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The two-dimensional stationary flow past a rotating cylinder is investigated for both two- and three-dimensional perturbations. The instability mechanisms are analysed using linear stability analysis and the complete neutral curve is presented. It is shown that the first bifurcation in the case of the rotating cylinder occurs for stationary three-dimensional perturbations, confirming recent experiments. Interestingly, the critical Reynolds number at high rotation rates is lower than that for the stationary circular cylinder. The spatial characteristics of the disturbance and a qualitative comparison with the results obtained for linear flows suggest that the stationary unstable three-dimensional mode could be of hyperbolic nature., QC 20130819

In this work, we investigate the power allocation (PA) problem aimed at minimizing the users' packet error rate (PER) over a noncooperative link, i.e., a link where the set of users, employing packet-oriented bit-interleaved coded (BIC) orthogonal frequency division multiplexing (OFDM) systems, compete for the same bandwidth. For these kind of systems, the PER is not available in closed-form, but a very efficient solution is offered by the effective SNR mapping (ESM) technique. This method allows each user to evaluate a single scalar value, the effective SNR (ESNR), accounting for all the SNIRs experienced over the active subcarriers, and to univocally map it into a PER value. Thus, in order to derive a decentralized strategy allowing each user to minimize its own PER, the problem is described as a strategic game, called min-PER game, with the set of player, utilities and strategies represented by the competitive users, the ESNRs and the set of feasible power allocations, respectively. We will show both the existence of at least one Nash Equilibrium (NE) for the min-PER game and its equivalence with a Nonlinear Variational Inequality (NVI) problem. Finally, relying on the theory of contraction mappings, we will derive a distributed algorithm to reach the NE of the game.

The availability of flexible radio interfaces capable of adapting their configuration to the time-varying operating environment is the key response to the demand encountered in modern wireless networks for high data rates under strict quality of service (QoS) constraints. To this end, this paper develops a novel link resource adaptation (LRA) scheme for soft-decoded multiantenna (MIMO) bit interleaved coded orthogonal frequency division multiplexing (BIC-OFDM) transmissions employing automatic repeat request (ARQ) mechanisms. As the first step, a simple link performance evaluation model based on the effective signal-to-noise ratio (SNR) mapping concept is derived in a closed-form expression that it is shown to yield better accuracy than previous techniques. Then, an effective LRA strategy is formulated taking advantage of that framework. The aim is maximizing the goodput (GP) metric, that is to say, the number of information bits delivered without error to the user by unit of time, over the available radio resources, such as the power distribution on the subchannels, coding rate, modulation order and the MIMO configuration. The numerical results demonstrate considerable performance gains compared with nonadaptive transmissions, while keeping the computational complexity at affordable levels in view of the specific structure of the GP objective function. © 2012 IEEE.

In next generation wireless networks, high data rates under strict quality of service (QoS) constraints call for flexible radio interfaces capable of adapting their configuration on the fly to the time-varying operating environment. Motivated by this need, this paper first derives a simple link performance prediction model for bit interleaved coded orthogonal frequency division multiplexing (BIC-OFDM) systems using incremental redundancy (IR) hybrid automatic repeat request (HARQ) mechanisms. Then, an adaptive HARQ strategy is formulated whose aim is maximizing the goodput (GP) metric, i.e., the number of error-free information bits delivered to the user by unit of time, over the coding rate, the bit distribution and an on-off power allocation across the active subchannels. Simulation results corroborate the GP performance gains of the proposed approach compared with non-adaptive transmissions, while keeping the computational complexity at affordable levels. © 2011 IEEE.

In this paper, an approximate analytical expression for the peak-to-average power ratio (PAPR) of a Multi-Carrier Code Division Multiple Access (MC-CDMA) signal is derived. Then, it is demonstrated that the PAPR of a MC-CDMA system employing Walsh-Hadamard (WH) codes can be suitably reduced by resorting to a judicious strategy for the allocation of the spreading signature codes. Eventually, a low-complexity implementation of the proposed strategy is presented and the relevant benefits, in terms of peak signal occurrence and robustness against nonlinear distortions, is numerically assessed over conventional random allocation strategy. © 2011 IEEE.

