Sara Teodoro

Virtual MIMO schemes for downlink space-frequency coding OFDM systems

In this paper we investigate the performance of single-relay cooperative scheme, where the source is equipped with a 2-antenna array and both the relay and destination with a single antenna. We consider Amplify-and-Forward, Decode-and-Forward and Selective Decode-and-Forward relaying protocols and a space-frequency block coding designed for DL OFDM based systems is employed. Signals expressions are derived for each relay protocol under various scenarios, considering power constraint imposed on cooperating nodes. The aim of this work is to evaluate this Virtual MIMO scheme in realistic scenarios (e.g. correlated antennas, precise channel models, Doppler effects) for implementation purposes in a system with LTE based parameters and considering channel turbo coding. The coded performance of the proposed Virtual MIMO scheme is evaluated, and compared against non-cooperative OFDM based systems, for typical pedestrian scenarios based on LTE specifications. The proposed cooperative scheme outperforms the co-located MISO and MIMO for some scenarios.

Iterative Frequency-Domain Detection for IA-Precoded MC-CDMA Systems

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative frequency-domain detection receivers based on the IB-DFE concept (Iterative Block Decision Feedback Equalization) can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this paper we combine iterative IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The receiver is designed in two steps: first a linear filter is used to mitigate the inter-user aligned interference, and then an iterative frequency-domain receiver is designed to efficiently separate the spatial streams in the presence of residual inter-user aligned interference at the output of the filter. The matrices for this non-linear space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our receiver structure is explicitly designed taking into account the residual inter-user interference, allowing both an efficient separation of the spatial streams and a reduction in the number of iterations of the IA procedure. We also propose a simple, yet accurate analytical approach for obtaining the performance of the proposed receiver structure. Our scheme achieves the maximum degrees of freedom provided by the IA precoding, while allowing an almost optimum space-diversity gain, with performance close to the matched filter bound (MFB).

Virtual reality simulator of transurethral resection of the prostate

In this work we present the current state of development of a virtual reality surgery simulation system for training Transurethral Resection of the Prostate (TURP). The prototype consist on a virtual environment of part of the urinary system which includes 3D models of the bladder, the prostate and the pennile urethra. The system simulates in real-time the endoscopic view of the resectoscope, allows realistic virtual navigation and exploration inside the anatomic structures of the virtual patient. The simulator includes a deformable tissue model of the prostate in order to simulate tissue resection and deformation. The interaction between the trainee and the system is done with a mechatronic interface that emulates a real resectosope and allows to perform the most important movements of the surgical tool during a TURP. We are currently working in incorporating tactile feedback to the interface by adapting an haptic robot device to the mechanism.

Performance Evaluation of Virtual MIMO Schemes for the UL OFDMA Based Systems

In this paper we propose and assess the performance of virtual MIMO or relay-assisted cooperative schemes designed for the UL OFDMA based systems. We consider the use of an antenna array at the base station and a single antenna at both the mobile terminal and relay. The proposed cooperative schemes emulate a MIMO channel with 2 transmit and M receive antennas. Two types of relays are considered: amplify-and-forward and selective decode-and-forward. We derive the instantaneous normalized capacities and both the outage capacities and bit error rate are compared against the non-cooperative SISO and SIMO systems, considering different scenarios. The proposed relay-assisted schemes are suited for existing and for the future broadband wireless systems to increase the system capacity and coverage.

Low-bit rate feedback strategies for iterative IA-precoded MIMO-OFDM-based systems.

Interference alignment (IA) is a promising technique that allows high-capacity gains in interference channels, but which requires the knowledge of the channel state information (CSI) for all the system links. We design low-complexity and low-bit rate feedback strategies where a quantized version of some CSI parameters is fed back from the user terminal (UT) to the base station (BS), which shares it with the other BSs through a limited-capacity backhaul network. This information is then used by BSs to perform the overall IA design. With the proposed strategies, we only need to send part of the CSI information, and this can even be sent only once for a set of data blocks transmitted over time-varying channels. These strategies are applied to iterative MMSE-based IA techniques for the downlink of broadband wireless OFDM systems with limited feedback. A new robust iterative IA technique, where channel quantization errors are taken into account in IA design, is also proposed and evaluated. With our proposed strategies, we need a small number of quantization bits to transmit and share the CSI, when comparing with the techniques used in previous works, while allowing performance close to the one obtained with perfect channel knowledge.

