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The 3rd Generation Partnership Project (3GPP) has defined Ultra-Reliable Low-Latency Communication (URLLC) as one of the main objectives of 5G development to satisfy the applications with stringent requirements of latency and reliability. Uplink (UL) configured-grant (CG) transmission where the user equipment (UE) transmits a packet without scheduling request (SR) and UL grant is standardized by 3GPP Release 15 to reduce latency. The UE also transmits automatically a configured number of repetitions without feedback from the base station (gNB) to increase reliability.Nevertheless, the repetitions are not allowed to transmit outside the hybrid automatic repeat request (HARQ) process containing the first repetition. It might cause a smaller number of transmitted repetitions than configuration that is harmful to the performance of URLLC.This paper uses the reserved resources where the UEs can transmit the repetitions outside the original HARQ process until the configured number is reached. The scheme is developed further when the gNB is equipped with a successive interference cancellation (SIC) receiver so it can decode multiple repetitions of the different UEs in the same reserved resource. The use of reserved resources ensures the performance of UL CG transmission while SIC receiver minimizes the reserved resource’s consumption. The numerical results show a higher transmission reliability and lower reserved resource consumption of the proposed scheme compared to the related works.

Ultra-reliable low-latency communication (URLLC) is one of the main services in 5G New Radio (NR) to serve the applications with the strict requirements of latency and reliability. Increase of mobile traffic and bandwidth requirements for new applications have resulted in a shortage of licensed spectrum so that even URLLC services are deemed to be running over the unlicensed spectrum. This may require a significant re-design of channel access, transmission and reception procedures over the unlicensed spectrum and is currently being investigated in The 3rd Generation Partnership Project (3GPP) Release 17.This paper focuses on the channel access mechanism for URLLC services over the unlicensed spectrum. It provides an analytical analysis of current channel access procedures, listen before talk (LBT). LBT states in random duration channel access are modeled through Markov chains which help characterize the closed form expressions for average channel access time. These expressions are evaluated using the parameters from currently standardized channel access priority classes. The evaluation shows the total inability of several current channel access classes to support URLLC services over the unlicensed spectrum even under low load conditions.The insights gained from this analysis lead the proposal of new channel access priority classes where new special classes are introduced for URLLC services. The results are provided demonstrating the improved performance of URLLC services in the unlicensed spectrum with the proposed changes in the channel access procedures.

Spectrum sharing has been recognized as a key component for 5G and beyond wireless networks. Recent trials have revealed the business value of spectrum sharing via the licensed shared access model, wherein both the incumbent and the licensee operators are protected from harmful interference by sharing the available spectrum using long-term spectrum access agreements. Further increase in capacity gains can be achieved by exploiting dynamic spectrum access, where access is granted in shorter time intervals, and the Spectrum Access System, which is a more flexible version of traditional LSA. This article first hints at LSA-type spectrum sharing in regulatory and standards bodies, then proposes a few relevant scenarios that would take advantage of LSA, and finally presents an LSA system architecture and some enabling technologies for LSA/DSA/SAS. These include novel sensing, dynamic and radioaware resource allocation, and advanced cooperative communication techniques.

In this paper we describe the simulation modeling of the OpenAirInterface (OAI) C-RAN testbed deployed at Eurecom and present the performance evaluation of a fast calibration (FC) scheme based on antenna grouping. We generate an indoor LOS radio wave propagation model using ray tracing technique and perform distributed channel reciprocity calibration which is required to exploit uplink (UL) channel estimates to infer the precoder performed on the downlink (DL) channel. We consider a channel based on the geometry of the area where the RRUs are distributed and compare the different choices to form the RRU groups. We validate that the simulation results match the experimental results from our previous work [1].

