In this paper we introduce projection-based precoders for secure transmission in cell-free massive multiple-input multipleoutput (MIMO) systems. Assuming that the channel state information (CSI) of both legitimate users (referred to as Bob) and passive eavesdroppers (Eve) is known, we derive three secure precoder designs using well-known linear precoders, such as maximum ratio transmission (MRT), zero-forcing (ZF) and minimum mean square error (MMSE). The first method projects these linear precoders onto the orthogonal complement space of Eve. The second method processes Bob's channel with the same projection operator before applying linear precoding to this newly formed "secure" channel matrix. Additionally, we propose a novel MMSE-based precoder with added security constraints, designed to reduce energy leaked towards Eve. By reformulating the proposed precoders, we provide several interpretations of the latter, which result in simpler expressions and equivalent projected precoding schemes. Analytical results are supported by simulations in realistic environments, and demonstrate that the proposed methods can completely eliminate Eve's received power. It results in improved secrecy performance compared to their non-secrecy aware counterparts.

This paper deals with a massive multiple-input multiple- output (mMIMO) full-duplex (FD) communication system. We propose new spatial domain self-interference cancellation (SIC) techniques. The main idea is to treat the self-interference (SI) signal emitted from the FD node as another spatial stream that needs to be precoded in the null space of the MIMO channel that includes that node as an output. To this end, an additional RF chain is needed at the transmitter to cancel the SI signal. On this basis, we derive the expressions of the spatial precoders with respect to the conventional zero forcing (ZF) and minimum mean square error precoding (MMSE) criteria. Furthermore, the originality of the analysis lies in the consideration of SWM for modeling the SI channel, which is justified by the close proximity of the transmit and receive antenna panels in the FD node. Simulations show the relevance of the study. We verify the SIC efficiency in the spatial domain using the derived ZF and MMSE precoders by analyzing their relative performance in terms of spectral efficiency. We also highlight the robustness of our SWM-based SI channel compared to classic planar wave model (PWM) and therefore the need for using it.

This paper deals with the implementation of a light fifth generation (5G) base station (gNB) intended  for specific use cases requiring an airborne deployment of the network. We focus on the radio unit (RU) and give details on the components, the implementations, and the technical choices that have been made, driven by the strong constraints inherent to the considered use cases.

This paper deals with a new spatial domain-based self-interference cancellation (SIC) method called constrained minimum mean square error (C-MMSE) for massive multiple-input multiple-output (mMIMO) full-duplex (FD) communication systems. The main idea is to treat the self-interference (SI) signal emitted from an FD node as a particular spatial stream arriving at the receiver part of that same FD node which needs to be spatially postcoded along with other useful signals coming from other transmitters, so that it falls into the null space of the MIMO channel that includes the FD node transmitter part as an input. On this basis, we first adapt the expressions of the spatial combiners with respect to the conventional zero forcing (ZF) and minimum mean square error combining (MMSE) criteria and show that the latter is not capable to efficiently cancel the SI signal unless an additional constraint is added to properly perform SIC. We hence design the new so-called C-MMSE combiner and derive its expression. In addition to our proposal, the originality of our work lies in the consideration of spherical wave model (SWM) for modeling the SI channel, which is justified by the close proximity of the transmit and receive antenna panels in the FD node. We examine and compare the SIC performance of the adapted ZF combiner, the adapted MMSE combiner and the newly introduced C-MMSE combiner by evaluating the obtained spectral efficiency (SE). We also highlight the robustness of the SWM-based SI channel modelling compared to conventional planar wave modelling (PWM) and therefore the relevance of using it.

In this paper, we study the behavior of a multiple-input multiple-output (MIMO) full-duplex (FD) system according to two different models of the self-interference (SI) channel  inherent to the FD communication strategy: the spherical wave model (SWM) and the planar wave model (PWM). More precisely, we evaluate the received SI power at the receiver antenna array by changing the relative geometrical position of the transmitter antenna array in respect to the latter. Our study shows a significant difference between the two models. In particular we theoretically show as well as through simulation that the SWM for the SI channel is more precise than the PWM, and should then be considered for practical applications of the FD concept.

This paper deals with multipath channel estimation and equalization in LoRa. It is suggested to take advantage of the cyclic property of the symbols in the LoRa frame preamble to obtain an interference-free version of the symbols in the frequency domain. Then, estimation methods used in multicarrier systems can be applied, such as the least square (LS), and the minimum mean square error (MMSE) estimators. It is shown that the cyclic property in LoRa is inherently independent of the length of the channel, making these estimation techniques robust to any frequency-selective channel. In addition the frequency domain zero-forcing (ZF) equalizer is used, and an original phase equalizer is introduced, taking advantage of the constant modulus property of LoRa symbols in the frequency domain. The performance of the investigated estimators and equalizers is shown through simulations, and applications to the presented results are further discussed.

This paper deals with time and frequency synchronization in LoRa system based on the preamble symbols. A thorough analysis of the maximum likelihood (ML) estimator of the delay (time offset) and the frequency offset shows that the resulting cost function is not concave. As a consequence the a priori solution to the maximization problem consists in exhaustively searching over all the possible values of both the delay and the frequency offset. Furthermore, it is shown that these parameters are intertwined and therefore they must be jointly estimated, leading to an extremely complex solution. Alternatively, we show that it is possible to recover the concavity of the cost function, from which we suggest a low-complexity synchronization algorithm, whose steps are described in detail. Simulations results show that the suggested method reaches the same performance as the ML exhaustive search, while the complexity is drastically reduced, allowing for a real-time implementation of a LoRa receiver.

This paper deals with a cloud radio access network (CRAN) architecture for the LoRa system. In the suggested design, the gateway embeds a limited remote radio head (RRH), including the analog radio-frequency (RF) analog part, the digital-to-analog and analog-to-digital conversion, and a digital front-end (DFE). The other LoRa network functions, including the physical (PHY) layer, the LoRaWAN medium access control (MAC) layer, and the application and customer servers are implemented as cloud resources. The presented approach leads to a flexible RAN that is robust to the variations of capacity needs. Furthermore, it allows us to test very specific LoRa features, such as the detection or demodulation, while bypassing the other ones including the hardware RRH. The methodology and tools we used to deploy a LoRa cloud RAN are detailed, and results concerning the performance indicator (CPU load, memory consumption) are provided as well.

This paper proposes a flexible multi-standard digital frontend (DFE) hardware architecture designed for three main low-power wide-area (LPWA) technologies: LoRa, Sigfox, and narrowband-Internet of things (NB-IoT).We demonstrate the feasibility of an unified DFE architecture that fits the requirements of these LPWA technologies. The proposed DFE architecture has been implemented on a Spartan-6 field programmable gate array (FPGA) within a base station receiver, and tested using a universal software radio peripheral (USRP) based platform. We show that the expected performance can be achieved with low hardware complexity, in terms of memory and logic requirements.