Velio Tralli

A theoretical characterization of nonlinear distortion effects in OFDM systems

This paper presents a theoretical characterization of nonlinear distortion effects in orthogonal frequency division multiplexing (OFDM) transmission systems. In the theoretical framework developed, it is shown that the effects on the decision variables of the in-band distortion introduced by a bandpass memoryless nonlinearity can be described by means of a complex gain and an additive Gaussian term with zero mean and suitable variance; analytical expressions for gain and variance are given. The conditions which allow this description are emphasized and discussed. As a consequence, a completely analytical procedure to evaluate error probability is also obtained and illustrated using OFDM/discrete multitone modulation (DMT) systems with rectangular pulse shaping; for the soft-envelope limiter nonlinearity, a closed form is derived. A comparison with simulation results is carried out to verify the accuracy of this method.

Wireless TCP performance with link layer FEC/ARQ

We investigate the performance of TCP Tahoe and NewReno on wireless links with a FEC/ARQ protocol at the link layer. The effect of link layer (LL) parameters like the LL packet size, number of LL transmission attempts, and FEC code rate on the TCP throughput is evaluated. It is shown that the retransmissions at the link layer must persist long enough to outlast the average bad state duration of the Rayleigh fading process in order to achieve performance better than TCP without an ARQ at the link layer. It is further shown that with a suitable link layer in place, TCP Tahoe offers as high a throughput as offered by other enhanced versions like NewReno in the considered wireless environment.

An analytical framework for CDMA systems with a nonlinear amplifier and AWGN

The design of code-division multiple-access (CDMA) transmission systems for satellite communications requires an appropriate consideration of the distortion effects due to on-board nonlinear amplification. The aim of this paper is to provide an analytical framework for the evaluation of the in-band nonlinear distortion effects on the performance of CDMA systems. Both synchronous systems with orthogonal codes and asynchronous systems are considered. It is first shown that, when the users accessing the channel have the same power and their number is sufficiently large, the nonlinear distortion in the decision variables at the receiver can be simply described by a complex scale factor, which depends on the high-power-amplifier (HPA) characteristics only, and an additive noise, which is uncorrelated to the useful signal. Moreover, an analytical formulation of the bit error probability and the total degradation as a function of the output back-off and number of users is given. In the results, which are obtained for three classes of HPA models (i.e., the traveling wave tube amplifier, the solid-state power amplifier, and the amplifier with ideal predistortion), the performance and the capacity of power-limited systems is also discussed.

High-speed indoor wireless communications at 60 GHz with coded OFDM

A high-speed indoor wireless communication system using coded orthogonal frequency-division multiplexing (OFDM) and working at 60 GHz is proposed and analyzed. An actual propagation environment consisting of a medium sized research laboratory, characterized by means of a ray-tracing technique, is considered for the analysis. In this contest the paper investigates and discusses the effects of frequency diversity, antenna sectorization, OFDM clustering, and different block coding strategies. Moreover, to characterize the communication between stationary indoor terminals at millimeter waves, a new definition of coverage is introduced. In order to evaluate the performance of the coded system in the actual environment, a suitable semianalytical algorithm is defined and applied. In the results the feasibility of a coded OFDM system for 155 Mbit/s packet transmission is checked. It is shown that all the line-of-sight (LOS) positions and 70% of the no LOS points can be covered in the scenario considered with a transmitted power of 10 dBm.

Log-Concavity Property of the Error Probability With Application to Local Bounds for Wireless Communications

A clear understanding of the behavior of error probability (EP) as a function of signal-to-noise ratio (SNR) and other system parameters is fundamental for assessing the design of digital wireless communication systems. We propose an analytical framework based on the log-concavity property of the EP which we prove for a wide family of multidimensional modulation formats in the presence of Gaussian disturbances and fading. Based on this property, we construct a class of local bounds for the EP that improve known generic bounds in a given region of the SNR and are invertible, as well as easily tractable for further analysis. This concept is motivated by the fact that communication systems often operate with performance in a certain region of interest (ROI) and, thus, it may be advantageous to have tighter bounds within this region instead of generic bounds valid for all SNRs. We present a possible application of these local bounds, but their relevance is beyond the example made in this paper.

