Leonardo S. Cardoso

Cooperative spectrum sensing using random matrix theory

In this paper, using tools from asymptotic random matrix theory, a new cooperative scheme for frequency band sensing is introduced for both AWGN and fading channels. Unlike previous works in the field, the new scheme does not require the knowledge of the noise statistics or its variance and is related to the behavior of the largest and smallest eigenvalue of random matrices. Remarkably, simulations show that the asymptotic claims hold even for a small number of observations (which makes it convenient for time-varying topologies), outperforming classical energy detection techniques.

Vandermonde frequency division multiplexing for cognitive radio

We consider a cognitive radio scenario where a primary and a secondary user wish to communicate with their corresponding receivers simultaneously over frequency selective channels. Under realistic assumptions that the primary user is ignorant of the secondary userpsilas presence and that the secondary transmitter has no side information about the primarypsilas message, we propose a Vandermonde precoder that cancels the interference from the secondary user by exploiting the redundancy of the cyclic prefix and the frequency selectivity of the channel. Our numerical examples show that VFDM, with an appropriate design of the input covariance, enables the secondary user to achieve a non-negligible rate while generating zero interference to the primary user.

Capacity of linear multi-user MIMO precoding schemes with measured channel data

In multi-user multiple-input multiple-output (MU-MIMO) systems, spatial multiplexing can be employed to increase the throughput without the need for multiple antennas and expensive signal processing at the user equipments. In theory, MU-MIMO is also more immune to most of propagation limitations plaguing single-user MIMO (SU-MIMO) systems, such as channel rank loss or antenna correlation. In this paper we compare the performance of different linear MU-MIMO precoding schemes using real channel measurement data. The measurement data has been acquired using Eurecompsilas MIMO Openair Sounder (EMOS). The EMOS can perform real-time MIMO channel measurements synchronously over multiple users. The results show that MU-MIMO provides a higher throughput than SU-MIMO also in the measured channels. However, the throughput in the measured channels is by far worse than the one in channels without spatial correlation. Of all the evaluated linear precoding schemes, the MMSE precoder performs best in the measured channels.

Why MAC Address Randomization is not Enough: An Analysis of Wi-Fi Network Discovery Mechanisms

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.

Orthogonal LTE Two-Tier Cellular Networks

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 projects (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 receiver analysis

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.

Cognitive interference alignment for OFDM two-tiered networks

In this contribution, we introduce an interference alignment scheme that allows the coexistence of an orthogonal frequency division multiplexing (OFDM) macro-cell and a cognitive small-cell, deployed in a two-tiered structure and transmitting over the same bandwidth. We derive the optimal linear strategy for the single antenna secondary base station, maximizing the spectral efficiency of the opportunistic link, accounting for both signal sub-space structure and power loading strategy. Our analytical and numerical findings prove that the precoder structure proposed is optimal for the considered scenario in the face of Rayleigh and exponential decaying channels.

Channel estimation impact for LTE small cells based on MU-VFDM

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.

Blind standard identification with bandwidth shape and GI recognition using USRP platforms and SDR4all tools

In this paper, focusing on identifying standards blindly, we propose a bandwidth shape sensor and a GI (guard interval) sensor using USRP (Universal Software Radio Peripheral) platforms and SDR4all tools. These sensors are fundamental parts of the so-called Blind Standard Recognition Sensor. The blind standard bandwidth sensor is based on a Radial Basis Function Neuronal Network designed in Matlab. We have presented first experience of using blind standard bandwidth sensor in a previous work. We will provide in this paper further details on the results of this sensor (simulations, preliminary implementations and validations). The GI sensor is implemented in order to improve the detection performance in the case of two identical bandwidth shapes. The SDR4all driver offers a simple yet efficient interface between the Matlab signal processing codes and the USRP transmitting and receiving platforms. These simple and easily accessible software defined radio tools were used to design and implement two sensors. The inducted simulations and experiments show that the designed system is indeed able to discriminate three standard-like spectrums (e.g., GSM-like, UMTS-like and OFDM-like) under simple yet real transmission conditions using their different bandwidth shapes and to identify a GI-OFDM-like system using cyclic autocorrelation method.

Blind Bandwidth Shape Recognition for Standard Identification Using USRP Platforms and SDR4all Tools

In this paper a blind standard bandwidth shape sensor is implemented on a USRP (Universal Software Radio Peripheral) platform using SDR4all tools. This sensor is a fundamental part of the so-called Blind Standard Recognition Sensor. The blind standard bandwidth sensor is based on a Radial Basis Function Neuronal Network designed in Matlab. The SDR4all driver offers a simple yet efficient interface between the Matlab signal processing codes and the USRP transmitting and receiving platforms. To the best of our knowledge, it is the first time that simple and easily accessible software defined radio tools were used to design and implement this sensor. Although the experiments were realized under line-of-sight transmission conditions the results show that the designed system is indeed able to discriminate several standard-like spectrums under real transmission conditions using their different bandwidth shapes.