Paulo Montezuma

Iterative Frequency-Domain Equalization for general constellations

IB-DFE (Iterative Block DFE) is a promising turbo equalization technique for SC-FDE schemes (Single-Carrier with Frequency Domain Equalization). However, typical IB-DFE implementations are tailored for a specific constellation (usually QPSK). In this paper we propose a general method for designing IB-DFE receivers for any constellation. Our approach relies on an analytical characterization of the mapping rule were the constellation symbols are written as a linear function of the transmitted bits. This method is then employed in both uniform and non-uniform QAM constellations.

Frequency-domain multipacket detection: a high throughput technique for SC-FDE systems

The traditional approach to cope with collisions is to discard the packets involved in it and to ask for their retransmission. However, since the signal associated to a collision has important information concerning the packets involved, we can efficiently resolve collisions with proper retransmissions. In this paper we consider severely time-dispersive channels and we propose a technique that allows efficient packet separation. We employ SC-FDE schemes (single-carrier with frequency-domain equalization) and we propose an iterative frequency-domain multi-packet detection. Since our technique requires uncorrelated channels for different retransmissions, we also propose an SP technique (Shifted Packets) for retransmissions in fixed channels. Since the total number of transmissions is equal to the number of packets involved in the collision (even when the channel remains fixed for the retransmissions), our technique allows high throughputs. The analysis of our detection technique combined with the NMDA (network-assisted diversity multiple access) MAC (medium access control) protocol shows significant throughput and delay improvements for low Eb/N0 values compared to a TDMA (time division multiple access) approach, where collisions are avoided. This technique is particularly appealing for the uplink of broadband wireless systems, since we consider SC-FDE schemes and the complexity is concentrated in the receiver. By employing the SP scheme we can use the same channel for the retransmissions, with only a small performance degradation.

Optimum and Sub-Optimum Receivers for OFDM Signals with Strong Nonlinear Distortion Effects

In this paper we consider the optimum detection of OFDM (Orthogonal Frequency Division Multiplexing) signals with strong nonlinear distortion effects. It is shown that the optimum performance with strong nonlinear distortion effects is not as bad as one might expect and can even be better than the performance with conventional, linear transmitters. To achieve these excellent performances we should employ receivers able to take advantage of the information associated to transmitted data symbols that is inherent to the nonlinear distortion component, in opposition to traditional OFDM implementations where nonlinear distortion effects are regarded as an undesirable noise-like component. We study the achievable gains of the optimum receiver both analytically and by simulation. Since the complexity of optimum receivers is extremely high when we have nonlinear distortion effects, even for OFDM signals with a small number of subcarriers, we propose several sub-optimum receivers and evaluate their performance. Our sub-optimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

A Closed-Form Solution for RSS/AoA Target Localization by Spherical Coordinates Conversion

This letter addresses the problem of target localization in a 3-D space, utilizing combined measurements of received signal strength and angle of arrival (AoA). By using the spherical coordinate conversion and available AoA observations to establish new relationships between the measurements and the unknown target location, we derive a simple closed-form solution method. We then show that the proposed method has straightforward adaptation to the case where the targets transmit power is also not known. Simulation results validate the outstanding performance of the proposed method.

Distributed algorithm for target localization in wireless sensor networks using RSS and AoA measurements

This paper addresses target localization problem in a cooperative 3-D wireless sensor network (WSN). We employ a hybrid system that fuses distance and angle measurements, extracted from the received signal strength (RSS) and angle-of-arrival (AoA) information, respectively. Based on range measurement model and simple geometry, we derive a novel non-convex estimator based on the least squares (LS) criterion. The derived non-convex estimator tightly approximates the maximum likelihood (ML) one for small noise levels. We show that the developed non-convex estimator is suitable for distributed implementation, and that it can be transformed into a convex one by applying a second-order cone programming (SOCP) relaxation technique. We also show that the developed non-convex estimator can be transformed into a generalized trust region sub-problem (GTRS) framework, by following the squared range (SR) approach. The proposed SOCP algorithm for known transmit powers is then generalized to the case where the transmit powers are different and not known. Furthermore, we provide a detailed analysis of the computational complexity of the proposed algorithms. Our simulation results show that the new estimators have excellent performance in terms of the estimation accuracy and convergence, and they confirm the effectiveness of combining two radio measurements.