Modern cellular networks are becoming denser, less regularly planned, and increasingly heterogeneous, making performance analysis challenging. We develop a flexible and accurate model of such heterogeneous networks (HetNets) consisting of $K$ tiers of randomly located base stations (BSs), with different densities, transmit powers, and radio access technologies (RATs). Our main goal is to understand the impact of flow level dynamics on such a system, assuming non-saturated users that randomly generate download requests (“flows”). We do so by deriving analytically the per flow delay, the load, the utilization and the congestion probability of BSs in different tiers. We base our analysis on stochastic geometry, to understand the impact of topological randomness and intra- and inter-tier interaction, and queueing theory, to model the competition between concurrent flows within the same BS, for each RAT. This allows us to model the interference more realistically as a function of network load. We apply our model to the case of a 2-tier network based on LTE and Wi-Fi and study different user inter-tier association criteria, such as off-load, max-SINR association, and min-delay association. Our results provide some interesting qualitative and quantitative insights about the impact of these association policies and different traffic intensities.

In this paper we describe the OpenAirInterface (OAI) C-RAN testbed deployed at Eurecom and present the real-time implementation and performance evaluation of a channel calibration scheme. The distributed MIMO operation in dense Time Division Duplex (TDD) radio networks requires accurate time and frequency synchronization and calibration for precoding. We achieve this by using a common reference and over-the-air (OTA) synchronization between remote radio units (RRUs), which is a much cheaper alternative to distributed synchronization using PTPv2-like protocols and special clock regeneration circuitry. Furthermore we perform channel measurements between the RRUs without interrupting the real-time operation in order to perform distributed channel reciprocity calibration which is required to exploit uplink (UL) channel estimates to infer the precoder performed on the downlink (DL) channel.

5G will be supporting new services that have remarkably higher requirements than LTE 4G and Ultra-reliable and low-latency communication (URLLC) is one of those emerged categories. Although various techniques have been proposed to improve the data reliability, there has been a gap in how to improve the reliability of control/scheduling information pointing to the scheduled data. In this paper, we propose an intelligent combining of retransmissions of physical downlink control channel (PDCCH) and the physical downlink data channel (PDSCH). In the proposed scheme, the downlink control information (DCI) on PDCCH already indicates the location of a potential retransmission of the corresponding PDSCH. Moreover, the retransmitted DCI can be combined with the first transmission so that resource consumption and latency are reduced compared to the conventional scheme. Theoretical calculations and simulation results show a decrease of resource consumption.

Machine-to-machine (M2M) communication is one of the vertical sectors that will benefit from 5G communication systems, but today, these systems are still dominated by technologies such as ZigBee and WiFi. An M2M scenario will experience dense deployment of ZigBee and WiFi nodes in order to route the data from one end to the other. In the 2.4 GHz industrial, scientific, and medical (ISM) band, both of the technologies perform co-channel overlapped operation and hence face severe cross technology co-channel interference (CCI). In contrast to cellular systems, which solve the CCI by centralized coordination through the base station, addressing CCI in the ISM band is non-trivial due to heterogeneous wireless technologies and the lack of centralized coordination. In this work, we first present interference mitigating receiver architectures for OFDM-based WiFi using single and multiple antennas. Our single antenna work is based on the localized estimation of excess noise caused by single and multiple co-channel narrowband interferers and scaling the log-likelihood ratios (LLRs) of the affected WiFi subcarriers. The simulation shows our method achieves a significant gain in SNR compared to the conventional method for a given packet error rate (PER) criterion. Next, we discuss maximal ratio combiner with LLR scaling (MLSC), which is a multi-antenna extension to our previous work. The simulation shows MLSC achieves diversity gain apart from the gain in SNR. Further, we propose soft-bit maximal ratio combiner with LLR scaling (SB-MLSC). SB-MLSC is an easy to implement version of MLSC. However, diversity combining in SB-MLSC is performed by combining the LLRs. Nonetheless, simulations show equivalence in performance by SB-MLSC and MLSC. Finally, as a significant part of this work, we implemented all our methods using a software-defined radio (SDR) and performed over-the-air (OTA) testing in the 2.4-GHz ISM band using standard WiFi and ZigBee frames. Results of OTA tests fall in complete agreement with our simulations indicating the practical applicability of our methods. Our methods apply to all the standards and related radio transmission techniques which are based on OFDM and face narrowband co-channel interference. Additionally, since our work focuses only on receiver side modifications, they can be integrated with the existing infrastructure with minimal modifications.

