Network Query Protocol (ANQP) requests using their real MAC address.

In practical implementations of single-antenna full-duplex (FD) radios, the imperfect isolation between the transmit (TX) and receive (RX) ports of the circulator causes the leakage of a strong self-interference (SI) component. This issue reduces both the spectral and energy efficiency of the device. State-of-the-art SI cancellation (SIC) solutions mitigate this problem, however they also constrain the maximum TX power of the FD radio. To address these issues and restrictions, we propose an energy recycling FD architecture that improves both the SIC capability and the spectral/energy efficiency of FD devices by means of a suitable radio frequency (RF) energy harvesting. The benefits brought by such solution are analyzed in an uplink (UL) scenario where two FD access points (APs), operating according to the proposed FD architecture, exchange signaling messages via a wireless link while communicating with two half-duplex (HD) mobile users (MUs). Our findings highlight non-negligible performance enhancements as compared to state-of-the-art approaches. Specifically, the proposed architecture is shown to yield a rate enhancement up to 29% for both UL transmission and signaling, and the recycling of up to 50% of the wasted energy at the circulator.

In this work, a novel energy-recycling single-antenna full-duplex (FD) radio is designed, in which a new three-port element including a power divider and an energy harvester is added between the circulator and the receiver (RX) chain. The presence of this new element brings advantages over the state of the art in terms of both spectral efficiency and energy consumption. In particular, it provides the means of performing both an arbitrary attenuation of the incoming signal, which in turn increases the effectiveness of the state-of-the-art self-interference cancellation strategies subsequently adopted in the RX chain, and the recycling of a non-negligible portion of the energy leaked through the nonideal circulator. The performance of this architecture is analyzed in a practically relevant four-node scenario in which two nodes operate in FD and two nodes in half-duplex (HD). Analytical approximations are derived for both the achievable rates of the transmissions performed by the FD and HD radios and the energy recycled by the FD radios. The accuracy of these derivations is confirmed by numerical simulations. Quantitatively, achievable rate gains up to 40% over the state-of-the-art alternatives, in the considered scenario, are highlighted. Furthermore, up to 50% of the leaked energy at the circulator, i.e., 5% of the energy of the transmitted signal, can be recycled.

In this contribution we propose a method to increase the energy efficiency of orthogonal frequency division multiplexing (OFDM)-based femtocells. This is accomplished with no impact to the current power consumption, radio frequency (RF) circuitry, link adaptation strategies, bandwidth and transmit power. The proposed technique recycles redundant resources of OFDM transmissions (e.g., guard bands and cyclic prefixes), introduced to combat frequency selectivity. We borrow the underlying idea from a technique called cognitive interference alignment (CIA). Interestingly, our novel approach does not suffer from the same issues inherent to CIA, such as synchronization at the primary receiver and channel knowledge related complications. Nevertheless, it introduces a new issue related to the interference from the OFDM signal, which prompted the adoption of an adequate linear receiver at the femtocell user equipment. Numerical findings demonstrate that spectral efficiency gains are achieved, improving the energy efficiency of the femtocell by up to 20% for the simulated scenario.

In this work, the problem of cross-tier interference in a two-tiered (macro-cell and cognitive small-cells) network, under the complete spectrum sharing paradigm, is studied. A new orthogonal precoder transmit scheme for the small base stations, called multi-user Vandermonde-subspace frequency division multiplexing (MU-VFDM), is proposed. MU-VFDM allows several cognitive small base stations to coexist with legacy macro-cell receivers, by nulling the small- to macro-cell cross-tier interference, without any cooperation between the two tiers. This cleverly designed cascaded precoder structure, not only cancels the cross-tier interference, but avoids the co-tier interference for the small-cell network. The achievable sum-rate of the small-cell network, satisfying the interference cancelation requirements, is evaluated for perfect and imperfect channel state information at the transmitter. Simulation results for the cascaded MU-VFDM precoder show a comparable performance to that of state-of-the-art dirty paper coding technique, for the case of a dense cellular layout. Finally, a comparison between MU-VFDM and a standard complete spectrum separation strategy is proposed. Promising gains in terms of achievable sum-rate are shown for the two-tiered network w.r.t. the traditional bandwidth management approach.

