Optimal radio resources and transmission formats assignment in OFDMA systems

The problem of assigning radio resources and transmission formats to users in the downlink of an OFDMA network is addressed. In particular, a single cell environment with a realistic interference model and a margin adaptive approach is considered, i.e., the aim is of minimizing the total transmission power while maintaining a certain given rate for each user. The computational complexity issues of the problem are discussed, and an approximation analysis is presented. Heuristic approaches, based on rounding techniques and graph models, are also proposed. Computational experiences show that, in a comparison with a commercial state-of-the-art optimization solver and with alternatives from the literature, the proposed algorithms are effective in terms of solution quality and CPU times.     

Noncoherent MLSE in DS-CDMA wireless systems with antenna arrays

This paper addresses the noncoherent detection problem for direct-sequence code-division multiple-access (DS-CDMA) signals in the presence of antenna arrays at the base station. An ideal open-loop power control, which compensates for the long-term variations due to path loss and shadowing experienced by the mobile users, is assumed. Then, the noncoherent maximum-likelihood sequence-estimation problem is analyzed under the assumption of spatially white interference. The contribution of this paper is to extend previous results obtained by the author in for the case of multiantenna receivers, including the derivation and analysis of two suboptimal detectors based on an approximate maximization of the transmitted symbols likelihood. The first detector works as a traditional squared envelope combiner, which fully exploits the K diversity degrees provided by the multiple antenna system. In the second case, the receiver assumes a perfect knowledge of antenna response vector, so that spatial filtering of the received signal is made possible. Simulation and theoretical results are provided for a widely used circular ring of scatterers spatial channel model, which allows analysis in a wide range of different environmental conditions.     

Noncoherent MLSE detection of M-DPSK for DS-CDMA wireless systems

This paper focuses on maximum-likelihood sequence estimation of noncoherent M-ary differential phase-shift keying (M-DPSK) receivers for code division multiple-access (CDMA) systems, which make use of direct-sequence spread-spectrum modulations. A typical frequency-selective Rayleigh environment with multipath diversity at the receiver is considered. In this scenario, the optimum noncoherent decision metric, which requires an estimation of the channel tap weights envelope, is derived. Then, in order not to increase the receiver implementation complexity, a joint channel and data estimation strategy is proposed, which does not require the transmission of a known training sequence (blind estimation). In this case, the decision metric becomes a simple equal gain combining of multiple-symbol square-law detection decision metrics. For this suboptimum noncoherent detector, useful bounds on the bit error probability are provided through a theoretical analysis. Nonconstant and constant multipath intensity profiles are both considered for this purpose. Simulations are also carried out in order to verify the accuracy of the theoretical bounds     

Air separation plant design

This paper presents a general review of the design of air separation plants from the initial specifications to the final details. References to key sources for data and for design criteria are presented in lieu of lengthy discussions.The effect of the product quantities and purities on the selection of the process cycle are discussed and general design guidelines and specific examples of various types of air separation plants are given. Process cycles for large tonnange plants, unattended nitrogen generators, small oxygen generators for sewage plants, and for all-liquid plants are described. Unit operations used in these cycles and materials of construction are reviewed briefly.     

Outage statistics in CDMA mobile radio systems

The statistics of the outage events in the reverse link of a direct sequence code-division multiple access (DS/CDMA) wireless system are investigated taking into account the effects of correlated Rician fading. The outage probability and the average fade duration are derived by assuming a constant multipath intensity profile, which includes a direct-path component in the first path. Simulations have been carried out which validate the analytical results     

Optimization of power control parameters for DS-CDMA cellularsystems

This paper envisages a cellular system based on code-division multiple access and investigates the performance of a strength-based closed-loop power control (CLPC) scheme on the basis of different parameters, such as the number of bits of the power command, the quantization step size, and the user speed. On the basis of a log-linear CLPC model, an analytical approach has been developed that has allowed to determine the optimum quantization step size to be used for each value of the number of power command bits. Simulation results have permitted to support the analytical framework developed in this paper     