To fulfill the exponential growth of mobile traffic demands, 3GPP has standardized the use of LTE in the 5 GHz unlicensed band in the Release 13 dedicated to LTE Licensed-Assisted Access (LTE-LAA). Simulations and field trials have shown that incumbent WiFi (802.11ac) in the 5 GHz unlicensed band will be the victim in such co-channel deployment with LTE-LAA. Hence a concern is being raised about fair co-existence. In this work, we investigate the limits of Successive Interference Cancellation (SIC) to recover the WiFi packets corrupted by LTE-LAA in the event of a collision. For an indoor deployment, which is the most probable battleground for WiFi and LTE-LAA, we propose a method to perform SIC by using stored clean channel estimates of stronger signals obtained during an interference-free period. Our approach leverages the slow fading channel and hence high coherence time of the indoor channel. For an experimental proof of this phenomenon, we simulated a scenario with interference between 20 MHz 802.11ac and 20 MHz LTE-LAA at different inter-frame-intervals (IFI) of LTE-LAA frames. Our results show that for an LTE-LAA IFI up to 2 milliseconds, the proposed method of using stored channel estimates to perform SIC can achieve significant transmit power gain (TPG) compared to the conventional way of using instantaneous channel estimates. Our method complements the conventional approach to ensure improved WiFi Frame Detection and Decoding while applying SIC. Moreover, our method does not require any coordination between WiFi and LTE-LAA transceivers and hence can be deployed within existing infrastructure with minimal modifications.

Narrowband transmitters such as IEEE 802.15.4 (ZigBee) operating in 2.4 GHz ISM band cause significant degradation of throughput to the co-channel IEEE 802.11g (WiFi). The phenomenon is common in smart factories and smart homes which generously use ZigBee sensors for process monitoring and automation. This interferes with omnipresent WiFi in the same 2.4 GHz ISM band. In our demonstration, we will present Software Defined Radio (SDR) based single antenna and multi-antenna prototype of standard-compliant IEEE 802.11g receiver. Our receiver prototype is capable of significantly reducing the packet error rate while facing multiple co-channel narrowband ZigBee interferers. We also demonstrate a real-time SDR implementation of Soft Bit Maximal Ratio Combiner capable of decoding frames from standard compliant IEEE 802.11g transmitters. This is a first of its kind implementation to the best of author's knowledge. The demonstrations use Ettus B210 as SDR hardware and a combination of signal processing modules from two different SDR packages: GNU Radio and Openairinterface.

The rapid increase in wireless devices and inherent limitation of RF spectrum is causing Co-Channel Interference (CCI). Effects of CCI are prevalent in Industrial Scientific Medical (ISM) bands which lack centralized control over devices operating on heterogeneous standards, a situation entirely different from cellular networks. In this study, we propose physical layer signal processing techniques for multi-antenna OFDM receivers in ISM band for mitigating narrowband CCI. Our work focuses only on receiver side modifications for interoperability with the existing infrastructure. We first analyze two such prominent multi-antenna interference mitigation methods: Optimal Combiner (OC) and Technology-Independent MIMO (TIMO). Optimal Combiner, although theoretically optimal, requires statistics of interferer which is difficult to obtain in practice. TIMO does not benefit from diversity gain despite having two antennas. We propose MLSC (Maximal Ratio Combiner with LLR Scaling) for multi-antenna OFDM receivers which mitigates CCI caused by narrowband interferers as well as benefits from diversity gain. For a given Packet Error Rate (PER), MLSC achieves comparable Transmit Power Gain (TPG) to OC without needing the statistics of the interferer. In addition, MLSC achieves significant TPG compared to TIMO. Further, we propose an improvement to TIMO: DC-TIMO (Diversity Combiner TIMO) which enables it to perform joint interference nulling and diversity combining.

