Browsing by Author "Samano-Robles, Ramiro"
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- A Multi-Ray Analysis of LOS V2V Links for Multiple Antennas with Ground ReflectionPublication . Farzamiyan, Amir Hossein; Gutiérrez Gaitán, Miguel; Samano-Robles, RamiroThis paper presents a vehicle-to-vehicle (V2V) geometric multi-ray tracing model for an improved line-of-sight (LOS) estimation. The model is especially suited for distributed antenna transceivers in the presence of ground reflections. The multiple antennas are assumed to be spaced regularly in horizontal and vertical directions over contiguous vehicles. The main focus of our study is the ability of the multiple antenna system to counteract or exploit, respectively, the destructive or constructive interference of multiple rays in the LOS channel component. This work is a complement to existing V2V channel models by providing more details on how ground reflections affect the LOS channel. The analysis is initially framed in the context of MIMO (multiple-input multiple-output) systems to investigate general aspects such as capacity limits and singular value distribution. The work then focuses on a scheme with single symbol repetition across the transmit antennas and two different strategies for signal combining at the receiver: maximum-ratio and equal-gain combining (MRC and EGC, respectively). These solutions are compared with a full diversity solution as well as with the information theoretical limits. An adaptive antenna selection mechanism is finally proposed that outperforms all other solutions. The paper shows both vertical and horizontal polarization results with corrected complex Fresnel reflection coefficients for lossy materials. Moreover, it is shown that multiple antenna design in V2V systems can be useful to counteract the destructive interference created by multiple rays on the LOS channel component.
- Network Diversity Multiple Access with Imperfect Channel State Information at the Transmitter SidePublication . Samano-Robles, RamiroNetwork diversity multiple access (NDMA) is the family of algorithms with the highest potential throughput in the literature of signal-processing assisted random access protocols. NDMA uses the concept of protocol-induced retransmissions to create an adaptive source of diversity. This diversity is used to resolve packet collisions employing signal separation tools without the explicit need (or as a complement) of a multiple antenna receiver. This paper proposes a further improvement on the performance of NDMA by allowing each terminal access to an outdated copy of its individual channel state information (CSI). Based on this decentralized CSI, each terminal conveniently decides to transmit only if the estimated channel gain surpasses a threshold that is optimized to maximize performance. This ensures that the probability of terminal presence detection, and thus the probability of correct estimation of the collision multiplicity are considerably improved at the receiver end. The paper is focused on the modelling of the receiver operational characteristic (ROC) of the terminal presence detector considering that the CSI used by each terminal is potentially inaccurate (outdated) due to feedback delay. The results indicate that when the correlation coefficient that describes the accuracy of the available CSI tends to zero, the scheme degrades into the conventional NDMA. By contrast, when the quality of the channel state information improves, the throughput can nearly achieve the nominal channel rate (minimum throughput penalty). The selection of the detector thresholds for channel gain and terminal presence is optimized to maximize system performance.
- On the central Chi-square distribution with even degrees of freedom and correlated multivariate complex componentsPublication . Samano-Robles, RamiroThis paper presents the derivation new expressions for the statistics of a Chi-square distribution with $n$ degrees of freedom and where n is an even number. The complex Gaussian components of the chi-square distribution are modelled with a linear correlated model using different statistics (multi-rate) for each component. We focus on the specific expressions for the probability density function (PDF) and complementary cumulative density function (CCDF). Unlike previous approaches, we use a frequency domain interpretation that allows us to derive a closed form expression for the characteristic function (CF) as an inverse polynomial equation. Using the roots of this polynomial equation, it is possible to decompose the CF as a partial fraction expansion (PFE). This allows us to obtain a simple expression for both the PDF and CCDF by simply using the inverse Fourier transform of PFE decomposition of the CF. The statistics derived here have a much lower complexity than the expressions obtained from conventional non-frequency domain methods at the expense of the complexity of the polynomial root solution scheme. In scenarios where the average statistics of the components do not change over some periods of time, the proposed expressions provide the lowest possible complexity, as the polynomial rooting process needs to be conducted only once and potentially offline.
- Orthogonal Space-Time Block Coding for V2V LOS Links with Ground ReflectionsPublication . Gaitán, Miguel Gutiérrez; Samano-Robles, RamiroThis work presents a capacity analysis of Space-Time Block Codes (STBC) for Vehicle-to-Vehicle (V2V) communication in Line-of-Sight (LOS). The aim is to assess how this type of coding performs when the V2V LOS channel is influenced by ground reflections. STBCs of various coding rates are evaluated using antenna elements distributed over the surface of two contiguous vehicles. A multi-ray tracing tool is used to model the multiple constructive/destructive interference patterns of the transmitted/received signals by all pairs of Tx-Rx antenna links. Simulation results show that STBCs are capable of counteracting fades produced by the destructive self-interference components across a range of inter-vehicle distances. Notably, the effectiveness in deep fades is shown to outperform schemes with exclusive receive diversity. Higher-order STBCs with rate losses are also evaluated, showing interesting gains even for low coding rate performance, particularly, when accompanied by a multiple antenna receiver. Overall, these results can shed light on how to exploit transmit diversity in slow fading vehicular channels.