This paper presents an accurate tool yielding a reliable prediction of the link quality for the down-link of a BIC-OFDM system featuring bandlimited pulse shaping to control spectral sidelobe emission, plus AMC, and operating over a nonlinear channel. The tool enables an effective design of the communication system, with special respect to the setting of the operating point of the transmitter's power amplifier. First, an analytical characterization of the nonlinear distortion effects caused by the amplifier on bandlimited BIC-OFDM signals is carried out by resorting to the extended version of the Bussgang theorem. Then, the equivalent additive noise model provided by the Bussgang-based analysis is taken into account in the derivation of a novel link quality prediction method, named BkappaESM. Finally, the novel BkappaESM tool is employed in the link design phase for choosing the best amplifier setting, aimed at maximizing the layer-3 offered traffic over a realistic wireless channel, featuring both multipath propagation and nonlinear distortions.

This paper develops cross-layer adaptive modulation and coding as an efficient method for bit interleaved coded OFDM packet transmission. As objective function, we adopt the goodput, or the number of data bits delivered without error per unit of time. After the formalization of the optimization problem through an accurate modeling on the link performance, an iterative solution is derived based on the framework of greedy algorithms. Simulation results show that the proposed method improves upon the performance of static transmission and uniform bit allocation by 2-4 dB., Anglais

This paper presents a novel technique for cross-layer adaptive modulation and coding, suited for bit interleaved turbo-coded OFDM packet transmission, and aimed at maximizing the goodput (i.e., the number of data bits delivered without error per unit of time). The optimization problem is formalized through an accurate modeling on the link performance, and a greedy iterative solution is derived. Simulation results demonstrate that performance improvement provided by the proposed method amount to 4 dB with respect to static transmission and about 1.2 dB with respect to adaptive transmission with uniform bit allocation. Copyright © The authors.

An efficient and flexible air interface is the necessary condition for enabling high data rates next generation wireless systems. A practical answer to this need consists in exploiting channel state information at the transmitter for an adaptive and efficient use of the available radio link resources. This paper contributes with a novel strategy for the allocation of the subcarrier transmit power in the case of a wireless system featuring: i) bit-interleaved coded modulation (BICM); ii) orthogonal frequency division multiplexing (OFDM); iii) multiple antenna profile (MIMO). The proposed power-allocation algorithm aims at maximizing the so-called goodput performance metric for packet transmissions employing automatic repeat request (ARQ) schemes. In addition, it is also proven to be near-optimum in maximizing the input/output mutual information of the MIMO-OFDM subchannels. Numerical simulations over typical wireless indoor scenarios confirm the effectiveness of the procedure when applied in the context of MIMO-BICM-OFDM systems. ©2009 IEEE.

Coded multi-antenna and multi-carrier techniques combined together with link resources adaptation algorithms are the key technologies toward efficient high-data-rate communications over wireless fading channels. The practicability of this concept, however, requires that the transmitter can perform accurate and simple evaluation of the actual link performance. This paper contributes with a novel method specifically developed to predict the link performance of bit-interleaved coded MIMOOFDM links, which offers improved accuracy at the price of lower complexity when compared with conventional techniques. Its effectiveness is confirmed through extensive simulation results obtained over typical wireless channel environments. © 2009 IEEE.

In multicarrier (MC) broadband wireless networks, nonlinear power amplification is one of the main performance limiting factor, and therefore its effects have to be properly quantified during the design phase of the radio links. This letter outlines an analytical framework for the forward-link of a bandlimited MC code division multiple access that intends to fulfill this need while improving the scarce efficiency of conventional simulation-based-only design procedures. Through the extended Bussgang theorem, the approach presented herein yields a theoretical description of several time- and frequency-domain system features, such as transmitted signal power spectrum, constellation warping, nonlinear distortion noise and BER metric. Extensive simulation results verify the accuracy of the proposed analysis under typical multiple-access operating conditions. © 2009 IEEE.

