In fifth-generation (5G) cellular networks, users feed back to the base station the index of the precoder (from a codebook) to be used for downlink transmission. The precoder is strongly related to the user channel and in turn to the user position within the cell. We propose a method by which an external attacker determines the user position by passively overhearing this unencrypted layer-2 feedback signal. The attacker first builds a map of fed back precoder indices in the cell. Then, by overhearing the precoder index fed back by the victim user, the attacker finds its position on the map. We focus on the type-I single-panel codebook, which today is the only mandatory solution in the 3GPP standard. We analyze the attack and assess the obtained localization accuracy against various parameters. We analyze the localization error of a simplified precoder feedback model and describe its asymptotic localization precision. We also propose a mitigation against our attack, wherein the user randomly selects the precoder among those providing the highest rate. Simulations confirm that the attack can achieve a high localization accuracy, which is significantly reduced when the mitigation solution is adopted, at the cost of a negligible rate degradation.

During the first phase of the deployment of new radio (NR) networks, base station (BS) and user equipment (UE) are expected to operate alongside the pre-existing long-term evolution (LTE) networks. This paper proposes a solution to be adopted by NR UE to perform an uplink transmission towards NR BS over the same time and frequency resources as the legacy LTE system, which would extend the uplink coverage when operating at the supplementary uplink band. In order to guarantee the absence of interference towards LTE BS, we design a new waveform termed as inverse discrete Fourier transform-based Vandermonde-subspace frequency division multiplexing (IDFT-VFDM) with the following important features. On one hand, an NR UE communicating with an NR BS through IDFT-VFDM would not generate any interference at the legacy LTE BS, regardless of the number of antennas present at both ends of the interference channel. On the other hand, an IDFT-VFDM signal can always be designed to occupy the same signal bandwidth as a legacy single-carrier frequency division multiple access signal for LTE uplink transmission. Numerical results demonstrate the merit of IDFT-VFDM and confirm its potential as a candidate solution to achieve LTE-NR co-existence in multi-antenna multi-user scenarios.

The majority of modern communication systems adopt quadrature amplitude modulation (QAM) constellations as transmission schemes. Due to their square structure, however, QAM do not provide satisfying protection to phase noise effects, as the number of constellation points grows, increasing at the same time their peak-to-average-power ratio. This requires an expensive power amplifier and an oscillator at the transmitter to guarantee low distortion, complicating the adoption of dense transmission schemes in practical high-data rate systems. In this paper, we construct a coded modulation scheme based on regular amplitude and phase shift keying modulations. We propose a novel multilevel coding labeling for the constellation points separating the amplitude and phase domains. We provide a novel multistage decoding scheme allowing for a low-complexity log-likelihood ratio calculation for soft-input decoding of component codes, along with a suitable rate design. Finally, we compare the proposed scheme with the state-of-the-art QAM constellations and optimize the constellations in the presence of phase noise.

We consider downlink communications between a Base Station (BS) and various mobile stations, equipped with multiple antennas, based on Orthogonal Frequency Division Multiplexing (OFDM). Transmission is compliant with the Long Term Evolution (LTE) standard operating in Frequency Division Duplex (FDD) mode. Since ideal feedback of channel state information to the BS may be cumbersome, we consider two suboptimal channel estimation algorithms, denoted as Resource Block (RB) and Resource Block Group (RBG). Both approaches approximate the channel as constant over multiples of the fundamental LTE block, known as Physical Resource Block (PRB). Our results show that RB and RBG incur a limited performance loss, yet guaranteeing significant saving in the amount of feedback information.

Caching popular contents at the edge of the network can positively impact the performance and future sustainability of wireless networks in several ways, e.g., end-to-end access delay reduction and peak rate increase. In this paper, we aim at showing that non-negligible performance enhancements can be observed in terms of network interference footprint as well. To this end, we consider a full-duplex small-cell network consisting of non-cooperative cache-aided base stations, which communicate simultaneously with both downlink users and wireless backhaul nodes. We propose a novel static caching model seeking to mimic a geographical policy based on local files popularity and calculate the corresponding cache hit probability. Subsequently we study the performance of the considered network in terms of throughput gain with respect to its cache-free half-duplex counterpart. Numerical results corroborate our theoretical findings and highlight remarkable performance gains when moving from cache-free to cache-aided full-duplex small-cell networks.

Self-interference (SI) significantly limits the performance of full-duplex (FD) radio devices if not properly cancelled. State-of-the-art SI cancellation (SIC) techniques at the receive chain implicitly set an upper bound on the transmit power of the device. This paper starts from this observation and proposes a transmit beamforming design for FD multiple-antenna radios that: i) leverages the inherent SIC capabilities at the receiver and the channel state information; and ii) exploits the potential of multiple antennas in terms of spatial SIC. The proposed solution not only maximizes the throughput while complying with the SIC requirements of the FD device, but also enjoys a very low complexity that allows it to outperform state-of-the-art counterparts in terms of processing time and power requirements. Numerical results show that our transmit beamforming design achieves significant gains with respect to applying zero-forcing to the SI channel when the number of transmit antennas is small to moderate, which makes it particularly appealing for FD small-cell base stations.

In this paper, we target a massive multiple-input/multiple-output (MIMO) system operating in frequency-division duplexing (FDD) mode, assuming the adoption of a two-level linear precoding strategy at the BS. We propose a novel strategy to effectively acquire the channel state information (CSI) at the base station (BS). In particular, we devise a cross-layer dynamic algorithm for user grouping, CSI acquisition and user scheduling that takes into account fairness considerations, application characteristics and quality of service (QoS) constraints of the users. We assess the merit of the proposed algorithm for a proportional fairness objective by comparing its performance with what is achieved by a relevant baseline algorithm in which user grouping is static and based only on the second order statistics, i.e., joint space division and multiplexing (JSDM). Our numerical findings illustrate that the proposed algorithm outperforms the baseline in terms of both fairness and speed of convergence to a steady state, and for different network topologies.