Millimeter waves for short-range multimedia communication systems

A framework for the performance characterization of short-range communications systems is developed with the intention of investigating the feasibility of new multimedia wireless services at millimeter waves (MMWs). Both narrow- and wide-band systems are considered for mobile and/or fixed users. This paper aims at defining and evaluating proper metrics to characterize the service quality for the user and jointly takes the propagation characteristics, the transmission techniques, and the multiple access protocols into account. The definition of service-oriented metrics is emphasized. Three different real scenarios operating at MMW are investigated with a unified perspective: intelligent transport systems, wide-band local-area networks, and local multipoint distribution systems for interactive video services. The role played by the MMW band in the development of these services is discussed. In each scenario, accurate propagation analysis is carried out and suitable countermeasure techniques are pointed out in order to join suitable service-quality levels. The methodology considered is based on both analytical and semianalytical tools for performance evaluation.

Distortion-Fair Cross-Layer Resource Allocation for Scalable Video Transmission in OFDMA Wireless Networks

The design of optimized video delivery to multiple users over a wireless channel is a challenging task, especially when the objectives of maximizing the spectral efficiency and providing a fair video quality have to be jointly considered. In this paper we propose a novel cross-layer optimization framework for scalable video delivery over OFDMA wireless networks. It jointly addresses rate adaptation and resource allocation with the aim of maximizing the sum of the achievable rates while minimizing the distortion difference among multiple videos. After having discussed the feasibility of the optimization problem, we consider a “vertical” decomposition of it and propose the iterative local approximation (ILA) algorithm to derive the optimal solution. The ILA algorithm requires a limited information exchange between the application and the MAC layers, which independently run algorithms that handle parameters and constraints characteristic of a single layer. In order to reduce the overall complexity and the latency of the optimal algorithm, we also propose suboptimal strategies based on the first-step of the ILA algorithm and on the use of stochastic approximations at the MAC layer. Our numerical evaluations show the fast convergence of the ILA algorithm and the resulting small gap in terms of efficiency and video quality fairness between optimal and suboptimal strategies. Moreover, significant individual PSNR gains, up to 7 dB for high-complexity videos in the investigated scenario, are obtained with respect to other state-of-the-art frameworks with similar complexity.

Further results on convolutional code search for block-fading channels

This correspondence presents results of code search for convolutional codes over block-fading channels (BFCs). Search criteria are based on a union bound approach which exploits the concept of generalized transfer function (GTF) of the error trellis diagram. A new asymptotic bound is derived, and the performance of the codes found using our search criterion is compared analytically and by numerical simulation with already existing codes.

On the capacity of slotted Aloha with Rayleigh fading: the role played by the number of interferers

The role played by the number of interferers and the modulation choice in the performance of slotted Aloha networks with Rayleigh fading is investigated and discussed; the case of multiple transmitted power classes is also considered. A new capture model which takes the number of actual interferers into account is proposed. The network capacity is analytically estimated, and significant differences (depending on the modulation scheme) are found with respect to previously published results carried out by means of more common (and less accurate) capture models, also when the case of different transmitted power classes is considered.

Power-shaped advanced resource assignment (PSARA) for fixed broadband wireless access systems

We propose and investigate a new resource allocation technique for the downlink of time-division multiple access (TDMA)-based fixed broadband wireless access systems (FBWA) with full frequency reuse. This technique, named power-shaped advanced resource assignment exploits an appropriate set of power profiles which limit (or shape) the power transmitted in each slot of the frame and are suitably reused among the cells, in order to efficiently distribute the intercell and intersector interference inside the frame and to make it partially predictable. In systems where base stations assign radio resources in an uncoordinated fashion, this allows the allocation algorithm to assign time slots to users on the basis of the power required to fulfill a predefined carrier-to-interference ratio, since worst case interference can be suitably estimated; moreover, different degrees of protection against interference are provided across the slots to efficiently accommodate users with different location-dependent channel conditions. Simulation results for a typical cellular FBWA system show that this technique significantly improves the capacity with respect to other techniques recently proposed, e.g., the enhanced staggered resource assignment, even when they use power control. Moreover, an analytical framework useful to understand the concept of power shaping and to discuss some guidelines for the design of power profiles is provided.