The Fifth Generation of mobile communications (5G) is being standardized in order to reach higher data rates and deploy new services. In this frame, researchers are looking for possible waveforms to improve the air interface. Orthogonal Frequency Division Multiplexing (OFDM) has high Out-of-Band Emissions (OBE) which force us to leave wider guard bands, reducing so the spectral efficiency. Recently, we have proposed the Power Adapted-OFDM which is capable of fulfilling the requirements of 5G and avoids the main issues of other proposed candidates. OpenAir Interface (OAI) is a powerful and flexible wireless technology platform based on the Long-Term Evolution (LTE) ecosystem, which offers the possibility of evaluating the effects of introducing a new technology on the entire mobile communication system. In this paper, we will evaluate and show the performance of the proposed technique using the OAI.

Software Defined Radio platforms are theoretically capable of operating more than one wireless standard simultaneously. The significant challenges arise when the operating standards use overlapping frequency bands. Co-channel interference (CCI) is one of such challenges which have been in focus of the cellular communication community, but not in the Industrial, Scientific and Medical (ISM) radio bands where multiple heterogeneous wireless standards operate without a centralized coordination. Recognizing Successive Interference Cancellation (SIC) as one of the proven mechanisms to mitigate CCI, we propose methods to improve Packet Error Rate (PER) of wideband OFDM-based systems in the presence of multiple cochannel narrowband interferers. Our methods achieve improvements in PER statistics at lower received power level compared to conventional methods. Reduction in PER of the dominant OFDM signal makes more packets available for regeneration and hence cancellation during SIC procedure which aides the decoding of weaker signal. We also propose a simple yet efficient method to detect the presence and positioning of multiple narrowband interferers. Extensive Monte-Carlo simulations show that our methods elevate a receiver sensitivity gain up to 6 dB and are capable of detecting multiple narrowband interferers simultaneously. Our methods require modifications in the physical layer of the receiver only and hence can be integrated into existing infrastructure.

This letter presents an analysis of intercarrier interference (ICI) in high-speed trains. The ICI is caused by the motion of the receiver together with the carrier frequency offset (CFO). It results in a degradation of the reliability of the OFDM transmission. The analysis is based on a channel sounding measurement campaign for cellular broadband wireless communications with high-speed trains that was carried out in the context of the CORRIDOR project. The considered scenario corresponds to a railway deployed network, where the base station is located directly next to the railway line. We present the ICI power obtained with the measurement campaign and some theoretical results for a simple channel model that captures the main effects observed in the measurements.

In this paper we introduce the design of the “Rebot” (Relaying Robot) for future wireless networks. The Rebot concept is first of its kind in providing enhanced end-to-end LTE connectivity to ground users from a fixed base station via a flying relay which is enabled with an autonomous placement algorithm. The Rebot that we have built is a customized integrated UAV relay and its communication layer is based on OpenAirInterface. The ground user carries an off-the-shelf commercial LTE mobile terminal. We also present a placement algorithm that updates the UAV position in real time based on user location and wireless channel conditions so as to maximize the throughput at all times. The experimental results show throughput gains by using this UAV relay and also illustrate the learning/tracking behavior of the Rebot.

One of the biggest challenges in operating massive multiple-input multiple-output systems is the acquisition of accurate channel state information at the transmitter. To take up this challenge, time division duplex is more favorable thanks to its channel reciprocity between downlink and uplink. However, while the propagation channel over the air is reciprocal, the radio-frequency front-ends in the transceivers are not. Therefore, calibration is required to compensate the RF hardware asymmetry. Although various over-the-air calibration methods exist to address the above problem, this paper offers a unified representation of these algorithms, providing a higher level view on the calibration problem, and introduces innovations on calibration methods. We present a novel family of calibration methods, based on antenna grouping, which improve accuracy and speed up the calibration process compared to existing methods. We then provide the Cram\'er-Rao bound as the performance evaluation benchmark and compare maximum likelihood and least squares estimators. We also differentiate between coherent and non-coherent accumulation of calibration measurements, and point out that enabling non-coherent accumulation allows the training to be spread in time, minimizing impact to the data service. Overall, these results have special value in allowing to design reciprocity calibration techniques that are both accurate and resource-effective., Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible

Hybrid analog-digital (AD) beamforming structure is a very attractive solution to build low cost massive multiple-input multiple-output (MIMO) systems. Typically these systems use a set of fixed beams for transmission and reception to avoid the need to obtain channel state information at transmitter (CSIT) for each antenna element individually. However, such a method can not fully exploit the potential of hybrid AD beamforming systems. Alternatively, CSIT can be estimated by assuming a model for the propagation channel, whereas this model is only validated in millimeter-wave (mmWave) band thanks to its poor scattering nature. In this paper, we focus on time division duplex (TDD) systems with hybrid beamforming structure and propose a reciprocity calibration scheme that allows to acquire full CSIT. Different to existing CSIT acquisition methods, our approach does not require any assumption on the channel model and can, in theory, estimate the CSIT up to an arbitrary small error., Submitted to IEEE Journal of Selected Topics in Signal Processing

Although the base station processing and the majority of the protocols of the wireless systems are standardized by 3GPP, the receiver architecture and implementation remain vendor-dependent. The receiver design should fulfill computational efficiency requirements, and be evaluated in practical simulators or emulators that are compliant with real-world wireless standards. In this paper, we provide a comparative study of two low complexity Maximum-Likelihood receivers with the Parallel Interference-Aware (PIA) detection and with the Successive Interference Canceling (SIC) detection. Given 5MHz of LTE bandwidth, the SIC receiver achieves up to 4Mbit/s throughput gain in Rician fading and 1.8Mbit/s in Rayleigh fading compared to the PIA receiver. The quantification of computational effort demonstrates short signal processing time, which makes our receivers suitable for real-time modems. Our empirical simulations deliver insights on the optimal SNR regimes and the modulation and coding schemes combinations for both codewords to maintain high level of throughput.

International audience; This paper presents a low-cost measurement method for MIMO antenna performance assessment based on the open-source OpenAirInterface initiative. MISO 8×1 measurements are presented to experimentally prove the reproductibility of the test method, using a prototype with 8 antennas at 2.6 GHz integrated into a 140×140×40mm femto cell. The results shows that measurements are repeatable with a good accuracy. Beamforming performance are presented to finalize the validation of the setup.

International audience; We study the transmit time reversal beamforming in a 8x1 MISO communication system at 2.68GHz. We consider the downlink time reversal transmission where a BS communicates with one user. A prototype composed by 8 antennas and designed by Orange labs acts as the BS while the user has a single antenna. The reciprocity property is destroyed by the non-symmetric characteristics of the RF electronic circuitry. We use relative calibration which is based exclusively on signal processing techniques to solve this issue. Utilizing a controlled test setup based on OpenAirInterface, the ExpressMIMO2 SDR boards, as well as a servo controlled rail, we show the feasibility of a relative calibration method through beamforming SNR measurements. We also evaluate the performance of an antenna selection scheme at the transmit side as a low-cost low-complexity alternative to capture many of the advantages of multi-antenna systems. The measurements show that the relative calibration method is performing almost optimal and that the complexity can be significantly reduced by using antenna selection.

International audience; This paper presents a low-cost measurement method for MIMO antenna performance assessment based on the open-source OpenAirInterface initiative. A first measurement is presented with a prototype with 8 antennas at 2,6 GHZintegrated into a 140x140x40 mm fento cell and using Laser Direct Structuring (LDS) technique. The setup is validated through a beamforming gain measurement in a MISO 4x1 configuration in LTE TDD mode. We achieved the expected 6 dB beamforming gain between a SISO and 4×1 MISO configuration.