Approaching the Maximum Likelihood Performance with Nonlinearly Distorted OFDM Signals

The high envelope fluctuations of OFDM signals (Orthogonal Frequency Division Multiplexing) make them very prone to nonlinear distortion effects. In typical OFDM implementations the nonlinear distortion component is regarded as a noise-like term that leads to performance degradation. To achieve optimum performance we should employ a ML (Maximum Likelihood) receiver where we take into account all the information associated to the transmitted signals that is in the nonlinearly-distorted signal. Although the performance of an ML is substantially better than traditional receivers, its complexity is prohibitively high. In this paper we consider nonlinearly distorted OFDM schemes and we present sub-optimum receivers that try to approach the ML performance. It is shown that, contrarily to what was expectable, the ML performance with nonlinear transmitters can be better than with ideal, linear transmitters. Our suboptimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the ML performance and the performance of typical OFDM receivers.

On the design of frequency-domain equalizers for OQPSK modulations

This paper considers frequency-domain receiver design for OQPSK signals (Offset Quaternary Phase Shift Keying). It is shown that conventional QPSK receivers are not suitable to OQPSK schemes since they lead to significant interference between in-phase (I) and quadrature (Q) components. To overcome this problem we propose several linear receivers where there is no I-Q interference. Our receivers are suitable to OQPSK schemes, significantly outperforming frequency-domain receivers designed for QPSK modulations.1

Efficient Channel Estimation for Single Frequency Broadcast Systems

In SFN (Single Frequency Network) broadcasting systems the equivalent CIR (Channel Impulse Response) can be very long but typically it has a sparse nature. This means that the equivalent CIR has several clusters of paths, each one associated to the CIR between a given transmitter and the receiver. These clusters have several multipath components and are typically well separated in time. In this paper we consider OFDM-based (Orthogonal Frequency Division Multiplexing) broadcasting systems with SFN operation and we propose an efficient channel estimation method that takes advantage of the sparse nature of the equivalent CIR. For this purpose, we employ low-power training sequences and we employ an iterative receiver with joint detection and channel estimation. The receiver can know the location of the different clusters that constitute the overall CIR or not. Our performance results show that we can have excellent performance, close to the performance with perfect channel estimation, even with employ low-power training blocks and also for the case where the receiver does not know the location of the different clusters that constitute the overall CIR.

On the Design of Linear Receivers for SC-FDE Schemes Employing OQPSK Modulation

This paper considers the design of linear frequency-domain receivers with oversampling for OQPSK (Offset Quaternary Phase Shift Keying) signals. Based on a receiver structure able to eliminate the residual interference between in-phase (I) and quadrature (Q) components at the sampling instants, a method that minimizes the overall residual ISI (Inter-Symbol Interference) plus IQI (I-Q interference) is proposed. Therefore, this receiver significantly outperforms previous frequency-domain OQPSK receivers.

Approaching the matched filter bound with block transmission techniques

Block transmission techniques, with appropriate cyclic prefixes and employing frequency-domain equalisation techniques, have been shown to be suitable for high data rate transmission over severely time-dispersive channels. The most popular techniques based on this concept are orthogonal frequency division multiplexing (OFDM) and single-carrier with frequency-domain equalisation (SC-FDE). In this paper, we consider OFDM and SC-FDE transmission schemes and study the impact of the number of multipath components and the diversity order on their performance. It is shown that when we have a high number of separable multipath components, the asymptotic performance of both schemes approaches the matched filter bound, even without diversity. When we have diversity, the performance approaches the matched filter bound faster, with a small number of separable multipath components. It was also observed that the SC-FDE has an overall performance advantage over the OFDM option, especially when employing the iterative block decision feedback equaliser with turbo equalisation and for high code rates. Copyright