On the analytical evaluation of closed-loop power-control errorstatistics in DS-CDMA cellular systems

This paper proposes an analytical study that aims at evaluating the power-control error statistics in wireless direct-sequence code-division multiple-access (DS-CDMA) cellular systems based on an ideal variable step closed-loop power-control scheme. In particular, the cumulative distribution function and the correlation coefficient of the power-control error are derived through a first-order Taylor expansion of the received signal envelope. A novel power-control scheme that exploits the autocorrelation properties of the fading is also proposed, and its performance is analyzed in terms of power-control error statistics. Rayleigh and Rice frequency-selective channel models, which involve the use of a diversity RAKE receiver at the base station, have been taken into account. The proposed analytical approach specifically applies to CDMA systems. A method that aims at estimating the capacity of a DS-CDMA cellular network is also given     

Capacity evaluation of a mixed-traffic WCDMA system in the presence of load control

This paper deals with the capacity analysis of a direct-sequence code-division multiple-access (DS-CDMA) cellular system in the presence of heterogeneous traffic. We consider a distributed resource allocation strategy that uses a closed-loop control scheme to manage the processing gain, the signal-to-interference ratio (SIR), and the system load. In particular, each base station aims at maintaining the uplink load to its allowable maximum value in order to achieve the maximum throughput. A typical urban cellular system is considered in which the path loss depends on the /spl alpha/th power of the distance, shadowing is log-normally distributed, and short-term fluctuations due to multipath fading are present. Besides, a radio resource-management scheme is considered, which is able to control transmission power and spreading gain values so as to maintain the total received power at the base stations at a maximum allowable level. In this scenario, this paper derives a novel analytical approach for the evaluation of system capacity of DS-CDMA systems in presence of different traffic sources. Bit-error-ratio (BER) and bit-rate requirements have been considered for the different traffic classes. Simulation results have permitted to validate the developed analytical framework on a very comprehensive range of cases. The proposed analysis is finally applied to universal mobile telecommunications system (UMTS)-like traffic classes in order to evaluate the system capacity.     

Multiple-symbol differential detection of GMSK for mobilecommunications

A new multiple differential detection (MDD) sequence estimator is described which uses a decision feedback for the demodulation of a GMSK signal. This technique is based upon a maximum-likelihood sequence estimation of the transmitted phases rather than on a symbol-by-symbol detection. An upper and a lower bound on the bit error probability of the described detector in the case of an AWGN channel and a two-ray Rayleigh fading channel are derived. The performance of the detection algorithm in a mobile radio communication system is obtained through computer simulation. Comparisons with the coherent detection algorithm show that the proposed detection algorithm is quite attractive both in an AWGN and in a multipath channel     

Radio resource allocation problems for OFDMA cellular systems

Orthogonal frequency division multiple-access (OFDMA) manages to efficiently exploit the inherent multi-user diversity of a cellular system by performing dynamic resource allocation. Radio resource allocation is the technique that assigns to each user in the system a subset of the available radio resources (mainly power and bandwidth) according to a certain optimality criterion on the basis of the experienced link quality. In this paper we address the problem of resource allocation in the downlink of a multi-cellular OFDMA system. The allocation problem is formulated with the goal of minimizing the transmitted power subject to individual rate constraint for each user. Exact and heuristic algorithms are proposed for the both the single-cell and the multi-cell scenario. In particular, we show that in the single-cell scenario the allocation problem can be efficiently solved following a network flow approach. In the multi-cell scenario we assume that all cells use the same frequencies and therefore the allocation problem is complicated by the presence of strong multiple access interference. We prove that the problem is strongly NP-hard, and we present an exact approach based on an MILP formulation. We also propose two heuristic algorithms designed to be simple and fast. All algorithms are tested and evaluated through an experimental campaign on simulated instances. Experimental results show that, although suboptimal, a Lagrangian-based heuristic consisting in solving a series of minimum network cost flow problems is attractive for practical implementation, both for the quality of the solutions and for the small computational times.