This editorial sums up relevant topics on the assessment of wireless communication systems covered by the especial issue entitled “Experimental Evaluation in Wireless Communications.” The topics include practical aspects on the implementation of distributed asynchronous non-linear kernel methods over wireless sensor networks; localization methods based on the exploitation of radio-frequency identification (RFID) wireless sensors and cellular networks or on sparsity approximations; channel sounding and assessment of broadband orthogonal frequency-division multiplexing (OFDM)-based wireless systems in high-speed vehicular communications; coexistence analysis of femtocell-based and outdoor-to-indoor systems; techniques for peak-to-average power ratio (PAPR) reduction; new solutions for baseband and radio frequency (RF) hardware impairments in full-duplex wireless systems; and, finally, suitability of interference alignment for broadband indoor wireless communications.

In this paper, we develop a flexible and accurate analytical model of large networks with random base station (BS) placement, in order to understand the impact of key network parameters like BS density and load on the network performance. The main goal is to understand the flow level dynamics of such a system, assuming non-saturated users and studying the congestion statistics for BSs and the per flow delay. To achieve this, we base our analysis on two main tools: (a)stochastic geometry, to understand the impact of topological randomness and coverage maps and (b) queueing theory, to model the competition between concurrent flows within the same BS. Our model is then applied the populars Radio Access Technologies (RATs), such as LTE and WIFi. Our results provide some interesting qualitative and quantitative insights about the performance of those networks.

Mobile networks will increasingly need to make use of heterogeneous access and spectrum opportunities to realize required capacity and quality of service. Moreover, aggregation of such resources will routinely be necessary, and there will be a clear need to develop a management architecture for that aggregation. Such an architecture should ascertain what can and should be aggregated by particular systems, networks, and terminals in view of better managing the collection of available resources on a heterogeneous system level, taking into account all systems', networks', and terminals' traffic requirements and technical capabilities. This article proposes such a management architecture and assesses its benefits, quantified by some particular examples.

Carrier aggregation (CA) is one of the key features of LTE-Advanced. On one hand it allows to boost the system capacity and on the other hand it enables dynamic spectrum access, such as licensed assisted access (LAA) and licensed shared access (LSA). Efficiently exploiting CA is however not a trivial task; the main challenges are scheduling, load balancing, dynamic carrier activation/deactivation, etc. Due to the complexity of these systems, experimentation is needed to evaluate and test these features. OpenAirInterface is a well know open-source software defined radio solution that allows for experimentation with LTE 4G systems. In this paper we are going present two extensions of the current version of OpenAirInterface: Carrier Aggregation and an application programming interface (API) for schedulers that allows a third party to plug-in their own scheduler. These two extensions allow for more control in the experimentation process than with commercial equipment.

We perform an analysis on the accuracy of the channel state information at the transmitter (CSIT) in a Massive MIMO time division duplex (TDD) system. In such a system, channel calibration is needed to compensate the hardware non-symmetry which breaks the TDD channel reciprocity. We consider the joint impact of calibration coefficients and uplink (UL) CSI on the calibrated downlink (DL) CSI accuracy. Moreover, we show the impact of these factors on both conjugate and zero-forcing (ZF) beamforming performance, which provides a useful tool to determine the accuracy level that TDD calibration coefficients should achieve. In this paper, we aim to answer the question: “How accurately should we calibrate a Massive MIMO TDD system to fully release its potential?”

