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This work considers the design of multiuser MIMO (MU-MIMO) schemes suitable for orthogonal frequency division multiplex (OFDM) systems such as those being currently discussed within IEEE 802.11ac. A general framework is proposed showing that the singular value decomposition (SVD), the geometric mean decomposition (GMD) and the uniform channel decomposition (UCD) are all viable mechanisms on which to lay the design. Nonetheless, some striking differences in terms of performance among these three schemes are uncovered. In particular, a novel adaptive transmission scheme based on the UCD is introduced that enables multiuser MIMO-OFDM gains to be realized, clearly outperforming those obtained using the SVD or GMD counterparts while remaining fully compliant with the latest IEEE 802.11ac specification. Relying on the UCD properties, the proposed technique takes into account the availability of a finite modulation and coding set and restrictions on transmission mode selection defined within the standard, to incorporate an optimisation step in charge of selecting the number of transmitted data streams to each user to maximise the system throughput. Comprehensive simulation results in the context of IEEE 802.11ac confirm the applicability and advantage of the adaptive UCD-based design.Index Terms-MU-MIMO, OFDM, IEEE 802.11ac, geometric mean decomposition (GMD), uniform channel decomposition (UCD), zero-forcing beamforming, fast link adaptation (FLA).

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The distributions of ratios of random variables are of interest in many areas of the sciences. In this brief paper, we present the joint probability density function (PDF) and PDF of maximum of ratiosμ1=R1/r1andμ2=R2/r2for the cases whereR1,R2,r1, andr2are Rayleigh, Rician, Nakagami-m, and Weibull distributed random variables. Random variablesR1andR2, as well as random variablesr1andr2, are correlated. Ascertaining on the suitability of the Weibull distribution to describe fading in both indoor and outdoor environments, special attention is dedicated to the case of Weibull random variables. For this case, analytical expressions for the joint PDF, PDF of maximum, PDF of minimum, and product moments of arbitrary number of ratiosμi=Ri/ri,i=1,…,Lare obtained. Random variables in numerator,Ri, as well as random variables in denominator,ri, are exponentially correlated. To the best of the authors' knowledge, analytical expressions for the PDF of minimum and product moments of{μi}i=1Lare novel in the open technical literature. The proposed mathematical analysis is complemented by various numerical results. An application of presented theoretical results is illustrated with respect to performance assessment of wireless systems.

Abstract-The transmission speed of optical networks strongly depends on the impact of higher order dispersion. In the presence of coherent interference which can't be kept under control by optical filtering, the impact of higher order dispersion becomes more serious. In this paper we give general expressions that describe pulse deformation due to even higher order dispersion in a single-mode fiber. The impulsive responses for even order dispersion in the presence of coherent interference are characterized by symmetrical waveforms with long trailing skirts. Individual and joint influence of second and fourth order dispersion on the transmission quality is studied. Pulse shape and eye diagram are obtained.

In this paper, the performance of L-branch selection combining receiver over correlated Weibull fading channels in the presence of correlated Weibull-distributed cochannel interference is analyzed. Closed-form expressions for probability density function and cumulative distribution function of the signal-to-interference ratio at the output of the selection combining receiver present main contribution of this paper. Numerical results are also presented to show the effects of various parameters as the fading severity, correlation and number of branches on outage probability. only one of the diversity branches. Because of that, SC is the least complicated of these three types of linear diversity combining. In general, SC chooses the branch with the highest signal-to-noise ratio (SNR) (or equivalently, with the strongest signal assuming equal noise power among the branches). In fading environments as in cellular communication systems, the level of the thermal noise can be sufficiently low as compared with the level of the cochannel interference (CCI). In that case SC receiver chooses the branch with the highest signal-to-interference ratio (SIR-based selection diversity) [4].In wireless communication systems several statistical models are used to describe fading. The most frequently used models are Rayleigh, Nakagami-m, Rice and Weibull fading channel model. Weibull fading model is a special case of a generalized − fading model [5] and it could be used in land-mobile satellite system and terrestrial radio system due to the fact that it confirms experimentally attained fading channel measurements for both indoor [6] and outdoor [7, 8] environments. However, this model has recently begun to receive some interest. The performance of a class of generalized-selection combining receivers over independent and nonidentical Weibull fading channels has been studied in [9]. Closed-form expressions for the average output SNR, the amount of fading, the outage probability, the switch rate and the average symbol error probability of dual switched and stay combining receivers over independent Weibull fading channels have been extracted in [10,11]. The performance of dual SC receiver over correlated Weibull fading channels has been presented in [12]. In that paper, obtained closed-form expressions for the probability density function (PDF), the cumulative distribution function (CDF), and the moments of the receiver output SNR have been used to consider important performance criteria. In [13], useful analytical expressions for the joint PDF, CDF, moment-generating function and product moments have been derived for both the bivariate and multivariate Weibull distribution with exponential correlation. Moreover, the performance analysis of dual-branch and multibranch SC, EGC and MRC receivers has been studied in that paper. Some important performance criteria of multibranch EGC and MRC receiver operating over nonidentical Weibull fading channels, for both independent and correlative fading, have been derived in [14].The performance analy...

System performances of dual selection combining over fading channels are analyzed. Fading between the diversity branches and between interferences is correlated and Rician distributed. Infinite series expressions for the probability density function, and the cumulative distribution function of the output signal-to-interference ratio are derived, which is the main contribution of this paper. Outage probability and the average bit error probability for noncoherent modulation schemes are also presented. Numerical results, presented in this paper, point out the effects of fading severity and correlation on the system performances.