Coded multicarrier techniques combined together with link resources adaptation algorithms are the key technologies toward efficient high-data-rate communications over wireless fading channels. This paper contributes with a novel accurate yet simple method to predict at the transmitter the link performance of bit-interleaved coded (BIC) orthogonal frequency division multiplexing (OFDM) links, which offers appealing features when compared with conventional techniques. Its effectiveness is confirmed through extensive simulation results obtained over typical wireless channel environments. © 2009 Springer-Verlag.

This paper deals with the power allocation problem for coded multicarrier transmission. Specifically, we focus on a bit interleaved coded modulation (BICM) packet transmission implemented with Orthogonal Frequency Division Multiplexing (OFDM) and in the presence of automatic repeat request (ARQ) protocol. Capitalizing on the binary-input output-symmetric (BIOS) nature of the BICM channel it is provided a simple upper-bound of the rate of information bits received without any error, the so called goodput. Based on this theoretical characterization, we develop a power allocation strategy among the different subcarriers so that the system goodput performance metric is maximized. The effectiveness of the proposed method is numerically testified for BICM-OFDM transmission in the context of the typical WLAN scenario. ©2008 IEEE.

In this contribution we study the optimal power allocation scheme for a coded multicarrier transmission. Specifically, we focus on a bit interleaved coded modulation (BICM) packet transmission implemented with Orthogonal Frequency Division Multiplexing (OFDM) and in the presence of automatic repeat request (ARQ) protocol. Based on a simple lower-bound of the rate of information bits received without any error, the so called goodput, we develop a power allocation strategy among the different subcarriers so that the system goodput performance metric is maximized. We also demonstrate the optimality of the solution for the proposed objective function under the given power constraint. The effectiveness of the power allocation policy is numerically testified for BIC-OFDM transmission in the context of typical indoor environment. © 2008 IEEE.

An efficient and flexible air interface is the necessary condition to achieve the ambitious goals of next generation broadband wireless networks. Multicarrier code division multiple access (MC-CDMA) is a practical answer to this need, but the considerable vulnerability to nonlinear distortions induced by high power amplifiers at the transmitter side hinders its appealing features, thereby demanding for adequate countermeasures. The current paper contributes to this issue with a novel multiuser data predistortion (MUDP) procedure aimed at mitigating the performance degradation of a MC-CDMA forward-link in the presence of a nonlinear channel. The proposed MUDP resides entirely at the transmitter of the base station, and simply requires a set of modified constellations, which are jointly and iteratively optimized so that the distortions experienced by each active user are significantly reduced. The required computational complexity is then kept at affordable levels by resorting to a suboptimal yet effective scheme based on the identification of the strongest interfering users combined with the set-partitioning and the symmetry properties of their constellations. Numerical simulation results corroborate the effectiveness of the MUDP in improving the link performance under typical demanding multiple-access operating conditions. © 2008 IEEE.

In this contribution we demonstrate that the peak-to-average power ratio (PAPR) of a Multi-Carrier Code Division Multiple Access (MC-CDMA) signal can be suitably minimized by resorting to a judicious strategy for the allocation of the spreading signature codes. A low-complexity implementation of the proposed strategy is presented and its performance gain over conventional random allocation strategy is numerically assessed. ©2007 IEEE.

In this contribution we demonstrate that the peak-to-average power ratio (PAPR) of a Multi-Carrier Code Division Multiple Access (MC-CDMA) signal can be suitably minimized by resorting to a judicious strategy for the allocation of the spreading signature codes. A low-complexity implementation of the proposed strategy is presented and its performance gain over conventional random allocation strategy is numerically assessed.

This paper presents an analytical framework for the theoretical characterization and performance evaluation of an MC-CDMA forward-link in the presence of nonlinear distortions induced by transmitter's high-power amplifiers. The analytical characterization of the signal features, both in the time and frequency domains, is carried out through the application of the Bussgang theorem. Simulation results are derived to validate the accuracy of the proposed method under typical operating conditions 1. © 2006 IEEE.

This paper presents an analytical framework for the performance evaluation of an MC-CDMA forward-link in the presence of nonlinear distortions induced by transmitter's high-power amplifiers. The statistical characterization of the decision variable used for data detection is carried out through the application of the Bussgang theorem. Simulation results validate the accuracy of the proposed method under typical operating conditions.