Vandermonde-subspace frequency division multiplexing (VFDM) is an overlay spectrum sharing technique for cognitive radio. VFDM makes use of a precoder based on a Vandermonde structure to transmit information over a secondary system, while keeping an orthogonal frequency division multiplexing (OFDM)-based primary system interference-free. To do so, VFDM exploits frequency selectivity and the use of cyclic prefixes by the primary system. Herein, a global view of VFDM is presented, including also practical aspects such as linear receivers and the impact of channel estimation. We show that VFDM provides a spectral efficiency increase of up to 1 bps/Hz over cognitive radio systems based on unused band detection. We also present some key design parameters for its future implementation and a feasible channel estimation protocol. Finally we show that, even when some of the theoretical assumptions are relaxed, VFDM provides non-negligible rates while protecting the primary system.

In a previous work, we introduced a spectrum sharing technique called Multi-User Vandermonde-subspace Frequency Division Multiplexing (MU-VFDM). This overlay technique allows the coexistence of a downlink Orthogonal Frequency Division Multiple Access (OFDMA) macro-cell and a cognitive multi-user small-cell system in time division duplex mode. In that work, MU-VFDM was shown to be able to completely cancel the interference towards a macro-cell system at the price of perfect channel state information (CSI) at the opportunistic small-cells. In this work we relax the perfect CSI constraint by introducing a channel estimation protocol that does not require cooperation between the two systems, but still provides harmless coexistence between them. The impact of this protocol is evaluated in terms of interference at the legacy and sum-rates at the opportunistic system. Simulation results show that, even with imperfect CSI estimation, MU-VFDM is able to achieve promising rates for the small-cells while incurring a small rate loss at the macro-cell due to interference.

In previous works, Vandermonde-subspace frequency division multiplexing (VFDM) has been shown to promote overlay networks by enabling a secondary transmitter to cancel its interference to a primary receiver, while simultaneously transmitting useful information to its own receiver at nonnegligible rates. Interference cancelation is achieved by exploiting the null-space of the channel from the secondary transmitter to the primary receiver. In the wake of a global deployment of the third generation partnership project's (3GPP) long term evolution (LTE), one of the open questions of VFDM concerns its applicability in a primary LTE-orthogonal frequency division multiple access (OFDMA) multi-user setting. In this work, we address this question by extending VFDM to the multi-user scenario where the primary system employs OFDMA, such as LTE. We show that by using at the secondary system a similar precoder structure to the ones previously studied, we are able to cancel the interference towards multiple primary receivers while still achieving acceptable rates for the secondary system.

Vandermonde-subspace frequency division multiplexing (VFDM) is a technique for interference cancellation in overlay networks that allows a secondary network to operate simultaneously with a primary network, on the same frequency band. VFDM can be applied to block transmission systems with a guard time (or cyclic prefix) over frequency selective channels. It achieves zero interference towards the primary system by employing a special precoder that aligns the data to the null space of the interfering channel from the secondary to the primary system. In this work, we extend the assessment of VFDM by analyzing the bit error rate and sum rate capacity of practical linear receiver structures for the VFDM-based secondary system. The study realized herein serves as a basis for the implementation of a VFDM prototype system on a real transmission testbed.

This paper presents the study of the Access Selection (AS) problem in a multi-access wireless network. The contributions of this paper could be classified in two groups. Firstly, we extend previous investigations in terms of performance results, regarding a system-level dynamic evaluation. Performance indicators like coverage, SNR, rate and offered traffic load are included as algorithm criteria. An AS algorithm based on theoretical throughput estimation is also evaluated. Secondly, we propose a sub-optimum access selection solution, in which the arriving user as well as a few ongoing users are (re) assigned according to the new systems' conditions, following the specific optimization criterion. This solution selects candidate users to a vertical handover and anticipates the user context transfer. The approaches presented here assume a generic system modeling and user throughput characteristics as performance indicator. Our simulation campaigns could demonstrate that the access selection with anticipatory vertical handover provides gains about 300% in the 10th percentile of the user session throughput.