- Outage Probability of V2V Multiple-Antenna Rice Fading Links with Explicit Ground ReflectionPublication . Gutiérrez Gaitán, Miguel; Samano-Robles, Ramiro; Rodriguez, JonathanThis paper investigates the improvement in terms of outage probability of a vehicle-to-vehicle (V2V) communication link with respect to the density of antennas used at each vehicle end. The objective is to find a trade-off between system complexity and communication performance considering that the deterministic component of the link is affected explicitly by multiple ground reflections (self-interference). The antennas are assumed to be located at regularly distributed positions across the surface of contiguous vehicles. Part of the work assumes symbol repetition at the transmitter side, and different signal combining mechanisms at the receiver side, namely, maximum-ratio and equal-gain combining (MRC and EGC, respectively). The objective is to minimize outage probability of the link with deterministic and stochastic channel components (Rice-distributed), where the line-of-sight (LOS) is affected by multi-ray ground reflections as an extension of the well-known two-ray model. This scenario is considered more realistic for V2V scenarios due to the potential proximity of ground to the antenna elements. The outage probability is calculated over a range of inter-vehicle distances with respect to the free-space loss solution. The results show that performance is improved even for a relatively small number of antennas and that a critical point is reached beyond which improvement is only differential. This suggests that an optimum trade-off can be obtained to ensure a value of outage probability with a complexity constraint over a range of inter-vehicle distances.
- Performance Analysis of Network Diversity Multiple Access with Sequential Terminal Detection and Non-Orthogonal Training SequencesPublication . Samano-Robles, RamiroThis paper presents a new approach for terminal presence detection in the family of algorithms called network diversity multiple access (NDMA). The new scheme is based on non-orthogonal sequences. In NDMA, system-induced retransmissions are used to resolve the conflicts between colliding terminals. The key initial aspect in NDMA is to use signal processing tools to identify the size of the collision, as well as the identity of the contending terminals. This information is used to calculate the number of required retransmissions. These retransmissions are stored in memory, thereby creating a virtual MIMO (multiple input multiple-output) system that can be used to resolve the collision via source separation or multi-user detection. These detection and source separation processes are based on a set of orthogonal training sequences, each sequence uniquely assigned to one terminal in the network. This paper proposes a new approach for presence detection in NDMA using non-orthogonal sequences. The number of available sequences is increased and the bandwidth used for training is therefore considerably reduced. This comes at the expense of multiple access interference (MAI) between contending terminals. Additionally, in NDMA the estimation of the collision multiplicity is conventionally achieved in the first time-slot of the collision resolution period. This paper extends the detector to include all the received copies of the original transmissions (the initial transmission and also subsequent retransmissions). This means that after each retransmission received by the access point, the estimation of the collision multiplicity and contending terminals identification must be updated. The analysis here presented includes the effects of MAI caused by non orthogonal training sequences and the effect of sequential collision multiplicity estimation. Results suggest a considerably decrease of performance with respect to the orthogonal case scenario, but a more flexible training sequence allocation that becomes relevant for large numbers of terminals.
- A Space-Time Correlation Model for MRC Receivers in Rayleigh Fading ChannelsPublication . Samano-Robles, RamiroThis paper presents a statistical model for maximum ratio combining (MRC) receivers in Rayleigh fading channels enabled with a temporal combining process. This means that the receiver effectively combines spatial and temporal branch components. Therefore, the signals that will be processed by the MRC receiver are collected not only across different antennas (space), \mbox{but also} at different instants of time. This suggests the use of a retransmission, repetition or space-time coding algorithm that forces the receiver to store signals in memory at different instants of time. Eventually, these stored signals are combined after a predefined or dynamically optimized number of time-slots or retransmissions. The model includes temporal correlation features in addition to the space correlation between the signals of the different components or branches of the MRC receiver. The derivation uses a frequency domain approach (using the characteristic function of the random variables) to obtain closed-form expressions of the statistics of the post-processing signal-to-noise ratio (SNR) under the assumption of equivalent correlation in time and equivalent correlation in space. The described methodology paves the way for the reformulation of other statistical functions as a frequency-domain polynomial root analysis problem. This is opposed to the infinite series approach that is used in the conventional methodology using directly the probability density function (PDF). The results suggest that temporal diversity is a good complement to receivers with limited spatial diversity capabilities. It is also shown that this additional operation could be maximized when the temporal diversity is adaptive (i.e., activated by thresholds of SNR), thus leading to a better resource utilization.