In this work we study the capacity of multi-user multiple-input multiple-output (MU-MIMO) downlink channels with codebook-based limited feedback using real measurement data. Several aspects of MU-MIMO channels are evaluated. Firstly, we compare the sum rate of different MU-MIMO precoding schemes in various channel conditions. Secondly, we study the effect of different codebooks on the performance of limited feedback MU-MIMO. Thirdly, we relate the required feedback rate with the achievable rate on the downlink channel. Real multi-user channel measurement data acquired with the Eurecom MIMO OpenAir Sounder (EMOS) is used. To the best of our knowledge, these are the first measurement results giving evidence of how MU-MIMO precoding schemes depend on the precoding scheme, channel characteristics, user separation, and codebook. For example, we show that having a large user separation as well as codebooks adapted to the second order statistics of the channel gives a sum rate close to the theoretical limit. A small user separation due to bad scheduling or a poorly adapted codebook on the other hand can impair the gain brought by MU-MIMO. The tools and the analysis presented in this paper allow the system designer to trade-off downlink rate with feedback rate by carefully choosing the codebook.

For time division duplexing (TDD) systems, the physical channel in the air is reciprocal for uplink (UL) and downlink (DL) within the channel coherence time. However when the transceivers' radio frequency (RF) hardware is taken into consideration, TDD channel reciprocity no longer holds because of the non-symmetric characteristics of RF transmit and receive chains. Relative calibration has been proposed to compensate this hardware impairment with a multiplicative matrix. In this paper we perform hardware measurements on this calibration matrix which gives a direct insight on the physical phenomenon of TDD transceivers. Especially, we inspect the assumption that this calibration matrix is diagonal, which is widely adopted in literature but has never been verified by experiments. This work can be regarded as an experimental base for TDD calibration or for theoretical analysis of non-perfect channel reciprocity of TDD systems.

Ray tracing (RT) is an accurate propagation prediction tool that has been widely used to simulate channel characteristics in indoor environments. To date, the developed RT tool includes not only specular reflection, penetration through dielectric blocks and diffraction, but also diffuse scattering mechanisms. The accuracy, provided by a detailed modeling of the environment, comes at the cost of a high computational complexity, which directly scales with the number of propagation paths considered. We are interested in simulating the radio propagation conditions for a mobile terminal, communicating in a frame based communication system indoors with several fixed nodes. This communication shall be used to obtain the position of the mobile terminal in indoor scenario. Therefore, the correlated temporal and spatial evolution of the channel impulse response is of utmost concern. In this paper, we propose a method to significantly reduce the computational complexity of RT by using a projection of all propagation paths on a subspace spanned by two-dimensional discrete prolate spheroidal (DPS) sequences. With this method the computational complexity can be reduced by more than one order of magnitude for indoor scenarios. The accuracy of our low-complexity DPS subspace based RT algorithm is verified by numeric simulations.

Cognitive radio, which enables smart use of wireless resources, is a key ingredient to achieve high spectral efficiency. LTE, the latest evolution of cellular standards, is widely adopted and also targets high spectral efficiency. Hence, to enable wide adoption of cognitive radio, using LTE as the physical layer is a natural choice. Targeting a real-time implementation of LTEbased cognitive radio, we focus on spatial interweave cognitive radio, in which a secondary user uses an antenna array to perform null-beamforming in the primary user's direction, hence reusing the spectrum spatially. To allow this, without any help from the primary system, we use the time-division duplex mode and take advantage of the channel reciprocity. However, this reciprocity is jeopardized by the mismatch between the RF front-ends. Hence, we design a calibration protocol to restore it. A key contribution is that this cognitive system calibration does not require cooperation from the primary user. The whole system is implemented and evaluated on EURECOM's experimental OpenAirInterface platform. Performance results are presented, showing the feasibility of spatial interweave cognitive radio on a real-time platform.

We present in this paper a software implementation of a cognitive radio (CR) communication based on long term evolution-time division duplex (LTE-TDD) channel reciprocity. We study the problem of calibration and beamforming design for a CR system in which a multiple-input single-output (MISO) secondary link, wants to opportunistically communicate without harming the primary SISO system. Specifically, we evaluate the CR communication through the EURECOM's experimental OpenAirInterface (OAI) software platform. We will show that it is feasible to restore the reciprocity after calibration in a non reciprocal channel, and provide an overview of challenges in the channel estimation for real conditions.

