There has been phenomenal interest in applying space-time coding techniques in wireless communications in the last two decades. In general, the benefit of applying space-time codes in multiple-input, multiple-output (MIMO) wireless channels is an increase in transmission reliability or system throughput (capacity). However, such a benefit cannot be obtained in some wireless systems where size or other constraints preclude the use of multiple antennas. As such, wireless relay communications has recently been proposed as a means to provide spatial diversity in the face of this limitation. In this approach, some users or relay nodes assist the transmission of other users information. This dissertation contributes to the advancement of wireless relay communications by investigating the performance of various relaying signal processing methods under different practical fading environments. In particular, it examines two main relaying methods, namely decode-and-forward (DF) and amplify-and-forward (AF).<p> For DF, the focus is on the diversity analysis of relaying systems under various practical protocols when detection error at relays is taken into account. In order to effectively mitigate the phenomenon of error propagation, the smart relaying technique proposed by Wang et al. in [R1] is adopted. First, diversity analysis of a single-relay system under the scenario that only the relay is allowed to transmit in the second time slot (called Protocol II) is carried out. For Nakagami and Hoyt generalized fading channels, analytical and numerical results are provided to demonstrate that the system always obtains the maximal diversity when binary phase shift keying (BPSK) modulation is used. Second, a novel and low-complexity relaying system is proposed when smart relaying and equal gain combing (EGC) techniques are combined. In the proposed system, the destination requires only the phases of the channel state information in order to detect the transmitted signals. For the single-relay system with M-ary PSK modulation, it is shown that the system can achieve the maximal diversity under Nakagami and Hoyt fading channels. For the K-relay system, simulation results suggest that the maximal diversity can also be achieved. Finally, the diversity analysis for a smart relaying system under the scenario when both the source and relay are permitted to transmit in the second time slot (referred to as Protocol I) is presented. It is shown that Protocol I can achieve the same diversity order as Protocol II for the case of 1 relay. In addition, the diversity is very robust to the quality of the feedback channel as well as the accuracy of the quantization of the power scaling implemented at the relay.<p> For AF, the dissertation considers a fixed-gain multiple-relay system with maximal ratio combining (MRC) detection at the destination under Nakagami fading channels. Different from the smart relaying for DF, all the channel state information is assumed to be available at the destination in order to perform MRC for any number of antennas. Upperbound and lowerbound on the system performance are then derived. Based on the bounds, it is shown that the system can achieve the maximal diversity. Furthermore, the tightness of the upperbound is demonstrated via simulation results. With only the statistics of all the channels available at the destination, a novel power allocation (PA) is then proposed. The proposed PA shows significant performance gain over the conventional equal PA.

There has been phenomenal interest in applying space-time coding techniques in wireless communications in the last two decades. In general, the benefit of applying space-time codes in multiple-input, multiple-output (MIMO) wireless channels is an increase in transmission reliability or system throughput (capacity). However, such a benefit cannot be obtained in some wireless systems where size or other constraints preclude the use of multiple antennas. As such, wireless relay communications has recently been proposed as a means to provide spatial diversity in the face of this limitation. In this approach, some users or relay nodes assist the transmission of other users information. This dissertation contributes to the advancement of wireless relay communications by investigating the performance of various relaying signal processing methods under different practical fading environments. In particular, it examines two main relaying methods, namely decode-and-forward (DF) and amplify-and-forward (AF).<p> For DF, the focus is on the diversity analysis of relaying systems under various practical protocols when detection error at relays is taken into account. In order to effectively mitigate the phenomenon of error propagation, the smart relaying technique proposed by Wang et al. in [R1] is adopted. First, diversity analysis of a single-relay system under the scenario that only the relay is allowed to transmit in the second time slot (called Protocol II) is carried out. For Nakagami and Hoyt generalized fading channels, analytical and numerical results are provided to demonstrate that the system always obtains the maximal diversity when binary phase shift keying (BPSK) modulation is used. Second, a novel and low-complexity relaying system is proposed when smart relaying and equal gain combing (EGC) techniques are combined. In the proposed system, the destination requires only the phases of the channel state information in order to detect the transmitted signals. For the single-relay system with M-ary PSK modulation, it is shown that the system can achieve the maximal diversity under Nakagami and Hoyt fading channels. For the K-relay system, simulation results suggest that the maximal diversity can also be achieved. Finally, the diversity analysis for a smart relaying system under the scenario when both the source and relay are permitted to transmit in the second time slot (referred to as Protocol I) is presented. It is shown that Protocol I can achieve the same diversity order as Protocol II for the case of 1 relay. In addition, the diversity is very robust to the quality of the feedback channel as well as the accuracy of the quantization of the power scaling implemented at the relay.<p> For AF, the dissertation considers a fixed-gain multiple-relay system with maximal ratio combining (MRC) detection at the destination under Nakagami fading channels. Different from the smart relaying for DF, all the channel state information is assumed to be available at the destination in order to perform MRC for any number of antennas. Upperbound and lowerbound on the system performance are then derived. Based on the bounds, it is shown that the system can achieve the maximal diversity. Furthermore, the tightness of the upperbound is demonstrated via simulation results. With only the statistics of all the channels available at the destination, a novel power allocation (PA) is then proposed. The proposed PA shows significant performance gain over the conventional equal PA.

Η χρήση διαφορισμού σε συστήματα κινητών επικοινωνιών είναι μία ευρέως χρησιμοποιούμενη τεχνική για την καταπολέμηση των διαλείψεων που παρατηρούνται στις ραδιοζεύξεις. Στην παρούσα διατριβή παρουσιάζονται διάφοροι τρόποι και τεχνικές για την μελέτη της απόδοσης συστημάτων που κάνουν χρήση διαφορισμού στον δέκτη. Οι τεχνικές διαφορισμού που μελετώνται είναι ο διαφορισμός ίσου κέρδους (equal-gain combining) και ο διαφορισμός επιλογής (selection combining). Επιπλέον, στα πλαίσια της διατριβής αυτής, υποθέτουμε ότι ο δέκτης λειτουργεί σε κανάλια μη-επιλεκτικών συχνοτήτων με βραδείες διαλείψεις (flat and slow fading channels), ενώ το εκπεμπόμενο σήμα υφίσταται διαλείψεις κατά Rayleigh, Nakagami-m, Rice ή Hoyt. Αποδεικνύεται ότι η απόδοση του συστήματος στο οποίο ο δέκτης κάνει χρήση διαφορισμού εξαρτάται σε μεγάλο βαθμό από: • τη συσχέτιση μεταξύ των καναλιών διαφορισμού και • τη διαφορά της ισχύος μεταξύ των καναλιών διαφορισμού. Για να μελετηθεί η απόδοση ενός ΔΕ που λειτουργεί σε συσχετισμένα Nakagami-m κανάλια παρουσιάζεται παρουσιάζονται για πρώτη φορά σε κλειστή μορφή η συνάρτηση πυκνότητας πιθανότητας (ΣΠΠ) Nakagami-m μεταβλητών με εκθετική συσχέτιση, ενώ η αντίστοιχη συνάρτηση αθροιστικής κατανομής (ΣΑΚ) που προκύπτει έχει τη μορφή πολλαπλών άπειρων αθροισμάτων. / 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 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.