- The DEWI high-level architecture: Wireless sensor networks in industrial applicationsPublication . Samano-Robles, Ramiro; Nordstrom, Tomas; Santonja, Salvador; Rom, Werner; Tovar, EduardoThis paper presents the high-level architecture (HLA) of the research project DEWI (dependable embedded wireless infrastructure). The objective of this HLA is to serve as a reference for the development of industrial wireless sensor and actuator networks (WSANs) based on the concept of the DEWI Bubble. The DEWI Bubble is defined here as a high-level abstraction of an industrial WSAN with enhanced interoperability (via standardized interfaces), technology reusability, and cross-domain development. This paper details the design criteria used to define the HLA and the organization of the infrastructure internal and external to the DEWI Bubble. The description includes the different perspectives, models or views of the architecture: the entity model, the layered model, and the functional view model (including an overview of interfaces). The HLA constitutes an extension of the ISO/IEC SNRA (sensor network reference architecture) towards the support of industrial applications. To improve interoperability with existing approaches the DEWI HLA also reuses some features from other standardized technologies and architectures. The HLA will allow networks with different industrial sensor technologies to exchange information between them or with external clients via standard interfaces, thus providing a consolidated access to sensor information of different domains. This is an important aspect for smart city applications, Big Data and internet-of-things (IoT).
- The DEWI High-Level Architecture: Wireless Sensor Networks in Industrial ApplicationsPublication . Samano-Robles, Ramiro; Nordström, Tomas; Kunert, Kristina; Santonja, Salvador; Himanka, Mikko; Liuska, Markus; Karner, Michael; Tovar, EduardoThis paper presents the High-Level Architecture (HLA) of the research project DEWI (Dependable Embedded Wireless Infrastructure). The objective of this HLA is to serve as a reference for the development of industrial Wireless Sensor and Actuator Networks (WSANs) based on the concept of the DEWI Bubble. The DEWI Bubble is a concept that can be used to integrate legacy industrial sensor networks with a modern, interoperable, and flexible IoT (Internet-of-Things) infrastructure. The DEWI Bubble is defined as a high-level abstraction of an industrial WSAN with enhanced interoperability (via standardized interfaces), dependability, technology reusability, and cross-domain development. The DEWI Bubble aims to resolve the issue of how to improve commercial WSAN technology to match the dependability, interoperability and high criticality needs of industrial domains. This paper details the criteria used to design the HLA and the organization of the infrastructure internal and external to the DEWI Bubble. The description includes the different perspectives, models or views of the architecture: the entity model, the layered model, and the functional model (including an overview of interfaces). The HLA constitutes an extension of the ISO/IEC 29182 SNRA (Sensor Network Reference Architecture) towards the support of wireless industrial applications in different domains: aeronautics, automotive, railway and building. To improve interoperability with existing approaches, the DEWI HLA also reuses some features from other standardized technologies and architectures. The DEWI HLA and the concept of Bubble allow networks with different industrial sensor technologies to exchange information between them or with external clients via standard interfaces, thus providing a consolidated access to sensor information of different industrial domains. This is an important aspect for smart city applications, Big Data, Industry 4.0, and the Internet-of-Things (IoT). The paper includes a non-exhaustive review of the state of the art of the different interfaces, protocols and standards of this architecture. The HLA has also been proposed as the basis of the European project SCOTT (Secure Connected Trustable Things) for security and privacy in the IoT.
- Ultra-Reliable Low Latency based on Retransmission and Spatial Diversity in slowly fading channels with co-channel interferencePublication . Samano-Robles, RamiroThis paper presents the analysis of the statistics of latency and information theoretic capacity of an adaptive link with retransmission-spatial diversity in a scenario with co-channel interference. The paper focuses specifically on the delay of the wireless transmission component, measured from the instant a packet at the head of the queue is first transmitted until it is correctly received by the destination (considering retransmissions). The objective is to evaluate the ability of temporal and spatial diversity tools to achieve ultra-low values of latency as desired in future 5G and machine-to-machine (M2M) networks with real-time requirements. It is assumed that the source transmits information towards the destination in a Rayleigh fading spatially correlated channel. In case the instantaneous signal-to-interference-plus-noise (SINR) ratio has not surpassed a predetermined reception threshold, then the source engages in a persistent retransmission protocol. All the copies of the original transmission and subsequent retransmissions are stored in memory and processed at the destination using maximum ratio combining (MRC) to obtain a more reliable copy of the signal (a scheme also called retransmission diversity). The retransmission scheme stops once the instantaneous post-processing SINR achieves the desired target threshold. This persistent retransmission scheme can also be regarded as a security mechanism against interference jamming attacks. Since retransmissions are assumed to take place in a short time interval in order to achieve very low values of latency, they are modelled with statistical temporal correlation, which is explicitly introduced in the embedded Gaussian channel distribution model. Results suggest that retransmission diversity can provide good latency results in moderate to high values of SINR. However, at low SINR, a combination with other diversity sources will be necessary to achieve the desired target value.