This demonstration1 highlights the capabilities of the OpenAirInterface platform in terms of Agile RF spectrum-access and rapidly-deployable networking technologies. With respect to application scenarios, the latter target broadband public-safety communications. We demonstrate the real-time baseband and RF capabilities of the platform components as well as the methodologies used for large-scale system emulation on PC-based computing platforms.

A relatively recent idea of extending the benefits of MIMO systems to multiuser scenarios seems promising in the context of achieving high data rates envisioned for future cellular standards after 3G (3rd Generation). Although substantial research has been done on the theoretical front, recent focus is on making Multiuser Multiple-Input Multiple-Output (MUMIMO) practically realizable. This paper presents an overview of the different MU-MIMO schemes included/being studied in 3GPP standardization from LTE (long-term evolution) to LTE Advanced. MU-MIMO system concepts and implementation aspects have been studied here. Various low-complexity receiver architectures are investigated, and their performance assessed through link-level simulations. Appealing performance offered by low-complexity interference aware (IA) receivers is notably emphasized. Furthermore, system level simulations for LTE Release 8 are provided. Interestingly, it is shown that MU-MIMO only offers marginal performance gains with respect to single-user MIMO. This arises from the limited MU-MIMO features included in Release 8 and calls for improved schemes for the upcoming releases.

Dynamic spectrum access supported by a low-latency wireless sensor network for spectrum usage monitoring is demonstrated using OpenAirInterface platform. The platform performs secondary communications in bands detected as free, thanks to its multi-band capability. With respect to application scenarios, the main target is to address agile broadband public safety communications. Firstly, this demonstration will highlight the newest hardware platforms offered by OpenAirInterface comprising the ExpressMIMO baseband engine and the AgileRF front-end. The second aspect of the demonstration aims to highlight OpenAirInterface's performance evaluation methodology allowing for scalable emulated real-time deployment of radio networks on generic PC-based computers.

International audience; Wireless communication standards are developed at an ever-increasing rate of pace, and significant amounts of effort is put into research for new communication methods and concepts. On the physical layer, such topics include MIMO, cooperative communication, and error control coding, whereas research on the medium access layer includes link control, network topology, and cognitive radio. At the same time, imple- mentations are moving from traditional fixed hardware archi- tectures towards software, allowing more efficient development. Today, field-programmable gate arrays (FPGAs) and regular desktop computers are fast enough to handle complete baseband processing chains, and there are several platforms, both open- source and commercial, providing such solutions. The aims of this paper is to give an overview of five of the available platforms and their characteristics, and compare the features and performance measures of the different systems.

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. However, in this paper we show that this is not always true. We compare the capacity and the correlation of measured MU-MIMO channels for both outdoor and indoor scenarios. 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 in most scenarios MU-MIMO provides a higher throughput than SU-MIMO also in the measured channels. However, in outdoor scenarios with a line of sight, the capacity drops significantly when the users are close together, due to high correlation at the transmitter side of the channel. In such a case, the performance of SU-MIMO and MU-MIMO is comparable.

In multi-user multiple-input multiple-output (MU-MIMO) systems, channel state information at the transmitter (CSIT) allows for multi-user spatial multiplexing and thus increases the system throughput. We assume that CSIT is obtained by means of a finite-rate feedback channel through channel vector quantization (CVQ) at the receiver. In this paper we use real channel measurements to study the effect of CVQ on the sum rate of a MU-MIMO system employing linear precoding. The measurement data has been acquired using Eurecom's MIMO Openair Sounder (EMOS). The EMOS can perform realtime MIMO channel measurements synchronously over multiple users. We consider CVQ using a Fourier codebook, a random codebook and a random codebook exploiting the second order statistics of the channel. For comparison, we also show the capacity of a single-user system using time division multiple access (TDMA) with no CSIT at all. The results show that the Fourier codebook shows very poor performance in the measured channels. Random codebooks - although suboptimal - provide a much better performance in the measured channels.