Víctor P. Gil Jiménez

Design and implementation of synchronization and AGC for OFDM-based WLAN receivers

An efficient implementation of several tasks at the receiver becomes crucial in OFDM-based high-speed WLAN systems, such as automatic gain control, time and frequency synchronization, and offset tracking. This paper deals with fixed point constraints and accuracy requirements for implementation of those algorithms. Also, a complete set of thresholds for the practical implementation of time and frequency synchronization sub-blocks is obtained. Moreover, a technique to mitigate the remaining frequency offset after coarse acquisition is proposed, yielding a good trade-off between performance and complexity. Finally, we propose the implementation of a simple and effective automatic gain control procedure.

High Power Amplifier Pre-Distorter Based on Neural-Fuzzy Systems for OFDM Signals

In this paper, a novel High Power Amplifier (HPA) pre-distorter based on Adaptive Networks - Fuzzy Inference Systems (ANFIS) for Orthogonal Frequency Division Multiplexing (OFDM) signals is proposed and analyzed. Models of Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA), both memoryless and with memory, have been used for evaluation of the proposed technique. After training, the ANFIS linearizes the HPA response and thus, the obtained signal is extremely similar to the original. An average Error Vector Magnitude (EVM) of 10-6 can be easily obtained with our proposal. As a consequence, the Bit Error Rate (BER) degradation is negligible showing a better performance than what can be achieved with other methods available in the literature. Moreover, the complexity of the proposed scheme is reduced.

Reduction of Power Envelope Fluctuations in OFDM Signals by using Neural Networks

One of the main drawbacks of Orthogonal Frequency Division Multiplexing (OFDM) are the large fluctuations of its power envelope. In this letter, a novel and efficient scheme based on Multilayer Perceptron (MLP) Neural Networks (NN) is proposed. The NN synthesizes the Active Constellation Expansion - (ACE) technique which is able to drastically reduce envelope fluctuations. This is achieved with much lower complexity, faster convergence, and better performance compared to previously available methods.

Field Measurements and Guidelines for the Application of Wireless Sensor Networks to the Environment and Security

Frequently, Wireless Sensor Networks (WSN) are designed focusing on applications and omitting transmission problems in these wireless networks. In this paper, we present a measurement campaign that has been carried out using one of the most commonly used WSN platforms, the micaZ from Crossbow©. Based on these measurements, some guidelines to deploy a robust and reliable WSN are provided. The results are focused on security and environmental applications but can also be extrapolated to other scenarios. A main conclusion that can be extracted is that, from the transmission point of view, a dense WSN is one of the best choices to overcome many of the transmission problems such as the existence of a transitional region, redundance, forwarding, obstructions or interference with other systems.

Reduction of the Envelope Fluctuations of Multi-Carrier Modulations using Adaptive Neural Fuzzy Inference Systems

In this paper, a novel scheme for reducing the envelope fluctuations in multi-carrier signals applying Adaptive Neural Fuzzy Inference Systems (ANFIS) is proposed and analyzed. Once trained with signals with very low envelope fluctuations, such as those obtained by the Active Constellation Expansion - Approximate Gradient Project (ACE-AGP) algorithm, ANFIS approximately reaches a similar reduction as with ACE-AGP for multi-carrier signals without the complexity and the large convergence time of conventional ACE-AGP. We show that our approach is less complex than other previous schemes and with better performance.

Subcarrier and Power Allocation for the Downlink of Multiuser OFDM Transmission

In this paper, a new algorithm for subcarrier and power allocation for the downlink of multiuser OFDM transmission is presented. The proposed algorithm is more stable and it offers a lower complexity and better performance than previous existing algorithms.

Multiuser Subcarrier and Power Allocation Algorithm for OFDM/Offset-QAM

In this letter, the multiuser bit and power allocation problem for an orthogonal frequency division multiplexing (OFDM)/offset-quadrature amplitude modulation (OQAM) system is investigated. The signal-to-interference-plus-noise ratio (SINR) and required power expressions for the formulation of the problem in an OFDM/OQAM system are developed. Next, a suboptimal algorithm based on this formulation is proposed.

Joint channel and Frequency Offset estimation in MIMO-OFDM systems with insufficient Cyclic Prefix☆

Abstract A Maximum Likelihood joint channel and Frequency Offset estimator is addressed for Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing systems in the presence of Inter Symbol Interference and Inter Carrier Interference due to an insufficient Cyclic Prefix. The use of known Pseudo Noise based Symmetric Sequences to perform the joint estimation is proposed. The periodicity of the cost function for the Frequency Offset estimator has been analytically derived. It has been found that the proposed Pseudo Noise based Symmetric Sequences achieve an acquisition range for normalized Frequency Offset within the interval ±0.5, while previously proposed symmetric sequences are limited to ±0.25. The performance of Frequency Offset and channel estimators has been evaluated by the Cramer–Rao Bound and the theoretical Mean Squared Error. Also, the analytical Mean Squared Error for channel estimation in case of an imperfect estimation of Frequency Offset has been developed to analyze its impact in the performance. Moreover, we have proven that the estimation with the second half of the symmetric preamble performs better than with the whole preamble.

A MIMO-OFDM testbed, channel measurements, and system considerations for outdoor-indoor wimax

The design, implementation, and test of a real-time flexible (Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing) MIMO-OFDM IEEE 802.16 prototype are presented. For the design, a channel measurement campaign on the 3.5 GHz band has been carried out, focusing on outdoor-indoor scenarios. The analysis of measured channels showed that higher capacity can be achieved in case of obstructed scenarios and that (Channel Distribution Information at the Transmitter) CDIT capacity is close to (Channel State Information at the Transmitter) CSIT with much lower complexity and requirements in terms of channel estimation and feedback. The baseband prototype used an (Field Programmable Gate Array) FPGA where enhanced signal processing algorithms are implemented in order to improve system performance. We have shown that for MIMO-OFDM systems, extra signal processing such as enhanced joint channel and frequency offset estimation is needed to obtain a good performance and approach in practice the theoretical capacity improvements.

Multi-user Synchronisation in ad hoc OFDM-based Wireless Personal Area Networks

In this paper, a procedure for dealing with the multi-user synchronisation problem in Orthogonal Frequency Division Multiple Access (OFDMA)-based systems for ad hoc environments is proposed and analysed. We show with this novel approach that it is possible to re-use much of the already extent literature for single-user synchronisation in OFDM and apply it to multi-user ad hoc scenarios. Also a distributed version of the procedure is proposed in order to fairly share out the power consumption among all the devices. The proposed procedure makes use of higher layer capabilities in a cross-layer design and it does not incur too much complexity or power. This issue is specially critical in wireless heterogeneous ad hoc networks where devices can be very limited in terms of transmission and/or computational power.