Μέθοδοι και τεχνικές βελτιστοποίησης της απόδοσης των ψηφιακών ασύρματων συστημάτων λειτουργούντων σε περιβάλλον με διαλείψεις [thesis]

Δημήτριος Ζώγας
Diversity reception has been successfully used in wireless communications systems to mitigate the negative effect of channel fading. This thesis deals with the performance analysis of wireless systems employing receiver diversity. The diversity schemes studied are selection combining (SC) and equal-gain combining (EGC). We assume that the receiver operates in flat and slow fading channels and that the signal undergoes Rayleigh, Nakagami-m, Rice or Hoyt fading. As it will be shown, the
more » ... hown, the performance of the diversity system depends greatly on: • the correlation among the diversity branches, • the average power imbalance of the received signals In order to study the performance of a SC receiver operating in correlated Nakagami-m fading channels, we first present a statistical analysis of correlated Nakagami-m random variables (rvs). The probability density function (pdf) of exponentially correlated Nakagami-m rvs is presented in closed-form, while the corresponding cumulative distribution function (cdf) has the form of multiple converging infinite series. Furthermore, bounds for the truncation of the infinite series are also derived. Into the following, using an approximation technique for the correlation matrix, the above analysis is expanded for the case of arbitrary correlated Nakagami-m rvs. Results show that the proposed approximation technique gives exact results for exponentially correlated rvs, since for other correlation models, such as the linearly arbitrary, circular and constant models, the results are quite accurate. Based on the pdf of exponentially correlated Nakagami-m rvs, we study the performance of a triple SC receiver operating over exponentially correlated Nakagami-m fading channels. The performance criteria considered are the outage probability and the average symbol error probability (ASEP) for several modulations. The improvement achieved by the triple SC compared to the dual diversity case is also presented. Moreover, using the proposed approximation technique, we study the average output signal-to-noise ratio (SNR) for a triple SC receiver operating over arbitrary correlated Nakagami-m channels. Results for the dual SC are also depicted. As far as the EGC receiver is concerned the following cases are studied. Firstly, the ASEP for several modulation schemes is studied for the dual EGC for correlated Nakagami-m fading. This is accomplished deriving the characteristic function (chf) for the sum of two correlated Nakagami-m fading. Using this chf the vi ASEP is studied deriving either closed-form expressions for several modulation schemes, either transforming the error integral into the frequency domain using the Parseval's theorem. Into the following, closed-form expressions are derived for important statistical parameters, such as the moments and the central moments of the output SNR, for an L-branch EGC receiver operating in correlated Nakagami-m fading channels. The moments and the central moments are used to study parameters, such as the average output SNR, the variance, the kurtosis and the skewness of the output SNR, which characterize the form of the output SNR pdf. Moreover, the performance of the EGC is studied via performance criteria, such as the amount of fading (AoF) of the output SNR and the spectral efficiency of the receiver at the low power regime. Using similar analysis, the moments and the central moments are derived in closed-form for an L-branch EGC receiver operating in independent but not necessarily identically distributed Rice and Hoyt fading channels. Moreover, using the Pade rational approximation to the moment generating function (mgf) of the output SNR, the ASEP for several modulation schemes and the outage probability are evaluated. vii
doi:10.12681/eadd/26354 fatcat:j5z33rxmzzhn5bbynyrw3i6udy