Jose Martin Luna-Rivera

Quality-of-Service Analysis for Linear Multiuser Detectors in the Uplink of a Wireless Network

In any wireless network is important to measure the quality-of-service (QoS) at the mobile units. Based on this information, the network will relocate their resources in order to achieve the performance specifications. Hence this objective motivates this paper, where analytical evaluations of the signal-to-noise plus interference ratio (SNIR) are carried out for each active user in a wireless communication system. In particular, this work studies the performance of linear detectors for direct-sequence code division multiple access (DS-CDMA) systems at different loading levels using singular value decomposition (SVD) techniques in the uplink channel. The QoS in terms of the SNIR are compared for the matched filter (MF), decorrelator, projector and minimum-mean-squared error (MMSE) detectors. In the analytical evaluations, two techniques are proposed to quantify the SNIR according to the multiple-access interference (MAI) and noise representation. The first one is based on an average of the MAI and noise information of all mobile users at the base station. The second approach relies on computing explicitly the interference for each active user. Simulation results show a performance comparison between both approaches, and a Monte-Carlo numerical evaluation in terms of the SNIR and the number of active users.

Power control in the uplink of a wireless multi-carrier CDMA system

This paper addresses the power allocation problem in the uplink of wireless multi-carrier code-division multiple-access (MC-CDMA) in order to guarantee QoS requirements. QoS is evaluated with respect to the signal to interference-noise ratio (SINR), estimated after the detection process at the base station (BS). First, departing of the received signal model in a MC-CDMA system, the SINR is computed by considering the application of linear multiuser detectors and a common transmission power through all subcarriers. By assuming that the measured SINR must follow an objective SINR value, an open-loop solution is introduced to the power allocation problem. Next, an iterative version is derived that relies on an integral correction of the required transmission power. By applying a loop transformation and the small-gain theorem, general distributed controllers can be proposed, where specific stability and performance conditions are introduced. A simulation evaluation is presented at different load levels in the MC-CDMA, and time-varying channel gains. In all the studied cases, the resulting power allocation system was able to allocate transmission power to achieve the objective SINR.

Geometry-Based Statistical Modeling of Non-WSSUS Mobile-to-Mobile Rayleigh Fading Channels

In this paper, we present a novel geometry-based statistical model for small-scale non-wide-sense stationary uncorrelated scattering (non-WSSUS) mobile-to-mobile (M2M) Rayleigh fading channels. The proposed model builds on the principles of plane wave propagation to capture the temporal evolution of the propagation delay and Doppler shift of the received multipath signal. This is different from existing non-WSSUS geometry-based statistical channel models, which are based on a spherical wave propagation approach, that in spite of being more realistic is more mathematically intricate. By considering an arbitrary geometrical configuration of the propagation area, we derive general expressions for the most important statistical quantities of nonstationary channels, such as the first-order probability density functions of the envelope and phase, the four-dimensional (4-D) time-frequency correlation function (TF-CF), local scattering function (LSF), and time-frequency-dependent delay and Doppler profiles. We also present an approximate closed-form expression of the channels 4-D TF-CF for the particular case of the geometrical one-ring scattering model. The obtained results provide new theoretical insights into the correlation and spectral properties of non-WSSUS M2M Rayleigh fading channels.

Geometry-Based Statistical Modeling of Non-Stationary MIMO Vehicle-to-Vehicle Channels

A novel geometry-based statistical model (GBSM) for non-stationary multiple-input multiple-output (MIMO) vehicle-to-vehicle (V2V) fading channels is presented in this paper. In contrast to the existing geometrical models for non-stationary channels, which are based on a spherical wave propagation (SWP) approach, our proposal builds on the principles of plane wave propagation (PWP). This modeling approach simplifies the mathematical analysis of the relevant statistics of non-stationary channels, such as the space-time-frequency cross-correlation function (STF-CCF). To demonstrate the mathematical tractability of the model, we derive a novel closed-form expression for the STF-CCF. For that purpose, we consider a geometrical one-ring scattering model, and assume that the angle of arrival (AOA) of the received multipath signal follows the von Mises distribution. The obtained results provide valuable theoretical insights into the cross-correlation properties of non-stationary MIMO V2V fading channels.