Distributed Space-Time Code Using Precoding for Cellular Systems

We propose a data precoded relay-assisted scheme, for a system cooperating with 2 relays. We assume that all the terminals are equipped with a single antenna. The use of relays is of significant interest to allow radio access in situations where a direct path is not available. However because of the half duplex constraint at the relays, transmission of a data rate equivalent to that of a modulation technique with m bits per symbol in the case a continuous link would be available from the BS to the UT requires the use of a constellation with 2m bits per symbol. This implies a penalty in power efficiency. The proposed simple precoding scheme exploits the relation between QPSK and 16-QAM, by alternately transmitting through the 2 relays, keeping the diversity of order 2. Numerical results show significant performance improvements relatively to the case of a relay-based distributed Alamouti employing 16-QAM, and a performance very close to the non-cooperative system employing 2x1 QPSK Alamouti coding with a continuous link available, when all the links have the same SNR.

Combined distributed turbo coding and space frequency block coding techniques

The distributed space-time (frequency) coding and distributed channel turbo coding used independently represent two cooperative techniques that can provide increased throughput and spectral efficiency at an imposed maximum Bit Error Rate (BER) and delay required from the new generation of cellular networks. This paper proposes two cooperative algorithms that employ jointly the two types of techniques, analyzes their BER and spectral efficiency performances versus the qualities of the channels involved, and presents some conclusions regarding the adaptive employment of these algorithms.

Distributed space-frequency block coding for a 2-antenna relay in downlink OFDM systems

In this paper we evaluate the coded and uncoded performance of a 2-antenna terminal relaying schem, for downlink OFDM based systems, using efficient space-frequency block coding protocols. We consider that the relay and the base station are equipped with 2 antennas and the user terminal has a single antenna. Two types of relay protocols are accounted for: Equalize-and-Forward and Decode-and-Forward. Signals expressions are derived and the bit error rates obtained for a pratical and preeminent scenario, with LTE parameters and using convolutional channel coding. The performance of the proposed relay-assited scheme is evaluated under realistic scenarios, considering typical pedestrian scenarios based on LTE specifications. Results are compared to those of other relay-assisted design approaches that have been recently proposed for the case of 1-antenna relay and the non-cooperative systems. For this comparison, we implement three propagation scenarios, with different SNR transmission conditions among links. Simulation results show that the availability of a bi-antenna array at the relay significantly improves the cooperative system performance, but improvements are highly dependent on the scenarios and relay protocol as well as of the cooperative second-hop quality.

Iterative interference alignment techniques for broadband wireless systems with limited feedback

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. In this paper we consider iterative IA techniques for the downlink of OFDM-based (Orthogonal Frequency Division Multiplexing) broadband wireless systems with limited feedback. A quantized version of the channel state information (CSI) associated to the different links between base station (BS) and user terminal (UT) is feedback from the UT to the BS, which sends it to the other BSs through a limited-capacity backhaul network. This information is employed by each BS to perform the overall IA design. Our channel quantization method requires much less complexity than random vector quantization based techniques and requires the quantization of a fraction of the channel frequency response samples. The results have shown that a small number of quantization bits per multipath component is enough to allow performance close to one obtained with perfect channel knowledge.

Data-precoded algorithm for multiple-relay-assisted systems

A data-precoded relay-assisted (RA) scheme is proposed for a system cooperating with multiple relay nodes (RNs), each equipped with either a single-antenna or a two-antenna array. The classical RA systems using distributed space-time/frequency coding algorithms, because of the half-duplex constraint at the relays, require the use of a higher order constellation than in the case of a continuous link transmission from the base station to the user terminal. This implies a penalty in the power efficiency. The proposed precoding algorithm exploits the relation between QPSK and 4 L -QAM, by alternately transmitting through L relays, achieving full diversity, while significantly reducing power penalty. This algorithm explores the situations where a direct path (DP) is not available or has poor quality, and it is a promising solution to extend coverage or increase system capacity. We present the analytical derivation of the gain obtained with the data-precoded algorithm in comparison with distributed space-frequency block code (SFBC) ones. Furthermore, analysis of the pairwise error probability of the proposed algorithm is derived and confirmed with numerical results. We evaluate the performance of the proposed scheme and compare it relatively to the equivalent distributed SFBC scheme employing 16-QAM and non-cooperative schemes, for several link quality scenarios and scheme configurations, highlighting the advantages of the proposed scheme.