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 paper we develop a new cross-layer framework for adaptive spectrum management in practical mobile ad hoc networks. We first propose a two-step spectrum sensing algorithm with reduced hardware complexity which improves energy efficiency without requiring knowledge of the spectrum sparsity. We then incorporate our spectrum sensing algorithm into our proposed distributed spectrum access protocol, based on local orthogonality and well suited for clustered ad hoc networks. We show via simulations that our proposed cross-layer scheme achieves near-optimal spectrum utilization under various operating scenarios, and we provide design guidelines for all system parameters. Simulations also verify the robustness of our scheme, showing that its performance is only marginally affected by errors in the spectrum sensing phase.

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 work, the self-sustainability of block transmission systems is analyzed. In particular, orthogonal frequency division multiple access (OFDMA) is taken as a reference, due to its popularity and rather simple signal model. More precisely, a generalized variant of this scheme in which the transmitted signal is obtained as the sum of an OFDMA and a cognitive interference alignment (CIA) component, acting as an energy bearer, is considered. In this scenario, the self-sustainability of the transmission is made possible by the flexibility of the adopted strategy and the introduction of a novel energy harvesting OFDMA receiver. Both the feasibility conditions for the self-sustainability and the optimal power allocation to maximize the effectiveness of the energy transfer performed through the CIA signal are derived. Numerical results show that full self-sustainability can be achieved for several system configurations and channel statistics. However, this comes at the cost of a rate penalty with respect to a standard classic OFDMA transmission, which is termed the price of self-sustainability. A study of the relationship between the performance of both the energy and the information transfer is carried out. A CP size that minimizes the price of self-sustainability can be found for all the considered configurations.

In this work, we consider a multiple-input single-output system in which an access point (AP) performs a simultaneous wireless information and power transfer (SWIPT) to serve a user terminal (UT) that is not equipped with external power supply. To assess the efficacy of the SWIPT, we target a practically relevant scenario characterized by imperfect channel state information (CSI) at the transmitter, the presence of penalties associated to the CSI acquisition procedures, and non-zero power consumption for the operations performed by the UT, such as CSI estimation, uplink signaling and data decoding. We analyze three different cases for the CSI knowledge at the AP: no CSI, and imperfect CSI in case of time-division duplexing and frequency-division duplexing communications. Closed-form representations of the ergodic downlink rate and both the energy shortage and data outage probability are derived for the three cases. Additionally, analytic expressions for the ergodically optimal duration of power transfer and channel estimation/feedback phases are provided. Our numerical findings verify the correctness of our derivations, and also show the importance and benefits of CSI knowledge at the AP in SWIPT systems, albeit imperfect and acquired at the expense of the time available for the information transfer.

In this work a wireless power transfer (WPT) scheme for orthogonal frequency division multiple access (OFDMA) systems is proposed, with the objective of prolonging the battery life of the receiver. A novel receiver that capitalizes on the presence of a redundant portion of the OFDMA signal, i.e, the cyclic prefix (CP), to harvest energy that can be used to reduce the power consumption of the OFDMA digital signal processing at the receiver, is described. The concept of self-sustainability, a condition achieved when the amount of harvested energy from the received signal is sufficient to perform the digital signal processing at the receiver, is introduced. A parametric model to study the feasibility of several levels of self-sustainability, is provided and an analytical condition to ensure their achievability is derived. Numerical findings show that, under certain reasonable conditions, the full self-sustainability can be achieved for CP sizes compliant with the existing telecommunication standards.

Recent advances in microwave technology and signal processing have unveiled the potential of the so-called wireless power transfer (WPT) from one device to the other. This has resulted in reducing the need of a local power source at the wireless devices. In this paper, we target a popular physical layer technology, namely the orthogonal frequency division multiplexing (OFDM) and propose a novel approach to prolong the battery life of an OFDM receiver, by exploiting the concept of WPT. We design a novel OFDM receiver architecture that does not discard the cyclic prefix (CP), but instead, exploits it to extract power from the received signal, effectively realizing a WPT between the transmitter and the receiver. The proposed technique does not require any change in the transmission protocol as compared to the legacy OFDM. We show that the amount of power carried in the CP could be made sufficient to decode the information symbols, making the transmission self-sustainable in terms of power consumption at the receiver. We analytically derive the feasibility condition for the self-sustainability of the transmission and analyze its impact on the performance of the OFDM system. Numerical findings provide encouraging results and confirm the potential of the proposed approach.

This paper proposes two novel strategies to extend the battery life of an orthogonal frequency-division multiplexing (OFDM) receiver by exploiting the concept of wireless power transfer (WPT). First, a new receiver architecture is devised that does not discard the cyclic prefix (CP) but instead exploits it to extract power from the received signal, realizing a WPT between the transmitter and the receiver. Subsequently, a flexible composite transmit strategy is designed, in which the OFDM transmitter transmits to the receiver two independent signals coexisting in the same band. It is shown that, by means of this approach, the transmitter can arbitrarily increase the power concentrated within the CP at the OFDM receiver, without increasing the redundancy of the transmission. Feasibility conditions for the self-sustainability of the transmission are derived, in terms of power consumption at the receiver, for both legacy and composite transmission. Numerical findings show that, under reasonable conditions, the amount of power carried in the CP can be made sufficient to decode the information symbols, making the transmission fully self-sustainable. The potential of the proposed approach is confirmed by the encouraging results obtained when the full self-sustainability constraint is relaxed, and partially self-sustainable OFDM transmissions are analyzed.

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.

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.