A cross-layer power allocation scheme for CDMA wireless networks

Cross-layer resource allocation represents an emerging technology to enable efficient resource management and quality of service (QoS) support for wireless networks. In particular, the rapidly rising energy prices and the increasing power consumption levels in current wireless network constitute a major concern for network operators. In this context, we introduce here a cross-layer approach that allows optimization of the overall power resource adaptively for a commercial DS-CDMA wireless network. First, this approach maximizes the networks utility, as a function of the signal to interference-plus-noise ratio (SINR), by considering adaptively the changes in the three most important network characteristics, that is, the total consumed power, the effective bandwidth and the average user throughput. Then the detector selection and closed-loop power control units are addressed as the main components to reach the required QoS with the lowest power consumption. Simulation results show that substantial utility gains may be achieved by improving the power efficiency in the network.

A Suboptimal LQ Power Control Algorithm for a CDMA Wireless System

In todays wireless systems, power control is an important problem which is related to the quality-of-service (QoS) and battery utilization at the mobile units. This problem is studied in this work for the uplink of a direct-sequence code- division multiple-access communication (DS-CDMA) system. By a proper selection of the error function, the resulting linear decoupled feedback loops can be controlled efficiently following a distributed power control approach, and these closed-loops are independent of the users channel variations. A suboptimal LQ strategy is suggested and derived for power update. This control law has a variable structure that depends on the transmission roundtrip delay in the feedback system, which is a key parameter to establish closed-loop stability. It is demonstrated that in the limit of some synthesis parameters, the LQ-controller tends to a Dead-Beat response, improving the time response but sacrificing robustness. Meanwhile, the LQ-controller can be adjusted to increase its robustness. Simulation results are presented to compare the control algorithms using a standard single-step power correction approach.

Dead-Beat and LQ-optimal power control algorithms in the uplink of wireless systems

Power control is an important problem in todays wireless systems, which is related to the battery utilization in the mobile units. In this work, this problem is addressed using a distributed approach. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied, and through a proper selection of the error function, the nonlinear coupling among the active users is transformed to individual loops, where the power references incorporate the information of the remaining users in the cell. It is concluded that the uplink channel variations do not destroy the stability of the feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. An LQ-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm is sensitive to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. However, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ weight parameter, an stable closed-loop system can be always guaranteed independently on the uncertainty in the estimation of the roundtrip delay. Simulation results are presented to compare the control algorithms using a standard single-step power correction approach.

FPGA-Based Educational Platform for Wireless Transmission Using System Generator

Configurable embedded systems sharing an intelligent sensor-based network are an important issue in electronic engineering curricula. This is particularly true in those with wireless communication features. The ability to plan, simulate and debug the overall communication system performance is useful in a number of situations. The authors report a hardware-software platform based on the Spartan-IIE XC2S200E coupled with both, Simulink/Matlab and Xil-inx/System generator. Results of implementation with Xil-inx tools are of valuable interest among the students community as it represents demanding skills on programming and integration of wireless communications-based systems

The Effects of Network Topology on Epidemic Algorithms

Epidemic algorithms can propagate information in a large scale network, that changes arbitrarily, in a self-organizing way. This type of spreading process allows rapid dissemination of information to all network nodes. However, the dynamics of epidemic algorithms can be strongly influenced by the network topology. In this paper, numerical simulations are used to illustrate such influences. We address networks with simple topologies for simplicity and in order to isolate other effects that occur in more complex networks.

Modeling of Non-WSSUS Double-Rayleigh Fading Channels for Vehicular Communications

This paper deals with the modeling of nonstationary time-frequency (TF) dispersive multipath fading channels for vehicle-to-vehicle (V2V) communication systems. As a main contribution, the paper presents a novel geometry-based statistical channel model that facilitates the analysis of the nonstationarities of V2V fading channels arising at a small-scale level due to the time-varying nature of the propagation delays. This new geometrical channel model has been formulated following the principles of plane wave propagation (PWP) and assuming that the transmitted signal reaches the receiver antenna through double interactions with multiple interfering objects (IOs) randomly located in the propagation area. As a consequence of such interactions, the first-order statistics of the channel model’s envelope are shown to follow a worse-than-Rayleigh distribution; specifically, they follow a double-Rayleigh distribution. General expressions are derived for the envelope and phase distributions, four-dimensional (4D) TF correlation function (TF-CF), and TF-dependent delay and Doppler profiles of the proposed channel model. Such expressions are valid regardless of the underlying geometry of the propagation area. Furthermore, a closed-form solution of the 4D TF-CF is presented for the particular case of the geometrical two-ring scattering model. The obtained results provide new theoretical insights into the correlation and spectral properties of small-scale nonstationary V2V double-Rayleigh fading channels.